Polarization Control Of High-power Vertical-cavity Surface-emitting Lasers

Vertical cavity surface emitting lasers become more and more attractive inoptical interconnection, optical signal processing, pumped solid state laser, lightcoupling output,etc, mainly because vertical cavity surface emitting laser has moresuperior performance than edge emitting laser including： very low thresholdcurrent;single mode output;circular spot;very convenient array integration, etc.Especially, high power vertical cavity surface emitting lasers have made outstandingachievements recent years, the output power of single device was refreshed again andagain,since ciomp reported the output power of single device reached1.95W.High-power VCSELs with stable single polarization are particularly attractive forapplications including laser display, communication and sensing. Strong polarizationselectivity is especially required for intra-cavity frequency doubling to achieve highefficiency and high power operation. Cylindrically symmetrical VCSELs are inclinedto polarization instabilities due to the symmetrical current distribution across theactive region and furthermore the symmetrical gain distribution. Several approacheshave been presented to improve the polarization properties of VCSELs. Theseapproaches can be divided into three categories: external cavity optical feedback,externally applied stress, and non-isotropic gain. However, the approaches above areall applied to small aperture device due to its simple mode characteristics. Apparentlythe output power is limited due to the small aperture of VCSEL. Few investigationshave been reported for the polarization control of large aperture VCSELs because of complex high order transverse mode characteristics, which are resulted from largeaperture and non-uniform current distribution across the active region of VCSELs.Rectangular aperture of non-isotropic gain was used by our group to increase thepolarization selectivity of large aperture VCSELs. An output power up to660mW wasrealized at a current of5A and the highest polarization ratio can be up to2. On theother hand, rectangular aperture of VCSELs has an obvious drawback of noncircularoutput beam, which might limit their application where high power density or highfiber coupling efficiency was required.After analysis，sub-wavelength metal-grating microstructure is an appropriatecandidate to introduce non-isotropic gain. When the period of the grating is muchsmaller than the incident wavelength, the light polarizing perpendicular to the stripesis transmitted through the grating and only the light polarizing along the stripes isreflected back to establish the laser oscillationFirst of all, analyse the merits and demerits of severval methods which cancontrol the polarization of VCSELs. And the metal-graing is a useful method whencontroling the polarization of high power VCSELs.Then analysed the poarization modes of VCSELs, the output and internalcharacteristics of sub-wavelength metal-grating VCSEL theroy in theroy. Putforward a new mathod oxide grating which can not only control the polarization butalso tanserve mode. And the method of oxide grating was simulated by comsolmulti-physics infinit elment software. As a result, oxide grating structure maycontrol the polarization of high power VCSEL well after simulated.In this work, we presented an approach to achieve higher non-isotropic gain andfurther highly selective polarization of large aperture high-power VCSELs, and thehigh-order of large aperture VCSEL can be suppressed to some extent. Here thepolarizations vibrating along and orthogonal to <110>. The pairs of P-side DBRswere reduced to decrease the reflectance of two orthogonal polarizations andaccordingly the threshold current of both orthogonal polarizations was increased. Bycoupling the additional reflectance which was provided by sub-wavelength metal-grating, the threshold gain of TE was decreased remarkably compared withTM.The grating with a period of186nm and a duty ratio of0.5was fabricated on theGaAs-cap layer to provide additional reflectance for H-polarization. The pairs ofP-DBRs were reduced to realize the maximum difference threshold gain of twoorthogonal polarizations. Polarization ratios of3, an output power of860mW atcontinuous-wave operation at room temperature were demonstrated, and thehigh-order modes were suppressed, as a result the far-field beam divergence wassuppressed to12°.We get the result that both two polarizations apear red shift phenomenon fromthe spectrogram. The center wavelength of two polarization are978.5nm，981.5nm；982.5nm,982.8nm；985.2nm，984.5nm respectively when the input current are1.2A、2.2A、3A. The obvious improvement is the far-field beam divergence of12.9°and12°[full-width at half-maximum(FWHM)] for metal-grating VCSEL compared withthe beam divergence of16.2°and15.6for conventional VCSEL device without themetal-grating at a current of5A.To forther improve the polarization ratio of two polarizations, the pairs ofP-DBRs were reduced and the GaAs-cap between grating stripes was etched to forcethe current to be injected linearly along grating stripes to realize the maximumnon-isotropic gain. A polarization ratio of4.8, an output power of780mW and hightemperature performance were demonstrated for a550um aperture device.