The study demonstrates C-band InP-based photonic-crystal surface-emitting lasers (PCSELs) as high-power, narrow-divergence light sources for lens-free free-space optical (FSO) communication. PCSELs leverage double-lattice photonic crystals in the InGaAsP multiple-quantum-well active layer to produce single-mode, near-diffraction-limited beams (M² ≈ 1.1) at ~1543 nm with output powers exceeding 350 mW (pages 3–4, Figures 1–2).
Device characterization shows a relaxation oscillation frequency (fᵣ) > 6 GHz and an intrinsic modulation bandwidth of ~9 GHz, extracted from relative intensity noise (RIN) measurements and small-signal frequency response (pages 3–4, Figures 3–4). This indicates potential for 10 GHz-class direct modulation suitable for high-speed communication.
Amplitude-modulation (AM) FSO experiments demonstrate lens-free transmissions up to 3 Gbaud NRZ, achieving link budgets of 43–47 dB over 20–80 cm without optical amplifiers or collimating lenses (pages 5–6, Figures 5–6). Increasing DC bias current alone does not improve the link budget due to reduced modulation depth, highlighting the importance of impedance matching and RF optimization for higher AM performance.
Frequency-modulation (FM) and coherent-detection experiments exploit the inherent chirp of PCSELs, enabling 1 Gbaud NRZ transmission with link budgets up to 67 dB (pages 6–7, Figures 7–9). FM modulation depth scales with RF drive amplitude, effectively leveraging high optical output power and achieving superior sensitivity compared to AM.
The study estimates virtual transmission distances of ~6 km for AM lens-free FSO and ~60 km for FM/coherent-detection schemes, indicating strong potential for terrestrial and inter-satellite communications. The authors note that scaling device aperture and optimizing in-plane optical coupling are necessary to reach watt-class C-band PCSELs for broader FSO deployment (pages 7–8).
Conclusion: C-band InP PCSELs exhibit high output power, narrow beam divergence, GHz-class modulation bandwidth, and lens-free FSO capability, offering a promising platform for high-speed, amplifier-free, and long-range optical communication systems.
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