Frequency-modulated high-power photonic-crystal surface-emitting lasers for long-distance coherent free-space optical communications

Abstract High-power coherent optical transmitters with high-speed controllability are in demand for a number of cutting-edge applications, including intersatellite communications and deep-space optical communications. The conventional transmitters used in these applications require many bulky optical components besides their semiconductor laser sources, such as fibre-optical amplifiers, external phase modulators, optical fibres and beam-collimation lenses, which are obstacles in achieving compact and efficient systems. Here we propose and experimentally demonstrate compact coherent optical transmitters based on frequency-modulated photonic-crystal surface-emitting lasers (PCSELs) towards achieving long-distance free-space optical (FSO) communications. We design two-section PCSELs that incorporate two photonic crystals with slightly different band-edge resonant frequencies, and we realize watt-class frequency modulation with suppressed amplitude modulation via anti-phase current injection into the two sections. Using the above two-section PCSELs as coherent optical transmitters, we demonstrate fibre-amplifier-free FSO communications with Gbps-class bandwidth, even when the laser power is attenuated by >80 dB. Our work opens avenues toward the realization of one-chip coherent optical transmitters whose volume and weight are several orders of magnitude smaller than conventional bulky systems for a wide variety of coherent free-space laser applications.