Principal Investigator Leslie Kolodziejski
Project Website http://web.mit.edu/cbegroup/www/SBRs.html
Broadband Saturable Bragg Reflectors (SBR) consisting of monolithically integrated absorbers onto GaAs-based Bragg-mirrors have been used in a variety of ultra-short pulse lasers. The absorber, high-index layers and layer thicknesses are selected based on the laser's wavelength. The low-index layer can either be aluminum oxide that was created by the wet oxidation of AlAs layers or high Al content AlGaAs layers.
Infrared SBRs are composed of AlGaAs/AlxOy mirrors with InGaAs-based absorbers which strain the structure and, depending on the absorber thickness, may lead to delamination during the AlAs oxidation process. For oxidation temperatures between 410°C and 435°C, delamination occurs between the absorber and mirror layers. More severe delamination occurs at higher oxidation temperatures. In an alternate SBR design, the additional strain introduced by the InP cladding layers generally increases the observed amount of delamination. A controlled temperature ramp before and after oxidation has greatly reduced the delamination of the SBR structures despite the presence of strain.
The same AlAs oxidation technique also enables the fabrication of visible SBRs. Using In0.5Ga0.15Al0.35P as the high-index layer and AlxOy, Bragg mirrors are created for operation below 800 nm. Along with a GaAs absorber layer, these visible SBRs are nominally unstrained and may mode-lock a variety of lasers including Ti:Sapphire, Cr:LiSAF, Cr:LiCAF, and Cr:LiSGaF.
A variety of SBRs with GaAs/AlGaAs distrubuted Bragg reflectors were grown for use in an Er-Yb laser. The saturable absorber sections contain either one or two InGaAs quantum well(s); the InGaAs quantum wells are clad with either GaAs or InP. The reflectivity characteristics of the different saturable Bragg reflectors were assessed. As the number of InGaAs quantum wells increased from one to two, the overall reflectivity decreased due to the absorption of light within the additional InGaAs quantum well. Although the SBRs were designed to mode-lock the Er-Yb laser, only one of the SBRs actually mode-locked the laser. The two SBRs with InP cladding layers and the SBR with a single quantum well with GaAs cladding layers exhibited both Q-switching as well as mode-locking. The two SBRs with two InGaAs quantum wells with GaAs cladding layers should have yielded similar results, however, only one of the samples successfully mode-locked the laser. Further optical measurements of the SBRs are underway.