This paper presents a review and discussion of the directly modulated semiconductor lasers and their applications to optical communications and microwave photonics. A detailed and comprehensive demonstration of directly modulated semiconductor lasers from development history to specific techniques on measurement, analysis, and packaging is provided for the first time to the best of our knowledge. A few typical applications based on directly modulated lasers are also illustrated, such as optical fiber communications, free-space optical communications and microwave photonics. Future directions of research are also highlighted.

We summarized the design, fabrication challenges and important technologies for multi-wavelength laser transmitting photonic integration. Technologies discussed include multi-wavelength laser arrays, monolithic integration and modularizing coupling and packaging. Fabrication technique requirements have significantly declined with the rise of reconstruction-equivalent-chirp and second nanoimprint mask technologies. The monolithic integration problem between active and passive waveguides can be overcome with Butt-joint and InP array waveguide grating technologies. The dynamic characteristics of multi-factors will be simultaneously measured with multi-port analyzing modules. The performance of photonic integration chips is significantly improved with the autoecious factors compensation packaging technique.

112Gbps per wavelength, amplification freeFour-level Pulse Amplitude Modulation (PAM-4)transmissions are experimentally demonstrated withadvanced digital signal processing (DSP) algorithms. TwoDSP architectures are investigated for Directly ModulatedLasers (DMLs): (1) the regular DSP with Feed ForwardEqualization (FFE) and Decision Feedback Equalization(DFE), and (2) the enhanced DSP with MaximumLikelihood Sequence Estimation (MLSE) equalizer. Theexperimental results show that with advanced DSPtechnologies the conventional DML can achieve 40kmtransmissions with Bit Error Rate (BER) under 2.4 × 10 $^{-4}$ that meets RS(544, 514) Forward Error Correction (FEC)requirement; with regular-DSP the advanced DML canachieve 40km transmissions with BER under 3.8 × 10 $^{-3}$ , withstronger FEC. 112Gbps and beyond per lambda DMLbased transmission is a promising low power, low-costsolution for 800GbE or 1.6TbE Ethernet.

Originating from the non-Hermitian quantum mechanics, the parity-time (PT) symmetry has been extensively explored to ensure a stable single-mode oscillation for optoelectronic oscillators (OEOs), while avoiding additional extremely challenging ultrahigh-Q filters. Enabled by the emerging photonics integrated circuit (PIC) technology, we report the tunable PT-symmetric OEO by using an integrated mode-locked laser, as a striking advance to reduce the footprint of OEOs. Stable single-mode oscillating signal at 24.5 GHz has been generated with a phase noise of -108 dBc/Hz @ 10 kHz frequency offset. In particular, a tunable frequency range from 24 to 25 GHz is achieved by manipulating the injection currents into the well-designed on-chip linear laser cavity. This work exhibits a breakthrough in conceptual integrated microwave photonic devices, and also accelerates the real-world applications of fundamental physical theory of PT symmetry to information industry.

Indoor optical wireless communication with optical beamsteering capability is currently attracting a lot of attention. One major two-dimensional (2D) optical beamsteering scheme is realized by 2D grating or its active counterpart, which is usually based on a spatial light modulator (SLM). However, there is a fundamental trade-off between the field of view (FoV) and power efficiency due to the inherent feature of gratings. In this Letter, we propose a new class of 2D beamsteering, named cyclically arranged optical beamsteering (CAO-BS), which can break that trade-off. Traditional 2D gratings extend the optical beam in the Cartesian coordinates (1D grating in horizontal + 1D grating in vertical), while CAO-BS extends the optical beam in the polar coordinates (1D grating + angular rotation). Since only 1D grating is engaged, the power efficiency increases with the number of grating lobes reduced. In the polar coordinates, the angle rotation tuning in a SLM is quasi-continuous in a full $2\pi$ range. The CAO-BS is demonstrated at the receiving end in an indoor experimental system. The FoV is 18° by 360° in polar coordinates without any additional mechanical parts. Based on the CAO-BS, 40 Gbit/s on-off keying data is also successfully transmitted over 1 km single-mode fiber and 0.5 m free space.

A widely tunable narrow-linewidth laser consisting of a sampled-grating distributed Bragg reflector (SG-DBR) laser and an external silica-based multi-period-delayed feedback (MPDF) photonic circuit is fabricated and experimentally demonstrated. The measured results show that a 58-nm lasing wavelength tuning range together with a more than 40 dB side-mode suppression ratio can be achieved. Notably, among the wavelength tuning range, all linewidths remain below 1 kHz, which are narrowed by more than three orders of magnitude compared to the average intrinsic linewidth of 3 MHz for the original SG-DBR laser. Besides, since the proposed MPDF structure has an ultra-dense distribution of the multi-period feedback, the linewidth reduction capability can be maintained over the entire operating wavelength range does not require free spectral range (FSR)-alignment or any extra phase match. Such widely tunable narrow-linewidth laser might offer a compact, cost-effective and easily-implemented solution for next-generation applications including ultra-wideband dense wavelength division multiplexing (DWDM) systems and light detection and ranging (LiDAR) systems.