Ch3 06 Optical Amplifiers,Regenerators.pptee.sharif.edu/~opticsicsys-AliF/Ch3_06.pdf · Optical Amplifiers vs Regenerators (1 of 2) • Transparent: Regenerators specific to bit rate

Chapter 3: Optical Devices

Amplifiers, Regenerators

Optical Communications: Circuits, Systems and Devices

Sep 2012 Sharif University of Technology 1

lecturer: Dr. Ali Fotowat Ahmady

Optical Amplifiers vs Regenerators (1 of 2)

• Transparent: Regenerators specific to bit rate and modulation format used; O-Amps are insensitive• Easily upgraded: A system with optical amplifiers can be more easily upgraded to higher bit rate without replacing the amplifiers• Optical amplifiers have large gain bandwidths → key enabler of DWDM

Chapter 3 Optical Devices Amplifiers, Regenerators


• Issues:- Amplifiers introduce additional noise that accumulates- Spectral shape of gain (flatness), output power, transient behavior need to be carefully designed

Optical Amplifiers vs Regenerators (2 of 2)



OEO Regenerator



The model of a generator and the three major functions, optical receiver,electronic amplifier, and optical transmitter

1R, 2R and 3R Regeneration



Optical Amplifier Varieties (1 of 2)

• Semiconductor Optical Amplifiers (SOA)- conventional SOA- GC-SOA (Gain-Clamped SOA)- LOA (Linear Optical Amplifier)

• Fiber Optical amplifiers (FOA)



• Fiber Optical amplifiers (FOA)- Rare earth-Doped Fiber Amplifiers

◦ Erbium-Doped Fiber Amplifiers (EDFA) : C,L-Band◦ Thulium-Doped Fiber Amplifiers (TDFA) : S-Band◦ Praseodymium-Doped Fiber Amplifiers (PDFA):O-Band

- Not based on stimulated emission but on nonlinear effects

Optical Amplifier Varieties (2 of 2)

- Fiber Raman Amplifiers◦ Discrete Raman Amplifiers◦ Distributed Raman Amplifiers (DRA)

- Hybrid EDFA/Raman Amplifier



Optical Amplification: mechanics



For sustained amplification, the rate of excitation should be equal to the rate of stimulation + the rate of spontaneous emission

Erbium-Doped Fiber Amplifier (EDFA)

• Length of fiber: core doped with (rare earth) erbium ions Er3+

• Fiber is pumped with a laser at 980 nm or 1480nm. • Pump is coupled (in- and out-) using a λ-selective coupler• An isolator is placed at the end to avoid reflections (else this may will convert into a laser!)



An EDFA amplifier consists of an erbium-doped silica fiber, an optical pump, a couplerand isolators at both ends

EDFA success factors

• Availability of compact and reliable high-power semiconductor pump lasers• EDFA is an all-fiber device → polarization-independent & easy to couple light in/out• Simplicity of device• No crosstalk introduced while amplifying!



EDFA Operation (1 of 2)

• When Er3+ ions introduced in silica, electrons disperse into an energy band around the lines E1, E2, E3 (Stark splitting)• Within each band, the ion distribution is non-uniform (thermalization)• Due to these effects, a large λ range (50 nm) can be simultaneously amplified & luckily it is in the 1530nm range• 980 nm or 1480nm pumps are used to create a population inversion



• 980 nm or 1480nm pumps are used to create a population inversion between E2 and E1 • 980 nm pump => E1 → E3 (absorption) & E3 → E2 (spontaneous emission) • 1480 nm pump => E1 → E2 (absorption, less efficient)• Lifetime in E3 is 1µs, whereas in E2 it is 10ms

EDFA Operation (2 of 2)



Energy Level States of Erbium: While the energy states are represented as horizontal lines,they are really “energy bands” centered around a specific energy state. This distribution ofenergy is called a “Fermi-Dirac Distribution”.

Semiconductor Optical Amplifiers (SOA)

• SOAs have severe crosstalk problems, besides others but used in switches etc• Semiconductor optical amplifiers are devices based on conventional laser principles.



Raman Amplifiers

• Power transferred from lower-λ to higher-λ channels (about 100nm)• Eg: 1460-1480nm pump → amplification at 1550-1600nm• Gain can be provided at ANY wavelength (all you need is an appropriate pump λ!)• Multiple pumps can be used• Lumped or distributed designs possible



• Lumped or distributed designs possible• Used today to complement EDFAs in ultra-long-haul systems

Principles of Raman amplification



Qualitative comparison between Raman and OFAs (1 of 2)



Qualitative comparison between Raman and OFAs (2 of 2)


Parameter EDFA RAMAN SOAGain ~30 dB ~20–25 dB ~10–20 dB

Output Power High High LowInput Power Moderate High High


Input Power Moderate High HighCross Talk Low Low Very highApplication Metro, long haul Long haul,

ultra long haulShort haul,

single channel

Long-Haul All-optical Amplification



All optical amplification (EDFA + Raman) and dispersion compensation modules (DCM)enable the optical signal to reach ultra long distances (~4,000 km) between end terminals.

Questions



Documents

Ch3 06 Optical Amplifiers,Regenerators.pptee.sharif.edu/~opticsicsys-AliF/Ch3_06.pdf · Optical Amplifiers vs Regenerators (1 of 2) • Transparent: Regenerators specific to bit rate