392 CHAPTER 8. MULTICHANNEL SYSTEMS
Figure 8.34: Receiver for a hybrid WDM–CDMA system sharing the same spectral bandwidth.
A notch filter is used within the decoder to remove the WDM signal. (After Ref. [267];©c 2001
IEEE; reprinted with permission.)
encoding and decoding. A single chirped Moir ́e grating can replace the grating array
because several gratings are written at the same location in such fiber gratings [46].
In a 2000 experiment, several Moir ́e gratings were used to demonstrate recovery of
622-Mb/s CDM channels [265].
In another approach calledcoherence multiplexing[258], a broadband optical source
is used in combination with an unbalanced MZ interferometer that introduces a delay
longer than thecoherence timein one of its branches. Such CDM systems rely on co-
herence to discriminate among channels and are affected severely by the optical beat
noise. In a demonstration of this technique, four 1-Gb/s channels were multiplexed.
The optical source was an SOA operating below the laser threshold so that its output
had a bandwidth of 17 nm. A differential-detection technique was used to reduce the
impact of optical beat noise. Indeed, bit-error rates below 10−^9 could be achieved by
using differential detection even when all four channels were operating simultaneously.
The coherent CDMA systems designed with spectral encoding make use of ultra-
short optical pulses with a relatively broad spectrum [259]. The encoder splits the
broad spectrum into many distinct wavelengths using a periodic optical filter (such as
an AWG) and then assembles different frequency signals according to the code used for
that channel. A matched-filter decoder at the receiver end performs the reverse opera-
tion so that a single ultrashort pulse is regenerated in a coherent fashion. This technique
has a distinct advantage that the CDMA signal can be overlaid over a WDM signal such
that both signals occupy the same wavelength range. Figure 8.34 shows schematically
how such a hybrid scheme works [267]. The spectrum of the received signal consists of
a broadband CDMA background and multiple sharp narrowband peaks that correspond
to various WDM channels. The CDMA background does not affect the detection of
WDM channels much because of its low amplitude. The CDMA receiver employs a
notch filter to remove the WDM signal before decoding it. The hybrid WDM–CDMA
scheme is spectrally efficient as it makes use of the unused extra bandwidth around
each WDM channel. In a 2002 experiment, a spectral efficiency of 1.6 (b/s)/Hz and a
capacity of 6.4 Tb/s were realized in the C band alone using the combination of CDMA
and WDM techniques [271].