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Doctoral Defense

High power, narrow spectrum and passively mode-locked GaSb-based type-I quantum well cascade diode lasers

Tao Feng

December 7, 2018
2:30 PM
Light Engineering room 250
Advisor: Prof. Leon Shterengas

The GaSb-based type-I quantum well (QW) diode lasers can operate in continuous wave regime (CW) regime at room temperature (RT) in spectral range from 1.9 to 3.5 µm. Cascade pumping of type-I QW gain sections was utilized to increase output power and efficiency of GaSb-based diode lasers. Two-stage cascade diode lasers operating near 2 µm demonstrated threshold current densities ~80 A/cm 2 and CW output power ~2 W per 100-µm-wide stripe and ~ 250 mW from ~ 5-µm-wide narrow ridge apertures. The use of three-stage cascade active region yielded devices with 350 mW in multimode beams at 3.25 µm. The optimization of the single spatial mode emitting lasers design included fabrication of the two-step shallow narrow and deep wide ridges. For 3.25-µm-emitting lasers, the single step deep etched ridges also generated tens of mW of output power level but showed higher thresholds compared to two-step designs. Notably all narrow ridge devices operated also in epi-up configuration generating tens of mW of CW power at 20 °C and above. Laterally coupled distributed feedback (LC-DFB) narrow spectrum lasers using 2 nd order gratings generated above 10 mW of tunable single frequency CW power at 20 °C near 3.22 µm. Tuning range from 3220 to 3230 nm with tuning rates of ~21 nm/A (epi-side-up) / ~11 nm/A (epi-side-down) and ~0.32 nm/K were observed. The 6th order LC-DFB devices generated near 4 mW of CW power at 15 °C.

Passively mode-locked type-I quantum well cascade diode lasers emitting near 3.25 µm utilizing split-contact architecture were demonstrated. The devices with 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 mode-locked regime comprised a smooth bell-shape-like frequency comb covering about 20 nm range at the full-width-at-half-maximum level. Second order interferometric autocorrelation studies revealed strong spectral chirp in pulse. The intermodal beat note of the Lorentzian shape with 180 kHz linewidth was observed corresponding to pulse-to-pulse timing jitter of about 110 fs/cycle.