Download - Optical Signal Amplification
Optical Amplifiers
Two types
¤ Opto-electronic conversion
¤ All Optical
ALL OPTICAL AMPLIFIERS
☼ Boosting of an optical signal without conversion of an optical signal into an electrical signal.
Why we go for such an amplifiers?
۩ Cheap
۩ Few Repeaters
۩ Noise Reduction
۩ No electronic restriction on Bandwidth
CHARACTERISTICS OF AN OPTICAL AMPLIFIER
♪ Gain
♪ Gain Efficiency
♪ Gain Bandwidth
♪ Gain Saturation
♪ Noise
TYPES OF OPTICAL AMPLIFIERS
¤ Semiconductor Optical Amplifiers
¤ Fiber Amplifiers¤ Erbium Doped Fiber Amplifier
¤ Raman Fiber Amplifier
MODES OF APPLICATION¤ Power Amplifier
¤ In Line Amplifier
¤ Preamplifier
¤ Functions of the Amplifier
ReceiverReceiver
Power Amplifier
Inline Amplifier
Transmitter A1
A2
ReceiverA3Pre
Amplifier
SEMICONDUCTOR OPTICAL AMPLIFIERS(SOA)
Laser diodes with or without end mirrors which have fiber attached to both the ends.
Two types Fabry perot SOA Travelling wave SOA
♫ They work for the optical windows both 1310 and 1550nm.
♫ Transmit bidirectionally.
SOA - OPERATION PRINCIPLE
ASE Photons
Ground state
Excited state
Metastable stateTransition
Transition
Pump signal
Excited state
Metastable stateTransition
Pump signal Stimulated emission1550 nm
Signal photon 1550 nm
Ground state
Merits♫Cheap solution♫Can be easily integrated with other devices
like MUX/DEMUX ( AWG’s )♫Good for use in Metro WDM
Limitations♪ High coupling loss♪ Polarization dependent♪ High Noise figure♪ Matching with the fiber is also a problem
Erbium Doped Fiber Amplifier
Why Erbium?
• Erbium ions (Er3+) have quantum levels that allows them to be stimulated to emit in the 1540nm band.
• Erbium's quantum levels also allow it to be excited by a signal at either 980nm or 1480nm.
Energy Level Diagram of EDFAs
• Thanks to: Optical Communications Laboratory
Gain
Output Gain Photons plus Signal Photon
GainSpontaneous Emission Noise
(1.53 << 1.56 m)
4Incoming
Signal Photon
3
2
1
Excited State
Metastable State
Ground State
0.98
-m
pu
mp
1.48
-m
pu
mp
Fast Decay
0
000
00
Excited
State
Ground State
1.48
-μm
pum
p
Fast Decay
0.98
-μm
pum
p
Metastable State
λ1
λ2
λ3
λ4
Incoming signal photon
λ0
Output gain photons
plus signal photon
λ0λ0λ0λ0λ0
Spontaneous Emission Noise
(1.53<λ<1.56μm)
Gain
Operation Details• Erbium atoms emit photons at same wavelength and in
same phase and direction as incoming photons Cascading photons effectively amplify incoming signal Signal amplified in direction of travel only Similar to laser action
• Isolator put at output to prevent reflections from returning to amplifier and disrupting operation
Thanks to: Applied Optoelectronic center
Input
1480 or 980 nm Pump Laser Erbium Doped Fiber
Output
IsolatorCoupler
• Advantages~ High gain per mW of pump power~ Low crosstalk~ Happen to operate in most transparent region of the spectrum for
glass fiber~ Extremely long excited state lifetime (of the order of 10 ms)
Limitations~ Can only work at wavelengths where Er+3 fluoresces~ Requires specially doped fiber as gain medium~ Three-level system, so gain medium is opaque at signal
wavelengths until pumped~ Requires long path length of gain medium (tens of
meters in glass)~ Gain very wavelength-dependent and must be flattened
Raman Fiber Amplifier
Raman Amplification
Ŕ Stimulated Raman scattering occurs when light waves interact with vibrations of atoms in a crystalline lattice ( optical fiber ). The atom absorbs the light and re-emits new photons with an energy which is lower than the original energy ( with a wavelength which is about 100nm longer than the original WL at 1550nm ).
Ŕ Raman amplification is possible for the S-band and even for the 2nd optical window ( pump WL about 13 THz higher frequency ).
Ŕ Raman amplification excellent for use in new ultra long haul DWDM systems: High channel count ( more than 80 ) High modulation speed ( 40 Gbit/s ) Longer distances between regeneration
Types
Ŕ 2 types of Raman amplifiers:Ŕ Discrete Raman amplifiers: Signal is not amplified in the
transmission fiber, but in a special fiber within a box with other components, like EDFA !
Ŕ Distributed Raman amplifiers: The transmission medium ( fiber ) is used to achieve gain.
Ŕ Distributed Raman amplifiers benefits:Ŕ Reduces the overall Noise Figure ( NF ) longer links without
regeneration & higher modulation rates become possible.Ŕ Flat gain can be achieved with the use of more than one pump
laser with different wavelengths ( Also possible with Discrete Raman amplifiers ).
Discrete Amplifier
Distributed Raman Amplification (I) Raman pumping takes place backwards over the fiber.
Gain is a maximum close to the receiver and decreases in the transmitter direction
Source: Master 7_5
TransmitterOptical
ReceiverEDFA
Raman Pump Laser
Long Fibre Span
Distributed Raman Amplifier
Thanks to: Applied Optoelectronic center
Advantages¤ Variable wavelength amplification possible¤ Compatible with installed SM fiber¤ Can be used to "extend" EDFAs¤ Can result in a lower average power over a
span, good for lower crosstalk¤ Very broadband operation may be possible
Limitations☼High pump power requirements, high pump
power lasers have only recently arrived☼Sophisticated gain control needed☼Noise is also an issue
Comparison:Property SOA EDFA Raman
Amplification Band depends on pump power
depends on dopant (Er, Y, Th)
depends on pump power
Gain BW 60nm ~90nm(extended range)
20-50nm per pump
Flat gain 15-20nm
NOISE FIGURE 8 5 5
Noise ASE ASE Raman scatter, double Rayleigh
Pump wavelength Electrical pump 980/1.480nm for erbium
by 100nm shorter than amplified signal
range
Pump power <400mA ~10-300mW < 300mW
Saturation power depends on Bias current
Depends on dopant and gain
~power of pump
Direction Unidirectional Unidirectional Bidirectional
Simplicity Simpler more complex
(EDFA needed)
Simpler (no special fiber needed)
Cost Low Medium high
References:• DWDM Pocket Guide, Ines Brunn, Acterna Eningen GmbH,Postfach 12
62, 72795 Eningen u. A., Germany.• Semiconductor Optical Amplifiers, Michael J. Connelly, Kluwer Academic
Publishers, New York.• Erbium-Doped Fiber Amplifiers Fundamentals and Technology, P.C.
Becker, N.A. Olsson and J.R. Simpson, Elsevier Academic Press, San Diego.
• Raman Amplification in Fiber Optical Communications System, Clifford Headley and Govind P. Agrawal, Elsevier Academic Press, Amsterdam.
• Electro-Optics Handbook, Ronald W. Waynant, Marwood N. Ediger, Second Edition, McGraw Hill, Inc., New York
Thank you