M-ary pulse-position modulation and frequency-shift keying with additional polarization/phase modulation for...

Area of Science:

• Optical communications
• Signal processing
• Information theory

Background:

• Advanced modulation formats are crucial for increasing optical communication capacity.
• Existing methods face limitations in receiver sensitivity and spectral efficiency.
• Optically pre-amplified receivers require highly sensitive modulation schemes.

Purpose of the Study:

• To introduce and analyze a new class of optical modulation formats.
• To improve receiver sensitivity in optically pre-amplified systems.
• To demonstrate the practical implementation and performance of these novel formats.

Main Methods:

• Combining m-ary pulse-position modulation (m-PPM) or m-ary frequency-shift keying (m-FSK) with polarization and/or phase modulation.
• Utilizing pilot-assisted frequency-domain equalization for channel estimation and compensation.
• Implementing coherent optical orthogonal frequency-division multiplexing (CO-OFDM) for spectral efficiency.

Main Results:

• PQ-mPPM (m-PPM with PDM-QPSK) shows superior receiver sensitivity near forward-error correction thresholds.
• Record sensitivities of 3.5 photons per bit (ppb) at BER=10⁻³ (PQ-16PPM) and 2.7 ppb at BER=1.5×10⁻² (PQ-4PPM) achieved.
• Successful 6.23-Gb/s PQ-4PPM transmission over 370-km unrepeatered fiber with 71.7-dB loss demonstrated.

Conclusions:

• The proposed optical modulation formats offer significant improvements in receiver sensitivity.
• These formats are suitable for long-haul, high-loss optical communication systems.
• The integration with CO-OFDM enhances spectral efficiency and channel compensation capabilities.