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Chapter 3: Optical Devices

Sources for Optical Communications

Optical Communications: Circuits, Systems and Devices

Sep 2012 Sharif University of Technology 1

lecturer: Dr. Ali Fotowat Ahmady

Source Types (1 of 4)

• Diode laser: high optical output, sharp spectrum, can be modulated up to tens of GHz, but turn-on delay, T instability, and sensitivity to back-reflection• LED: longer lifetime and less T sensitive, but broad spectrum and lower modulation limit• DFB distributed feedback laser: even sharper spectrum but more

Chapter 3 Optical Devices Sources


• DFB distributed feedback laser: even sharper spectrum but more complicated to make• MQW Multiple-Quantum Well laser: less T dependence, low current, low required bias, even more complicated• VCSEL vertical-cavity surface-emitting laser: single mode and easy fabrication, best for arrays, but higher current required


• Light-emitting diodes (LEDs)- Incoherent sources- Modulation bandwidth up to 100–200 MHz- Low power ( 10−2 mW)- Low cost



• Laser diodes- Coherent sources

◦ Oscillator = amplifier with positive feedback and enough gain to cancel loss

- Modulation bandwidth up to 25 GHz- Medium to high power ( 1 mW to 1 W)- Moderate to high cost


Light Emitting Diodes(LED)


(LED)

LED Types (1 of 2)

• Homojunction- Single p–n junction created by doping a single semiconductor material- Thermal equilibrium built-in electric field and potential difference- Current increases and light is emitted when junction is forward-



- Current increases and light is emitted when junction is forward-biased

- Problem: Light-emitting volume is large◦ Poor coupling efficiency to fibers◦ Low carrier density

( ) 1BqV k TsI V I e = −

LED Types (2 of 2)

• Double-heterostructure- Improve carrier confinement by sandwiching a thin layer of a different semiconductor material between the p- and n-type cladding layers

◦ Bandgap in the thin layer < bandgap in the cladding layers



layers◦ must (nearly) match lattice constants

Band gaps and emission wavelengths



Bandgaps and emission wavelengths (at 300◦ K) of semiconductors used as sources and detectors for optical communications

Modulation response

• Rate equation for carrier density N:

- τc = carrier lifetime, v = active volume• Modulation of injected current


c

dN I Ndt qv τ

= −


• LED optical transfer function and 3-dB bandwidth:

• 3-dB electrical bandwidth (determined by |Hoptical(ωm)|2) is f3dB, electrical = 1/2π τc

( ) ( ) ( )

, ( )1

m mi t i tb m b m m

c b c mb m m

m c

I t I I e N t N N eI I qv

N Nqv i

ω ωωτ τ

ωω τ

− −= + ⇒ = +

= =−

3 ,1 2( )

1 2optional n dB optionalm c c

H fi

ωω τ πτ

= ⇒ =−

LED structures

• Edge-emitting (ELEDs)- Beam divergence FWHM 30◦ in the direction perpendicular to the plane of the p–n junction- Used at 1300 and 1550 nm

• Surface-emitting (SLEDs)- Lambertian source with angular distribution



- Lambertian source with angular distribution- Beam divergence FWHM 120◦- Used at 850 and 1300 nm

LED packaging

• Transistor-style header- Metal cap with a transparent cover or lens- Poor light-gathering efficiency

• Microlens placed directly on LED• Attached-fiber



• Attached-fiber- Burrus SLEDs- Pigtailed ELEDs- Power coupled into system in Lambertian approximation:

Pin = Psource(NA)2

NA (Numerical apperture) is a measure effective area orcone of acceptance of the fiber

LED Output Characteristics (1 of 2)

• Typical Powers - 1 to 10 mW

• Typical beam divergence- 120 degrees FWHM: Surface emitting LEDs- 30 degrees FWHM: Edge



- 30 degrees FWHM: Edge emitting LEDs

• Typical wavelength spread- 50 to 60 nm

Output power vs. drive current for typical LED source

LED Output Characteristics (2 of 2)



Output Power versus forward current and operating temperature for an LED


LASERs


Introduction to LASERS (1 of 2)

• LASER is an acronym- Light Amplification by Stimulated Emission of Radiation- A laser is actually an oscillator

◦ LOSER was thought to be a poor acronym for attracting funding

- First lasers and their inventors: Ruby – Ted Maiman; HeNe – Ali



- First lasers and their inventors: Ruby – Ted Maiman; HeNe – Ali Javan; CO2 – Kumar Patel- In the early days of lasers, they were often called “optical masers”- MASER=Microwave Amplification by Stimulated Emission of Radiation

Introduction to LASERS (2 of 2)

• Oscillator = amplifier with positive feedback and enough gain to cancel loss- In lasers, feedback is accomplished by reflection at mirrors or gratings that enclose an amplifying medium- The physicist Arthur Schawlow thought of putting mirrors on an optical or microwave amplifying medium to create an oscillator- Prof. Willis Lamb founded the area of theoretical and computational



- Prof. Willis Lamb founded the area of theoretical and computational analysis of lasers and laser-pulse propagation

LASERs vs. LEDs

• LEDs- Broad spectrum (covers a large range of wavelengths)- Low output power- Poor directivity- Incoherent

No speckle



◦ No speckle• LASERs

- Narrow spectrum (covers a small range of wavelengths)- High output power- Good directivity- Coherent

◦ Speckle

LASERs Classified by Modal Properties


• Multiple-longitudinal-mode lasers: Broad spectrum

- Lasing occurs on several longitudinal (cavity) modes- Separated in frequency


- Separated in frequency by c/2nL

• Single-longitudinal-mode lasers: Narrow spectrum- Distributed-feedback (DFB) lasers- Distributed-Bragg-reflector (DBR) lasers- Quantum-well (QW) lasers- Vertical-cavity surface-emitting lasers (VCSELs)

Semiconductor LASERs for Communications (1 of 3)

• Edge-emitting lasers- Fabry-Pérot lasers

◦ Feedback occurs through reflection at the end surfaces◦ Each Fabry-Pérot mode has a unique wavelength λ, such that

2n L = mλ for some integer m



2ncL = mλ for some integer mwhere nc is the refractive index of the laser medium◦ More than one Fabry-Pérot mode lies under the gain curve


- Distributed-feedback (DFB) lasers◦ Feedback occurs through Bragg reflection from gratings




• Vertical-cavity surface-emitting lasers (VCSELs)- Feedback occurs through reflection from multilayer mirrors- Modes are widely spaced → usually only one mode under the gain curve



Emission and Absorption of Light (1 of 5)

• Consider an idealized atomic or molecular system with only two energy levels: E0 (ground state) and E1 (excited state) that interact significantly with light at the frequency


( )1 0E Eω = − h


• The rate at which photons are absorbed, per unit volume, is

where N0 is the number of ground-state atoms per unit volume, and ρph(ω) is the power spectral density of the light at frequency ω

( )0'abs phR B N ρ ω=


• Einstein discovered that there are two kinds of processes by which light is emitted:

- Spontaneous emission (happens in the dark)Rspont = AN1

where N1 is the number of excited-state atoms per unit volume, and A is the spontaneous emission rate per atom (units: s−1)



1A is the spontaneous emission rate per atom (units: s−1)- Stimulated emission (happens only when other photons are present)

Rstim = BN1ρph(ω)- A and B are called the Einstein coefficients


• In thermal equilibrium, the ratio of the excited-state to the ground-state population is given by the Boltzmann distribution,

- Since there’s no net population change in equilibrium, the “down” rate equals the “up” rate:


1

0

Bk TN eN

ω−= h


rate equals the “up” rate:AN1 + BN1ρph(ω) = B N0ρph(ω)

- The optical power spectral density must equal the Planck black-body spectral density,

where ν = ω/2π is the circular frequency (in Hz)- Then A = (8πhν3/c3)B and B’= B

( )3 38( )

'/ 1 1B Bph k T k T

A B hv cB B e eω ω

πρ ω = =

− −h h


• Stimulated emission produces gain- Rate at which new photons are emitted ∝ power spectral density of existing photons- Ratio of gain to loss is Rstim/Rabs = N1/N0- Net amplification and oscillation (laser action) require a population inversion (N > N ) and are not possible in thermal



population inversion (N1 > N0) and are not possible in thermal equilibrium

◦ Must pump excited-state population with an external energy source

• Spontaneous emission depletes the excited state and acts as a noise source


• Ratio of stimulated to spontaneous emission rates in thermal equilibrium:

- Spontaneous emission dominates when the photon energy, hω, is much larger than the thermal energy, kBT

For communications at λ = 1550 nm, hω ≈ 1 eV >> k T


11B

stimk T

spont

RR e ω=

−h


B◦ For communications at λ = 1550 nm, hω ≈ 1 eV >> kBT ≈ 25 meV

Condition for LASER Oscillation (1 of 2)

• Optical electric field amplitude after n round trips in the laser cavity:An(ω) = ρ1ρ2e2[−jβ(ω)+g(ω)−α]LAn−1(ω)ρ1, ρ2 = amplitude reflection coefficients at end mirrorsL = cavity length, α = attenuation coefficient- Condition for oscillation: Gain ≥ loss


1 1


- Equality holds at laser threshold1 2

1 1( ) ln2

gL

ω αρ ρ

≥ +

Condition for LASER Oscillation (2 of 2)

• Constructive interference of An, An−1, ... occurs when the cavity length is an integral number of half-wavelengths

- For each value of m, there is a longitudinal mode of the laser cavity


2 ( ) 22 ( )

mL m Ln

λβ ω π

ω= ⇒ =


cavity- Frequency spacing between adjacent longitudinal modes is

2 ( )cf

n Lω∆ =

Turn on Delay (1 of 2)

• For an applied current pulse of amplitude Ip, the turn on delay is given by:


τ τ

=−

ln pd th

p th

II I


- With a bias current Ib applied:

where τth is the delay at threshold (2ns Typ.)

τ τ

=+ −

ln pd th

p b th

II I I

Turn on Delay (2 of 2)



• To reduce the turn on delay:- Use a low threshold laser and make Ip large- Bias the laser at or above threshold

Chirping

• Current modulation causes both intensity and frequency modulation (chirp)• As the electron density changes, the gain (imaginary part of refractive index ni) and the real part of the refractive index (nr) both change. • The susceptibility of a laser to chirping is characterized by the alpha parameter.


nn ∂∂


where N is the electron density• Large α implies lots of chirping• α=1-3 is expected for only the very best lasers.• Chirping gets worse at high frequencies• Correctly adjusting the material composition and laser mode volume can reduce α

ir nnN N

∂∂α

∂ ∂=

Effects of Current and Temperature

• Applying a bias current has the same effect as applying a pump laser; electrons are promoted to conduction band. Fc and Fv get farther apart as well• Increasing the temperature creates a population distribution rather than a sharp cutoff near the Fermi levels



Quantum Efficiency

• Internal quantum efficiency ηi, photons emitted per recombination event, determined empirically to be 0.65±0.05 for diode lasers• External quantum efficiency ηe given by

Equal to emitted optical power divided by applied electrical power, or


( )i the

th

gg

η αη

−=


Equal to emitted optical power divided by applied electrical power, or hνηe/qV• For GaAs lasers, TQE ≈ 50%• For InGaAsP lasers, TQE ≈ 20%

thg

Laser Reliability and Aging (1 of 2)



Changes in the operating current as a function of aging time for a 1.3 μm InGaAsP laser aged at 60ºC with 5 mW of output power.

Laser Reliability and Aging (2 of 2)



Typical laser diode lifetime test data. (the curve has been displaced for clarity.) device A has failed; device B has significant output power but has not failed yet.

Bathtub curve for device failure rates

Laser Diode Transmitter Block Diagram



Functional block diagram of a transmitter

Questions



sources for optical communications - ee.sharif.eduee.sharif.edu/~opticsicsys-alif/ch3_01.pdf ·...

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