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This document summarizes Ashok Prabhu Masilamani's Ph.D. presentation on advanced silicon microring resonator devices for optical signal processing. It introduces microring resonators and their use in optical filters. It outlines Masilamani's research goals to explore new coupled microring topologies that can realize complex transfer functions. The document demonstrates experimental fabrication and testing of microring filters in silicon-on-insulator material. It also shows thermal tuning of microring resonances using integrated microheaters. The research contributes new coupled microring architectures and synthesis techniques for advanced optical signal processing.
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- 1. 1 Advanced silicon microring resonator devices for optical signal processing Ph.D. Presentation By Ashok Prabhu Masilamani Department of Electrical and Computer Engineering, University of Alberta, Canada Supervised by Dr. Vien Van
- 2. 2 Content Introduction Background and research goals New microring coupling topologies Experimental demonstration of microring filters Thermal tuning of microrings Contributions Future directions
- 3. 3 Introduction Explosive growth in information technology industry Enormous data transfers between computing nodes RC delay and skewing Power dissipation Electrical interconnect bottleneck I/O 1 I/O N WDM Mux WDM DeMux Clock Optical waveguide Processor core 1 Processor core 2 The fiber optic Wavelength Division Multiplexing (WDM) technology used in long distance communications offers: very high bandwidth interconnects WDM based low interference interconnects An optical interconnect layer
- 4. 4 Silicon photonics Silicon based photonic devices are gaining interest as devices of the future Especially the high index contrast SOI material system offers very compact device sizes - Sub Micron dimension wires - Tight bending radii Conventional photonic devices Lasers: Inp, GaAs, InGaAs etc Modulators: LiNbO3 Detectors: Inp, InGaAs, GaAs, SiGe Monolithic Silicon components A EO Modulator A WDM filter Q. Xu, B. Schmidt, S. Pradhan, and M. Lipson, Nature 435, 325 (2005) Y. Vlasov et.al., Nature Photonics 2, 242 (2008) Hybrid Silicon components A hybrid Si Laser Di Liang et. al., Opt. Express 17, 20355 (2009) A hybrid Si photodetector H. Park et. al., Optics Express 15, 6044 (2007). Various material systems & Integration issues
- 5. 5 Microring resonators 2πR = mλ/neff θin ≥ sin-1 (1/N) Total internal reflection happens near the dielectric-air boundary. Resonance condition of the microring resonator Ray optics view θin R A common building block seen in the silicon photonic functionalities is the microring Microrings are compact high-Q optical microcavities Dielectric waveguide bent into a ring structure Typically have radius in the range of tens to hundreds of microns
- 6. 6 Microring add-drop filter Add drop microring spectral response Schematic of a microring resonator add-drop filter. Input Si Through St Add Sa Drop Sd R λ1, λ2 ... λd…λΝ λ1 , λ2 ... λa …,λΝ λd λa κο τ ο κi τ i Input waveguide Output waveguide λd
- 7. 7 Higher order filters Existing microring-based filter architectures Filters without transmission zeros Filters with transmission zeros Serially-coupled microring filters Parallel cascaded microring filters MZI loaded with all-pass microring filters Disadvantage Can realize all-pole transfer functions only Disadvantage Poles cannot be independently controlled Disadvantage The round-trip phases have to be precisely controlled st si sd sa 1 2 3 N st si sd sa 1 2 3 N sd si st sa 1 2 3 N sd si st sa 1 2 3 N si s2 1 2 N/2 N/2+1 NN/2+2 s1si s2 1 2 N/2 N/2+1 NN/2+2 s1 ϕ1 ϕ2 ϕΝ/2 ϕΝ/2+1 ϕΝ/2+2 ϕΝ ψsi s2 1 2 N/2 N/2+1 NN/2+2 s1si s2 1 2 N/2 N/2+1 NN/2+2 s1 ϕ1 ϕ2 ϕΝ/2 ϕΝ/2+1 ϕΝ/2+2 ϕΝ ψ -50 0 50 -100 -50 0 Four Transmission zerosButterworth, Chebyshev Elliptic, Inverse Chebyshev
- 8. 8 Research Goals To explore, propose and demonstrate new coupled resonator topologies based on microring resonators in SOI material system which can be used to realize complex optical transfer functions. Explore new coupled microring architectures that could overcome the disadvantages of existing microring filter architectures. Develop analysis and synthesis techniques for these new coupled microring architectures. Experimental demonstration of microring filters in the SOI platform. Explore the limits of miniaturization of microring resonators in SOI to reduce the device foot prints. Develop the fabrication process for SOI material system with integrated micro heater elements. Demonstrate thermo-optic control and tunability of microring filters using micro heaters. Specific research goals:
- 10. 10 General 2D Coupling Topology N N-1 1 2 m 2m m+1 μi μ1,2 μo μN-1,N μ1,2m St Sr Si µi,j = energy coupling coefficient between microrings i and j. µi, µo = couplings to input and output bus waveguides Si, Sr, St = input, reflected and transmitted signals Energy coupling matrix: A direct synthesis procedure was developed which allows arbitrary filter shapes to be designed and realized This topology can realize a filter with N poles and N-2 transmission zeros. ∆+− ∆ ∆ ∆+− = NoNNN N N Ni j j ωµµµµ µωµµ µµωµ µµµωµ 2/ 2/ M 2 ,3,2,1 ,333,23,1 ,23,222,1 ,13,12,11 2 2D array of N asynchronously tuned microring resonators mutually coupled to each other via energy coupling coefficients μi,j. V. Van, J. Lightw. Technol. 25, 584 (2007). A. M. Prabhu and V. Van, Optics Express 15, 9645, (2007).
- 11. 11 Synthesized transmission (black) and reflection (blue) characteristics along with a Butterworth response (red) of the same order Filter layout for 7 pole 3 zero 25 GHz asymmetric filter Numerical example 4 5 6 1 2 3 7 ]0.16340.3955][0.62470.2967][9066.00.1600[ ]1.02510.0476][0.37510.3910][0.81600.266][1.04450.0902[ )1.669)(1.4)(995.1( )( jsjsjs jsjsjsjs jsjsjs sH −+−+−+ −+++++++ −−+ = -30 -20 -10 0 10 20 30 -80 -60 -40 -20 0 frequency detune, ∆f (GHz) transmission(dB) −− − − 8923.0 2088.427650.2 01662.453269.35 005723.416984.58 00005104.33 2088.420004642.447966.4 02088.420002088.428923.0 Synthesized Energy coupling matrix: To realize transmission zeros located on the imaginary axis jω, the filter requires negative couplings between neighbour microring resonators. To realize a negative coupling coefficient, the coupling angle between adjacent resonators must be between 3π/2 and 2π, so that micro-racetracks with long coupling lengths are required g κ RLc
- 12. 12 Generalized ladder microring topology Cross port Bar portInput port M1 M2 Mk MN γ2 γk a0 b0 dk ck ak bk aN bN stage k M1 M2 Mk MN γ2 γk a0 b0 dk ck ak bk aN bN stage k γ2 γ3input TB TX 1 2 3 4 1 2 3 5 5 3 4 2 1 6 γ2 γ3input TB TX 1 2 3 4 1 2 3 4 5 3 4 2 1 6 A filter architecture based on a parallel-ladder array of symmetric microring networks with inter-stage phase shifts. Can realize general transfer functions with transmission zeros symmetric microring networks Use only synchronous microring resonators All-positive coupling coefficients Each stage can be independently optimized Can be reduced to only 1 phase shift element even with multiple stages ⇒ less demand on fabrication The detailed analysis and synthesis procedures are available in reference – the differential phase shift in the upper branch .1,0 ±=⇒= kk γπθ kj k e θ− =γ A. M. Prabhu, H. L. Liew and V. Van, J. Opt. Soc. Am. B 25, 1505 (2008).
- 13. 13 A numerical example 1.40862.8863.29752.1307 4017.124064.0 )( )( )( 234 2 ++++ − == ssss s sQ sP sTt 1.40862.8863.29752.1307 13926.00276.1 )( )( )( 234 24 ++++ ++ == ssss ss sQ sR sTr -40 -30 -20 -10 0 10 20 30 40 -80 -70 -60 -50 -40 -30 -20 -10 0 Frequency detune, ∆f (GHz) Transmission(dB) μ10 μ20 μ11 μ21 μ12 μ22 bk-1 bk ak-1 ak γk ' ka ' kb µk0 = mk2 µk1 γk Stage 1 0.7357 1.1923 Stage 2 1.2607 0.5575 -1 Synthesized transmission and reflection characteristics of a 25 GHz 4th order elliptic filter
- 15. 15 Fabrication Process silicon oxide Silicon-on-insulator (SOI) (e) (d) (f) (a) Si SiO2 Substrate (b) PMMA (c) Exposure Clean SOI wafer Spin on PMMA E-beam Expose PMMA Develop PMMA ICP RIE Etch Resist removal Fabrication process Silicon = 340nm Oxide = 1µm
- 16. 16 Fabrication results Surface roughness of a 400x340nm2 Waveguide Cleaved edge of the Waveguide
- 17. 17 Microring fabrication Device Parameters Waveguide width = 400nm Coupling gap = 200nm Microring Radius = 5µm 1530 1535 1540 1545 1550 1555 -40 -30 -20 -10 0 wavelength (nm) transmission(dB) Through Drop Measured parameters FSR = 16.5 nm 3dB Bandwidth = 0.2 nm Insertion loss = 1dB through-port extinction = 10dB TM polarisation
- 18. 18 Microring Miniaturization 1.5μm Radius 1.0μm Radius 1540 1560 1580 1600 1620 -30 -25 -20 -15 -10 -5 0 wavelength (nm) transmission(dB) R = 1.0µm m = 9 m = 8 1520 1530 1540 1550 1560 1570 1580 -40 -30 -20 -10 0 wavelength (nm) transmission(dB) R = 1.3µm m = 12 m = 11 FSR = 52 nm 3dB Bandwidth = 1.2 nm Insertion loss = 0.95 dB FSR = 80.5 nm 3dB Bandwidth = 3.3 nm Insertion loss = 1 dB TM polarisation
- 19. 19 Loss performance of the devices 0.6 0.8 1 1.2 1.4 1.6 0 0.2 0.4 0.6 0.8 radius (µm) roundtriploss(dB) Theoretical bending loss Theoretical coupling loss Extracted bending loss Theoretical and extracted bending losses are in very good agreement Coupling loss dominates for microring radius < 1.3 µm
- 20. 20 Fourth-order double-microring ladder filterFourth-order double-microring ladder filter Stage 1 Stage 2 g0 ( κ0 ) g1 ( κ1 ) g2 ( κ2 ) g3 ( κ3 ) g4 ( κ4 ) g5 ( κ5 ) Designed Values 200nm (0.2654) 340nm (0.0534) 200nm (0.2654) 265nm (0.1470) 265nm (0.1154) 265nm (0.1470) Designed and measured coupling gaps and the corresponding power coupling coefficients of a 4th-order elliptic microring ladder filter with two stages Port 2Port 2 1 2 3 4 L = 25µm g1 , κ1 g2 , κ2 g5 , κ5 g4 , κ4 Lt LtLc g0 , κ0 g3 , κ3 R = 8µm, Lt = 5µm, W = 300nm, Wc = 600nm, Lc = 1.65µm WWcPort 1Port 1 InputInput dropdrop thruthru ππ - phase shift- phase shift Tt Tr R
- 21. 21 Experimental ResultsExperimental Results All four microrings – detuned from each other To correct the resonance mismatches thermo-optic tuning 1 2 3 4 Port 1Port 1 ThroughThrough DropDrop π-phase shifter SEM image of an SOI 4th -order double-microring ladder filter 1530 1535 1540 1545 1550 1555 1560 -50 -40 -30 -20 -10 0 Wavelength (nm) Transmission(dB) FSR
- 23. 23 Microheater design Silicon substrate SiO2 (1 μm) Cladding (1 μm) Silicon (340 nm) Thin film heater ~ (150 nm) Optical Cladding layer Microring layer 100 ºC 60 ºC PECVD SiO2 or SU8 SOI wafer Au / Al / Ti / TiW Optical Cladding layer A top view of the microheater A cross sectional view of the microheater Thin metal films drawm into wires acts as resistive elements and can produce heat due power dissipation
- 24. 24 Heater fabrication process Cleaned SOI chip with microring patterns 10μm PECVD SiO2 deposition Etch back to 1μm thick PECVD SiO2 E-beam lithography Metal deposition based on RF sputtering Lift - off and remaining heater layer Top view of gold micro heaters. Width = 800nm Thickness = 150nm Maximum resistance = 150Ω
- 25. 25 Optimized TiW based heaters 1 2 3 4 5 Port 1 Port 2 Port 3 Port 4 Micro heater 1 = 5.8 kΩ Micro heater 2 = 4.2 kΩ Micro heater 3 = 5.7 kΩ Micro heater 4 = 4 kΩ Micro heater 5 = 2 kΩ Measured heater resistances Optimized microheaters Mounted chip Ag epoxy Bond PCB The wire bonded microring chip with microheaters
- 26. 26 Thermally tuned fourth order filter 1530 1532 1534 1536 1538 1540 1542 0 0.2 0.4 0.6 0.8 1 Wavelength (nm) NormalizedPower -100 -50 0 50 100 -25 -20 -15 -10 -5 0 frequency (GHz) Transmission(dB) Bandwidth ~ 100 GHz FSR = 10nm Heater 1 & 2 = 6.25 mA Heater 3 = 2.1 mA Heater 4 = 1.1 mA Stage 1 Stage 2 κ0 κ1 κ2 κ3 κ4 κ5 Extracted Values 0.342 0.234 0.342 0.558 0.0785 0.558 1530 1532 1534 1536 1538 1540 1542 0 0.2 0.4 0.6 0.8 1 Wavelength (nm) NormalizedPower Ring 1 & 2 Ring 3 Ring 4 1 2 3 4 Port 1Port 1 ThroughThrough DropDrop Drop Response without heating Drop Response with thermal tuning
- 27. 27 Key Contributions General 2D array of direct-coupled asynchronous microring resonators: -Arbitrary filter shapes can be designed and realized. - Requires negative couplings to realize transmission zeros General cascaded microring network topology: - Modular design approach. - All positive coupling elements Demonstration of ultra-compact silicon microring resonators Demonstration of a fourth order silicon microring ladder filter
- 28. 28 Future directions Permanent post fabrication tuning methodology based on laser ablation of microring (D. Bachman et. al., IPR 2010) Demonstration of advanced optical signal processing functions such as dispersion compensators maximally flat group delay filters for optical buffering Demonstration of extremely high order filters (e.g., N > 10) using the modular design approach with the ladder architecture.
- 29. 29 Thank you
Hinweis der Redaktion
- Current growth in IT industry has lead to massive data transfers between computing nodes this has choked the electrical interconnects (bandwidth limitation) One solution proposed is to use the well established WDM based optical interconnects to chip level as depicted in this picture where the critical electrical bus signals are multiplexed and sent over on optical waveguide
- To bring optical interconnects to chip level we need integration of photonic devices which has a integration botteleneck due to the various material systems used. However recently si based photonic devices are gaining interest which could solve the integration problem. Some examples of Si photonic devices are shown below. Note that some are plainly si based and some of Si hybrid. Also we can note that microring resonator has been a common photonic functionality in these demonstratiuons.
- So what is a microring? – compact high Q optical cavity Formed by bending a dilelectric waveguide in to a ring structure as shown. When light is coupled in, it travels around the circumference as shown for wavelengths that satisfy the given resonance condition
- Microrings are used to add and drop data channels in WDM systems. A single microring based add-drop filter configuration is shown here. Muliplexed datachannels go in to the IP WG. The drop - channel whose resonates into the ring and goes out via the drop arm. Similarly we can add a channel. If we plot te spectral response at the through and drop ports it would look as shown. We can see that at the drop channel wavelength there is a dip in the thru and peak in the drop port signifying the dropped channel. How well this operation is performed can be measured by the IL, TPE. In real scenarios, often we need dense WDM systems. DWDM systems need much more sharp roll than this which brings in higher order filters (i.e multiple coupled microrings)
- Some existing coupled microring architectures: Serial coupled – only all pass transfer functions Parallel coupled and MZI loaded micorings – can realize filters with transmission zeros – i.e. sharp roll off characteristics needed in DWDM systems. Problem is that in PC filters poles cannot be indep controlled and MZI one needs the phases of microrings precisely controlled.
- With this background we aim to propose new multiple coupled microring architectures which over come the disadvantages seen in previous slide. Also we target to demonstrate microing add-drop filters in the SOI platform. Not just propose new topologies but also develop analysis and synthesis techniques for them. To demonstrate these filters we aim to develop the fabrication process We also aim to explore the limits of miniaturizing these mircoring devices.
- 1st architeture we propose is a 2D array of coupled microrings (which are de-tuned with each other as well) as shown in the fig. We developed the analysis and synthesis procedures for this topology which allows us to realize complex spectral shapes. The synthesis provides powerful way to extract the coupling matrix for a given filter transfer function.
- Here we show a synthesized filter example for the 7th order asymmetric filter transfer function, The 2D synthesized layout is shown in the figure along with the corresponding extracted coupling matrix. The values in the diagonal are the de-tunings from the center frequency. Needs negative coupling elements which require the racetrack approach (long coupling lengths). Big constarint on the layout.
- So we proposed a new architecture called the cascaded ladder architecture. Here each arm in the ladder is a 2D microring network. The analysis and synthesis procedures are available in the references. Main advantages – all positive coupling coefficients. Modularized design approach i.e. each stage can be independently designed and optimized. It requires a phase shift element which are much easier to design.
- Here we see a simple ladder architecture filter synthesized. It’s a elliptic filter with two transmission zeros. 4 rings, 2 stages as shown in figure.
- To demonstrate these topologies we needed to develop a fabrication process - @ UofA Nanfab facility. We used a SOI material system which had 340 nm thick Silicon on top with a 1um buffer oxide. The process flow has 2 important steps – E – beam lithograpy and plasma etching. E-beam required for high resolution pattering (we need coupling gaps as small as ~ 200nm) Plasma etching for vertical sidewall for the waveguides.
- These two figure show the achievable quality of the fab process we developed. 1st is a 400x340 waveguides. We notice the vertical sidewalls as well as the smooth surface. The second picture shows the cleaved edge of the waveguide into which the light can be coupled in.
- Armed with this fab process we attempted to make a compact microring i.e. with a radius of 5um. SEM shows a top view of the ring. The measured spectral response of the microring is shown here. Notably it had a low insertion loss of 1dB 0.2 nm = how many GHz
- Once we made a compact ring we decided to explore the further miniaturization by attempting to make rings from 1.5 to 1um radii (i.e wavelength scale). Theoretically these ultra-compact rings were suppose to have high bending losses. Despite the device losses, the ultracompact add-drop microrings delivered excellent filter characteristics like low insertion loss, wide bandwidth and wide FSR
- Blue curve loss due to tight bending per one round trip (theory). We were able to get experimental values close to the theory except for 1.1um radius ring (due to a fabrication anomaly). Also we noted that the loss at coupling junctions dominate more for radius less than 1.3 um
- Next we wanted to demonstarte a higher order fileter. The Ladder topology chosen over the 2D microring topology because it is much simpler to implement a single phase shift element connecting two stages than realizing a negative coupling coefficient required in the latter architecture We took the synthesized example from the ladder architecture and obtained the physical parameters like waveguide widths, ring radius, coupling gaps etc. We also designed the phase shift element as a tapered waveguide shown.
- The SEM picture shows the image of one of the devices we fabricated. As shown in the figure input is at port 1 and we show the thru and drop ports. Measurement at the drop port shows a spectral response like this. We saw that the errors of 5-10nm in the fabrication can de-tune the microring resonances which resulted in this spectral shape. So we need to tune these resonance back in place. We decided to use thermo-optic control for this.
- To do thermal control of the microring resonance we planned to install micro-sized heaters on top as depicted in the picture. The second picture gives an idea of additional process required to install them. Here the heater material is a metal which when current is passed through them starts heating up due to the resistance. This heat then travels thru cladding and reaches the ring WG and changes the effective index – i.e. shifts the resonance of the ring. Cladding should provide opical isolaotion but also leads to thermal loss.
- Process flow steps to install heaters. PECVD SiO2 dep. and etch back. E-beam to define heater. Metal was sputtered and using lift-off the heater was installed. The figure shows the installed gold heaters. Provided max resistance of 150 ohms. Experimental work.
- We then optimized to heater material and settled for TiW which provided higher resistances and more reliable. The lower figure shows a chip with heaters on which was then wire bonded using silver epoxy to a PCB custom designed to pass DC current to the heaters.
- Then we conducted experiments with the wire-bonded chip by supplying DC currents to the heaters using LM 334 based current control circuits. With heaters 1 and 2 maintained at 6.25 mA and heaters 3and 4 with 2 and 1 mA currents we were able to tune all the microring resonances in to one single peak as shown in the figure. We performed curve fitting to extract the corresponding device parameters which confirmed that the peak was a fourth order filter response. But this wasn’t the same 4th order filter we designed. While attempting to fine tune the peak, three of the 4 heaters broke down due to over heating. Nevertheless the earlier achieved peak is a 4th order filter.