Optical Amplifier

BY:AMIT SINGHGURCHARANJEET SINGH

INTRODUCTION

• What is Optical Amplifiers?

• Why Optical Amplifiers are used instead of Repeaters?

• Types of Optical Amplifiers?

Applications of optical amplifiers

Comparison of Real and Ideal Amplifier

SEMICONDUCTOR OPTICAL AMPLIFIERS(SOA)

Laser diodes with or without end mirrors which have fiber attached to both the ends.

• They work for the optical windows both 1310 and 1550nm.

• Transmit bidirectionally.

Four-level lasersSOA - OPERATION PRINCIPLE

Types of SOAFabry-Perot Amplifier

High gain but non-uniform gain spectrum

Traveling wave amplifierBroadband but very low facet reflectivities are needed

Gain as a function of frequencyRipples are caused by the cavity modesThe overall gain curve is due to the width of the atomic transition in the semi-conductorFundamentals fo Multiaccess Optical Fiber Networks

Dennis J. G. Mestgagh

Semiconductor amplifier advantages

• Used for all logic gate• Can easily be integrated as preamplifiers at the

receiver end• Use same technology as diode lasers• Gain relatively independent of wavelength• Are pumped with current, not another laser

Semiconductor amplifier disadvantages

• Polarization dependence• Self-phase modulation leading to chirp• Cross-phase modulation • Four-wave mixing and crosstalk• Extremely short (ns) excited state lifetimes

All-Optical Logic Gates Based on an SOA

Wave length based on SOA

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.

• 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 AmplifierRaman 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

Discrete Amplifier

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

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 possibleLimitations• 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

THANKS