Directly Modulated C-band High-Power Photonic-Crystal Surface-Emitting Lasers for Free-Space Optical Communications

Photonic-crystal surface-emitting lasers (PCSELs) are a new type of semiconductor laser employing two-dimensional photonic crystals, offering high output power, narrow divergence angles, and coherent light generation, making them highly suited for free-space optical (FSO) communications. However, studies in the C-band remain limited despite its industrial importance. Here, we report the direct modulation performance of C-band InP-based PCSELs and demonstrate lens-free FSO communication. Both amplitude modulation (AM) with direct detection and frequency modulation (FM) with coherent detection are investigated to demonstrate the broad applicability of C-band PCSELs. Analysis of the relative intensity noise (RIN) spectra and small-signal frequency responses reveals a relaxation oscillation frequency of 6 GHz and a corresponding intrinsic bandwidth of approximately 9 GHz at output powers of ∼ 350 mW, indicating potential for high-power direct modulation. In FSO experiments, lens-free AM transmission achieves up to 3 Gbaud non-return-to-zero (NRZ) signaling with a link budget exceeding 43 dB. Furthermore, FM/coherent transmission utilizing the frequency change caused by the PCSELs' chirp effect, with heterodyne detection, achieves larger link budgets exceeding 67 dB for 1 Gbaud NRZ. These results highlight the potential of our C-band PCSELs for lens-free, optical-amplifier-free FSO communication in both terrestrial and satellite systems.