Crystal quartz for high-intensity wavelength conversion was evaluated. The pure durability of crystal quartz for a sub-ns pulse region at 1.064µm irradiation was measured as 602 GW/cm2, which was 2-times higher than undopedd YAG crystal. QPM-structured quartz constructed by multi-plate stacking was evaluated by sub-ns high-energy MCL-MOPA pumping. Availability of crystal quartz for high-intensity pumping at 17.3 GW/cm2 could be demonstrated. QPM quartz is expected for both high-intensity operation and short-wavelength conversion.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
The sub-nanosecond (sub-ns) pulses possess practical characteristics of both narrow spectral bandwidth compared to femtosecond pulses and high-peak intensity compared to conventional nanosecond pulses [1,2]. High-energy pulse sources with sub-ns pulse durations are becoming a practical tool for various applications, such as laser machining, laser-induced breakdown spectroscopy, and laser ignition [3–5]. Because of the characteristics of the sub-ns pulses, requirements of nonlinear crystal for effective wavelength conversion are also changing. Ferroelectric and semiconductor crystals, as LiNbO3 [6,7], KTiOPO4 , GaAs [9,10], and ZnGeP2 , have been used by both birefringent-phase matching (BPM) and quasi-phase matching (QPM) scheme as practical and efficient materials [12–14], though their damage thresholds against intense-laser irradiation stays relatively low. On the other hand, borate-type nonlinear crystals such as LiB3O5 and CsLiB6O10 have been used for high-intensity BPM wavelength conversion [15,16], though their phase-matching scheme is limited to BPM because of their birefringent characteristics. Also, the borate-type crystals have a drawback of hydroscopic susceptibility.
We are recently focusing on a crystal quartz as a nonlinear material for high-intensity QPM devices . Crystal quartz can be used for various applications, such as optical window by its high transparency, optical wave plate by birefringence, and crystal oscillator by piezoelectric property. Crystal quartz is also famous as a nonlinear optical material used in the first second harmonic generation (SHG) by Franken et al. in 1961 . Although crystal quartz has desirable properties for intense-laser pumped wavelength conversion as short absorption edge in ultraviolet (UV) region, small absorption, and high laser-damage threshold, its small birefringence have prevented effective wavelength conversion by BPM scheme. Compared to BPM, QPM enables all nonlinear materials effective wavelength conversion by artificial inversion structure of their nonlinear coefficient. Realization of QPM device using crystal quartz can be expected to realize a new scheme of wavelength conversion, especially in high-energy, sub-ns pulse sources [17,19,20].
We are currently developing a compact-sized, high-energy, sub-ns pulse source with output energy around 1 J at 1.064 µm operation, which is constructed by a microchip-laser-based master-oscillator power amplifier (MCL-MOPA) configuration, for both beam-mode cleaning and output-energy increasing .
In this paper, we present characteristics of crystal quartz for high-intensity wavelength conversion. Laser-induced damage characteristics for high-intensity 1.064 µm irradiation is evaluated using a bulk-shaped material, which can eliminate surface effects of materials such as adhesive dirt and polishing quality. Pure damage intensity of crystal quartz for various quality was evaluated and compared with related optical materials. Also, SHG by a QPM-structured crystal quartz device is demonstrated using a high-intensity MCL-MOPA. Multi-stacked quartz by optical contact method is evaluated against high-intensity laser irradiation.
2. Evaluation of laser-induced damage characteristics using bulk-shaped material
Damage evaluation using a plate-shaped material is a basic method for laser-induced damage characterization, although it includes its plate-surface effects. Various surface conditions as adhesive dirt, surface degradation by oxidation and de-oxidation, and polishing quality affect the plate-shaped evaluation, especially in intense-laser irradiation. Even if the evaluation is demonstrated under vacuum condition, complete elimination of the surface effects cannot be possible. On the other hand, damage evaluation using a tightly focused laser beam inside of a bulk-shaped material can suppress such surface effects as shown in Fig. 1(a). Laser beams is tightly focused at center of the material, which gives a limited laser intensity at both input and output facet and no surface damages occurs at the both facet. On the other hand, a laser intensity at the focusing point inside of the bulk-shaped material can be easily increased to make damages without the surface effects in case of the plate-shaped material. Resulted line-shaped damages exist only inside of the bulk-shaped material as in Fig. 1(b), and evaluated results presents a pure damage characteristics of the material without the surface effects.
For the damage evaluation using a bulk-shaped material, intense laser sources with high and stable beam quality are required. We prepared a Nd:YAG microchip laser (MCL) with pulse duration Δt = 0.7 ns of 1.064 µm wavelength. Linearly polarized beam with maximum energy of 3 mJ at 30Hz operation and beam quality M2 ~3 was used for the damage evaluation. Further information on the pump source is shown in our previous report .
Focused 1/e2-diameter D of the laser beam was around 34 µm with assuming a spatial Gaussian beam shape. Laser intensity I was calculated as I = 8Ep / (π D 2·Δt) using an input pump energy Ep. Each measurements were done by continuous irradiation of pulse trains, and damage threshold intensity ID was evaluated from multiple measurements for each material.
3. Damage characteristics of crystal quartz and related optical materials
Crystal quartz can be grown by hydrothermal growing method , and resulting quartz have some classification of type and quality. We evaluated two types of crystal quartz, which are commercially available for materials of optical and piezoelectric devices. Typical size of bulk-shaped quartz is 40 mm x 20 mm x 15 mm. Both quartz are uncoated. Figure 2 shows generation and expansion of line-shaped damages in the bulk-shaped quartz. Laser-induced damage firstly occurs at the focusing point in the middle of the material, grows to the backward direction of the laser beam as a line-shaped damage, and terminates automatically at the condition of laser intensity < laser damage threshold. Because residual laser beam is depleted by absorption and scattering at the focusing point, the line-shaped damage does not grow to the forward direction of the laser beam. Laser intensity was gradually increased, and a laser intensity that damage occurred was measured as the damage-occurred intensity. After one measurement finished, the focusing point was moved to proceed another measurement.
Distribution of the damage-occurred intensity in multiple measurements for both quartz were summarized in Fig. 3. Total number of measurement was 39 for both quartz. From these results, ID of crystal quartz for optical device could be evaluated as 602 GW/cm2 (with standard deviation, SD = 16 GW/cm2), and ID for piezoelectric device was 459 GW/cm2 (SD = 53 GW/cm2). Crystal quartz for optical devices showed higher ID compared to piezoelectric devices. Also, we can see a small SD of measured results for the optical quartz compared to the piezoelectric quartz, which means that crystal uniformity the optical quartz is superior to the piezoelectric quartz. Improvements of the hydrothermal growing method for crystal quartz can be expected by evaluation of the distribution of the damage-occurred intensity.
For comparison, borosilicate glass (BG) as conventional optical glass, undoped Y3Al5O12 crystal (YAG) as laser material were also evaluated by the same method. Size of each materials are 20 mm x 20 mm x 15 mm for BG, and 10 mm (diameter) x 30 mm (length) for YAG, and both are uncoated. Total number of measurement was 30 and 21 for BG and YAG. Distribution of the damage-occurred intensity is shown in Fig. 4, and damage threshold intensity ID could be evaluated as 428 GW/cm2 (SD = 17 GW/cm2) and 287 GW/cm2 (SD = 12 GW/cm2) for BG and YAG, respectively, as shown in Fig. 4. Although ID of BG and YAG were evaluated lower compared to the crystal quartz, their SD of measured results showed comparable or smaller, which means a superior crystal uniformity of BG and YAG as optical and laser materials.
4. SHG using QPM-structured quartz pumped by high-energy MCL-MOPA
For construction of a QPM device, periodically inverted structure of nonlinear coefficient is required [12–14]. The QPM period Λ depends on refractive index of nonlinear material, and the Λ for 1st-order QPM-SHG by a nonlinear coefficient d11 or d12 of crystal quartz at 1.064 µm pumping can be calculated to 42 µm. We have previously fabricated a QPM-structured quartz device by optical contact method using quartz plates with thickness of 0.12 mm , which is not a 1st-order but a high-order QPM structure. Although conversion efficiency of the high-order QPM results in a degraded efficiency compared to the 1st-order QPM, high durability of crystal quartz enables high-intensity pumping by a sub-ns laser source, which can compensate the degradation of efficiency.
We evaluated a SHG characteristics of the QPM-structured quartz pumped by a high-energy, MCL-MOPA source of 1.064 µm operation. Experimental set up is shown in Fig. 5. The MCL-MOPA is constructed by three components as master oscillator (MO), pre amplifier (PA1) for beam cleaning, and main amplifier (PA2) for power scaling , and generates linearly-polarized, high-energy pulses of Δt = 0.75 ns at 10Hz repetition frequency. The beam quality factor was measured as M2x ~1.34 and M2y ~1.40. Maximum energy used for this experiment was 55 mJ. The QPM quartz device was fabricated by multi stacking of x-cut quartz plates, and several devices with different plate-stacking number N of 1, 4, 20 and 48 were evaluated. High-energy pulses from the MCL-MOPA with pumping energy Ep was weakly focused to the QPM quartz device, and generated SH green energy ESH was measured after several filters. The QPM quartz was operated at room temperature, and precise condition of phase matching was realized by rotation of the QPM quartz.
Figure 6(a) shows characteristics of ESH on Ep at N = 48. Maximum SH energy of 250 µJ could be obtained at Ep = 52 mJ with conversion efficiency of 0.48%, and output SH energy could be improved > 30 times higher than out previous report . Peak power of SH beam can be estimated > 0.4 MW by assuming a SH-pulse duration as 80% of a pump-pulse duration from previous work . Focused area size S is 0.8 x10−2 cm2 (beam 1/e2-diameter D = 1.01 mm), and maximum laser intensity I reached 17.3 GW/cm2 at Ep = 52 mJ.
Figure 6(b) presents an increasing characteristics of maximum ESH on N at conditions of S = 0.5~0.8 x10−2 cm2 and Ep = 50~55 mJ. Dotted line in Fig. 6(b) denotes N2-proportional characteristics in low efficiency SHG theory, and our experimental results well fitted the N2-characteristics.
5. Discussion on crystal quartz for a high-intensity QPM device
Comparison of damage intensity, SD, and relative durability against YAG are summarized in Table 1. As expected, crystal quartz grown for optical device showed highest laser-damage durability at 1.064 µm irradiation, which was 2-times higher than YAG. The laser damage characteristics of materials depend on both laser wavelength and pulse duration, and we expect a superiority of crystal quartz can be shown more at shorter-wavelength lasers as visible and UV region.
From the result of Fig. 6(b), future possibility of the QPM quartz device can be also estimated. Although we could demonstrate an availability of crystal quartz for high-intensity pumping as I = 17.3 GW/cm2, maximum conversion efficiency remained low because maximum plate-stacking number N in this evaluation is as low as 48. Typical QPM device has larger periodic structure of N > 1000 . Therefore, as shown in Fig. 6(b), higher conversion efficiency for the QPM quartz can be expected simply by increase N, such as 1% efficiency (lower dashed line in Fig. 6(b)) for N ~60. Although much higher efficiency > 10% (upper dashed line in Fig. 6(b)) can be also expected at N > 300, characteristic deviation from the N2-proportional theory at the low-conversion efficiency region becomes larger, and total optical loss as scattering and reflection at interface of each plates becomes large, which prevent an efficient wavelength conversion.
In general, construction of QPM device by plate-stacking method become severe as increasing number of stacking plates. Recently, material bonding technique by using an ion-beam / atomic-beam irradiation has been reported for optical applications. Distributed face cooling device by YAG and sapphire plates for efficient cooling of microchip laser system , and room-temperature bonding of GaAs plates for infrared QPM device  are already demonstrated. Improving of the QPM quartz with increased N > 300 can be also expected by using these low-loss plate-stacking technique.
Crystal quartz is a suitable material for high-intensity QPM devices. Damage characteristics of the crystal quartz was evaluated at sub-ns pulse region by 1.064 µm laser irradiation, and compared with related optical materials. Crystal quartz grown for optical device showed highest durability, which was 2-times higher than undoped YAG crystal. Also, high-energy-laser pumped SHG was demonstrated using QPM-structured quartz devices. The QPM quartz could be used for high-intensity pumping > 10 GW/cm2, and output SH energy of 250 µJ could be obtained.
Future improvement of wavelength-conversion efficiency can be expected by increasing number of plate stacking combined with low-loss plate-bonding technique. QPM quartz has a potential for future material of high-intensity, nonlinear-optic wavelength conversion in shorter wavelength region as visible and UV region.
JSPS Grand-in-Aid for Scientific Research (A)15H02030, Photon-Frontier-Consortium project by MEXT of Japan; ImPACT program by cabinet office of Japan.
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