Laser and LED

1. MODULE – 3
OPTICAL TRANSMITTERS AND
RECEIVERS

2. Fiber Optical Sources
 High Radiance output
 Optical power radiated per unit solid angle per unit area of the
emitting surface
 Fast emission response time
 Time delay between application of current pulse and the onset
of optical emission
 High quantum efficiency
 Efficiency of conversion from electrical to optical form
 Optical Sources
Incoherent Optical Source LED
Coherent Optical Source Laser Diodes
 Direct Intensity Modulation / External Modulation

3. LEDs & LDs
 LED
 Incoherent output
 No wavelength
selectivity
 Broad spectral width
 1 to 2KBT
 Large beam
divergence
 MMF
 LASER DIODE
 Coherent output
 Spatial
 Temporal
 Highly
monochromatic
 Resonant cavity
 Directional beam
 MMF / SMF

11. Pure-crystal energy-band diagram
(Intrinsic Semiconductor)

16. hole
electron
Electron diffusion across a pn junction
?
•Depletion Region
•Barrier Potential

17. Forward bias condition

18. Reverse bias condition

19. Luminescence = emission of optical radiation as a result of an electronic excitation
Photoluminescence: optical excitation
Catholuminescence: excitation by electron irradiation
Electroluminescence: excitation by current
hole
electron
absorption
non-radiative
transition
Radiative transition:
emission of photon
CB
VB
What happens under forward bias in an LED ?
Eg = hν = hc/λ
λ ( μm ) = 1.24 / Eg (eV)

20. Choice of material
 Direct Bandgap Semiconductor
 Lattice Matched Materials

23. What is the difference between Phonon and Photon?
• A photon is a form of energy but the phonon is a mode of
oscillation that occurs in lattice structures.
• A photon can be considered as a wave and a particle, which are
physically observable entities. A phonon is a mode of vibration,
which is neither a wave nor a particle.

25. k
E


CB
VB
k
E


CB
VB
? “missing” k-vector
by phonons or by
defect states
Energy and momentum conservation
Direct bandgap Indirect bandgap
Efficient light - matter Inefficient light –matter
interaction interaction
Optical transitions:
EC-EV= h
k  0 vertical transition in band structure diagram
(Electron crystal momentum p = 2πhk)

26. Direct and indirect bandgap materials
Semiconductor
material
Energy
bandgap (eV)
Recombination
coefficient (cm3/s)
GaAs Direct: 1.43 7.21 x 10-10
GaSb Direct: 0.73 2.39 x 10-10
InAs Direct: 0.35 8.5 x 10-11
InSb Direct: 0.18 4.58 x 10-11
Si Indirect: 1.12 1.79 x 10-15
Ge Indirect: 0.67 5.25 x 10-14
GaP Indirect: 2.26 5.37 x 10-14

29. Light emitting diode (LED)
Direct bandgap semiconductor device
with 
spontaneous light emission
resulting from radiative recombination
of injected minority carriers
across a lattice matched
pn-junction

53. Power coupling into fiber
Lambertian source to step index fiber :
I () = Io Cos 
a = Sin –1 (NA)

54. Power coupling into fiber
Considering source smaller than fiber, in close proximity and
Assuming cylindrical symmetry 
Coupling efficiency:
c = Sin2 a = NA2

55. External quantum efficiency
The external quantum efficiency ext is defined similar to the
internal quantum efficiency, but considers the number of photons
actually leaving the LED structure.
ext  no. of photons emitted from LED
no. of photons internally generated
Loss mechanisms that affect the external quantum efficiency:
• Absorption within LED
• Fresnel losses: part of the light gets reflected back,
Reflection coefficient: R={(n2-n1)/(n2+n1)}
• Critical angle loss: all light gets reflected back if  >  C with
 C=sin-1(n1/n2); [e.g.  C=17° for GaP/air interface with n2=3.45, n1=1]

61. Spectral width
•Spectral width and center wavelength
 effect of doping concentration
•center wavelength increases with doping
•energy spread increases with doping
•SLED  heavily doped
•ELED  lightly doped
•SLD  Superluminescent LED

62. Spectral width
•Spectral width and center wavelength
 effect of temperature in AR
•higher temperature  more energy spread
•SLED  0.1 to 0.3 nm / Co
•Peak emission  shift :
GaAlAs 0.3 to 0.4 nm / Co
InGaAsP 0.6 nm /Co

63. Spectral width
•Spectral width and center wavelength
Energy spread : between 1.5 KT & 3.5 KT ~ 2.4 KT
 = hc/ph   = - ( hc/2
ph ) ph
•Fractional spectral linewidth
 = |  | = ph ~ 2.4 KT = 2.4KT  (m)
 ph ph 1.24 ( eV)
•At room temperature ph = 2.4 KT ~ 0.062 eV
 0.85 m ;   0.048 ;   36 nm
1.30 m ;   0.065 ;   85 nm
1.55 m ;   0.078 ;   120 nm

64. LED spectral patterns
SLD  30 nm

66. LED output power
0.1
1
10
P
o
(mW)
Temperature
0 20o 40o 60o
SLD
ELED
SLED
0
5
10
P
o
(mW)
Current (mA)
0 100 200 300
ELED
SLED

67. Modulation Capability of LED
 Modulation Bandwidth 
 Doping level in AR
 Radiative lifetime of injected carriers
 Doping concentration
 Number of injected carriers
 Thickness of AR
 Parasitic capacitance of device

68. Modulation Capability of LED
 SLED  high doping concentration  lifetime
reduces (R & NR)  IQE decreases
 AR 2 - 2.5 m, doping < 5 x 1017cm-3 20-50 MHz
 1 - 1.5 m, doping < 0.5-1.0 x 1018cm-3 50-100 MHz
 doping > 5 x 1018cm-3  100-200 MHz
 ELED  very thin ( 50 nm AR) but lightly doped
 By increasing Injected current  145 MHz
 r  ( d / J )1/2
 To achieve high mod. BW w/o reducing IQE
 Minimize r in AR by increased doping- limited to 1024 cm-3

69. Modulation Capability of LED
 P() = Po [ 1 + ( r )2 ] –1/2
Po  Optical power at dc

76. Reliability
 Rapid degradation mode  dislocation
of defects in AR
 Slow degradation mode  exponential
 GaAlAs SLEDs  100 to 1000 years CW
at room temperature
 InGaAsP SLEDs  > 109 hours
 Half power lifetime  50 % drop in Pout

77. Example
 A Burrus type p-n GaAs LED is coupled to a step-index fiber. The LED
is forward biased with a current of 120 mA and a voltage of 1.5 V.
The LED has a circular emitting area of radius 35 μm and a
lambertian emission pattern.The mean lifetimes corresponding to
radiative and non-radiative recombinations are taken to be the same
for GaAs and are equal to 100 ns and the emitted photons possess
energy Eph = 1.43 eV. Calculate :
 (a) the internal quantum efficiency of the LED,
 (b) the internal power efficiency of the device,
 (c) the external power efficiency of the device, if emitting in air, &
 (d) the overall source fiber power coupling efficiency and the optical
loss in dB. Assume that the refractive indices of the core and cladding
of the optical fiber are 1.5 and 1.48, respectively.

78. Solution
 Internal quantum efficiency
ηint = τ nr / { τ nr + τ r } = 0.5
 Optical power generated internally within the LED
P int = photon generation rate x photon energy
= ηint x Rate of carriers crossing junction x Eph
= ηint x ( I / e ) x Eph ; (Eph = 1.43 eV )
= 85.8 mW
 Total power consumed by the device = 120 mA x 1.5 V
= 180 mW
 Internal power efficiency = 85.8 / 180 = 0.48

79. Solution
 External quantum efficiency
ηext ≈ 1 / n ( n+1)2 ; n  RI of outer medium
 Optical power emitted from LED
PLED = ηext P int = 0.25 x 85.8 = 21.45 mW
 Source fiber coupling efficiency
ηc = NA 2 = 0.0596
 PLED, Step = ηc ηext Pint = 1. 27842 mW
 Optical Loss ( dB ) = 10 log10 ( PLED ) = 12.25 dB
PLED, Step