Optical Time Division Multiplexing (OTDM) At the inputs to the network, lower-speed data streams are multiplexed optically into a higher speed stream, and at the outputs of the network, the lower-speed streams must be extracted from the higher-speed stream optically by means of a demultiplexing function. Functionally, optical TDM (OTDM) is identical to electronic TDM. The only difference is that the multiplexing and demultiplexing operations are performed entirely optically at high speeds. The typical aggregate rate in OTDM systems is on the order of 100 Gb/s.

OTDM is illustrated in Figure 5.1. Optical signals representing data streams from multiple sources are interleaved in time to produce a single data stream. The interleaving can be done on a bit-by-bit basis as shown in Figure 5.1(a). Assuming the data is sent in the form of packets, it can also be done on a packet-by-packet basis, as shown in Figure 5.1(b).

If the packets are of fixed length, the recognition of packet boundaries is much simpler. We will assume that fixed-length packets are used. In both the bit-interleaved and the packet-interleaved case, framing pulses can be used. In the packet-interleaved case, framing pulses mark the boundary between packets. In the bit-interleaved case, if n input data streams are to be multiplexed, a framing pulse is used every n bits. These framing pulses will turn out to be very useful for demultiplexing individual packets from a multiplexed stream of packets.

Figure 5.1: (a) Function of a bit-interleaved optical multiplexer. (b) Function of a packet-interleaved optical multiplexer. The same four data streams are multiplexed in both cases. In (b), the packet size is shown as 3 bits for illustration purposes only; in practice, packets are much larger and vary in size. Note that in both cases, the data must be compressed in time.

Note from Figure 5.1 that very short pulses—much shorter than the bit interval of each multiplexed stream—must be used in OTDM systems. Given that we are interested in achieving overall bit rates of several tens to hundreds of gigabits per second, the desired pulse widths are on the order of a few picoseconds. A periodic train of such short pulses can be generated using a mode-locked laser, or by using a continuous-wave laser along with an external modulator. Since the pulses are very short, their frequency spectrum will be large. Therefore, unless some special care is taken, there will be significant pulse broadening due to the effects of chromatic dispersion. For this purpose, many OTDM experiments use suitably shaped return-to-zero (RZ) pulses.