The interest in the use of borate crystals in nonlinear optics (NLO) has increased because all solid-stateshort-wavelength lasers obtained with NLO are in highly demand. Much effort has been spent on developingborates series, such as β-BaB₂0₄ (BBO) and LiB₃O₅ (LBO), in this decade. Recently another new boratecrystals, CsLiB₆O₁₀ (CLBO), GdCa₄0 (B0₃) ₃ (GdCOB), YCa₄0 (B0₃) ₃ (YCOB) and Sr₂Be₂B₂O₇ (SBBO) families, have been discovered and developed. Here, the growth, NLO and frequency conversion properties ofthese new borate crystals are reviewed, with emphasis on the history of the development of this crystal group.

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BBO (β-BaB₂O₄) exhibits excellent performance when applied to wavelength conversion devices, specially inthe UV region. While BBO is usually grown by the flux method, we have successfully grown it by the Czochralski (CZ) method from a BaB₂O₄ composition melt. Seed-crystals in the phase matching direction are used to thepulling direction of CZ growth. The advantages of CZ-BBO are its high purity, high growth rate, and high damage threshold. For stable output of BBO-generated UV-laser, polishing and coating, as well as crystalquality, are important. This paper reviews newly developed growth and processing techniques for CZ-BBO and discuss the future imnrovements in BBO performance.

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Absorption band of Cr: LiSAF crystal is wide so out put laser power from the crystal does not so sensitive fora wavelength change of LD used as excited source. Though non-linear constant of LBO crystal is small, itshigh transparency and wide tolerance for phase-matching are suitable to an intra-cavity resonator. As a result of these characters of the two crystals out put stability of the blue SHG laser ICD-430 shows a level ofpractical use.

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LiNbO₃ waveguide devices for quasi-phase-matched nonlinear-optic wavelength conversion have beenstudied actively in many research groups. This paper reviews our research works of LiNbO₃ waveguide second harmonic generation (SHG) devices for implementation of compact short wavelength coherent lightsource, including descriptions of LiNbO₃ ferroelectric domain inversion by voltage application, SH blue/ultraviolet light generation with a normalized efficiency of 150%/W, enhancement of the efficiency utilizingresonant effect, generation of third harmonic wave of Nd: YAG laser light by cascading SHG and sumfrequencygeneration. Recent research topics of waveguide nonlinear-optic wavelength conversion devicesare also reviewed briefly.

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A periodically domain inverted crystal has several advantages, such as 1) high conversion efficiency 2) widetemperature tolerance 3) no walk off. Periodically poled MgO-LiNbO₃ for blue and green were successfullyobtained with uniform periodicity by the novel corona discharge method. Using these crystals in intracavityfrequency doubling of a diode-pumped device made it possible to control temperatures of LD and periodicallypoled MgO-LiNbO₃ simultaneously. As a result, we have developed high beam quality, high power stabilityand compact blue and green solid state lasers. Using these blue and green solid state lasers, a new digital photoprinting system and a new fluorescent image analyzer were accomplished.

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This paper explains the tunable 214.5-nm continuous-wave coherent radiation generated by using an all-solidstatelight source system. The light source consists of the oscillator and amplifier system based on laser diodesand the two-stage frequency doubling system with KNbO₃ and β-BaB₂O₄ crystals. The power of the generated 214.5-nm light amounts to more than 0.64 mW. This light source will be used for laser-cooling of Cd⁺ ions.

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The automatic and rapid crystal growth method was developed for water-soluble crystal, KDP. Theover-heating of the water solution of crystal enlarges the metastable zone during the temperature reducingprocess as a result of eliminating an initial cluster density. The higher level of supersaturation up to 200% onKDP crystal was achieved resulting in an average growth rate of 54 mm/day at supersaturation of 38%. Theconcentration of the growth solution was measured by the conductivity of the solution. The supersaturationwas controlled with the dependency of conductivity on temperature by the computer. The 60-mm class KDP single crystal was grown automatically by the method with an average growth rate of 30 mm/day. The spectraltransmittance and laser damage threshold of the rapidly grown crystals were measured. The lower laserdamage threshold of the rapidly grown crystal was improved completely by a thermal conditioning to be 17J/cm² of 1 ns pulse at 1-μm wavelength.

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We have compensated for thermal lensing of nonlinear crystal and Nd: YAG rods by advanced resonator designin a high-power diode-side-pumped intracavity-frequency-doubled CW Nd: YAG laser. The maximum greenoutput power of 27.0 W was generated with the beam quality of M²=7.9. This value is, to our knowledge, thehighest green output power in CW diode-pumped solid-state lasers. It was found that the green powerfluctuation is caused by the unstable zone of the resonator due to the thermal lensing of the nonlinear crystal (KTP). TEM₀₀-mode green output power of 16.3 W was also generated.

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A high power SHG blue laser with direct modulation capability at high frequency has been demonstrated usinga tunable distributed Bragg reflector (DBR) laser diode and a waveguide-type quasi-phase-matched secondharmonic generation (QPM-SHG) device in x-cut Mg doped LiNbO₃. Continuous blue light power of 12 mWand pulsed peak power of 26 mW were generated. The SHG blue laser was installed to an optical head, andreadout of ROM disk and recording on phase change optical disk were performed. A butt-coupling-type 0.8cc SHG blue laser was also fabricated. It was indicated that SHG blue laser is applicable to high-density opticaldisk system.

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When visible laser light passes through a β-BaB₂O₄ (BBO) single crystal, the Tyndall phenomenon can beobserved along the beam path. BBO samples were prepared in three ways: Czochralski (CZ) grown in thex-axis direction, CZ-BBO grown in the SHG (θ=23°) direction, and flux grown-BBO. The intensity of scattered light in the first samples was significantly lower that in the other two. Intensity distribution measurements with a polarized He-Ne laser show that the intensity of scattered light is determined by thestrength of the dipole vibration along the z-axis. Additionally, when the UV laser light (Kr-F Excimer laser: 248 nm 700 mW) passes through the BBO, heat propagation is also observed, which indicates that the somepart of the dipole vibration energy from UV light is exciting thermal phonons.

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Accurate instrumentation is described which is capable of measuring mass in-10μg of an electrodynamically levitated pellet. From the voltage required to counterbalance the gravitational-buoyant forces and the electrostatic force in a three dimensional quadrupole, the pellet mass, along with its charge, is determined. Bythis method, the mass is determined within an error of 0.21μg.

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Spectrophotometry for determining the metal thickness of a laser-fusion pellet is proposed. The technique, which uses Ni ion solution from Ni-coated glass micro-balloon, is demonstrated within ±5% error. Using thistechnique, a conventional quartz crystal monitor is calibrated for spherical pellets.

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The Q-switch, high peak power oscillation of a laser-diode pumped Er, Yb: glass laser at eye-safe wavelengthhas been realized using a three-mirror coupled-cavity, low-loss and fast piezoelectric actuator Q-switch. Highpeak power of 3 kW was obtained with a pulse width of 4 ns and output energy of 12 μJ. The resonancecondition of the threshold gain of the coupled-cavity laser was calculated. In the case where the free spectralrange (FSR) of the external cavity is selected to be equal to the gain cavity FSR, the multi-axial mode pulse canoscillate and resulted in a high peak output power. By scanning the external mirror, the single-axial modeoscillation was step-tunable from 1535 nm to 1565 nm.

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