A Novel Differential Polarization-shift Keying Scheme for Fiber-optic Communication System

Abstract

<p>The rapid development of fiber-optic communication system requires a increasing transmission data rate and reach. One of the challenge for long-haul high-speed fiber-optic system is how to reduce the impairments and degrading effects from fiber dispersions and nonlinearity. Although the advances of digital signal processor (DSP) make impairment compensation for the coherent systems become possible, the implementation of coherent system is still expensive. Among the direct detection systems, differential phase-shift keying (DPSK) system shows great advantages over other modulation schemes. Although the recent commercial fiber-optic communication systems are based on DPSK, polarization-mode dispersion and fiber nonlinearity are still limiting factors for DPSK systems. In 2004, a differential polarization-phase-shift keying (DPolPSK) system in which information is encoded in both polarization and phase with multi-level direct detection is proposed, and it is found out the DPolPSK system greatly reduces the effect of nonlinear polarization scattering. With the expectation of getting a better nonlinear tolerance than DPSK system, a novel differential polarization-shift keying (DPolSK) system with balanced direct detection receiver is proposed and compared with DPSK system in terms of fiber dispersion and nonlinearity tolerance.</p> <p>The DPolSK system encodes information in the polarization angle difference between two adjacent symbols. To transmit bit '1' ('0'), the polarization angle of the current symbol is shifted by π/2 [-/2 (-π/2) with respect to the previous symbol. A balanced detector based on optical delay interferometer and Faraday rotator is used to demodulate the DPolSK signals. Ideally without dispersion and nonlinearity, the DPolSK has the same receiver current level as DPSK system.</p> <p>Monter-carlo simulations are conducted to evaluate the DPolSK system in the presence of chromatic dispersion (CD), polarization-mode dispersion (PMD), and fiber nonlinearity (FNL). The simulation results show DPolSK system has the same bit-error-rate (BER) and PMD tolerance as DPSK system when fiber nonlinearity is ignored. However, when nonlinearity is taken account in the system, DPolSK shows overall superiority to DPSK system. The reason that DPolSK has higher nonlinear tolerance than DPSK is also explored. The intra-channel four-wave mixing (IFWM) effect on the DPolSK and DPSK systems are investigated and the simulation results show the ghost pulse generation induced by IFWM is suppressed in DPolSK system as compared to DPSK system, leading to higher nonlinear tolerance.</p>