Includes bibliographical references (page 756).Direct measurement of scattered fields from oxide confinement apertures in vertical-cavity lasers is presented. Diffraction fringes associated with each transverse lasing mode are detected in the far field from devices with varying oxide aperture dimensions and with quantum efficiencies as high as 48%. The diffracted pattern symmetries match the rectangular symmetry of the oxide apertures present in the devices and fringe locations are compared to Fraunhofer theory. The fraction of power diffracted from the lasing mode remains roughly constant as a function of relative pump rate, but is shown to depend on both transverse mode order and oxide aperture size.This work was supported by the NSF–EPSCOR, NASA–EPSCOR, ILX Lightwave, and the DOE

Carlsten, J. L.

Kilper, D. C.

Lear, Kevin L.

Roos, P. A.

Direct measurement of scattered fields from oxide confinement apertures in vertical-cavity lasers is presented. Diffraction fringes associated with each transverse lasing mode are detected in the far field from devices with varying oxide aperture dimensions and with quantum efficiencies as high as 48%. The diffracted pattern symmetries match the rectangular symmetry of the oxide apertures present in the devices and fringe locations are compared to Fraunhofer theory. The fraction of power diffracted from the lasing mode remains roughly constant as a function of relative pump rate, but is shown to depend on both transverse mode order and oxide aperture size.This work was supported by the NSF–EPSCOR, NASA–EPSCOR, ILX Lightwave, and the DOE

Lear, Kevin L., author

Kilper, D. C., author

Carlsten, J. L., author

Roos, P. A., author

American Institute of Physics, publisher

Mountain Scholar (Digital Collections of Colorado and Wyoming)

APPLIED PHYSICS LETTERS VOLUME 75, NUMBER 6 9 AUGUST 1999Diffraction from oxide confinement apertures in vertical-cavity lasersP. A. Roos and J. L. Carlstena)Montana State University, Department of Physics, Bozeman, Montana 59717D. C. KilperUniversity of North Carolina at Charlotte, Department of Physics, Charlotte, North Carolina 28223K. L. LearMicroOptical Devices Inc., 5601C Midway Park Pl. NE, Albuquerque, New Mexico 87109~Received 22 February 1999; accepted for publication 12 June 1999!Direct measurement of scattered fields from oxide confinement apertures in vertical-cavity lasers ispresented. Diffraction fringes associated with each transverse lasing mode are detected in the farfield from devices with varying oxide aperture dimensions and with quantum efficiencies as high as48%. The diffracted pattern symmetries match the rectangular symmetry of the oxide aperturespresent in the devices and fringe locations are compared to Fraunhofer theory. The fraction of powerdiffracted from the lasing mode remains roughly constant as a function of relative pump rate, but isshown to depend on both transverse mode order and oxide aperture size. ©1999 AmericanInstitute of Physics.@S0003-6951~99!03532-9#icaeinladnsifutolaaneict aarrodonsungAobanion.areareicens-six,tsever,nce,-. 1,ve-eicalob-l istheas atereAllasi-bernt.a-Selective oxidation as a means of electrical and optconfinement has dramatically enhanced the performancvertical-cavity surface-emitting lasers~VCSELs!. The ben-efits of oxide confinement are manifest in properties rangfrom low laser threshold1 and decreased intensity noise2 tohigh differential quantum efficiencies and increased modution bandwidth3 as compared to other confinement methosuch as proton implantation. By growing the laterally confiing oxide aperture dimensions smaller and thereby increaoptical losses to higher order transverse modes, singledamental mode laser operation has also been achieved.4But such lateral confinement is not without cost. Duethe abrupt nature of the index step between the oxide cding region and the unoxidized optical cavity,4 there can ex-ist a phase discontinuity in the field at this boundary. Sctering losses due to such discontinuities are predictedincrease with decreasing aperture dimensions.5 Since aper-ture dimensions on the order of several wavelengths areessary to obtain single transverse mode operation, a delbalance ensues between the desired optical confinementhe unwanted scattering from the confinement boundSuch scattering losses have been shown to play a criticalin laser performance,5,6 but have not been directly measureIn this letter, we present direct observation of diffractipatterns from oxide confinement boundaries and we meadiffracted field intensities relative to the undiffracted lasimode.The devices used in this experiment consist of AlGalaser cavities grown to match a nominal lasing wavelength960 nm. Distributed Bragg reflectors serve as the top~out-put! and bottom mirrors with reflectivities of.99.0% and.99.9%, respectively. The laser cavities are sandwichedtween two oxide confinement layers with square or rectgular apertures varying between;4.5mm34.5mm and;12mm312 mm ~see Ref. 3 for similar device construction!.a!Electronic mail: carlsten@physics.montana.edu7540003-6951/99/75(6)/754/3/$15.00Downloaded 29 Oct 2004 to 129.82.229.173. Redistribution subject to AIlofg-s-ngn-d-t-toc-atendy.le.resfe--Smaller device apertures were not available for investigatThe oxide layers are of a quarter wave thickness andpositioned on the first low index layer. The active regionsthree 8 nm InGaAs quantum wells. Although each devoperates in a single longitudinal mode, the number of traverse lasing modes typically varies between one anddepending on the drive current.With traditional methods of beam profiling it is difficulto extract the individual transverse mode profiles for thedevices due to the spatial overlap of the modes. Howesince each transverse mode has a distinct cavity resonafollowing previous work,7 we utilize the frequency separation of these modes as a discriminator. As depicted in Figa multimode optical fiber is scanned along the curved wafront of the uncollimated VCSELs in the far field. Thsource-to-fiber distance is 2.4 cm for all data taken. Optfeedback was avoided and associated effects were notserved. At each angular position of the fiber, the signaspectrally resolved using a spectrometer. In this manner,optical power of each transverse mode can be monitoredfunction of azimuthal and polar angles. If the spectromeoutput is logarithmically amplified, this method can providmore than five orders of magnitude in dynamic range.results were highly reproducible even when the laser wrotated or translated slightly from its original position indcating that coupling into and transmission through the fiwas not greatly affected by polarization or precise alignmeFIG. 1. Light collection apparatus. Light from the fiber is spectrally anlyzed using a spectrometer.© 1999 American Institute of PhysicsP license or copyright, see http://apl.aip.org/apl/copyright.jspityiesthsobethoteteednoEaactenodatr-filevesntalsity-udetos inusesenters astheontheorn-fors ofchpiteigh.ltheizes.in-funiswdizea.a.as755Appl. Phys. Lett., Vol. 75, No. 6, 9 August 1999 Roos et al.Figure 2 shows two-dimensional logarithmic intensprofiles of the fundamental transverse mode for a device w;4.5 mm34.5 mm aperture, operating at relative pump rat(I 2I th)/I th of ~a! 1.76 and~b! 17.8. Note that the plot axeare the two angles of spherical polar coordinates andrepresentation on a flat, square plane necessarily causesdistortion. Interference maxima and minima are clearly oserved in both plots. Although the absolute diffracted powof the device scales roughly with relative pump rate,fractional diffracted power@(4.460.2)31023 for this de-vice# and the locations of the interference minima did nmeasurably depend on pump rate. Fractional diffracpower was determined by taking the ratio of the integradiffracted optical power to that contained in the undiffractmode. Note, however, that this fractional power doesinclude the losses due to additional reflections in the VCSmesa structure, nor does it include losses due to the baperture in the cavity. The observed radiation pattern wcommon to all devices studied and matches the expepattern for the rectangular oxide apertures present in thdevices.Fraunhofer theory was used to verify the diffractiosource. On a logarithmic scale, Fig. 3 shows data~squares!for a single slice through the center of the fundamental mprofile of the same device at zero polar angle, operatingrelative pump rate of 17.8. A simple calculation of the faFIG. 2. Detected optical power vs polar and azimuthal angles for thedamental~0,0! transverse mode. Relative pump rates (I 2I th)/I th of ~a! 1.76and ~b! 17.8 are shown. The lowest contour represents296 dBm and thehighest represents253 dBm resulting in a 4.3 dBm contour spacing. Thdata indicates that fringe position is independent of relative pump rate. Tdimensional integration over the mode profiles showed that fractionalfracted power remained constant for the two relative pump rates.FIG. 3. Optical power vs azimuthal angle. The polar angle was held atdegrees. The fundamental~0,0! mode is shown on a log scale~squares! fora relative pump rate (I 2I th)/I th of 17.8. The dotted curve representsFraunhofer diffraction of a Gaussian beam truncated by an opaque slitDownloaded 29 Oct 2004 to 129.82.229.173. Redistribution subject to AIthsatme-retddtLcksedseeafield diffraction pattern due to a Gaussian beam prowithin the cavity, truncated by an opaque aperture, gigood agreement with the features present in the experimedata. The dashed curve in Fig. 3 shows the resulting intenprofile calculated for a1/e field diameter of 4.19mm and anaperture size of 4.53mm, in good agreement with the expected values. The calculation assumes a field amplitmaximum centered within the aperture and refraction duethe laser medium/air interface was included. Discrepanciethe diffracted maxima amplitudes can be attributed to ourof an opaque aperture as compared to the index step prein the actual device. Also, attenuation of the field at highangles is expected due to the distributed Bragg reflectorwell as the laser medium/air interface and the gain oflaser. A slight asymmetry was observed in the diffractipattern of some device profiles, indicating that eitherpeak of the field amplitude is not centered in the aperturethe propagation vector of the outgoing field is not perpedicular to the plane of the aperture. This effect may occurexample if the oxide apertures on the top and bottom sidethe cavity were slightly misaligned with respect to eaother. We also note that diffraction is observed here desthe high differential quantum efficiency~up to 48%! of somedevices. Therefore, the diffraction is observable even in hfficiency devices with presumably low scattering lossesFigure 4 shows logarithmic intensity profiles for~a! the~0,0! fundamental and~b! the ~2,1! higher order transversemodes for a device with;6 mm36 mm aperture. A driveFIG. 5. Detected optical power of the~0,0! mode vs polar and azimuthaangles for~a! a ;4.5mm34.5mm and~b! a ;6 mm36 mm aperture device.The lowest contour represents296 dBm and the highest represents255dBm resulting in a 4.1 dBm contour spacing. This plot indicates thatdiffracted maxima positions move to larger angles for smaller aperture sIntegration over the profiles showed that fractional diffracted powercreases for smaller aperture sizes.-o-f-roFIG. 4. Detected optical power vs polar and azimuthal angles for~a! thefundamental~0,0! mode and~b! the ~2,1! higher order transverse mode of;6 mm36 mm aperture device. The lowest contour represents296 dBmand the highest represents255 dBm resulting in a 4 dBm contour spacingIntegration over the profiles showed that fractional diffracted power wlarger for the higher order mode.P license or copyright, see http://apl.aip.org/apl/copyright.jspwaiotadicdarn-rwonaafromn.dif-ni-R,pl.r.,ne,l.ol.ntum756 Appl. Phys. Lett., Vol. 75, No. 6, 9 August 1999 Roos et al.current was chosen such that the total optical powerroughly the same for the two modes. As expected, diffractfrom the oxide layers is not restricted to the fundamenmode. Furthermore, the higher order transverse modefracts a larger fraction of its power@(8.560.5)31023# ascompared to the fundamental mode@(3.760.2)31023#.This observation qualitatively matches theoretical predtions, and is expected considering the higher order mohave more power concentrated near the aperture boundFigure 5 shows logarithmic intensity profiles of the fudamental transverse mode for devices with~a! ;4.5 mm34.5 mm and~b! ;6 mm36 mm apertures. Pump rates foeach device were chosen such that the optical powerroughly equal for the two plots. As expected, the diffractimaxima positions move to larger angles with decreasingerture size. Furthermore, fractional diffracted power wmeasured to increase with decreasing aperture size(3.760.2)31023 to (4.460.2)31023, representing an in-crease of 1964%. Uncertainty in these values resulted froDownloaded 29 Oct 2004 to 129.82.229.173. Redistribution subject to AIsnlif--esy.asp-smuncertain first order minima positions used for integratioOptical losses from the fundamental mode due to suchfraction can therefore play a role in determining the mimum aperture dimensions allowable for laser operation.This work was supported by the NSF–EPSCONASA–EPSCOR, ILX Lightwave, and the DOE.1K. D. Choquette, K. L. Lear, R. P. Schneider, Jr., and K. M. Geib, ApPhys. Lett.66, 3413~1995!.2D. C. Kilper, P. A. Roos, J. L. Carlsten, and K. L. Lear, Phys. Rev. A55,R3323~1997!.3K. L. Lear, A. Mar, K. D. Choquette, S. P. Kilcoyne, R. P. Schneider, Jand K. M. Geib, Electron. Lett.31, 208 ~1995!.4K. L. Lear, K. D. Choquette, R. P. Schneider, Jr., and S. P. KilcoyAppl. Phys. Lett.66, 2616~1995!.5E. R. Hegblom, D. I. Babic, B. J. Thibeault, and L. A. Coldren, AppPhys. Lett.68, 1757~1996!.6K. L. Lear, S. P. Kilcoyne, and S. A. Chalmers, IEEE Photonics TechnLett. 6, 778 ~1994!.7C. J. Chang-Hasnain, J. P. Harbison, and G. Hasnain, IEEE J. QuaElectron.27, 1402~1991!.P license or copyright, see http://apl.aip.org/apl/copyright.jsp

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Diffraction from oxide confinement apertures in vertical-cavity lasers

Diffraction from oxide confinement apertures in vertical-cavity lasers

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