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Light Amplification optical feed back and light amplification or

oscillation

• Light amplification in the LASER occurs when a photon colliding with an atom in the excited state, causes the stimulated emission of a second photon and then both of these release two more. This process can effectively creates avalanche multiplication.

• Coherency is achieved by confiscating the light produced with in the two mirrors acts as the +ve feed back.

• One mirror is semi-transparent which is used as o/p to be coupled to required devices.

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Fabry perot resonator

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Fabry perot resonator

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Fabry perot resonator

• Light having different frequency cannot gets amplified in the fabry perot resonator.

• Single frequency of multiple harmonics of fundamental frequency can only exist.

• The standing wave formation must satisfy the following condition:

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Fabry perot resonator

• Length defines the number of frequencies which can exist whereas, width defines the density of o/p.

• The standing wave can only exist at the frequencies for which the distance b/w the mirrors are integer multiple of “λ/2”

• The expression can be written as:

L= λq/2n

• Where

– Λ=wavelength– N=refractive index of the medium– q=integer

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• Discreet which can exist:• Frequencies generated as a function of length

“L” are called “longitudinal or axial modes” • In terms of fractional frequency we can write

nL

qcf

2

f

nL

c

2

nL

getwefinally

nl

c

cnL

equationfrom

fc

f

nL

cf

2

:

2.....

2

2

cf& ff where

f

t wavelenghof In terms

2............2

2

22

2

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Modes patterns

TEM00

TEM11

TEM10

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Threshold Condition For Laser Oscillation

• THE sustained stimulated emission is obtained when the amplifying medium balance the total loss:

• Loss coefficient per unit length : cm/

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21

21

1ln

2

1

:

12exp2exp

:

2exp

/:

2exp

rrLg

obtainedbemaylengthunitperthresholdthe

LrrLg

nstimulatiosustainedforHence

Lggainfractional

cmgtcoefficiengainThe

Lrrlossfractional

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Optical emission from semiconductor

• Protperties of semiconductors:• Intrinsic s/c• Extrinsic s/c

– Intrinsic s/c: a perfect semiconductor material with no impurities or lattice defects are called intrinsic s/c. the intrinsic s/c the energy distribution is as follow:

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• The thermally excited electron in conduction band and holes left in valance band allow conduction through the material.

• In intrinsic type s/c the energy distribution is fermi-dirac and probability of electron excitation is given as:– P(E)=1/[1+EXP(E-EF )/KT]

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• Extrinsic s/c:– To create the extrinsic type s/c the material is dopped

with impurities to create more free electrons (donor impurities) or holes (acceptor impurities).

– Intension is to provide the capability to produce free electrons or holes in s/c material.

– The energy distribution is as follow:• The energy distribution near the conduction band in donor

type s/c• The energy distribution is near the valance band in acceptor

type s/c.

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N-type

P - type

Shift of fermi-D

energy

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Laser through heterojunction type material

• LASER generated by using through single p-n junction fabricated from single s/c material are called homojunction.

• The radiative properties of junction diode can be improved by using hetero-junction.

• Heterojunction is an interface between two single adjoining crystal s/c.

• H/junctions are classified as isotype (n-n,p-p) or anisotype (p-n).

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Laser through hetrojunction type material

• The isotype H/junction provide potential barrier within the structure which confines the minority carrier to a small radiative region.– It effectively reduces the carrier diffusion and

volume within the structure where carrier recombination may take place.

– Heterojunction is widly used in injection LASER.

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DH junction LASER

• DH junction LASER structure provides the optical confinement in vertical direction through the refractive index step at the Heterojunction interface.

• LASING takes place across the broad area of the entire surface.

• Broad emission area creates several problems, difficult heat sinking, unsuitable light geometry, difficult coupling.

• This requires to develop a suitable geometry.

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Strip geometryBroad area optical o/p

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Strip geometry

• Optical o/p is confined in the horizontal direction.• Major current fllows through the device and

optical o/p and active region is within the strip.• Strip is formed by high resistance material on

either side by photon bombardment or oxide isolation.

• Strip acts as the guiding mechanism which over come the broad area problems

• The o/p beam divergence is 45 deg perpendicular to the plane and 9 deg h/plane.

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- Broad area injection LASER

- Sub peaks due to higher order horizontal transverse lateral modes

- Longitudinal mode obtained by using strip geometry

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Light Emitting Diode

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Spontaneous emission• The increased concentration of minority

carriers in opposite type region in the forward biased p-n diodes lead to the recombination of carriers across the band gap.– Normally empty electrons states in conduction

band of p-type material.– Normally empty hole states in valance band of

n-type material.

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Spontaneous emissionConduction band in p-type material

Valance band in p-type material

Conduction band in n-type material

Valance band in n-type material

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Spontaneous emission

• Carrier recombination i.e.electron – hole recombination….releases the energy (radiative mode).

• The released energy is equal to band gap energy.

• E = hf ….=.hc/λ…….. λ=1.24/Eg• Where Eg is in ev,

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Spontaneous emission

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Optical source: LEDs• Characteristics of LEDs:

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• Disadvantages of LEDs:

• incoherent light due to which probability of light coupling into the fiber is less.

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Types of LEDs

• There are five types of LEDS based on their structure and material used.– Planar type– Dome type – Surface emitter– Edge emitter– Super luminescent type

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Types of LEDs

• Planar LED;– The planar LED is the simplest of the

structure.– Fabricated by liquid or vapor-phase epitaxial

process over the whole surface of GaAs substrate.

– P type difusion into n-type substrate to create junction.

– Forward current give spontaneous emission from all surface.

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Planar LED

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Dome Type LED

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

• Dia of dome is chosen to maximize the amount of internal emission reaching the surface within the critical angle of GaAs-air interface

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Surface emitter LED

• Wavelenght is from 0.8-0.9 micro meter• o/fiber is etched to reduce the refractive index mismatch

and to increase internal power efficiency.• Mainly used for graded index fiber.

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Surface emitter LED

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Edge emitter LED

• Similar to strip geometry injection LASER• It takes advantages of transparent guiding layers

with a very thin active layer (50-100μm)•

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Super luminescent LED• Provides high o/p power

• Directional o/p power.

• Narrow spectral o/p.

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Characteristics of LEDS

IDEAL charateristics

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Spectral o/p