We present a general quantum theory capable of describing photon statistics under the combined effects of four-wave mixing and Raman scattering inside optical fibers. Our theory is vectorial in nature and includes all polarization effects. Our analysis shows that spontaneous Raman scattering degrades the pair correlation in all cases but the extent of degradation depends on the pumping configuration employed. In a single-pump configuration, photon pairs can be created with polarization either parallel or orthogonal to the pump. Our results show that the orthogonal configuration can improve the extent of quantum correlation considerably over a broad bandwidth. In the case of a dual-pump configuration, we show that imbalance of two pump powers can be used to improve the quality of photon pairs. We show that orthogonally polarized pumps can generate photon pairs automatically in a polarization-entangled state. In particular, orthogonal pumping with circular polarizations produces such an entangled state with relatively high quality. We also quantify the quality of polarization entanglement as well as energy-time entanglement constructed using correlated photon pairs.

A universal post-compensation scheme for fiber impairments in wavelength-division multiplexing (WDM) systems is proposed based on coherent detection and digital signal processing (DSP). Transmission of 10 x 10 Gbit/s binary-phase-shift-keying (BPSK) signals at a channel spacing of 20 GHz over 800 km dispersion shifted fiber (DSF) has been demonstrated numerically.

Abstract:We demonstrate mode-division multiplexed WDM transmission over 50-km of few-mode fiber using the fiber's LP 01 and two degenerate LP 11 modes. A few-mode EDFA is used to boost the power of the output signal before a few-mode coherent receiver. A 6×6 time-domain MIMO equalizer is used to recover the transmitted data. We also experimentally characterize the 50-km few-mode fiber and the few-mode EDFA.

Abstract-Effects of zero-dispersion wavelength (ZDWL) fluctuations on dual-pump fiber-optic parametric amplifiers are investigated analytically and numerically. It is found that the signal gain varies considerably from fiber to fiber even though each fiber may have the same ZDWL on average. Moreover, the gain spectrum becomes highly nonuniform for a given fiber because of such dispersion fluctuations. Numerical simulations show that this problem can be solved to a large extent by reducing wavelength separation between the two pumps, but only at the expense of a reduced amplifier bandwidth.Index Terms-Dispersion fluctuations, four-wave mixing (FWM), parametric amplification.F IBER-OPTIC parametric amplifiers (FOPAs), based on four-wave mixing (FWM), can provide uniform gain over a relatively wide bandwidth when they are pumped at two wavelengths located on each side of the zero-dispersion wavelength (ZDWL) of the fiber [1]- [5]. However, the core diameter of any fiber exhibits random variations related to manufacturing, resulting in a randomly varying ZDWL along the fiber. Since FWM is sensitive to phase mismatch among the interacting waves, it is important to know how ZDWL variations affect the FOPA performance. This issue was addressed in [6] for single-pump FOPAs. In this letter, we study the effects of ZDWL fluctuations on dual-pump FOPAs and show that the amount as well as the uniformity of gain reduces considerably because of ZDWL fluctuations. We also show that the problem can be solved to a large extent by reducing the wavelength separation between the two pumps at the expense of a reduced gain bandwidth.Although a complete description of dual-pump FOPA is quite complicated [4], the uniform portion of the gain spectrum actually stems from the nondegenerate FWM process for which , where ( -) are the frequencies of the two pumps, signal, and idler waves, respectively. We focus on this process and assume that the pump powers and are not depleted. The evolution of the signal and idler waves is then governed by the following two equations where is the nonlinear parameter, describes the total phase mismatch, and is the wave-vector mismatch related to dispersion parameters as (3) where is the center frequency of the two pumps and is half their frequency difference. The parameters and govern the second-and fourth-order dispersion at the center frequency . They are related to the third-and fourth-order dispersion at the fiber's ZDWL as and , where . When the ZDWL is constant along the fiber, a broad gain spectrum is obtained by optimizing FOPAs such that remains close to zero over a relatively broad spectral range. Random variations in the ZDWL cause to vary randomly along the fiber, and it becomes difficult to maintain . As a result, the FOPA gain spectrum becomes considerably nonuniform even if the fiber is otherwise perfect. As FWM is sensitive to local phase mismatch, optimization of other design parameters (such as average ZDWL, pump powers and wavelengths, strength of nonlinearity, etc.) does not guarantee a...

Using multimode fibers for long-haul transmission is proposed and demonstrated experimentally. In particular few-mode fibers (FMFs) are demonstrated as a good compromise since they are sufficiently resistant to mode coupling compared to standard multimode fibers but they still can have large core diameters compared to single-mode fibers. As a result these fibers can have significantly less nonlinearity and at the same time they can have the same performance as single-mode fibers in terms of dispersion and loss. In the absence of mode coupling it is possible to use these fibers in the single-mode operation where all the data is carried in only one of the spatial modes throughout the fiber. It is shown experimentally that the single-mode operation is achieved simply by splicing single-mode fibers to both ends of a 35-km-long dual-mode fiber at 1310 nm. After 35 km of transmission, no modal dispersion or excess loss was observed. Finally the same fiber is placed in a recirculating loop and 3 WDM channels each carrying 6 Gb/s BPSK data were transmitted through 1050 km of the few-mode fiber without modal dispersion.