Research: Integrated Photonic Devices and Materials Group

Research Activities of the Integrated Photonic Devices and Materials Group

Components for Ultra-Short Pulse Laser Systems

SBR suitable for a
Ti:sapphire laser.

SEM Cross Section of a
SBR with a Al_xO_y/GaAs
distributed Bragg reflector.

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. Reflectivity measurements of a variety of fabricated SBRs are shown in below.

Infrared SBRs are composed of AlGaAs/Al_xO_y 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 In_0.5Ga_0.15Al_0.35P as the high-index layer and Al_xO_y, 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.

Reflectivity measurements of SBR structures fabricated for three different laser systems: Ti:Sapphire, Cr:Forsterite, and Er-doped fiber (EDF) laser with theoretical mirror reflectivity greater than 99% over 294nm, 466nm, and 563nm respectively.

Last update:
9/1/07