Proton Radiation Effects on Mode-Locked Operation of Three-Section AlGaInAs/InP Lasers for Space Applications

We report, for the first time, an experimental investigation of proton radiation-induced effects on a three-section AlGaInAs/InP mode-locked laser diode (MLLD), using 1.8 MeV protons at a fluence of 1 × 10¹² /cm². Post-irradiation characterization shows stable passive mode-locking operation with improved overall performance. The threshold current increased and slope efficiency decreased due to defect-induced nonradiative recombination and absorption, yet the pulse width remained nearly unchanged at 1.44 ps with an improved time–bandwidth product of 0.438. Optical spectra exhibited a redshift, linewidth broadening, and better optical frequency comb tooth coherency. Fixed-point analysis demonstrated altered dispersion characteristics and enhanced tunability, indicating potential benefits for drift compensation and Doppler shift management in intra-satellite optical links. Multi-tone optical injection locking further confirmed robust post-irradiation performance, with stronger coherency transfer, improved RF sideband suppression (13.9 dB to 34.6 dB), narrower pulses (1.60 ps to 1.44 ps), and reduced time–bandwidth products, albeit requiring slightly higher injection power. These results indicate that this specific three-section MLLDs maintains functionality after proton exposure and exhibits features advantageous for compact, high-coherence comb generation in satellite and deep-space optical communication systems.