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We report a high-power optically pumped semiconductor vertical external cavity surface emitting laser operating at 2-µm wavelength. The gain material consisted of 15 GaInSb quantum-wells placed within a threelambda GaSb cavity and grown on the top of an 18-pairs AlAsSb/GaSb Bragg reflector. For thermal management we have used a transparent diamond heat spreader bonded on the top of the structure. When cooled down to 5°C, the laser emitted up to 1 W of optical power in a nearly diffraction-limited Gaussian beam demonstrating the high potential of antimonide material for VECSEL fabrication.
©2006 Optical Society of America
1. Introduction
Optically pumped semiconductor vertical external cavity surface emitting lasers (OPS-VECSELs), first introduced by M. Kuznetsov et al. [1], combine many of the advantages of traditional solid-state lasers with the versatility offered by semiconductor gain materials. Although they are less compact than the vertical cavity surface emitting lasers (VCSEL) or the edge-emitting lasers, the VECSELs are very attractive because they can deliver diffraction limited beams with power scalable to multi-Watt levels [2]. The external cavity conveniently enables the use of intra-cavity filters to achieve narrow linewidth [3] or wavelength tunable operation [4]. Non-linear elements can also be easily included within the cavity for short pulse generation [5] or to achieve efficient frequency conversion [6]. The advantages offered by VECSEL-based optical sources are becoming increasingly apparent for many applications spanning from medical investigations to display technologies and environmental monitoring. To date, the most impressive results have been obtained from OPS-VECSELs operating around the 1-µm wavelength range. Here the rapid progress was possible owing to a combination of several advantageous factors such as the availability of high power semiconductor pump diodes and suitable semiconductor materials, which allow for facile fabrication of the gain region and Bragg mirror. Recently, the growing interest for VECSEL has triggered their developments at other wavelength domains such as the red [7], 1.3-µm [8] and 1.55-µm [9]. Other spectral range of interest for example in gas spectroscopy and environmental monitoring, is located around 2-µm [10]. The 2-µm wavelength range can be reached by using Sb-based compound semiconductors [11, 12]. While essential progress has been obtained in the development of Sb-based edge emitting laser diodes [13, 14], the output power delivered by the first antimonide-based OP-VECSELs was limited to mW levels [15, 16].
In this letter we demonstrate that antimonide compound semiconductors are suitable for fabrication of high performance VECSELs. A 2-µm VECSEL with 1-W output power is demonstrated.
2. The laser design
The VECSEL structure was grown in a single epitaxial step on GaSb substrate by using molecular beam epitaxy. It consists of a distributed Bragg reflector (DBR) and a quantum-well (QW) gain section. The distributed Bragg reflector includes 18 pairs of quarter-wave AlAsSb and GaSb layers. Owing to the high contrast between the refractive indices of the constituting layers, the DBR has a reflectance of over 99.8% in a broad wavelength range; the stop-band spans from ~1850 nm to ~2150 nm. The active region comprises 5 groups of 3 Ga0.78In0.22Sb quantum wells with a width of 8 nm. Each QWs group was placed at an antinode of the optical field in the 3-λ GaSb Fabry-Perot cavity defined by the DBR and the semiconductor-air interface. An additional AlAsSb layer was grown on the top of the gain section to ensure good confinement of the photocarriers generated within the GaSb layers by optical pumping and to avoid non-radiative recombination on the surface. The structure was terminated with a 30-nm thick GaSb cap layer used to protect against oxidation of the AlAsSb layer. Figure 1 shows the reflectivity of the structure revealing a Fabry-Perot resonance at the wavelength of 2035 nm. Across the 2″ wafer, the resonant wavelength had a variation of ~1% between 2010 nm and 2038 nm.
A 2.5×2.5 mm2-size chip was scribed off the wafer and capillary bonded with water to a type IIa natural diamond heat spreader with slightly larger dimensions of 3×3×0.3 mm3. The diamond was bonded on the top of the gain structure close to the QWs, thus enabling efficient heat removal from the active region. The bonded chip was finally pressed between two copper plates. The top plate had a small hole in it to allow the passage of pump and signal. The mounted sample was attached to a water cooled copper heat sink.
The laser cavity, shown in Fig. 2, had a V-type configuration consisting of the mounted gain chip, a high reflective folding mirror with radius of curvature of 150 mm, and a plane output coupler. A fiber coupled 790-nm diode laser from LIMO GmbH was used for optical pumping of the gain region. The pump beam was focused to a spot of about 180 µm in diameter at an angle of 35° to the surface normal. The pump absorption occurs only within the GaSb and Ga0.78In0.22Sb QW layers.
Fig. 2. Schematic representation of the VECSEL cavity. RoC: radius of curvature; OC: output coupler.
3. Results and discussions
The cavity length was carefully adjusted to match the laser mode size with the pumped area. Free-running laser emission was observed at wavelength near 2025 nm. First, we have studied the influence of the output coupler on the emission characteristics for couplers with 1%, and 2% transmission. The dependence between the output power and the pump power for operation near room temperature (15°C) is shown in Fig. 3. The maximum output powers were 500 mW and 700 mW for the 1% and 2% couplers, respectively. The profile of the output beam was measured at 300 mm and 430 mm distances from the output coupler with a 5-µm pinhole and a photodetector. The beam profile had a Gaussian shape (shown as inset in Fig. 3) and a divergence of about 0.12°. The corresponding beam quality factor M2, determined using the knife-edge technique, was less than 1.45.
Fig. 3. Output characteristics of the VECSEL for operation near room temperature. Inset: the spatial profile of the laser beam measured at 430 mm distance from the output coupler.
Next we measured the laser emission characteristics as a function of the mount temperature. For this study we used a 2% output coupler. The results are presented in Fig. 4. With moderate cooling of the mount temperature to 5°C, we achieved a maximum output power of 1 W. The measurements show a strong dependence of the threshold pump power, roll-over point and efficiency on the mount temperature. The result indicates that the heat transfer from the gain section to the heat sink is not sufficient for optimal operation of the laser. A key element for improving the laser performance would be to reduce the quantum defect caused by the large energy difference of the pump and laser photons. Pumping at longer wavelength could significantly improve the optical-to-optical conversion efficiency and reduce the heat load to the gain section. We should also note that about 24% of the pump power was reflected from the device. By appropriate coating of the intracavity diamond and the gain chip, the output power and slope efficiency would be further improved. With optimized pumping and heat management we believe that it is possible to increase the output power to the multi-Watt level.
The typical spectral characteristics of the device, measured with a spectrometer having a resolution of 0.6 nm, are presented in Fig. 5. The emission spectrum was shaped by the etalon effect owing to reflection from the diamond surface. Indeed, the separation between the different lines of the comb corresponds to the free spectral range of the ~300 µm diamond.
Fig. 4. Light output characteristics of the VECSEL for different temperatures of the mount. The output coupler was 2%. Inset: the dependence between the threshold pump power and the mount temperature.
4. Conclusions
We have demonstrated a 1-W continuous wave VECSEL emitting at 2-µm wavelength range. Emission was obtained for a low pump threshold in a nearly diffraction-limited beam with a M2 value below 1.45. The overall performance of the laser shows that Sb-based active material has a high potential for the development of VECSELs operating at 2-µm. By further optimization of the cavity and pumping scheme, we expect to scale the power up to few Watts. Further studies would be focused on improving the thermal management and demonstrating operation in a passive mode-locking regime.
Acknowledgments
This work was supported by EU project NATAL (IST-016769), Finnish Funding Agency for Technology and Innovation TEKES, Jenny and Antti Wihuri foundation, National Graduate School of Nanosciences, Emil Aaltonen’s foundation and Nokia foundation.
References and links
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