390 CHAPTER 8. MULTICHANNEL SYSTEMS
chip pattern matches precisely. An array of fiber Bragg gratings, designed with identi-
cal stop bands but different reflectivities, can also act as encoders and decoders [261].
Different gratings introduce different delays depending on their relative locations and
produce a coded version of the signal. Such grating-based devices provide encoders
and decoders in the form of a compact all-fiber device (except for the optical circulator
needed to put the reflected coded signal back onto the transmission line).
The CDM pulse trains consisting of 0 and 1 chips suffer from two problems. First,
only unipolar codes can be used simply because optical intensity or power cannot
be negative. The number of such codes in a family of orthogonal codes is often not
very large until the code length is increased to beyond 100 chips. Second, the cross-
correlation function of the unipolar codes is relatively high, making the probability of
an error also large. Both of these problems can be solved if the optical phase is used
for coding in place of the amplitude. Such schemes are being pursued and are called
coherent CDMA techniques [264]. An advantage of coherent CDM is that many fam-
ilies of bipolar orthogonal codes, developed for wireless systems and consisting of 1
and−1 chips, can be employed in the optical domain. When a CW laser source is used
in combination with a phase modulator, another CW laser (called local oscillator) is
required at the receiver for coherent detection (see Chapter 10). On the other hand, if
ultrashort optical pulses are used as individual chips whose phase is shifted byπin
chip slots corresponding to a−1 in the code, it is possible to decode the signal without
using coherent detection techniques.
In a 2001 experiment, a coherent CDMA system was able to recover the 2.5 Gb/s
signal transmitted using a 64-chip code [268]. A sampled (or superstructured) fiber
grating was used for coding and decoding the data. Such a grating consists of an ar-
ray of equally spaced smaller gratings so that a single pulse is split into multiple chips
during reflection. Moreover, the phase of preselected chips can be changed byπso
that each reflected pulse is converted into a phase-encoded train of chips. The decoder
consists of a matched grating such that the reflected signal is converted into a single
pulse through autocorrelation (constructive interference) for the signal bit while the
cross-correlation or destructive interference produces no signal for signals belonging
to other channels. The experiment used a NOLM (the same device used for demulti-
plexing of OTDM channels in Section 8.4) for improving the system performance. The
NOLM passed the high-intensity autocorrelation peak but blocked the low-intensity
cross-correlation peaks. The receiver was able to decode the 2.5-Gb/s bit stream from
the 160-Gchip/s pulse train with less than 3-dB penalty at a BER of less than 10−^9.
The use of time-gating detection helps to improve the performance in the presence of
dispersive and crosstalk effects [269].
8.6.2 Spectral Encoding
Spectrum spreading can also be accomplished using the technique of frequency hop-
ping in which the carrier frequency is shifted periodically according to a preassigned
code [256]. The situation differs from WDM in the sense that a fixed frequency is
not assigned to a given channel. Rather, all channels share the entire bandwidth by
using different carrier frequencies at different times according to a code. A spectrally
encoded signal can be represented in the form of a matrix shown schematically in