Mode Locked GaAs/AlGaAs Quantum Well Lasers With External Reflectors for Tunable THz Signal Generation

This dissertation is concerned with the design, fabrication and characterization of 850 nm GaAs/AlGaAs quantum well mode locked lasers with tunable repetition rates. The 850 nm wavelength is located in the operation range of the commonly used low temperature grown GaAs photomixer for THz generation. A novel approach to achieve a tunable repetition rate was developed by forming a tunable semiconductor-air compound cavity using external reflectors. To improve the reflection effect, an optimized far-field reduction layer was inserted into the n side cladding layer to reduce the far field divergence. The relation between the repetition rate and the compound cavity length was studied at first. Then the results from mode locked lasers with different saturable absorber lengths were compared. Furthermore, the influence of the saturable absorber position in the laser cavity on the mode-locking behavior were investigated. Finally, a 500 μm mode locked laser was fabricated and measured. The device was able to tune from the fundamental mode locking frequency 76 GHz to the 32nd harmonic mode locking frequency at 2.4 THz, which is the highest harmonic mode locking frequency for a semiconductor mode locking laser at 850 nm to author’s best knowledge. With the harmonic frequencies achieved, the device developed in this dissertation shows potential to work with low temperature grown GaAs photomixers for tunable THz signal generation.