The present invention is directed toward optical communication system. In particular, optical communication systems providing dispersion slope compensation.
Optical signals transmitted in a fiber optic communication system typically constitute a series of pulses of digital information. Although the pulses are usually at a single nominal wavelength, each pulse is actually composed of different spectral components. These spectral components propagate through the transmission fiber at different speeds with higher frequency components traveling slower than lower frequency components. This effect, known as xe2x80x9cchromatic dispersionxe2x80x9d, can result in spectral components of one pulse arriving at a receiver at substantially the same time as a succeeding pulse, thereby causing degraded receiver sensitivity. Chromatic dispersion becomes increasingly pronounced at higher bit rates, e.g. 10 Gbit/sec. such as those associated with synchronous optical network (SONET) OC192 transmission speeds. Unique fibers have been developed to offset or compensate chromatic dispersion. These fibers, referred to as dispersion compensated fiber or DCF, are commercially available from Corning Inc. and Lucent Technologies, for example. DCF fibers are typically specified with a dispersion coefficient having an opposite sign to that of the transmission fiber. The net dispersion experienced by the transmitted optical signal is thus the sum of the dispersion accumulated through the transmission fiber and the DCF. If the product of dispersion coefficient times length in the DCF is equal in magnitude and opposite in sign to that of the transmission fiber, the net dispersion is zero, and the dispersion is said to be compensated.
DCF typically provides compensation at a particular wavelength. As a result, single wavelength optical transmission systems incorporating DCF and operating at the specified wavelength can have relatively low error rates, even at OC192 transmission speeds.
In order to further increase fiber capacity, wavelength division multiplexed (WDM) systems have been developed for carrying multiple wavelengths on a single fiber. At OC192 rates, each wavelength or channel of a WDM system must be dispersion compensated. Typically, the dispersion coefficient of the transmission fiber, as well as that associated with DCF, varies with wavelength; the relationship of dispersion and wavelength being referred to xe2x80x9cdispersion slopexe2x80x9d. Since the dispersion slope associated with the transmission fiber and DCF may not be identical, not all channels may be adequately dispersion compensated.
Theoretically, one approach to providing spectrally uniform dispersion compensation involves fabricating DCF uniquely tailored to have a slope of the same magnitude, but opposite sign, as the transmission fiber. As a practical matter, however, such DCF would be difficult to fabricate with precisely the required slope and magnitude.
Alternatively, since the amount of dispersion compensation depends in part on the length of the DCF, varying lengths of DCF can be provided in a WDM system downstream from the point where individual wavelengths have been separated. Each channel can then be compensated individually prior to detection at a photodetector. An additional segment of DCF, however, must be provided for each channel, thereby complicating system design and increasing costs.
Consistent with the present invention, an optical device is provided comprising an optical splitter having an input port and a plurality of output ports, said input port being configured to receive a plurality of optical signals, each at a respective one of a plurality of wavelengths. The optical signals are output from each of the plurality of output ports, but in attenuated form. The optical device further includes a plurality of in-fiber Bragg gratings, each of which being coupled to a respective one of the plurality of output ports. The in-fiber Bragg gratings are configured to reflect a selected one of the plurality of optical signals and compensate for a dispersion associated with the selected one of the plurality of optical signals.