We report on a GaInNAs/GaAs semiconductor disk laser frequency-doubled to produce orange-red radiation. The disk laser operates at a fundamental wavelength of 1224 nm and delivers an output power of 2.68 W in the visible region with an optical-to-optical conversion efficiency of 7.4 %. The frequency-converted signal could be launched into a single-mode optical fiber with 70–78 % coupling efficiency, demonstrating good beam quality for the visible radiation. Using a Fabry-Pérot glass etalon the emission wavelength could be tuned over an 8 nm spectral range.
©2007 Optical Society of America
High power lasers emitting at visible wavelengths are needed for a number of applications including optical pumping, life sciences and laser projection displays. Semiconductor disk lasers are capable of producing multi-watt output power with good beam quality [1,2]. These light sources take advantage of semiconductor gain material, optical pumping with low-cost multi-mode pump diodes, and an optical configuration that allows the use of non-linear crystals inside the laser cavity.
Multi-watt visible emission has been produced from frequency-doubled semiconductor disk lasers at blue , green  and yellow  wavelengths using the InGaAs/GaAs material system. Recently, direct generation of red emission at 674 nm wavelength from a GaInPAlGaInP/GaAs-based source pumped by a green solid-state laser  was demonstrated. However, it is increasingly difficult to extend the GaInP technology towards wavelengths below 635 nm, because of a lack of suitable direct bandgap barrier materials that would provide sufficient carrier confinement in the quantum wells. On the other hand, increased lattice mismatch between InGaAs and GaAs hinders the use of this materials system at the wavelengths above 1180 nm that are needed for generation of amber emission through frequency-doubling. As a consequence, the amber-orange-red spectral region between 590 and 635 nm is difficult to approach utilizing InGaAs and GaInP compounds. Gerster et al. have demonstrated a frequency-doubled orange-red (610 nm) source based on GaAsSb/GaAs material . Alternatively, conventional InGaAs technology can be extended to longer wavelengths by introducing a small amount of nitrogen into the crystal lattice . Dilute nitride GaInNAs disk lasers operating at 1.3 µm have been reported  previously. Recently, we have obtained 617 nm emission from an intracavity frequency-doubled disk laser based on GaInNAs material [10,11].
In this letter we demonstrate that dilute nitride semiconductor is suitable for generating multi-watt orange-red output with high efficiency and good beam quality.
2. Semiconductor structure and device processing
The gain mirror was grown by molecular beam epitaxy equipped with a RF-plasma source for incorporating the nitrogen into the semiconductor crystal. Apart from the quantum wells (QWs) and barriers composition, the gain mirror structure is typical for any semiconductor disk laser. It includes a high reflective Bragg stack and a multiple quantum wells gain region grown on an n-type GaAs substrate. The distributed Bragg reflector (DBR) was comprised of 30 pairs of ¼-λ thick AlAs/GaAs layers. The active region included ten 9-nm-thick GaInNAs QWs uniformly distributed in five pairs. For strain compensation/mediation we used 4-nmthick GaAsN layers surrounding each QW. Pump absorbing GaAs spacer layers were used to position the QW pairs in-line with the antinodes of the standing wave formed between the DBR and semiconductor-air interface. A large bandgap AlGaAs window layer was grown on top of the gain section to avoid non-radiative recombination of photo-carriers at the surface of the gain mirror. The structure was completed with a thin GaAs layer that protects the window layer from the oxidation.
Since the gain mirror operates under intense pumping conditions, thermal management is a crucial issue for the disk laser. In our laser the generated heat was conducted to a heat sink using a transparent intracavity heat spreader [12,13]. A 2.5×2.5 mm2 laser chip was scribed from the as-grown wafer and capillary bonded with de-ionized water to a 3×3×0.3 mm3 natural type IIa diamond heat spreader. In this bonding technique , two flat and smooth surfaces are pulled into close contact by the surface tension of water, methanol or another suitable liquid, and bonded together by intermolecular surface forces. The bonded component was assembled between two metallic plates with indium foil in between to ensure good thermal and mechanical contact. The topmost metal plate had a circular aperture for signal and pump beams. The mounting assembly was cooled with circulating water down to 7°C.
3. Experimental and results
The laser includes the gain mirror and three curved mirrors forming a Z-type cavity, as shown in Fig. 1. All curved mirrors are highly reflective for the fundamental wavelength of 1.2 µm and have transmission over 90 % for the second-harmonic radiation. The gain medium was pumped with a fiber-coupled 788 nm pump diode laser at an angle of 35° to the surface normal of the gain mirror. The cavity was designed to match the size of the laser mode with the 290-µm diameter of the pump spot at the gain mirror. A 4-mm long non-linear BBO crystal was placed between mirrors M2 and M3 in the waist of the cavity transverse mode. The waist diameter was calculated to be approximately 160 µm with a Rayleigh length of 18 mm. The BBO crystal was critically, type-I, phase matched and antireflection coated for 1220 nm and 610 nm on both facets. Red emission appears as two beams behind mirrors M2 and M3. In order to measure the output power at different wavelengths independently, the residual infrared power was spatially separated from red emission with a dichroic beam splitter.
The output of the laser was delivered to an optical spectrum analyzer using a multimode fiber. A typical spectrum of the fundamental radiation around 1.2 µm is shown in Fig. 2a. The multiple-line laser spectrum with a spectral period of 0.9 nm originates from the Fabry-Pérot etalon effect induced by the uncoated 337-µm-thick intracavity diamond heat spreader. The corresponding frequency-doubled spectrum at 612 nm is presented in Fig. 2b.
The output characteristics of the laser are presented in Fig. 3a. The total output power from mirrors M2 and M3 was 2.68 W at 612 nm with absorbed pump power of 36.1 W. Power decay has not been found on an hour time scale. The measured total power at the fundamental wavelength of 1224 nm that leaked from the cavity was negligible. The conversion efficiency into the red emission was 7.4 % indicating a significant improvement as compared to the earlier report .
The polarization of the fundamental and the frequency-doubled beams was also studied. The outputs at both wavelengths, shown in Fig. 3b, are plotted as a function of the polarizer angle and indicate that the signals are linearly polarized in orthogonal directions.
Spectrally narrow-line operation was achieved with a 25-µm solid glass etalon inserted in the cavity. The etalon was placed between mirrors M1 and M2. Tilting the etalon enabled a discrete tuning of the operation wavelength with the step corresponding to 0.9-nm freespectral range of the intracavity heat spreader at 1.2 µm. The tuning range at the fundamental wavelength was 16 nm, limited by the free-spectral range of the Fabry-Pérot glass etalon. The corresponding tuning range in the visible was 8 nm, as presented in Fig. 4. Over 40 % of the free-running output power has been obtained in a tunable regime with the thin glass etalon.
The beam profile of the red emission was studied with a CCD-camera. The transverse beam profiles are shown in Fig. 5a. The beam was found to be elliptical with a Gaussian profile for both directions. The ellipticity is likely to originate from spatial walk-off in the BBO crystal. In order to quantify the quality for the elliptical beam, we launched the red emission from the output to a fiber with single-mode guiding in the visible and determined the coupling efficiency for different powers. Launching the laser beam to single-mode fiber has been chosen for output beam characterization since this method automatically accounts for all possible beam distortions, e.g. due to thermal lensing effect, beam ellipticity etc. Specifically, the minor degradation of M2 parameter ascribed to the thermal lens is always observed in SDL with an increase in a pump power. The coupling efficiency into the fiber was 78 % at low power and gradually decreased down to 70 % with an increase in the output power. Coupling efficiency as a function of the fiber-coupled power at 612 nm is shown in Fig. 5b. The ellipticity also exhibited by the beam due to pumping at the angle to the gain mirror is unlikely to progress significantly with pump power. Therefore, the observed reduction in the coupling to the single-mode fiber likely occurs owing to thermally-induced effects.
In conclusion, we have demonstrated an optically-pumped GaInNAs semiconductor disk laser with intracavity second-harmonic generation producing 2.68 W in the orange-red spectral range. To our knowledge this represents the highest power reported to date from a semiconductor disk laser at this wavelength. The optical-to-optical conversion efficiency of the laser was 7.4 %. Narrow band operation with an 8-nm tuning range was obtained with a 25-µm Fabry-Pérot etalon. The visible emission was coupled to a single-mode optical fiber with 70-78 % efficiency demonstrating good beam quality. The dilute nitride GaInNAs quantum well material provides high gain at 1.2 µm and takes advantage of low-cost GaAs technology.
The authors acknowledge the support from EU FP6 project NATAL (contract number IST-016769), the Academy of Finland, and Jenny and Antti Wihuri foundation.
References and links
1. M. Kuznetsov, F. Hakimi, R. Sprague, and A. Mooradian, “High-power (>0.5-W CW) diode-pumped vertical-external-cavity surface-emitting lasers with circular TEM00 beams,” IEEE Photon. Tech. Lett. 9, 1063–1065 (1997). [CrossRef]
2. S. Lutgen, T. Albrecht, P. Brick, W. Reill, J. Luft, and W. Spath, “8-W high-efficiency continuous-wave semiconductor disk laser at 1000 nm,” Appl. Phys. Lett. 82, 3620 (2003). [CrossRef]
3. L. Fan, T.-C. Hsu, M. Fallahi, J. T. Murray, R. Bedford, Y. Kaneda, J. Hader, A. R. Zakharian, J. V. Moloney, S. W. Koch, and W. Stolz, “Tunable watt-level blue-green vertical-external-cavity surfaceemitting lasers by frequency doubling,” Appl. Phys. Lett. 88, 251117 (2006). [CrossRef]
4. J. H. Lee, S.M. Lee, T. Kim, and Y. J. Park, “7 W high-efficiency continuous-wave green light generation by intracavity frequency doubling of an end-pumped vertical external-cavity surface emitting semiconductor laser,” Appl. Phys. Lett. 89, 241107 (2006). [CrossRef]
5. S. Hilbich, W. R. Seelert, V. G. Ostroumov, C. Kannengiesser, R. von Elm, J. Mueller, E. S. Weiss, H. Zhou, and J. L. A. Chilla, “New wavelengths in the yellow orange range between 545 nm and 580 nm generated by intracavity frequency-doubled optically pumped semiconductor lasers,” Proc. SPIE 6451, 64510C (2007). [CrossRef]
6. J. E. Hastie, S. Calvez, D. Dawson, T. Leinonen, A. Laakso, J. Lyytikäinen, and M. Pessa, “High power CW red VECSEL with linearly polarized TEM00 output beam,” Opt. Express 13, 77–81 (2005). [CrossRef] [PubMed]
7. E. Gerster, I. Ecker, S. Lorch, C. Hahn, S. Menzel, and P. Unger, “Orange-emitting frequency-doubled GaAsSb/GaAs semiconductor disk laser,” J. of Appl. Phys. 94, 7397–7401 (2003). [CrossRef]
8. C. S. Peng, T. Jouhti, J. Konttinen, and M. Pessa, “InGaAsN/GaAs lasers: high performance and long lifetime,” Proc. SPIE 5738, 204 (2005). [CrossRef]
9. J.-M. Hopkins, S. A. Smith, C. W. Jeon, H. D. Sun, D. Burns, S. Calvez, M. D. Dawson, T. Jouhti, and M. Pessa, “0.6 W CW GaInNAs vertical external-cavity surface emitting laser operating at 1.32 µm,” Electron. Lett. 40, 30–31 (2004). [CrossRef]
10. A. Härkönen, J. Rautiainen, M. Guina, J. Konttinen, P. Tuomisto, L. Orsila, M. Pessa, and O. G. Okhotnikov, “High power frequency doubled GaInNAs semiconductor disk laser emitting at 615 nm,” Opt. Express 15, 3224–3229 (2007). [CrossRef] [PubMed]
11. J. Rautiainen, A. Härkönen, P. Tuomisto, J. Konttinen, L. Orsila, M. Guina, and O. G. Okhotnikov, “1 W at 617 nm generation by intracavity frequency conversion in semiconductor disk laser,” Electron. Lett. 43, 980–981 (2007). [CrossRef]
12. W. J. Alford, T. D. Raymond, and A. A. Allerman, “High power and good beam quality at 980 nm from a vertical external-cavity surface-emitting laser,” J. Opt. Soc. Am. B 19, 663–666 (2002). [CrossRef]
13. A. J. Kemp, G. J. Valentine, J.-M. Hopkins, J. E. Hastie, S. A. Smith, S. Calvez, M. D. Dawson, and D. Burns, “Thermal Managenemt in Vertical-External-Cavity Surface-Emitting Lasers: Finite-Element Analysis of a Heatspreader Approach,” J. of Quant. Electr. 41,148–155 (2005). [CrossRef]
14. Z. L. Liau, “Semiconductor wafer bonding via liquid capillarity,” Appl. Phys. Lett. 77, 651–653 (2000). [CrossRef]