Laser phase noise effect and reduction in self-homodyne optical OFDM transmission system

Area of Science:

• Optical Communications
• Signal Processing
• Photonics

Background:

• Laser phase noise (PN) in self-coherent optical OFDM systems induces phase-modulation-to-intensity-modulation (PM-IM) conversion noise and noise pedestals.
• These impairments degrade the performance of orthogonal frequency division multiplexing (OFDM) subcarriers in self-homodyne transmissions.

Purpose of the Study:

• To analyze the statistical effects of PN-induced impairments on received symbols in optical OFDM.
• To propose and demonstrate a novel compensation technique for PN-induced noise and its associated effects.
• To evaluate the performance improvements in terms of quadrature error, OSNR requirements, and dispersion tolerance.

Main Methods:

• Statistical analysis of received symbols using histograms to quantify PN-induced phase rotation and inter-subcarrier interference.
• Implementation of a simple time delay method to realign subcarrier phases and compensate for phase walk-off.
• Experimental validation using 16-QAM and 64-QAM with varying laser linewidths, transmission lengths, and OFDM bandwidths.

Main Results:

• Significant quadrature improvements of 6.82 dB (5 MHz linewidth, 720 km) and 5.38 dB (20 MHz linewidth, 240 km) were achieved.
• Optical-signal-to-noise ratio requirement was reduced by 16.15 dB for 64-QAM over 160 km at a 1x10^-3 BER.
• The time delay compensation technique enabled the system to tolerate three times the chromatic dispersion-length product.

Conclusions:

• The proposed time delay compensation effectively mitigates laser phase noise-induced PM-IM conversion noise and noise pedestals in optical OFDM.
• This technique offers substantial improvements in signal quality, enabling longer transmission distances and higher-order modulation formats.
• The method enhances system robustness against chromatic dispersion, paving the way for more efficient optical communication systems.