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We demonstrate high-power 355 nm ultraviolet (UV) picosecond (ps) laser using a type I phase-matching nonlinear optical crystal of La2CaB10O19 (LCB), which possesses the characteristic of non-hygroscopicity. The high-power third harmonic generation was successfully achieved from two types of 1064 nm ps fundamental lasers. The maximum output power of 7.81 W of 355 nm UV laser was obtained with a pump of 35.2 W 1064 nm ps laser (80 MHz repetition rate, 10 ps pulse width) with optical conversion efficiency of 22.2%. The experimental results show that the LCB crystal is a promising candidate for generating high-power UV laser.
© 2014 Optical Society of America
1. Introduction
All-solid-state laser processing technology is a typical representative in the field of advanced manufacturing technology. Laser welding, cutting, cleaning and cladding are widely used in shipbuilding, aerospace, automotive manufacturing and other industries. As an important branch of laser processing, picosecond (ps) laser precision processing is widely used in flexible circuit board manufacturing, wafer cutting, cardiovascular stents, solar cell manufacturing, and so on [1,2]. These application areas require high processing precision. The key method to improve processing precision is to minimize the thermal diffusion in laser processing. Many studies indicate that 355 nm ultraviolet (UV) ps ultrafast laser pulses can effectively reduce the thermal diffusion, reduce the pulse energy and average power required for processing, and achieve good processing results [3,4]. UV ps laser is employed in silica glass processing, indium-tin oxide processing, resistor trimming, and memory repair [5]. An effective way to generate 355 nm ps laser is sum-frequency mixing of the fundamental ps laser and the second harmonic ps laser. L. Guo et al. reported high average power third harmonic generation (THG) of a mode-locked ps laser in a CsB3O5 crystal in 2007. The ps laser at 1064 nm is produced by a 30W master oscillator power-amplifier Nd:YVO4 laser system. The maximum THG output at 355 nm is up to 5.4W [6]. In 2009, Y. Zhou et al. demonstrated high-efficiency UV laser generation in a nonlinear optical crystal BaAlBO3F2 (BABF). 355 nm UV laser was generated by using a type I BABF crystal as a sum-frequency mixing of the fundamental light and the second harmonic from a ps Nd:YAG laser operated at a repetition rate of 10 Hz with a pulse duration of 20 ps. An output of 0.635 mJ per pulse for the third harmonic was obtained. The conversion efficiency was 26.4% [7]. In 2013, L. Y. Chen et al. reported a THG ps laser at 355 nm in nonlinear optical materials LiB3O5 (LBO). The single pulse energy of third harmonic was up to 2 mJ at the repetition rate of 1 kHz. The conversion efficiency was up to 33.3% from 1064 nm to 355 nm [8]. In order to achieve high-power 355 nm ps laser through sum-frequency mixing, the choices of nonlinear optical crystal and high-power high-energy 1064 nm ps fundamental laser play a very important role. LCB is a new type of nonlinear optical crystal which possesses moderate birefringence, a wide transparency range, a relatively large nonlinear optical coefficient, a high laser damage threshold, and the characteristic of non-hygroscopicity [9–12]. There are only few papers reported UV ps based on LCB crystal. In 2011, J. X. Zhang et al. reported that a 355 nm UV ps laser output of 5.0 mW corresponding to the conversion efficiency of 28.3% was generated using LCB crystal [13].
In this paper, we used type I phase-matching LCB crystal as a nonlinear optical crystal. High-power THG was successfully achieved from two types of 1064 nm ps fundamental lasers. An average output power of 1.243 W of 355 nm UV laser was generated from a 10.5 W 1064 nm ps laser (100 kHz repetition rate, 30 ps pulse width) with optical conversion efficiency of 11.8%. A maximum output power of 7.81 W of 355 nm UV laser was obtained by frequency tripling of a 35.2 W 1064 nm ps laser (80 MHz repetition rate, 10 ps pulse width) with optical conversion efficiency of 22.2%. The output is the highest reported so far.
2. Experimental setup
The experimental setup of 355 nm UV ps laser using LCB as a nonlinear optical crystal was shown in Fig. 1.We used two types of 1064 nm ps pump source. One of the pump sources is a 10.5 W 1064 nm ps laser with 100 kHz repetition rate and 30 ps pulse width. Another pump source is 35.2 W 1064 nm ps laser with 80 MHz repetition rate and 10 ps pulse width. The 1064 nm pump laser was focused by lens f1, which was anit-reflection (AR) coated at 1064 nm. The frequency doubling crystal LBO was a type I noncritical phase-matching crystal with θ = 90°, φ = 0° and dimensions of 3 × 3 × 30 mm3. Both sides of LBO crystal were AR-coated at 1064 nm and 532 nm. The LBO crystal was heated in an oven with an accuracy of temperature of ± 0.1 °C. Lens f2 was the focusing lens for both 1064 nm laser and 532 nm laser which was AR-coated at 1064 nm and 532 nm. The nonlinear optical crystal for sum-frequency mixing is type I phase-matching LCB with θ = 49.4°, φ = 0° and dimensions of 4 × 4 × 17.6 mm3. LCB crystal is a new type of crystal which possesses moderate birefringence, a wide transparency range, a relatively large nonlinear optical coefficient, a high laser damage threshold, and the characteristic of non-hygroscopicity. The LCB crystal was optically polished. Both sides of it were not coated to avoid damage at high power density. LCB crystal was also heated to a certain temperature in an oven whose temperature is strictly controlled. According to Ref. [13], The temperature bandwidth of LCB crystal is 5.96°C·cm. That is to say, for 17.6 mm length of LCB crystal, when the temperature variation range is ± 1.7 °C, the output power will decrease to half the maximum output power. Therefore, the LCB temperature must be strictly controlled with an accuracy of ± 0.1°C. For type I phase-matching, the fundamental laser at 1064 nm and the frequency-doubled output at 532 nm need to have the same polarization. Therefore, a dual wavelength wave plate (AR-coated at 1064 nm and 532 nm) was used in our experiment. It allows rotation of the polarization of 532 nm laser by 90 degrees and keeping the polarization of 1064 nm laser unchanged. Two dichroic mirrors M1 and M2 were high reflection (HR) coated at 355 nm, HT-coated at 1064 nm and 532 nm at the angle of 45°to separate 355 nm laser from the residual 1064 nm laser and 532 nm laser.
3. Experimental results and discussion
In the experiment of frequency doubling, the frequency doubling crystal LBO was a type I noncritical phase-matching crystal. The type I noncritical phase matching has some advantages over the type II critical phase matching. In a type II phase-matching, the fundamental laser need to have different polarization, one is horizontal polarized, while the other is vertical polarized. The refractive indexes are different because of different polarization. Therefore, there’s group velocity mismatch and walk-off effect. The horizontal and vertical laser will be partially separated both temporally and spatially after propagating through the nonlinear optical crystal. The conversion efficiency of frequency doubling will decrease. On the contrary, such problems do not occur in type I noncritical phase matching of frequency doubling because the fundamental laser has the same polarization. The temperature of LBO crystal was controlled at 155.6°C. Experimentally, we first used the pump source of 10.5 W 1064 nm ps laser with 100 kHz repetition rate, 30 ps pulse width and 150 μJ pulse energy. The focal length of lens f1 was 400 mm. The output power and conversion efficiency of 532 nm ps laser as a function of 1064 nm pump power was shown in Fig. 2.When the pump power was 10.5 W, the average output power of 532 nm laser was 6.5 W with the optical conversion efficiency of 61.9%. When the pump power was 8.3 W, the maximum optical conversion efficiency was 63.9%.
Fig. 2 Output power and conversion efficiency of 532 nm ps laser as a function of 1064 nm pump power.
For THG in LCB crystal, four lenses with different focal lengths of 50 mm, 100 mm, 150 mm and 200 mm were employed, respectively. At the exit of LBO, the 1064 nm fundamental laser was vertical polarized and 532 nm frequency doubling laser was horizontal polarized. After passing though the dual wavelength wave plate, the 1064 nm laser and 532 nm laser were both vertical.
Figure 3 shows the output power of 355 nm UV ps laser as a function of 1064 nm pump power with various focal length of f2. In the experiment of THG, the temperature of LBO and LCB was 155.3°C and 79°C, respectively. From Fig. 3, we can see that the maximum output power of 355 nm ps laser was obtained using the lens with focal length of 150 mm. With the pump power of 10.5 W, the output of 355nm was as high as 1.243 W.
Figure 4 shows the conversion efficiency from 1064 nm to 355 nm as a function of 1064 nm pump power. It can be seen that the maximum conversion efficiency was obtained by using a lens with focal length of 150 mm. At a pump power of 6.6 W, the maximum conversion efficiency was 14.2%.
Then, we did the THG experiment using the 1064 nm fundamental laser with 35.2 W output power, 80 MHz repetition rate, 10 ps pulse width, 440 nJ pulse energy, and 44 kW peak power. The optimized focal lengths of lens f1 and lens f2 were 200 mm and 100mm, respectively. The 1064 nm fundamental laser was horizontal polarized and 532 nm frequency doubling laser was vertical polarized. The 1064 nm laser and 532 nm laser were all horizontal polarized after the dual wavelength wave plate. The temperature of LBO and LCB was 155.2°C and 69.5°C, respectively. The output power and conversion efficiency of 355 nm ps laser as a function of 1064 nm pump power was illustrated in Fig. 5.From Fig. 5, we can see that the output power and conversion efficiency increased with the increasing of pump power. The maximum output power of 355 nm UV laser was as high as 7.81 W when it was pumped with 35.2 W 1064 nm ps laser with optical conversion efficiency of 22.2%. For an ideal THG, the photon number ration between 1064 nm laser and 532 nm laser should be 1:1. That is to say, it requires a power ratio of 1:2. In our experiment, the beam radiuses of 1064 nm laser and 532 nm laser in LCB crystal were 110 μm and 115 μm, respectively. In our experiment, the group velocity mismatch of 1064 nm laser and 532 nm laser in LCB crystal is 1.1 ps. Since the group velocity mismatch, 1064 nm laser and 532 nm laser cannot completely overlap. Energy loss occurs in the process of THG. So the conversion efficiency of THG will reduce.
Fig. 5 Output power and conversion efficiency of 355 nm ps laser as a function of 1064 nm pump power.
In the experiment, two sides of LCB were not coated, so that there are some Fresnel losses at the entrance and exit of the crystal. A higher conversion efficiency from 1064 nm to 355 nm laser could be expected if both sides of LCB crystal AR-coated at 1064 nm, 532 nm and 355 nm. In addition, we can also optimized the conversion efficiency by adjusting the intensities of the fundamental and second harmonic beams, and improving the temporal and spatial overlapping in the LCB crystal.
4. Conclusion
In conclusion, we have demonstrated a high-power 355 nm UV ps laser using type I phase-matched LCB crystal. The high-power THG was successfully achieved from two kinds of 1064 nm ps fundamental lasers. The average output power of 1.243 W of 355 nm UV laser was generated from 10.5 W 1064 nm ps laser (100 kHz repetition rate, 30 ps pulse width) with optical conversion efficiency of 11.8%. The maximum output power of 7.81 W of 355 nm UV laser was obtained from 35.2 W 1064 nm ps laser (80 MHz repetition rate, 10 ps pulse width) with optical conversion efficiency of 22.2%. Higher output power and conversion efficiency of 355 nm laser could be expected if using AR-coated LCB crystal, adopting separated focusing lenses to control the spot sizes of 1064 nm laser and 532 nm laser, and compensating the group velocity mismatch. The experimental results show that the LCB crystal is a promising crystal to generate high-power UV laser.
Acknowledgments
The authors would like to thank Prof. Jingyuan Zhang at Georgia Southern University for helpful suggestions. This research has been supported by the National Natural Science Foundation of China (61205133, 61308033, 61308032, 61172010 and 51205380), and the National Key Scientific Research Project of China (2012CB934200).
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