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Chapter 1:
Introduction to Optics and
optoelectronics
Light has many properties that make it very
attractive for information processing
1. Immunity to electromagnetic interfere...
4. High speed/high bandwidth
– Potential bandwidths for optical communication systems
exceed 1013 bits per second.
(1250 G...
Radiation/Light Sources
Classification of radiation source by
Flux Output
1. A point source
• An LED or a small filament clear bulb with small emi...
Radiation spectrum
1. A continuous spectrum source
• Has a wavelength of emission that ranges from
ultraviolet to infrared...
The nature of light
Light wave in a homogenous
medium
Ex
z
Direction of Propagation
By
z
x
y
k
An electromagnetic wave is a travelling wave wh...
k
Wave fronts
r
E
k
Wave fronts
(constant phase surfaces)
z


Wave fronts
P
O
P
A perfect spherical waveA perfect plane...
z
Ex
= Eo
sin(wt–kz)
Ex
z
Propagation
E
B
k
E and B have constant phase
in this xy plane; a wavefront
E
A plane EM wave tr...
Light as plane electromagnet (EM) wave
• We can treat light as an EM wave with time varying
electric and magnetic fields
...
Electromagnet (EM) wave
• We indicate the direction of propagation with a vector
k, called the wave vector.
– whose magnit...
y
z
k
Direction of propagation
r
O
q
E(r,t)r
A travelling plane EM wave along a direction k.
© 1999 S.O. Kasap, Optoelect...
Maxwell’s Equation
2 2 2 2
2 2 2 2
0x x x x
o o r
E E E E
x y z t
  
   
   
   
2
2
0xE
x



2
2
0xE
y
...
Phase velocity
• During a time interval t, this constant phase
moves a distance z.
– The phase velocity of this wave is...
Phase Velocity
2 2
( ) cos( ) 0o o r o ok E t kz   w w    
2
2
1
o o rk
w
  

 
1/2
o o rv   

 

w
f...
Group Velocity
• There are no perfect monochromatic wave in practice
– All the radiation source emit a group of waves diff...
Group velocity
         
       
, cos cos
, 2 cos cos
,v
x o o
x o
g
E x t E t k k z E t k k z
E x t E ...
w
w
w + w
w – w
kEmax
Emax
Wave packet
Two slightly different wavelength waves travelling in the same
direction result...
What is refractive index, (n) ?
Interaction between dielectric medium and EM
wave
• When an EM wave is traveling in a dielectric medium,
– the oscillating...
Phase velocity in dielectric medium
• For EM wave traveling in a non-magnetic dielectric
medium of r , the phase velocity...
Definition of Refractive Index
• For an EM wave traveling in free space (r= 1)
velocity
(1)
• The ratio of the speed of l...
Example: phase velocity
• Considering a light wave traveling in a pure silica
glass medium. If the wavelength of light is ...
Refractive Index in Materials
• In free space, k is the wave vector (k=2 /)
and  is the wavelength
• In medium, kmedium...
Refractive Index in non-Crystal Materials
• In non-crystalline materials (glass & liquids),
the material structure is the ...
Refractive Index in Crystal Materials
• In crystals, the atomic arrangements and inter-
atomic bonding are different along...
Cubic crystal
hexagonal crystal
Refractive index and phase velocity
• For example: a wave traveling along the z-direction
in a particular crystal with its...
Chapter 1a
Chapter 1a
Chapter 1a
Chapter 1a
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Chapter 1a

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Chapter 1a

  1. 1. Chapter 1: Introduction to Optics and optoelectronics
  2. 2. Light has many properties that make it very attractive for information processing 1. Immunity to electromagnetic interference – Can be transmitted without distortion due to electrical storms etc 2. Non-interference of crossing light signals – Optical signals can cross each other without distortion 3. Promise of high parallelism – 2D information can be sent and received.
  3. 3. 4. High speed/high bandwidth – Potential bandwidths for optical communication systems exceed 1013 bits per second. (1250 GigaByte/second) 5. Signal (beam) steering – Free space connections allow versatile architecture for information processing 6. Special function devices – Interference/diffraction of light can be used for special applications 7. Ease of coupling with electronics – The best of electronics & photonics can be exploited by optoelectronic devices
  4. 4. Radiation/Light Sources
  5. 5. Classification of radiation source by Flux Output 1. A point source • An LED or a small filament clear bulb with small emission area 2. An area source • An electroluminescence panel or frosted light bulb with an emission area that is large 3. A collimated source • A searchlight with flux lines that are parallel 4. A coherent source • A laser which is either a point source or a collimated source with one important difference: the wave in coherence source are all in phase
  6. 6. Radiation spectrum 1. A continuous spectrum source • Has a wavelength of emission that ranges from ultraviolet to infrared. 2. A line spectrum source • Has a distinct narrow bands of radiation throughout the ultraviolet to infrared range. 3. A single wavelength source • Radiates only in a narrow band of wavelength 4. A monochromatic source • Radiates at a single wavelength/a very narrow band of wavelength.
  7. 7. The nature of light
  8. 8. Light wave in a homogenous medium Ex z Direction of Propagation By z x y k An electromagnetic wave is a travelling wave which has time varying electric and magnetic fields which are perpendicular to each other and the direction of propagation,z. © 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
  9. 9. k Wave fronts r E k Wave fronts (constant phase surfaces) z   Wave fronts P O P A perfect spherical waveA perfect plane wave A divergent beam (a) (b) (c) Examples of possible EM waves © 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
  10. 10. z Ex = Eo sin(wt–kz) Ex z Propagation E B k E and B have constant phase in this xy plane; a wavefront E A plane EM wave travelling along z, has the same Ex (or By) at any point in a given xy plane. All electric field vectors in a given xy plane are therefore in phase. The xy planes are of infinite extent in the x and y directions. © 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
  11. 11. Light as plane electromagnet (EM) wave • We can treat light as an EM wave with time varying electric and magnetic fields Ex and By perpendicular to each other propagating in z direction. Ex (z, t) = Eo cos (w t – kz + o) Ex =electric field at position z at time t, k = 2/λ is the propagation constant, λ is the wavelength and w is the angular frequency, Eo is the amplitude of the wave and o is a phase constant. Ex (z, t) = Re[ Eo exp (jo) exp j(wt – kz)]
  12. 12. Electromagnet (EM) wave • We indicate the direction of propagation with a vector k, called the wave vector. – whose magnitude, k = 2/λ • When EM wave is propagating along some arbitrary direction, k, then electric field at a point r is Ex (r, t) = Eo cos (wt – k ∙ r + o) – Dot product (k ∙ r) is along the direction of propagation similar to kz. – In general, k has components kx , ky & kz along x, y and z directions: (k ∙ r) = kx x + ky y + kz z
  13. 13. y z k Direction of propagation r O q E(r,t)r A travelling plane EM wave along a direction k. © 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
  14. 14. Maxwell’s Equation 2 2 2 2 2 2 2 2 0x x x x o o r E E E E x y z t                2 2 0xE x    2 2 0xE y    Given wave equation: Ex (z, t) = Eo cos (w t – kz + o) 2 2 2 cos( )x o o E k E t kz z w        2 2 2 cos( )x o o E E t kz t w w        2 2 cos( ) cos( ) 0o o o o r o ok E t kz E t kzw     w w        2 2 ( ) cos( ) 0o o r o ok E t kz   w w    
  15. 15. Phase velocity • During a time interval t, this constant phase moves a distance z. – The phase velocity of this wave is therefore z/t. • Phase velocity, f is the frequency (w = 2f )  w f kdt dz v 
  16. 16. Phase Velocity 2 2 ( ) cos( ) 0o o r o ok E t kz   w w     2 2 1 o o rk w       1/2 o o rv        w f kdt dz v 
  17. 17. Group Velocity • There are no perfect monochromatic wave in practice – All the radiation source emit a group of waves differing slightly in wavelength, which travel along the z-direction • When two perfectly harmonic waves of frequency w–w &w+w and wave vectors k–k &k+k interfere, they generate wave packet. • Wave packet contains an oscillating field at the mean frequency w that is amplitude modulated by a slowly varying field of frequency w. • The maximum amplitude moves with a wavevector k and the group velocity is given Vg = dw/dk
  18. 18. Group velocity                   , cos cos , 2 cos cos ,v x o o x o g E x t E t k k z E t k k z E x t E t k z t kz dz d dt k dk w w  w w  w  w w w                      
  19. 19. w w w + w w – w kEmax Emax Wave packet Two slightly different wavelength waves travelling in the same direction result in a wave packet that has an amplitude variation which travels at the group velocity. © 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
  20. 20. What is refractive index, (n) ?
  21. 21. Interaction between dielectric medium and EM wave • When an EM wave is traveling in a dielectric medium, – the oscillating Electric Field (E-field) polarizes the molecules of the medium at the frequency of the wave. • The field and the induced molecular dipoles become coupled – The net effect: The polarization mechanism delays the propagation of the EM wave. – The stronger the interaction, the slower the propagation of the wave – r: relative permittivity (measures the ease with which the medium becomes polarized).
  22. 22. Phase velocity in dielectric medium • For EM wave traveling in a non-magnetic dielectric medium of r , the phase velocity, • If the frequency  is in the optical frequency range, – r will be due to electronic polarization as ionic polarization will be too slow to respond to the field. • At the infrared frequencies or below, – r also includes a significant contribution from ionic polarization and phase velocity is slower oor   1 
  23. 23. Definition of Refractive Index • For an EM wave traveling in free space (r= 1) velocity (1) • The ratio of the speed of light in free space to its speed in a medium is called refractive index n of the medium, n= c/v =  r (2) 18 103 1   mscv oo
  24. 24. Example: phase velocity • Considering a light wave traveling in a pure silica glass medium. If the wavelength of light is 1m and refractive index at this wavelength is 1.450, what is the phase velocity ? The phase velocity is given by v= c/n = 3108ms–1/1.45 =2.069108ms–1
  25. 25. Refractive Index in Materials • In free space, k is the wave vector (k=2 /) and  is the wavelength • In medium, kmedium=nk and medium = /n. – Light propagates more slowly in a denser medium that has a higher refractive index – The frequency f remains the same – The refractive index of a medium is not necessarily the same in all directions
  26. 26. Refractive Index in non-Crystal Materials • In non-crystalline materials (glass & liquids), the material structure is the same in all directions – Refractive index, n, is isotropic and independent on the direction
  27. 27. Refractive Index in Crystal Materials • In crystals, the atomic arrangements and inter- atomic bonding are different along different directions • In general, they have anisotropic properties except cubic crystals. – r is different along different crystal directions – n seen by a propagating EM wave in a crystal will depend on the value of r along the direction of the oscillating E-field
  28. 28. Cubic crystal hexagonal crystal
  29. 29. Refractive index and phase velocity • For example: a wave traveling along the z-direction in a particular crystal with its E-field oscillating along the x-direction – Given the relative permittivity along this x-direction is rx then , – The wave will propagate with a phase velocity that is c/nx • The variation of n with direction of propagation and the direction of the E-field depends on the particular crystal structure rxxn 

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