Light Sources for Optical

Communications

Considerations with Optical Sources

Physical dimensions to suit the fiber 9 micron fiber core diameter

Narrow radiation pattern (beam width) to launch enough light into low NA fiber

Linearity (output light power proportional to driving current) important for analog systems

Considerations … Ability to be directly modulated by varying

driving current output light varies with injected current

Fast response time (wide band) for high speed links

Adequate output power into the fiber to go further without repeaters

Considerations…

Narrow spectral width (or line width) to reduce ___________ ?

Stability LED better than LASER Driving circuit issues impedance

matching Reliability and cost

Solid State (Semiconductor) Light Sources Considering all these factors following SLS

are used in fiber optics Light Emitting Diode (LED) Forward biased

PN junction LASER LED with stimulated emission to

provide (1) low line width (2) low beam width (3) high bandwidth (4) high power and (5) coherency

Theory of Operation A PN junction (that consists of direct band gap

semiconductor materials) acts as the active or recombination region

When the PN junction is forward biased, electrons and holes recombine either radiatively (emitting photons) or non-radiatively (emitting heat). This is simple LED operation.

In an LASER, the photon is further processed in a resonance cavity to achieve a coherent, highly directional optical beam with narrow linewidth

Energy-Bands

In a pure Gp. IV material, equal number of holes and electronsexist at different energy levels.

n-type material

Adding group V impurity will create an n- type material (more electrons than holes)

p-type material

Adding group III impurity will create a p-type material

Light Emission

Basic LED operation: When an electron jumps from a higher energy state (Ec) to a lower energy state (Ev) the difference in energy Ec- Ev is released either as a photon of energy E = h (radiative

recombination) as heat (non-radiative recombination)

The Light Emitting Diode (LED) For fiber-optics, the LED should have a

high radiance (light intensity), fast response time and a high quantum efficiency

Emitted wavelength depends on band gap energy Eg

Eg depends on the type of material (ratio between them)

eV)(

24.1m

hchEg

Physical Design Double hetero structure is used to improve

light output (2 p type and 2 n type) Each region shall also have the right

refractive index to guide the light (optical property)

Light is confined in the active region (high ref. index) due to waveguide operation

Double-Heterostructure configuration

Surface-Emitting LED larger emitting area

Edge-Emitting LED

The active region is embedded into a waveguide structure so that the light is directed an edge Larger active region More directional radiation (similar to LASER)

LED Spectral Width

Generally LED is a broadband light source (125 nm)Edge emitting LED’s have slightly narrow line width

Quantum EfficiencyInternal quantum efficiency is the ratio

between the radiative recombination rate and the sum of radiative and non-radiative recombination rates

For exponential decay of excess carriers, the radiative recombination lifetime is n/Rr and the non-radiative recombination lifetime is n/Rnr

)/(int nrrr RRR

Internal Quantum Efficiency

If the current injected into the LED is I, then the total number of recombination per second is,

Rr+Rnr = I/q where, q is the charge of an electron. That is, Rr = intI/q.

Since Rr is the total number of photons generated per second, the optical power generated internal to the LED depends on the internal quantum efficiency

External Efficiency Not all the light internally generated exits

the LED The actual light output depends on the

optical properties of the active region and surrounding material as well as incident angle of light

Fresnel Reflection and Transmission Coefficients At the surface of any two material with n1

and n2 ref indices, there will be F. LossFresnel Loss = -10 Log (T)

tCoefficien Reflection2

21

21

nnnnR

tCoefficienon Transmissi)(

42

21

21

nnnnT

External Efficiency

External Efficiency for air n2=1, n1 = n

2)1(1

nnext

n1

n2

Lightemission cone

Im

P ext )(24.1

int0

Optical Power Emitted

Half Power Beam Width (θ1/2)

The angle at which the power is half of its peak value

L = 1 For Lambertian source

)(Cos)( LoBB /2)( 2/1 oBB

3-dB bandwidths

Optical Power I(f); Electrical Power I2(f)

2)2(1/)( fPfP o

Electrical Loss = 2 x Optical Loss

Drawbacks of LED

Large line width (30-40 nm) Large beam width (Low coupling to the

fiber) Low output power Low E/O conversion efficiencyAdvantages Robust Linear

The LASER

Light Amplification by ‘Stimulated Emission’ and Radiation (L A S E R)

Coherent light (stimulated emission) Narrow beam width (very focused beam) High output power (amplification) Narrow line width because only few

wavelength will experience a positive feedback and get amplified (optical filtering)

Fundamental Lasing Operation Absorption: An atom in the ground state might

absorb a photon emitted by another atom, thus making a transition to an excited state.

Spontaneous Emission: random emission of a photon, which enables the atom to relax to the ground state.

Stimulated Emission: An atom in an excited state might be stimulated to emit a photon by another incident photon.

In Stimulated Emission incident and stimulated photons will have

Identical energy Identical wavelength Narrow linewidth

Identical direction Narrow beam width

Identical phase Coherence and Identical polarization

Laser Transition Processes(Stimulated and Spontaneous Emission)

Energy absorbed from the incoming photon

Random release of energy

Coherent release of energy

Stimulated Emission

Fabry-Perot Laser (resonator) cavity

Mirror Reflections

How a Laser Works

Multimode Laser Output Spectrum

Longitudinal Modes

ModeSeparation

(Center Wavelength)

g(λ)

Optical output vs. drive current of a laser

Threshold Current

External Efficiency Depends on the slope

Laser threshold depends on Temperature

Distributed Feedback Laser (Single Mode Laser)

The optical feedback is provided by fiber Bragg Gratings Only one wavelength get positive feedback

DFB Output Spectrum