We use the iterative split-step Fourier method to simulate the eight quadrature amplitude modulation all-optical orthogonal frequency division multiplexing (OFDM) system employing different constellation design schemes. It is found that properly narrowing down the modulus difference between constellation points has an impact on the whole system’s ability to resist nonlinear effects. The results show that by means of changing the modulus values of the constellation points, the system bit error rate will be significantly reduced when the optical signal-to-noise ratio is high. Furthermore, by comparing four different constellation diagrams, we propose an alternative solution to reduce the distortion brought by the nonlinear effects on all-optical OFDM systems, that is, the constellation points can be appropriately redesigned to improve the performance of high nonlinear effect systems.

A hybrid shaping scheme, carrier-less amplitude and phase modulation geometric-probabilistic hybrid shaping 16QAM (CAP-GPS-16QAM), is proposed for data center networks. The probabilistic shaping is achieved by applying the proposed probabilistic subset shaping to the constellation after geometric shaping (GS). An intensity modulation/direct detection experimental system of 5 GBaud is built to verify the proposal. The results show that the gains of the proposed scheme are 2.4, 2.6, and 3 dB compared with CAP-GS-16QAM, CAP-PAS-16QAM, and CAP-square-16QAM, respectively, at BER  =  10^−  3. Meanwhile, in terms of generalized mutual information, the proposal improves the performance by 0.078, 0.125, and 0.153 bits/symbol compared with CAP-GS-16QAM, CAP-PAS-16QAM, and CAP-square-16QAM, respectively.