Monolithic integration of dual-layer optics into broad-areasemiconductor laser diodes
Laurent Vaissié, Waleed Mohammed, and Eric G. Johnson
Microphotonics Laboratory, School of Optics/Center for Research and Education in Optics and Lasers,University of Central Florida, Orlando, Florida 32816-2700
Photonic devices have made tremendous strides overthe past decade, largely based on the developments ofwafer-based manufacturing methods for the fabrica-tion of active and passive devices. However, activedevices are still fabricated separately from passiveoptics with an additional alignment step based onan active process. Recently several research effortsfocused on integrating beam-shaping elements withlaser diodes to avoid the cost and alignment processof external optics. For example, the integration of alens on the end facet of an edge-emitting laser diodewas recently demonstrated by use of focused-ion beametching1 and deposition.2 However, the end facetintegration is not suitable for low-cost wafer-basedfabrication. Other devices based on a single beam-shaping element in a vertical-cavity surface-emittinglaser have been introduced for low-power applications.3
For higher-power devices, multiplexed grating schemeshave been demonstrated through the use of grating-coupled semiconductor lasers for beam-shaping and-splitting applications.4,5 One major disadvantage ofthe multiplexed grating coupler is that the complexgrating creates a high level of feedback into thediode cavity, which leads to increased filamentation,resulting in severe wave-front distortion of the outputbeam.6 To minimize the feedback into the cavity, onemust decouple the beam-shaping function from theoutput coupling grating. The solution to this problemis to couple the diffracted light through the substrateand out of the opposite side. In this approach asecond optical surface is added for additional beamshaping and correction. Moreover, this allows thedevice to be mounted p-side down on a heat sink,helping with thermal management of the device to ob-tain stable output by keeping the active region closerto the cooling system. It was indeed shown thatheating induces thermal defocusing, thus distortingthe output beam in addition to changing the emissionwavelength.7
In this Letter we introduce the dual-element conceptinto a high-power large-area grating-coupled surface-emitting laser. The two optical functions consist of agrating coupler combined with a multilevel refractivelens processed on the GaAs substrate for beam shap-
ing (Fig. 1). The second-order diffraction grating pro-vides both feedback for lasing and outcoupling. Thelow ref lectivity and large emitting area from the de-tuned output-coupler grating make possible continuoushigh optical output power up to several watts withoutany risk of catastrophic optical damage.
The laser diode material was metal-organic chemi-cal-vapor deposition–grown AlGaAs-GaAs graded-index structure with a single 6-nm-thick InGaAsquantum well for an emission wavelength near970 nm. After top contact fabrication, the gratingarea was defined by selective wet etching. Thebroad-area gain stripe is 100 mm wide by 2 mm long.A 100-nm-thick Si3N4 layer was deposited on thep-doped surface to be used as a mask to transferthe pattern. The 275-nm period grating was thenpatterned in poly(methyl methacrylate) by use of theEBMF Leica/Cambridge 10.5 electron-beam lithogra-phy system at the Cornell Nanofabrication Facility.The pattern was successively transferred into theSi3N4 with a reactive ion etcher and into the laserdiode with an electron cyclotron resonance plasmaetcher that uses chlorine-based chemistry. Afterthe grating was fabricated, the wafer was thinnedand an n-electrode was patterned on the backside.Lift-off technique was used to open a nonmetallizedwindow on the substrate side, allowing the transferof the second optical element. The wafer was thencleaved, and the devices were tested p-side downunder quasi-continuous conditions (pulse width of
Fig. 1. Dual-layer integrated optics semiconductor laserdiode. Light is diffracted by a detuned second-orderdiffraction grating toward a multilevel lens transferredinto the GaAs substrate.
500 ns at 1 kHz). A CCD camera and a zoom wereused to characterize near-f ield intensity profile on thesubstrate interface for accurate placement of the lens.
The slope efficiency of the substrate output was de-termined to be 0.37 W�A. No attempt was made toredirect the light coupled in the air toward the sub-strate; however, previous work has shown that gratingmetallization would increase the directionality of thedevice.8 A focused ion beam (FIB) inspection showedthat the duty cycle was near 20% and the depth was240 nm (Fig. 2). Given the grating parameters, a per-turbation technique algorithm was used to predict theinteraction of the electric field with the grating fol-lowed the relation9
P �z� � P �0�exp�22az� , (1)
where P is the power of the electric field propagatingin the waveguide and a is the decay factor obtained bythe perturbation theory. A comparison of the inten-sity profiles predicted by the theory and the experi-mental data is shown in Fig. 2.
To demonstrate the backside processing of theoptical element, we mounted the diode p-side downfor FIB patterning. This approach to direct millingappeared to be the best fabrication technique availableto demonstrate the dual-element concept on a cleavedlaser diode. However, the backside fabrication pro-cess may clearly be extended to wafer scale withphotolithography techniques if the backside opticalelement is integrated before cleaving and alignmentmarks are patterned during n-side metallization.10
The lens design consisted of a 40-level 250-mm-diameter lens. We fabricated the multilevel lens bymilling 40 rings of different inner diameters. Themilling depth of each ring is given by
di �R 2
µR2 2
W 2
4
∂1�2
N, (2)
where R is the lens radius of curvature, W is the lensdiameter, and N is the number of levels. The innercircle radius ri of the ith ring is given by
ri � �R2 2 �R 2 idi�2�1�2. (3)
Customized software was used to program an FEI 200TEM FIB system at the materials characterizationfacility of the University of Central Florida. Thelens was patterned at a current of 7 nA. The millingrate was determined to be 5.8 3 1023 mm3�min. AFIB picture of the backside of the device is shown inFig. 3. The calculated curvature of the lens, obtainedby white-light Zygo interferometer inspection, corre-sponds to a 1-mm focal length.
The device was again tested under quasi-continuouspumping conditions at 400 mA. Near-f ield imagesof the output were captured on a CCD camera orientedalong the diffraction angle at 15± with respect to
the normal. The device was tested at 1 A with apulse width of 500 ns and 1-kHz frequency. First, animage of the lens was taken and used for calibrationpurposes. Then the device and probe were translatedvertically, and we recorded images by focusing thecamera on a fixed 0.5-mm grain-size diffuser, locatedat the initial position of the lens. Density f ilterswere used to avoid saturation of the CCD camera.Recorded images were normalized and FWHM con-tours were calculated. The effect of the lens on theFWHM of the longitudinal intensity profile (X) andtwo corresponding two-dimensional plots are shown inFig. 4.
The experiment confirms that the lens has an ac-tual 1-mm focal length, similar to what was calculatedfrom the profile measurements. The output power inthe focal spot was 180 mW. The output beam profileshows a two-fold reduction of the longitudinal FWHM1 mm from the lens. The experimental results wereconfirmed by use of the Fresnel propagation kernel topropagate the theoretical amplitude profile through a
Fig. 2. Photograph of the 275-nm outcoupler grating ob-tained by FIB at 45± tilt (insert). The grating was pro-tected by a 1-mm-thick layer of platinum coating during thecross-section milling. The corresponding near-f ield nor-malized intensity prof ile is shown along with the theo-retical exponential decay predicted by perturbation theory.The near-field profile was obtained from p-side imaging ofa device before backside processing.
Fig. 3. Photograph of the backside of the device obtainedby FIB (left). Three-dimensional profile obtained from aZygo white-light interferometer (right). The aberrationterms were obtained from 36 Zernike coefficients.
Fig. 4. FWHM variation of the intensity profile with re-spect to distance from the lens (left). The four-level con-tour images captured with (bottom right) and without (topright) the lens after 1-mm propagation illustrate the focus-ing effect of the dual-layer optics.
lens with the profile obtained from the Zygo measure-ments (Fig. 4). The 75-mm FWHM near-circular out-put may be suitable for direct pumping of multimodefiber or solid-state laser rods.
This Letter has demonstrated the integration oftwo optical elements into a high-power semiconductorlaser diode by use of front and backside processing.The low ref lectivity of the f irst optical layer elementeliminates the catastrophic optical damage risk bydistribution of the coupling surface over a larger area.The second optical layer, the diode substrate, maybe used for monolithic integration of optical lenses orcomplex beam-shaping functions. The experimentalresults confirmed a significant improvement of thebeam profile when a multilevel refractive lens wasused. An additional optical function may also bemultiplexed on the top-layer element and used in com-bination with the second optical surface to perform avariety of optical mapping functions for a wide range of
applications. This configuration also permits bettertemperature cooling by allowing one to keep the activeregion only a couple of micrometers from the heatsink. This configuration, combined with the addedoptical functionality, opens up numerous possibilitiesfor highly integrated semiconductor laser diode baseddevices.
The authors thank CEO, Inc., for processing thewafers before the integration of the optical elements.We also thank E. Venus and O. Smolski from In-finite Photonics for discussions on grating-coupledsemiconductor lasers and the staff of the CornellNanofabrication Facility. E. G. Johnson’s e-mailaddress is [email protected].
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