In recent years, Wavelength Division Multiplexed (WDM) and Dense Wavelength Division Multiplexed (DWDM) optical transmission systems have been increasingly deployed in optical networks. Although DWDM optical transmission systems have increased the speed and capacity of optical networks, the performance of such systems, especially those providing high bit rates, has traditionally been limited by various factors such as chromatic dispersion and the non-linearity in an optical fiber's refractive index, which can cause spectral broadening of optical pulses and degrade the transmission of high speed optical signals. Because such optical signal degradation tends to accumulate along transmission paths, chromatic dispersion and non-linearity can significantly limit the transmission distance of high speed optical signals.
Chromatic dispersion refers to the fact that different wavelengths of light pass through an optical fiber at different speeds, thereby causing a pulse of light propagating through the optical fiber to broaden. Chromatic dispersion is an inherent property of the glass fiber and arises from two sources, the material properties of the glass and the waveguide structure (i.e., the refractive index profile of the glass fiber). For example, a single mode fiber through which light propagates at a wavelength of 1550 nm operates in the so-called anomalous dispersion regime. In the anomalous dispersion regime longer wavelengths (i.e. lower frequencies) of light travel at slower speeds than shorter wavelengths (i.e. higher frequencies) of light. As a result, the different wavelengths in pulse of light will be broadened as it travels through the single mode optical fiber.
Several solutions have been proposed to mitigate the effects of dispersion in transmission fibers. One technique involves the use of a compensating optical fiber having an appropriate length and which has a dispersion that is opposite to the dispersion characteristic of the transmission fiber. As a result, the dispersion in the transmission fiber is substantially canceled by the total dispersion in the compensating fiber. The dispersion of the compensating fiber is generally selected to be much greater in magnitude than the dispersion of the transmission fiber. In this way the length of the dispersion compensating fiber may be much less than the length of the transmission fiber. For example, to compensate for dispersion in a single mode fiber 100 km in length, a typical dispersion compensating fiber may need to be about 10 km in length. Such fiber is generally wound on a mandrel and provided for use as a dispersion compensating module.
Since high dispersion fiber has a relatively small core area (e.g., about 25 μm2 for a −100 ps/nm-km fiber), the optical power density in the dispersion compensating fiber will be relatively high, which leads to signal degradation arising from non-linear interactions. Accordingly, there is limit to the optical power that can be directed into the dispersion compensating fiber if nonlinear penalties are to be avoided. For example, it has been determined that a safe power level for light launched into one common dispersion compensating fiber is about 0 dBm/wavelength. Thus, for a WDM system employing 40 channels or wavelengths, the maximum permissible power that can be launched into the dispersion compensating fiber while avoiding non-linear penalties is about 16 dBm.
In many cases WDM networks are often initially deployed at less than their maximum capacity. That is, a system designed to transmit 40 channels or more, for instance, initially may be more lightly loaded with only 2, 4, or 8 channels. As demand increases, network capacity can be increased by increasing the number of wavelengths that are used. In addition, the number of wavelengths that are used may be changed dynamically as demand increases or decreases. Such dynamic networks will need to dynamically control the input power to the dispersion compensating fiber to avoid nonlinearities.
Accordingly, it would be desirable to provide a method and apparatus for automatically adjusting the optical power directed to a dispersion compensating fiber so that nonlinear interactions are avoided.