Independent component analysis for impairment mitigation in direct-detected Stokes vector modulation

Area of Science:

• Optical communications
• Signal processing
• Information theory

Background:

• Direct-detection optical systems using Stokes vector modulation (SVM) face challenges with simultaneous cross-polarization and inter-symbol interference (ISI).
• Conventional digital signal processing (DSP) techniques struggle to effectively resolve these multiple impairments.
• High symbol rates exacerbate signal distortion and interference, degrading system performance.

Purpose of the Study:

• To develop and validate a novel adaptation of independent component analysis (ICA) for compensating impairments in direct-detection SVM links.
• To address the limitations of conventional DSP in mitigating simultaneous cross-polarization and ISI.
• To enhance the performance of optical communication systems operating at high symbol rates.

Main Methods:

• A six-dimensional adaptation of independent component analysis (ICA) was theoretically and experimentally developed.
• The ICA method was applied to simultaneously de-rotate the SVM constellation, correct constellation shape distortion, and mitigate ISI.
• Experimental validation was performed at symbol rates of 7.5 Gb/s, 15 Gb/s, and 30 Gb/s.

Main Results:

• The ICA-based equalizer achieved power penalties below ~1 dB compared to ideal bit-error rate (BER) curves at 7.5 Gb/s and 15 Gb/s.
• At a more challenging 30 Gb/s, the ICA method successfully enabled polarization de-rotation.
• A BER below 10^-5 was achieved before error correction at 30 Gb/s, demonstrating significant ISI mitigation.

Conclusions:

• The proposed ICA adaptation offers an effective solution for compensating cross-polarization and ISI in direct-detection SVM systems.
• This method significantly improves system performance, particularly at high symbol rates where conventional techniques fall short.
• The experimental results validate the practical applicability and effectiveness of the ICA-based equalizer for future optical communication.