Optical Amplifiers and its Future Uses

By :- Paul Sourya ChatterjeeECE – 1034

6th Sem.Academy Of Technology

INTRODUCTION

An optical amplifier is

a device

that amplifies an

optical

signal

directly, without the need to first convert

it to an

electrica

l signal. An

optical

amplifier may be

thought

of as a

laser

without an

optical

cavity, or one in

which

feedback from

the cavity is suppressed. Stimulated

emission

in the amplifier

's gain medium caus

es amplification of

incomin

g light

. The

y are important in optical

communication

and lase

r physics.

The optical fiber amplifier was invented by H. J. Shaw and Michel Digonnet at Stanford University, California (1980s).

OPTICAL COMMUNICATIONA typical communications system includes a transmitter, an optical fiber, a receiver, multiplexers and demultiplexers, amplifiers, switches and other components. The transmitter incorporates information to be communicated into an optical signal and transmits the optical signal via the optical fiber to the receiver. The receiver recovers the original information from the received optical signal.

Require no conversion to electrical signals

.

Can amplify

many

optical signals at a time.

Insensitive to

various bit

rates

No matter what type of modulation is used.

Can be used for multiple

wavelengths of optical

signal.

OPTICAL AMPLIFIERS VS. REGENERATORS

AMPLIFIER COMPARISONS

SEMICONDUCTOR OPTICAL AMPLIFIER

p

Single pass chip gain: G increases exponentially with length

GAIN ~ WAVELENGTH CURVESingle SOA

• InGaAs/InGaAsP.• Spanning from 1250-1650 nm

Output Power (db)

SOAs are linear for small input powers.

Gain Dynamics

ERBIUM DOPED FIBER AMPLIFIER

1st demonstrated by a group including David N. Payne, R. Mears, & L. Reekie, from the University of Southampton & a group from AT&T Bell Laboratories, E. Desurvire, P. Becker, & J. Simpson.

All optical and fiber compatible

Wide bandwidth 20 ~ 70 nm High gain, 20 ~ 40 dB High output power,

> 200mW Bit rate, modulation format,

power and wavelength insensitive

Low distortion and low noise (NF < 5dB)

EDFA Circuit Board

• Approximately 1525 nm - 1565 nm

Conventional, or C-band

• Approximately 1570 nm to 1610 nm

Long, or L-band

980 nm band has a higher absorption

cross-section and is generally used

where low-noise performance is

required.

1480 nm band has a lower, but broader,

absorption cross-

section and is generally

used for higher power

amplifiers.

Distributed Raman Amp.

• A Distributed Raman amplifier is one in which the transmission fibre is utilised as the gain medium by multiplexing a pump wavelength with signal wavelength

Lumped Raman Amp.• A Lumped Raman

amplifier utilises a dedicated, shorter length of fibre to provide amplification. Lumped Amplifier is highly nonlinear fibre with a small core, utilised to increase the interaction between signal and pump wavelengths and thereby reduce the length of fibre required.

2 Types of Raman Amplifier

However, Distributed Amplifier requires 500 mW power and Lumped Amplifier require upto 1W.

NOISE IN OPTICAL AMPLIFIERS

Principle source is Amplified Spontaneous Emission (ASE).

Noise figure in an ideal DFA is 3 dB, while practical amplifiers can have noise figure as large as 6-8 dB.

A proportion of spontaneously emitted photons will be emitted in a direction that falls within the Numerical Aperture of the fibre and are thus captured and guided by the fibre. Those photons captured may then interact with other dopant ions, and are thus amplified by stimulated emission.

ce

Must be measured electrically.

Noise figure

Nois

e F

igu

re

(dB

)

• There is a 3 dB NF limit for complete inversion & high gain.

Gain is achieved in a fibre amplifier due to population inversion of the dopant ions.

As the signal power increases, or the pump power decreases, the inversion level will reduce and thereby the gain of the amplifier will be reduced. This effect is known as Gain Saturation.

The amplifier saturates and cannot produce any more output power, and therefore the gain reduces.

Saturation is also commonly known as Gain Compression.

Gain Saturation

Output saturation power is defined as the output power when gain drops by 3db

Power amplifiers usually operate at saturation.

Amplifier Gain vs. Power

Future Uses

Power (Booster) Amplifier[Saturation

Power]

In-Line Amplifier[Saturation

Power, Noise Figure]

Pre- Amplifier[Gain, Noise

Figure]

THEIR APPLICATIONS

References• www.google.com

• Wikipedia Encyclopedia

• Britannica Encyclopedia

• JSTOR Philosophical Transactions of the Royal Society of London Series A, Mathematical and Physical Sciences, Vol_ 329, No_ 1603 (Sep_ 28, 1989)

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