Method and apparatus for providing dispersion compensation

#The present invention relates to optical fibers and, more particularly, to providing very accurate dispersion compensation over an entire range of wavelengths.

#Dispersion in a glass fiber causes pulse spreading for pulses that include a range of wavelengths, due to the fact that the speed of light in a glass fiber is a function of the transmission wavelength of the light. Pulse broadening is a function of the fiber dispersion, the fiber length and the spectral width of the light source. Dispersion for individual fibers is generally illustrated using a graph having dispersion on the vertical axis (in units of picoseconds (ps) per nanometer (nm), or ps/nm) or ps/nm-km (kilometer) and wavelength on the horizontal axis. There can be both positive and negative dispersion, so the vertical axis may range from, for example, −250 to +250 ps. The wavelength on the horizontal axis at which the dispersion equals zero corresponds to the highest bandwidth for the fiber. However, this wavelength typically does not coincide with the wavelength at which the fiber transmits light with minimum attenuation.

#For example, typical single mode fibers generally transmit best (i.e., with minimum attenuation) at 1550 nm, whereas dispersion for the same fiber would be approximately zero at 1310 nm. The theoretical minimum loss for glass fiber is approximately 0.16 db/km, and that occurs at the transmission wavelength of about 1550 nm. Because minimum attenuation is prioritized over zero dispersion, the wavelength normally used to transmit over such fibers is typically 1550 nm. Also, Erbium-doped amplifiers, which currently are the most commonly used optical amplifiers for amplifying optical signals carried on a fiber, operate in 1530 to 1565 nm range. Because dispersion for such a fiber normally will not be zero at a transmission wavelength of 1550 nm, attempts are constantly being made to improve dispersion compensation over the transmission path in order to provide best overall system performance (i.e., low optical loss and low dispersion).

#FIG. 1 is a graph illustrating RDS distributions for a first DCF, a second DCF and the combination of the two versus the percentages of DCMs that are used to match the RDS of a transmission fiber at 1580 nanometers (nm).

#FIG. 2 is a graph illustrating the distribution of the RDSs of the resulting combination of the DCFs shown in FIG. 1.

#FIG. 3 is a graph illustrating the RDS distribution for a DCF having a negative dispersion slope.

#In accordance with the present invention, it has been determined that overall dispersion compensation over a transmission band can be further improved by simultaneously matching the RDSs of the DCM and transmission fiber and compensating the dispersion slope and dispersion of the transmission link. The present invention not only improves over dispersion compensation of the transmission link by ensuring simultaneous compensation of the center wavelength of a broad transmission band and of all of the other wavelengths of transmission band.

#In accordance with the preferred embodiment of the present invention, multiple DCFs having particular RDSs are combined in a single DCM to obtain an effective RDS for the DCM that matches the RDS of the transmission fiber. In general, this is accomplished by matching the effective RDS of the DCM with the RDS of the transmission fiber. The effective RDS of the DCM is the RDS produced by combining multiple DCFs having particular RDSs. Selecting the lengths of the DCFs enables the dispersion slope and the dispersion of the transmission link to be compensated. The manner in which this can be accomplished will first be mathematically described and then graphically demonstrated.

#The dispersion and the dispersion slope of a DCM can be obtained from Equations 5 and 6 below, respectively, as follows: