An important application of optical signal processing is for regenerating optical signals degraded during transmission through fibers and amplifiers. An ideal optical regenerator transforms the degraded bitstream into its original form by performing three functions: reamplification, reshaping, and retiming. Such devices are referred to as 3R regenerators to emphasize that they perform all three functions. With this terminology, optical amplifiers can be classified as 1R regenerators because they only reamplify the bitstream. Devices that perform the first two functions are called 2R regenerators. Since 2R and 3R regenerators have to work at time scales shorter than the bit slot in order to carry out pulse reshaping and retiming, they must operate at time scales of 10 ps or less, depending on the bit rate of the optical signal. As nonlinear effects in optical fibers respond at femtosecond time scales, highly nonlinear fibers are often employed for such devices. However, the use of SOAs is also being pursued in view of their low-power requirements.
Wavelength converters switch the entire bitstream at one wavelength to a different wavelength without affecting its temporal content. Some applications require selective switching of one or more bits to a different port. An example is provided by packet switching in which a packet of tens or hundreds of bits is selected from a bitstream. Another example is provided by the OTDM technique in which a selected bit of a high-speed bit stream is periodically sent to another port. Such applications require time-domain switches that can be turned on for a specific duration using external control.
Optical networks in which WDM channels are switched based on their carrier wavelengths require a device that can change the carrier wavelength of the channel without affecting its bit pattern that contains the information being transmitted. A large number of such devices have been developed based on both optical fibers and semiconductor materials. This tutorial considers some of them and focuses on their performance from a system standpoint.
Optical flip-flops constitute time-domain switches that can be turned on and off using an external control. Such devices attracted considerable attention during the 1980s because they mimic the functionality of electrical flip-flops and provide the most versatile solution for optical switching, optical memory, and optical logic elements.
Current lightwave systems perform signal processing mostly in the electric domain. This approach is acceptable if signal processing is done at the transmitter and receiver ends but becomes impractical if it needs to be carried out at intermediate nodes of an optical network. For example, switching of individual WDM channels at an intermediate node may require a change in its carrier wavelength. An electric-domain implementation would require recovering the electric bit stream with an optical receiver and then recreating the WDM channel using an optical transmitter operating at the new wavelength. An all-optical approach would simply send the channel to a nonlinear optical device (called the wavelength converter) that changes the carrier wavelength without affecting its data contents. Another example is provided by optical regenerators that clean u pan optical signal and amplify it without any optical to electrical conversion. This tutorial focuses on a variety of signal processing devices that make use of the same nonlinear effects, such as self-phase modulation (SPM), cross-phase modulation (XPM), and four-wave mixing (FWM), that are otherwise harmful for lightwave systems.