Investigations On The Vertical-cavity Surface Emitting Laser Operating At High Temperatures

The unique geometry of VCSELs results in several significant advantages overtheir edge-emitting counterparts, including lower threshold current, circularoutput-beam profile, longitudinal single-mode and stable operation at hightemperatures et al. Due to these good performances, VCSEL has become the keylight source for the Chip-scale Atomic Clock （CSAC） system, which was developedsince2000. Requirements for VCSELs used in CSAC were as follows: firstly, theVCSEL must operate at high temperature of65°C-80°C reliably; secondly itsthreshold current should be at the magnitude of mA to realize the low powerconsumption. To meet these requirements, investigations focused on the analysis of795nm VCSEL performances at high temperatures, structure design for hightemperature operation and device fabrication et al were carried out in this thesis. Themain research and its achievement were as follows:1. To satisfy the demand for high temperature operation, the VCSEL structurewith gain peak-cavity mode offset was designed. The major design was as follows.The active region was finely considered, including the theoretical analysis on theconfinement factor of carrier in quantum well at high temperature. Then the gain ofactive region changing with the temperature was gained. The reflectivity ofDistributed Bragg Reflectors was well designed to achieve the high efficiency andlow threshold current of VCSEL device. And the temperature dependence of cavity mode was showed utilizing the Transfer Matrix Method （TMM）. The designedVCSEL structure included the8nm Al_0.09Ga_0.91As quantum well and8nmAl_0.36Ga_0.64As barrier, with779nm gain peak wavelength at room temperature. Thecavity mode of VCSEL at room temperature was designed at790nm, with the DBRreflectance production of98.5%.2. By analyzing the mechanism of heat generation in active region, thermalmodel for calculating the self-heating induced by the injected current was built upand coupled into the COMSOL software to analyze the temperature distributionwithin VCSELs. The thermal model proposed by us was firstly tested and verified byanalyzing an edge emitting laser. The reliability of our thermal model wasdemonstrated from the deviation of less than0.5K between the simulation andexperiment result. This thermal model was then used to compare the thermaldistribution within VCSELs with different mesa structures. Thermal simulationsdemonstrated that the enhanced lateral heat dissipation and decreased seriesresistance within VCSELs could be gained by employing the self-planar mesa. Thusthe self planar mesa structure was more favored in fabricating VCSELs operating athigh temperatures.3. VCSEL devices with different oxide apertures were fabricated utilizing theself-planar mesa structure. The VCSEL with7μm oxide aperture performed the2mAthreshold current at80°C, and themaximum output power of VCSEL with9μm oxideaperture was1.1mW. The threshold current firstly decreased and then increased withincreasing temperature, corresponding with the theoretical prediction. The outputwavelength of795.2nm was gained at80°C, and0.09nm Full Width at HalfMaximum （FWHM） was gained.4. The DBR microlens was proposed to be integrated on the VCSEL structure toimprove its optical mode characteristics of VCSELs, and related experiments werecarried out. The fabricated DBR microlens of35μm aperture showed500μm radiusof curvature and the roughness less than2nm was obtained. These initialexperimental results laid the foundation for directly integrating DBR microlens on VCSEL devices. In addition, Vertical external cavity surface emitting lasers（VECSEL） fabricated by the VCSEL with DBR microlens was proposed, and morenarrow linewidth of output spectrum was expected.