Light has several properties that make it useful for information processing and optical communication systems. It can be transmitted without interference from electrical signals or other light beams crossing its path. Optical signals also allow high parallelism and bandwidth exceeding 1013 bits per second. Radiation sources can be classified by their flux output and spectrum. Light behaves as an electromagnetic wave that propagates through space as oscillating electric and magnetic fields. In a material medium, the light's phase velocity decreases and is characterized by the medium's refractive index. Crystalline materials exhibit anisotropic refractive indices depending on the propagation and polarization directions.

1. Chapter 1:
Introduction to Optics and
optoelectronics

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. 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

5. Radiation/Light Sources

6. 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

8. 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.

11. The nature of light

12. 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)

13. 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)

14. 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)

15. 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)]

16. 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

17. 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)

18. 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    

19. 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 

20. 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 

21. 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

22. 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

             
    
 

23. 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)

24. What is refractive index, (n) ?

25. 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).

26. 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


27. 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

28. 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

29. 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

30. 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

31. 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

32. Cubic crystal
hexagonal crystal

33. 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 