Telecommunications technology can use optical signals to communicate information from one location to another. In particular, optical signals can be communicated over optical fibers that connect one location to another. One particular protocol used in optical communications technology is wavelength division multiplexing (WDM), which permits the concurrent transmission of multiple information channels over a common optical fiber, thus expanding available bandwidth of information that can be transmitted over the fiber. Optimally exploiting the capabilities of optical communication, including WDM technology, requires dealing with various transmission impairments. In particular, attenuation and chromatic dispersion of optical signals occur as the signals propagate through a length of optical fiber. Eventually, after optical signals have propagated over a long distance through an optical fiber and have suffered a certain amount of attenuation and/or chromatic dispersion, the signals must be amplified and/or regenerated so that the information carried by the optical signals can continue to propagate through the fiber.
Erbium-doped fiber amplifiers (EDFAs) can be used to amplify an optical signal having a wavelength of about 1525-1565 nanometers (nm), which propagates in an optical fiber. Because an EDFA provides gain over a relatively wide of wavelengths, and EDFA can provide simultaneous amplification of all wavelengths in a composite WDM signal. Using this type of amplification, the WDM composite signal may be transmitted large distances, e.g., more than 600 km, without regeneration.
Another type of optical communication impairment is chromatic dispersion, which leads to a widening of an optical pulse as the pulse propagates along the fiber and is caused by different spectral components of the pulse propagating through the fiber at different velocities. Because of chromatic dispersion, the modulation of optical pulses that encode data spread out in the time domain as the pulses propagate along the fiber and can start to overlap overlapping one another, which can lead to bit errors. Generally, the amount of chromatic dispersion suffered by an optical signal depends on the characteristics of the fiber and the length of the fiber span over which the signal propagates.
The amount of chromatic dispersion suffered by an optical signal that travels over a fixed length of optical fiber can be compensated by inserting a tunable dispersion compensation unit into the transmission path. The dispersion compensation unit deliberately introduces a chromatic dispersion that is opposite in sign to the dispersion caused by transmission of the signal through the fiber and therefore effectively cancels out the dispersion caused by the optical fiber. In order to tune the dispersion compensation unit to provide a negative amount of chromatic dispersion equal in magnitude to the positive dispersion caused by transmission through the optical fiber the amount of dispersion accrued while the signal propagates through the optical fiber must be known.