Passive Mode-Locking of 3.25 μm GaSb-Based Cascade Diode Lasers

Passively mode-locked type-I quantum well cascade diode lasers emitting near 3.25 μm were designed, fabricated, and characterized. The deep etched ∼5.5-μm-wide single spatial mode ridge waveguide design utilizing split-contact architecture was implemented. The epi-side-up mounted 3-mm-long ridge lasers with both 2.7-mm-long gain and 300-μm-long absorber sections forward biased operated in the continuous wave regime with above 12 mW of output power at 20 °C. The devices with a reverse-biased absorber section generated ∼10 ps pulses at the repetition rate of ∼13.2 GHz with average power exceeding 1 mW. The laser emission spectrum in the mode-locked regime comprised a smooth bell-shape-like frequency comb covering about a 20 nm range at the full-width at half-maximum level. Second-order interferometric autocorrelation studies revealed a strong spectral chirp in the pulse. The intermodal beat note of the Lorentzian shape with a 180 kHz line width was observed, corresponding to pulse-to-pulse timing jitter of about 110 fs/cycle. The small degree of the phase-locking was observed in autocorrelation scans even without reverse bias applied to the absorber section. The mapping of the radio frequency spectrum over variable gain and absorber section biases showed switching between two bistable mode-locking regimes at low absorber bias section voltages.