Polarization-Insensitive and Receiver-Sensitivity-Gain Format Conversion for PDM Signals Based on Dual-Orthogonal-Pump Four-Wave Mixing in Highly Nonlinear Fiber

All-optical format conversion plays an important role in future transparent optical networks in order to enhance reconfigurability. On the other hand, the polarization-division multiplexing (PDM) technique has been extensively studied for achieving high-speed and high-spectral-efficiency optical networks. Since the state of polarization of PDM signals are randomly changed along the transmission optical link, it is important to elaborately design a polarization-insensitive conversion scheme in such a network. In this paper, we proposed and experimentally demonstrated an all-optical nonreturn-to-zero-return-to-zero format conversion for PDM on-off keying (OOK), binary phase-shift keying (BPSK), and quadrature phase-shift keying (QPSK) signals. Polarization-insensitive operation is achieved by utilizing the merit of the dual-orthogonal-pump four-wave mixing (FWM) process in a highly nonlinear fiber, and receiver-sensitivity-gain operation is observed in the conversion process. The bit error ratio (BER) results show that receiver-sensitivity gains of ~1.5, ~0.8, and 0.3 dB for the OOK, BPSK, and QPSK PDM signals at BER values of 10^-9, 10^-9, and 10^-3 are obtained, respectively.