PART II:- OPTICAL FIBRE SOURCES AND DETECTORS • Materials • Construction • Working • Efficiencies and response time • Modulation • Drawbacks and Limitations • Power Launching Efficiencies • Coupling to fibre • Photo-detector noises OPTICAL FIBER COMMUNICATION
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Optical fiber communication Part 2 Sources and Detectors
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PART II:-
OPTICAL FIBRE SOURCES AND
DETECTORS
• Materials
• Construction
• Working
• Efficiencies and response time
• Modulation
• Drawbacks and Limitations
• Power Launching Efficiencies
• Coupling to fibre
• Photo-detector noises
OPTICAL FIBER
COMMUNICATION
FIBER OPTIC SOURCE
CHARACTRISTIC LED LASER
Coherence Non-Coherent Coherent
Chromaticity Many wavelengths Highly Monochromatic
Spectral Width 36 to 40nm 2nm
Divergence Cosine power distribution Narrow pencil beam
Output Power Low (pW) High
Modes Feeds MM Fiber Only Can feed MM and SM
Bit Rate < 100-200Mbps > 2Gbps
Cost Less expensive More expensive
Construction Simple- pn junction Complex–Laser cavity
Emission Spontaneous Stimulated
CHOICE OF SOURCE
Parameters for choice – geometry of fiber,
attenuation, group velocity, group delay distortion,
modal characteristics.
LED – Low power, Multimode, Less precision
requirement.
LASER – High power, Single/Multimode, High
precision, Fiber with high attenuation, Longer
distance application etc.
P-N JUNCTION
• If proper material chosen, recombination energy release is light.
• p-side lightly doped and n-side highly doped.
• Major recombination in p-side.
SPONTANEOUS EMISSION
• h- Plank’s constant = 6.625 x 10-34 Js
Frequency of radiation
IN- DIRECT BAND GAP MATERIALS
DIRECT BAND GAP MATERIALS
MATERIAL FOR LED
Spontaneous Emission:
Electron is excited from valance band to conduction
band using external bias.
Electron stays there for carrier lifetime and then falls
back to valance band, emitting energy equal to band-
gap energy.
In p-n junction in forward bias, electrons and holes
cross junction and recombine to emit energy equal to
band-gap energy.
MATERIAL FOR LED
In-Direct band-gap materials: Momentum of
electrons in valance band and conduction band are not
same. (Higher/lower)
Electrons in conduction band have to search for
Phonon(high energy lattice vibration) to balance
momentum to convert to photon.
This requires generation of phonon and photon
simultaneously for every recombination.(Highly unlikely)
This results in non-radiative recombination. Si, Ge
Direct band-gap materials: Momentum of electrons
in valance band and conduction band are same.
This does not require generation of phonon and photon
simultaneously for every recombination.
This results in most recombinations radiative.
CHOICE OF MATERIAL No pure semiconductor is direct band gap material.
Binary, Ternary and quaternary combination of band
III and band V materials can give direct band gap
material.
Can give almost all recombination radiative.
Band III – Al, Ga, In
Band V – P, As, Sb
GaAs, GaAlAs, InGaAsP
CHOICE OF MATERIAL
Alloy Ga1-xAlxAs has ratio x of Aluminum Arsenide and
Gallium Arsenide.
With x = 0.08, peak wavelength is 810nm.
CARRIER LIFETIME At positive biased p-n junction, carrier injection occurs.
Excess electrons and holes created in p and n- type material
(minority carriers).
Δn = Δp, as carriers form and recombine in pairs.
When injection stops, carrier return to equilibrium value.
Excess carrier density decays exponentially with time.
• Δno initial injected excess electron density.
• Time constant τ is carrier lifetime or bulk
recombination life time, time between creation and
recombination.
DIFFUSION LENGTH
Distance moved by carrier after diffusion and before
recombination.
Can be defined for electrons and holes as Le and Lh.
Le and Lh are electron and hole diffusion coefficients.
τ is carrier lifetime.
Electric current due to electrons and holes is result of
non uniform carrier distribution in material.
Flows even in absence of electric field.
INTERNAL QUANTUM EFFICIENCY In radiative recombination, photon of energy hν is released.
Non radiative recombination releases energy as heat(lattice
vibration).
IQE in active region is fraction of electron-hole pairs which
recombine radiatively.
Rr and Rnr are radiative and non radiative recombination
rate per unit volume.
Bu
t
an
d
LED STRUCTURE - HOMOJUNCTION
n+
p
n+ substrate
n+
Dielectric SiO2
Ohmic Contact
p
LED STRUCTURE - HOMOJUNCTION p-n junction formed by diffusion or epitaxial
technique.
Specially designed to enable most radiative recombination at junction side nearer to surface.
Done when major current flow carried by carriers injected into surface layer.
By making n-side heavily doped.
Major junction crossing is due to electrons to p-side.
Light in p-region radiated out.
Light in n-region may be absorbed.
Both p and n-type semiconductor are made of same base material. (e.g. GaAs).
Called Homo Junction.
HETERO JUNCTION
n-side made of n-type GaAs on n-type GaAlAs.
GaAs – Smaller and direct band gap – Larger electron affinity.
GaAlAs – Larger and direct band gap – Smaller electron affinity.
Electrons flows into GaAs layer.
GaAs becomes collection layer of electrons.
N-GaAlAs - Depletes.
Reduces diffusion length and carrier life time.
Increases bandwidth.
P GaAlAs
N GaAlAs
n GaAs
DOUBLE HETERO JUNCTION
Lower band gap GaAs sandwiched between two larger
band gap GaAlAs layers.
Central GaAs layer becomes active layer.
Placed closest to surface.
Gives carrier confinement and light confinement.
P GaAlAs
N GaAlAs
n or p GaAs
p GaAs
n GaAs
Contact layer
Contact layer
Confining layer
Confining layer
Active layer
DOUBLE HETERO JUNCTION
5 layer structure.
n-N and p-P on two sides.
Ohmic resistive element
Gives good ohmic contact of active layer to conduction layer.
Narrow band gap material at device contact.
Low resistance at device terminal.
Central layers make active layer p or n-type GaAs
sandwiched between N-GaAlAs and P-GaAlAs.
CARRIER
CONFINEMENT
At n-N, electrons flow from N to n higher band gap to
lower band gap.
n-GaAs becomes collection region of electrons.
These electrons do not enter P-GaAlAs as higher BG
even in forward bias.
In forward bias, holes from P-GaAlAs come to active
region.
All recombination take place in active layer.
Gives narrow output.
Flow of electrons from higher BG to lower BG more
efficient than same BG.
P GaAlAs
N GaAlAs
n GaAs
OPTICAL
CONFINEMENT
Refractive Index inversely proportional to BG energy.
GaAs – Higher RI
GaAlAs – Lower RI
Higher RI layer sandwiched between two lower RI.
Acts as slab wave guide.
Light generated inside active region remains guided
through total internal reflection.
Optical confinement.
Required for preventing absorption of emitted
radiation by material around p-n junction.
High efficiency, high radiance.
P GaAlAs
N GaAlAs
n GaAs
N-type
Ga1-
xAlxAs
n-type
GaAs
P-type
Ga1-
xAlxAs
DOUBLE
HETERO
JUNCTION
SURFACE EMITTING LED
BURRUS/FRONT EMITTER
LED
SURFACE EMITTING LED Plane of active light emitting
region perpendicular to axis of fiber.
Fiber cemented into well.
Active region approximately 50μm dia and 2.5 μ.m thick.
Emission pattern isotropic with 120⁰ half power beam width.
Lambertian pattern.
Power decreases as cosine of θ.
Source is equally bright when viewed from any direction.
As projected area decreases as cosθ.
Coupling not good.
Highly divergent.
EDGE EMITTING LED
EDGE EMITTING LED Active region RI greater than side
layers.
Forms waveguide channel that directs
optical radiation towards side into fiber.
Active region 50-70μm wide, 100-150μm
long.
Emission pattern-
Lambertian 120⁰ horizontally.
With proper choice of waveguide thickness, it
can be 25⁰ to 35⁰ vertically.
Better than Surface Emitter.
RADIANCE AND EMISSION RESPONSE TIME
Radiance – (Brightness)
Measure in watts, of the optical power radiated into
a unit solid angle per unit area of the emitting
surface.
High radiance necessary to couple sufficiently high
power levels into a fiber.
Emission response time –
Time delay between application of current pulse and
the onset of optical emission.
OPTICAL OUTPUT
Highly divergent, high power Less divergent, low power
MODULATION CAPABILITY OF LED
Light output from LED can be modulated by wideband
information signal.
Response time > 1µs.
Sufficient for common applications.
Not suitable for communication application as
response time required < 1ns.
Modulation capability restricted by –
Diffusion capacitance
Parasitic diode space charge capacitance
DIFFUSION CAPACITANCE
During forward bias storage of charge carriers in
active region cause diffusion capacitance.
Cdiff = dQ/dV
dQ is change in number of minority carriers stored
outside the depletion region when a change in voltage
across the diode dV is applied.
Delays storage of injected carriers.
Shows how fast change in charge takes place for a
particular change in voltage.
Very large in F.B.(8000pf to 20µf )
PARASITIC DIODE SPACE CHARGE CAPACITANCE
Delays charge injection process itself.
It determines emission response time.
C = εA/d
Emission response time due to this Capacitance can be
made negligible by applying a small constant forward
bias.
Varies more slowly with current that Diff Capacitance.
Considered constant.
Typical value – 350 to 1000pf.
FREQUENCY RESPONSE OF LED Then Frequency Response is entirely determined by
Diffusion Capacitance.
Drive current is modulated by frequency ω, output
optical intensity is -
• Io is intensity emitted at zero modulation frequency.
• τeff is effective carrier life time.
OPTICAL OUTPUT BANDWIDTH
3DB ELECTRICAL VS OPTICAL BANDWIDTH For electrical bandwidth, we feed Iin and receive Iout.
We plot electrical Pout /Pin α (Iout / Iin)2.
Electrical 3dB bandwidth is when output current falls to 70.7% of peak value.
For optical bandwidth, again we feed Iin and receive Iout.
We plot optical Pout /Pin α (Iout / Iin) .
Electrical 3dB bandwidth is when output power falls to 50% of peak value.
Fictitiously gives Optical BW > Electrical BW.
Both BWs are normally mentioned to avoid confusion.
ELECTRICAL BANDWIDTH OF LED
It is frequency band over which –
P(ω) = P(0)/2
I2(ω) = I2(0)/2
Using I(ω) and ω = Δω
Δω = 1/τeff
Higher BW if τeff is lower.
Effective carrier lifetime can be reduced by
increasing doping level in active region.
Controlling injected carrier density.
TRANSIENT RESPONSE
Square pulse when applied to LED gives rise time and fall time due to Diffusion capacitance.
Junction space charge capacitance
To avoid the above current peaking is achieved using peaking coil in parallel to LED.
TRANSIENT RESPONSE
A current 2I is fed.
At t=0, current through coil =0.
Double current through LED enhances injection and recombination rate, reducing rise time.
Current gradually distributed in L and D.
At t=t1, I=0, coil tries to flow current in same direction ½ LI2.
Negative current I through diode brings injected carriers to equilibrium faster, reducing fall time.
TEMPERATURE DEPENDENCE
TEMPERATURE DEPENDENCE
Internal quantum efficiency of LED decreases
exponentially with increasing temperature.
Light emitted decreases.
Edge emitting LED has lower output power than
surface emitting LED.
Edge emitting LED are more temperature dependent.
EXTERNAL QUANTUM EFFICIENCY
Fresnel Reflection – When light strikes boundary between two homogeneous media
with different refractive indices, a portion reflects back and rest transmits further
through refraction. It is not total internal reflection.
EXTERNAL QUANTUM EFFICIENCY
Ratio of the number of photons finally emitted to
number of carriers crossing junction.
• Not same as Internal Quantum Efficiency. as – 1. Only light emitted in the direction of the semiconductor air surface
is useful.
2. Out of light in 1, only light striking emitting surface at angle less
than critical angle will be transmitted through.
3. Some of this light in 2, will be reflected back at semiconductor-air
surface due to Fresnel reflection.
4. There is absorption of light along the path till emitting surface.
ɳext < ɳint
LED POWER AND EFFICIENCY
Excess minority carrier Δn = Δnoe-t/τ
Equilibrium established at constant current flow into junction.
Total carrier generation rate
= externally supplied + thermally generated rate
Current density in ampere/sq m = J
Electrons injected across p-n junction per cubic meter per second = J/qd
q = charge on electron
d = thickness of recombination region.( cubic meter hence include d)
Rate equation for carrier recombination in LED is –
d(Δn)/dt = J/qd - Δn/τ m-3s-1
At equilibrium d(Δn)/dt = 0
Δn = J τ /qd (steady state electron density at constant current into
junction.)
LED POWER AND EFFICIENCY
Total R = Δn/τ = J /qd = Rr + Rnr
Total number of recombination per second R = i/q
i = Forward bias current into device.
(All excess carriers recombine either radiatively or non-
radiatively)
ɳint = Rr/R
Rr = ɳint i/q
= Photons generated/second
Total optical power generated = Rr hν
Pint = ɳint hν i/q watts
Pint = ɳint hc i/qλ watts
LED POWER AND EFFICIENCY
External power efficiency =
• Optical power emitted externally Pe / Electrical power
provided
Pe /P x 100%
Optical power emitted Pe into medium of low RI n from
the face of planer LED fabricated from material of RI
nx is appox
Pe = (Pint F n2)/ 4 nx 2
F is transmission factor of semiconductor – external interface.
(Due to Fresnel reflection, all power will not transmit outside)
LASER
LIGHT AMPLIFICATION BY STIMULATED EMISSION OF
RADIATION
• h- Plank’s constant = 6.625 x 10-34 Js
Frequency of radiation
STIMULATED EMMISSION
Electron at higher excited energy level E2, is impinged
with external stimulation = photon energy = hν12
Electron is forced to come down to stable state E1,
radiating energy hν12
Electron can be stimulated mush before its natural
spontaneous transition time.
Emitted photon by stimulation emission has same
frequency, phase and polarization as the incident
photon.
POPULATION INVERSION
In thermal equilibrium, density of electrons in non-excited lower level E1 is much more than excited level E2.
Most photons emitted will be absorbed. Stimulated emission negligible.
Stimulated emission will exceed absorption only if population of excited stage is greater than that of ground state.
Called Population Inversion.
Inverted population is not an equilibrium condition.
Hence requires pumping techniques.
In semiconductor LASER, it is achieved by injecting electrons into material at device contact to fill lower energy state of conduction band.
In pn junction diode, forward bias applied to inject e into conduction band of p-region or holed are injected into valance band of n-region.
POPULATION INVERSION
Boltzmann Distribution-
Thermal Equilibrium Non equilibrium Distribution-
Population Inversion
LASING ACTION
Two processes:-
Stage one:-
FB applied to active layer and confining layer forming pn
junction.
Hole-electron pair created , recombine after carrier lifetime
to emit spontaneous emission.
FB is gradually increased causing more pairs and more
emission.
Some of these photons are re-absorbed to create more pairs