Receiver implemented RF pilot tone phase noise mitigation in coherent optical nPSK and nQAM systems

Gunnar Jacobsen,Sergei Popov,Yimo Zhang,Jie Li,Ari T Friberg,Tianhua Xu

doi:10.1364/oe.19.014487

Abstract

In this paper, a novel method for extracting an RF pilot carrier signal in the coherent receiver is presented. The RF carrier is used to mitigate the phase noise influence in n-level PSK and QAM systems. The performance is compared to the use of an (ideal) optically transmitted RF pilot tone. As expected an electronically generated RF carrier provides less efficient phase noise mitigation than the optical RF. However, the electronically generated RF carrier still improves the phase noise tolerance by about one order of magnitude in bit error rate (BER) compared to using no RF pilot tone. It is also found, as a novel study result, that equalization enhanced phase noise--which appears as correlated pure phase noise, amplitude noise and time jitter-cannot be efficiently mitigated by the use of an (optically or electrically generated) RF pilot tone.

Highlights

Fiber impairments, such as chromatic dispersion (CD) and phase noise, severely impact the performance of high speed optical fiber transmission systems [1,2]
The performance is compared to the use of an optically transmitted radio frequency (RF) pilot tone
As a novel study result, that equalization enhanced phase noise - which appears as correlated pure phase noise, amplitude noise and time jitter - cannot be efficiently mitigated by the use of an RF pilot tone

Summary

Introduction

Fiber impairments, such as chromatic dispersion (CD) and phase noise, severely impact the performance of high speed optical fiber transmission systems [1,2]. Adaptive dispersion compensation - using the least mean square (LMS) method – has a different impact than using fixed filter implementations such as a time domain fiber dispersion finite impulse response filter or a frequency domain equalizer (FDE) [21] When it comes to the system impact of phase noise, it is important to understand that moving to higher constellations (which is a way of increasing the system capacity using the same symbol rate) tends to increase the phase noise impact [21,22]. This means that higher constellation systems (i.e. realized as n-level PSK or QAM implementations with n = 4,8,16,64,...) put more stringent requirements to the spectral purity of the system transmitter (Tx) and local oscillator (LO) lasers [23] This can be further enhanced by the total EEPN effect for systems using discrete signal processing for chromatic dispersion compensation.

Principle and structure for extracting an RF pilot tone in the Rx

Phase noise analysis

D L fLO TS

Simulation results

Conclusions