8.2. WDM COMPONENTS 357
its low-cost potential (bubble-jet technology is used routinely for printers) but requires
a careful design for reducing the crosstalk and insertion losses.
Electroholographic switches are similar to 2-D MEMS but employ a LiNbO 3 crys-
tal for switching in place of a rotating mirror. Incident light can be switched at any
point within the 2-D array of such crystals by applying an electric field and creating a
Bragg grating at that location. Because of the wavelength selectivity of the Bragg grat-
ing, only a single wavelength can be switched by one device. This feature increases
the complexity of such switching fabrics. Other issues are related to the polarization
sensitivity of LiNbO 3 -based devices.
Optical fibers themselves can be used for making OXCs if they are combined with
fiber gratings and optical circulators [116]. The main drawback of any OXC is the
large number of components and interconnections required that grows exponentially as
the number of nodes and the number of wavelengths increase. Alternatively, the sig-
nal wavelength itself can be used for switching by making use of wavelength-division
switches. Such a scheme makes use of static wavelength routers such as a WGR in
combination with a new WDM component—thewavelength converter. We turn to this
component next.
8.2.7 Wavelength Converters.....................
A wavelength converter changes the input wavelength to a new wavelength without
modifying the data content of the signal. Many schemes were developed during the
1990s for making wavelength converters [119]–[129]; four among them are shown
schematically in Fig. 8.20.
A conceptually simple scheme uses an optoelectronic regenerator shown in Fig.
8.20(a). An optical receiver first converts the incident signal at the input wavelengthλ 1
to an electrical bit pattern, which is then used by a transmitter to generate the optical
signal at the desired wavelengthλ 2. Such a scheme is relatively easy to implement as
it uses standard components. Its other advantages include an insensitivity to input po-
larization and the possibility of net amplification. Among its disadvantages are limited
transparency to bit rate and data format, speed limited by electronics, and a relatively
high cost, all of which stem from the optoelectronic nature of wavelength conversion.
Several all-optical techniques for wavelength conversion make use of SOAs [119]–
[122], amplifiers discussed in Section 6.2. The simplest scheme shown in Fig. 8.20(b)
is based on cross-gain saturation occurring when a weak field is amplified inside the
SOA together with a strong field, and the amplification of the weak field is affected
by the strong field. To use this phenomenon, the pulsed signal whose wavelengthλ 1
needs to be converted is launched into the SOA together with a low-power CW beam
at the wavelengthλ 2 at which the converted signal is desired. Amplifier gain is mostly
saturated by theλ 1 beam. As a result, the CW beam is amplified by a large amount
during 0 bits (no saturation) but by a much smaller amount during 1 bits. Clearly, the
bit pattern of the incident signal will be transferred to the new wavelength with reverse
polarity such that 1 and 0 bits are interchanged. This technique has been used in many
experiments and can work at bit rates as high as 40 Gb/s. It can provide net gain to the
wavelength-converted signal and can be made nearly polarization insensitive. Its main
disadvantages are (i) relatively low on–off contrast, (ii) degradation due to spontaneous