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.

A broadband and compact TE-pass/TM-stop polarizer is presented based on hybrid plasmonic grating (HPG) on an x-cut Lithium-Niobate-on-isolator (LNOI) platform. By comprehensively analyzing the effects of metal width on mode effective index, mode similarity, and mode conversion, we demonstrate the structure with a narrower metal width. The simulation results indicate that the polarizer with a compact length of 9 μm achieves an extinction ratio over 20 dB within the wavelength range from 1470 nm to 1700 nm. The insertion loss is below 2.3 dB in C-band. Furthermore, the polarizer exhibits large fabrication tolerance to current fabrication technology.

Radar warning receiver (RWR) plays a significant role in detecting weapon radar. The instantaneous frequency measurement (IFM) section with fast measuring response, large measurement bandwidth, moderate accuracy, and sensitivity is important for an all-threat-covered RWR. In this IFM method, a phase modulator (PM) in combination with a differentiation structure is employed to get rid of the DC drifting and makes the power response is a strict monotonous function of the input frequency. Therefore, the unambiguous determination can be made in a very large range, which guarantees the fast measuring response. According to the theoretical analysis, a strict monotonous amplitude comparison function (ACF) with the covering range higher than 40 GHz can be achieved. Besides, no filter or dispersion elements (DE) such as optical fibers are required, which makes the structure be very concise to meet the potential for integration in any receiving systems. Due to the limit operation range of the PM, only the range from 0.1 GHz to 25 GHz is demonstrated. The measurement error can be lower than 3%. Especially, benefited by the balance photo-diode (BPD), the sensitivity is good without any noise cancellation processing. The error of lower than 10% will be promised when the RF power level is - 40 dBm.

Semiconductor lasers are widely used in incoherent/coherent optical communications, optical fiber sensing, and lidar due to their small size, high efficiency, and immunity to vibrations. To support the continuous evolution of these high-performance application systems, the noise characteristics of semiconductor lasers play a key role and the noise levels need to be reduced to unprecedentedly low values (such as quantum limit). In this paper, a comprehensive overview on the principles, measurements, and suppressions of semiconductor laser noises are provided. First, we introduce the analysis models and key performance indicators of semiconductor lasers related to noise. Then, different noise measurement systems are investigated for semiconductor lasers and the noise characteristics are fully analyzed. Next, the noise suppression methods towards semiconductor lasers are demonstrated from three aspects: the chip structures, the external cavity optical feedback technology and the driving and controlling circuits. Finally, we summarize the recent advances on the noise measurement and suppression of semiconductor laser, future prospects are also discussed.

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.

In this article, a reconfigurable mixer based on only one dual-polarization quadrature phase shift -keying modulator (DP-QPSKM) is proposed and comprehensively demonstrated for a radio-over-fiber (RoF) system. By changing the detection unit and adjusting the dc voltages, the single-ended, double-balanced, image (IM)-free, mixing spurs suppressed, and dispersion-immune mixing can be implemented. Experimental results show that the ~60-dB IM rejection ratio and the spur suppression ratio are achieved. Moreover, the dispersion-induced power fading is successfully compensated by the dc voltage adjustment. The spurious-free dynamic range (SFDR) is about 100.56 dB· Hz2/3. Only one optical path is required without extra optical filters or phase-related devices, which makes the structure concise and stable. This structure would be more compact to meet the potential of integration if a dispersion medium with smaller size is available.