Efficiency and power scaling of in-well and multi-pass pumped AlGaInP VECSELs Cherry May N. Mateo*1a,c, Uwe Braucha,c, Hermann Kahleb,c, Roman Bekb,c, Thomas Schwarzbäck b,c, #, Michael Jetterb,c, Marwan Abdou Ahmeda,c, Peter Michlerb,c, and Thomas Graf a,c a
Institut für Strahlwerkzeuge, University of Stuttgart, Pfaffenwaldring 43, 70569 Stuttgart, Germany Institut für Halbleiteroptik und Funktionelle Grenzflächen, University of Stuttgart, Allmandring 3, 70569 Stuttgart, Germany c Stuttgart Research Center of Photonic Engineering SCoPE, University of Stuttgart, Pfaffenwaldring 9, 70569 Stuttgart, Germany # current address is TRUMPF Lasersystems for Semiconductor Manufacturing GmbH, JohannMaus-Straße 2, 71254 Ditzingen, Germany
b
ABSTRACT We report a continuous wave operation of a quantum-well and multi-pass-pumped AlGaInP based red vertical-externalcavity surface-emitting laser emitting at 660 nm. The laser output power was 1.5 W with a slope efficiency of 35 %. The critical role of optimizing the sample design both for the pump and laser wavelengths, pump spot size, and the number of pump light passes were experimentally investigated. Keywords: VECSEL, quantum-well pumping, multi-pass pumping, AlGaInP
1. INTRODUCTION Vertical-External-Cavity Surface-Emitting-Lasers (VECSELs) 1, have been extensively studied because of their advantageous characteristics such as multi-Watt output in circularly symmetric, fundamental TEM00 mode and diffraction-limited beam quality (M2 ~ 1.0 - 1.2). Because of the wavelength flexibility of the gain region, these compact, power scalable sources have been demonstrated in continuous wave (CW) and pulsed operation with output wavelengths spanning from the ultraviolet (UV) to the mid infrared (IR) in fundamental wavelength or in combination with intracavity frequency mixing. In the red spectral range, the developments in VECSELs are motivated by several applications where red lasers are useful including medicine, bio-photonics, atomic spectroscopy, and photodynamic therapy. 2 - 4 Additionally, by exploiting the external resonator configuration, UV emissions can be reached via nonlinear frequency doubling.5 VECSELs with red fundamental wavelength are practically based on the AlGaInP material system grown on GaAs substrate. 5 - 10 A typical design uses GaInP quantum wells or InP quantum dots with AlGaInP barrier layers, with the compositions such that it is nearly lattice matched to the GaAs substrate. The AlGaAs material system is usually used for the mirrors. Because of the small conduction band offset, thermal carrier overflow limits high-temperature stability, and the output power in this spectral range. Another limitation is in the composition of the AlGaAs mirror material which should have an AlAs mole fraction of greater than 0.4. This leads to a reduced available range of refractive index and requires several mirror periods to achieve high reflectivity. Furthermore, AlGaAs has a poor thermal conductivity which makes heat removal through the substrate of the sample problematic.11 Although epitaxial design optimization and the use of quantum dots as active material for the barrier pumped AlGaInP based VECSELs, can improve the output power, the optical efficiency of these lasers were only 25 % or lower and the operating temperatures are fairly low despite the effective intracavity cooling scheme. Last year, we have demonstrated that the efficiency with respect to the absorbed power increased to 60 % and the operating temperature can reach up to 50 °C when direct pumping of the quantum well (QW or in-well pumping) is used instead of barrier pumping. 12 While 1
[email protected]; phone + 49 711 685-69735; fax +49 711 685-66842 Vertical External Cavity Surface Emitting Lasers (VECSELs) VI, edited by Keith G. Wilcox, Proc. of SPIE Vol. 9734, 973410 · © 2016 SPIE · CCC code: 0277-786X/16/$18 · doi: 10.1117/12.2212162
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QW pumpingg reduces the excessive e heatt load drasticaally because th he energy of thhe pump phottons can be clo ose to the QW W bandgap enerrgy and hencee the laser phooton energy, itt suffers from reduced pumpp absorption. To compensaate for this, thee use of a multii-pass for the pump light is required. Tw wo possible options can be done; d one is thhe use of interrnal multi-passs wherein the semiconductor s r disk is used as an etalon and a then the pump p wavelenngth resonancee is shifted by y adjusting thee pump angle of o incidence according a to the t resonant absorption a sch heme or by chhanging the ssample design n (see ref. 13)). However, thhis requires a fairly highh reflection coefficient c off the semiconnductor frontt surface wh hich makes it incompatible with intra-caavity cooling where the heeatspreader sh hould be in direct d contact with the actiive layer. Thee second option is to use external e pumpp optics to ree-image the pump p light seeveral times bback onto thee disk. In thiss contribution, we used an external multi--pass to make several pump p reimaging looops on the quuantum well (QW) ( pumpedd AlGaInP – VECSEL with a dye laser as a pump sourcce.
2. EPITAXIAL E L DESIGN AND A GROW WTH The VECSEL L chip was deeposited via metal-organic m v vapor-phase epitaxy e (MOV VPE). The gainn region, grow wn on top of a highly reflectting distributeed Bragg refleector (DBR) mirror m consistting of 55 AlA As/AlGaAs laayer pairs, waas designed ass resonant perioodic gain (RP PG) structure with w an emisssion waveleng gth of 670 nm. It consists of 5 groups of quantum-welll packages. Each package haas 4 compresssively-strainedd GaInP quan ntum wells sanndwiched in A AlGaInP barriier layers. Thee p with w barrier pumping p are ffound in ref. 9. In order too in-depth detaails of the laseer’s growth structure and performance dissipate mosst of the heatt introduced on o the gain reegion from th he pump enerrgy, a single-ccrystal CVD diamond withh SiO2/Al2O3 antireflection coating on one side, opptimized for the 638 nm pump waveelength, and 665 nm laserr wavelength, was w liquid-caapillary bondeed 14 onto thee surface of a cleaved 2.5 mm × 2.5 mm m piece of th he disk. Afterr bonding, the diamond/diskk structure was w fixed to a water-cooled d brass mounnt. This mounnt acts as heaat sink for thee diamond and the rear surfaace of the VEC CSEL.
3. EXPE ERIMENTA AL SETUP a 4-pass ex xperiments. A Coherent dyye laser which h delivered ann Fig. 1 showss the laser settup used in thhe two-pass and output powerr of 4.5 W att a wavelengtth of 638 nm was used as an excitationn source. In the 4-pass ex xperiments thee remaining puump light whicch was not abssorbed after thhe first 2 passses through thee gain region was reflected d back onto thee VECSEL usiing a mirror. The laser cavvity was form med by the sem miconductor'ss DBR as onee end mirror and an outpuut coupler with a transmittance of 1% on the t other end.. The radius of o curvature of the output ccoupler was 10 00 mm. Pumpp spot size wass varied by adj djusting the foocusing distancce. For each measurement, m pump spot siize was measu ured by fittingg the intensity profile p of the photoluminesscence as imagged by a CCD D camera to a Gaussian assuuming the 1/ee2 definition of the beam diam meter.
wtput mirror
17
Figure 1. Schematics of the t 4-pass pumpping in a linearr resonator conffiguration.
To further inncrease the nuumber of passees to 24 a typpical multi-paass pump opticcs which is w well known fro om solid-statee disk lasers was w used. 15 Figg 2(a) shows the t schematicc of the setup used in the exxperiment. It consists of tw wo prism pairss for pump beaam displacemeent, and a paraabolic mirror,, f = 32.5 mm m, with 50 mm m diameter for beam focusin ng. A photo of the setup is shown in Figg 2(b). To alllow multiple pump-light passes, p the DB BR has to addditionally co over the pumpp wavelength for f the corressponding rangge of angles of o incidence. In the case of multi-passs pump opticss the angle of incidence of the t pump beam m is approxim mately 30°.
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Figure 2. (a) Schematicss of the multi-pass pump opticcs 15 in a linear resonator conffiguration. (b) A Actual experimental setup for multi-ppass pumping.
4. RESUL LTS AND DIISCUSSION N 4.1 Spectrosscopy Before the saample was useed in the laserr experimentss, angle depen ndent reflectivvity with unpoolarized light was measuredd as shown in Fig. 3. This provides p infoormation on thhe position off the Bragg reflectance bannd (the stop band) b and thee resonance poositions for puumping and laasing (the stroong angle-dependent absorpption dips witthin the stop band). b For thee pump waveleength of 638 nm n the pump resonance is optimum wheen the pump incidence anggle is 60°. When W using thee multi-pass puump optics with w an anglee of incidencce of 30°, th he reflectivity spectrum inndicates that the excitationn wavelength iss covered by the t stop band but is alreadyy quite far offf the cavity ressonance. The pump absorpttion resonancee is further reduuced in the lasser experimennts where the intracavity i diaamond is in coontact with thee sample surfaace. 1.0
Reflectivity
0.8 0.6 0.4 0.2 0.0 62 20
15° 30° 45° 50° 60°
630
λpump = 638 nm
640
650
660
670
680
W Wavelength h (nm) Figure 3. Room temperaature reflectivityy spectrum of thhe sample at different angles of o incidence.
nt. 4.2 Power sccaling by incrreasing the pump absorpttion coefficien To achieve high h pump abssorption of thhe VECSEL at a our pump source, two asspects were taaken into acco ount. The first relates to thee pump resonaance, by meaasuring laser performance p at a pump angles of incidence of 45° and d 55° for a ppolarized pum mp light.The second conceerns the numbber of pump recycling loopps. Fig. 4 com mpares the pum mp absorptionn efficiency off the sample under u laser opperation as a function f of pu ump power att different pum mp angles of incidence andd mp wavelengthh of 638 nm. T The pump spo ot diameter onn number of puump passes thhrough the gaiin region at a constant pum the VECSEL L was 52 ± 2 μm. μ The heat sink temperattures for the two t and four pump p passes was 16 °C wh hile for the 244
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pump passes, the heatsink temperature was 5 °C. At the maximum pumping power for two pump passes, the pump absorption efficiency was increased from 18 % to 25 % when the pump angle of incidence was increased from 45° to 55°. Increasing the number of pump passes from 2 to 4 and maintaining the pump angle of incidence increases the pump absorption efficiency by a factor of two. The highest absorption efficiency of 88 % was obtained with the 24 pump passes. Even though the 30° pump angle of incidence for the 24 pump passes is not optimal for 638 nm pump wavelength as indicated in Fig. 3, the large number of pump passes enables further enhancement of the pump absorption.
Pump absorption efficiency
1.0 0.8
2x, 45° 2x, 55° 4x, 45° 24x, 30°
Outcoupling = 1 % λpump = 638 nm
0.6 0.4 0.2 0.0
0
1
2
3
4
5
Pump power (W) Figure 4. Pump absorption coefficient of the VECSEL under laser operation.
The output power of the VECSEL as a function of the pump power and the absorbed pump power at different pump angles of incidence and number of pump passes through the gain region is shown in Fig. 5. 2.0
2x, 45°, ηslope = 53 %, Pth = 0.05 W
1.6
2x, 55°, ηslope = 50 %, Pth = 0.07 W
1.2
24x, 30°, ηslope = 40 %, Pth = 0.26 W
(a)
4x, 45°, ηslope = 43 %, Pth = 0.15 W
Output power (W)
0.8
Outcoupling = 1 % λpump = 638 nm
0.4 0.0
0
1
2
3
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5
Absorbed pump power (W) 2.0 2x, 45°, ηslope = 9 %, Pth = 0.16 W
1.6
2x, 55°, ηslope = 12 %, Pth = 0.13 W
1.2
24x, 30°, ηslope = 34 %, Pth = 0.28 W
(b)
4x, 45°, ηslope = 14 %, Pth = 0.18 W
0.8 0.4 0.0
0
1
2
3
Pump power (W)
4
5
Figure 5. Laser output power versus (a) the absorbed and (b) the incoming pump power at pump angles of 30°, 45°, and 55°, and for 2, 4, and 24 passes. The pump wavelength was fixed at 638 nm.
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A maximum output power of 1.5 W was achieved at a pump power of 4.6 W which corresponds to an absorbed pump power of 4.1 W for the setup with 24 pump passes. No thermal rollover was observed which indicates that the output power was only limited by the input power. Its slope efficiencies with respect to the pump and absorbed pump powers are 34 % and 40 % respectively. Comparing the results for the 2 and 4 pump passes at a pump angle of incidence of 45°, an additional double pass through the QW gain region resulted in an increase of the laser power by a factor or 1.5. With respect to the pump power as shown in Fig 5(b), the threshold power is almost identical but is a little more than a factor of two higher when the absorbed threshold power is considered as shown in Fig 5(a). The slope efficiency based on the pump power increased by 55 % and slope efficiency which is based on the absorbed pump power decreased by 18 %. Since the laser threshold based on the absorbed pump power should be the same with or without retroreflection, the increase in the threshold based on the absorbed pump power when the number of pump passes was doubled can be explained by a non-perfect overlap of the re-imaged pump light and the original pump spot. If this is the case, the decrease in slope efficiency can be possibly due to a worsened matching of laser mode to the overall pump area. 4.3 Power scaling with respect to the pump spot size The plot of the slope efficiency and the threshold absorbed power density as a function of the pump spot size radius for the VECSEL with 4 pumping passes is shown in Fig. 6. The highest slope efficiency with respect to the absorbed pump power was 62 % for a pump-spot radius of 22 µm and decreases to 8 % for a pump-spot radius of 82 µm. The decrease in the slope efficiency in combination with the slightly nonlinear increase in the threshold power density with respect to the absorbed power when the pump spot size was increased can be attributed to either lower internal efficiency, or higher resonator losses due to suboptimal sample quality of the VECSEL on a larger scale.
70
10 9 Slope
40
Outcoupling = 1 % λpump = 638 nm
ρthreshold
2
50
THS = 16 °C
8
45 ° pump angle
30
4x pump passes
7
20 6
10 0 20
25
30
35
40
ρthreshold (kw/cm )
Slope (%)
60
5
45
Radius (μm) Figure 6. The slope efficiency and the threshold absorbed power density of the VECSEL with respect to absorbed pump power as a function of the pump spot radius. The pump incidence angle was kept at 45° and the number of pump passes through the gain region was four.
Output-versus-input power curve of the VECSEL with 24 pump passes for pump spot sizes of 52 μm and 92 μm is shown in Fig. 7. At the same heatsink temperature of 5 °C, the slope efficiency decreased by 20 % when the pump spot size was increased by approximately 80 %. As mentioned above, higher resonator losses due to sub-optimum sample quality covered by a larger pump spot area, can explain this slope efficiency decrease. Pump spot diameters reaching 300 μm have been used by Mateo et al.16 to pump an AlGaInP VECSEL with 16 pump passes and with a 640 nm fiber coupled diode laser as a pump source. They have achieved a laser output of 2.5 W with a slope efficiency of 17 %. Furthermore, in-well pumped laser operation can be extended to high heat-sink temperatures. In Fig. 7 the output power characteristics of the VECSEL with a pump spot diameter of 90 μm are shown for different heat sink temperatures reaching 30 °C. Even at such a heat-sink temperature, thermal rollover was not observed. The threshold slightly increased and the slope decreased with temperature resulting in a decrease in the output power.
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1.6 φ = 52 μm
Output power (W)
1.4
5 °C, ηslope = 35 %
1.2 1.0
THS = 5 °C
0.8
Outcoupling = 1 % λpump = 638 nm
0.6
24x pump passes
30 ° pump angle
φ = 92 μm
0.4
5 °C, ηslope = 28 %
0.2 0.0
15°C, ηslope = 26 % 30°C, ηslope = 21 %
0
1
2
3
4
5
Pump power (W) Figure 7. Laser performance of the VECSEL with multi-pass pumping and different pump spot sizes, at 1 % of output coupling and a heatsink temperature of 5 °C.
5. SUMMARY AND OUTLOOK We demonstrated an output power of 1.5 W from a multi-pass and quantum-well pumped red AlGaInP VECSEL operated in CW and fundamental-wavelength mode. Quantum-well pumping combined with the multi-pass pumping resulted in an increased output power of AlGaInP VECSELs without the need to cool the sample below 0 °C. For further power scaling homogeneity of the samples should be increased. Additionally, a design specifically optimized for external multi-pass pumping with a second resonance for the pump can be used. This can be achieved by increasing the cavity length such that two neighboring resonances fulfill the conditions for the pump and laser resonance at their respective pump angle. Furthermore, the QWs have to be positioned where approximately both the laser and the pump fields have their antinodes preferably near the sample surface and hence the heatsink. In combination with adapted multipass pump optics, it should be possible to increase both the pump absorption efficiency and the pump power density, and by this the overall efficiency.
6. ACKNOWLEDGEMENT This work was supported by the German Research Foundation (DFG) through the project "Diode-pumped GaInP disk laser with frequency doubling into the UV region", Project Nos.: Br 3606/4-1 and Mi 900/24-1.
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REFERENCES [1] Kuznetsov, M., Hakimi, F., Sprague, R., and Mooradian, A., “High-power (> 0.5-W CW) diode-pumped verticalexternal-cavity surface-emitting semiconductor lasers with circular TEM00 beams,” IEEE Photon. Technol. Lett. 9(8), 1063-1065 (1997). [2] Agustinis, P., Berg, K., Cengel, K.A., Foster, T.H., Girotti, A.W., Gollnick, S.O., Hahn, S.M., Hamblin, M.R., Juzeniene, A., Kessel, D., Korbelik, M., Moan, J., Mroz, P., Nowis, D., Piette, J., Wilson, B. C., and Golab, J., “Photodynamic therapy of cancer: an update,” CA Cancer J. Clin. 61(4), 250-81 (2011). [3] Chung, H., Dai, T., Sharma, S. K., Huang, Y.-Y., Carroll, J. D., Hamblin, M. R., “ The Nuts and Bolts of LowLevel laser (Light) Therapy,” Annals of Biomedical Engineering 40 (2), 516-533 (2012). [4] Kuramoto, K., Nishida, T., Abe, S., Miyashita, M., Mori, K., and Yagi, T., “High power operation of AlGaInP red laser diode for display applications,” Proc. SPIE 9348, 93480H (2015). [5] Kahle, H., Bek, R., Heldmaier, M., Schwarzbäck, T., Jetter, M., and Michler, P., “High optical output power in the UVA range of a frequency-doubled, strain-compensated AlGaInP-VECSEL,” Appl. Phys. Express 7(092705), 1-4 (2014). [6] Müller, M. I., Linder, N., Karnutsch, C., Schmid, W., Streubel, K., Luft, J., Beyertt, S. S., Giesen, A., and Dohler, G. H., “Optically pumped semiconductor thin-disk laser with external cavity operating at 660nm,” Proc. SPIE 4649, 265–271 (2002). [7] Hastie, J., L. Morton, A. Kemp, M.Dawson, A. Krysa, and J. Roberts, "Tunable ultraviolet output from an intracavity frequency-doubled red vertical-external-cavity surface-emitting laser" Appl. Phys. Lett. 89, 061114 (2006). [8] Baumgärtner, S., Kahle, H., Bek, R., Schwarzbäck, T., Jetter, M., and Michler, P., “Comparison of AlGaInPVECSEL gain structures,” J. Cryst. Growth 414, 219-222 (2015). [9] Schwarzbäck, T., Eichfelder, M., Schulz, W-M., Roßbach, R.. Jetter, M., and Michler, P., “Short wavelength redemitting AlGaInP-VECSEL exceeds 1.2W continuous-wave output power,” Appl. Phys. B 102, 789-794 (2011). [10] Schwarzbäck, T., Bek, R., Hargart, F., Kessler, C. A., Kahle, H., Koroknay, E., Jetter, M., and Michler, P., “Highpower InP quantum dot based semiconductor disk laser exceeding 1.3W,” Appl. Phys. Lett. 102(092101), 1-4 (2013). [11] Afromowitz, M., “Thermal conductivity of Ga1-xAlxAs alloys,” J. Appl. Phys., 44, 1292-1294 (1973). [12] Mateo, C. M. N., Brauch, U., Schwarzbäck, T., Kahle, H., Jetter, M., Abdou Ahmed, M., Michler, P., and Graf, T. "Enhanced efficiency of AlGaInP disk laser by in-well pumping," Opt. Express 23, 2472-2486 (2015). [13] Beyertt, S.-S., Brauch, U., Demaria, F., Dhidah, N., Giesen, A., Kübler, T., Lorch, S., Rinaldi, F., and Unger, P., “Efficient gallium-arsenide disk laser,” IEEE J. Quantum Electron. 43(10), 869–875 (2007). [14] Liau, Z. L.,” Semiconductor wafer bonding via liquid capillarity,” Appl. Phys. Lett. 77, 651-653 (2000). [15] Giesen, A., (2011), retrieved from: http://elib.dlr.de/73040/1/2011-11-17_DEPS_High_Power_TDL.pdf. [16] Mateo, C. M. N., Brauch, U., Schwarzbäck, T., Kahle, H., Jetter, M., Abdou Ahmed, M., Michler, P., and Graf, T. "2.5 W output at 665 nm from a multi-pass and quantum-well pumped AlGaInP VECSEL," submitted to Optics Letters (2016)
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