Multi-watt orange light generation by intracavity frequency doubling in a dual-gain quantum dot semiconductor disk laser

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1 Invited Paper Multi-watt orange light generation by intracavity frequency doubling in a dual-gain quantum dot semiconductor disk laser J. Rautiainen* a, I. Krestnikov b, J. Nikkinen a, O. G. Okhotnikov a a Optoelectronics Research Centre, Tampere University of Technology, Korkeakoulunkatu 3, Tampere, Finland; b Innolume GmbH, Konrad-Adenauer-Allee 11, Dortmund, Germany ABSTRACT We demonstrate a frequency doubled dual-gain quantum dot semiconductor disk laser operating at 590 nm. The reflective gain element, grown by molecular beam epitaxy, has active region composed of 39 layers of InGaAs Stranski- Krastanov quantum dots. The gain mirrors produce individually 3 W and 4 W of output power while the laser with both elements in a single cavity reveals 6 W at 1180 nm with beam quality factor of M 2 <1.2. The loss induced by the nonlinear crystal is compensated by gain boosting in the dual-gain laser and 2.5 W of output power at 590 nm was achieved after frequency conversion. Keywords: Semiconductor disk laser, frequency conversion, quantum dot 1. INTRODUCTION Optically pumped semiconductor disk lasers (OP-SDLs), also known as vertical external cavity surface emitting lasers (VECSELs), have recently gained much interest among scientific and industrial communities due to the unique properties that are unlikely achievable with VCSELs or in-plane semiconductor lasers 1. Specifically the capability of producing high optical power with nearly diffraction limited beam is a combination that makes the SDL desirable in numerous applications. In addition, SDL concept allows wide spectral coverage ranging from visible wavelengths to the mid-ir 2,3. The optical pumping by low-cost high-power multimode diode laser provides high potential for power scaling while preserving good beam quality 4. The emerging demand for laser projection displays has created a need for a laser that can produce visible radiation with good beam quality in order to form a high-contrast image 5. Though the direct generation of visible light from a semiconductor disk laser is basically limited to red wavelengths 2, the high-q cavity of an SDL allows for efficient intracavity frequency conversion of infrared emission in a nonlinear crystal to cover the RGB spectral range 6. The efficiency of frequency doubling can be high even with continuous wave operation making these sources attractive option for a laser projection display. This technology producing orange-red emission can be used in medical applications 7 and laser guide stars for correcting the atmospheric distortions of images obtained with terrestrial telescopes 8. Orange emission can be generated by frequency doubling of an 1180 nm radiation. There are few options available for producing this wavelength from an SDL, each having certain constraints that limit their practical value. The InGaAsbased gain materials, widely used in SDLs operating at nm spectral range 4, are usually grown monolithically on top of a GaAs/AlGaAs distributed Bragg reflector (DBR). This material composition offers high refractive index contrast resulting in high reflectivity for the signal wavelength. High power 1180 nm InGaAs SDL with frequency doubling has been reported 9. Further increase in the emission wavelength is, however, limited due to high strain in the suitable structures. *jussi.rautiainen@tut.fi; phone ; fax Nonlinear Frequency Generation and Conversion: Materials, Devices, and Applications X, edited by Konstantin L. Vodopyanov, Proc. of SPIE Vol. 7917, SPIE CCC code: X/11/$18 doi: / Proc. of SPIE Vol

2 The strain inflicted by the InGaAs can be moderated by adding nitrogen to the crystal lattice. The resulted InGaAsN material composition is commonly referred as dilute nitride compound 10. A few percent of nitrogen could reduce the bandgap considerably, and consequently extends the available wavelength to 1.55 μm 11.The adding of nitrogen could increase the rate of nonradiative recombination that in turn can reduce the laser efficiency and thus making the epitaxial growth of these structures to be a critical matter. A post growth thermal annealing should be usually applied to prevent the degradation of the crystal quality 12. One promising alternative for generating 1180 nm emission is to use quantum dot (QD) based active media, which exhibit loosen requirement for the lattice matching 13,14. The increased localization of charge carriers induces unique properties such as low threshold, temperature insensitive operation, wide gain bandwidth, significant tunable operation and short pulse generation 15. On a downside, the reduced overlapping of the light with the active material results in lower gain that can be compensated to certain extent by increasing the quantum dot density. Here we report a quantum dot semiconductor disk laser frequency doubled to 590 nm in a dual-gain cavity. The gain mirror has 39 layers of InGaAs QDs grown in Stranski-Krastanov growth mode. The dual-gain configuration shared the pump induced thermal load among two active media and produces gain sufficient for efficient intracavity frequency conversion. 2. GAIN MIRROR STRUCTURE The semiconductor gain element designed for 1180 nm was fabricated with molecular beam epitaxy (MBE) on a GaAs substrate. The DBR, consisting of 35 layer pairs of λ/4 thick GaAs/AlAs, was grown first on top of the substrate. On top of the mirror is the active region with 39 layers of InGaAs Stranski-Krastanov quantum dot layers that were designed to operate at the first exited state. The QD layers were arranged in 13 identical groups at the antinodes of the standing wave. A 35 nm thick GaAs layer was positioned between each QD layer and the same material was also used as a pump absorbing spacer layer between each group. An AlGaAs window layer was grown on top of the active region, which was protected against oxidation with 42.6 nm thick GaAs cap layer. The room temperature reflectivity for beam propagating normal to the surface and the photoluminescence spectrum, measured from the edge of the wafer, are presented in Fig. 1. It can be observed that the reflectivity has a stopband bandwidth of ~120 nm and the photoluminescence peaks around 1170 nm. More detailed description of the gain mirror is described elsewhere 14. Reflectivity Reflectivity Photoluminescence Photoluminescence intensity (a.u.) Figure 1. Room temperature reflectivity and photoluminescence intensity spectrum of the gain mirror. Proc. of SPIE Vol

3 The as-grown wafer was cut into 2.5 mm 2.5 mm chips and two of the chips were water bonded to ~300 μm thick transparent intracavity diamond heat spreaders 16. The assemblies were then sandwiched between copper blocks with indium foil that operates as a fitting layer. The top surfaces of the diamond heat spreaders were antireflection coated for the laser wavelength with SiO 2 /TiO 2 layers. The reflectivity for the pump wavelength was ~10 %, which takes into account the reflectivity from both surfaces of the diamond. 3. GAIN MIRROR CHARACTERIZATION First, the two gain mirrors were tested separately in linear cavity configurations, in which the gain mirror operates as one cavity mirror and a spherical mirror as an output coupler. The gain mirrors #1 and #2 were pumped optically with 808 nm and 788 nm fiber-coupled diode lasers, respectively, to a spot diameter of 180 μm. The spherical mirror has a 75 mm radius of curvature (RoC) and the distance to the gain mirror ensures the mode size to be equal to the pump spot at the gain mirror. Mirrors, with output coupling (OC) in a range from 0.2 % to 0.8 %, were tested. The temperature of the gain mirrors was set to 9 C with a Peltier element. The output power as a function of the launched pump power is plotted in Fig. 2 for the both gain mirrors. The maximum output power with the gain mirror #1 was more than 4 W, while the SDL with gain mirror #2 produced 3 W. The beam quality factor M 2 was measured for the both gain mirrors, for two orthogonal directions, with the 0.7 % output coupler. It was found that the M 2 value for the gain mirror #1 varied between 1.1 and 1.2 at different powers. The laser with gain mirror #2 exhibited somewhat larger M 2 factors of Output power (W) OC=0.2 % OC=0.4 % OC=0.7 % OC=0.8 % OC=0.2 % OC=0.4 % OC=0.7 % OC=0.8 % Input pump power (W) Input pump power (W) (a) (b) Figure 2. (a) Output power of the disk laser #1 and (b) disk laser #2 with different output couplers. Output power (W) The next step was to characterize the wavelength tunability of the device. The tuning was performed with the gain mirror #1 in a V-type cavity, where the gain mirror and a plane mirror operated as end mirrors and a spherical mirror as a folding mirror. The temperature and pump spot diameter were kept the same as in previous measurements. The tuning was performed with a 1.5 mm thick birefringent filter, placed at the Brewster angle between the spherical and the plane mirrors. The pump power was 20 W. When all the mirrors were highly reflective, 68.8 nm tuning range was achieved with 80 mw of maximum output power at the center of the tuning range. The replacing of the highly reflective plane mirror to a mirror with a transmission of 0.4 %, resulted in a smaller tuning range of 22 nm but with increased output power of 270 mw, as shown in Fig. 3. Proc. of SPIE Vol

4 Intensity (10dB/div.) Output power (mw) Figure 3. Wavelength tuning observed with 0.4 % output coupler. Output power (W) DUAL-GAIN LASER AND FREQUENCY DOUBLING Next, the dual-gain configuration was characterized in a Z-cavity built with the gain mirror #1 and a curved output coupler as cavity end reflectors. The gain mirror #2 and a highly reflective spherical mirror acted as folding mirrors. The cavity maintains equal mode sizes on both gain mirrors. In this configuration, the 0.8 % output coupler was found to enable the highest output power of 6 W, as shown in Fig. 4(a). Since the Z-cavity exhibits higher losses, the optimal output coupling transmission was only slightly higher as compared to the single-gain setup. As expected, more total pump power can be launched to the gain mirrors, before the thermal rollover appears resulting in higher output power. Typical spectrum of the laser, shown in Fig. 4(b), reveals the spectral components induced by the etalon effect of the heat spreaders. The beam quality factor was found to be less than 1.2 and was fairly independent of the output power. OC=0,4 % OC=0,7 % OC=0,8 % OC= % Total pump power (W) Intensity (10 db/div.) (a) (b) Figure 4. (a) Output power characteristics of the dual-gain laser with different output couplers. (b) Output spectrum of the laser measured at 6 W of output power with 0.8 % output coupler. Proc. of SPIE Vol

5 The cavity with nonlinear crystal for second harmonic generation, shown in Fig. 5, uses the dual-gain scheme. The spherical output coupler was replaced here with a highly reflective mirror and one more cavity arm was assembled where the nonlinear crystal was placed. The waist diameter at the location of the 4 mm long BBO crystal was ~180 μm. The crystal was antireflection coated for both, the fundamental and frequency doubled wavelengths. The laser produced 2.5 W of power at the 590 nm wavelength, as shown in Fig. 6. Figure 5. Dual-gain cavity configuration for the second harmonic generation. All the mirrors are highly reflective for the fundamental wavelength. 100 mm RoC end mirror has high reflectivity for the second harmonic wavelength of 590 nm. The output is taken after 100 mm RoC folding mirror which has a high transmission for the orange wavelength. Output power at 590 nm (W) Total pump power (W) (a) (b) Figure 6. (a) Output power at 590 nm as a function of total launched pump power. (b) Optical spectrum of the laser recorded at 2.4 W of output power. Intensity (10 db/div.) Proc. of SPIE Vol

6 5. CONCLUSIONS We have demonstrated a dual-gain quantum dot disk laser. The laser produced 6 W of power at 1180 nm with beam quality factor M 2 less than 1.2. The dual-gain laser with frequency doubling in a BBO crystal demonstrates 2.5 W of power at orange wavelength of 590 nm. The multiple gain approach is particularly useful with quantum dot based gain media, in order to enhance the total gain and compensate the losses induced by the intracavity frequency conversion element. 6. ACKNOWLEDGEMENTS This work has received funding from the European Community s Seventh Framework Programme (FAST-DOT) under grant agreement , Walter Ahlström Foundation and HPY Research Foundation. The authors would like to thank Edik Rafailov and Mantas Butkus from the University of Dundee for useful discussions and technical help. 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 TEM 00 beams," IEEE Photon. Tech. Lett. 9(8), (1997). [2] Hastie, J. E., Calvez, S., Dawson, M. D., Leinonen, T., Laakso, A., Lyytikäinen, J. and Pessa, M., "High power CW red VECSEL with linearly polarized TEM 00 output beam, " Opt. Express 13(1), (2005). [3] Schulz, N., Hopkins, J., Rattunde, M., Burns, D. and Wagner, J., "High-brightness long-wavelength semiconductor disk lasers," Laser & Photon. Rev. 2(3), (2008). [4] Chilla, V., Shu, Q., Zhou, H., Weiss, E., Reed, M. and Spinelli, L., "Recent advances in optically pumped semiconductor lasers," Proc. SPIE 6451, (2007). [5] Chellappan, K., Erden, E. and Urey, H., "Laser-based displays: a review," Appl. Opt. 49(25), F79 F98 (2010). [6] Calvez, S., Hastie, J., Guina, M., Okhotnikov, O. and Dawson, M., "Semiconductor disk lasers for the generation of visible and ultraviolet radiation," Laser & Photon. Rev. 3(5), (2009). [7] Dougherty, T., Gomer, C., Jori, G., Kessel, D., Korbelik, M., Moan, J. and Peng, Q., "Photodynamic therapy," JNCI J. Natl. Cancer Inst. 90(12), (1998). [8] Wizinowich, P., Le Mignant, D., Bouchez, A., Campbell, R., Chin, J., Contos, A., van Dam, M., Hartman, S., Johansson, E., Lafon, R., Lewis, H., Stomski, P., Summers, D., Brown, C., Danforth, P., Max, C. and Pennington, D., "The WM Keck Observatory laser guide star adaptive optics system: overview," PASP 118(840), (2006). [9] Fallahi, M., Fan, L., Kaneda, Y., Hessenius, C., Hader, J., Li, H., Moloney, J., Kunert, B., Stolz, W., Koch, S., Murray, J. and Bedford, R., "5-W yellow laser by intracavity frequency doubling of high-power verticalexternal-cavity surface-emitting laser," IEEE Photon. Tech. Lett. 20(20), (2008). [10] Kondow, M., Kitatani, T., Nakatsuka, S., Larson, M. C., Nakahara, K., Yazawa, Y., Okai, M. and Uomi, K., "GaInNAs: a novel material for long-wavelength semiconductor lasers," IEEE J. Sel. Top. Quantum Electron. 3(3), (1997). [11] Sun, H., Clark, A., Liu, H., Hopkinson, M., Calvez, S., Dawson, M., Qiu, Y. and Rorison, J., "Optical characteristics of 1.55 μm GaInNAs multiple quantum wells," Appl. Phys. Lett. 85(18), (2004). [12] Tournié, E., Pinault, M. and Guzmán, A., "Mechanisms affecting the photoluminescence spectra of GaInNAs after post-growth annealing," Appl. Phys. Lett. 80(22), (2002). [13] Germann, T. D., Strittmatter, A., Pohl, U. W., Bimberg, D., Rautiainen, J., Guina, M. and Okhotnikov, O. G., "Quantum-dot semiconductor disk lasers," J. Cryst. Growth 30(23), (2008). [14] Rautiainen, J., Krestnikov, I., Butkus, M., Rafailov, E. U. and Okhotnikov, O. G., "Optically pumped semiconductor quantum dot disk laser operating at 1180 nm," Opt. Lett. 35(5), (2010). [15] Blood, P., "Gain and Recombination in Quantum Dot Lasers," IEEE J. Sel. Top. Quantum Electron. 15(3), (2009). [16] Liau, Z. L., "Semiconductor wafer bonding via liquid capillarity," Appl. Phys. Lett. 77(5), (2000). Proc. of SPIE Vol