In fiber optic communication systems, optical signals are modulated to carry symbols of data and are transmitted on an optical fiber from an optical transmitter to a receiver, such that a given symbol is transmitted during a symbol period. Although the optical signals are usually at a single nominal wavelength, each signal can include different spectral components. The spectral components of each optical signal can propagate through the transmission fiber at different speeds. This effect, known as “chromatic dispersion”, can result in spectral components of one symbol period arriving at a receiver at substantially the same time as a succeeding symbol period, thereby causing degraded receiver sensitivity. Chromatic dispersion becomes increasingly pronounced at higher bit rates.
Early fiber optic communication systems included transmitters that output light having a wavelength of 1310 nm, which is the wavelength at which conventional single mode optical fiber has a substantially zero dispersion. The absorption of silica, a material from which optical fibers are made, is greater at 1310 nm than the absorption at 1550 nm. Accordingly, subsequent systems were developed to transmit optical signals at or near 1550 nm. Since conventional single mode optical fiber has significant chromatic dispersion at such wavelengths, so-called “dispersion shifted fiber” (DSF) was developed that has zero or substantially zero dispersion at 1550 nm.
In order to increase the data carrying capacity of fiber optic communication systems, wavelength division multiplexing has been deployed in which multiple transmitters output modulated optical signals at different wavelengths. The optical signals are then combined onto an optical fiber and transmitted as a wavelength division multiplexed (WDM) signal.
When a WDM signal is transmitted on DSF, however, optical signals that are spectrally close to one another in wavelength may remain correlated with one another over a long distance (phase-matching) due to the low dispersion in the fiber. Under such phase-matching conditions, the optical signals at different wavelengths can strongly interact with one another to generate additional optical components at other wavelengths (mixing products). Such mixing products may have the same or substantially the same wavelength as other optical signals in the WDM signal, and the magnitude of the mixing products is related to the distance over which the optical signals propagate and the frequency or wavelength spacing between such optical signals. Thus, if two spectrally close and phase matched optical signal propagate over long distances on DSF, the resulting mixing products may increase in magnitude, which may be observed as significant noise at one of the optical signal wavelengths. This non-linear effect, referred to as “four wave mixing”, can introduce significant distortions and result in relatively high error rates. Optical signal wavelengths close to the zero dispersion wavelength of DSF are particularly susceptible to the effects of four wave mixing, such that a limited number of optical signals having wavelengths in the C-band are typically transmitted on DSF. Accordingly, optical networks including DSF often have substantially limited capacity. In addition, since the magnitude of the mixing products is related to the power of the optical signals, such optical networks typically launch optical signals with reduced power over shorter distances.
Therefore, non-zero dispersion-shifted fibers (NZDSF) have been developed that have a small chromatic dispersion in a wavelength range about 1550 nm (the “C-band”, 1530 nm-1565 nm), with the zero dispersion wavelength lying just outside this range. Accordingly, the C-band wavelengths are not spectrally close to the zero-dispersion wavelength of NZ-DSF fibers, such that phase matching, as well as four wave mixing, is substantially reduced.
DSF fiber plants, however, are still in use, primarily due to the cost of replacing such fiber once it has been deployed. In order to increase the capacity of fiber optic networks including DSF, systems have been developed that include multiple transmitters which output optical signals in a wavelength range of 1565 nm to 1625 nm (the “L-band”). L-band transmitters, however, include lasers that can be more expensive than those provided in C-band transmitters.
Accordingly, there is a need for a high capacity, long-distance optical communication system that transmits optical signals in the C-band over DSF.