360 CHAPTER 8. MULTICHANNEL SYSTEMS
to be changed is injected into a tunable laser directly. The change in the laser threshold
resulting from injection translates into modulation of the laser output, mimicking the
bit pattern of the injected signal. Such a scheme requires relatively large input powers.
Another scheme uses the low-power input signal to produce a frequency shift (typically,
10 GHz/mW) in the laser output for each 1 bit. The resulting frequency-modulated CW
signal can be converted into amplitude modulation by using a MZ interferometer. An-
other scheme uses FWM inside the cavity of a tunable semiconductor laser, which also
plays the role of the pump laser. A phase-shifted DFB laser provided wavelength con-
version over a range of 30 nm with this technique [120]. A sampled grating within a
distributed Bragg reflector has also been used for this purpose [123].
Another class of wavelength converters uses an optical fiber as the nonlinear medium.
Both XPM and FWM can be employed for this purpose using the last two configura-
tions shown in Fig. 8.20. In the FWM case, stimulated Raman scattering (SRS) af-
fects the FWM if the frequency difference|ν 1 −ν 2 |falls within the Raman-gain band-
width [124]. In the XPM case, the use of a Sagnac interferometer, also known as the
nonlinear optical loop mirror [40], provides a wavelength converter capable of oper-
ating at bit rates up to 40 Gb/s for both the return-to-zero (RZ) and nonreturn-to-zero
(NRZ) formats [126]. Such a device reflects all 0 bits but 1 bits are transmitted through
the fiber loop because of the XPM-induced phase shift. In a 2001 experiment, wave-
length conversion at the bit rate of 80 Gb/s was realized by using a 1-km-long optical
fiber designed to have a large value of the nonlinear parameterγ[129]. A periodically
poled LiNbO 3 waveguide has provided wavelength conversion at 160 Gb/s [128]. In
principle, wavelength converters based on optical fibers can operate at bit rates as high
as 1 Tb/s because of the fast nature of their nonlinear response.
8.2.8 WDM Transmitters and Receivers
Most WDM systems use a large number of DFB lasers whose frequencies are chosen
to match the ITU frequency grid precisely. This approach becomes impractical when
the number of channels becomes large. Two solutions are possible. In one approach,
single-frequency lasers with a tuning range of 10 nm or more are employed (see Sec-
tion 3.4.3). The use of such lasers reduces the inventory and maintenance problems.
Alternatively, multiwavelength transmitters which generate light at 8 or more fixed
wavelengths simultaneously can be used. Although such WDM transmitters attracted
some attention in the 1980s, it was only during the 1990s that monolithically inte-
grated WDM transmitters, operating near 1.55μm with a channel spacing of 1 nm
or less, were developed using the InP-based optoelectronic integrated-circuit (OEIC)
technology [131]–[139].
Several different techniques have been pursued for designing WDM transmitters. In
one approach, the output of several DFB or DBR semiconductor lasers, independently
tunable through Bragg gratings, is combined by using passive waveguides [131]–[134].
A built-in amplifier boosts the power of the multiplexed signal to increase the trans-
mitted power. In a 1993 device, the WDM transmitter not only integrated 16 DBR
lasers with 0.8-nm wavelength spacing, but an electroabsorption modulator was also
integrated with each laser [132]. In a 1996 device, 16 gain-coupled DFB lasers were
integrated, and their wavelengths were controlled by changing the width of the ridge