All-Solid-State 228.5 nm DUV Light Source by quadrupling of an Acousto-Optically Q-Switched Nd:YVO4 Laser

All-solid-state 228.5 nm Deep Ultraviolet (DUV) laser has been studied with two-step second harmonious generation (SHG) of Nd:YVO4 914 nm fundamental laser, by optimizing its resonator design and SHG configurations. Adopting three-mirror folded V cavity and acousto-optical Q-switching, high peak power 457 nm laser output has been achieved by intracavity frequency-doubling of LD pumped Nd:YVO4 914 nm fundamental laser. At pump power of 41 W, the average output power for 457 nm laser has reached 600 mW at repetition frequency 10 kHz, 50 ns pulse width. With Type-I phase matching BBO crystal, externally frequency-doubling of 457 nm blue output was realized and optimized. Under LD pump power of 41 W, DUV laser at 228.5 nm with average output power of 35 mW has been achieved, at repetition frequency 10 kHz and pulse width of 46 ns. Under these conditions, the frequency doubling conversion efficiency is 5.8%, and the DUV laser output power instability is less than 2% in 2-hour test.


I. INTRODUCTION
DUV lasers, owing to their special wavelength range, have found many applications in material processing and spectral analysis [1]- [6]. At present, these applications mainly adopt excimer lasers as the laser source. In comparison, DUV allsolid-state laser based on nonlinear frequency conversion has gained extensive attention due to its high efficiency, compactness and low cost of system maintenance [7], [8]. The most common way of realizing all-solid-state DUV lasers is quadruple-frequency conversion of Nd:YAG 1064 nm laser, achieving DUV 266 nm output [9]- [12]. However, for some applications, such as 5G electronic material, grating and waveguide processing, wave-lengths shorter than 250 nm can be more appealing owing to higher efficiencies [13]. Still, some special spectral applications need specific DUV wavelengths. Lasers at about 228 nm wave-lengths have proved to be especially suitable for testing genome DNA Methylation [14], protein structure [15], and Raman spectral analysis of explosives [16]. One way of achieving ~228 nm lasers is the quadruple-frequency conversion of either the 914 nm output of Nd:YVO4 crystal, or 912 nm output of Nd:GdVO4 crystal. Sergei V. Bykov et al. [17] reported acousto-optically Q-Switched, frequency quadrupled 912 nm Nd:GdVO4 laser with the average output power of 30 mW. Nd:YVO4 and Nd:GdVO4 have some advantages in common, including high absorption cross section, broad absorption bandwidth and polarization output. Although there exist some thermal lens effect inside Nd:YVO4 crystal as the laser gain medium, this thermal lens effect can be taken into consideration in the cavity design, thermal stable cavity can be achieved using the Nd:YVO4 crystal pumping facet as one mirror of the laser cavity. This not only simplifies the cavity system, the overall system cost is also lower. What is more, Nd:GdVO4 crystal is ~3 times more expensive than Nd:YVO4 crystal, that is one of the reasons we conduct DUV laser study by quadrupling of Nd:YVO4 laser. So far, the 914 nm spectral line of Nd:YVO4 crystal has been mainly reported to generate the second harmonic to obtain the CW 457 nm laser [18], [19]. The 914 nm laser output of Nd:YVO4 crystal corresponds to a Quasi-three-level system, with much smaller stimulated emission cross section than four-level system, and thus lower efficiency. In order to enhance the quadruple frequency conversion efficiency and thus the DUV 228.5 nm output, it is necessary to optimize the whole laser system. In this work, three-mirror folded V cavity and acousto-optical Q-switching were used to achieve 914 nm fundamental laser pulse operation, LBO crystal was adopted to perform the intracavity SHG and achieve 457 nm output. Finally, BBO crystal was used to achieve the external-cavity frequency-doubled 228.5 nm pulsed laser output.

A. OPTIMIZATION OF RESONANT CAVITY PARAMETERS
The three-mirror folded V cavity adopted in this work is shown in Fig.1. The pumping facet of Nd:YVO4 crystal was used as one of the reflecting mirrors of the laser cavity. The thermal focal length was taken into consideration in the selection of resonant cavity parameters. In this work, thermal lens focal length of the laser crystal ℎ was calculated numerically according to [20], [21]: Where ℎ is the thermally induced power; n/ T is the coefficient of change of refractive index with temperature; is the thermal conductivity; is pump beam radius; is absorption coefficient, is the crystal length.
In this work, Nd:YVO4 length is 5 mm, Nd 3+ atomic doping concentration is 0.1%. For this crystal, = 0.0523 W/cm/K, = 4.1 cm -1 , n/ T is 2.2 × 10 -6 K -1 , the power conversion percentage from pump power to heat is 15%. From these parameters and for different pump beam radius = 200 m, 300 m and 400 m, the heat lens focal lengths have been obtained under different pump powers, as shown in Fig.2. For pump power of 41 W, the heat lens focal lengths are 50 mm, 109 mm and 194 mm, respectively. Putting these focal lengths into V cavity parameters, using ABCD Matrix and cavity stability condition, by Matlab numerical calculations, the cavity beam waist radius for M1 and M2 as the cavity mirrors are 118 m, 125 m and 128 m, respectively. So from the perspective of mode matching, the pump beam radius of 200 m has been adopted in this design. The effects of L1 and L2 on optical beam radius inside the cavity have been obtained, as depicted in Fig.3 and Fig.4. From these figures, it is clear that L1 has little effect on the beam radius at different positions inside the cavity, while L2 has bigger impact on the beam radius. Therefore in this work, L2 needs to be carefully adjusted. Based on the above analysis and device parameters, L1 and L2 have been set at 83 mm and 31 mm, respectively.

B. SELECTION OF NONLINEAR CRYSTAL
Lithium triborate LiB3O5 (LBO) and Bismuth borate BiB3O5 (BiBO) are both good nonlinear optical crystals for achieving blue light efficiently from near infrared wavelength through second harmonious generation (SHG). Though BiBO has larger nonlinear coefficient, it has smaller acceptance angle, larger walk off angle, resulting in an elliptical output beam and lower frequency doubling efficiency. In addition, BiBO is a hygroscopic crystal, not suitable for storing in air for a longer period of time. On the contrary, though LBO has smaller nonlinear coefficient, it has larger acceptance angle, smaller walk off angle, and better optical beam quality. Besides, LBO has larger damage threshold, it is not easy to deliquesce. Therefore, LBO was selected as the nonlinear crystal to achieve 457 nm laser wavelength through SHG. The optimum length of LBO crystal can be calculated as [22]: Where is the round trip loss of the fundamental light in the cavity, 2 is the refractive index of LBO crystal, is the fundamental light frequency, = ℎ 0 ⁄ is the saturation light intensity of fundamental light, 0 is the absorption cross section of lasing medium, 1, 2 are the beam waists of fundamental light at laser medium and frequency doubling crystal, respectively. When = 0.02, the optimum LBO crystal length of about 16.4 mm for 457 nm light generation can be obtained. Considering the V cavity effective space and the free adjustment range of various devices, the LBO crystal length of 15 mm has been set in the experiments. Using SNLO software, the nonlinear characteristics of LBO and BiBO crystals can be obtained for the generation of 457 nm wavelength, as shown in Table  1. Barium borate BaB2O4 (BBO) is one of the most common nonlinear crystals for quadrupling application to generate UV laser, which has larger nonlinear coefficient, moderate birefringence, higher damage threshold, wider phase matching range, and better temperature stability. In the case of BBO crystal for the generation of 228.5 nm from 457 nm through SHG, which has relatively large nonlinear coefficient of 1.38 pm/V, a phase matching angle of 61.4° can be obtained using SNLO software and numerical calculation. In addition to BBO, RbBe2BO3F2 (RBBF) and KBe2BO3F2 (KBBF) can also be used to generate UV light through fourth harmonic generation, but with poor performance [23], [24].

III. EXPERIMENTAL SETUP
Our experimental setup is shown in Fig.1. The pump source is a 808 nm fiber-coupled LD with a core diameter of 400 μm and a numerical aperture of 0.22, with maximum CW power of 110 W. The wavelength shift coefficient is 0.25 nm/℃. The pumping wavelength can be finely tuned to match the central absorption wavelength of Nd:YVO4 through temperature adjustment. The pumping light was focused and collimated into 200 m in radius through collimation and focusing system, and injected into the Nd:YVO4 crystal. The coupling system was composed of two flat convex mirrors with focal lengths of 10 mm, and 45° polarizer. The parameters of Nd:YVO4 crystal are: Nd 3+ atomic doping concentration is 0.1%; 4 mm × 4 mm × 5 mm in size; the left facet was antireflection coated at 808 nm and 1064 nm and high reflection coated at 914 nm; the right facet was antireflection coated at 914 nm, 1064 nm and 1342 nm wavelengths. The laser crystal was wrapped in a layer of indium foil on the side and secured on a copper heat sink, which is capable of controlling the temperature through circulating water cooling. The output mirror M was a flat concave mirror with radius of curvature of 50 mm, in which the concave surface was antireflection coated at 457 nm, 1064 nm and 1342 nm and high reflection coated at 914 nm; the flat surface was antireflection coated at 457 nm, 914 nm, 1064 nm and 1342 nm. The high reflection mirror M2 was a flat concave mirror with 200 mm in radius of curvature which was high reflection coated at 457 nm and 914 nm. The V shape cavity was formed by the left facet of Nd:YVO4 crystal M1 and M and M2, where the angle between the two arms is 10°. The long arm was inserted the acousto-optical Q-switch, while the short arm was inserted LBO crystal for SHG, which was put about 1 mm in front of the reflection mirror. The size of LBO crystal is 4 mm × 4 mm × 15 mm, both facets of which were antireflection coated at 457 nm, 914 nm and 1064 nm. The above cavity has an oscillation wavelength of 914 nm, which was converted by SHG in LBO into 457 nm wavelength. M3 was a focusing mirror for 457 nm, which was antireflection coated at 457 nm, at the focus of which BBO crystal was put for quadrupling wave generation. Both facets of BBO crystal were antireflection coated at 457 nm and 228.5 nm, and out from the BBO crystal was the 228.5 nm wavelength. The function of mirror M4 is to isolate 457 nm and 228.5 nm wavelengths.

A. 457 NM PULSED LASER OUTPUT
In order to obtain the highest peak power 457 nm pulsed output, the repetition frequencies were set at 5 kHz, 10 kHz, 15 kHz and 20 kHz, respectively, and average output powers and pulse widths were measured at these frequencies. The results show that 457 nm laser has good performance at 10 kHz and 15 kHz, as shown in Fig.5 and Fig.6, from which we note the average power increases with increase of injection power, while the pulse width decreases with the increase of the injection power. At 10 kHz and 41 W injection power, maximum average power of 600 mW was obtained for the 457 nm laser when the pulse width was 50 ns, corresponding to a peak power of 1.2 kW.
At 41 W injection power, maximum average power of 661 mW was obtained for the 457 nm pulse laser output at 15 kHz when the pulse width was 62 ns, corresponding to a peak power of 710 W. These results show that highest peak power can be achieved at 10 kHz repetition frequency. Fig.7 shows the 457 nm laser spot and beam quality at 10 kHz and maximum average power of 600 mW, where TEM00 mode can be clearly seen but with somewhat ellipse spot, which we believe, is the results of the definite angle exists in the V shape folded cavity resulting in the astigmatism. By performing Binomial fitting of beam radius at different positions, M 2 factors for X and Y directions can be obtained as 1.32 and 1.15, respectively.

B. 228.5 NM DUV PULSED LASER OUTPUT
The layout for generating 228.5 nm DUV pulse laser by extra cavity frequency doubling is shown in Fig.1. In order to enhance the efficiency for quadrupling wave generation, the optimum focusing condition derived by Boyd and Kleinman (3) was used to calculate the focal length of the focusing lens and the nonlinear crystal length [25]: Where L is the nonlinear crystal length, is the Rayleigh length of the focusing beam. From the above relationship, a focal length of 150 mm for M3 for focusing 457 nm is obtained, and BBO crystal length is 8 mm. The OCEAN OPTICS HR4000CG-UV-NIR spectrometer was used to measure the laser spectrum with resolution of 0.7 nm. The 228.5 nm DUV laser spectrum was obtained at repetition frequency of 10 kHz as depicted in Fig.8. As shown in Fig.9, the 228.5 nm average output power increases with the 457 nm laser average power, and maximum average power of 35 mW was obtained for 228.5 nm laser, with pulse width of 46 ns.
The reason for the narrower 228.5 nm DUV laser pulse width than the 457 nm blue laser pulse width is that, at the beginning and end of 457 nm laser pulse, the power is too low for BBO crystal to generate DUV laser output. Shown in Fig.10 is the 228.5 nm laser spot with somewhat elliptic shape, which is the result of the non-negligible walk off angle, about 75.68 mrad at 228.5 nm laser. The laser output stability has been measured for the 228.5 nm DUV laser at maximum average output power of 35 mW, as shown in Fig.11. It is seen that the output power stability is within 35 mW 1% within 2 hours.

V. CONCLUSION
This paper reports the all-solid-state pulsed 228.5 nm laser design and characterization, in which the 914 nm laser from the Nd:YVO4 laser gain medium was used as the fundamental frequency. Pulsed blue laser at 457 nm was obtained when V shape folded cavityand acousto-optical Q-switching technology were adopted, and intra cavity SHG was performed with LBO crystal under type-I phase matching. Further, 228.5 nm DUV pulsed laser, with typical output power of 35 mW, was realized using extra cavity frequency conversion with BBO crystal, with stable output power and good output beam quality. To the best of our knowledge, it is the highest average output power of acousto-optically Q-