Experimental Study on Deep Ultraviolet 222 nm Lamp-Pumped Solid-State Laser
5.2.2 Continuous-Wave 222 nm Lamp Laser Output
The 457 nm continuous-wave laser was frequency-doubled outside the cavity using a Type-I phase-matched BBO crystal. A simple lens-focusing scheme was adopted for the extracavity frequency doubling. To maximize the second harmonic generation (SHG) efficiency of the 457 nm laser, the optimal focusing condition defined by Boyd and Kleinman was followed [1]:
2Z_R L = 2.84 (5.10)
where L is the length of the nonlinear crystal, and Z_R is the Rayleigh length of the focused beam. Based on this condition and the measured beam quality of the 457 nm output, appropriate focal length of the focusing lens M3, BBO crystal length, and their relative positions were selected. In this experiment, the focusing lens M3 had a focal length of 150 mm, and the BBO crystal length was 8 mm.
The 457 nm continuous-wave laser was focused by lens M3 and passed through the BBO crystal to generate 222 nm lamp deep-ultraviolet laser. The laser spectrum was measured using an Ocean HR4000CG-UV-NIR spectrometer, as shown in Figure 5.10. The spectrum clearly shows the 457 nm fundamental, the 222 nm lamp second-harmonic, and the residual 808 nm pump lines. After passing through a dichroic beam splitter, the separated 457 nm and 222 nm lamp beams produced visible fluorescence spots on white paper, as shown in Figure 5.11.
When the input 457 nm continuous-wave power was 2.6 W, the output power of the 222 nm lamp deep-ultraviolet laser reached 6 mW. The relationship between the 222 nm lamp output power and the injected 457 nm power is shown in Figure 5.12. Figure 5.13 shows the beam profile of the 222 nm lamp laser at maximum output power, measured with a THORLABS BP209-VIS scanning slit beam profiler. The beam was elliptical due to the large walk-off angle of the 222 nm second-harmonic light generated in the BBO crystal. Figure 5.14 shows the stability test of the 222 nm lamp laser over 2 hours at 6 mW output power, yielding a power stability of 2.2%.
5.2.3 457 nm Pulsed Laser Output
To increase the extracavity frequency-doubling efficiency for generating 222 nm lamp laser, an acousto-optic Q-switch was inserted into arm L1 of the resonator, building upon the continuous-wave 457 nm laser described above. Before enabling Q-switching, the position and angle of the acousto-optic Q-switch crystal were carefully adjusted to ensure the oscillating beam passed through the optimal diffraction region, so that the continuous-wave 457 nm output power remained nearly unchanged compared to the case without the Q-switch.
To achieve the highest peak power 457 nm pulsed laser, the repetition rate was set to 5 kHz, 10 kHz, 15 kHz, and 20 kHz, and the average output power and pulse width were measured at different pump powers. The performance was poor at 5 kHz and 20 kHz because, for this laser, repetition rates that are either too high or too low are unfavorable for efficiently converting the inverted population into laser output.
Figures 5.15 and 5.16 show the average power and pulse width of the 457 nm pulsed laser as functions of injected pump power at repetition rates of 10 kHz and 15 kHz, respectively. The average power increased with pump power, while the pulse width decreased:
At 10 kHz repetition rate and 41 W pump power, the maximum average power was 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 pump power, the maximum average power was 661 mW, with a pulse width of 62 ns, corresponding to a peak power of 710 W.
The highest peak power of the 457 nm pulsed laser was therefore obtained at 10 kHz repetition rate. Figure 5.17 shows the beam profile and beam quality at 10 kHz and 600 mW average power. The laser operated in TEM₀₀ mode, but the spot was slightly elliptical due to astigmatism introduced by the angled folding mirror in the V-shaped cavity. By fitting the beam radius data at different positions with a quadratic function, the M² factors were approximately 1.15 (horizontal) and 1.31 (vertical).
5.2.4 222 nm Lamp Deep-Ultraviolet Pulsed Laser Output
Using the optimized 457 nm pulsed laser (pump spot radius ≈ 200 μm, arm lengths L1 ≈ 83 mm and L2 ≈ 31 mm, repetition rate 10 kHz) that provided the highest peak power, frequency doubling was performed with the same BBO crystal to generate 222 nm lamp pulsed laser.
Because the beam quality of the pulsed 457 nm laser differed slightly from the continuous-wave case, the positions of focusing lens M3 and the BBO crystal were fine-adjusted to ensure the Rayleigh length matched the crystal length, thereby maximizing SHG efficiency. Ultimately, a 222 nm lamp pulsed laser with maximum average power of 35 mW and pulse width of 36 ns (measured directly at the detector) was obtained. The ultraviolet laser spectrum is shown in Figure 5.18. The average output power of the 222 nm lamp pulsed laser increased with the injected 457 nm pulsed power, as shown in Figure 5.19. Figure 5.20 shows the beam spot of the 222 nm lamp pulsed laser at the highest average output power.
Figure Captions (translated)
Fig. 5.9 Beam quality and spot profile of the 457 nm continuous-wave laser at maximum output power (see color plate): labeled "● x-direction M²=1.2, ■ y-direction M²=1.13"; x-axis: position along propagation axis (mm); y-axis: beam diameter (mm).
Fig. 5.10 Laser spectrum; x-axis: Wavelength (nm); y-axis: Intensity.
Fig. 5.11 Fluorescence spots excited on white paper: labeled "dichroic beam splitter, 457 nm, 222 nm lamp".
Fig. 5.12 222 nm lamp continuous-wave output power vs. injected 457 nm power; x-axis: 457 nm injected power (W); y-axis: 222 nm lamp output power (mW).
Fig. 5.13 Beam profile of 222 nm lamp continuous-wave laser at maximum power (see color plate).
Fig. 5.14 Stability test of 222 nm lamp continuous-wave laser at 6 mW over 2 hours; x-axis: time (min); y-axis: average power (mW).
Fig. 5.15 At 10 kHz repetition rate, 457 nm pulsed laser average power and pulse width vs. pump power; labeled "f=10 kHz".
Fig. 5.16 At 15 kHz repetition rate, 457 nm pulsed laser average power and pulse width vs. pump power.
Fig. 5.17 Beam profile and beam quality of 457 nm pulsed laser at 10 kHz and 600 mW average power (see color plate): labeled "● x-direction M²=1.15, ■ y-direction M²=1.32"; x-axis: position along propagation axis (mm); y-axis: beam diameter (mm).
Fig. 5.18 Ultraviolet laser spectrum; x-axis: wavelength (nm); y-axis: intensity.
Fig. 5.19 At 10 kHz, 222 nm lamp pulsed laser average output power vs. injected 457 nm pulsed power; x-axis: 457 nm injected power (mW); y-axis: 222 nm lamp output power (mW).
Fig. 5.20 Beam spot of 222 nm lamp pulsed laser at maximum average power.
