Non-uniform bit-manipulation 3D probabilistic shaping method over hollow-core fiber transmission

This paper proposes a three-dimensional (3D) probabilistic shaping (PS) method based on non-uniform bit manipulation (NUBM) for hollow-core fiber transmission. The proposed NUBM involves bit-level preprocessing to achieve a non-uniform manipulation of '0's and '1's in the amplitude bits. Subsequently, Gray mapping is employed to generate PAM4 symbols, resulting in a higher probability for the two inner PAM4 signal levels. This effectively lowers the average transmission power and moves the system nearer to the channel capacity limit. Three such probabilistically shaped PAM4 (PS-PAM4) signals are then combined and mapped into a 3D-64QAM constellation with shaping gain. The scheme was experimentally validated over a 1.5 km hollow-core fiber link, achieving a 15.75 Gb/s signal transmission. Under the condition of maintaining the same net data rate, the experimental results show that at the 7% hard-decision forward error correction (HD-FEC) threshold (BER = 3.8 × 10^-3), the proposed 3D-PS-64QAM scheme gains receiver sensitivity improvements of 1.7 dB and 3.2 dB compared to conventional uniform 3D-64QAM and 2D-PS-64QAM, respectively. This study validates the effectiveness of the described 3D non-uniform bit manipulation in a hollow-core fiber environment, offering a high-performance transmission solution for next-generation high-speed optical communication systems.