Two-Stage Linearization Filter for Direct-Detection Subcarrier Modulation

A novel digital two-stage linearization filter is proposed for direct-detection (DD) systems and assessed experimentally for the first time. The performance improvement is quantified by experiments with a $7\times25$ Gb/s wavelength division multiplexing DD single sideband 16-QAM Nyquist-shaped subcarrier modulation system with a net optical information spectral density of 2.3 (b/s)/Hz. The results indicate that this technique can effectively compensate the nonlinearity caused by square-law detection, while, at the same time reducing the DSP complexity, avoiding the need to perform the multiple iterations which are required in previously proposed beating interference cancellation methods.

I. Introduction

Short- and medium-haul optical links/networks with increased throughputs are needed to meet the continuously increasing traffic demand. However, it is not currently practical to install coherent technologies in such applications due to their cost-sensitive nature [1]. Thus, it is still favorable to employ simpler direct-detection (DD) wavelength division multiplexing (WDM) systems for many inter-data center, access, metro and back-haul applications [2].