Capacity-achieving symbol distributions for directly modulated laser and direct detection systems

Area of Science:

• Optical Communications
• Digital Signal Processing

Background:

• Directly Modulated Laser (DML) and Direct-Detection (DD) systems are widely used in optical communication.
• These systems often face constraints on average optical power and peak electrical amplitude due to components like bias tees and electrical amplifiers.
• Probabilistic Constellation Shaping (PCS) is a technique to optimize signal distributions for improved performance.

Purpose of the Study:

• To determine the capacity-achieving symbol distributions for DML-DD systems using PCS.
• To analyze the impact of PCS on system capacity under power and amplitude constraints.
• To experimentally validate the theoretical capacity calculations.

Main Methods:

• Utilized the Blahut-Arimoto algorithm to compute channel capacity and optimal symbol distributions.
• Investigated systems employing Probabilistic Constellation Shaped Pulse Amplitude Modulation (PCS-PAM).
• Conducted experimental demonstrations to verify computational findings.

Main Results:

• PCS marginally increases capacity for DML-DD systems when the optical modulation index (OMI) is below 1.
• PCS enables operation with OMI greater than 1, avoiding clipping distortions.
• PCS significantly increases DML-DD system capacity compared to uniformly distributed signals.

Conclusions:

• PCS is a valuable technique for enhancing the capacity of DML-DD systems.
• The ability to exceed OMI=1 without distortion is a key benefit of PCS.
• PCS offers a pathway to higher data rates in intensity-modulated/direct-detection optical systems.