10 W reliable operation of 808 nm broad-area diode lasers by near field distribution control in a multistripe contact geometry

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1 W reliable operation of 88 nm broad-area diode lasers by near field distribution control in a multistripe contact geometry K. Paschke*, S. Einfeldt, Chr. Fiebig, A. Ginolas, K. Häusler, P. Ressel, B. Sumpf, and G. Erbert Ferdinand-Braun-Institut, Gustav-Kirchhoff-Str.4, Berlin, Germany ABSTRACT High-power diode lasers operating at 88 nm and consisting of a multiple ridge-waveguide structure have been fabricated. Lasers with this structure show a more stable far and near field pattern in comparison to conventional single stripe broad area lasers. A reliable continuous wave operation at room temperature over 8 h at and 8 h at W has been achieved with 2 µm stripe width devices. Keywords: High-power diode lasers, broad area lasers, multiple stripe lasers, reliability 1. INTRODUCTION Due to their high efficiency and non-sophisticated manufacturing process broad-area (BA) devices are the most important type of high power diode lasers. Devices emitting at 88 nm have come into wide use across the industrial, medical applications and for optical pumping of solid-state lasers as well as materials processing. Currently, there is much effort put into the improvement of the performance of these diode lasers. Recently, lasers with a stripe width of µm and a cavity length of 2 mm operated at 2 C have a maximum output power of W. The quantum well of these devices consist of InAlGaAs. A reliable operation at an output power of 2. corresponds to facet load of 5 mw/µm over 25 h has been reported [1]. Applying non-absorbing mirrors manufactured with a two step epitaxy process the level of catastrophically optical damage (COD) could be increase and the life time was increased. High power single stripe emitters and laser bars emitting at 88 nm and having a similar AlGaAs based structure with a GaAsP active region as the devices investigated in this paper were already reported in Ref. [2]. A COD power level of 9 W was obtained with a µm stripe width device. A cm-bar having a filling factor of 28 % exhibited a maximum output power of 14. These devises had already a narrow vertical far field divergence (26 FWHM). The facet load can be reduced by a waveguide design with larger vertical spot size. Such design is described in Ref. [3]. An important issue for these devices is the increase of the mode size resulting in a reduced optical power density. BA devices with such super large optical cavity (SLOC) structure emitting at 88 nm reached a maximum output power of 1 at 25 C with a vertical far field divergence of 18 FWHM from a 2 µm aperture. The latter raises the power level where catastrophic optical damage (COD) occurs and therefore increases the device lifetime. However, at high output powers wide stripe devices suffer from intensity spikes in the near field. The segmentation of the single contact stripe into an array of multiple narrow stripes is known to suppress the excitation of higher order lateral modes and the formation of filaments [4], which results in a more uniform distribution of the current and the power. In this paper we present results for BA lasers with a SLOC structure emitting at 88 nm and consisting of a multiple ridgewaveguide (RW) structure in comparison to conventional single stripe BA lasers with same vertical structure having a stripe width of 2 µm. Details of the structure, the laser design and the fabrication process are presented in section 2. The electro-optical properties and the results of the reliability tests will be described in section 3. Finally, the results are summarized in section DEVICE STRUCTURE, FABRICATION, AND MOUNTING The AlGaAs/GaAsP layer structure is based on a SLOC grown by metal-organic chemical vapor phase epitaxy. The active region consists of an aluminum-free tensile-strained GaAsP single quantum well which results in a transverse- High-Power Diode Laser Technology and Applications V, edited by Mark S. Zediker, Proc. of SPIE Vol. 6456, 6456H, (27) X/7/$18 doi:.1117/ Proc. of SPIE Vol H-1

2 magnetic polarized laser emission. The total width of the AlGaAs waveguide core is 3. µm. The thickness of the cladding layers was optimized to obtain a minimal series resistance and small radiation losses of the fundamental mode [2]. The SLOC structure reduces the facet load and gives a low vertical divergence angle of about 19 (FWHM). About 96 % of the output power is included in a vertical far field angle of 32.4 as shown in Fig. 1. The profile is nearly of Gaussian shape which should simplify the coupling of the laser emission into optical systems. The reduction of the vertical far field divergence angle to a FWHM about 19 permits the use of optical elements with a small numerical aperture. The internal losses are about.6 cm -1 as determined from the dependence of the differential quantum efficiency on the resonator length. Typically, the differential quantum efficiency is 92 %. The manufactured devices which consists either of a single stripe with a width of 2 µm or of 18 RWs with a ridge width and a pitch of 7 µm and 11 µm, respectively. The single stripes were defined by ion implantation. The RWs were fabricated by reactive ion etching. The etching depth was adjusted to provide an effective index step of -3. After backside thinning, polishing and n-metallization, the wafers were cleaved to obtain laser bars with a cavity length of 4 µm. The coating of the facets included a facet passivation as described by Ressel et al. [6]. The front facets were anti-reflection coated with reflection coefficients between % and 2.5 %. The rear facets were high-reflection coated with a reflection coefficient of 95 % vertical far field θ vert / degree Fig. 1: Profile of the vertical far field intensity of the SLOC structure lasers. The lasers were finally mounted epitaxial side down on CuW submounts by using AuSn soldering [7]. The substrat-side was contacted by wire bonding. The subessemblies were then mounted with PbSn solder on specially designed conduction cooled packages with dimensions 25 x 25 x 7.6 mm 3. A thermal resistance about 3.5 KW -1 was obtained. 3. EXPERIMENTAL RESULTS Figures 2 and 3 show the power-current, power-voltage and conversion efficiency-current characteristics of a singlestripe and a multiple-rws device operated in continuous wave mode at 15 C (dashed lines) and 25 C (solid lines), respectively. For both types of device the maximum wall-plug efficiency is 51 % at a temperature of 25 C. This value is reached at A (~12 W) with the single stripe device and at 9 A (~12 W) with the multiple-rws device. Up to these power levels the slope efficiency, which was determined slightly above threshold, is 1./A for both cases, despite the long cavity. The threshold currents of the single-stripe and the multiple-rws devices are 2.13 A and 4 A, respectively. The threshold current of the multiple RWs structure device is 15 % higher than expected from a geometrical scaling of the contact area. This may be caused by current spreading in the regions between 7 µm wide RW stripes. For single-stripe devices output powers about 17 W at 15 C and higher than 16 W at 25 C were measured. The output powers were limited by thermal roll-over rather than by COD. The high output power illustrate the superiority of a SLOC structure in comparison to a more conventional broadened waveguide structures. Proc. of SPIE Vol H-2

3 Maximum output powers of 16 W at 15 C and 1 at 25 C, respectively, were achieved for the multiple-rws devices. In comparison with the single-stripe devices, the thermal roll-over becomes obvious at a lower current for the multiple- RWs devices. This effect can be attributed to the higher current density at the p-contact and higher series resistance, due to the lower contact area for the multiple-rws devices which results in a higher temperature in the active region. 2 2 voltage U /V T = 15 C T = 25 C current I /A Fig. 2: Power-voltage-current characteristics of a singlestripe laser at a heat sink temperature of 15 C and 25 C, respectively output power P opt /W wall-plug efficiency η C voltage U /V 15 C 25 C current I /A 15 5 output power P opt /W wall-plug efficiency η C Fig. 3: Power-voltage-current characteristics of a multiple- RWs laser at a heat sink temperature of 15 C and 25 C, respectively. The lateral intensity profiles of the optical beam were measured at different output powers under quasi-continuous wave conditions with a pulse length of 1 ms (repetition rate 25 Hz) at 25 C. Figure 4(a) shows the lateral near-field distribution of a single-stripe laser which consists of multiple peaks of varying width and intensity. The total width of the profile is close to the stripe width of 2 µm. At all output powers wide stripe devices suffer from intensity spikes in the near field. The profiles indicate the formation of current filaments in the stripe. The intensity maxima vary about 2% for the three investigated power levels. Their position is randomly distributed and varies dependent on optical power. The local enhancement of power density may cause unpredictable device failure due to generation of defects in the waveguide or at the facet. The near-field distribution of a multiple-rws laser is shown in Fig. 4(b). It consists of 18 nearly identical peaks which correspond to the 18 RWs. The lateral near field is homogenous over all single stripes. There is no indication for filamentation in this structure. By increasing optical power the relative variation of peak intensity decreases from % at 1W to 7% at 8W. Compared to the single stripe laser the random variation of peak intensity is considerably smaller, i.e. the peaks are more homogeneously distributed among the stripes. The lateral intensity profiles of the far field for the single-stripe and the multiple-rws devices are shown in Fig. 5(a) and (b), respectively. A top hat profile with a relatively strong modulation is found for the single-stripe laser at an output power of. This type of beam profile can be attributed to the non-uniform distribution of near-field intensity. The modulation depth of the peaks in the far field profile is more pronounced with increasing power. The peak to valley depth increases from 24% at to 46% at. The far field angle calculated from 1/e 2 -level decreases with increasing optical power from 5.1 at to 7.9 at. In contrast, the multiple-rw laser exhibits a Gaussian like profile of the far field. Even at higher power of or the profile remains Gaussian-like with small modulation. The modulation depth (peak to valley) is only 7% at 5W and 12% at 8W which is much smaller compared to the single stripe laser. This beam profile can be attributed to the homogeneous near-field intensity distribution of the multiple-rws. On the other hand compared to the standard single stripe laser the far field angles of the multiple RWs laser are larger. Their 1/e 2 -values vary from 11.7 at 1W to 17.1 at 8W. All these data illustrate the excellent properties of the multiple-rws laser structure, with power-current characteristics comparable to those of conventional BA single stripe devices, but with controlled distribution of the beam profile. Proc. of SPIE Vol H-3

4 position / µm position / µm (a) (b) Fig. 4: Lateral profile of the near field intensity for different output powers; (a) for the same device as in Fig. 2 and (b) as in Fig. 3, respectively angle θ / degree angle θ / degree 2 (a) (b) Fig. 5: Lateral profile of far field intensity for different output powers; (a) for the same device as in Fig. 2 and (b) as in Fig. 3, respectively. Reliability tests have been carried out at 25 C by operating three lasers at 7 W, two lasers at and four lasers at W under cw-conditions. None of the devices was preselected by burn in. Figure 6 shows that the single-stripe devices are operating without failure at output powers of 7 W and for 8 h. The degradation rate is -6 / h for those lasers. It is defined as relative increase of the current per unit time and obtained by linear fit over the full data range. Two singlestripe lasers are still running at an output power of W corresponds to facet load of 5 mw/µm after 5 h, which is the best reliability for lasers of this wavelength reported so far. One of them shows a sudden increase of current at 25 h, as the device was switched off and on again due to maintenance work of the equipment. After this current step the average degradation rate is 12x -6 / h (obtained from linear fit between 25h and 5h). The degradation rate of the other laser is 2x -6 / h. The results of aging tests of the multiple-rws device are shown in Fig. 7. A sudden failure of one device, which operated at, occurred after 66 h. The laser at 7 W continue operating for over 8 h. The average aging rate is -6 / h for the surviving laser. Two lasers are operating at W for 8 h. The average degradation rate is 2x -6 / h. The life time tests show the excellent long term stability of the devices based on the technology for fabrication of multiple ridge waveguides and single ridge broad area lasers. Current / A P = W P = P = 7 W 5 single stripe lasers aging at T = 25 C Time / h Fig. 6: Aging test of single stripe lasers at 7 W optical power and 25 C heatsink temperature. Current / A 12 P = W P = P = 7 W 4 multiple RWs aging at T = 25 C Time / h Fig. 7: Aging test of multiple-rws devices at 7 W optical power. The heatsink temperature was 25 C. Proc. of SPIE Vol H-4

5 4. CONCLUSION High power BA lasers for the wavelength range around 88 nm whose 2 µm wide p-contact consisted either of a single stripe or of multiple RWs were developed. The SLOC design results in a vertical far field distribution of 19 FWHM. The single-stripe and the multiple-rws structures showed a maximum output power of 17 W and 1, respectively, and the wall-plug efficiency is 51% when operated in cw-mode at 25 C. Distribution of the beam profile is more controllable for the multiple-rws. Laser with single stripe design shows uncontrollable intensity spikes. A reliable operation at an output power of was achieved over more than 8 hours, which is the best reliability for lasers of this wavelength reported so far. ACKNOWLEDGMENT The authors acknowledge the technical support of P. Brade, O. Fink, A. Krause, R. Olschewski, S. Wiechmann, and P. Wochatz,. REFERENCES 1. R.M.Lammert, M.L.Osowski, S.W. Oh, C. Panja, and J.E Ungar, High power (>W from µm aperture) high reliability 88 nm InAlGaAs broad area laser diodes IEE Electronics Letters, 42, No. 9, (26) 2. J. Sebastian, G. Beister, F. Bugge, E. Buhrandt, G. Erbert, H.G. Hansel, R. Hülsewede, A. Knauer, W. Pittroff, R. Staske, M. Schröder, H. Wenzel, M. Weyers, and G. Tränkle, IEEE J. Sel. Top. Quantum Electron., 7, 334, (21) 3. A. Knauer, G. Erbert, R. Staske, B. Sumpf, H. Wenzel and M. Weyers High-power 88 nm lasers with a superlarge optical cavity, Semicond. Sci. Technol., 2, (25). 4. C. J. Chang-Hasnain, E. Kapon, and E. Colas, Spatial mode structure of index-guide Broad area quantum well lasers,ieee J.Quantum Electron., 26, No., , (199) 5. P. Ressel, G. Erbert, U. Zeimer, K. Häusler, G. Beister, B. Sumpf, A. Klehr, G. Tränkle, Novel Passivation Process for the Mirror Facets of High-Power Semiconductor Diode Lasers, IEEE Photonics Technology Letters, 17, (25) 6. W. Pittroff, G. Erbert, G. Beister, F. Bugge, A. Klein, A. Knauer, J. Maege, P. Ressel, J. Sebastian, R. Staske, and G. Tränkle "Mounting of High Power Laser Diodes on Boron Nitride Heat Sinks Using an Optimized Au/Sn Metallurgy", IEEE Trans. on Advanced Packaging, 24, No. 4, , (21). Proc. of SPIE Vol H-5