376 CHAPTER 8. MULTICHANNEL SYSTEMS
Figure 8.26: Design of an OTDM transmitter based on optical delay lines.then combined to form a composite signal. It should be clear that the multiplexed bit
stream produced using such a scheme has a bit slot corresponding to the bit rateNB.
Furthermore,Nconsecutive bits in each interval of durationB−^1 belong toNdifferent
channels, as required by the TDM scheme (see Section 1.2).
The entire OTDM multiplexer (except for modulators which require LiNbO 3 or
semiconductor waveguides) can be built using single-mode fibers. Splitting and recom-
bining of signals inNbranches can be accomplished with 1×Nfused fiber couplers.
The optical delay lines can be implemented using fiber segments of controlled lengths.
As an example, a 1-mm length difference introduces a delay of about 5 ps. Note that
the delay lines can be relatively long (10 cm or more) because only the length differ-
ence has to be matched precisely. For a precision of 0.1 ps, typically required for a
40-Gb/s OTDM signal, the delay lengths should be controlled to within 20μm. Such
precision is hard to realize using optical fibers.
An alternative approach makes use of planar lightwave circuits fabricated using
the silica-on-silicon technology [41]–[45]. Such devices can be made polarization in-
sensitive while providing a precise control of the delay lengths. However, the entire
multiplexer cannot be built in the form of a planar lightwave circuit as modulators
cannot be integrated with this technology. A simple approach consists of inserting
an InP chip containing an array of electroabsorption modulators in between the silica
waveguides that are used for splitting, delaying and combining the multiple channels
(see Fig. 8.26). The main problem with this approach is the spot-size mismatch as the
optical signal passes from Si to InP waveguide (and vice versa). This problem can
be solved by integrating spot-size converters with the modulators. Such an integrated
OTDM multiplexer was used in a 160-Gb/s experiment in which 16 channels, each
operating at 10 Gb/s were multiplexed [218].
An important difference between the OTDM and WDM techniques should be ap-
parent from Fig. 8.26: The OTDM technique requires the use of the RZ format (see
Section 1.2.3). In this respect, OTDM is similar to soliton systems (covered in Chap-
ter 9), which must also use the RZ format. Historically, the NRZ format used before
the advent of lightwave technology was retained even for optical communication sys-
tems. Starting in the late 1990s, the RZ format began to appear in dispersion-managed
WDM systems in the form of CRZ format. The use of OTDM requires optical sources
emitting a train of short optical pulses at a repetition rate as high as 40 GHz. Two types