Communications and data transmission systems that transmit information signals in the form of optical pulses over a dielectric waveguide such as an optical fiber are now commonplace, and optical fiber systems have become the physical transport medium of choice in long distance telephone and data communication networks. While improvements in the sources of the optical pulses and in the optical fiber waveguides have increased the range over which such signals can be transmitted to between 100 and 200 kilometers, a problem with optical fiber systems is that whenever a digital optical data signal is generated, transmitted, switched, multiplexed, demultiplexed, or otherwise processed, the signal invariably is subject to some degree of distortion. Distortion is typically cumulative and if the original signal is not periodically restored, data can become riddled with errors or become completely incomprehensible.
Optical data regenerators are utilized to periodically restore the quality of an original data signal. Once a signal (e.g., comprising bits of ones and zeros) has propagated through some distance of an optical fiber, it loses power and occasionally must be re-amplified. However, each time the signal is amplified noise is added, and the signal to noise ratio is constantly reduced as the signal propagates through the fiber. The role of regenerators, which are situated at locations along the signals' transmission, is to restore the original information content and improve optical signal noise reduction by reducing amplitude fluctuations of the signal as it existed when launched into the optical fiber. Thus, once a signal is detected and analyzed, a copy of the signal can be re-launched, thereby ensuring that a clean, undistorted signal continues in the transmission path.
U.S. Pat. No. 6,141,129, to Mamyshev (the ‘Mamyshev patent’), describes a method and apparatus for all-optical regeneration of return-to-zero (RZ) data streams through the use of self-phase modulation (SPM) of a data signal which passes through a non-linear medium (NLM). Essentially, the Mamyshev patent discloses a method and apparatus which enhances the power level of a propagated optical signal and then passes the signal through an NLM to create spectral broadening which is subsequently utilized by a filter to reduce amplitude fluctuations (noise) and to regenerate the originally transmitted signal.
More specifically, the invention disclosed in the Mamyshev patent performs regeneration by utilizing a nonlinear effect, where the output of the regenerator varies or differs depending on the power of the signal that was incident (received) at the regenerator. An NLM causes spectral broadening in individual data pulses which are then filtered to pass a selected bandwidth centered at a frequency ωf shifted with respect to the input data carrier frequency ω0. Generally, the higher the input power of a signal, the further new frequencies due to spectral broadening will be from the input signal carrier ω0, and to some extent, saturation will occur. The new spectral components are created by the physics of the NLM, and are created by a process of self-phase modulation (SPM). Thus, if a 1 bit is launched into the NLM, a large spectral broadening will result, whereas small spectral broadening will result if a 0 bit is launched into the NLM. In the method and apparatus of Mamyshev, noise in null values (data ‘zeros’) possess insufficient intensity to cause the requisite amount of spectral broadening to encompass the selected filter bandwidth centered around ωf, and the noise is suppressed. On the other hand, noise or amplitude fluctuations in set values (data ‘ones’) possess sufficient intensity to cause the requisite spectral broadening to encompass the selected filtered bandwidth centered around ωf, and that portion of the spectrally broadened pulse contained within the bandwidth centered around ωf is subsequently passed.
As noted above, the SPM induced spectral broadening in the optical fiber depends on the optical power launched into the fiber. For a given launch power, the spectral broadening also depends on the fiber dispersion (see, e.g., G. P. Agrawal, “Nonlinear fiber optics”, Academic Press, Inc, 1989, pp. 75–94). The optical regenerator of Mamyshev works best with a small normal dispersion between −0.25 and −0.75 ps/nm/km. However, in this range of dispersion, a fixed launch power may not result in the same spectral broadening for different fiber dispersion values due to the inability to effectively produce fiber with exact and highly fine tolerances. Therefore, the performance of the optical regenerator is not optimized and can result in inaccurate signal regeneration.
Therefore, what is needed is a method and apparatus for optical regeneration that will allow a range of fiber dispersion values while simultaneously resulting in the same or similar spectral broadening, thus maximizing the effectiveness of an optical signal regenerator.