With the advancement of photonic integration technology, ultra-low linewidth frequency-stabilized lasers have demonstrated significant potential in precision measurement, quantum communication, atomic clocks, etc. This review summarizes the latest developments in integrated photonics for achieving ultra-low linewidth lasers, particularly breakthroughs made by integrating Brillouin lasers. We discuss the design principles, manufacturing processes, performance characteristics, and potential value of these lasers in various applications.

Nonlinearity impairments and distortions have been bothering the bandwidth constrained passive optical network (PON) system for a long time and limiting the development of capacity in the PON system. Unlike other works concentrating on the exploration of the complex equalization algorithm, we investigate the potential of constellation shaping joint support vector machine (SVM) classification scheme. At the transmitter side, the 8 quadrature amplitude modulation (8QAM) constellation is shaped into three designs to mitigate the influence of noise and distortions in the PON channel. On the receiver side, simple multi-class linear SVM classifiers are utilized to replace complex equalization methods. Simulation results show that with the bandwidth of 25 GHz and overall bitrate of 50 Gbit/s, at 10 dBm input optical power of a 20 km standard single mode fiber (SSMF), and under a hard-decision forward error correction (FEC) threshold, transmission can be realized by employing Circular (4, 4) shaped 8QAM joint SVM classifier at the maximal power budget of 37.5 dB.

We propose an access network that integrates fiber and wireless for mobile fronthaul (MFH) with simple protection capabilities, using dual-mode orbital angular momentum (OAM) multiplexing. We experimentally demonstrate a 3.35 Gbit/s DMT-32QAM pre-equalized system with 10 km and 15 km fiber links in the 5.9 GHz band; then there is a link of two channels with a 0.5 m wireless link.