Study on thermal characteristics of 266 nm deep ultraviolet laser generated by BBO crystal - 04

03 Experimental results and analysis

When the green light input power is lower than 4 W, the matching temperature of the BBO crystal has little effect on the output power of the quadrupled 266 nm laser, and when the optimal power of ultraviolet light output is achieved, the temperature offset ΔT of the heating device also tends to be consistent; when the green light input power is greater than 8 W, the higher the matching temperature of the BBO crystal (www.wisoptic.com), the smaller the temperature offset ΔT of the heating device, and the higher the output power of the 266 nm laser. Therefore, it is necessary to establish a suitable theoretical model to explore the root cause of heat generation of the BBO crystal during the quadrupled process, and to conduct theoretical research on the thermal characteristics of crystals with different matching temperatures.

In order to better understand the output characteristics of the deep ultraviolet 266 nm laser at different matching temperatures, the beam quality of the obtained 266 nm deep ultraviolet laser was measured using an ultraviolet beam profiler when the crystal matching temperature was 60 °C. It was found that the beam quality factors in the x and y directions were 1.66 (M²_x) and 1.68 (M²_y), respectively, as shown in Figure 3 (a). In addition, the power stability of the 266 nm deep ultraviolet laser obtained at this time was recorded, as shown in Figure 3 (b). The power stability of the 266 nm laser within 1 h was better than 4.7% (RMS). After the crystal matching temperature was increased to 180 °C, the beam quality factors in the x and y directions can be increased to 1.62 and 1.52, respectively, and the power stability can be better than 4.0% (RMS). As can be seen from Figure 3, increasing the matching temperature can also improve the beam quality and long-term stability of the generated deep ultraviolet laser to a certain extent.

Finally, the central wavelength of the ultraviolet pulse laser was measured by a spectrometer to be 265.7 nm, and the line width was 0.354 nm, as shown in Figure 4 (a). The single pulse width of the 266 nm deep ultraviolet laser was measured by a photodetector to be 4 ns at a repetition frequency of 20 kHz, and the pulse sequence was uniform and stable, as shown in Figure 4 (b).

Fig. 3 Experimental results. (a) Beam quality and (b) power stability of the DUV 266 nm laser when phase matching temperature of the crystal is 60 ℃; (c) beam quality and (d) power stability of the DUV 266 nm laser when phase matching temperature of the crystal is 180 ℃

Fig. 4 Output characteristics of the DUV 266 nm laser. (a) Spectral width; (b) pulse width