494
12 Nonlinear Processes in Optical Fibers
signal beam modulates the gain of the SOA by depleting the carriers, which produces
a modulation of the refractive index. When the CW beam encounters the modulated
gain and refractive index, its amplitude and phase are changed, so that it now carries
the same information as the input signal. As shown in Fig. 12.11, the SOAs are
placed in an asymmetric configuration so that the phase change in the two amplifiers is
different. Consequently, the CW light is modulated according to the phase difference.
A typical splitting ratio is 69/31 percent. These types of converters readily handle
data rates of at least 10 Gb/s.
A limitation of the XPM architecture is that it only converts one wavelength at
a time. In addition, it has limited transparency in terms of the data format. Any
information that is in the form of phase, frequency, or analog amplitude is lost
during the wavelength conversion process. Consequently, this scheme is restricted
to converting digital signal streams.
12.9.2 Wavelength Converters Based on Wave-Mixing
Wavelength conversion based on nonlinear optical wave mixing offers important
advantages compared to other conversion methods. The advantages include a multiwavelength conversion capability and transparency to the modulation format. The
mixing arises 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 these parameters of
the interacting waves. Therefore, the wave mixing preserves both amplitude and
phase information, and consequently is the only wavelength-conversion category
that offers strict transparency to the modulation format.
Two wavelength-conversion schemes are four-wave mixing in either a passive
waveguide or SOA, and difference-frequency generation in waveguides. For wavelength conversion, the FWM 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 mechanisms that can form a grating. These
are 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 contains amplitude, phase, or frequency information and the other
two waves are constant, then the generated wave will contain the same information.
Difference-frequency generation in waveguides is based on the mixing of two
input waves. In this case, the nonlinear interaction of the material is with a pump and
a signal wave.
12 Nonlinear Processes in Optical Fibers
signal beam modulates the gain of the SOA by depleting the carriers, which produces
a modulation of the refractive index. When the CW beam encounters the modulated
gain and refractive index, its amplitude and phase are changed, so that it now carries
the same information as the input signal. As shown in Fig. 12.11, the SOAs are
placed in an asymmetric configuration so that the phase change in the two amplifiers is
different. Consequently, the CW light is modulated according to the phase difference.
A typical splitting ratio is 69/31 percent. These types of converters readily handle
data rates of at least 10 Gb/s.
A limitation of the XPM architecture is that it only converts one wavelength at
a time. In addition, it has limited transparency in terms of the data format. Any
information that is in the form of phase, frequency, or analog amplitude is lost
during the wavelength conversion process. Consequently, this scheme is restricted
to converting digital signal streams.
12.9.2 Wavelength Converters Based on Wave-Mixing
Wavelength conversion based on nonlinear optical wave mixing offers important
advantages compared to other conversion methods. The advantages include a multiwavelength conversion capability and transparency to the modulation format. The
mixing arises 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 these parameters of
the interacting waves. Therefore, the wave mixing preserves both amplitude and
phase information, and consequently is the only wavelength-conversion category
that offers strict transparency to the modulation format.
Two wavelength-conversion schemes are four-wave mixing in either a passive
waveguide or SOA, and difference-frequency generation in waveguides. For wavelength conversion, the FWM 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 mechanisms that can form a grating. These
are 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 contains amplitude, phase, or frequency information and the other
two waves are constant, then the generated wave will contain the same information.
Difference-frequency generation in waveguides is based on the mixing of two
input waves. In this case, the nonlinear interaction of the material is with a pump and
a signal wave.