Unit-3Light Light Sources

LEDs1. Requirements for a Light source (or Transmitter)• High intensity output so that sufficient energy is

transmitted on a fiber to overcome losses.• Light source must be highly directional to focus

it into the fiber.• Output of light source must be monochromatic

to reduce dispersion.• Light source should have long life and good

stability of operation.• Light source must be small and easily coupled to

fiber to reduce coupling losses.

Cont..

• Wavelength of light source must lie in the low loss region of the fiber.

• Sources must be capable of being easily modulated giving linear input-output characteristics.

• Light source must be inexpensive to manufacture.

• Light source must be capable of maintaining a stable optical output.

Source used in opticaloptical fiber communication Germanium and Silicon, useful in semiconductor

electronics, are not useful in optics communication due to following reasons :

• They emit light very poorly• Their absorption coefficients are low• They possess a small energy gap• Their conversion efficiency is very lowThere are two important optical sources, employed in

optical fiber communication :

1. Light Emitting Diode 2. LASER Diode.

Advantages with LEDs as optical Source

1. Simpler Fabrication.2. Low Cost.3. Reliability : Immunity from self pulsation and

modal noise problems.4. Less temperature dependence.5. Simpler driver circuitry: Lower drive currents

and reduced temperature dependence.6. Linearity: Linear light output against current

characteristic.

Heterojunctions• A heterojunction is an interface between two

adjoining single crystal semiconductors with different bandgap energies. Classified in two types:

1)Isotype (n-n or p-p)heterojunction2)Anisotype (p-n) heterojunction.These techniques are used for fabrication of

high radiance LEDs and injection LASERS.

Properties of Heterojunctions• Selective etching of layers: material can be

etched away precisely to the heterojunctions without critical control of etching process.

• Improvements of ohmic contacts.• High injection efficiency.• Provide radiation confinement to the active

region.• Heterojunction provide the potential barrier

and much higher rate of recombination can be established within the active layer for a given injection current density.

Internal Quantum efficiency• Excess minority carrier density decays exponentially

with time t:

Where is the initial injected excess electron density, τ is total carrier recombination lifetime, is only a small fraction of the majority carriers and comprises all minority carriers.

Rate equation for carrier recombination in the LED :

When a constant current flow into the diode, an equilibrium condition is reached,

Cont..• So, by setting the derivative equal to zero in eq(2):

• In the steady state the total number of carrier recombinations ( radiative and non radiative) per second or recombination rate:

• When a current i is flowing; then total no. of recombinations per second:

Cont..• Now the LED internal quantum efficiency:

On rearranging with eq.6 we get, is also equal to no. of photons generated per second.E=hf, therefore optical power generated internally by the LED,Pint.,

And f=c/λ.

From the exponential decay of excess carriers , the radiative minority carrier lifetime is and

Cont..

for non nonradiative minority carrier lifetime.

Therefore the internal quantum efficiency:

Coupling Efficiency

Cont..

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Double Heterojunction LED

Contt.

LED Structures

Planar LED

• Fabricated by liquid or vapour phase epitaxial processes over the whole surface of a GaAs substrate.

• Involves a p-type diffusion into the n-type substrate in order to create the junction as shown in fig.(above).

• Forward current flow through the junction gives Lambertian spontaneous emission and the device emits light from all surfaces. However only a limited amount of light escapes the structure due to total internal reflection. Radiance is low.

Dome LED

• A hemisphere of n-type GaAs is formed around a diffused p-type region.

• The diameter of the dome is chosen to maximize the amount of internal emission reaching the surface within the critical angle of the GaAs –air interface.

• This device has higher external power efficiency than the planar LED.

Basic LEDs

The basic LED types used for fiber optic communication systems are

• Surface-emitting LED (SLED),

• Edge-emitting LED (ELED), and

Surface-Emitting LEDs • The surface-emitting LED is also known as the Burrus LED

in honor of C. A. Burrus and Dawson, its developer.

• In SLEDs, the size of the primary active region is limited to a small circular area of 20 m to 50 m in diameter.

• The internal absorption in this device is very low due to larger bandgap confining layers, and the reflection coefficient at the back crystal face is high giving good forward radiance.

• The active region is the portion of the LED where photons are emitted. The primary active region is below the surface of the semiconductor substrate perpendicular to the axis of the fiber.

• A well is etched into the substrate to allow direct coupling of the emitted light to the optical fiber. The etched well allows the optical fiber to come into close contact with the emitting surface.

Cont..• Power Coupled to multimode step index optical

fiber:

Surface-emitting LED

Edge emitter LEDs• Another high radiance structure currently used in optical

communications.• It takes advantage of transparent guiding layers with a

very thin active layer (50 to 100μm) in order that the light produced in the active layer spreads into the transparent guiding layers, reducing self absorption in the active layer.

• Most of the propagating light is emitted at one end face only due to a reflector on the other end face and an antireflection coating on the emitting end face. The effective radiance at the emitting end face can be very high giving an increased coupling efficiency into small N.A. fiber compared with surface emitter.

Edge-emitting LED

Advantages of Edge LEDs• The active region is very thin so self absorption is

reduced.• The beam radiating from the LED has narrow beam

width in the direction perpendicular to the junction so its coupling with fiber using lens coupler is good. And it can be used with multimode and single mode fibers (upto 20 Kms).

• Light output has emerged in the lambertian pattern of elliptical shape having major axis of 1200 and minor axis of 300 as shown in fig.

• Edge emitters couple more power into low N.A. than surface emitter, where opposite is true for large N.A.

Cont..• Coupling efficiency can be increased by lens coupling

as in surface emitters.• Edge emitter allows very high carrier injection

densities for given drive currents.• Edge emitters have also been found to have a

substantially better modulation bandwidth of the order of hundreds of megahertz than other.

• It gives narrow spectral line width as compared to Surface LED. It is about 25nm at 0.9μm and 70nm at 1.3 μm.

• Now a days InGaAsP/InP is also used in fabricating for longer wavelengths.

Comparison

Surface LED1. Easy to fabricate.2. Easy to mount and

handle.3. Require less critical

tolerances.4. Cheaper in cost.5. Less Reliable.6. Low system

Performance.

Edge LED1. Difficult to fabricate2. Difficult to mount and

handle mechanically.3. Need critical tolerances

on fabrication.4. More expensive.5. Highly Reliable.6. High System

Performance.

Lens Coupling to Fiber• It is apparent that much of the light emitted from LEDs is not

coupled into the narrow acceptance angle of the fiber. Even with the well surface and Edge LEDs coupling efficiency is poor due to low Numerical Aperture.

However greater coupling efficiency can be obtained if lenses are used to collimate the emission from the LED, particularly when fiber core diameter is significantly larger than the width of the emission region. The function of the lens is to magnify the emitting area of the source to match exactly the core area of the fiber end face.

There are several lensing scheme used: 1) spherical ended fiber, tapered fiber lenses, truncated spherical lens and integral lens LED. The main limit in coupling efficiency is determined by the fraction of emitted radiation that can be collected by the lens.

LED Characteristics1. Optical Output Power

Cont..

3. Modulation bandwidth• The Modulation bandwidth in optical communications may be

defined in either electrical or optical terms. However, in optic communication it is used where the electrical signal power has dropped to half its constant value due to modulated portion of the optical signal.

Cont..

Modulation