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.

In this paper, a 200-Gb/s free-space optical (FSO) wavelength division multiplexing (WDM) transmission architecture based on multiwavelength directly modulated transmitter optical sub-assemblies for 5G wireless networks has been proposed and demonstrated experimentally. Pulse amplitude modulation-4 (PAM-4) modulation format is adopted in our scheme to further improve system capacity. In order to demonstrate the feasibility of our scheme, the outdoor experiments with 25 Gb/s PAM-4 are tested in free space. With 8 × 25 Gb/s ~50-m outdoor transmission between the two buildings, the eye diagrams and bit error rates (BERs) are tested for one-hop system and forwarding system, respectively. For one-hop system, clear eye diagrams and stable BER performance are demonstrated in all the wavelengths. For the forwarding system, the values of BER with eight channels are measured lasting for 30 min that range from 1E-2 to 1E-3. The experimental results show that the scheme is feasible in FSO transmission system for 5G wireless networks. The advantages of integration, channelization, miniaturization, and low power consumption of the FSO WDM scheme present a promising result on constructing the future fronthaul/backhaul link for wireless cellular networks and optical access network extension.

This work presents a simple microwave photonic downconversion channelizer based on multi-wavelength laser sources. The design of two laser diode (LD) arrays enables signal multiplexing and simultaneous multichannel downconversion processing, which provide stable, relatively flat, and strong multi-frequency combs. A proof-of-concept experiment was taken, showing that 14.375–17.75 GHz broadband radio frequency (RF) signals were successfully downconverted to the same intermediate frequency (IF) and were sliced into four subchannels with 875 MHz bandwidth showing excellent image rejection and channel uniformity, which agrees with the simulation results. The spurious free dynamic range (SFDR) of the proposed RF channelizer is 100 d B H z 2 / 3 , the image rejection is over 28 dB, and the frequency measurement error is less than ± 6 M H z . Replacing optical filters with electrical filters, the proposed simple optoelectronic hybrid reconfigurable microwave photonic channelizer system acquits stable performance and high maturity and meets the application requirements, behaving with stupendous potential in fields such as radar, satellite communication, electronic warfare, and others.

A novel approach to simultaneous radiofrequency self-interference cancellation (SIC) and frequency down-conversion in a unified system is proposed and demonstrated experimentally based on cascade Mach—Zehnder modulators (MZM). The local oscillator signal is injected into the MZM1, whereas the received RF signal and the reference signal are fed into MZM2 and MZM3. After photoelectrical conversion by a balanced photodetector, the desired intermediate frequency signal with SIC is obtained. The cancellation depth for the single frequency SIC with frequency down-conversion is around 38 dB, and that for a wideband signal is around 19 dB. No electrical delay and attenuation are used in the proposed system, so it can potentially reach a wider frequency band and a higher cancellation depth.

A novel microwave photonic true-time-delay (TTD) system using carrier compensation technology is proposed and experimentally demonstrated. Wavelength division multiplexing combines ten lasers into a single beam. We separate one channel from the laser as a compensating carrier, and the compensation carrier is combined with the time-delayed optical signals to be detected. Meanwhile, sideband signals are amplified effectively thanks to carrier-suppressed double-sideband (CS-DSB) modulation. Therefore, the power of both the central optical carriers and sidebands is guaranteed, which produces a better beat frequency result than the TTD system without carrier compensation. The simulation results confirm that the signal amplitude has an order of magnitude improvement due to the compensation. With employing the delay fibers based on multiple-fiber Bragg gratings (MFBGs), the experimental delay and response time reach 90.160 μs and 160.80 ns. The proposed technique can find applications in time-delay beamforming of phased array antennas (PAAs).

A 16-channel photodetector array receiving module with good performance as low loss, fine tuning and high reliability is manufactured in our laboratory. The whole module was consisted with a silica arrayed waveguide grating chip (AWG), a thermo-optical type variable optical attenuator (VOA), a photodetectors (PDs) array with direct-current (DC) bias circuit, all components were integrated via 3D package method. In the article, the structure parameters of Y branch and edge couplers were optimized for lower loss. In order to realize excellent tuning ability, Titanium was selected as heating material for precisely tuning and its electric attenuation range could reach at a level more than 20 dB. Periodically segmented waveguide coupler designed in passive chips could minimize coupling loss less than 0.4 dB/coupling. The high-performance receiving module could effectively improve detection quality and significantly enlarge capacity in communication system.