This application relates to techniques for reducing or mitigating polarization-related signal distortion effects in optical media such as optical fibers.
Some optical transmission media such as optical fibers are optically birefringent, i.e., exhibiting different refractive indices for light with different polarizations along two orthogonal principal directions. An optical signal having polarization components along the two orthogonal principal directions for each frequency, thus, may be distorted after propagation through such a transmission medium since the two components propagate in different group velocities. This optical dispersion is generally referred to as polarization-mode dispersion (xe2x80x9cPMDxe2x80x9d).
The degree of PMD may be approximately characterized by the average differential group delay (xe2x80x9cDGDxe2x80x9d) between two principal states of polarization. Typical causes for such birefringence in fibers include, among others, imperfect circular core and unbalanced stresses in a fiber along different transverse directions. The axis of birefringence of the optical fiber may change randomly, e.g., in the time period from milliseconds to hours depending on the external conditions in some fiber systems. Therefore, the DGD in an actual PMD fiber is not a fixed value but a random variable that fluctuates over time.
Such polarization-mode dispersion is undesirable in part because the associated pulse broadening may adversely limit the transmission bit rate, the transmission bandwidth, and other performance factors of an optical communication system. In fact, PMD is one of key limitations to the performance of some high-speed optical fiber communication systems at or above 10 Gbits/s per channel due to the fiber birefringence.
This application includes techniques for generating optical pulses with two orthogonal polarizations within each pulse for reducing or mitigating polarization-related distortion effects, e.g., PMD and cross-phase modulation, on the pulses at the receiving end after transmission through an optical link. The portion of the transmitted optical signal in either one of the two principal states of polarization of the transmission medium is selected at the receiving end for detection.
Accordingly, in one embodiment, optical pulses are transmitted through a birefringent optical link, where each optical pulse has a first pulse portion in a first optical polarization and a second pulse portion immediately adjacent to the first pulse portion without overlap and in a second, orthogonal optical polarization. The transmitted light from the birefringent optical link is then received to select a portion of the received transmitted light for optical detection, where the selected portion is polarized in one of two orthogonal principal states of polarization of the birefringent optical link.