362 CHAPTER 8. MULTICHANNEL SYSTEMS
using individual modulators.
A unique approach to WDM sources exploits the technique of spectral slicing
for realizing WDM transmitters and is capable of providing more than 1000 chan-
nels [141]–[145]. The output of a coherent, wide-bandwidth source is sliced spec-
trally using a mutipeak optical filter such as an AWG. In one implementation of this
idea [141], picosecond pulses from a mode-locked fiber laser are first broadened spec-
trally to bandwidth as large as 200 nm through supercontinuum generation by exploit-
ing the nonlinear effects in an optical fiber [59]. Spectral slicing of the output by an
AWG then produces many WDM channels with a channel spacing of 1 nm or less.
In a 2000 experiment, this technique produced 1000 channels with 12.5-GHz chan-
nel spacing [143]. In another experiment, 150 channels with 25-GHz channel spacing
were realized within the C band covering the range 1530–1560 nm [145]. The SNR of
each channel exceeded 28 dB, indicating that the source was suitable for dense WDM
applications.
The generation of supercontinuum is not necessary if a mode-locked laser produc-
ing femtosecond pulses is employed. The spectral width of such pulses is quite large
to begin with and can be enlarged to 50 nm or more by chirping them using 10–15 km
of standard telecommunication fiber. Spectral slicing of the output by a demultiplexer
can again provide many channels, each of which can be modulated independently. This
technique also permits simultaneous modulation of all channels using a single modula-
tor before the demultiplexer if the modulator is driven by a suitable electrical bit stream
composed through TDM. A 32-channel WDM source was demonstrated in 1996 by us-
ing this method [142]. Since then, this technique has been used to provide sources with
more than 1000 channels [144].
On the receiver end, multichannel WDM receivers have been developed because
their use can simplify the system design and reduce the overall cost [146]. Monolithic
receivers integrate a photodiode array with a demultiplexer on the same chip. Typically,
A planar concave-grating demultiplexer or a WGR is integrated with the photodiode
array. Even electronic amplifiers can be integrated within the same chip. The design
of such monolithic receivers is similar to the transmitter shown in Fig. 8.21 except that
no cavity is formed and the amplifier array is replaced with a photodiode array. Such
a WDM receiver was first fabricated in 1995 by integrating an eight-channel WGR
(with 0.8-nm channel spacing), eightp–i–nphotodiodes, and eight preamplifiers using
heterojunction-bipolar transistor technology [147].
8.3 System Performance Issues
The most important issue in the design of WDM lightwave systems is theinterchannel
crosstalk. The system performance degrades whenever crosstalk leads to transfer of
power from one channel to another. Such a transfer can occur because of the nonlinear
effects in optical fibers, a phenomenon referred to asnonlinear crosstalkas it depends
on the nonlinear nature of the communication channel. However, some crosstalk occurs
even in a perfectly linear channel because of the imperfect nature of various WDM
components such as optical filters, demultiplexers, and switches. In this section we