OTDM at High Bit Rates
Growing network traffic requires a higher bit rate from each optical fiber. One way to achieve this is to aggregate several lower-rate streams into a faster stream. In TDM, each stream receives its own time slots, so pulses in the aggregate signal occur more often. Each bit consequently occupies a shorter interval in the resulting signal. This approach, known as , organizes those streams in time. At rates around 40 Gb/s and above, the electronics required to combine and separate the streams and to perform modulation and detection becomes a technical and cost constraint. This constraint is called the electronic bottleneck. Optical TDM (OTDM) moves the interleaving into the optical domain.
ETDM and OTDM in an Optical Link
In an optical link, TDM can be performed before electrical-to-optical conversion or at the level of optical pulses. The first arrangement is called ETDM, and the second is called OTDM.
Electrical TDM (ETDM)
In ETDM, several lower-speed electronic data streams are combined by a high-speed electronic . This stream drives one laser at the aggregate rate. The limit is set by the fastest MUX, DEMUX, modulator, and detector, since each element must handle the combined stream.
Optical TDM (OTDM)
In OTDM, each lower-rate electronic data stream first modulates its own laser. The resulting optical pulse streams are then combined and interleaved in the optical domain using an optical multiplexer. The aggregate-rate combination is optical, so the electronic path does not need one circuit operating at that rate. The transmitters, delays, multiplexer, and receiver must share a timing reference.
How OTDM Operates
OTDM creates a faster stream by interleaving pulses from lower-rate streams in time. The transmitters, delays, and receiver must share a timing reference so that each pulse occupies the intended slot.
- Pulse interleaving: An OTDM multiplexer takes optical pulse streams from several input channels, for example four 10 Gb/s streams, and applies a defined optical delay to each one. The bit period of one 10 Gb/s stream is 100 ps, while adjacent slots in the aggregate 40 Gb/s stream are 25 ps apart. The delays shift the stream phases so that the pulses fill successive slots and form one 40 Gb/s stream.
- Optical demultiplexing: At the receiver, the demultiplexer selects one phase of the interleaved slots. In the 40 Gb/s example, slots are 25 ps apart, while pulses from one 10 Gb/s tributary recur every 100 ps. The optical gate is synchronized to the selected phase and passes those pulses while suppressing the other three phases.
- Dispersion: One of the main OTDM challenges is . OTDM places pulses in tightly packed time slots, so broadening in the fiber can cause inter-symbol interference and errors. The effect depends on fiber parameters, dispersion compensation, and link length.
OTDM and WDM: Time and Wavelength
OTDM and Wavelength Division Multiplexing (WDM) increase capacity in different dimensions. OTDM assigns time slots, while WDM separates channels by wavelength.
- OTDM divides the time domain, packing more bits into the same second on a single color of light.
- WDM divides the frequency domain, sending multiple independent data streams simultaneously, each on a different color (wavelength) of light.
High-capacity systems can combine both technologies. For example, a DWDM system can use 80 wavelengths and carry an OTDM signal on each one. At 40 Gb/s per wavelength, the multiplication gives 3.2 Tb/s before overhead and link limitations are included.