Dual modulation of phase and intensity based on cascaded DML and EAM

We propose a dual modulation scheme that integrates phase and intensity modulation, enabled by a directly modulated laser (DML) and an electro-absorption modulator (EAM). Unlike conventional intensity modulation schemes, we employ the frequency chirp of the DML for phase modulation through the time integral. The EAM is utilized for intensity modulation due to its advantages in frequency chirp and extinction ratio performance. To address the phase distortion induced by chromatic dispersion (CD) and bandwidth constraints, we derive a phase damage formula and propose a modified equalization algorithm based on the second-order Volterra equalizer (VE), which can effectively mitigate the differential phase interference. We experimentally validate the feasibility of generating, transmitting, and recovering the dual-modulation signals with high spectral efficiency (SE). Results indicate that compared with the traditional single-stage modulation schemes that employ only a DML or an electro-absorption modulated laser (EML), the dual modulation scheme achieves the maximum transmission rate of 84 Gb/s. Moreover, after applying the second-order VE, the system exhibits strong robustness to variations in DML chirp values, eliminating the need for precise measurement of DML chirp parameters. Additionally, the bit error rate (BER) for BtB and 25-km SSMF transmission exhibits negligible differences. The proposed dual modulation scheme enhances the transmission rate and SE and ensures high compatibility with existing passive optical networks (PONs), providing a cost-effective solution for next-generation optical access networks.