1. UNIT III SOURCES AND DETECTORS

2. Direct Band Gap Semiconductors

3. Indirect Band Gap Semiconductors

4. Light-Emitting Diodes (LEDs)
For photonic communications requiring data rate Mb/s with multimode fiber with tens of microwatts, LEDs are usually the best choice.
LED configurations being used in photonic communications:
1- Surface Emitters (Front Emitters)
2- Edge Emitters

5. LED PRINCIPLE OF OPERATION
In FOC LEDS must have
high radiance o/p or brightness
Fast emission response time
High Quantum Efficiency
RADIANCE
Measure of optical power radiated into a unit solid angle per unit area of the emitting surface.(unit is Watts.)

6. Emission Response Time
Time delay between application of a current pulse and respective optical emission.
Quantum efficiency
Related to fraction of electron hole pairs recombine radiatively.

7. LED structures For high radiance and quantum efficiency
The LED must have
Optical confinement
Achieve high level Radiative Recombination in the active region of the device-yield high quantum efficiency
Carrier confinement
Preventing absorption of the emitted radiation by the material surrounding the PN junction.

8. HETRO JUNCTION STRUCTURE
Used to achieve Carrier and optical confinement
Consists of two adjoining semiconductor with different band gap energies
Band gap energy difference of adjacent layers confines charge carriers.
RI differences of adjoining layers confines optical field to the central active layer.

9. Cross-section drawing of a typical GaAlAs double heterostructure light
emitter. In this structure, x>y to provide
for both carrier confinement and optical
guiding.
b) Energy-band diagram showing the
active region, the electron & hole
barriers which confine the charge carriers
to the active layer.
c) Variations in the refractive index; the
lower refractive index of the material in
regions 1 and 5 creates an optical barrier
around the waveguide because of the higher
band-gap energy of this material.
[4-3]
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000

10. LED CONFIGURATION Surface emitters LED Edge emitters LED
Data rates above 1000bps
Light emitting region perpendicular to fiber axis
Edge emitters LED
Emit more directional light than SELED
Homo structure
Use an etched well in GaAs substrate in order to prevent heavy absorption.
Double hetero junction structure
Giving increased efficiency from optical and electrical confinement

11. Surface-Emitting LED High radiance etched well is 0.8 to 0.9 um
Due to large band gap in confining layer- very low internal absorption.
GaAs is used in well- to avoid heavy absorption of emitted light.
Circular active area of surface emitter is 50 um dia and 2.5um thick.
Emission pattern is isotropic with 120 half power beam width.
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000

12. LAMBERTIAN PATTERN Isotropic pattern from surface emitter.
Source equally bright in all directions-view
It decides coupling efficiency
Source equally bright but power dismisses cosᶿ.
Power 50% down in its peak when ᶿ= 60.
So total half power beam width is 120

13. Coupled power The power coupled to MMSI is PC =π(1-r)A RD(NA)2
R- fresenel coefficient of fiber surface
A- emission area of the source
RD - radiance of the source
They allow more power to be coupled- more difficult, expensive.

14. DH Edge Emitter LED Emit more directional light
Reduce lose by absorption and more directional –light collected from edge.
Has transparent guiding layers with very thin active layer of 50 um to 100 um – reducing self absorption.
Guiding layer RI < surrounding material(core&cladding)> outer surrounding material.
Form wave guide channel that directs the optical radiation towards fiber.
To match fiber core diameter (50 to 100um) the contact stripes for the edge emitters are 50-70um wide.
In the plane parallel to the junction no waveguide effects.
In the plane highly directional perpendicular to the junction-when half power beam width is 25 to 35 by proper choice of waveguide thickness.

15. Edge-Emitting LED
Schematic of an edge-emitting double heterojunction LED. The output beam is
lambertian in the plane of junction and highly directional perpendicular to pn junction.
They have high quantum efficiency & fast response.
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000

17. Full Width Half Power Maximum
Width of spectral pattern and its half power point.
Fundamental quantum relation E=hv
= h (c/λ)
V=c/λ
λ= hc/E
Peak emission wavelength λ(um)=1.240/Eg(eV)

18. Light Source Material
Most of the light sources contain III-V ternary & quaternary compounds.
by varying x it is possible to control the band-gap energy and thereby the emission wavelength over the range of 800 nm to 900 nm. The spectral width is around 20 to 40 nm.
By changing 0<x<0.47; y is approximately 2.2x, the emission wavelength can be controlled over the range of 920 nm to 1600 nm. The spectral width varies from 70 nm to 180 nm when the wavelength changes from 1300 nm to 1600 nm. These materials are lattice matched.

19. Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000

20. Spectral width of LED types
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000

21. Quantum efficiency Intro..
Excess of electrons and holes in p and n type material created in semiconductor by carrier injection at device contacts.
Excess carrier can be recombine radiatively or non radiatively.
Total carrier generated rate is sum of externally supplied and thermally generated rates.
Externally generated rate = j/qd
j – current density
q – electron charge
d – thickness of region
Themally generation rate = n/Ԏ
Ԏ- carrier life time,N – excess carrier

22. Rate equations, Quantum Efficiency & Power of LEDs
When there is no external carrier injection, the excess density decays exponentially due to electron-hole recombination.
n is the excess carrier density,
Bulk recombination rate R:
Bulk recombination rate (R)=Radiative recombination rate + nonradiative recombination rate
[4-4]
[4-5]

23. In equilibrium condition: dn/dt=0
With an external supplied current density of J the rate equation for the electron-hole
recombination is:
[4-6]
In equilibrium condition: dn/dt=0
[4-7]

24. INTERNAL QUANTUM EFFICIENCY & OPTICAL POWER
Fraction of electron and holes recombine radiatively
Ratio of radiative recombination to total recombination
[4-8]
Optical power generated internally in the active region in the LED is:
[4-9]

25. External Quantum Eficiency
In order to calculate the external quantum efficiency, we need to consider the reflection effects at the surface of the LED. If we consider the LED structure as a simple 2D slab waveguide, only light falling within a cone defined by critical angle will be emitted from an LED.
[4-10]

26. [4-11]
[4-12]
[4-13]
[4-14]

27. Modulation of LED The frequency response of an LED depends on:
1- Doping level in the active region
2- Injected carrier lifetime in the recombination region, .
3- Parasitic capacitance of the LED
If the drive current of an LED is modulated at a frequency of the output optical power of the device will vary as:
Electrical current is directly proportional to the optical power, thus we can define electrical bandwidth and optical bandwidth, separately.
[4-15]
[4-16]

28. [4-17]
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000

29. LASER (Light Amplification by the Stimulated Emission of Radiation)
Laser is an optical oscillator. It comprises a resonant optical amplifier whose output is fed back into its input with matching phase. Any oscillator contains:
1- An amplifier with a gain-saturated mechanism
2- A feedback system
3- A frequency selection mechanism
4- An output coupling scheme
In laser the amplifier is the pumped active medium, such as biased semiconductor region, feedback can be obtained by placing active medium in an optical resonator, such as Fabry-Perot structure, two mirrors separated by a prescribed distance. Frequency selection is achieved by resonant amplifier and by the resonators, which admits certain modes. Output coupling is accomplished by making one of the resonator mirrors partially transmitting.

30. LASER
Single wavelength- related to molecular characteristics of material
Lasing medium – gas,liquid,insualting crystal

31. Principle of operation
Two energy levels E1(lower) and E2(higher)
Three main process for laser action:
1- Photon absorption
photon with energy E2-E1 incident on the atom in E1,atom excited into E2 through absorption of photon
2- Spontaneous emission
Atom return to E1 by random manner.
3- Stimulated emission
Photon have equal energy b/w two states(E1-E2) interact with atoms causing it to lower state with creation of second photon. Give coherent radiation.
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000

32. Lasing in a pumped active medium
In thermal equilibrium the stimulated emission is essentially negligible, since the density of electrons in the excited state is very small, and optical emission is mainly because of the spontaneous emission.
Stimulated emission will exceed absorption only if the population of the excited states is greater than that of the ground state. This condition is known as Population Inversion.
Population inversion is achieved by various pumping techniques.
In a semiconductor laser, population inversion is accomplished by injecting electrons into the material to fill the lower energy states of the conduction band.

33. ILD ADVANTAGES DISADVANTAGES Coherent light Less coupling loss
High o/p power
Used High bit rates
Monochromatic light
Good spatial coherence
DISADVANTAGES
10 times expensive than LED
Shorter life time due to high power operation
Temperature dependent

34. SEMICONDUCTOR INJECTION LASER
BW Greater than 200Mhz.
Its have response time less than 1ns
HOptical BW 2nm or less
igh coupling efficiency
Multilayered
Smaller temp dependence

36. Fabry-Perot Resonator
[4-18]
R: reflectance of the optical intensity, k: optical wavenumber

37. Hetero junctions
Semiconductor lasers cleaved facets are used instead of external mirrors
Reflectivity Rm=(n-1/n+1)^2
In DH structures-carrier and optical confinement reduces threshold currents for lasing by a factor 100
DH forward bias applied by +ve to p & -ve to n.
Voltage correspond to band gap energy applied –large no of es are injected into active layer lasing commences.
Small amount of energy required for operation of laser.
Realized only by laser pumped above threshold
Threshold current- current needed to reach threshold

38. Fabry Perot resonator cavity for Laser Diode
Laser diode is an improved LED, in the sense that uses stimulated emission in semiconductor from optical transitions between distribution energy states of the valence and conduction bands with optical resonator structure such as Fabry-Perot resonator with both optical and carrier confinements.
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000

39. Cavity much smaller
250 – 500 um long
5 – 15 um wide
0.1 – 0.2 um Thick

40. Laser Diode Characteristics
Nanosecond & even picosecond response time (GHz BW)
Spectral width of the order of nm or less
High output power (tens of mW)
Narrow beam (good coupling to single mode fibers)
Laser diodes have three distinct radiation modes namely, longitudinal, lateral and transverse modes.
In laser diodes, end mirrors provide strong optical feedback in longitudinal direction, so by roughening the edges and cleaving the facets, the radiation can be achieved in longitudinal direction rather than lateral direction.

41. Modes of cavity
The radiation with in the resonance cavity of a laser diode set up a pattern of electric and magnetic lines
Modes
TE modes
TM modes
Each modes described in lateral, longitudinal and transverse.

42. Longitudinal modes Transverse modes Related to length L of the cavity.
Much larger then lasing wavelength approximately 1um
Lin in the plane of p-n junction.
Depends on side wall preparation, width of cavity.
Transverse modes
Associated with electromagnetic field and beam profile perpendicular to the plane of p-n junction.

43. Laser Operation & Lasing Condition
To determine the lasing condition and resonant frequencies, we should focus on the optical wave propagation along the longitudinal direction, z-axis. The optical field intensity, I, can be written as:
Lasing is the condition at which light amplification becomes possible by virtue of population inversion. Then, stimulated emission rate into a given EM mode is proportional to the intensity of the optical radiation in that mode. In this case, the loss and gain of the optical field in the optical path determine the lasing condition. The radiation intensity of a photon at energy varies exponentially with a distance z amplified by factor g, and attenuated by factor according to the following relationship:
[4-19]
LASING – Light amplification possible in laser diode

44. [4-20]
Z=0
Z=L
[4-21]
Lasing Conditions:
[4-22]

45. Optical output vs. drive current
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000

46. Threshold gain & current density
[4-23]
For laser structure with strong carrier confinement, the threshold current
Density for stimulated emission can be well approximated by:
[4-24]

47. External quantum efficiency
Number of photons emitted per radiative electron-hole pair recombination above threshold, gives us the external quantum efficiency.
Note that:
[4-29]

48. Threshold current Density & excess electron density
At the threshold of lasing:
The threshold current needed to maintain a steady state threshold concentration of the excess electron, is found from electron rate equation under steady state condition dn/dt=0 when the laser is just about to lase:
[4-26]
[4-27]

49. Semiconductor laser rate equations
Rate equations relate the optical output power, or # of photons per unit volume, , to the diode drive current or # of injected electrons per unit volume, n. For active (carrier confinement) region of depth d, the rate equations are:
[4-25]

50. DFB(Distributed FeedBack) Lasers
In DFB lasers, the optical resonator structure is due to the incorporation of Bragg grating or periodic variations of the refractive index into multilayer structure along the length of the diode.
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000

51. Laser operation beyond the threshold
The solution of the rate equations [4-25] gives the steady state photon density, resulting from stimulated emission and spontaneous emission as follows:
[4-28]

52. Laser Resonant Frequencies
Lasing condition, namely eq. [4-22]:
Assuming the resonant frequency of the mth mode is:
[4-30]
[4-31]

53. Spectrum from a laser Diode
[4-32]

54. Laser Diode Structure & Radiation Pattern
Efficient operation of a laser diode requires reducing the # of lateral modes, stabilizing the gain for lateral modes as well as lowering the threshold current. These are met by structures that confine the optical wave, carrier concentration and current flow in the lateral direction. The important types of laser diodes are: gain-induced, positive index guided, and negative index guided.

55. (a) gain-induced guide (b)positive-index waveguide (c)negative-index waveguide

56. Laser Diode with buried heterostructure (BH)

57. Single Mode Laser
Single mode laser is mostly based on the index-guided structure that supports only the fundamental transverse mode and the fundamental longitudinal mode. In order to make single mode laser we have four options:
1- Reducing the length of the cavity to the point where the frequency separation given in eq[4-31] of the adjacent modes is larger than the laser transition line width. This is hard to handle for fabrication and results in low output power.
2- Vertical-Cavity Surface Emitting laser (VCSEL)
3- Structures with built-in frequency selective grating
4- tunable laser diodes .

58. VCSEL

59. Frequency-Selective laser Diodes: Distributed Feedback (DFB) laser
[4-33]

60. Frequency-Selective laser Diodes: Distributed Feedback Reflector (DBR) laser

61. [4-35]
Output spectrum symmetrically distributed around Bragg wavelength in an idealized DFB laser diode

62. Frequency-Selective laser Diodes: Distributed Reflector (DR) laser

63. Modulation of Laser Diodes
Internal Modulation: Simple but suffers from non-linear effects.
External Modulation: for rates greater than 2 Gb/s, more complex, higher performance.
Most fundamental limit for the modulation rate is set by the photon life time in the laser cavity:
Another fundamental limit on modulation frequency is the relaxation oscillation frequency given by:
[4-36]
[4-37]

64. Relaxation oscillation peak

65. Pulse Modulated laser
In a pulse modulated laser, if the laser is completely turned off after each pulse, after onset of the current pulse, a time delay, given by:
[4-38]

66. Temperature variation of the threshold current

67. Linearity of Laser Information carrying electrical signal s(t)
LED or Laser diode
modulator
Optical putput power:
P(t)=P[1+ms(t)]

69. Nonlinearity x(t) Nonlinear function y=f(x) y(t)
Nth order harmonic distortion:

70. Intermodulation Distortion
Harmonics:
Intermodulated Terms:

71. Laser Noise
Modal (speckel) Noise: Fluctuations in the distribution of energy among various modes.
Mode partition Noise: Intensity fluctuations in the longitudinal modes of a laser diode, main source of noise in single mode fiber systems.
Reflection Noise: Light output gets reflected back from the fiber joints into the laser, couples with lasing modes, changing their phase, and generate noise peaks. Isolators & index matching fluids can eliminate these reflections.