Wavelength conversion efficiency to the UV region is limited by a host of factors. To overcome several of these constraints, we use a fluxless-grown BBO crystal for fourth harmonic conversion of a linearly polarized Nd:YAG microchip laser, passively Q-switched with  cut Cr4+:YAG. The high quality BBO crystal used in the picosecond pulse width regime enables 60% conversion efficiency to give 3.4 MW peak power, 250 ps, 100 Hz pulses at 266 nm.
©2012 Optical Society of America
Several scientific and industrial applications, such as, spectroscopy, photolithography and micromachining, can benefit from compact and rugged solid-state ultraviolet (UV) sources.
Passively Q-switched microchip lasers offer advantages of simple, rugged construction providing high peak power in a compact size [1–6]. Quasi-continuous-wave (QCW) pumping of the microchip laser reduces the heating effects, enabling high pulse energy even in a small, air-cooled configuration [3,7].
Recently, we have reported the use of a high peak power microchip laser for efficient wavelength conversion from 1064 nm to 532 nm , and 266 nm . The high peak power of several megawatts, achievable even at a moderate pulse energy of a few millijoules, by operating in the sub-nanosecond pulse-gap region , has significantly enhanced the wavelength conversion efficiency.
However, conversion to the UV is still limited by the availability of suitable nonlinear crystals and their properties. At present, the commonly used crystals for fourth harmonic generation areβ-BaB2O4 (β-barium borate, BBO) and CsLiB6O10 (CLBO). Since CLBO requires special precautions due to its highly hygroscopic nature, we have chosen BBO for fourth harmonic generation.
Normally, a BBO crystal is grown by using a flux to lower its melting temperature. Oxide Corporation has been able to grow the BBO crystal without using a flux. This greatly enhances its quality, reducing linear and nonlinear absorption in the crystal.
In this paper, we report, for the first time, the use of a BBO crystal grown by a fluxless method to obtain 60% FHG efficiency. We compare the results with those for a flux-grown BBO crystal, under identical experimental conditions. Using the fluxless-grown BBO, we achieve a stable pulse train having 3.4 MW peak power with 250 ps pulse width and 100 Hz repetition rate at 266 nm wavelength. We believe that this is the highest conversion efficiency reported for a BBO crystal in the generation of deep ultraviolet (DUV).
2. Laser structure
We used a Nd:YAG/ Cr4+:YAG microchip laser structure as shown in Fig. 1 . A 4 mm-thick 1.1 at. %  cut Nd:YAG crystal (Scientific Materials Corp.) was pumped in the QCW regime by a fiber-coupled, 120 W, 808 nm laser diode (600 μm core diameter, 0.22 NA, JOLD-120-QPXF-2P of Jenoptik) at 100 Hz.
A 30% initial transmission,  cut Cr4+:YAG crystal (Scientific Materials Corp.) was used for passive Q-switching. The use of  cut Cr4+:YAG, instead of the normally used  cut Cr4+:YAG, enables a stable linearly polarized output which is parallel to the <001> crystallographic axis . The stable linearly polarized output is essential for high wavelength conversion efficiency. A low initial transmission Cr4+:YAG was used to obtain a short pulse width and a high peak power .
A flat coupler with a transmission of 50% was used at the output. The total cavity length was 11 mm. A TE cooler was used to maintain the Nd:YAG and Cr4+:YAG crystal temperature at 25°C. The laser was air-cooled.
3. Fundamental wavelength characteristics
Using the laser structure described above, we could obtain stable, linearly polarized laser oscillation at 1064 nm. An output pulse train of 3 mJ, 365 ps pulses at 100 Hz was obtained for 100 W, 300 μs width, 100 Hz, QCW pumping. This resulted in an output peak power of 8.2 MW. The polarization of the output beam was stable with a polarization ratio of better than 100:1.
The output beam diameter was 1 mm approximately and the M2 factor was measured to be 3.4. We aimed for maximum output energy and peak power, rather than an ideal Gaussian beam. An ideal Gaussian beam does not give maximum wavelength conversion efficiency, since the energy in the wings of the Gaussian beam does not contribute to the wavelength conversion.
4. Second harmonic generation
We performed SHG in the critical phase matching (CPM) regime using Type I LiB3O5 (Lithium Triborate, LBO). We chose LBO crystal for SHG due to its high enough damage threshold and a relatively large angular acceptance bandwidth that permits effective SHG with multi-mode laser radiation.
We obtained best results for a 10 mm-long LBO, by focusing the fundamental beam to a spot-diameter of 0.72 mm. We obtained 1.7 mJ pulse energy with 265 ps pulse width. This resulted in a peak power of 6.3 MW at 532 nm for an input peak power of 7.4 MW at 1064 nm. The SHG conversion efficiency was 85%. The M2 factor of the 532 nm beam was measured to be 3.
For the SHG results under various other conditions of crystal length and focusing, and for a discussion on the optimum conditions, the reader is referred to our paper on SHG .
5. Fourth harmonic generation
We have recently reported results for FHG using a commercial, flux-grown BBO crystal and discussed the factors important to achieve high conversion efficiency .
Here, we report, for the first time, the FHG results obtained by using a fluxless-grown BBO crystal and compare them with the results for a commercial, flux-grown BBO crystal, under identical experimental conditions.
The fluxless-grown Type I BBO crystal had dimensions of 3x3x6 mm. The commercial, flux-grown crystal (M/s Kaston) used for comparison was of identical dimensions. The 532 nm input beam was softly focused, as shown schematically in Fig. 2 . The spot diameter of the focused beam was 1.35 mm approx., and the confocal length (2 x Rayleigh range, Zr) was 160 mm. Hence, for the BBO crystal, the input beam was almost parallel.
The focusing conditions were slightly different from those used in our earlier experiments reported in , since the conditions were optimized for a longer crystal used in the experiments being reported here. We arranged a longer crystal anticipating a lower two-photon absorption in the fluxless-grown BBO crystal.
The experimental results for FHG are shown in Fig. 3 . Under identical experimental conditions of input power, focusing of the input beam, crystal dimensions, etc., the FHG conversion efficiency for the fluxless-grown BBO crystal was 60% as against 35% for the flux-grown BBO crystal, at the maximum input peak power of 5.63 MW. Using the fluxless-grown BBO crystal, we could obtain 840 μJ pulse energy, with 250 ps pulse width, at a repetition rate of 100 Hz. This results in a peak power of 3.4 MW, which is 1.7 times that obtained for the flux-grown crystal, under identical experimental conditions.
The fluxless BBO crystal was grown at Oxide Corporation using a modified Czochralski (CZ) method. The techniques used to improve the crystal quality are outlined in Fig. 4 .
In a flux-grown BBO crystal, traces of the flux creep into the crystal as impurities. The fluxless-grown BBO crystal avoids these impurities. Further, the double-crucible technique reduces the temperature fluctuations in the melt during the crystal growth, and the aqueous solution technique for melt preparation enables a uniform melt . This improves the micro-spatial uniformity of the growth surface, reducing point defects and sub-grain defects. These factors result in much smaller scattering losses in the BBO crystal.
Grown crystal samples were characterized by X-ray topography with Mo Kα1 radiation in order to confirm the crystal quality. Figure 5 shows a topographic image of the fluxless-grown BBO sample (cross-section 3 mm x 3 mm) used in our experiments.
It is seen that there are no sub-grain defects that are commonly seen in a normal CZ grown BBO crystal. This result indicates that the double-crucible and aqueous solution techniques are useful for the growth of BBO crystal with a high uniformity.
Figure 6 shows the linear transmission characteristics of the flux-grown and fluxless-grown BBO crystals used in our experiments. Both the crystals were 6 mm long. The characteristics were measured using a Hitachi U-3500 spectrophotometer. It is seen that the transmission of the fluxless-grown crystal is higher than that of the flux-grown crystal. At 266 nm, the absorption coefficient for the flux-grown BBO is calculated to be 0.143 cm−1. For the fluxless-grown BBO, the absorption coefficient is 0.009 cm−1, which is a significant improvement. Though the spectrophotometer specifications allow measurement only till 200 nm, it can be deduced from the shown transmission characteristics that the fluxless growth does not extend the transmission window of the BBO crystal.
Further, if we look at the conversion efficiency characteristics shown in Fig. 3, we find that the conversion efficiency, η, starts dropping with increasing input peak power, P2ω, at some point. This phenomenon can be attributed to a combination of pump depletion and either two-photon absorption, walk-off or thermal dephasing. We note that, since the confocal length (160 mm) for the input beam at 532 nm is much longer than the length (6 mm) of the BBO crystal, the diffraction and walk-off effects are negligible under our experimental conditions. We have also found that at a low repetition rate of 100 Hz, the thermal effects in the BBO crystal are insignificant. This was confirmed by further lowering the repetition rate. Hence, under our experimental conditions, the drop in conversion efficiency, with increasing input peak power, is due to a combination of pump depletion and two-photon absorption. So, we can compare the two-photon absorption in the fluxless-grown BBO with that in the flux-grown BBO by considering the points at which the drop in conversion efficiency starts, i. e., dη/dP2ω = 0. It is at this point that the two-photon absorption starts becoming appreciable.
For the flux-grown BBO crystal, the output peak power, P4ω = 1.18 MW at dη/dP2ω = 0. This works out to a UV intensity of 165 MW/cm2, for a beam diameter of 1.35 mm. For the fluxless-grown crystal, the output peak power, P4ω = 3.08 MW at dη/dP2ω = 0, giving a UV intensity of 430 MW/ cm2. This indicates that the fluxless-grown crystal exhibits much lower two-photon absorption than the flux-grown crystal, since the two-photon absorption starts becoming appreciable at a much higher (2.6 times) UV intensity.
Therefore, the significant improvement in the FHG efficiency in the case of fluxless-grown BBO, as compared to the flux-grown BBO, is due to the reduced scattering losses and the lower two-photon absorption in the fluxless-grown BBO.
In this paper, we have given a relative comparison of the two-photon absorption in the flux-grown and fluxless-grown BBO crystals. In the near future, we plan to measure the two-photon absorption coefficients using the Z-scan technique on thin crystal samples.
We have demonstrated 3.4 MW peak power, 250 ps, 100 Hz pulse generation at 266 nm from a compact microchip laser. This could be achieved due to the high peak power of the fundamental beam, and the high quality fluxless-grown BBO crystal. The fluxless-grown BBO crystal exhibits significantly lower linear and two-photon absorption loss, as compared to the flux-grown crystal.
The SHG and FHG conversion efficiencies were 85% and 60%, respectively. This gives a conversion efficiency of 51% from the fundamental to the fourth harmonic. We believe that this will be a significant step towards efficient generation of DUV for a variety of applications.
We acknowledge the support of SENTAN, JST (Japan Science and Technical Agency) for this work.
References and links
1. J. J. Zayhowski, C. Dill III, C. Cook, and J. L. Daneu, “Mid-and high-power passively Q-switched microchip lasers,” in Proceeding of Advanced Solid-State Lasers, M. M. Fejer, H. Injeyan, and U. Keller, eds., Vol. 26 of OSA Trends in Optics and Photonic Series (Optical Society of America, Washington, D.C., 1999), pp. 178–186.
2. N. Pavel, J. Saikawa, S. Kurimura, and T. Taira, “High average power diode end-pumped composite Nd:YAG laser passively Q-switched by Cr4+:YAG saturable absorber,” Jpn. J. Appl. Phys. 40(Part 1, No. 3A), 1253–1259 (2001). [CrossRef]
4. S. Hayashi, K. Nawata, H. Sakai, T. Taira, H. Minamide, and K. Kawase, “High-power, single-longitudinal-mode terahertz-wave generation pumped by a microchip Nd:YAG laser [Invited],” Opt. Express 20(3), 2881–2886 (2012). [CrossRef] [PubMed]
5. M. Tsunekane, T. Inohara, A. Ando, N. Kido, K. Kanehara, and T. Taira, “High peak power, passively Q-switched microlaser for ignition of engines,” IEEE J. Quantum Electron. 46(2), 277–284 (2010). [CrossRef]
6. N. Pavel, M. Tsunekane, and T. Taira, “Composite, all-ceramics, high-peak power Nd:YAG/Cr4+:YAG monolithic micro-laser with multiple-beam output for engine ignition,” Opt. Express 19(10), 9378–9384 (2011). [CrossRef] [PubMed]
7. H. Kan, A. Sone, H. Sakai, T. Taira, N. Pavel, and V. Lupei, “Laser light source,” U. S. Patent No. 6,931,047 B2 (dated Aug. 16, 2005).
10. T. Taira, “Domain-controlled laser ceramics toward giant micro-photonics [Invited],” Opt. Mater. Express 1(5), 1040–1050 (2011). [CrossRef]
11. M. Takahashi, A. Osada, A. Dergachev, P. F. Moulton, M. Cadatal-Raduban, T. Shimizu, and N. Sarukura, “Effects of pulse rate and temperature on nonlinear absorption of pulsed 262-nm laser light in β-BaB2O4,” Jpn. J. Appl. Phys. 49(8), 080211 (2010). [CrossRef]
12. A. Dubietis, G. Tamošauskas, A. Varanavičius, and G. Valiulis, “Two-photon absorbing properties of ultraviolet phase-matchable crystals at 264 and 211 nm,” Appl. Opt. 39(15), 2437–2440 (2000). [CrossRef] [PubMed]
13. N. Kondratyuk and A. Shagov, “Nonlinear absorption at 266 nm in BBO crystal and its influence on frequency conversion,” Proc. SPIE 4751, 110–115 (2002). [CrossRef]
14. G. Kurdi, K. Osway, J. Klebniczki, M. Divall, E. J. Divall, A. Peter, K. Polgar, and J. Bohus, “Two-photon-absorption of BBO, CLBO, KDP and LTB crystals,” in Proceedings of Advanced Solid State Photonics, Technical Digest (Optical Society of America, Washington, D.C., 2005), paper MF18.
15. D. Eimerl, L. Davis, S. Velsko, E. K. Graham, and A. Zalkin, “Optical, mechanical, and thermal properties of barium borate,” J. Appl. Phys. 62(5), 1968–1983 (1987). [CrossRef]
16. R. DeSalvo, A. A. Said, D. J. Hagan, E. W. Van Stryland, and M. Sheik-Bahae, “Infrared to ultraviolet measurements of two-photon absorption and n2 in wide bandgap solids,” IEEE J. Quantum Electron. 32(8), 1324–1333 (1996). [CrossRef]
17. H. Sakai, H. Kan, and T. Taira, “Passive Q-switch laser device,” U. S. Patent No. 7,664,148 B2 (dated Feb. 16, 2010).
18. M. Nishioka, A. Kanoh, M. Yoshimura, Y. Mori, and T. Sasaki, “Growth of CsLiB6O10 crystals with high laser-damage tolerance,” J. Cryst. Growth 279(1-2), 76–81 (2005). [CrossRef]