# Advanced Modulation Formats for 400 Gbps Optical Networks and AI-Based Format Recognition
### Prior Works for Optical Modulation Technologies  
Advanced optical modulation formats are crucial to the development of high-speed optical networks. They can meet the continuously growing demand for system data throughput and improve spectrum efficiency. However, multi-level modulation not only requires a higher optical signal-to-noise ratio (OSNR) but is also more sensitive to fiber nonlinear effects and laser phase noise, both of which limit the maximum transmission distance of optical networks. To date, there has been much new research progress in the modulation format field of high-speed optical communication systems in the industry [3; 4; 5; 6]. Studies have shown that 50 Gb/s alternate polarization frequency shift keying (Apol-FSK) signals exhibit better transmission performance than carrier-suppressed return-to-zero frequency shift keying (CSRZ-FSK) and return-to-zero differential phase shift keying (RZ-DPSK) signals, making them one of the most promising solutions for future high-speed transmission systems [3]. A transceiver system using radio over fiber-based polarization division multiplexing differential quadrature phase shift keying (RoF-based PDM-DQPSK) DWDM was developed, and simulation results indicated that a maximum data rate of 1.792 Tbps can be achieved using 64-channel DWDM technology, with a maximum transmission distance extendable to 1600 km at a bit error rate (BER) of \(10^{-12}\)[4]. An all-optical wavelength and format conversion scheme based on a delay line interferometer (DLI) and a pump-assisted nonlinear optical loop mirror (NOLM) can convert a 20 Gbps differential quadrature phase shift keying (DQPSK) signal to a four-level pulse amplitude modulation (PAM4) signal. This scheme can be deployed in all-optical gateways to bridge long-distance and short-distance optical networks [5]. A 50 Gbps quadrature phase shift keying (QPSK) and a 100 Gbps sixteen quadrature amplitude modulation (16 QAM) signal have been demonstrated through all-optical conversion while maintaining an error vector magnitude (EVM) below 10% with a required bandwidth of 55 GHz. The results showed that the converted QPSK and 16 QAM signals can be transmitted to 19,500 km and 4000 km, respectively, in single-channel and eight-channel transmission by maintaining a sufficient 62.5 GHz channel spacing [6]. Currently, more and more research efforts are focused on modulation format conversion, and there is still room for further exploration in the research of advanced modulation formats with high nonlinear resistance. FSK modulates signals by different frequencies with constant optical power, inherently exhibiting high nonlinear resistance, and has received more attention from researchers in recent years [7; 8; 9]. By integrating technologies such as FSK, alternating polarization, and CRZ, solutions can be provided for the development of high-speed computing optical networks. This paper proposes a noncoherent solution based on the Apol-CRZ-FSK modulation format for achieving a 4 \(\times\) 100 Gbps DWDM optical network. The narrow channel spacing in a DWDM system exacerbates nonlinear effects, necessitating a modulation format with high nonlinear tolerance to enhance system performance. To address this challenge, we conducted comprehensive research on Apol-CRZ-FSK in a DWDM context and compared to traditional CRZ-FSK and DQPSK modulation formats, Apol-CRZ-FSK exhibits superior nonlinearity resistance, supporting longer transmission distances and better transmission performance. Specifically, when the input optical power is low and nonlinear effects are negligible, the performance advantage of the Apol-CRZ-FSK modulation format in a 4 \(\times\) 100 Gbps DWDM system is not significant. However, as the optical power increasesand the transmission distance extends, the Apol-CRZ-FSK signal demonstrates better nonlinearity resistance, especially when nonlinear effects become the primary factor affecting system performance.