# Beam Control in an Intracavity Frequency-Doubling Semiconductor Disk Laser
## 3 Experimental Results and Discussion  
A standard V-shaped laser cavity configuration is used in our experiment as shown in Figure 3, which is formed by the semiconductor's DBR as one end mirror, a curved folding mirror (M1) and a planar mirror (M2). The mirror M1 is highly reflective (HR) at 1030 nm but highly transmissible for the second-harmonic wavelength of 515 nm, which has a curvature radius of 250 mm and is placed at a distance of 345 mm from the SDL chip. The measured reflectivity of M1 is R \(>\) 99.8% from 650 nm to 1100 nm and R \(<\) 5% from 488 nm to 532 nm. In order to form the resonator, M1 has a tilting angle of about 9\({}^{\circ}\) related to the axis of the SDL. Before inserting the SHG crystal, the output power at the fundamental wavelength was measured at the followed situation. A plane mirror was used as M2 mirror with a 3% transmission for the fundamental wavelength of around 1030 nm, and plays a role of output coupler. Then the output power of SDL was measured and shown in Figure 4. The TEC temperature was set as 13 \({}^{\circ}\)C, and the temperature of cooling water is 12 \({}^{\circ}\)C. The pump beam of 808 nm diode laser was incident at 40\({}^{\circ}\) angle relative to the surface normal and was focused on the SDL chip with a spot diameter of 300 \(\upmu\)m. This angle produced an elliptical pump spot, which may not only reduce the pumping efficiency compared with nearly circular beam shape [20], but also degrade the beam quality. The continuous wave (CW) output power is over 3.5 W under 25 W pump power. As about 30% of the pump power was reflected on the un-coated SDL surface, the optical-to-optical efficiency related to the absorbed pump power is about 20%. No thermal rollover occurred within the available range of pump powers, indicating the efficient heat dissipation of our packaged method. Figure 1: Schematic diagram of the designed semiconductor disk laser (SDL). Figure 2: Schematic diagram of the bonding structure. For the SHG experiment, we replace the plane output coupler described as above with a plane end mirror M2. This mirror M2 is highly reflective for both the fundamental and the second-harmonic laser. The pump laser and the spot size are not changed. The mode diameter on the SDL is about 500 \(\upmu\)m. M2 is located at the position of the beam waist, which is 130 mm far from M1 and has a spot size around 80 \(\upmu\)m. To produce a green laser beam, a 3 mm long beta-barium borate (BBO) crystal is inserted near to the mirror M2, which is coated with moisture-proof antireflection film on both sides. The full experimental setup is schematically shown in Figure 3.  
Figure 5 shows the lasing spectra of the 1036 nm fundamental wave and the green light at 516 nm, which are measured with a fiber-coupled spectrometer. The linewidths of the fundamental and frequency-doubling laser defined as the full-width at half-maximum (FWHM) of lasing spectra are 4.5 nm and 1.5 nm, respectively. In this experiment, we do not use the birefringent filter in order to reduce the power loss, but the wide spectral width of the fundamental laser will sacrifice the conversion efficiency. The CW output characteristic of green laser is shown in Figure 6. The TEC temperature was set to 13 \({}^{\circ}\)C. The pump threshold is about 6 W, and the slope efficiency is 9.4% for the frequency-doubling generation. The maximum output power is 1.3 W under the input power of 25 W, corresponding to the maximum optical to optical conversion efficiency is 6.3%. Also, this corresponds to a frequency conversion efficiency of 27% with respect to the maximum fundamental power. Figure 4: Output power at 1036 nm as a function of the 808 nm pump power. Figure 3: Schematic of the V-shaped resonator for green laser. We also investigated the beam quality M\({}^{2}\) of the fundamental and the green laser respectively. The M\({}^{2}\) factors were measured by a commercial beam propagation analyzer M2 MS-BP209 according to the ISO11146 standard.