1. Course: Electronic Devices
paper code: EC301
Course Coordinator: Arpan Deyasi
Department of Electronics and Communication Engineering
RCC Institute of Information Technology
Kolkata, India
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Topic: Optical Transmitter
2. Optical Transmitter
converts electrical signal into optical signal
Optical
Transmitter
I hν
Types: LASER
[Light Amplification by Stimulated Emission of Radiation]
LED
[Light Emitting Diode]
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8. Semiconductor LASER: Drawbacks
Improper depletion width ---- ill-defined active region
---- carrier confinement is not possible
Improper modal volume
---- mode confinement is not possible
Larger active region --- higher threshold current
density ------- continuous room temperature operation
is not possible
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Steady state electron density in LASER & LED
( )
p
d J n
n
dt qd τ
∆
∆ = −
Rate equation for recombination
Under equilibrium 0
d
dt
=
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Steady state electron density in LASER & LED
0
p
J n
qd τ
∆
− =
pJ
n
qd
τ
∆ =
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Recombination rate in LASER & LED
t r nr
J
r r r
qd
= + =
Total recombination rate
Total number of recombination in the device under forward bias
fi
R
q
=
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Internal Quantum Efficiency in LASER & LED
Internal Quantum Efficiency =
Number of photons generated inside the device
Number of electrons injected
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External Quantum Efficiency in LASER & LED
External Quantum Efficiency =
Number of photons emitted
Number of EHPs responsible to produce photocurrent
r r
ext
t r nr
r r
r r r
η = =
+
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Recombination rate in LASER & LED
r r
ext
t t
r R
r R
η = =
Rr: number of radiative recombination in the whole
volume of active region
Rt: total number of recombination in the whole
volume of active region
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Recombination rate in LASER & LED
r ext tR Rη=
f
r ext
i
R
q
η=
18. Generated optical power
o rP R hν=
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Generated optical power in LASER & LED
f
o ext
i
P h
q
η ν=
f
o ext
i c
P h
q
η
λ
=
19. Output power
0 1.24
f exti
P
η
λ
=
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Generated optical power in LASER & LED
20. Responsivity --- ratio of emitted optical power
to injection current
1.24 ext
R
η
λ
=
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Responsivity in LASER & LED
0
f
P
R
i
=
21. Difference between LASER and LED
Parameter LASER LED
Working Principle Stimulated
Emission
Spontaneous
emission
Driving Current Lower
(~ 5 -40 mA)
Higher
(~ 50-100 mA)
Nature of emitted light Coherent,
monochromatic
Incoherent,
polychromatic
Power to light
Conversion Efficiency
Approx 80 % Approx 20 %
Numerical Aperture Extremely low large
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Difference between LASER and LED
Parameter LASER LED
Response faster slower
Junction Area Narrower wider
Lifetime longer smaller
Spectrum width narrower larger
Cost high economical