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By Frbiz Tian

Optical time division multiplexing (TDM) technology is a high-speed transmission technology (up to 100Gb / s), it uses very narrow pulses have a greater bandwidth, more efficient use of optical fiber spectrum. TDM will slip stream of light directly into the optical domain multiplexing, resulting in the synthesis of very high bit rate data stream. TDM allows multiple signals simultaneously access to the same fibers, but the realization method of TDM and WDM is entirely different.

TDM can be used to achieve a single light 40Gb / s over rate.

The light source (such as mode-locked laser) produces a series of very narrow optical pulses into N-zero data stream, N a data stream for each channel signal by XGb / s (such as STM-n) respectively, the slip signal modulation, delay one clock cycle, and then by multiplexing the data resulting from the total rate of (N X) Gb / s.

Present study shows that, TDM overall rate of up to 200Gb / s or higher. Are running at such high speed optical transmission system, the fiber dispersion is a very important issue, generally use two methods to resolve, one is to use dispersion management (through the use of dispersion compensation fiber), bringing the total of the dispersion Jie Jin to zero; Another way is to use soliton transmission. Soliton transmission technology must be carefully considered signal power and pulse shape, and compression using self-phase modulation of the dispersion effect to solve the problem. Therefore, TDM is to support long-distance, high-capacity transmission better technical solutions.

Since the 90s since the 20th century, the United States, Britain, Japan and other countries some of the research carried out on the TDM system, extensive research and have made great progress.

Used in optical transmission network TDM, must consider the synchronization problem, not only have to consider synchronization in time domain, frequency domain synchronization.

That is, for TDM networks, through various channels to reach any node to synchronize the pulse sequences needed to ensure the accurate demultiplexing, while they have the same spectral characteristics needed to ensure that when the data is handed to the network of another node, with zero offset between them. TDM networks for large-scale major difficulty has been inserted into the data path and data path of the optical-line properties match, a possible solution is to use a single distribution in the network to send laser signals.

TDM high-speed optical communication system is composed of ultra-short pulse light source, optical time division multiplexing / de-multiplexing equipment, optical clock recovery and synchronization, optical receivers, and optical fiber components.

TDM system requirements to produce high repetition rate light source, small super narrow pulse duty ratio. The more narrow width of the large ones can be reused more bandwidth, the more wide. Sources to meet these requirements mainly locked ring laser, mode-locked semiconductor lasers.

TDM time-division multiplexing optical transmission system technology easier to achieve, and optical demultiplexing up difficult to achieve. Optical demultiplexing TDM requirements are: fast and stable work without error, control, low power, polarization independent, timing jitter is small. Currently the main use of optical fiber nonlinear optical loop mirror (NOLM – based on semiconductor laser amplifier loop mirrors) and FWM (four wave mixing) two demultiplexer demultiplexing.

Extracted from the TDM clock signal system is very important. At present, the main optical clock extraction circuit resonance, injection mode-locked semiconductor laser and phase-locked loop (PLL), etc. The more mature the PLL.

In order to achieve a drop multiplexing function of optical networks, in addition to the clock extraction, but also need to frame and channel calibration. In the optical transmission network, the environmental impact, will produce jitter and drift, so these systems will require precise alignment and channel phase control.

TDM is a realization of a powerful high-capacity high-speed transmission technology, it can effective use of optical wavelength, and can run at different distances and capacity of the network. TDM can also use non-linear soliton transmission, and a host of other technologies, high-speed transmission systems to eliminate the dispersion effects. With the large-scale integration technology, such as InP and silicon planar waveguide technology and semiconductor optical amplification technology, TDM technology will make significant progress and into the practical.

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