Download - Lecture3 Light Amplification
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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
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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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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