High efficiency 12 . 5 J second-harmonic generation from CsLiB 6 O 10 nonlinear crystal by diode-pumped Nd : glass laser

A 12.5 J second-harmonic generation with 71.5% conversion efficiency at 0.6 Hz repetition rate from a diode-pumped Nd:glass laser system has been demonstrated by using a CsLiB6O10 (CLBO) nonlinear optical crystal as a frequency doubler. The CLBO has aperture of 40 mm x 40 mm and thickness of 14 mm with Type-II phase matching. The CLBO is mounted into a housing which flows dry nitrogen gas on the CLBO’s face. There is no significant reduction of conversion efficiency by exposing of over 600,000 shots for intermissive experiment during 3 years. In our knowledge, these experimental results of output energy and conversion efficiency are highest performance as second-harmonic generation of a diode-pumped solid state laser by using one CLBO nonlinear crystal. In this paper, potential of the CLBO as a frequency converter for repetitive kJ class laser is discussed. ©2013 Optical Society of America OCIS codes: (140.3480) Lasers, diode-pumped; (140.3515) Lasers, frequency doubled. 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Introduction
Frequency conversion is useful technique for extending utility of high power lasers [1,2].Second-harmonic generation (SHG) of near infrared lasers is essential technique to use for a pumping source of Ti:sapphire lasers and optical parametric amplifiers [3][4][5].Recently, frequency doubling technique for a 10 kW class high average-power lasers realized by high repetition rate diode-pumped solid state laser and CW output fiber laser is required.These lasers are actually used for several applications such as annealing process of solar cells and frat panel displays or cutting and welding for metal materials.In these high average power lasers, large temperature-bandwidth is the most important characteristic for a nonlinear crystal.On the other hand, requirements of nonlinear crystals for a high average-power laser reach comparable specifications of a frequency converter of unit-beamlet of laser drover for inertial fusion energy (IFE) reactor which minimum output is assumed 10 kW (1 kJ at 10 Hz) [6].Unique requirement but the most important specification for IFE driver is scalability of crystal growth.
In 1990's, large scalable nonlinear crystal with broad temperature-bandwidth of YCa 4 O(BO 3 ) 3 (YCOB) and CsLiB 6 O 10 (CLBO) were developed in Osaka university, Japan [7,8].A 317 W average-power SHG from 61 J at 10 Hz with 52% conversion efficiency output diode-pumped Yb:S-FAP laser was reported by using a YCOB nonlinear crystal with aperture of 5.5 x 8.5 cm 2 [3,9,10].As a frequency conversion experiment using a CLBO, a 25 J of green pulse output with 74% conversion efficiency at single shot experiment was demonstrated with flash-lamp-pumped Nd:glass laser [11].In the report, quadrature scheme SHG [12] was experimented by using two CLBO crystals.
We have developed a 20 J at 10 Hz in 1053 nm output diode-pumped solid-state laser (DPSSL) system [13].This paper reports experimental results of SHG from the DPSSL by using one CLBO crystal.As a result, output of 12.5 J in 527 nm at 0.6 Hz repetition rate has been demonstrated with the highest conversion efficiency of 71.5% from one CLBO crystal.Then maximum input intensity of fundamental pulses are estimated approximately 500 MW/cm 2 at 10 ns pulse duration.The high conversion efficiency has been kept over 600,000 shots with 8 J level outputs during intermissive experiments for 3 years.Stable DPSSL input and nitrogen-gas cooling techniques to the CLBO have contributed this reliable result.In addition, this paper reports calculated evaluation of frequency doubling of 1 kJ at 10 Hz class laser by a CLBO nonlinear crystal.

Experimental setup
Experimental setup of a frequency doubling from the diode-pumped Nd:glass laser and beam diagnostics is shown in Fig. 1.A single-longitudinal and -transverse mode seed pulse with duration of 10 ns is generated from a frontend system which consists of a fiber laser oscillator and a Pockels cell electro-optic modulator.The seed pulse is amplified from nano-Joule to typical 300 mJ by a high gain diode-pumped Nd:YLF ring-type multi-pass preamplifier at repetition rate of 10 Hz [14].The output pulse from the preamplifier is boosted up to 20 J by a diode-pumped zigzag slab Nd:glass multi-pass main-amplifier [13].Filling factor of a near field pattern (NFP) of the output pulse from the main amplifier was 0.55, and encircled energy of far field pattern (FFP) in 5 times of diffraction limit area was evaluated 80%.An apodized NFP at the frontend system by a serrated aperture is transferred to each amplifier heads both of the preamplifier and the main-amplifier through every passes.The high aspect NFP of 44 mm x 8 mm specified by aperture of the zigzag slab amplifier is image-relayed onto a CLBO frequency converter by a Galileo type anamorfic telescope which re-formed the NFP to 22 mm x 16 mm.Converted second-harmonic pulses by the CLBO are separated from fundamental pulses by two dichroic mirrors (DM1, DM2) (Showa Optronics Co., Ltd.:112-J01474-701).Reflectivity of the dichroic mirrors is 1% for fundamental and 99.8% for second-harmonic.Second-harmonic pulse energies are evaluated measuring of leakage pulses from dielectric-coated mirror (M) by a energy meter (Ophir Optronics Ltd.:PE50).Fundamental pulses transmitting from first dichroic mirror (DM1) are dumped energy by a water-absorbing beam dumper (WBD).Reflected fundamental pulses at a non antireflection (AR) coated window of the WBD are used for evaluation of pulse energy of non-converted fundamental pulses by a energy meter (Coherent Inc.:LM100).A NFP of second-harmonic pulses is measured by slightly leaked pulses from the DM1.Then fundamental pulses are removed by a heat-absorbing filter (SIGMA KOKI Co., Ltd.:HAF-50S-30H) with transmissivity of 0.1% for fundamental wavelength.A experimental setup of second-harmonic generation and beam diagnostics.Where, IP is image-point of telescopes in system, VSF is a vacuumed spatial filter, AT is a Galileo type anamorfic telescope, QR is a 45 degree quarts polarization rotator, DM is a dichroic mirror, BD is a beam dumper, WBD is a water absorbing beam dumper, EM is a energy meter and HA is a heat-absorbing filter, and CCD is a imaging camera.
Figure 2(a) shows a photograph of 10 cm class large ingot of a CLBO nonlinear crystal (Hamamatsu Photonics K. K.).The CLBO ingot was cut at Type-II phase match angle with aperture of 40 mm x 40 mm square shape with length of 14 mm (Kogakugiken Corp.), shown in Fig. 2(b).Initial transmitted wavefront distortion was λ/4.7 at peak-to-valley measured by a Zygo interferometer.The CLBO which is not anti-reflection (AR) coated on its faces was mounted into a non-heated housing [shown in Fig. 2(c)].This housing is equipped AR coated two optical windows.Nitrogen gas with room temperature flow both gaps between windows and the CLBO surfaces in order to prevent moisture absorption of the CLBO and to cool the CLBO.Then flow rate is 3-5 l/min.

Theoretical simulation and experimental results
Converted second-harmonic output power as a function of fundamental wave input power was calculated by using following equations of complex amplitude for three wave mixing [15].
where, E is amplitude of electric-field of the light, z is position at propagating direction in the Cartesian coordinate, k Δ is total wave mismatch, n is index of refraction and d eff is effective nonlinear coefficient.Wave mismatches caused by beam divergence of fundamental laser and temperature distribution in the CLBO due to absorption of fundamental are included in simulation by using following equations [15]. 1.772 Here, angl  Temperature distribution inside of the CLBO was assumed to distribute on z axis because Nitrogen gas flows on both face of the CLBO.Temperature distribution was calculated by following equations.
( ) ( ) where, Q is heating density, P is average power of fundamental laser, α is absorption coefficient, A is cross section of heating area on crystal, T c is coolant temperature, κ is thermal conductivity of nonlinear crystal.Beam divergence angle is defined as a function of input fundamental pulse energy to the CLBO.Because a wavefront of the fundamental pulses are distorted by thermal lens effect in the laser mediums.In our experiment, the wavefront distortion in this laser is about 1 μm at peak-to-valley with 10 J at 0.6 Hz operation.If this wavefront aberration distribute in uniform spherical, a maximum increment of the beam divergence is estimated approximately 160 μrad.We assume that the beam divergence is proportionate to fundamental pulse energy with factor of 10 μrad/Joule.In calculation, walk-off effect is ignored because beam size is enough large.Wave mismatch comes from spectral bandwidth is also ignored because the fundamental laser is single longitudinal mode with frequency of approximately 50 MHz.Used some physical constants and experimental parameters in calculation are shown in Table 1.Absorption coefficient and thermal conductivity are assumed a same value with KD 2 PO 4 (DKDP) nonlinear crystal.Experimental results of calculated theoretical curve of second-harmonic energy and conversion efficiency as a function of fundamental input energy are shown in Fig. 4. Repetition rate of laser was 0.6 Hz in this experiment.Experimental result is good agreement with theoretical curve.In this experimental result, a 12.5 J second-harmonic pulse was generated from 17.5 J fundamental pulse with 71.5% frequency conversion efficiency.Then this efficiency includes energy losses caused by Fresnel reflection at non-AR coated surfaces of the CLBO.Actual maximum conversion efficiency inside of the CLBO reached to 76.2% at input energy of 13.5 J.In this experiment, conversion efficiency from pump energy of LD to SHG output energy was reached to 8.6%.In my knowledge, this efficiency is the highest optical efficiency in 10 Joule class DPSSL.A NFP of second-harmonic pulse is shown in Fig. 5. Filling factor defined as the ratio of the average intensity in the beam area to the peak intensity was evaluated to be 44.2%.This uniformity of the NFP has contributed to prevent an optical damage at the condition of input peak-power of near 500W/cm 2 .We have used this green pulses for some intermittent experiment as a pump source of Ti:sapphire laser.There is no significant reduction of conversion efficiency of SHG by exposing over 600,000 shots during 3 years.Maximum temperature increment inside the CLBO crystal was evaluated 0.16 °C by calculation.This evaluation indicates that high average power operation at 10 Hz in this system is enough feasible because estimated temperature increase of 1.6 °C at 10 Hz operation is twentieth part of temperature bandwidth of a CLBO.

Evaluation of second-harmonic generation by CLBO for 1 kJ at 10 Hz laser
A CLBO is suitable material for frequency conversion of high average power lasers with high pulse energy because of its scalability of crystal size and large temperature bandwidth.We evaluated a potential of a CLBO as frequency doubler of 1 kJ at 10 Hz laser by calculation.In this calculation, simulation model shown in Fig. 3 and some parameters shown in Table 2 were used.A DKDP which can be grown to large size is conventional major candidate for SHG of kJ class laser system.We also have evaluated a DKDP as comparison.Then the crystal length of 12 mm for DKDP was optimized for same calculating condition of 1 kJ at 10 Hz input average power.Calculation results of second-harmonic conversion efficiency as a function of input fundamental pulse energy is shown in Fig. 6. Results of single shot condition which defined no heat effects are described in dashed line as a reference.In the single shot results, both of DKDP and CLBO achieved high conversion efficiency of over 75%.In the case of 10 Hz condition, however, conversion efficiency of DKDP shown in blue solid line was decrease until 40%.On the other hand, conversion efficiency of CLBO still remained over 75%.Then maximum temperature was estimated to 1.9 °C for DKDP and 1.1 °C for CLBO.This significant difference of conversion efficiency depends on difference of temperature bandwidth between DKDP and CLBO.This result indicates that CLBO is a candidate of frequency converter for repetitive pulse output kJ class laser.

Conclusion
A 12.5 J SHG with 71.5% conversion efficiency at 0.6 Hz repetition rate from diode-pumped Nd:glass laser system was demonstrated by using a CLBO nonlinear crystal as a frequency doubler.Filling factor of NFP was evaluated to be 44.2%.There was no significant reduction of conversion efficiency by exposing of over 600,000 of intermittent experiment during 3 years.In our knowledge, this result is the highest output characteristic of energy and conversion efficiency by SHG of diode-pumped laser from a CLBO.And also potential of CLBO for a frequency converter of 1 kJ at 10 Hz laser was indicated from numerically calculated simulation.A CLBO is used for frequency converter of third-harmonic generation of near infrared lasers if we use two crystals which are cut to Type-I and Type-II phase matching.In addition, CLBO has a large spectral bandwidth of 5.6 nm.These characteristics are more suitable for application of IFE driver than other candidate crystals.For actually applying of CLBO to industrial applications, development of remained practical issue of AR coating technique on hygroscopic crystals is required.
Fig.1.A experimental setup of second-harmonic generation and beam diagnostics.Where, IP is image-point of telescopes in system, VSF is a vacuumed spatial filter, AT is a Galileo type anamorfic telescope, QR is a 45 degree quarts polarization rotator, DM is a dichroic mirror, BD is a beam dumper, WBD is a water absorbing beam dumper, EM is a energy meter and HA is a heat-absorbing filter, and CCD is a imaging camera.

Fig. 2 .
Fig. 2. Photographs of a CLBO frequency doubler.(a) a ingot (b) a cut and polished (c) a mounted into nitrogen gas purged housing.
sensitivity of nonlinear crystal for incidence beam divergence and crystal temperature.δθ and T δ are actual beam divergence and temperature distribution inside of the crystal in z direction.θ Δ and T Δ are angular bandwidth and temperature bandwidth.L is a length of nonlinear crystal.Calculation model based on actual experimental condition is shown in Fig. 3.

Fig. 4 .
Fig. 4. Experimental and calculated results of second-harmonic pulse energy and conversion efficiency as a function of fundamental pulse energy.

Fig. 6 .
Fig. 6.Calculation result of second-harmonic conversion efficiency as a function of input fundamental pulse energy.