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  1. 1. March 2009 Vol. 20 No. 3 | $8.25 www.osa-opn.org RISK AND RESEARCH: HOW TO MAINTAIN A DIVERSE PORTFOLIO Optics & Photonics News A New Era in Optical Integration C.V. Raman and the Raman Effect Amateur Astronomy Gets Professional Optical Fiber Sensors
  2. 2. OPN March 2009 | 1 FEATURES | CONTENTS [ COVER STORY ] 20 A New Era in Optical Integration The Internet is increasingly taxing optical networks, and conventional network architecture cannot provide the scalability required to meet this demand. These authors advise telecommunications professionals to follow the lead of the microelectronics industry— by focusing on integrated solutions. Jacco L. Pleumeekers, Peter W. Evans, Wei Chen, Richard P. Schneider Jr. and Radha Nagarajan 26 Optical Fiber High-Temperature Sensors Optical fiber sensors allow researchers and engineers to make accurate, reliable measurements under high-temperature conditions. Anbo Wang, Yizheng Zhu and Gary Pickrell 32 The Professional World of Amateur Astronomy The work of today’s amateur astronomers goes far beyond peering through a telescope on a lonely mountaintop. Thanks to advances in solid-state imaging, software and inexpensive optics, they are collecting professional-quality data and making their own discoveries. Patricia Daukantas 40 C.V. Raman and the Raman Effect Barry Masters describes the life and legacy of one of the most important optical scientists of the 20th century. Barry R. Masters New technologies will be needed for photonic integration to scale to a “photonic Moore’s Law.” COVER PHOTO: Infinera’s Sheila Hurtt holds a tray containing 16 photonic integrated circuits. Photo by Gene Lee. OPN March 2009 Vol. 20, No. 3 Infinera’s Leigh Wade configures a system at the company’s system lab. The DTN system can accommodate four photonic-integrated-circuit- based line cards, each with data transmission capacity of 100 Gb/s. Gene Lee/Infinera
  3. 3. 2 | OPN March 2009 www.osa-opn.org CONTENTS | DEPARTMENTS 8 Scatterings Nanoscopy uncovers cells’ secrets; lasing mechanism depends on electron momentum; powerful light source for X-ray microscopy. Yvonne Carts-Powell 10 Optics Innovations CREOL’s tech-transfer success stories. Jenna Reiser and James Pearson 12 Light Touch The yellow sun paradox. Stephen R. Wilk 14 Conversations in Optics OPN talks with Philippe Morin, president of Metro Ethernet Networks at Nortel and OFC/NFOEC keynote speaker. 16 Viewpoint Risk and research: Maintaining a diverse portfolio. Ken Baldwin 18 The History of OSA George Ellery Hale and the Yerkes Observatory. John N. Howard 48 In Memory Remembering Richard E. Grojean, a professor emeritus at Northeastern University; Robert Hilbert, president and chief executive officer of ORA; and James L. Fergason, the father of the liquid crystal industry. 4 President’s Message 7 Letters 46 OSA Today 50 Book Reviews 52 Product Profiles 54 Marketplace 56 After Image 10 OPN Optics & Photonics News (ISSN 1047-6938), Vol. 20, No. 3 © 2009, Optical Society of America. OPN is published monthly except bimonthly July-August by the Optical Society of America, 2010 Massachusetts Ave., N.W., Washington, D.C. 20036; 202.223.8130; Fax 202.223.1096; Email opn@osa.org; Web www.osa-opn.org. OPN was published as Optics News from 1975-1989. (CODEN OPPHEL; GST #133618991; IPM #0895431). OSA is a not-for-profit society founded in 1916. Authorization to photocopy items for internal or personal use, or the internal or personal use of specific clients, is granted by the Optical Society of America, provided that the base fee of $6 per copy is paid directly to the Copyright Clearance Center, 27 Congress St., Salem, Mass. 01970-5575. For those organizations that have been granted a photocopy license by CCC, a separate system of payment has been arranged. The fee code for users of the Transactional Report Service is 0098-907X/99 $6. Permission is granted to quote excerpts from articles in this publication in scientific works with the customary acknowledgment of the name of the publication, page, year and name of the Society. Reproduction of figures and tables is likewise permitted in other articles and books provided that the same information is printed with them and notification is given to the Optical Society of America. 2009 nonmember and library subscription rates (domestic): $100/year. Membership in the Optical Society of America includes $7 from membership dues to be applied to a member subscription. Application to mail at Periodicals Postage pending at Washington, DC, and additional mailing offices. POSTMASTER: Send address changes to OPN Optics & Photonics News, 2010 Massachusetts Ave., N.W., Washington, DC 20036. Subscriptions, missing copies, change of address: Optical Society of America, Subscription Fulfillment Services, 2010 Massachusetts Ave., N.W., Washington, D.C. 20036; 800.766.4672. Back numbers, single issue, and foreign rates on request. Printed in the United States. OSA is a registered trademark of the Optical Society of America. ©2009. The content and opinions expressed in feature articles and departments in Optics & Photonics News and its occasional supplement, OPN Trends, are those of the authors and guest editors and do not necessarily reflect those of OPN or the Optical Society of America.   Jacquephoto.com Fraunhofer Institute for Laser Technology ILT OPN talks with Philippe The History of OSA Fraunhofer Institute 8
  4. 4. CODE V’s fast wavefront differential tolerancing gives you the accuracy of a long Monte Carlo analysis—in just seconds. Not only does CODE V provide an accurate statistical analysis of the expected RMS or MTF performance with selected tolerances and compensators, it also identifies the most sensitive, performance- driving tolerances. Utilizing a new Singular Value Decomposition algorithm, CODE V can even identify the best compensators for your system from all the possible compensators. CODE V’s unique Interactive Tolerancing leverages the speed of wavefront differential tolerancing to allow you to make changes to the tolerance values and instantly see the impact on performance. In addition, CODE V supports slope error and RMS surface error tolerance types for specifying tolerances on aspheric surfaces in a way that can be modeled in the software and measured by your fabricator. Try CODE V on your tolerancing problem.You’ll love the results. • Achieve fast system tolerancing with the accuracy of a Monte Carlo analysis of thousands of trials. • Let CODE V automatically select the most leveraged subset of compensators from a larger list for improved manufacturability and lowered costs. • Tolerance aspheric surface errors that are easily measured in production using surface profilometry. • Instantly see the impact of tolerance changes on system performance. • Create tolerance-insensitive designs by optimizing for best fabricated performance. Corporate Headquarters: 3280 East Foothill Boulevard, Pasadena, CA 91107-3103 (626) 795-9101 Fax (626) 795-0184 E-mail: info@opticalres.com Web: www.opticalres.com Offices: Tucson, AZ | Westborough, MA ©2008 Optical Research Associates. CODE V is a registered trademark of Optical Research Associates. Fewer Trials…No ErrorsDesign manufacturable, high-performance optics with CODE V’s comprehensive tolerancing features. OPTICAL DESIGN SOFTWARE www.opticalres.com CODEV_OPN.indd 1 3/25/08 12:27:28 PM
  5. 5. 4 | OPN March 2009 www.osa-opn.org PRESIDENT’S MESSAGE t several crisis points over the last century, large teams of high-level scientists and engineers mobilized to spearhead intense efforts to solve critical societal problems. These efforts not only produced the desired scientific breakthroughs, but also led to significant investments in basic and applied research and renewed public awareness of the scientific community’s tremendous capacity for innovation. The current energy crisis calls for just such a massive, coordinated effort. We are at a unique moment in time, with a new admin- istration in the United States committed to supporting initiatives focused on overcoming the energy and environmental crises. Recently, President Barack Obama, in his speech nominating OSA member Steven Chu as the new U.S. Secretary of Energy, announced that the pursuit of alternative and renew- able energy sources would be a “guiding purpose of the Department of Energy as well as a national mission.” Noting that energy independence lies “in the power of wind and solar [and]...in the innovation of our scientists and entrepreneurs,” Obama called for a “sus- tained, all-hands-on-deck effort” to address global energy concerns. Renewable energy based on solar, wind and biomass offers viable alternatives to fossil fuels. These options can greatly diminish a nation’s dependence on foreign energy, reduce greenhouse gas emissions, protect and preserve natural resources and stimulate economic growth through the development of new industries and technologies. The OSA community is uniquely positioned to play a prominent role in the further devel- opment of solar and other renewable energy technologies. We have the knowledge, expertise and resources to achieve significant advances in both research and applications—but we can only be successful if we marshal our resources and make the commitment to join in the “all- hands-on-deck effort” that President Obama and Secretary Chu are organizing. In June 2008, OSA held a very successful two-day Solar Energy topical meeting at Stanford University. At this meeting, an international group of leading scientists reported on new photovoltaic materials in combination with nanostructured electrodes, flat panel photovoltaic devices incorporating plasmonic resonances and nonimaging concentrators, all of which have the potential to significantly enhance solar energy efficiency. We are currently planning a second solar meeting to be held at MIT from June 24-25, 2009. I encourage all OSA members interested in this area to attend. In addition, we are forming an officers’ advisory group chaired by OSA President-elect Jim Wyant to lead our activities as OSA expands its efforts in solar energy. I invite all OSA members as well as the greater optics and photonics community to volunteer to put your expertise to work on this vital challenge. If you’d like join me in this effort, please send a message to osapresident@osa.org. Work- ing together, we can be a significant force in solving the world’s energy needs. — Thomas M. Baer OSA President We are forming an officers’ advisory group chaired by OSA President-elect Jim Wyant to lead our activities as OSA expands its efforts in solar energy. I invite all OSA members to put your expertise to work on this vital challenge.” A “
  6. 6. OPN March 2009 | 5 OEM Biomedical R&DImaging Laser Applications NEED STOCK OR CUSTOM FILTERS? need a volume or custom quote? ContaCt our sales department today or to receive your FREE catalog! more optics more technology more service • More than 1,300 Stock Filters Available for Same Day Shipping • Custom Designs & Coatings Available - Over 25 Years Coating Experience • High Transmission, Deep Blocking Fast Delivery Free Technical Support 800.363.1992 | www.edmundoptics.com/OP Visit us at Defense,security,& sensing Booth 1029 MANAGING EDITOR Christina E. Folz CREATIVE DIRECTOR Alessia Hawes Kirkland SENIOR WRITER/EDITOR Patricia Daukantas GRAPHIC DESIGNER Marko G. Batulan PRODUCTION MANAGER Stu Griffith PRODUCTION ASSISTANT Carlos X. Izurieta PUBLISHER John Childs ASSOCIATE PUBLISHER Alan N. Tourtlotte ADVERTISING SALES Anne Jones 202.416.1942 adsales@osa.org EDITORIAL ADVISORY COMMITTEE CHAIR James Zavislan University of Rochester EDITORIAL ADVISORY COMMITTEE Judith Dawes Macquarie University, Australia Madeleine Glick Intel Research Julio Gutierrez-Vega Tecnologico de Monterrey, Mexico Rongguang Liang Carestream Health Carlos Lopez-Marsical National Institute of Standards and Technology Lynne Molter Swathmore College Brian Monacelli the Optical Sciences Company Ali Serpenguzel Koç University, Turkey Maria Yzuel University Autonoma de Barcelona, Spain CONTRIBUTING EDITORS François Busque Fovea Technologies Inc. Alexandre Fong Optronic Laboratories Inc. G. Groot Gregory Optical Research Associates Bob D. Guenther Duke University John N. Howard Air Force Geophysics Laboratory (Retired) Bob Jopson Bell Labs, Lucent Technologies R. John Koshel Photon Engineering LLC Brian Monacelli the Optical Sciences Company Stephen R. Wilk Cognex Corp. OSA Board of Directors President Thomas M. Baer President-Elect James C. Wyant Vice President Christopher Dainty 2008 President Rod C. Alferness Treasurer Stephen D. Fantone Executive Director Elizabeth A. Rogan Chair, Publications Council Govind P. Agrawal Chair, Board of Editors Tony F. Heinz Chair, Corporate Associates Paul M. Crosby Chair, MES Council Irene Georgakoudi Co-Chairs, Science and Engineering Council David N. Fittinghoff and Edward A. Watson Chair, International Council Satoshi Kawata Directors-at-Large Neal S. Bergano, Thomas Elsaesser, Alexander L. Gaeta, Christoph S. Harder, Wilhelm G. Kaenders, Lenore McMackin, Masataka Nakazawa, Bishnu Pal, Philip St. J. Russell and David F. Welch Optics & Photonics News THE MAGAZINE OF THE OPTICAL SOCIETY
  7. 7. Prices subject to change. www.cambridge.org/us/physics 1-800-872-7423 New and Forthcoming Titles in Optics from Cambridge University Press Forthcoming… Polarization Holography Ludmila Nikolova and P. S. Ramanujam $115.00: Hb: 978-0-521-50975-6: 280 pp. Forthcoming… Cambridge Illustrated Handbook of Optoelectronics and Photonics Safa Kasap, Harry Ruda, and Yann Boucher $250.00: Hb: 978-0-521-81596-3: 576 pp. Forthcoming… High-Speed Electronics and Optoelectronics Devices and Circuits Sheila Prasad, Hermann Schumacher, and Anand Gopinath $90.00: Hb: 978-0-521-86283-7: 496 pp. New! Classical Optics and its Applications Masud Mansuripur $90.00: Hb: 978-0-521-88169-2: 720 pp. Fundamentals of Photonic Crystal Guiding Maksim Skorobogatiy and Jianke Yang $120.00: Hb: 978-0-521-51328-9: 280 pp. Geometrical and Trigonometric Optics Eustace L. Dereniak and Teresa D. Dereniak $80.00: Hb: 978-0-521-88746-5: 424 pp. Laser Fundamentals William T. Silfvast $80.00: Pb: 978-0-521-54105-3: 666 pp. Introduction to Nanoelectronics Science, Nanotechnology, Engineering, and Applications Vladimir V. Mitin, Viatcheslav A. Kochelap, and Michael A. Stroscio $80.00: Hb: 978-0-521-88172-2: 348 pp. Introduction to the Theory of Coherence and Polarization of Light Emil Wolf $45.00: Hb: 978-0-521-82211-4: 236 pp. 2nd Edition 2nd Edition
  8. 8. Please direct all correspondence to the Editor, Optics & Photonics News, The Optical Society, 2010 Massachusetts Ave., N.W., Washington, D.C. 20036. E-mail: opn@osa.org. OPN March 2009 | 7 FEEDBACK | LETTERS Photorealistic Rendering I most certainly enjoyed your article on photore- alistic rendering (January 2009). It is mind- boggling to think of how far we’ve come, and I was interested to learn about the techniques and technology that have made it possible. I do have one small nit to pick, though. In the interest of technical accuracy, I would like to mention that, although motion pictures are taken at 24 frames per second and the film runs through the projector at the same rate, each image on the screen is inter- rupted once during its residence in the projector gate. Thus, the sentence in your article that reads “The audience sees a single frame for only 1/24 of a second,” might be better expressed as, “The audience sees a single frame twice for about 1/96 of a second each time for a total viewing time of about 1/48 of a second.” Projectors have a circular shutter— with two open quadrants and two opaque quadrants—that rotates once per frame. One of the opaque quadrants blocks the light from the screen while the film is being pulled down to the next frame. The other opaque quadrant brings the “flicker” frequency up to 48 per second, which is above the threshold at which the viewer will see flicker on the screen at normal screen brightness. (Note: For silent film projectors, where the film runs at a nominal 16 frames per second, the shutter has six segments, three open and three opaque.) Here’s another statistic: A two-hour film will have 172,800 frames. Woodlief Thomas, Jr. Merriwood@aol.com Naples, N.Y., U.S.A. OSA Historians I am a longtime fan of your articles and lectures on the history of optics, and I partic- ularly enjoyed your latest article on OSA historians (January 2009). However, there is a minor error in your discus- sion of Hilda Kingslake. Hilda died in February 2003, some 20 months before the 75th anniversary of the Institute of Optics, and she had been incapacitated for a few years prior to that. Unfortu- nately, she was thus not able to write the history of the Institute of Optics for its 75th anniversary. I assumed the task of editing that volume, which included, among its 75 essays, 12 from Hilda’s earlier 50-year history. Carlos Stroud stroud@optics.rochester.edu Rochester, N.Y., U.S.A. JOHN HOWARD REPLIES: I did indeed make a dumb mistake in the January history column. I simply got a bit mixed up by reading her column on the His- tory of the Institute just as I was writing about OSA’s 50th Anniversary. I read her column two or three months ago and just didn’t keep good enough notes. Sorry about that! John N. Howard Contributing Editor, The History of OSA alistic rendering (January have made it possible. I do have one small nit to pick, though. In the interest of technical accuracy, I would like to mention that, although motion pictures are taken history of optics, and I partic- ularly enjoyed your latest article on OSA AdvAnces in imAging REGISTER TODAY Digital Holography and Three-Dimensional Imaging (DH) Fourier Transform Spectroscopy (FTS) Hyperspectral Imaging and Sensing of the Environment (HISE) Novel Techniques in Microscopy (NTM) Optical Trapping Applications (OTA) Advances in Imaging includes five Topical Meetings to bring together leaders from these technical areas to network, discuss, and share information and research. Take this opportunity to learn more about your own and other related fields. Register today for the Advances in Imaging 2009 OSA Optics & Photonics Congress! April 26-30, 2009 Sheraton Vancouver Wall Centre Hotel vancouver, bc, canada hotel reservation deadline: MArCH 25, 2009 pre-registration deadline: AprIl 1, 2009 FOr THE lATEST INFOrMATION ON All MEETINgS VISIT www.osa.org/congresses Five ColloCAted topiCAl Meetings And exhibit 2009 osA optics & photonics Congress
  9. 9. 8 | OPN March 2009 www.osa-opn.org SCATTERINGS | NEWS that wavelength. The minimum resolu- tion for normal microscopy is 200 nm. However, that is problematic for cell biologists who would like to view struc- tures smaller than that. Electron microscopy and X-rays image smaller objects—but they can only be used on dead, fixed cells. For live cells, even ultra- violet light is too energetic, ionizing molecules and disrupting normal functioning. “For biologists look- ing at live cells,” Hell says, “optical microscopy is the only option.” All optical fluorescence meth- ods depend in one way or another on being able to turn fluorescent mol- ecules on and off and build up an image over time. STED micros- copy incorporates confocal imag- ing to reduce the resolution out of Fluorescent optical microscopy is allowing researchers to image features much smaller than the diffrac- tion limit of visible light. This imaging technique, called nanoscopy, can yield resolutions down to hundredths of a micron—the size of large molecules. These methods have been in develop- ment for the past 15 years, since Stefan Hell, now director at the Max Planck Institute for Biophysical Chemistry in Göttingen, Germany, and head of the department of nano-biophotonics, reported the stimulated emission deple- tion microscopy (STED) method (Opt. Lett. 19, 780). Now Hell is educating the optics and biotech communities about nanoscopy. How does it work? The Abbe limit claims that objects illuminated with the light of a certain wavelength cannot be resolved to distances smaller than half Nanoscopy Uncovers the Secrets of Cells the imaging plane, then excites the fluo- rophores in the small volume with a laser pulse at the absorption wavelength. Before the sample fluoresces, how- ever, a second laser pulse, at a longer wavelength, depletes the energy from most of the confocal area. This pulse may be donut-shaped, leaving a small spot untouched in the center. Fluores- cence then comes from only the center area. The smaller that area, the better the resolution. In theory, the lower limit to the reso- lution is a single molecule. The tradeoff, however, is the reduction in signal (and the cost of the equipment). In practice, Hell’s group has demon- strated resolutions of less than 20 nm (Opt. Express 16, 4154). STED requires relatively high laser intensities, but other methods have different intensity require- ments, depending on the lifetimes of their on and off states. The journal Nature Methods recently named super-resolution fluorescence microscopy the 2008 Method of the Year (Nature Methods 6, 1), and many research groups are using methods such as this to image structures and pro- cesses inside living cells.“Molecular-level resolution with visible light is, of course, possible,” says Hell. — Yvonne Carts-Powell As the dendritic structures of neurons twist, the tips (indicated by arrows) turn cup-shaped. Images were obtained with stimulated emission depletion microscopy. (a) Nanoscale image of the endoplasmic reticulum of living mam- malian cells labeled with a fluorescent protein. (b) The 2D anisot- ropy histogram reveals only mobile fluorescent proteins. (c) Image of b-actin in living cells. (d) This histogram reveals static and free rotating molecules. (f) Images of immobile (green) and mobile (red) b-actin are created based on their position in the histogram. MaxPlanckInstituteofNeurobiology Opt.Express16,21093(2008) 1 µm 1 mm 1 mm 1 mm (a) (b) (d) (f) (c) (e)
  10. 10. OPN March 2009 | 9 Researchers at the Fraunhofer Institute for Laser Technology ILT (Aachen, Germany) have devel- oped a powerful soft X-ray light source and collector lens system smaller than 2 m3. The small size allows it to be integrated directly into a microscope. The hollow-cath- ode-triggered pinch plasma—gen- erated from ionized nitrogen—emits at 2.88 nm. A prototype microscope captures 3D images of several- micrometer-thick samples in tens of seconds. Klaus Bergmann, who leads the team, adds, “We will be able to bring the exposure time down to below 10 s for the larger samples too, by optimizing the design of the condenser mirror.” Anew lasing mechanism was recently reported from a quantum cascade laser that appears to depend on non- equilibrium electrons with high momen- ta. Kale Franz and others at Princeton University (N.J., U.S.A.) noticed a second lasing wavelength, with notably different characteristics from the design wavelength (Nature Photon. 3, 50). Franz and others in Claire Gmachl’s Mid-Infrared Technologies for Health and the Environment Center at Princeton designed and built a laser composed of interleaved AlInAs barriers and InGaAs quantum-well layers. But when they test- ed the quantum cascade laser designed to emit at 9.5-µm light, they discovered a second lasing peak at 8.2 µm. “With population inversion, you’d normally think of having a pool of electrons in that upper laser state so that they can contribute to lasing,” said Franz. “But we’ve shown that we can achieve a population inversion—and lasing—even at a point where we don’t have that electron pool.” The second wavelength is generated by the transition from a different energy level but still acts as a quantum cascade laser. It had bizarre characteristics: The power output increased with rising tem- perature (over a certain range of tempera- tures); it competed for electrons with the primary wavelength; and it had a lower threshold current than the primary wavelength. Mid-infrared lasers that offer higher efficiency and work at higher temperatures could be tremendously use- ful. One driver for the devel- opment of quantum cascade lasers is that they operate in the mid- and far-infrared range (roughly 3 to 300 µm in wave- length), which can be used to detect traces of water vapor, ammonia, nitro- gen oxides and other gases that absorb infrared light. Applications include air quality monitoring, medical diagnostics, homeland security, free-space commu- nications and defense countermeasures. The new discovery should help make these devices smaller, more efficient and more sensitive, Gmachl said. To explain the phenomenon, the researchers had to move away from the standard assumption that the wave- vector of electrons in both the low and excited states was zero. In most semi- conductor lasers, including quantum cascade lasers, stimulated emission occurs only from electrons with nearly zero momentum. One effect of using a “high k-space” transition for lasing is that the shape Lasing Mechanism Depends on Electron Momentum DID YOU KNOW? Princeton graduate students Kale Franz (left) and Stefan Menzel have uncovered a new lasing mechanism in quantum cascade lasers. Frank Wojciechowski Yvonne Carts-Powell (yvonne@nasw.org) is a freelance science writer who specializes in optics and photonics. Inset: A diatom imaged with a prototype X-ray microscope. Fraunhofer Institute for Laser Technology ILT of seconds. Klaus Bergmann, who leads the team, adds, “We will be able samples too, by optimizing the design of the condenser mirror.” primary wavelength; and it had the mid- and far-infrared range length), which can be used to detect 2 µm of the energy sub-bands changes from their parabolic shape at k=0. This leads to suppression of optical absorption by 90 percent, says Franz. Because of the physical properties of this new energy space, Franz says, “our laser emission wavelength and re-absorption wave- length are different. This means less loss, lower thresholds, more power, higher efficiencies...all the things that make lasers better.” The researchers are figuring out how to optimize the new lasing process. The mechanism may also be applicable to other types of semiconductor lasers.
  11. 11. 10 | OPN March 2009 www.osa-opn.org OPTICS | INNOVATIONS CREOL’s Tech-Transfer Success Stories Jenna Reiser and James Pearson Savvy industrial leaders know that they need to build strong partner- ships with research institutions in order to capture the best products and talent. But the transfer of technology from lab bench to business plan isn’t always as straightforward as it might seem. That is why the Center for Research in Optics and Lasers (CREOL) at the University of Central Florida (UCF) in Orlando, Fla., U.S.A., has dedicated itself to not only nurturing scientific discoveries in optics, but to promoting the growth and com- mercialization of the applications that result from them. When executives or entrepreneurs from a photonics company need tech- nology for a new product, they often seek help from university research- ers who are skilled in both basic and applied research. UCF fosters these collaborations through its Photonics Incubation Program, which is housed in the CREOL building on UCF’s main campus and is part of the UCF Technol- ogy Incubator (UCFTI) within the UCF Business Incubation program. The UCF Business Incubation Pro- gram began in 1999 and, in total, it has helped more than 90 clients to generate more than 900 new jobs and over $200 million in annual revenue. The program to be one of the largest photonic crystal growers in the world. Another successful spin-off, Optium, was created in 2000 by CREOL Profes- sors Guifang Li and Patrick LiKamWa, along with Paul Yu at the University of California, San Diego. In 2006, Optium went public, and, in 2008, it was acquired by and merged with Finisar, a global technology leader of fiber optic subsystems and network test systems. The combined company is one of the biggest suppliers of optical components, modules and subsystems for the communications industry in the world, with more than $660 million in annual revenues. Leon Glebov, CREOL senior research scientist, helped found OptiGrate in 2007 using his photo-thermal refractive glass technology. OptiGrate produces unique holographic volume Bragg gratings for optical beam control in high-power laser systems for myriad applications, including military laser devices, optical telecommunication systems, entertainment systems and medical and security sensing devices. A recent CREOL spin-off is BD Displays, which was founded by Profes- sors Michael Bass and Dennis Deppe. The two founders had each conducted Jacquephoto.com works with start-up and early-stage com- panies to provide mentoring and training in business development, networking opportunities, access to UCF faculty and labs, and other tools that are needed to create financially stable, high-growth, technology-driven enterprises. CREOL and the UCFTI have spun off many photonics-based companies, all of which continue to spark new areas of growth. For example, one of the earliest CREOL spinoff companies is Crystal Photonics Inc. (CPI), a manufacturer of optical crystals for many applications in biophotonics, microelectronics and photonics. Founded in 1995 by CREOL Professor Bruce Chai, CPI is now a multi-million-dollar enterprise located in Sanford, Fla. The company is positioned CREOL made significant contributions to the intellectual capital of photonics research. In fact, in 2008, it helped propel UCF to a national patent ranking. The Center for Research in Optics and Lasers (CREOL) at the University of Central Florida prides itself on its strong focus on technology transfer—and it has the multi-million-dollar success stories to show for it. CREOL Professor Peter Delfyett (center) helped start Raydiance, a photonics company based on CREOL-developed ultrashort- pulse laser technology.
  12. 12. OPN March 2009 | 11 independent research, which, when combined, provided the foundation for next-generation high-resolution and high- brightness micro-displays for various applications, including training, personal entertainment and gaming devices. CREOL Professor Martin Richardson (who also leads CREOL’s new Townes Laser Institute) helped form LP Photon- ics in 2008 to hasten the commercializa- tion of extreme ultraviolet (EUV) optical source technology. This new venture will provide the powerful, reliable, EUV light source that is needed by the semiconduc- tor manufacturing industry for the next generation of optical lithography that will allow for continued advancement of Moore’s law. Barry Schuler, a former CEO and chairman of AOL, started Raydiance Inc. in 2004 using ultrashort-pulse laser tech- nology developed primarily by CREOL Professor Peter Delfyett. The company has since raised more than $25 million of venture capital. Enabled by CREOL photonics patents, and by software and rugged fiber optic technology, Raydiance products provide a versatile, compact, plug-and-play platform to enable innova- tion and commercial applications. CREOL has also made significant contributions to the intellectual capital of photonics research. In fact, in 2008, it helped propel UCF to a national patent ranking. UCF joined other prestigious research universities, including the Massachusetts Institute of Technology and Stanford University, in the top 10 of the “2008 Patent Scorecard for U.S. Universities.” UCF also ranked third in the industry impact category, which measures the role that university patents play in serving as a foundation for other patents and technologies. The rankings were published in the September 2008 issue of Intellectual Property Today. One of the ways that UCF cultivates its intellectual property is through its Office of Technology Transfer (OTT), which is part of the UCF Office of Research and Commercialization (UCFORC). The OTT has licensing associates who work with UCF research- ers to help them protect, manage and license their intellectual property. CREOL also has an industrial affiliates program with more than 60 member companies. These partners benefit from CREOL’s strong alliance with other UCF research units, including the Nanoscience Technology Center, the Burnett School of Biomedical Sciences, the Advanced Materials Processing and Analysis Center, the Institute for Simula- tion and Training, the Florida Solar Energy Center, and UCF’s new College of Medicine. Joe Giampapa, the director of technology transfer in the UCFORC, speaks highly about CREOL’s talent for commercializing its research. “CREOL’s optics research generates a large number of breakthrough inventions, which pave the way for greater licensing and spin-out opportunities,” he said. CREOL has a long tradition of engag- ing in technology transfer, beginning with the vision of its founder Bill Schwartz, a laser pioneer who proposed creating a university-based center that would give Florida’s high-tech industries access to leading research and facilities in optical and laser sciences and engineering. Today, under the leadership of its new dean, Bahaa Saleh, CREOL is continu- ing its emphasis on strong research and partnerships. CREOL hosts two new research centers funded by the state of Florida’s Center of Excellence program: The Florida Photonics Center of Excel- lence (FPCE), which was started in 2003 with a $10 million Florida grant, and the Townes Laser Institute, which opened its doors in 2007 after having been estab- lished with a $4.5-million grant from the state, plus matching funds and funding for UCF faculty. “Preparing students to function well in the technological world is essential,” said Saleh, “and maintaining our strong links with industry and forging new links will continue to be of paramount importance.” t James Pearson (jpearson@creol.ucf.edu) is the director of research and administration at CREOL, The College of Optics & Photonics, in Orlando, Fla. Jenna Reiser is a communications consultant for CREOL. OPTICAL ISOLATORS Free Space Isolators • Epoxy-Free Optical Path • All Isolators are Set for a Horizontal Input Polarization • Units are Available Without Polarizers to Function as Faraday Rotators • Custom Wavelengths, Polarizers, and Apertures Available Fiber Isolators • High-Power, Polarization-Independent or Polarization Dependent Models • SM or PM Fibers Used on the Input and Output • Wavelength Ranges from 780 to 1550 nm New TOOLS OF THE TRADE Catalog! Request Online at www.thorlabs.com 30 W to 50 W SHIPPING FROM STOCK OFR_Isolators_2.qxd 12/4/08 4:19 PM Pa
  13. 13. www.osa-opn.org12 | OPN March 2009 LIGHT | TOUCH The Yellow Sun Paradox Stephen R. Wilk Though I am old with wandering Through hollow lands and hilly lands, I will find out where she has gone, And kiss her lips and take her hands; And walk among long dappled grass, And pluck till time and times are done The silver apples of the moon, The golden apples of the sun. — The last verse of The Song of Wandering Aengus by William Butler Yeats (1865-1939) If you ask preschool children to draw the sun, they’ll make a yellow circle, often with visible rays emanating from it (and maybe a smiling face). The “gold ball” in the story The Princess and the Frog represents the Sun, says mytholo- gist Joseph Campbell, because gold is the solar metal. Egyptian, Celtic, Chinese and Aztec representations of the sun are made of gold as well. And if you ask an average person on the street what color the sun is, he or she would say it was yellow. And yet the sun is not yellow. In fact, sunlight is the very definition of white light. If the sun were truly yellow, the colors of everything we see would be we perceive. But, as Plait points out, if white objects appeared yellow in the sky, then clouds would seem yellow, and they’re not. A third possibility is that yellow is the most accurate representation of the sun’s color when it is low in the sky—the only time we can look at it without hurting our eyes. When the sun is high, it’s too bright to look at. As it approaches the horizon, more of its light gets scattered away by the atmosphere, so you can glance at it more easily. The sun’s color changes because of that scattering: It goes from yellowish to orange to red and finally magenta. Plait finds this claim interesting but he has some doubts. He remembers the sun most when it is glow- ing magenta on the horizon, yet on the whole he does not perceive it as red. To get to the bottom of this, I mod- eled the passage of light from the sun through an atmosphere that scattered according to a strict Rayleigh scatter 1/l4 law. I assumed Illuminant D65 (noon daylight), a scattering cross-section that depends upon the inverse fourth power of the wavelength (and a loss expo- nential in the product of this cross-section times the optical path length, multiplied subtly altered. As anyone who works in a lithographic facility knows, working under truly yellow light can be unnerv- ing. The CIE coordinates of the standard illuminants all lie close to (0.3, 0.3), the white locus of the color diagram. So the sun is undoubtedly white, yet everyone seems to perceive it as yellow. What gives? Phil Plait, who manages the Web site “The Bad Astronomer” (on which he exposes examples of bad astronomy), has put forth some possible explanations for why people perceive the sun as yellow. One is that the same Rayleigh scatter- ing that is responsible for the sky’s blue- ness also makes the sun appear yellow, since some of the blue has been scattered out. (This is the most common sugges- tion I hear when I mention the paradox to people.) But the amount of blue light scattered out is far too small to have a noticeable effect on the sun’s color. The CIE standard illuminants already have the effects of scattering built into them, and they predict a white sun. A second suggestion is that the sun seems yellow because we are comparing it to a blue sky. Perception studies show that the background can affect the color There are a few things you can count on in this world: The sky is blue; grass is green; and the sun is yellow…right? Nicolas Raymond
  14. 14. OPN March 2009 | 13 by a constant), and the usual three stan- dard color functions. I then calculated the CIE chromaticity coordinates (x,y) in the usual fashion by numerical integration of the product of the illuminant, scattering function and color over wavelength space, then normal- izing the chroma- ticity coordinates X, Y and Z. The results were interesting. The starting point, with negligible scatter, was the Illuminant D65 “white point” of (0.313, 0.329). However, as soon as the path lengthened, the trajectory of the locus of the apparent sun color started moving directly toward the spectral locus at about 570 nm, which is about as yel- low as you can get. It continued toward this point for some time before veering off slowly toward 580 nm, which is still well within what is generally termed “yellow.” Then it gradually turned orange and then red, and asymptotically approached the deep red terminus of the spectral locus. Its trajectory superficially resembles the Planckian locus, representing the perceived color of a blackbody radiator as it cools—but the differences are sig- nificant. The blackbody starts not at the white center, but at the limiting point of (0.328, 0.502), at the light-blue color of blue heat. It then arcs across, skirting the edge of the white region at about 6,000 K before cutting across the yellow range, between about 4,000 K and 2,500 K, and asymptotically approaching the red end of the spectral locus. The difference is that the Planckian locus curve starts in the blue and spends much less of its length in the yellow portion of the color diagram. So, until the sun gets very low in the sky and starts to change from orange to red, it spends all of its time as either white or yellow. As soon as it is attenu- ated enough to look at even fleetingly, it appears yellow, and it remains this way until it rapidly begins to change color at sunset. It’s not just coincidental that the sun appears mostly yellow—this color is the complement of the blue sky. On the chromaticity diagram, it is diametrically opposite the blue sky locus, which this calculation sets at (0.2279, 0.2312) in the limit of small amounts of scatter. The chromatic- ity coordinates of the blue sky change very slowly with increased scatter distance, ulti- mately moving toward the white locus as the scattering length approaches infinity. When you subtract this blue from the white, you get yellow as a residue. So, in essence, each of the possible explanations put forth by Plait are, in a sense, correct. Another possible reason for why we view the sun as yellow could arise from our ancestors. Early humans would naturally view the sun as a “fire in the sky,” since they were accustomed to using fire to warm themselves and prepare food. They would believe the sun to be yellow-orange—the most prominent color in flames consisting of soot heated by combustion. And their experience would confirm this belief; they would see a yellow sun in the sky—as soon as it was dim enough to be viewed directly. t Stephen R. Wilk (swilk@comcast.net) is an optical engineer based in Saugus, Mass., U.S.A. References and Resources >> F. Birren. The Story of Color: from Ancient Mysticism to Modern Science, Crimson Press, Westport, Conn., U.S.A. (1941). >> M.N. Perrin. “Calibrating the Color Tem- perature Relation: The B-V Color of the Sun,” Annales de Physique 6 (1-2), 115-20 (1981). >> P.C. Plait. Misconceptions and Misuses Revealed, from Astrology to the Moon Landing “Hoax,” Wiley and Sons. (2002). Also see his Web site at http://blogs. discovermagazine.com/badastronomy/. >> The Munsell Color Laboratory Resources Page: www.cis.rit.edu/mcsl/online/cie. php. As soon as the sun is attenuated enough to look at even fleetingly, it appears yellow, and it remains this way until it rapidly begins to change color at sunset. OPTOMECHANICS New TOOLS OF THE TRADE Catalog! Request Online at www.thorlabs.com NEW PRODUCT IDEAS WELCOME www.thorlabs.com Hungry for your thoughts… Bringing Automated Control to the Table NEW Gimbal Mount • Over 1,200 Products • Shipping From Stock • Mounting Essentials • Cage Systems • High-Precision Positioning • OEM Designs and Production Mech_OPN_rev3.qxd 11/3/08 5:07 PM Page
  15. 15. 14 | OPN March 2009 www.osa-opn.org OPTICS | CONVERSATIONS OPN Talks with … Philippe Morin President of Metro Ethernet Networks at Nortel and OFC/NFOEC Keynote Speaker and high-performance digital signal processing techniques that result in easily deployable 40 G/100 G transmis- sion systems. The solution integrates dispersion compensation technology into the world’s first coherent 40 Gbps receiver, for both chromatic and polar- ization mode dispersion. This is a novel way of solving high-speed networking for our customers—and frankly, this is the only technique being discussed by standards organizations as a practi- cal option for 100 G transmission in 50 GHz systems. The solution is being deployed in metropolitan, long-haul and submarine applications that operate over a variety of line systems, including foreign line systems. How does Nortel stay at the fore- front of emerging technology? It is part of our DNA. We invest and participate in all the relevant standards bodies to ensure that we continue to distinguish ourselves. We recently launched our WDM PON Ethernet access solution in the fourth quarter; it For the past 20 years, Philippe Morin has watched the field of optical communications grow into a multi-billion- dollar industry that offers cutting-edge solutions to meet the demands of today’s super-fast, high- bandwidth networks. Morin is the president of Metro Ethernet Networks for the Canadian telecommunications equipment-maker Nortel. He leads the company’s production and logistics, research and product development, as well as business operations for Nortel’s optical and carrier Ethernet portfolios. What are the latest trends in Metro Ethernet? What is needed to make 100 GbE a deployable reality? In recent years, the Metro Ethernet busi- ness has been focused on consolidating Ethernet and optical capabilities, where Carrier Ethernet and WDM technologies are being recognized as the most efficient and cost-effective means of transporting today’s traffic across the network. We expect to see a lot of innovation behind these technologies as we move forward. The 100 GbE (that’s 100 Gigabits per second of Ethernet traffic on a single port) will provide a means for operators to both scale and simplify their networks. The technology will be deployed when the 100 GbE standards are finalized and volume-deployable 100 G optical solu- tions become available. Tell us about the 40 G/100 G Adaptive Optical Engine, one of Nortel’s most recent innovations. In our 40 G/100 G Adaptive Optical Engine, we use advanced modulation
  16. 16. OPN March 2009 | 15 provides a dedicated wavelength of high- capacity bandwidth per user. E-SPRING is an imminent technology that we are developing by transposing some of the traditional carrier grade SONET/SDH shared ring values to Ethernet. What changes has Nortel seen over the past 20 years? Where is it headed? After having survived both the boom and bust cycles of this business, I can say that the overall pace of the industry has increased dramatically over the last 15 years; companies need to move faster to stay abreast of each other in this crowded market. At Nortel, the combining of our Carrier Ethernet and optical businesses has allowed us to extend our innovation capa- bilities with Ethernet solutions and maintain our leadership posi- tion as one of the few global optical vendors with solutions in each market segment. What does the future of optical technology in telecommunications look like? The increased connectivity of high- bandwidth applications between a larger number of users signifies that optical systems will need to scale in a simple fashion and be very flexible so that a wavelength can easily be routed anywhere in the network. As digital signal processing techniques become the increasingly popular choice for correcting signal degradations, clumsy custom-engineered optical compensation devices such as dispersion compensation modules or PMD compensators will become obsolete. How did you get involved in optical communications? I have worked in the optical business for the past 20 years in various positions, including product management, sales and marketing. With my engineer- ing background, I have found it very interesting to remain close to a technol- ogy that we knew from early on would revolutionize the speed of global business operations and increase the power of personal networking. What has been the most significant advance in telecom that you’ve witnessed? What’s been the most surprising trend? The biggest advance in the optical industry is the incorporation of wireless transmission techniques, in particular coherent detection, after 20 years of systems that used intensity modulation detection. The new tech- niques enable deployable high-capacity transmis- sion systems. With respect to a surprising trend, I don’t think anyone realized the speed at which net- work bandwidth would be depleted, with video being the key gobbler in a variety of applications, including peer-to-peer net- working, personal video recorders, home theater TV systems, and even gaming systems such as the Xbox LIVE. Can you give us a sneak preview of what you will discuss in your plenary presentation? I will present my vision of where the optical network industry is headed. I will also speak to the innovations that I believe are necessary as we move forward in this difficult economic environment. Philippe Morin will deliver his plenary talk on Tuesday, March 24, 2009, at the OFC/ NFOEC conference in San Diego, Calif., U.S.A. For more information or to register, visit www.ofcnfoec.org. Angela Stark (astark@osa.org) is OSA’s public and government relations specialist. The biggest advance in the optical industry is the incorporation of wireless transmis- sion techniques, in particular coherent detection.” “ NEW! • Dual-Channel Power and Energy Meters • Analog and Digital Displays • High-Power and High-Sensitivity Models - Si and Ge Sensors - Thermal Sensors - Energy Sensors - Integrating Spheres • Interchangeable Sensor Heads • NIST-Traceable Calibration OPTICAL POWER METERS NEW PRODUCT IDEAS WELCOME www.thorlabs.com Hungry for your thoughts… New TOOLS OF THE TRADE Catalog! Request Online at www.thorlabs.com PM100D NEW! 2_PowerMeters_LFW_07.qxd 10/3/08 5:23 P
  17. 17. 16 | OPN March 2009 www.osa-opn.org Risk and Research: Maintaining a Diverse Portfolio Ken Baldwin How can funding agencies strike the right balance between financing “high- risk, high-reward” research—work that is initially uncertain but that could translate into major breakthroughs—and more modest studies that will likely produce solid, incremental results? c Capacity risk: The chance that research funding, staff or equipment may not be sufficient to perform the required task. (As we all know, some- times the true cost of research turns out to be far different from that in initial proposals.) c Failure risk: The risk that research might fail to meet a particular objective. There are rarely complete failures, however, since research can always point to avenues for further work. c Collaborative risk: The possibility that research is constrained by “safe” choices of collaborators, such as those from lead institutions or with strong reputations. This is a very real issue in multi-disciplinary research. c Precedence risk: The chance that funded research will become obsolete because another investigator or group makes the finding first. c Regulatory risk: The risk (particu- larly to institutions) that research transgresses certain ethical, regula- tory and commercial constraints. The FASTS forum focused primar- ily on transformative risk as the key VIEWPOINT The global financial crisis has brought the concept of risk to the forefront of our collective consciousness. It has also put pressure on the public and private entities that fund scientific research, given the competing needs of other sectors of the world’s economy. Yet now is arguably the time to invest in research that can help us secure a better future. It is also an ideal opportunity to reflect on how we can create a robust and flexible fund- ing system. In Australia this year, the Federation of Australian Scientific and Technological Societies (FASTS) held a forum on Risk Aware Research, which focused especially on the challenges faced by small nations. It explored how risk should be addressed across a limited range of research funding programs. Risk in research has many defini- tions. We therefore developed, per- haps for the first time, a “taxonomy of risk” to provide a starting point for discussion: c Transformative risk: The risk that funding for ideas that could transform the way we think may be delayed or denied because the research violates existing interests or views. VIEWPOINT challenge to research funding programs. The U.S. National Science Foundation (NSF) has also recently studied transfor- mative research. According to the NSF, transformative research is “characterized by its challenge to current understand- ing or its pathway to new frontiers.” The NSF report recognized the importance of fostering transformative risk. Its key recommendation was “that the NSF develop a distinct, foundation- wide transformative research initiative distinguishable by its potential impact on prevailing paradigms and by the potential to create new fields of science, to develop new technologies and to open new frontiers.” However, the report recognized that the peer-review process is often risk-averse. It notes that transformative research does not “fare well wherever a review system is dominated by experts highly invested in current paradigms or during times of especially limited budgets.” There are ways that researchers can counter the conservatism of the peer- review process, however. For example, they can game the system by putting forward “safe” proposals based on incremental work for which they have already achieved results. Then, after
  18. 18. OPN March 2009 | 17 peer reviewers recom- mend funding, the investigators can go on to pursue more specula- tive research, since they already have the incremental results “in the bag.” Such gaming gives researchers built-in agil- ity. However, allowing perverse behavior to counter inherent short- comings is bad policy. It is better to establish the right incentives in the first place. So how can transformative risk be encouraged? The FASTS forum identi- fied the following approaches to embed risk-awareness in funding programs: c Aggregation: The sheer size of research programs can provide the flexibility needed to encourage transformative risk. Centers of excel- lence and other large programs allow the quarantining of discretionary funds for more risky projects. Yet it is important to recognize that risk can- not easily be borne at the individual project level, particularly in tight funding environments. c Diversity: A portfolio approach that provides a large choice of funding bodies and programs can create a range of risk-friendly mechanisms to encourage transformative research. c Time: Agility is also encouraged through longer timeframes for re- search programs, particularly when coupled with aggregation. c Flexibility: Programs need to ensure that the funding rules allow research to change direction if necessary. This is sometimes achieved by not requir- ing a project to report against the original objectives, thereby encour- aging researchers to move in new directions. c Rewards: Contracts should encour- age the handing back of funds in cases where research reaches a dead Funding programs that encourage risk are as much about developing the human capacity to push the boundaries of knowledge as they are about the research outputs themselves. end. This should be treated as good profes- sional practice, where favorable consideration is given to provid- ing additional funds for future successful applications by the same investigators. c Costs: By minimiz- ing the regulatory and transaction costs of grant applications, researchers will be encouraged to apply more often for grants that might be more risky. c Context: A flexible risk-evaluation framework will encourage risk in the appropriate context. For example, the evaluation of research in a commer- cial setting must be different than that for fundamental research at universities. DARPA (the Defense Advanced Research Projects Agency) in the United States funds high-risk research aimed at identifying potentially disruptive threats and challenges. The NSF SGER (Small Grants for Exploratory Research) fund is also aimed at testing new, high-risk ideas; this option is particularly useful for early-career researchers. Funding programs that encourage risk are as much about developing the human capacity to push the boundar- ies of knowledge as they are about the research outputs themselves. Encour- aging risk-aware research can help scientists and engineers to investigate bold new ideas and free us all to create an exciting future. t Ken Baldwin (Kenneth.Baldwin@anu.edu.au) is a professor of physics at the Australian National University and president of FASTS. He is a cur- rent member of the Public Policy Committee. [ References and Resources ] >> The Federation of Australian Scientific and Technological Societies: www.fasts.org. >> “Enhancing Support of Transformative Research at the National Science Foundation,” NSF document NSB-07-32, May 7, 2007. Revolutionizing data shaRing in publishing n Interact with large 2D and 3D images and datasets associated with peer-reviewed journal articles. n Explore image data quickly and easily over the internet. Free access to Interactive Science Publishing articles now in Optics Express and Journal of the Optical Society of America A. ISPInteractive Science Publishing OSA Introduces Learn more at www.opticsinfobase.org/isp Interactive Science Publishing (ISP)
  19. 19. www.osa-opn.org18 | OPN March 2009 OSA | HISTORY George Ellery Hale served as the Society’s first vice president from 1916 to 1917, during the presidency of Perley Nutting. He was a charter member and he became an Ives Medalist in 1935. (Hale never became president; it wasn’t until 1922 that OSA leadership decided that VPs would automatically advance to that post at the next election.) Hale was interested in solar astronomy, and, in 1888, while still an undergraduate at MIT, he invented the spectrohelio- graph, with which he discovered solar vor- tices and the magnetic fields of sunspots. After MIT, he returned to his father’s house in Kenwood, Ill.—a district in south Chicago not far from the universi- ty—where he continued his studies from his own private observatory using a 12-in. refractor telescope. His efforts caught the eye of William Rainey Harper, president of the University of Chicago, who wrote on July 1, 1892, to ask if Hale would be interested in joining the university. The 24-year-old Hale asked his father for advice—and got a lot more than that in return. The elder Hale wrote to Harper that same day, offering to donate the Kenwood Observatory and its 12-in. tele- scope to the University of Chicago if the university would: 1) appoint his son as an associate professor of astrophysics; 2) raise $250,000 over the next three years for an even larger observatory; and 3) appoint George as director of the observatory. Harper took that response to the next board of trustees meeting, and George’s appointment (although with no salary) was approved on July 26, 1892. Hale was pleased with his new academ- ic status, and he took off in early August of aperture and a gain of 23 percent in light-gathering ability. The new telescope would then be the world’s largest, prob- ably for many years! (It is still the world’s largest refractor.) Hale said the mounting and tube could be finished in time to be displayed at the Columbian Exposition in Chicago in May 1893. Yerkes’ name would be remembered for all time! All of this went to Yerkes’ head, and he agreed to support the project. He said: “I don’t care what the cost; just send me the bill!” The Chicago papers headlined that Yerkes will spend a million dollars to “Lick the Lick.” Within a week, a contract was awarded to Clark & Sons to finish the disks. Another contract went to War- ner & Swasey of Cleveland to build the mounting and 40-in. telescope tube. Donors came forward offering land to site the observatory; altogether, 27 sites were proposed. Hale and Harper decided that the site should be within 100 miles of the university. Hale wrote to nine well- known astronomers for advice about the effects of smoke, electric lights, vibrations from passing trains and bad weather. They finally chose Williams Bay, on Lake Geneva, Wis.—76 miles from Chicago. In December 1892, Yerkes hired Henry Ives Cobb, the architect of the University, to design the Observatory. (Construction began in 1895, and the telescope’s first observations were made in 1897.) All in all, it was a busy six months for the young Hale! t John N. Howard (johnnelsonhoward@gmail.com) is the founding editor of Applied Optics and retired chief scientist of the Air Force Geophysics Laboratory. for a vacation in upstate New York, while he prepared a paper for an AAAS meet- ing in Rochester in September. After his talk, he was relaxing one evening in the lobby of the Powers Hotel, talking with Edwin Frost, the Dartmouth astronomer. Nearby, Alvan C. Clark, the well-known optician from Cambridgeport, Mass., was telling a story about two large disks of optical glass, 42-in. in diameter, that had been cast by Mantois of Paris in 1889 for the University of Southern California. USC had been planning an observato- ry on Mt. Wilson, and Clark was asked to figure the disks. To finance the telescope, one of the USC trustees had pledged a large tract of land; but a real estate bubble had burst, and that land was now almost worthless. USC had defaulted on its pay- ments and owed Clark $16,000 for work he had already done. Hale cut short his vacation and hurried back to the university to talk to Harper about financing Clark’s work to build a large telescope. (By coincidence, big telescopes were in the news: On Septem- ber 9, the newspapers carried a story that the astronomer Edward Barnard had just discovered a fifth satellite of Jupiter—one more than the four seen by Galileo. Bar- nard made this discovery using the world’s largest telescope, the 36-in. refractor at Lick Observatory.) Harper sent a note to Charles Tyson Yerkes, the financier who had built the Chicago electric railway system, and, on October 4, Harper and Hale met with Yerkes in his office. Hale told Yerkes that, using Clark’s optical disks, they could build a telescope even larger than the one at Lick—with four more inches George Ellery Hale and the Yerkes Observatory John N. Howard How OSA’s first vice president “licked the Lick.” George E. Hale observing with the spectrograph of the Snow telescope. The Hale Observatories, courtesy AIP Emilio Segre Visual Archives
  20. 20. san jose, california, Usa Technical conference: october 11-15, 2009 exhibiT: october 13-14, 2009 Visit www.frontiersinoptics.org for information and to submit papers. call for PaPers SubmiSSion DeaDline: Tuesday, May 26, 2009 12:00 p.m. noon eDT (16.00 GMT) 2009 PhAST/Laser Focus World Innovation Awards Award Sponsors: ® Announcing CALL FOR ENTRIES Submission Deadline: March 27, 2009 Submission information and entry form: www.phastconference.org/innovation PhAST is collocated with ConferenCe May 31–June 5, 2009 Baltimore Convention Center Baltimore, Maryland, USA exhibition June 2–4, 2009 Baltimore Convention Center Baltimore, Maryland, USA PhAST and Laser Focus World announce the fifth annual Innovation Awards. This award was established to honor exhibitors who have demonstrated outstanding leadership and made significant contributions in advancing the field of optics and photonics. Receiving this award provides exhibitors with the opportunity to reach top industry decision makers at the premier laser conference. The winning entry will be presented the PhAST/Laser Focus World Innovation Award during the CLEO/IQEC Plenary Session on Monday, June 1. In addition, the chosen entry’s submitter will participate in the prestigious Press Luncheon with other prominent speakers at the conference. Four honorable mentions will also be acknowledged during the Plenary Session.All winners will be highlighted in official conference materials on-site (Conference Program, Exhibit Buyers’ Guide).
  21. 21. www.osa-opn.org20 | OPN March 2009 Jacco L. Pleumeekers, Peter W. Evans, Wei Chen, Richard P. Schneider Jr. and Radha Nagarajan A New Era in Optical Integration
  22. 22. OPN March 2009 | 21 1047-6938/09/03/0020/6-$15.00 ©OSA Sheila Hurtt, an epitaxy and materials engineer at Infinera, studies a microscope image of a photonic integrated circuit. The Internet is increasingly taxing optical networks, and conventional network architecture cannot provide the scalability required to meet this demand. These authors advise telecommunications professionals to follow the lead of the microelectronics industry—by focusing on integrated solutions. Gene Lee/Infinera iber optics is one of the most crucial aspects of modern communications. Data transmission through optical fiber is very efficient over long distances (up to thousands of kilometers), and a single fiber can carry multiple data streams represented by different wavelengths of light simultaneously. However, there are some bottlenecks on the information superhighway. In order to build an optical transport system, many components are needed, and each of them must be optimized independently and then combined into a system by an optical fiber connection. Lasers provide the source of coherent light at the wavelength needed for efficient trans- mission; modulators encode the data (in the simplest case, as “1”’s and “0”’s) onto the transmitted light by modulating the amplitude (intensity) of the light at a rate up to many billions of bits per second (Gb/sec); detectors convert the optical signal to electronic data streams; and other components—including splitters, combiners, amplifiers and attenuators—route and further refine the data signals. As the Internet drives further demand for fiber capacity, the disadvantages of a discretized architecture have become glaring—namely, the cost, complexity and reliability risk associated with many independent components and couplings. There is also the inability to scale such an architecture. For example, assume bandwidth grows at 75 percent annually (the current growth rate of many networks) for 10 years. Using today’s discretized systems, or even the discretized systems targeted for commercial introduction in the next five years, the Internet would require millions more line cards, thousands more engineers to install them, and it would consume more than 3 gigawatts of electricity—the equivalent of seven new midsized power plants. With the budget constraints faced by telecom companies, this is clearly not a sustainable scenario. However, the introduction of large-scale photonic integrated circuits into telecom networks heralds an alternative approach to building scalable networks. F
  23. 23. www.osa-opn.org22 | OPN March 2009 [ Bandwidth growth ] [ Transmitter & receiver photonic integrated components ] [ 100 Gb/s transmitter and receiver chips ] Bandwidth growth (worldwide long-haul DWDM) over the past four years. CAGR=compound annual growth rate. Source: Dell’Oro Group (1Q08 DWDM report) Micrographs of the Infinera transmitter (TX) and receiver (RX) PICs (a few mm on a side), compared with all the discrete components they replace (several cm per component). In this block diagram, the TX chip consists of 10 tunable lasers, 10 3 10 Gb/s electro-absorption modulators (EAMs), and 10 variable optical attenuators (VOAs), all coupled to an arrayed- waveguide grating (AWG) multiplexer. In addition, 10 optical power monitors (OPMs) are also integrated monolithically on the transmitter chip. The RX chip consists of 10 3 10 Gb/s high- speed photodetectors coupled to an AWG demultiplexer. Researchers have long dreamed of integrating optical components into monolithic optoelectronic integrated circuits (OICs) or photonic integrated circuits (PICs) (see, for example, Miller, 1969), allowing for continued density scaling similar to that in the silicon microelectronics industry and for greater flexibility in network architecture. However, integrating opti- cal communications components poses significant challenges, due to the diversity of components and functions required for creating, modulating, detecting and routing light; the relatively immature state of indium phosphide manufacturing technology; and the limitations on scaling set by the fixed opti- cal wavelength (which is large relative to electron wavelengths in electronics). As a result, progress in optical integration has been slow, even as the rate of microelectronics scaling has been increasing according to Moore’s Law. Researchers took the first steps toward InP integration in the late 1980s and early 1990s, when several Japanese compa- nies (NTT, NEC and Hitachi, among others) pioneered the electroabsorption modulated laser (EML). It consisted of two discrete components (modulator and laser) on a single chip. These chips enabled very high data rate transmission, and early development (Kawamura, 1987; Soda, 1990) led quickly to commercialization (Aoki, 1991). More recently, the level of InP integration increased to three or four devices per chip, with the realization of widely tunable transmitters that integrated multi-section sampled-grating lasers with on-chip semiconductor optical amplifiers. Here, too, development at institutions and companies—including UC Santa Barbara, Agility Communications and Bookham, among others—led rapidly to commercialization (Mason, 1999; Akulova, 2002; and Ward, 2005). Establishing commercial viability for more complex integration schemes has proven to be a significant challenge. Researchers took a key step forward when they invented frequency-selective arrayed waveguide gratings (AWG) filters. These were developed at the Technical University at Delft, at NTT and at AT&T Bell Labs (Smit, 1988; Takahashi, 1990; Dragone, 1991). Using this technology alongside arrays of both transmitters and receivers, researchers have made substantial progress toward developing more complex chip architectures at a number of institutions. Examples of key demonstrations include multiple-wavelength high-speed laser chips, in which multiple signals are multiplexed into a single-output, multi-wavelength modulation, and wavelength selection and conversion. This and related research has established a solid founda- tion for InP-based integration technology and continues to provide innovation in the field. Another potential route to the realization of integration in communications is the develop- ment of transmitters and modulators on silicon substrates; this work offers the hope of leveraging the very sophisticated materials integration technology available on Si substrates. However, while researchers have achieved significant milestones at Intel (Rong, 2005) and Stanford University (Kuo, 2005; 1,000 100 10 1Q04 1Q05 1Q06 1Q07 1Q08 1Q09 LHDWDMaddedto networks[Tb/s] New LH DWDM 100% CAGR 75% CAGR 50% CAGR 10 3 10 Gb/s electrical input Optical output Optical input 1 ... 10 10 3 10 Gb/s 10310Gb/s electricaloutput CH1 CH1 CH10 CH10 DCelectrical biasandcontrol AWGmultiplexer OPMarray TunableDFB array EAMarrary VOAarray Pin Photodiode array AWGmultiplexer TX PIC 10- DWDM mux 10 3 10 G modulators 10 3 DWDM lasers 10- DWDM demux 10 3 10 G receivers RX PIC ~few mm per side ~few mm per side 100 Gb/s transmit (conventional) 100 Gb/s receive (conventional)
  24. 24. OPN March 2009 | 23 Roth 2008), they have demonstrated only limited basic func- tionality of discrete devices to date, and the path to commer- cialization is uncertain. In 2004, Infinera deployed monolithic InP-based large- scale PICs with more than 50 discrete components in live telecom networks—a milestone that established commercial viability for InP-based large-scale PICs. The transmitter (TX) chip outputs 10 channels of 10 Gb/s NRZ (non-return-to- zero) optical signals, each converted from electronic inputs using an array of EMLs and multiplexed into a single out- put fiber, and the receiver (RX) chip outputs 10 channels of 10 Gb/s electronic signals, converted from optical signals that are demultiplexed from a single input fiber using an array of waveguide photodetectors. The center figure on the facing page shows the block diagram of these OICs. The TX chip contains more than 50 optical components monolithically integrated onto a single InP chip that is smaller than a human thumbnail. The RX chip is even smaller, and it uses more than ten discrete, highly func- tional components. The impact of this integration is illustrated in the bottom figure on the facing page, which shows these PICs alongside the discrete components that they replace. These OICs have demonstrated the performance require- ments of a digital transport network system, enabling a big step forward in network flexibility and cost reduction. They also meet the stringent reliability criteria for telecommuni- cations networks: So far, the OICs have accumulated more than 130 million field hours with zero failures, and they have achieved a FIT rate (the standard industry metric for Failures in Time) that exceeds industry expectations for single dis- crete optical components. Integration complexity and scaling to meet network growth Now that large-scale PICs have been demonstrated, we can make scaling predictions for photonics-based chips that are akin to Moore’s Law for microelectronics. In February 2008, Infinera announced a roadmap for photonic integration, predicting the doubling of chip capacity every three years for the next 10 years. PICs have been shown in lab demonstrations to follow the next stage of the roadmap, but new technologies will be needed for photonic integration to continue to scale to a “photonic Moore’s Law” and meet the growing network capacity demand. To understand this evolutionary path, it is helpful to review modulation formats in optoelectronic devices. Traditionally, optical chips use a standard modulation format known as NRZ, based on on/off keying (OOK), to generate binary data in an optical fiber. In early modulation approaches, engineers used lasers that were directly modulated up to a few Gb/s. However, the electrons and holes that create gain in a laser diode desta- bilize laser gain at rates greater than 1.0 to 2.5 Gb/s, degrading the quality of the signal in long-haul transmission applications (>80 km). For 10 Gb/s long-haul applications, electro-absorption modulators with integrated lasers (EMLs) represented the first small-scale component integration on a chip. EMLs modulate a dc-powered diode laser by applying a modulated electric field to a waveguide that contains a reverse- biased diode; this absorbs and extinguishes light traveling through the waveguide, converting continuous laser output into an encoded binary string. The EML is a fundamental building block for a large-scale TX PIC. It has achieved an aggregate data rate of up to 1.6 Tb/s in a chip composed of 40 channels (each at 40 Gb/s). However, simply increasing the NRZ modulation data rate is not a viable path for next- generation networks, since optical signals modulated faster than 20 Gb/s are known to suffer nonlinear penalties over long distances due to dispersion and distortion in the fiber. Advanced modulation formats are required to extend per- wavelength data rates to 40 Gb/s and beyond for recoverable data transmission over large distances. Simple OOK modula- tion formats must give way to the encoding of more than just a 1 or a 0 per bit. One way to increase the capacity is to phase- modulate continuous beams of the same laser and detect them by appropriate separation of the phases prior to detection. This way, multiple data streams may be encoded on the same wavelength at the same data rate and power level. Another way is to use polarization multiplexing, where the laser is split and As the Internet drives further demand in fiber capacity, the disadvantages of a discretized architecture have become glaring—namely, the cost, complexity and reliability risk associated with many independent components and couplings. 1990 2000 2010 2020 Year Datacapacityperchip[Gb/s] Large-scale DWDM Tx PICs 10 x 40 Gb/s (DQPSK) 10 x 10 Gb/s (OOK) EML PIC roadmap (projected) [ PIC capacity scaling history and roadmap ] Scaling of InP-based transmitter photonic integrated circuits in telecommunications networks. 4,000 2,000 1,000 400 100 10 1 4,000 2,000 1,000 400 100 10 1
  25. 25. www.osa-opn.org24 | OPN March 2009 encoded in orthogonal TE and TM components, doubling the data rate with a minor penalty from cross-talk. Polarization multiplexing and phase multiplexing can also be used in tandem to further enhance data capacity per wavelength for a given encoding speed. Thus, the polarization- multiplexed differential quadrature phase-shift keying (PM- DQPSK) format can encode four streams of 10 Gb/s data, yielding 40 Gb/s per PIC wavelength, and only the transmitter and receiver portions of the system are modified. Presently, this PM-DQPSK format has been used to create PICs with a data rate of 400 Gb/s over 1,600 km of fiber (including in-path amplification to compensate for fiber attenuation). To accomplish phase modulation on a PIC, Mach-Zehnder interferometer-based modulators split light into separate paths and modulate the reverse-biased electric field on semiconductor optical waveguides, modulating the bandgap, refractive index, and therefore the optical path length prior to combining the same light paths. By nesting Mach-Zehnder interferometers and phase-delaying the encoded light streams, one can achieve higher degrees of encoding. Eye diagrams of DQPSK-encoded data streams prior to appropriate phase delay have ripples that correspond to transi- tions between quadrature states. However, after appropriate phase shifting and interference, original data encoding is reproduced. The figures on the left show eye diagrams from DQPSK signals encoded with 21.5 Gb/s (electrical) data streams that produce a 43 Gb/s aggregate DQPSK data capac- ity per wavelength. Advanced encoding schemes such as DQPSK or PM- DQPSK may require as many as 45 optical elements per wavelength; thus, integration of ten wavelengths on a single PIC would require hundreds of optical devices, heralding the next level of integration complexity and data capacity on opti- cal chips. Planar lightwave circuits One can gain significant signal advantages by incorporat- ing passive optical elements for switching, routing, filtering, multiplexing and power leveling onto an integrated chip. The primary function of such chips is to passively route and process incoming optical signals, so that they may be made with mate- rials other than InP. The use of silicon chips allows for leverage of existing substrates, processing tools, fabrication processes and manufacturing knowledge from the microprocessor indus- try. Silicon-based integrated optical chips are known as planar lightwave circuits (PLCs). To realize Si PLCs, a waveguide core layer is required that has a refractive index larger than the surrounding cladding layer. The index contrast between waveguide core and cladding determines the minimum bend radius for the waveguides and sets the waveguide dimensions for single mode performance. A high index contrast keeps the devices small and allows for effi- cient integration of many elements on one PLC. However, too [ Nested Mach-Zehnder modulator ] [ Signal eye diagrams ] Nested Mach-Zehnder modulator used in DQPSK optical data transmission systems. Light can be split into four equal paths and modulated in two different data streams in this example. To achieve phase quadrature, one branch needs to be rotated a quarter-turn (or /2). (Left) A 21.5 Gb/s electrical signal eye modulates the arms of a Mach-Zehnder interferometer in a DQPSK transmitter. (Right) A 43-Gb/s DQPSK signal eye received from a transmitter PIC. Constituent 21.5 Gb/s data streams are normally extracted from this signal separately in the receiver PIC. (Left) Photograph of a triplexer WDM chip. (Center) photo- graph of a packaged 16-channel tracking-demultiplexer based on microring resonators. (Right) A dynamic optical dispersion compensator circuit. PBS and PBC are polarization beam splitter and combiner. The dispersive elements are ring reso- nators having free spectral range of 50 GHz. The full physical circuit, including PBS and PBC, resides on a PLC chip mea- suring 9 mm 3 11 mm. (Left) Compound ring resonator. (Center) PLC chip with dense functionality. (Right) A silicon wafer with hundreds of PLC chips. [ Components made with the Hydex platform ] [ Ring resonator based PLC chip ] PBS PBC Thin film heaters /2 /2
  26. 26. OPN March 2009 | 25 high an index contrast will lead to tiny waveguide sizes that are more sensitive to process variability (i.e., critical dimension control), making efficient fiber coupling more difficult. As an example, Infinera’s novel PLC material system is based on a proprietary glass-based Hydex platform that uses conventional commercial silicon processing technology, and has an adjust- able index contrast of up to 20 percent. The process results in waveguide dimensions of approxi- mately 1.5 µm 3 1.5 µm and a bend radius of 35 µm. These dimensions enable a dramatic leap in component integration density. Integrated PLC chips can now be designed with a footprint of roughly 100 mm2, and hundreds to thousands of devices can be fabricated onto Si wafers that are 4 to 8 in. in diameter. An important aspect of this platform is that it exhibits low loss throughout the optical transmission window (< 0.16 dB/cm over 1,530-1,630 nm); this is a critical prerequi- site for making high-performance, low-loss PLCs for telecom applications. On this platform, compact mode transformers have been designed to enable efficient coupling to optical fibers. Further- more, there are now manufacturing processes that are compat- ible with thin film heaters. These heaters can be used for phase adjustments and active electronic control of the optical ele- ments by means of thermally tuning the local refractive index. Engineers can now realize many fundamental optical processing components, such as filters, beamsplitters, interfer- ometers, (de)multiplexers, polarization controlling elements, attenuators, etc. These components can be integrated into com- plex, large-scale PLCs, and they exhibit improved footprint, functionality, performance, cost and reliability compared with their corresponding bulk components. Researchers have dem- onstrated a wide variety of devices to date, including AWGs, tunable bandwidth micro-ring resonator filters, tunable optical dispersion compensators, triplexer filters and ring-resonator- based spectrometers. The bottom figure on the facing page shows how elementary building blocks like the microring resonators are assembled to form large functional chips in the PLC platform. These chips are then fabricated by the hundreds to cover a large Si wafer and individually packaged. For example, commercially avail- able 16-channel demultiplexers are larger than PLC devices by more than a factor of 10. These devices also eliminate multiple, manually assembled fiber splices between discrete components in complex, multi-channel systems, leading to improvements in component reliability and reductions in cost. Clearly, the Si-based PLC technology holds great promise for further density scaling. PLCs are expected to find more applications within optical networks, reducing cost and complexity while improving the flexibility and reliability of optical telecom systems. A new era of optical integration has arrived, and it is one that will provide improvements in capacity, speed, density and reliability concurrent with reductions in cost and power consumption. We have a solid foundation for optical network growth for decades to come. t The authors are with Infinera Corp. in Sunnyvale, Calif., U.S.A. Jacco L. Pleumeekers is a manager in the PIC integration engi- neering department. Peter W. Evans is a member of the technical staff for PIC development. Wei Chen is a member of the technical staff for PLC development. Richard P. Schneider Jr. (rschneider@infinera.com) is a senior director of PIC platform engineering. Radha Nagarajan is a senior director of optical component technology. PLCs are expected to find more applications within optical networks, reducing cost and complexity while improving the flexibility and reliability of optical telecom systems. Member [ References and Resources ] >> S.E. Miller. Bell Syst. Tech. J. 48, 2059–69, 1969. >> Y. Kawamura et al. J. Quant. Elec. QE-23, 915-8 (1987). >> M. K. Smit. Electron. Lett. 24(7), 385–6 (1988). >> H. Soda et al. Electron. Lett. 26, 9-10 (1990). >> H. Takahashi et al. Electron. Lett. 26(2), 87–8 (1990). >> M. Aoki et al. Electron. Lett. 27, 2138-40 (1991). >> C. Dragone. IEEE Photon. Technol. Lett. 3, 812–15 (1991). >> T.L. Koch and U. Koren. IEEE J. Quant. Electron. 27, 641-53 (1991). >> B. Mason et al. IEEE Photon. Tech. Lett. 11, 638-40 (1999). >> C.G.P. Herben et al. Photon. Technol. Lett. 11(12), 1599 (1999). >> Y.A. Akulova et al. IEEE J. Sel. Top. Quant. 8(6), (2002). >> Y. Suzaki et al. IPRM (Sweden), 681 (2002). >> Y. Yoshikuni. J. Sel. Top. Quantum Electron. 8(6), 1102 (2002). >> M.L. Maˇ sanovi´c et al. Photon. Technol. Lett. 15(8), 1117 (2003). >> B.E. Little. Proc. Optical Fiber Communications Conf. 2, 444-5 (2003). >> R. Nagarajan et al. IEEE J. Select. Topics Quantum Electron. 11(1), 50-65 (2005). >> Y.-H. Kuo et al. Nature 437, 1334-6 (2005). >> A.J. Ward et al. IEEE J. Sel. Top. Quant. 11(1), (2005). >> H. Rong et al. Nature 433, 725-8 (2005). >> W. Chen et al. Proc. Optical Fiber Communication Conf. 2006, paper PDP12. >> R. Nagarajan et al. IEE Electron. Lett. 42(13), 771-3 (2006). >> D.F. Welch et al. IEEE J. Lightwave Technol. 24(12), 4674–83 (2006). >> Z. Zhu et al. Proc. CLEO/QELS Conf. 2006, paper CThS5. >> D.F. Welch et al. IEEE J. Sel. Topics Quantum Electron. 13(1), 22–31 (2007). >> W. Chen et al. Proc. ECOC Conf. (2007). >> W. Chen. “Integrated Polarimeter Assisted Ring Scanning Spec- trometer,” Proc. ECOC Conf. 2008, paper P.2.17. >> B. Little. Proc. ECOC Conf. 2008, paper Th.2.C.2. >> D. van den Borne et al. Proc. Optical Fiber Communication Conf. 2008, paper OMQ1. >> J.E. Roth et al. Electronics Lett. 44(1), 49-50 (2008).
  27. 27. www.osa-opn.org26 | OPN March 2009 Optical Fiber High-Temperature SENSORS Anbo Wang, Yizheng Zhu and Gary Pickrell GettyImages YizhengZhu Viewed from the end, a fiber tip sensor reveals its 2 mm thick sensing diaphragm with a diameter of 125 mm.
  28. 28. OPN March 2009 | 27 1047-6938/09/03/0027/6-$15.00 ©OSA Optical fiber sensors allow researchers and engineers to make accurate, reliable measurements under high-temperature conditions. ressure and temperature are two of the most com- mon quantities that need to be measured for a wide range of scientific and industrial applications. Many of these applications involve high temperatures, which often impose a great challenge for researchers and engineers. For pressure measurement, sensors with an electric output are usually fabricated using sili- con. However, their maximum operat- ing temperature is generally limited to below or around 500° C by the intrinsic property of the semiconductor. Perhaps the most common device for high-temperature measurement is the thermocouple. There are different types of thermocouples that can be used at temperatures well above 1,000° C, and they are small in size, easy to use and relatively low cost. But they exhibit sig- nificant random drifts at high tempera- tures. Also, like other electronic sensors, they are susceptible to electromagnetic interference (EMI). P Compared with their electronic counterparts, optical fiber-based sensors have a number of intrinsic advantages, including insusceptibility to EMI, non-electrical conductibility, higher operating temperatures, remote and passive measurement, and the capability of sensor multiplexing. Fiber sensors are thus attractive for applications in which traditional electronic sensors are difficult to apply due to harsh environments, such as those posed by high temperatures. Fabry-Perot white-light interferometry In the high-temperature regime, a field- viable fiber sensor faces great optical, mechanical and chemical challenges, which limit design flexibility and allow only simple and robust sensors to sur- vive. Among them, white-light Fabry- Perot (FP) interferometry has emerged as a leading candidate in recent studies. To make the sensors capable of operation at high temperatures, engineers often form the optical reflectors in the FP cavity by bare glass/air interfaces, which provide about 4 percent reflectance for normal incidence. Therefore, higher order reflec- tions by the cavity are negligible so they have a low finesse and can be approxi- mated as a two-beam interferometer. The figure on p. 28 shows the concept of such an FP cavity; it consists of two partially reflective surfaces separated by a distance L and filled with a medium of refractive index n. The cavity generates two back reflections with amplitudes A1 and A2, respectively, and a differential phase delay ∆w of 4pnL/l, where l is the wavelength. The total reflected light intensity is given by I(l)=A1 2 + A2 2 + 2A1A2cos(4pnL/l). Depending on ∆w, the two reflections can interfere constructively (in phase) or destructively (out of phase) to produce maximum or minimum intensities at the detector. Any environmental variables that can induce changes in either n or L or both can be observed through I(l) and subsequently measured. Bo Dong GettyImages Anbo Wang in the lab.
  29. 29. www.osa-opn.org28 | OPN March 2009 Silica fiber pressure sensor The design of a high-temperature pres- sure sensor has remained one of the toughest aspects of fiber-optic sensing. Many of the significant challenges are related to the materials’ performance, including finding a way to create hermetic bonding that can survive high temperatures. Researchers have found that adhesive-free direct bond- ing between similar materials can be an excellent approach for avoiding a possi- ble thermal expansion mismatch, which can break the seal. For fiber sensors, this is often reduced to how to directly bond silica to silica or a single crystal to the same material with the same crystal axis orientations. One of the effective pressure sensor structures is shown in the figure on the bottom left. The two partial reflectors are the cleaved endfaces of the two fibers encapsulated in a silica glass capillary tube. The fibers and the capillary tube are then thermal-fusion-bonded circum- ferentially by a CO2 laser. This structure is often referred to as the extrinsic Fabry- Perot interferometer. Analysis has shown that the distance L between the thermal fusion points can be varied by an exter- nally applied pressure P, and the resulted distance change DL, which is also the FP cavity change, can be expressed as DL = LDPro 2(1–2m)/[E(ro 2–ri 2)], where ro and ri are the outer and inner radii of the tube, E is the Young’s modulus of the glass and m is the Poisson’s ratio. Since the tube is made of silica, which is nearly the same as the fiber material, the thermal expansion of the tube is mostly countered by the expan- sions of the fibers toward each other so the air gap separating the two fibers is intrinsically insensitive to temperature variations. This is especially true when single mode fibers are used that have a smaller core and less dopant concen- tration than multimode fibers. The epoxy-free thermal fusion fabrication of the sensor allows its operation at a high temperature. This type of sensor has been shown to be especially effective for large pressure measurements and has been field tested in an oil well. [ FP and white-light interferometry ] [ Fusion bonding of silica ] (Top) An FP interferometer consists of two partially reflecting surfaces that modulate the detected signal by their phase differential. (Bottom) White-light interferometry interrogates the sensor by acquiring its spectrum over a broad wavelength band. Fusion bonding of silica greatly improves high-temperature perfor- mance. (Top) A tube-based structure is very effective at measuring large pressure. (Center) A diaphragm- based sensor can also be used for dynamic measurement. (Bottom) An ultra-miniature tip sensor is fabricated using a fusion splicer. It is useful for minimally invasive applications. An FP sensor cavity can be interro- gated with a variety of signal demodu- lation methods. The most robust and reliable way is perhaps the so-called white-light interferometry method, which permits not only high resolu- tion but also absolute measurement. Absolute measurement means that the signal demodulation does not require knowledge of the sensor history. The typical layout of an FP white-light interferometric sensing system includes a light source, a fiber coupler, a sensing interferometer and a detector. The light from the source travels through the coupler to the sensor. The other arm of the coupler is terminated to prevent any optical reflection. The light to the FP cavity is partially reflected at the first partial reflector. The remainder con- tinues to propagate to the second partial reflector, where the second reflection is generated. The two reflections then travel back through the same fiber and coupler to the detector. The sensor is designed so that an environmental variation can effec- tively change the differential optical path length between the two reflections. FP white-light interferometry is essentially a method used to interrogate the FP interferometric cavity at differ- ent wavelengths over a certain spectral range. This can be done using a tunable laser and a single photodetector or with a broadband light source and an optical spectrometer. For a given FP cavity, constructive or destructive interference between the reflections from the FP cav- ity takes place at different wavelengths. For a low-finesse FP cavity, the returned optical power varies with wave- number (1/l) sinusoidally. A change in the FP cavity varies not only the phase but also the periodicity of the sinusoids. Researchers have developed various methods to detect very small cavity changes in an absolute and reliable man- ner. Since the cavity distance is deter- mined by the measurement of the light spectrum, white-light interferometry is thus insensitive to source power varia- tions and fiber-bending-induced losses, so it offers excellent reliability, even in real engineering conditions. Df DL Broadband source Spectrometer Anti- reflection 50/50 Coupler FP sensor Laser Laser Fiber SM fiber Core-etched MM fiber Ferrule Tube
  30. 30. OPN March 2009 | 29 Researchers have devel- oped various structures for relatively small pressure measurements. One such structure consists of a fiber, a ferrule with a recessed cavity and a diaphragm. Epox- ies are traditionally used to bond these parts together, but this limits the sensor use to relatively low tem- peratures. Researchers have demonstrated that, if all the parts are made of fused silica, a CO2 laser can be used to fuse them together to form an epoxy-free all- silica sensor body whose temperature limit would be dictated by the thermal properties of fused silica alone. Researchers have fabricated sensors of this type using a 1.8-mm fused silica fer- rule with a 1.5-mm diameter cavity. They fused a 125-µm silica wafer to the edge of the cavity using a focused CO2 laser beam while the parts are rotating. Then, they inserted a single mode fiber into the ferrule that has a bore only slightly larger than the fiber. Finally, the laser is focused on the fiber from the side and fuses its cladding with the bore while the whole assembly is under rotation, providing a hermetically sealed cavity. For some applications, sensor size may be a concern. To reduce the size while retaining the high-temperature capability, we recently developed another method for building a pressure-sensitive FP cavity on the fiber tip. The fabrica- tion utilizes the chemical properties of fibers that were previously investigated for manufacturing sharp fiber probes for near-filed scanning optical microscopy. Most fibers consist of a pure silica clad- ding and a silica core doped with germa- nium (Ge) to slightly raise the refractive index. The doped core, however, can be etched more than ten times faster than the cladding in hydrofluoric (HF) acid, making it possible to create miniature structures on a fiber tip. First, we spliced a Ge-doped 62.5-mm core multimode fiber to a single-mode nearly identically. At higher temperatures, however, creep becomes noticeable with a 0.6 percent relative repeatability. At low temperatures, fused silica behaves similar to a perfect elastic solid: Strains produced by an applied stress are completely and instantaneously recov- ered upon removal of the stress. As the temperature of the fused silica increases, the deviation from the perfectly elastic behavior is evident. Fused silica has a soften- ing point around 1,600° C depending on its purity and water content. The closer the tempera- ture to that point, the more viscously flowable and inelastic it becomes. After experiencing this inelastic strain, the diaphragm is unable to fully recover from the mechanical deforma- tion, even after the load is removed. The sensors therefore show less repeat- ability. Exactly when this issue becomes unacceptable is application-specific, depending on the operating pressure, temperature, environmental conditions and duration. In lab tests, we found that the sensors will generally perform reasonably well up to 700° C, far beyond previous technologies. White-light interferometry is limited for detection of slowly varying signals due to the response time of the spectrometers, but the diaphragm pressure sensors are not. All-fused-silica sensors are well suited for high-temperature dynamic pressure measurement for a wide range of frequencies. The ferrule-based sensor has a resonant frequency at 400 kHz, which can also be adjusted by tailoring the dia- phragm diameter and the thickness. Sapphire fiber temperature sensors The maximum operating temperature of silica fiber is limited by the temperature at which the protective polymer coating degrades. The epoxy acrylate, which Researchers have developed various structures for relatively small pressure measurements. One such structure consists of a fiber, a ferrule with a recessed cavity, and a diaphragm. fiber using a standard arc fusion splicer and cleaved it to a short length. We then used HF acid to remove the core quickly, with only little thinning to the cladding, and to produce a cavity on top of the single-mode fiber. The next step was to splice to the cavity a special pure silica rod, which is also cleaved to a very short length, serving as the diaphragm. Fur- ther etching of the diaphragm reduces its thickness and enhances the sensitivity. The sensor is made entirely of fused silica and reduced to an ultra-miniature size, the same as the 125-mm fiber. The figure above shows the sensor’s interference fringe pattern and its pres- sure response at 24° C and 611° C, each repeated three times. At room tem- perature, the repeatability is excellent, with all three measurements agreeing [ High-temperature pressure response of a fiber-tip sensor ] Measured Theory Wavelength [mm] Pressure [psi] 1.35 1.4 1.45 1.5 1.55 1.6 1.65 1.7 15 20 25 30 24° C 611° C 1 0.5 0 16.34 16.335 16.33 16.325 16.32 16.315 16.36 16.355 16.35 16.345 16.34 16.335 Intensity[a.u.] Cavitylength[mm]