Optical communications

Chapter 3.2Chapter 3.2

Pham Quang Thai

Pqthai.hcmut@gmail.com

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Content

• Semiconductor physics

• Light emitting diode (LED)

• Laser diode (LD)

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LED: light emission from radiative recombination of

carriers injected in forward biased p-n junction

• Video

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LED wavelength and spectral width: depend on

the semiconductor band gap Eg

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LED power-current characteristic

• Internal quantum efficiency:

Rrr: radiative recombination rate

Rnr: non-radiative recombination rate

Rtot: total recombination rate

N: carrier injection rate

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N: carrier injection rate

τrr: radiative recombination lifetime

τnr: non- radiative recombination lifetime

Nph: photon generation rate

Eph: energy of photon

I: bias current

e: electron charge

h: Planck constant

c: speed of light

λ: wavelength

int

int int int

1.24( )

( )ph ph ph

II hcP W N E NE

e m

ηη η

λ λ µ

= = = ≈

LED power-current characteristic

• External quantum efficiency: internal absorption and

internal total reflection

• LED emits about 1-2% of its internal optical power

• Epoxy lenses can increase ηext by 2-3 times

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LED power-current characteristic

• Coupler efficiency2

couple NAη =

• In practice, about 1% of internal optical power is

coupled into fiber

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Output power-current: linear across a small region!

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LED modulation bandwidth

2.2rise fall ct t τ= ≈

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LED modulation bandwidth: modulated signal

frequency where output power is reduced by 3dB

• Rate equation:

• Transfer function:

c

dN I N

dt ed τ= −

τc: carrier lifetime

• Transfer function:

• Output power:

• 3dB modulation bandwidth

10

2( )

(1 )

DCm

C

PP ω

τ=

+

3

1

2dB

c

fπτ

=

Modulation bandwidth example

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Examples for chapter 3.2

• Some commercial LEDs

• Problem 4-6:

– A double-heterojunction InGaAsP LED emitting at

a peak wavelength of 1310nm has radiative and a peak wavelength of 1310nm has radiative and

nonradiative recombination times of 25 and 90 ns,

respectively. The drive current is 35 mA. Find the

internal quantum efficiency and internal power

level

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