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.