Ultraviolet 222 nm Solid-State Laser Experiments
5.2.1 457 nm Continuous Laser Output
It is known that the pump spot size and cavity length affect laser output. To achieve high-performance 457 nm continuous laser output, experiments were conducted under different pump spot sizes and laser resonator arm lengths L1L_1L1, L2L_2L2. Figure 5.7 shows the relationship between injected pump power and 457 nm continuous laser output power when the resonator arm lengths L1L_1L1 and L2L_2L2 are 82 mm and 30 mm, respectively, with pump spot radii wpw_pwp of approximately 200 μm, 300 μm, and 400 μm.
Figure 5.7 Relationship between 457 nm continuous laser output power and injected pump power under different pump spot sizes
(curves: wpw_pwp=200 μm (squares), wpw_pwp=300 μm (circles), wpw_pwp=400 μm (triangles); vertical axis: output power / W; horizontal axis: injected pump power / W; range 10–45).
From Figure 5.7, at an injected pump power of 41 W, the maximum 457 nm continuous laser output powers are 1.6 W (wpw_pwp=300 μm), 2.2 W (wpw_pwp=200 μm), and 1.2 W (wpw_pwp=400 μm). The beam quality at wpw_pwp=200 μm is better than that at wpw_pwp=300 μm or 400 μm. Compared to wpw_pwp=300 μm or 400 μm, the 457 nm laser output performance is superior at wpw_pwp=200 μm. The reason is that the ratio of the pump spot size to the 914 nm oscillating beam waist is more appropriate at wpw_pwp=200 μm. These experimental results are consistent with the theoretical analysis of the laser in previous chapters, which also informs optimization strategies for related 222 nm far UVC systems.
Next, the resonator arm lengths were optimized. First, the arm length L2L_2L2 was fixed at 30 mm, and the effect of varying the arm length L1L_1L1 on the 457 nm continuous laser output power was experimentally studied. The relationships between injected pump power and 457 nm continuous laser output power at L1L_1L1 = 82 mm, 83 mm, and 84 mm are shown in Figure 5.8(a). As seen in Figure 5.8(a), the output power curves are quite similar regardless of whether L1L_1L1 is 82 mm, 83 mm, or 84 mm. Therefore, small changes in the long arm L1L_1L1 have negligible impact on the 457 nm continuous laser output power. The reason is that variations in L1L_1L1 cause minimal changes in the oscillating spot size at different positions within the laser resonator. To ensure sufficient adjustment space inside the cavity for subsequent experiments-including potential integration with UVC 222 frequency conversion stages-L1L_1L1 was set to 83 mm.
Figure 5.8 Relationship between 457 nm continuous laser output power and injected pump power under different arm lengths:
(a) Arm L1L_1L1: L1L_1L1=82 mm (squares), L1L_1L1=83 mm (circles), L1L_1L1=84 mm (triangles); (b) Arm L2L_2L2: L2L_2L2=32 mm (triangles), L2L_2L2=31 mm (squares), L2L_2L2=30 mm (circles); vertical axis: output power / W; horizontal axis: injected pump power / W.
Then, with wpw_pwp ≈ 200 μm and the resonator arm L1L_1L1 fixed, the effects of varying the resonator arm L2L_2L2 on the 457 nm continuous laser output power and beam quality were experimentally investigated. The relationships between injected pump power and 457 nm continuous laser output power at L2L_2L2 = 30 mm, 31 mm, and 32 mm are shown in Figure 5.8(b). From Figure 5.8(b), at an injected pump power of 41 W, the maximum 457 nm continuous laser output powers are 2.2 W (L2L_2L2=30 mm), 2.6 W (L2L_2L2=31 mm), and 1.95 W (L2L_2L2=32 mm). Additionally, the beam quality at L2L_2L2=31 mm is better than at L2L_2L2=30 mm or 32 mm, with the beam profile and quality shown in Figure 5.9. Furthermore, Figure 5.8(b) indicates that the 457 nm continuous laser output power is relatively sensitive to changes in L2L_2L2. The reason is that variations in L2L_2L2 significantly affect the oscillating spot size at different positions in the resonator, altering the spatial mode matching between the pump spot and the laser oscillation spot, thereby influencing laser output performance. This is consistent with the theoretical analysis in previous chapters. Therefore, during experiments, the pump spot size and the resonator arm L2L_2L2 length must be carefully adjusted to maintain optimal mode overlap, a principle equally critical in far UV 222 nm laser designs.
Figure 5.9 Beam quality and spot profile at maximum 457 nm continuous laser output power
(see color figure) (curves show spot diameter vs. propagation distance; x-horizontal M2M^2M2=1.2, y-vertical M2M^2M2=1.13; spot image inserted in the middle).
5.2.2 222 nm Continuous Laser Output
A Type-I phase-matched BBO frequency-doubling crystal was used for extracavity frequency doubling of the 457 nm continuous laser. The extracavity doubling section employed a simple lens focusing scheme. To improve the frequency-doubling efficiency of the 457 nm laser, the optimal focusing condition defined by Boyd and Kleinman was applied⁽¹⁾: 2ZrL=2.842Z_r L = 2.842ZrL=2.84 (5.10), where LLL is the nonlinear crystal length and ZrZ_rZr is the Rayleigh length of the focused beam. Based on this condition and the 457 nm output beam quality, an appropriate focal length for the focusing lens M3, BBO crystal length, and their placement positions were selected. In this experiment, the focal length of lens M3 was 150 mm, and the BBO crystal length was 8 mm. The 457 nm continuous laser was focused by lens M3 and passed through the BBO crystal to generate 222 nm laser light. The laser spectrum was measured using an Ocean HR4000CG-UV-NIR spectrometer, with results shown in Figure 5.10. The spectrum shows lines at 457 nm, 222 nm, and the pump light at 808 nm. After passing through a dichroic prism, the 457 nm and 222 nm laser spots excited on white paper are shown in Figure 5.11.
A 2.6 W 457 nm continuous laser, after passing through the BBO crystal, produced 6 mW of 222 nm deep ultraviolet laser-comparable in germicidal potential to emerging 222 nm far UVC sources but at a distinct wavelength. The relationship between 222 nm laser output power and 457 nm injected power is shown in Figure 5.12. Figure 5.13 shows the 222 nm laser spot profile measured using a THORLABS BP209-VIS scanning slit beam profiler at maximum output; the spot is elliptical due to the large walk-off angle of the 222 nm doubled light after frequency doubling of the 457 nm laser in the BBO crystal. Figure 5.14 shows the stability test of the 222 nm laser output power at 6 mW over 2 hours, yielding a stability of 2.2%.
Figure 5.10 Laser spectrum
(horizontal axis: wavelength / nm; vertical axis: intensity / counts; showing 222 nm, 457 nm, and 808 nm lines).
Figure 5.11 Laser spot effects excited on white paper
(labeled: dichroic prism, 457 nm spot, 222 nm spot).
Figure 5.12 Relationship between 222 nm continuous laser output power and injected 457 nm laser power
(vertical axis: 222 nm output power / mW; horizontal axis: 457 nm injected power / W; curve shows an upward trend).
Figure 5.13 222 nm continuous laser spot profile at maximum output power
Figure 5.14 Stability test of 222 nm continuous laser at maximum output power
(vertical axis: 222 nm average power / mW; horizontal axis: time / min; curve stable around 6 mW).
5.2.3 457 nm Pulsed Laser Output
To improve the efficiency of extracavity frequency doubling of the 457 nm laser to produce 222 nm laser-potentially extendable to UV 222 nm pulsed sources with modified nonlinear optics-an acousto-optic Q-switch was inserted into arm L1L_1L1 based on the above continuous 457 nm laser output. Before enabling the Q-switch, the position of the acousto-optic Q-crystal was first adjusted to ensure the oscillating light passed through its optimal diffraction position and pitch angle, such that the continuous 457 nm output power was nearly equal to that before inserting the acousto-optic Q device. Pulsed 457 nm laser output experiments were then conducted. To achieve the highest peak power pulsed 457 nm laser, repetition rates of 5 kHz, 10 kHz, 15 kHz, and 20 kHz were set, and the average output power and pulse width of the pulsed 457 nm laser were measured. Results showed that pulsed 457 nm laser performance was poor at repetition rates of 5 kHz and 20 kHz. The reason is that for this laser, excessively high or low repetition rates are not conducive to effectively converting the inverted population at the upper energy level into oscillating light output.
Figures 5.15 and 5.16 show the average power and pulse width of the 457 nm output versus injected pump power at repetition rates of 10 kHz and 15 kHz. As seen in Figures 5.15 and 5.16, the average power increases with injected pump power, while the pulse width decreases gradually: at 10 kHz repetition rate and 41 W injected pump power, the maximum average power of pulsed 457 nm laser output is 600 mW, with a pulse width of 50 ns, corresponding to a peak power of 1.2 kW; at 15 kHz repetition rate and 41 W injected pump power, the maximum average power is 661 mW, with a pulse width of 62 ns, corresponding to a peak power of 710 W.
The results indicate that a repetition rate of 10 kHz yields the highest peak power for the pulsed 457 nm laser. Figure 5.17 shows the spot profile and beam quality of the 457 nm laser at a repetition rate of 10 kHz and maximum average output power of 600 mW. The laser spot is TEM₀₀ mode; however, the spot is slightly elliptical due to astigmatism caused by the angular separation of the V-shaped cavity folding mirror. By performing quadratic fitting on the beam radius data at different positions, the M2M^2M2 factors in the X and Y directions are approximately 1.15 and 1.31, respectively-beam characteristics that would also be scrutinized in far UV 222 nm pulsed systems for similar cavity-induced distortions.
Figure 5.15 Average power and pulse width of 457 nm pulsed laser versus injected pump power at 10 kHz repetition rate
(left vertical axis: 457 nm average power / mW; right vertical axis: pulse width / ns; horizontal axis: injected pump power / W; two curves: average power (rising) and pulse width (falling)).
