Optical communications

Chapter 3.3Chapter 3.3

Pham Quang Thai

Pqthai.hcmut@gmail.com

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Content

• Semiconductor physics

• Light emitting diode (LED)

• Laser principles

• Laser diode• Laser diode

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The first semiconductor laser: Fabry Perot

homojunction laser (FB laser)

(a) at thermal equilibrium

(b) under forward bias (spontaneous emission)

(c) under high-injection condition (population inversion - lasing)

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P-I characteristic

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P-I characteristic

• Rate equation

Electron rate = injected electron - spontaneous recombination – stimulated emission

Photon rate = Stimulated emission + spontaneous emission – photon loss

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sp

sp

ph

dN I NCN

dt ed

dCN R

dt

τ

τ

= − − Φ

Φ Φ= Φ + −

N: number of electrons

I: bias current

e: electron charge

d: active region volume

τsp: spontaneous recombination lifetime

C: absorption coefficient

φ: number of photons

Rsp: spontaneous emission rate

τph: photon lifitime

P-I characteristic

• At lasing threshold:

0

0

dN

dt

d

=

Φ=

6

1 2

0

0

1 1 1ln

2

th thth

sp

dt

edN gI

L R Rα

τ β β

=

Φ =

= = = +

β: gain coefficient of cavity

α: loss coefficient of cavity

L: cavity length

R1, R2: mirror reflection coefficient

P-I characteristic

• Above lasing threshold

( )

1 1 1ln

ph

thI Ied

c hcP

τΦ = −

= Φ

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1 2

int

1 1 1ln

2 2

( )2 /

/

thext

th

c hcP

n L R R

gP hc

I ed g

λ

αλη η

= Φ

−= =

P-I characteristic

• Temperature dependent of Ith

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P-I characteristic

• Temperature dependent

1

( )

thth

sp

th

edNI

IN

τ

τ

=

→ ∼

• As temperature increases, Auger recombination

increases exponentially -> N decreases

• Ith increases with temperature

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2

( )

( )

th

sp

sp

IN

N N

τ

τ

→ ∼

∼

Modulation characteristic

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Modulation characteristic

• Modulation bandwidth for small signal

3

3( )

2dB th

Cf I I

edπ= −

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2

1 11

2relaxation

sp ph th

ed

If

I

π

π τ τ

= −

Modulation characteristic

• Modulation of large signal:

Large change in bias current

-> change in N

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-> change in N

-> change in n

-> change in phase

-> change in both amplitude

and phase of output signal

-> Chirping !

Signal disprtion at the optical source

• Distortion

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Relative intensity noise (RIN)

• Cause by amplified spontaneous emission

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Examples for chapter 3.3

• Problem 4-12:

– A GaAs laser emitting at 800 nm has a 400-μm

cavity length with a refractive index n=3.6. If the

gain g exceeds the total loss α, throughout the gain g exceeds the total loss α, throughout the

range 750nm<λ<850nm, how many modes will

exist in the laser?

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Examples for chapter 3.3

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Issues of homojunction

• Threshold current density: 105 A/cm2

• External quantum efficiency: << 1%

Methods for improvement:

1. Carrier confinement. Confine the injected 1. Carrier confinement. Confine the injected electrons and holes to a narrow region about the junction -> reduce current to achieve population inversion.

2. Photon confinement. Construct a dielectric waveguide around the optical gain region -> increase quantum efficiency

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Carrier confinement: single and double

heterojunction

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Carrier confinement: single and double

heterojunction

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Photon confinement: waveguide

structure

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Single frequency laser: distributed

feedback laser (DFB laser)

• Only one frequency satisfies Bragg condition

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Single frequency laser: Vertical Cavity Surface

Emitting Laser (VCSEL)

• Only one frequency satisfies Bragg condition and cavity length

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Light sources comparison

LEDIncoherence light

<10mW output power

LDCoherent light

~100 mW output power<10mW output power

~10THz spectral width

100-200 MHz modulation BW

30-50° beam divergence

~1% couple efficiency

<100 mA bias current

~100 mW output power

5-10MHz spectral width

~25 GHz modulation BW

3-5° beam divergence

>50% couple efficiency

~300-400mA bias current

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