High Power 490 nm Laser Based on Semiconductor Disk Intracavity Frequency Doubling

Objective The attenuation coefficients of blue and green light in the 470-580 nm band are the smallest in seawater, especially at the peak of transmittance near 490 nm. Therefore, blue-green lasers have important application prospects in underwater communications, laser detection, and radars. Currently, blue-green lasers can be realized using a middle -infrared laser quadruple frequency, solid-state laser sum frequency, and gas laser and AlGaN semiconductor laser direct excitation. However, these methods have low energy conversion efficiency and poor beam quality. The advantages of semiconductor disk lasers used to produce blue and green lasers are good beam quality, high -frequency doubling efficiency, and improved stability and reliability. The thermal problem is a key factor affecting the performance of semiconductor disk lasers and must be improved by optimizing the packaging structure. The semiconductor disk packaging process uses Ti -Pt -Au as the bonding layer and realizes bonding between the chip and diamond by the solid -liquid diffusion bonding of gold and indium. Pt acts as a diffusion medium for bonding. The experiment conducted herein identified that this method has some problems. Pt tends to spread onto the diamond surface and condense to form points during electron -beam evaporation. Packaging quality decreases and thermal resistance increases, limiting laser performance improvement.Methods The epitaxial structure of the 980 nm semiconductor disk consists of 26 pairs of distributed Bragg reflectors with undoped AlAs/GaAs layers, six pairs of active regions with InGaAs double quantum wells, and a high bandgap energy cap layer (Fig. 1). The quantum well spatial position in the epitaxial structure of the semiconductor disk must coincide with the standing -wave peak at the designed wavelength (Fig. 2). Based on the Ti -Pt -Au packaging technology, a Cu-Sn alloy with high thermal conductivity is selected as the barrier layer to increase the thickness between Pt and diamond. Pt is prevented from condensing on the diamond surface and the packaging process is improved. A 490 nm laser with high power is obtained by constructing a V-shaped cavity and using an LBO crystal cavity with intracavity frequency doubling (Fig. 6). Results and discussions A direct cavity is used to test the performance of the semiconductor disk laser. The output coupler M1 is a concave mirror with curvature radius of 77.5 mm and reflectance coating of 97%. The resonator cavity length is 90 mm (Fig. 4). A fiber laser of 808 nm wavelength of is used as the pump source and the spot size is 400 p.m. The temperature of the chip is controlled using a thermoelectric cooler (TEC) and the temperature is set to 10 degrees C . The laser slope efficiency reaches 47.3%. When the absorption pump power reaches 52.7 W, the output power will reach 22.5 W. The total optical -to -optical conversion efficiency is 42.7% (Fig. 5). The V-shaped cavity is used for second harmonic generation output. The output coupler M1 is a concave mirror with curvature radius of 77.5 mm, the reflection film of 996 nm 99.5% and antireflection film of 498 nm 99.5% are coated. M2 is a parallel -plane mirror -plated 996 and 498 nm 99.5% reflection film. The size of the LBO crystal is 3 mm x 3 mm x 10 mm (Fig. 6). The temperature of the crystal is controlled using a thermoelectric cooler (TEC) and the temperature is set to 10 degrees C. The slope efficiency of the blue and green light output is 17.8%, the maximum output power is 4.8 W and the total optical -optical conversion efficiency is 15. 4% (Fig. 8). After frequency doubling, the wavelength of the blue and green light is 496. 1 nm (Fig. 9). The pump spot on the surface of the disk has a 400 p.m diameter. Under the spot size, the maximum output power of blue and green light produces a frequency -doubling light intensity of 3.8 kW/cm2 per unit pumping area. This study compares the experimental results of a 490 nm optically pumped semiconductor disk laser at home and abroad (Table 1). In this research, a high fundamental frequency optical power and higher frequency doubling light intensity per unit area are obtained under a higher pump power density, indicating that the proposed chip unit area has an improved heat dissipation capacity. The frequency -doubling light power and efficiency reported in this study can be improved and the pump spot area can be further increased in the future.Conclusions A packaging process is developed that significantly improves the heat dissipation capacity of semiconductor disk lasers. This packaging technology can suppress Pt condensation on diamond surfaces during packaging. This packaging process bonds the laser chip and diamond heat sink more closely, reduces device thermal resistance, and improves heat -dissipation capacity. A fundamental -frequency optical output of 22.5 W with a pump spot diameter of 400 p.m is obtained using the packaging process. The optical conversion efficiency is 42.7% at the maximum output power. A blue and green light output of 4.8 W is obtained through frequency doubling. The total optical -optical conversion efficiency is 15.4%, and the intensity of blue and green light produced per unit pumping area is 3.8 kW/cm2.