High-power CW operation of AlGaInP laser-diode array at 640 nm

zyxwvutsrqpo zyxwvutsrq IEEE TRANSACTIONS ON PHOTONICS TECHNOLOGY LElTERS, VOL. 7, NO. 2, FEBRUARY 1995 I33 High-Power CW Operation of AlGaInP Laser-Diode Array at 640 nm zyxwvuts zyxwvut J. A. Skidmore, M. A. Emanuel, R. J. Beach, W. J. Benett, B. L. Freitas, N. W. Carlson, R. W. Solarz, D. P. Bour, Member-, IEEE, and D. W. Treat r Visible diodes that emit low power levels presently exist, but it has been considerably more challenging to produce high-average-power laser diodes and laser arrays. Pushing the lasing emission to shorter wavelength reduces the band offsets thereby increasing carrier leakage, which translates into a lower differential quantum efficiency [5]. Additionally, the electrical resistance is two to three times that of comparable AlGaAs lasers, and the p-cladding layer of the AIGaInP-laser ISIBLE LASER diodes are becoming attractive for an increasing number of applications [I]. The reliability of structure is at least twice as thermally resistive as AIGaAsthe AIGaInP-material system has been steadily improving, and based structures. This relatively low conversion efficiency. laser diodes operating at -635 nm will be replacing the He- coupled with higher thermal impedance, results in continuous Ne lasers used in such applications, as barcode readers. Diode wave (CW) performance that is much poorer than pulsed emission near 630 nm can also be used for certain medical operation for visible-emitting diodes. Similarly, due to the applications, such as dermatology and photodynamic therapy. problem of heat dissipation, the normalized CW output power For higher power applications, diodes can be stacked into of an array of emitters from a laser bar may also be lower than linear arrays. Of particular interest is the optical pumping of that obtained from a corresponding single emitter. Therefore, solid-state laser crystals, in which laser diode arrays offer to operate visible lasers efficiently under high-average power superior performance over flash lamp pumps. Diode arrays necessitates not only the diode’s highest possible light-output not only allow new solid-state laser systems to be conceived, performance, but also an extremely aggressive heatsink. For managing high thermal loads, the silicon microchannel but also permit the duty factor and repetition rate to greatly cooler offers superior performance in a practical architecture exceed what was previously possible using flash lamps. In the 630-690 nm range, AIGaInP-lasers can pump several materials (normalized thermal impedance <0.015 “C-cm*/W [6]. The for applications requiring tunability, including Cr3+:LiSrA1F6 salient performance advantage of this heatsink comes from (LiSAF) [2] and alexandrite [3]. The thermal fracture limit the incorporation of “microchannels” that lie just beneath and thermal conductivity of alexandrite are five and two times the diode; their narrow width (20 pm) minimizes the staggreater, respectively, than those found in yttrium aluminum nant boundary layer of water which contributes most of the garnet (YAG) [4]; hence, alexandrite may particularly benefit package’s overall thermal impedance (the thermal impedance from the high-average power densities afforded by laser diode of water at room temperature is -250 times greater than arrays. Although alexandrite can be pumped by the R1 ab- silicon). Unlike diamond or copper heatsink materials, the silicon microchannels can be wet-etched using conventional sorption line at 680.4 nm [3], shorter wavelengths (i.e., -640 nm) would allow more efficient pumping of compact radially- means for ease of fabrication, and the resulting laminar flow pumped rod geometries, depending on the Cr3+ concentration. consumes less hydraulic power than the turbulent flow found Furthermore, precise wavelength control at 640 nm becomes in impingement coolers [7]. Finally, the system design is unnecessary since the emission lies beneath a broad absorption modular, which allows for the stacking and maintenance band (unlike the narrow absorption line at 680.4 nm), making of arbitrarily large 2D arrays at a moderate density (10 a 640 nm diode-pumped system more practical to implement. barsjinear cm). The design and characterization of the silicon microchannel heatsink are given elsewhere [6]. The separate confinement heterostructure (SCH) laser diode used in this work is shown in Fig. 1; the design is simManuscript received July 13. 1994: revised October I I . 1994. This work ilar to those reported previously. which produced recordwas supported by the U S . Department of Energy. Lawrence Livermore National Laboratory under Contract W-7405-Eng-48. Material growth was level threshold current densities and slope efficiencies [5]. funded by DOC-ATP-70NANB2H 1241. The material was grown by metal organic chemical vapor J. A. Skidmore. M. A. Emanuel, R. J. Beach, W. J. Benett. B. L. Freitas, N. W. Carlson. and R. W. Solarz are with Lawrence Livermore National deposition (MOCVD) on a misorientated [(loo) 10” off Laboratory. Livermore. CA 945.50 USA. toward ( 1 1 l)]n+ GaAs substrate. A single tensile-strained D. P. Bour and D. W. Treat are with Xerox Palo Alto Research Center. Ga0.6Ino.lP quantum well (120 A) was confined by a pair of Palo Alto. CA 94304 USA. IEEE Log Number 940795 I . 1200 A-thick ( A l ~ ~ , ~ G a o . ~ ) ~ . j layers. I n ~ , . ; Pand the Al[1,;Ino.jP Abstract-Visible-emitting high-power laser bars are investigated at an emission wavelength of 640 nm. AIGaInPIGaInP, single tensile-strained quantum well, separate confinement heterostructures are fabricated into one cm long laser bars using a 0.7 fill factor. The low threshold current of the diode, combined with the aggressive heatsinking of a silicon microchannel cooler has resulted in more than 12 W of continuous wave output power. 4 zyxwvut V zyxwvutsrqponmlkj .( 4 zyxwvuts 1041-1 13.5/9.5$04.00 0 199.5 IEEE b zyxw zyxw zyx zyxwvu IEEE TRANSACTIONS ON PHOTONICS TECHNOLOGY LETTERS. VOL 7. VO 2. FEBRL 4RI IW5 GaAs p+-cap 0.05 pm ' A \ Alo,51n,5Pp-cladding 0.8 pm - Ga0.51n0.5PP+ 0.075 pm . A1,51n,5Pn-cladding 0.8 pm GaAs:Si substrate (100) - 10" toward (1 11)A LR... 10% HR 85% "0 0.1% duty,..' zyxwvu zyxwvu zyxwvu 20 40 60 80 100 120 Current (A) Fig. 2. Light output characteristics for a one-centimeter-long AlGaInP laser bar with a 0.7 fill factor and a 750 ktm cavity length. The device was tested pulsed (0.1% duty cycle) at 23 OCcoolant temperature, and run CW at 1 1 OC coolant temperature, with a ramp rate of 4 Ah. The microchannel cooler was operated with 50 psi pressure at 150 cm3/min flow rate, consuming 0.86 W of hydraulic power. zyxwvut Fig. 1. Schematic diagram of the (Al,Gal-,)o 5Ino 5P/Gw (,In0 4 P strained quantum well, separate confinement heterostructure laser diode, cladding layers were 0.8 pm thick. To reduce the series resistance, a Gao.Jno.sP barrier-reduction layer (750 A) was grown between the p-cladding layer and the 500 A-p+-GaAs cap layer. The laser bars consist of 100 pm wide emitters spaced on 140 pm wide centers, producing a fill factor of 0.7. Each emitter was optically isolated from its neighbors by etching v-grooves down to the lower n-cladding layer. For v-groove fabrication, the GaAs cap was removed using a mixture of H2S04:H202:H20 (5: 1: 1) at room temperature, and the remaining epitaxial layers were etched in H2S04 acid (100 "C), which stops when the GaAs substrate is reached. Elevating the temperature of the H2S04 acid helps reduce the etch rate selectivity between the AlInP and GaInP layers [8]. Si02 (grown by chemical vapor deposition) produces a durable wet-etch mask, and is also used for the electrical isolation. The p-side was metallized with Ti/Pt/Au, and afterwards, the wafer was thinned to a thickness of -100 pm. Ge/Au/Ni/Au was deposited onto the n-side and the wafer was then alloyed. After cleaving, the laser bar facets were coated using electronbeam evaporation. At an emission of 640 nm, the low reflector (LR) had a reflectivity of -10% using a single layer of Al2O3, and the high reflector (HR) had a reflectivity of -8.5% using an alternating quarter wavelength stack of Si02miO2 layers (not optimized for 640 nm). Once the laser bars were coated, they were indium-soldered, p-side down, to silicon microchannel heatsinks. Au wire bonds connected the n-side of the bar to the n-side contact of the package. The light output (L-I) characteristics for a one cm long laser bar is shown in Fig. 2, using a 750 pm cavity length. Under pulsed operation (100 ps at 10 Hz), the threshold current density ( Jtk,)is roughly 525 A/cm2 and the slope efficiency is 0.68 WIA; note that individual broad area lasers (uncoated) had a Jth as low as 4.55 Alcm2 and a slope efficiency as high as 1.1 W/A (sum of both facets). As shown in the figure, 600 620 640 660 680 Wavelength (nm) Fig. 3. Emission spectrum of a one-centimeter AlGaInP laser bar. The spectrum was taken at a current of 40 A. The microchannel cooler was operated at 1 1 OC with SO psi pressure at IS0 cm3/min flow rate. - 45 W was obtained under pulsed conditions at 100 A, which was supply limited. When the laser bar was operated CW, a maximum output power of 12.2 W was attained, which represents a twofold improvement over previous reports at the longer 6.50 nm wavelength [9]. This CW power level (12.2 W) corresponds to a maximum total conversion efficiency of only 7.4%; attesting the importance of efficient heat removal. The CW output spectrum of the same package is shown in Fig. 3. At a drive current of 40 A (power level -3 W), the peak emission intensity is centered near 640 nm, and the full width at half maximum (FWHM) is 4.9 nm. A wavelength shift equal to 0.16 nm/"C was measured using low duty cycle pulses while varying the heatsink temperature (A = 636 nm at room temperature). Based on the measured shift of wavelength versus temperature, the thermal impedance of the package is only 0.020"C-cm2/W for this cavity length. zyxwvutsrq SKIDMORE i zyxwvutsrqponmlk zyxwvutsrqponmlkjihgfedc zyxwvutsrqponmlkj PI ul : HIGH-POWER CW OPERATION I35 We have fabricated high-power, visible-emitting AlGaInP laser bars to demonstrate their potential as solid-state laser pumps. Due to the inherently poor conversion efficiency of AlGaInP devices with short wavelength (-635 nm), it is crucial that laser bars have the lowest possible threshold current and are cooled aggressively to dissipate the concomitant high thermal loads. By employing a silicon microchannel heatsink, we have generated more than 12 W of CW output power from a one cm bar, which to our knowledge, is the highest reported to date. Laser diode arrays capable of generating high-average power in the visible range will allow new architectures and uses of diode-pumped solid-state laser systems to be explored. zyxwvut [ 2 ] R. Sheps, J. F. Meyers. H. B. Serreze. A. Rosenberg. R. C. Morris and M. Long, "Diode-pumped Cr:LiSrAIF,, laser." Opt. Lett. vol. 16. p. 820, 1991. [3] R. Sheps, J. F. Myers and H. B. Serreze. "Monochromatic end-pumped operation of an alexandrite laser," Opt. Coninr.. vol. 97. p. 363. 1993. 141 W. Koechner. Solid-State Luscr Eiigirieei.iti,q New York: SpringerVerlag, 1992. p. 70. (The comparison of alexandrite to YAG is compelling; the latter host is the predominant choice of high-average power solid-state laser systems.) [5] D. P. Bour, R. S. Geels, D. W. Treat, T. L. Paoli. F. Ponce. R. L. Thomton, B. S. Krusor. R. D. Bringans and D. F. Welch. "Strained Ca., In, _ I P/(AIGa)o ;In0 ;P heterostructures and quantum-well laser diodes," lEEE J . Qriuiifiini €lec.ti-oii.. vol. 30. p. 593. 1994. [6] R. J. Beach, W. J. Benett. B. L. Freitas. D. Mundinger. B. J . Comaskey. R. W. Solarz and M. A. Emanuel. "Modular microchannel cooled heatsinks for high average power laser diode arrays." lEEE J . Qitmtrini Electron.. vol. 28, p. 966. 1992. (The thermal impedance <O.0ljoCcm2/W was measured at the footprint of a 330 I'm cavity length diode bar supplied by Siemens.) [7] J . G. Endriz, M. Vakili. G. S. Browder. M. DcVito. J. M. Haden. G. L. Hamagel, W. E. Plano, M. Sakamoto. D. F. Welch. S. Willing. D. P. Worland and H. C. Yao. "High power laser diode arrays." /€E€ J . Qi~utitiiniElertrnrr. vol. 28. p. 952. 1993. 181 T. R. Stewart and D. P. Bour. "Chemical etching of (AI, Gal - , )o ;In0 ;P using sulfuric and hydrochloric acids." .I. E/~Ctro(.hPnlSoc.. vol. 139. p. 1217, 1992. [9] R. S. Geels, M. Sakamoto. D. F. Welch. D. P. Bow. D. W. Treat and R. D. Bringans, "High-power visible lasers from 630 to 690 nm." CLEO '94 Tech. Dig., p. 24. zyxwvutsrqp ACKNOWLEDGMENT The authors thank D. Hudson, B. Comaskey and C. Hamilton for several illuminating discussions, and acknowledge the technical support provided by C. Reinhardt and E. Utterback. REFERENCES zyxwvutsrqpo zyxwvutsrq [ I ] J . Hecht, "Semiconductor diode lasers span the rainbow." Laser Fociis World, p. 199. Apr. 1993. and references therein.