Page 203 - Optical Communications Essentials

Optical Amplifiers

Optical Amplifiers 193

CW signal ( c )
SOA1
Input signal ( s )

SOA2
Converted signal ( c )
(a)

CW signal ( c )
SOA1
Input signal ( s )

SOA2
Converted signal ( c )
(b)
Figure 11.15. (a) Mach-Zehnder interferometer and (b) Michelson
interferometer setups using a pair of SOAs for implementing the
cross-phase modulation wavelength conversion scheme.

inside the SOA. This affects the phase of the probe and creates significant pulse
distortion.

11.6.2. Wave-mixing wavelength converters
Wavelength conversion based on nonlinear optical wave mixing offers impor-
tant advantages compared to other methods. This includes a multiwavelength
conversion capability and transparency to the modulation format. The mixing
results from nonlinear interactions among optical waves traversing a nonlinear
material. The outcome is the generation of another wave whose intensity is
proportional to the product of the intensities of the interacting waves. The
phase and frequency of the generated wave are a linear combination of those of
the interacting waves. Therefore the wave mixing preserves both amplitude and
phase information, and consequently is the only wavelength conversion cate-
gory that offers strict transparency to the modulation format.
Two successful schemes are four-wave mixing (FWM) in either passive wave-
guides or SOAs and difference-frequency generation in waveguides. For wave-
length conversion, the four-wave mixing scheme employs the mixing of three
distinct input waves to generate a fourth distinct output wave. In this method,
an intensity pattern resulting from two input waves interacting in a nonlinear
material forms a grating. For example, in SOAs there are three physical mech-
anisms that can form a grating: carrier-density modulation, dynamic carrier
heating, and spectral hole burning. The third input wave in the material gets
scattered by this grating, thereby generating an output wave. The frequency of
the generated output wave is offset from that of the third wave by the frequency
difference between the first two waves. If one of the three incident waves con-
tains amplitude, phase, or frequency information and the other two waves are
constant, then the generated wave will contain the same information.

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.