Free spectral range optimization of return-to-zero differential phase shift keyed demodulation in the presence of chromatic dispersion

Xiaoxia Wu,Jeng-Yuan Yang,Raman Kashyap,Yannick K Lizé,Scott Nuccio,Alan E Willner,Louis Christen,Teng Wu

doi:10.1364/oe.15.006817

Abstract

Optical differential phase shift keying is normally demodulated in a delay-line interferometer with a 1-bit delay such that the free-spectral-range of the demodulator is equal to the transmitted bitrate. We show using Karkunen-Loeve expansion simulation that free-spectral-range optimization leads to increased chromatic dispersion tolerances. The optimized delay inversely scales with the amount of chromatic dispersion such that a delay slightly shorter than the bit period increases tolerances with no adverse effect on the polarization-mode-dispersion tolerance or frequency offset penalty at the receiver.

Highlights

Due to its increased receiver sensitivity and increased tolerance to various fiber-based impairments, differential-phase-shift-keying (DPSK) has been pursued aggressively as an alternative to on-off keying (OOK) [1,2,3,4,5,6]
We show using Karkunen-Loeve expansion simulation that free-spectral-range optimization leads to increased chromatic dispersion tolerances
The optimized delay inversely scales with the amount of chromatic dispersion such that a delay slightly shorter than the bit period increases tolerances with no adverse effect on the polarization-mode-dispersion tolerance or frequency offset penalty at the receiver

Summary

Introduction

Due to its increased receiver sensitivity and increased tolerance to various fiber-based impairments, differential-phase-shift-keying (DPSK) has been pursued aggressively as an alternative to on-off keying (OOK) [1,2,3,4,5,6]. It has been shown that DLI degradations such as bit delay mismatch and frequency offset [10,11,12,13], transmission impairments such as chromatic dispersion (CD), polarizationmode-dispersion (PMD), and nonlinearities [14,16] or the combination of DLI degradations and transmission impairments [17] can distort the phase of the DPSK signal and reduce receiver sensitivity. It might be advantageous to optimize the FSR of the DLI to counteract the phase degradation of the transmission impairments in order to enhance the DPSK receiver sensitivity. In this paper we demonstrate that in the presence of CD, offsetting the FSR of the DLI to obtain partial bit delay in the demodulation of a RZ-DPSK signal increases CD tolerance with no adverse effect on the PMD tolerance or frequency offset penalty. We show that some of the increased degradation stemming from the combination of transmission impairments and frequency offset [17] is mitigated by using partial bit delay demodulation

Concept and Theory

Results

Chromatic dispersion and bit delay mismatch

Conclusion