1. Introduction

The strong demand for large area flat displays has motivated a large development effort aimed at compact laser sources producing high-power visible radiation in red, green and blue (RGB). Semiconductor disk lasers (SDLs) [1], also known as VECSELs, are a promising candidate that can deliver Watt-level output powers [2] at near-infrared wavelengths with superb beam quality. Visible radiation can be obtained conveniently from infrared SDLs using frequency doubling technique or directly at the fundamental wavelength. Impressive results from SDLs emitting in green and blue have been published by a number of research groups using frequency doubling technique [3–5]. The rapid progress in this area was enabled by mature GaAs-based semiconductor technology and the availability of high-power 808 nm pump diodes.

Among visible wavelengths, the generation of red emission looks more challenging. In reference [6] direct generation of high power red emission was demonstrated using green pump radiation from frequency doubled diode-pumped solid state laser. Despite reasonable laser performance, this approach suffers from the lack of commercially available high power short wavelength pump diodes, which makes the pump scheme complicated and expensive. With this technique it is also difficult to achieve high power emission at the shorter wavelengths of the red spectrum. On the other hand, the development of frequency doubled SDLs with red emission has been slow, due to the absence of suitable semiconductor materials for fabricating gain regions and distributed Bragg reflectors (DBRs) operating at around 1200–1250 nm. Owing to their high refractive index difference, GaAs/AlGaAs compound semiconductors are preferred for DBR fabrication over InP-based reflectors. This in turn limits the range of semiconductor materials suitable for the fabrication of gain regions to either use of GaAsSb/GaAs or dilute nitride GaInNAs/GaAs. Frequency doubled emission at 610 nm was previously demonstrated from a GaAsSb/GaAs based disk laser [7]. However, it was observed that due to weak electron confinement in this material system, the gain suffers from strong temperature sensitivity limiting the output power at room temperature [7]. The GaInNAs/GaAs QWs, instead, exhibit good electron confinement, but until now, the development of dilute nitride based SDLs has concentrated on lasers operating close to 1.3 μm [8].

In this paper we present a frequency doubled GaInNAs SDL emitting around 615 nm with total converted power of about 320 mW. The results show that GaInNAs can be used for building high power red sources.

2. Epitaxial design and processing

The gain mirror structure was grown monolithically on a GaAs substrate by solid-source molecular beam epitaxy (SS-MBE) with an RF nitrogen plasma source. The structure consists of a Bragg mirror designed for maximum reflectivity at λ=1220 nm, quantum well (QW) based gain section and a window layer. The Bragg reflector incorporating 30 pairs of GaAs/AlAs layers with $\frac{1}{4}-\lambda$ optical thickness was grown first on the substrate. It was followed by the gain section with 12 GaInNAs quantum wells of 7 nm thickness. The QWs were grouped in six pairs and each pair was positioned at an antinode of the standing-wave field in the micro-cavity defined by the DBR and the semiconductor-air interface. The quantum well groups were separated by pump absorbing spacer layers made of GaAs material. A 1-λ thick high bandgap Al_0.37Ga_0.63As window layer was grown on the top of the gain section to avoid diffusion of carriers to the semiconductor surface. The epitaxial structure was finished with a 5-nm GaAs cap layer protecting the window layer from oxidation.

The laser sample was prepared by attaching a 3 x 3 x 0.3 mm³ sized natural diamond heat spreader on a 2.5 x 2.5 mm² sized as-grown gain mirror by liquid capillary action of water [9]. This technique allows effective heat transfer from the quantum wells operated under intensive optical pumping, and therefore, increases the threshold of thermal rollover. The mounted gain mirror was pressed between two copper plates serving as a heat sink. Thin indium foil was used between the sample and the copper plates in order to ensure good thermal contact and to avoid excessive mechanical stress. The top most copper plate had a 1.5 mm diameter opening in it, which allowed the passage of pump and signal beams. A two-layer TiO₂-SiO₂ antireflective coating was deposited onto the diamond surface through the opening to reduce reflection of the pump and signal wavelengths. The mounted sample was attached to a water cooled metal block that ensured proper cooling.

3. Experimental and results

The laser cavity was formed of the semiconductor gain mirror and three curved high reflective mirrors M1, M2, and M3, arranged in a Z-type configuration, as shown in Fig. 1. The gain mirror was pumped with an 808 nm fiber-coupled diode laser at an angle of 32° relative the surface normal. The pump spot diameter on the gain mirror was approximately 180 μm. The operation wavelength of the laser was controlled with a 25-μm thick uncoated Fabry-Perot glass etalon placed in the cavity between mirrors M1 and M2. We used a 4 mm long type-I critically phase matched barium borate (BBO) crystal for the wavelength conversion to the red. Both ends of the crystal were anti-reflection coated. The crystal location between mirrors M2 and M3 corresponds to location of minimum waist cross-section of the laser mode. The beam diameter within the crystal was calculated to be about 190 μm. During all measurements the laser mount temperature was maintained at 15°C.

 

Fig. 1. Schematics of laser setup: The laser cavity is formed of the gain mirror and three high reflective curved mirrors. Frequency doubled emission was observed from the outputs of mirrors M2 and M3. RoC=Radius of Curvature, D=distance between adjacent mirrors, BBO=barium borate crystal

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Without the nonlinear crystal and the glass etalon, free running laser emission around 1230 nm was observed. The free running laser spectrum, illustrated in Fig. 2, shows a comb of lines separated by the free spectral range of the 300 μm thick diamond heat spreader acting as an intracavity etalon. This feature emerges from the residual reflection of the coated diamond.

 

Fig. 2. Laser spectrum around 1230 nm without the glass etalon and BBO crystal inserted in the cavity.

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Narrow-line operation at near-infrared and red wavelengths was achieved with the intracavity Fabry-Perot glass etalon used as a tuning element. The output at fundamental wavelength was linearly polarized and it could be adjusted within 9 nm spectral range by turning the etalon. Consequently, 4.5 nm tuning range from 612.5 nm to 617 nm was achieved for the red emission, as plotted in Fig. 3. Tuning was discrete with a step of about 0.5 nm that is half of the tuning step set by the diamond etalon to the fundamental wavelength.

 

Fig. 3. Ten narrow-line emission spectra at the red wavelength tuned with the Fabry-Perot glass etalon.

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The fundamental and frequency converted radiations were separated using prisms to perform power measurements. The combined power of red emission from mirrors M2 and M3 was found to exceed 170 mW over the entire tuning range. Maximum combined output power of 320 mW was observed at 614.5 nm with 18.2 W of pump power. Under these conditions the output from M2 was 148 mW and 172 mW from M3. Light output characteristics for the red and residual IR emission are shown in Fig. 4.

 

Fig. 4. Light output characteristics at fundamental and frequency doubled wavelength.

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Maximum output power obtained at fundamental wavelength from this sample with 1% out-coupler was 1.46 W. Therefore, conversion efficiency from IR to visible was estimated to be about 20%. It should be noted, that the two output scheme presented in this study could be avoided by using a nonlinear crystal with proper high reflective coating on one crystal surface. The laser could be also further compacted with use of electrical pumping [10]. Optical pumping was used in our experiments for its simplicity in laboratory condition and low demand for sample processing. Based on these experiments it is too early to say how well GaInNAs material is suited for use in electrically pumped SDLs, but we see no fundamental factor preventing the operation of such lasers.

3. Conclusion

We have demonstrated tunable red emission from a frequency doubled semiconductor disc laser based on GaInNAs material system and grown by MBE. The laser was pumped with an 808 nm diode laser and it produced 320 mW of narrow-line red radiation. The emission wavelength could be tuned 4.5 nm near 615 nm using an intra-cavity Fabry-Perot glass etalon. The practical significance of this approach is determined by the use of mature and relatively low cost GaAs technology. This study demonstrates that GaInNAs based frequency doubled SDLs have potential to produce high power orange-red emission that is difficult to achieve directly from semiconductor lasers.