We report broad tunability of a femtosecond (fs) diode-pumped Cr:LiSAF laser mode-locked with a broadband saturable Bragg reflector (SBR). The SBR had seven pairs of Al_xO_y(n∼1.5) and Al_0.17Ga_0.83As(n∼3.5) layers in its Bragg stack, enabling a 250 nm reflectivity bandwidth around 850 nm. A 6-nm-thick strained In_0.15Ga_0.85As quantum well placed between Al_0.17Ga_0.83As cladding layers was used as a broadband saturable absorber in the 800-920 nm wavelength range. The laser was pumped by 6 single mode diodes-four at 640 nn and two at 660 nm. Mode locking was self-starting and fs pulses could be continuously tuned from 800 nm to 905 nm by an intracavity birefringent filter with an out-of-plane optic axis. The pulse widths varied from 70 fs to 255 fs as the laser was tuned. The laser had an 85.5 MHz repetition rate and the output power varied from 80 mW to 180 mW with tuning.

A model of the thermal oxidation kinetics that is based on the continuity equation is adapted to the fabrication of circular saturable Bragg reflector (SBR) mesa structures, as well as inverted mesa array structures. A comparison between theoretical calculations and experimental measurements shows good agreement, making the model a useful tool for the controllability of the structure-dependent oxidation process. The model is applied to the wet oxidation of buried AlAs layers for the fabrication of ultra-broadband, high index contrast, III-V/Al x O y SBRs. The rate of oxidation is found to be dependent not only on the oxidation temperature, but also on oxidation time, mesa geometry, and AlAs layer thickness. The two distinct large-area SBR structures, the circular mesa and the inverted mesa, are fabricated.

A semiconductor Saturable Bragg Reflector (SBR) is a mirror structure comprising alternating layers of high and low refractive index materials with an incorporated saturable absorber. SBRs can be used to initiate and sustain ultra-short pulses in various laser systems. In order to form ultra-short pulses, SBRs with high reflectivity over a broad wavelength range are required. Furthermore, large-area SBRs facilitate easy integration in a laser cavity. One of the key elements for the realization of broad band SBRs is the development of the thermal oxidation process that creates buried low-index AlxOy layers over large areas. The design, fabrication, characterization, and implementation of broad band, high index contrast III-V/AlxOy SBRs in the form of circular mesas, as well as inverted mesa structures, is presented.

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