EECB 483 Optoelectronics & Fiber Opticslecture5v2
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Transcript of EECB 483 Optoelectronics & Fiber Opticslecture5v2
8/19/2019 EECB 483 Optoelectronics & Fiber Opticslecture5v2
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EECB 483 Optoelectronics &Fiber Optics
Lecture 5:
Optical sources and Amplifiers
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Optical sources
optical sources generates light signal as transmission medium for
optical communication system
most typical source
light emitting diode (LED):
incoherent output ;
no need optical cavityThe output radiation has broad spectral width, since the
emitted photons energies range over the energy distribution
of the recombining electrons and holes
Emitted into hemisphere according to a cosine powerdistribution and has large beam divergence
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Laser diode (LD):coherent output;
the optical energy is produced in an
optical resonant cavity.
energy released is spatial and temporal
coherence
highly monochromatic and
Output beam is very directional
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Choosing an optical Source Compatibility with Optical
Waveguide:
The following characteristics need to be consider:
i. Its geometry
ii. Its attenuation as function of wavelength
iii. Its group delay distortion (bandwidth)
iv. Its modal
The interplay of these factors with optical source power, spectral width,
radiation pattern and modulation capability
The spatially directed coherent optical output of Laser Diode can be coupledinto either single mode or multimode fibers.
The LED are used in multimode, because the incoherent output power can be
coupled in sufficient quantities.
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Light emitting diode (LED)
a pn-junction semiconductor that emit light when forward
biased
two energy bands separated by a gap energy, Wg
upper band is conduction band has free electrons
lower band is valance band has free holes
free electron can recombine with free holes to return
to neutral state and release energy in the process
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/Wg
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Material Symbol Band gap (eV) @ 300K
Silicon Si 1.11 [1]
Germanium Ge 0.67 [1]
Silicon carbide SiC 2.86 [1]
Aluminum phosphide AlP 2.45 [1]
Aluminum arsenide AlAs 2.16 [1]
Aluminium antimonide AlSb 1.6 [1]
Gallium(III) phosphide GaP 2.26 [1]
Gallium(III) arsenide GaAs 1.43 [1]
Gallium(III) nitride GaN 3.4 [1]
Gallium(II) sulphide GaS 2.5 (@ 295 K)
Gallium antimonide GaSb 0.7 [1]
Indium(III) phosphide InP 1.35 [1]
Indium(III) arsenide InAs 0.36 [1]
List of band gaps
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LED
when p-and n-doped material put close together,
energy band gap is produced without any applied voltage
free electron and free hole cannot jump the barrier
due to insufficient energy
when forward biased about the same as Wg, freeelectron and free hole has the sufficient energy to move
into the junction
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LED
electron can fall into valance band andrecombine with holes to release energy in the
form of photon
the wavelength of photon released depends onenergy band gap, Wg given by this formula
The photon energy and frequency are related
by
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LED
homojunction – a pn-junction formed from single
semiconductor. photon emitted over an extensive region
and diverge
heterojuction – pn-junction formed by dissimilar
material. two material with different band gap energies
and different refractive indices. recombination occurs
only in small well-define active layer and differentrefractive index formed waveguide.
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LED
output power of LED is linearly proportional to forwarddriving current
the current i is the injected charge per second
# of charge/second, ; e is the magnitude of charge
on each electron
if fraction of recombine charge and produce photon, η , the output
power will be P = ηNWg = (η¡Wg)/e this will be joules
In electron volt: P= η¡Wg
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LASER
Individual lower energy electrons absorb light wave energy and change theirspatial electric charge configuration (they “move” to a higher energy level) atrandom time intervals
Individual higher energy level electrons radiate light wave energy at random
time intervals. When light waves are present at a frequency corresponding tothe normal radiation frequency (f =∆E/h) for that downward energy change, thenumber of electrons that randomly radiate per unit time is increased. Thisincrease is called “stimulated emission.”
To continue amplifying, there must be a ready supply of high energy electrons.Electrons are continuously “pumped” up from a still lower energy level to thehigh energy level by constant irradiation using a much higher frequency,
shorter wavelength optical source. Arthur L. Schawlow (1921-1999) and Charles H. Townes first built an amplifier
using stimulated emission for amplifying microwaves. Gordon Gould is alsocredited with theoretical invention of the LASER in the patent office. Samemethod later applied to visible and infra-red light. Schawlow and Townesreceived Nobel prize with others.
Terminology:
MASER (Microwave Amplification via Stimulated Emission of Radiation)
LASER (Light Amplification via Stimulated Emission of Radiation)
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The electrons are generally found in the ground state or the lowest energy level.They can occupy higher energy levels leaving lower levels vacant. They change
from one level to another by (1) absorption or (2) emission of energy. This
changing of energy levels is called radiative transition. There are three types of
radiative transition.
A. Stimulated emission
B. Spontaneous emissionC. Absorption
E1(ground state)
E3 (Excited State)
Spontaneous
Absorption Stimulated
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Laser principle
atom/ion/molecule of the active material will
absorb energy supplied to raise from ground state toexcited state
once in excited state, the atom/ion/molecule will release
its energy to return back to ground state in two situation:
first process is called spontaneous emission (without external energy), theoutput are isotropic and of random phase, appear as narrowband Gaussian
output
Second process is called Stimulated emission, the electron is induced to
downward transition by an external stimulation; that is a photon energy (hv)
impinges the electron while in excited state to immediately stimulated to drop
to the ground state . The emitted photon is in phase with the incident photon.
the energy release is in the form of photon or light
energy
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Laser Principle
in order to keep the stimulated emission high, number ofatom/ion/molecule in excited state must be more than number of
atom/ion/molecule in ground state
this is to make sure more emission than absorption
this condition is called population inversion
In a Semiconductor laser, population inversion is accomplished by
injecting electrons into the material as the device contacts to fill the
lower energy states of the conduction band.
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a laser come from oscillation of photon
generation within a resonant cavity
the mirrors at both end provide reflection so
that light will be amplified as it pass throughamplifying medium back and forth
the traveling light will interfere and createstanding wave
LASER
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Laser
to produce standing wave, cavity length must be and
integral number of half wavelength long, L = mλ/2
resonant frequencies are found from f = mc/2nL, where
n is the refractive index of the material within cavity
various resonant frequency is called longitudinal modes
∆fc = c / 2Ln
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Laser
relation with wavelength
∆fc/f = ∆λc/λo
λo is free space mean wavelength
f is mean frequency
since f = c/λo, ∆λc = (λo2/c)∆fc
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Laser
we will discuss four types of laser
semiconductor laser
gas laser
solid state laserfiber laser
all use the same principle, oscillation in a cavity
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Principal components:1. Gain medium
2. Laser pumping energy
3. High reflector
4. Output coupler
5. Laser beam
http://en.wikipedia.org/wiki/Laser_beam
LASER Amplification
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A laser consists of a gain medium inside a highly reflective optical cavity,
also use to supply energy to the gain medium.
cavity consists of two mirrors arranged such that light bounces back and
forth, each time passing through the gain medium.
The gain medium is a material with properties that allow it to amplify
light by stimulated emission.
The output coupler, is partially transparent. The output laser beam is
emitted through this mirror.
LASER Amplification cont…
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Light of a specific wavelength that passes through the gain medium is
amplified (increases in power);
the surrounding mirrors ensure that most of the light makes many passes
through the gain medium, being amplified repeatedly. Part of the light
that is between the mirrors (that is, within the cavity) passes through the
partially transparent mirror and escapes as a beam of light.
The process of supplying the energy required for the amplification is
called pumping.
The energy is typically supplied as an electrical current or as light at a
different wavelength. Such light may be provided by a flash lamp orperhaps another laser.
Most practical lasers contain additional elements that affect properties
such as the wavelength of the emitted light and the shape of the beam.
LASER Amplification cont…
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Laser type
Gas laser
active medium : Helium & Neon gas
wavelength : 633nm
Semiconductor laser
Active medium : AlGaAs, InGaAsP
Wavelength : 800-900nm, 1000-1700nm
Solid State laser
Active medium : Neodymium yttrium-aluminium-garnet (Nd:YAG)
Wavelength : 1060nm, 1350nm
Fiber laser
Active medium : Erbium ions
Wavelength : around 1550nm
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Optical Repeaters
In general, optical repeaters perform functions similar toelectrical digital repeaters: the “Three Rs”:◦ Regenerate (amplify, compensate for power loss)
◦ Reshape (correct pulse wave-shape for distortions due to time dispersion)
◦ Retime (correct for jitter)
OEO Repeaters◦
Historical optical repeaters use an OE detector, electrical amplification andpulse shaping, and a EO LED or LD to transmit the repeated pulse streaminto the next span.
◦ Only one wavelength can be processed by a single OEO repeater.
◦ Many optical and electronic components and some manual adjustment atinstallation time are required. This is a relatively complicated and costlydevice.
◦
A simpler type of all-optical repeater, particularly one that amplifies all theinfrared wavelengths that are present, is desirable
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EDFA: Direct Optical Amplification
Erbium-doped fiber amplifier (EDFA) is an Infra-Red LASER(Light Amplification by Stimulated Emission of Radiation)
which converts shorter wavelength IR source (“pump”)power into greater power at signal wavelength(s).
Advantages:
◦ Simpler, uses less components than electro-optics,particularly for multiple wavelengths on same fiber
(WDM)◦ Amplifies many different optical wavelength signals
present in WDM (present and future as well)
◦ Compensates for optical losses due to filters and opticalcombiners used in WDM
But… optical amplification does not correct timing or waveshape (two of the “3 Rs”)
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Why Erbium?
Erbium atoms have three important energy levels. The top twolevels differ by an energy difference E= E3-E2 corresponding to~1300 nm wavelength, the desired wavelength of the amplifiedsignal. The lowest of the three energy levels differs from the topenergy level (E3-E1 ) by an amount corresponding to the “pump”signal wavelength.
A section of glass fiber made with Erbium doping is spliced intothe signal-carrying fiber. This Erbium section is continuouslyilluminated with a “pump” optical signal of wavelength
corresponding to E3-E1
Radiative energy level transitions occur from level E3 down to E2in proportion to the incoming (signal) light power level. Theoutgoing light power level is stronger.
◦ Electrons eventually fall in energy from level E2 backto E1 as well, but produce light of a different
wavelength than the 1300 nm wavelength used foroptical signals. These other wavelengths areeventually absorbed by “colored” filters that onlysubstantially pass 1300 nm infra red light.
E1
E2
E3
Pump
Action
(sche-
Matic)
Radiation
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E1(ground state)
E2
E3 (Excited State)
Pump
Action(schematic)
Radiation
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Optical amplifier
amplifier is needed to boost amplitude of signal after long
transmission
three types of optical amplifier
semiconductor amplifier
erbium-doped amplifier
raman amplifier
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Semiconductor amplifier
it is essentially a semiconductor laser without the oscillation
SOA has one major disadvantages which is too much noise and
gain of polarization dependent
SOA is usually used in application using 1300nm wavelength
range
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Raman amplifier
use raman scattering phenomena
raman scattering is the scattering of a photon by and optical
phonon which is the vibration of a crystal or molecule
the phonon will cause the downshift in frequency called Stoke
shift
usually operating in S-band (1460 -1530nm)
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Erbium-doped amplifier
two types
fiber amplifier (EDFA)
waveguide amplifier (EDWA)
different between these two is just the medium hosting
the Erbium ions.
EDFA uses silica fiber while EDWA uses silica waveguide
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EDFA
uses laser diode as source of energy (called pump). twowavelength : 980nm & 1480nm
pump energy is absorbs by Er ions and input signal (information
carrying signal) stimulated the energy release
Er will release energy (photon) which is exactly similar in
wavelength and phase of the photon that stimulates to release
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Noise figure
measure of noise characteristic of EDFA
it is the ratio of input SNR to output SNR
it give an indication of signal degradation due to amplification
since noise is amplified with the signal
the lesser the value the better
NF is measured in unit dB
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EDF laser
if the EDFA is closed-loop, it can become a laser, EDFL
two setups
ring laser – where the oscillation is in circular motion
linear laser – where the oscillation is linear
similar principle as other lasers