Theoretical investigation of polarization-insensitive data format conversion of RZ-OOK to RZ-BPSK in a nonlinear birefringent fiber

C C Wei,J Chen,Y.-J Chen,G M Carter,W Astar

doi:10.1364/oe.17.004306

Abstract

Polarization-insensitive conversion of return-to-zero (RZ) ON-OFF keying (RZ-OOK) to RZ binary phase-shift keying (RZ-BPSK) has been achieved by cross-phase modulation (XPM) in a nonlinear birefringent fiber. This work presents a theoretical analysis of the dependence of format conversion on pump-probe detuning, and the pump state-of-polarization (SOP) that can fluctuate unpredictably in a realistic system. An investigation of the impact of pump polarization fluctuation on receiver sensitivity and receiver optimal threshold for the converted RZ-BPSK probe is also carried out. It was found that although the desired XPM-induced pi phase shift can be achieved by launching both the RZ-OOK pump and the probe along the same birefringent axis of the fiber, the phase shift degrades to pi/3 if the SOP of the RZ-OOK pump unpredictably switches to the other axis of the fiber, resulting in a large receiver sensitivity penalty fluctuation of 14 dB. By contrast, launching the probe at 45 degrees relative to the birefringent axes can reduce the polarization-dependent receiver sensitivity penalty fluctuation to about 2 dB as the SOP of the RZ-OOK pump is swept over the Poincaré sphere. These conclusions are in good agreement with recently.

Highlights

Compared with traditional RZ-OOK, RZ differential phase-shift keying (RZ-DPSK) requires about 3-dB lower optical signal-to-noise ratio (OSNR) to achieve the same bit-error rate (BER), but a more complex transmitter and receiver [1]
It was found that the desired XPM-induced π phase shift can be achieved by launching both the RZ-OOK pump and the probe along the same birefringent axis of the fiber, the phase shift degrades to π/3 if the SOP of the RZ-OOK pump unpredictably switches to the other axis of the fiber, resulting in a large receiver sensitivity penalty fluctuation of 14 dB
By contrast, launching the probe at 45 relative to the birefringent axes can reduce the polarization-dependent receiver sensitivity penalty fluctuation to about 2 dB as the SOP of the RZ-OOK pump is swept over the Poincaré sphere

Summary

Introduction

Compared with traditional RZ-OOK, RZ differential phase-shift keying (RZ-DPSK) requires about 3-dB lower optical signal-to-noise ratio (OSNR) to achieve the same bit-error rate (BER), but a more complex transmitter and receiver [1]. RZ-OOK can seriously distort RZ-DPSK by cross-phase modulation (XPM) [3], but this impairment may be mitigated by implementing RZ-OOK-to-RZ-BPSK format conversion at the OXC level. The XPM effect in a linearly birefringent photonic crystal fiber (PCF) has been proposed to achieve polarization-insensitive RZ-OOK-to-BPSK conversion [8]. In this scheme, the local probe beam is launched to both birefringent axes of the fiber to reduce polarization sensitivity, obviating the necessity of sophisticated and costly polarization tracking of the RZ-OOK signal. The penalty induced by insufficient XPM-induced phase shift is analytically investigated

The XPM effect in a birefringent fiber

Penalty induced by insufficient phase shift

Conclusion