Optical converting method and converter device using the single-crystal lithium tetraborate, and optical apparatus using the optical converter device

Abstract: Single-crystal lithium tetraborate (single-crystal Li 2 B 4 O 7 or single-crystal LBO) as an optical converter device which stably operates for a long term, has a long life, and is compact, light and inexpensive, an optical converter device using the single-crystal LBO, especially, for emitting light having a wavelength of 500 nm or less, and an optical apparatus using the optical converter device. Coherent light is irradiated to the light incident face of the single-crystal LBO at a predetermined incident angle, which single-crystal LBO is cut at a predetermined plane to the optical axis so as to satisfy a predetermined phase matching angle, and light having a half wavelength of the incident light is emitted. Preferably, the single-crystal LBO has a refractive index variation of 10 -5 /mm or less and/or an etch-pit density of approximately 1×10 3 /cm 2 . A first coherent light beam having a first wavelength and a second coherent light beam having a one-second or one-fourth wavelength of the first coherent light beam are irradiated to the incident face of the single-crystal LBO at a predetermined angle, and at least a coherent light having a one-third or one-fifth wavelength of the first coherent light beam is emitted by sum frequency generation, for example, when light having a wavelength of 209 nm is emitted, the wavelength of the first coherent light is 981 nm and that of the second coherent light is 266 nm, for example.

Description: FIELD OF THE INVENTION The present invention relates to single-crystal lithium tetraborate (single-crystal Li 2 B 4 0 7 or single-crystal LBO) and a method for making the single-crystal lithium tetraborate, a method for converting a wavelength of light and an optical converter device using the single-crystal LBO, and an optical device using the optical converter device. The optical apparatus using the single-crystal LBO in accordance with the present invention as an optical converter device may includes wavelength converter devices for generating second harmonic, fourth harmonic, fifth harmonic and the like, various laser apparatuses and optical parametric oscillators using the wavelength converter devices, and other apparatuses using the single-crystal LBO as non-linear optical converter devices. DESCRIPTION OF THE RELATED ART Recording density can be improved by using a laser light beam having a shorter wavelength. Therefore, light sources which emit a laser light beam of a shorter wavelength are needed for reading data from and writing those to recording media. When a laser light beam of a shorter wavelength is used for material processing, finer processing can be performed. Further, light sources which emit a laser light beam of a shorter wavelength are needed for medical instruments and lithography of ultra LSIs. No practical light source emitting a light beam having a wavelength of 500 nm or less, however, has been developed. For example, semiconductor lasers can emit a laser light beam up to about 400-nm wavelength, but its output power is extremely low. Excimer lasers have been known as short wave, high output lasers. The excimer laser was first realized by Basov et al. in USSR in 1970 by means of electron beam excitation of liquid xenon (Xe), and then was performed by discharge excitation in 1976. In discharge excitation-type excimer lasers, a laser light beam is obtained by amplifying an ultraviolet light beam emitted from ArF (193 nm), KrF (248 nm), XeCl (308 nm) or the like by an optical resonator with pulse repeating oscillation laser. Applications of excimer lasers have been expected in abrasion processing, surface modification, and marking of polymer materials, thin film preparation, production of medical supplies and isotope separation. However, the use of excimer lasers is still unsatisfactory in processing or film forming process, because the light emission time is significantly short in relative to the interval, for example, a pulse light emission of 10 -9 sec per 10 -3 sec, in a pulse laser of a few hundred ppm (pulse per second). Further, in excimer lasers, gas media have shorter lives and are toxic, laser apparatuses barely miniaturized and maintained, with higher operation costs. Accordingly, no semiconductor laser which stably emits ultraviolet light for a long time has been developed. Non-linear optical devices such as secondary harmonic-wave generation (SHG) devices have been intensively investigated. Since SHG devices emit a light beam having a wavelength of one-half of the incident light beam, an ultraviolet light beam can be emitted from an infra red laser light beam, for example. Thus, SHG devices have significant industrial advantages. Compact, light apparatuses which can stably emit a visible or ultraviolet light beam for a long time is needed in the industrial fields, such as optical information processing for recording and reading high density information, displays, optical measurements, material processing, medical cares, and LSI productions. MATERIAL LETTERS, vol. 20, no. 3/04, 1 July 1994, pages 211-215, XP000483610 KWON T Y ET AL: "CHARACTERISTICS OF CRITICALLY PHASE-MATCHED SECOND-HARMONIC GENERATION OF A LI2B407 CRYSTAL GROWN BY THE CZOCHRALSKI METHOD" describes a second-harmonic generation experimental set-up and a method for transducing the wavelength of light. A coherent light beam is irradiated to a light incident face of a single-crystal lithium tetraborate (Li 2 B 4 O 7 ) at a predetermined. angle which is cut at the predetermined plane to the optical axis so as to satisfy a predetermined phase matching angle. Due to the second harmonic generation a light beam having a half wavelength of the incident light beam is output from the single-crystal. The European patent application EP 0 644 636 A2 describes a wavelength conversion device where a first and a second infra-red light with wavelengths of 0.86 µm and 1.3 µm are combined by a dichroic mirror and are then irradiated to a lithium-niobate (LiNbO 3 ) substrate. Due to non-linear optical effects in the substrate light having a wavelength of the sum frequency 0.52 µm is output from the substrate. Another example of crystalline used for wavelength converter devices such as SHG devices is KTP (KTiOPO 4 ) disclosed in Japanese Unexamined Patent No. 3-65597. Although a wavelength converter device using KTP has a wide transparent region of 0.25 to 4.5 µm to the wavelength of an incident laser, phase matching is not carried out at a region of less than 1 µm. Thus, when a YAG laser is used as an incidental laser light source, the wavelength converter device using KTP can emit only the light beam having the two-fold frequency (or the half wavelength) of the incident light. On the other hand, light having a shorter wavelength, for example, fourth harmonic and fifth harmonic of a YAG laser, recently has been needed. In addition, a large KTP crystal is barely obtained, and the refractive index varies inside the crystal. Therefore, different KTP devices made of a piece of crystal have different refractive indices, and thus have different phase matching angles, resulting the difficulty in the high precision wavelength converter device production. Moreover, the KTP crystal often has some voids and causes the difficulty in the mass production of KTP devices with high quality. Another example of crystal used as the wavelength converter device such as SHG device is BBO (β-BaB 2 O 4 ) disclosed in Japanese Unexamined Patent No. 63-279231. Although BBO has a durability to the laser higher than that in KTP, it is difficult to handle the crystal and make a large crystal piece because of its strong deliquescence. SUMMARY OF THE INVENTION It is the object of the present invention to provide an optical converter device and a converting method with which a stable conversion operation can be carried out for a long term and in a compact, light and inexpensive manner. This object is solved by claim 1 and by claim 4. It is an advantage of the present invention that an optical converter device or optical converter apparatus can effectively provide ultraviolet light. It is a further advantage of the present invention that various optical apparatuses using the optical converter device or optical converter apparatus set forth above, for example, an optical apparatus can convert the frequency (or wavelength) of the output light from YAG laser to ultraviolet light including four-fold and five-fold frequencies (or one-fourth and one-fifth wavelengths) of the output light. It is a still further advantage of the present invention that various optical apparatuses, e.g. laser apparatuses, using the optical converter device and/or optical converter device set forth above, can be provided. Further advantageous embodiments and improvements of the invention are listed in the dependent claims. In accordance with the invention a first and second coherent light beams each having a different wavelength are irradiated in the optical converter to the light incident face of the above-mentioned single-crystal lithium tetraborate (Li 2 B 4 O 7 ) and a light beam having a wavelength of sum frequency generation of the wavelengths of these two coherent light beams is emitted. Specifically, when the wavelength of the above-mentioned first coherent light beam is w and the wavelength of the above-mentioned second coherent light beam is 1/[(1/x)-(1/w)], a light beam having a wavelength of x is emitted from the above-mentioned single-crystal Li 2 B 4 O 7 . In accordance with one embodiment the first coherent light beam and the second coherent light beam which has a one-second or one-fourth wavelength of the first coherent light beam are irradiated to the incident face of the single-crystal Li 2 B 4 O 7 at a predetermined angle, and at least a coherent light beam having a one-third or one-fifth wavelength of that of the first coherent light beam is emitted. In accordance with the present invention, a method for converting a light wavelength comprises the steps of: irradiating a first and second coherent light beams each having a different wavelength to a light incident face of single-crystal lithium tetraborate (Li 2 B 4 O 7 ) at a predetermined angle, which is cut at a predetermined plane to the optical axis so as to satisfy a predetermined phase matching angle; and emitting a light beam having a wavelength of sum frequency generation of the wavelengths of two coherent light beams. When the wavelength of the first coherent light beam is w and the wavelength of the second coherent light beam is 1/[(1/x)-(1/w)], a light beam having a wavelength of x is emitted from the single-crystal Li 2 B 4 O 7 . Specifically, the first coherent light beam and the second coherent light beam which has a one-second or one-fourth wavelength of the first coherent light beam are irradiated to the incident face of the single-crystal Li 2 B 4 O 7 at a predetermined angle, and at least a coherent light beam having a one-third or one-fifth wavelength of that of the first coherent light beam is emitted. In accordance with another embodiment of the present invention, a method for converting a light wavelength comprises the steps of: irradiating a coherent light beam having a predetermined wavelength to a light incident face of single-crystal lithium tetraborate (Li 2 B 4 O 7 ) at a predetermined angle, which is cut at a predetermined plane to the optical axis so as to satisfy a predetermined phase matching angle and has & refractive index variation of 10 -5 /mm or less and an etch-pit density of 1×10 3 /cm 2 or less; and emitting a coherent light beam having a half wavelength of the incident light beam. In accordance with another embodiment of the present invention, a laser apparatus comprises: the wavelength converter set forth above; a first light source for emitting the first coherent light beam set forth above having a first wavelength; a second light source for emitting the second coherent light beam set forth above having a second wavelength; and an optical device for introducing the first and second coherent light beams to a wavelength converter; wherein the laser apparatus emits a light beam having a wavelength of sum frequency generation of the wavelengths of the first and second coherent light beams. Preferably, the second light source has two wavelength converters set forth in the fourth embodiment; and the first coherent light beam emitted from the first light source is irradiated to at least one wavelength converter and a light beam having a half or one-fourth wavelength of the incident light is emitted from the wavelength converter. In accordance with yet another embodiment of the present first wavelength converter set forth above; a second wavelength converter set forth above provided at a position parallel to the first wavelength converter; a first half mirror, provided at the incident side of the first wavelength converter, for irradiating the first coherent light beam to the first wavelength converter; a second half mirror for receiving a light beam emitted from the second wavelength converter to deflect it by a predatermined angle; a third mirror for deflecting the light beam emitted from the second half mirror to the incident face of the second wavelength converter; and a fourth mirror for deflecting the light beam emitted from the second wavelength converter to the first half mirror to irradiate the light beam to the first wavelength converter; wherein the first coherent light beam having a predetermined wavelength is irradiated to the first half mirror and a coherent light beam having a one-fifth wavelength of the first coherent light beam is emitted from the second half mirror. In accordance with still another embodiment of the present invention, an optical parametric oscillator comprises: a light source for emitting a coherent light beam having a pumping frequency of ω p ; and an optical parametric oscillating section having the wavelength converter set forth above and first and second plano-concave half mirrors for receiving the coherent light beam from the light source to achieve optical parametric oscillation in front of and behind the wavelength converter. In accordance with a still further embodiment of the present invention, a laser apparatus for lithography comprises: a light incident means for receiving a first coherent light beam having a first wavelength and a second coherent light beam having a one-second or one-fourth wavelength of the first coherent light beam; a wavelength converter set forth above; a first plano-concave half mirror provided at the light incident side of the wavelength converter; a second plano-concave half mirror provided at the light emitting side of the wavelength converter; and an optical filter provided behind the second half mirror. Preferably, the first coherent light beam irradiating to the light incident means is a laser beam from a light source selected from Nd:YAG laser, Nd:YVO 4 laser, Cr:LiSAF laser, ruby laser, glass laser, alexandrite laser, garnet laser, sapphire laser, and semiconductor laser. More preferably, the laser apparatus for lithography further comprises an integrator for smoothing the intensity of the light beam, provided behind the optical filter. In accordance with a still further embodiment of the present invention, a short wavelength solid-state laser apparatus for processing comprises: a first light source for emitting a first coherent light beam having a first wavelength; a second light source for emitting a second coherent light beam having a wavelength of one-second or one-fourth of the- first coherent light beam from the first light source; and the wavelength converter set forth above for receiving the first coherent light beam from the first light source and the second coherent light beam from the second light source; wherein the first light source emits the first coherent light beam in a predetermined energy for processing for a predetermined time; and the wavelength converter emits a light beam having a one-fifth wavelength of the first coherent light beam. In accordance with another embodiment of the present invention, the single-crystal lithium tetraborate (single crystal Li 2 B 4 O 7 ) is cut at a predetermined plane to the optical axis, and has a refractive index variation of 10 -5 /mm or less and/or an etch-pit density of 1×10 3 /cm 2 or less. In accordance with another further embodiment of the present invention, an optical-resonance-type laser apparatus comprises: a first wavelength converter set forth above; a second wavelength converter set forth alcore, provided at a position parallel to the first wavelength converter; a first half mirror provided at the incident side of the first wavelength converter so as to irradiate the first coherent light to the first wavelength converter; a second half mirror for receiving the light emitted from the first wavelength converter and for deflecting the light by a predetermined angle; a third mirror for deflecting the light from the second half mirror toward the incident face of the second wavelength converter; and a fourth mirror for deflecting the light emitted from the second wavelength converter toward the first half mirror to irradiate the light to the first wavelength converter; wherein the first coherent light beam having a predetermined wavelength is irradiated to the first half mirror to emit a coherent light beam having a one-fifth wavelength of the first coherent light from the second half mirror. In accordance with another embodiment of the present invention, an optical parametric oscillator comprises: a light source for emitting a coherent light beam having a pumping frequency of ω p ; and an optical parametric oscillating section having a wavelength converter set forth above and first and second plano-concave half mirrors, provided in front of and behind the wavelength converter, for receiving the coherent light beam from the light source to achieve optical parametric oscillation. In accordance with yet another embodiment of the present invention, a laser apparatus for lithography comprises: a light incident means for receiving a first coherent light beam having a first wavelength and a second coherent light beam having a one-second or one-fourth wavelength of the first coherent light beam; a wavelength converter set forth above; a first plano-concave half mirror provided at the light incident side of the wavelength converter; a second plano-concave half mirror provided at the light emitting side of the wavelength converter; and an optical filter provided behind the second half mirror. In accordance with a further embodiment of the present invention, a short wavelength solid-state laser apparatus for processing comprises: a first light source for emitting a first coherent light beam having a first wavelength; a second light source for emitting a second coherent light beam having a one-second or one-fourth wavelength of the first coherent light beam from the first light source; a wavelength converter set forth above for receiving the first coherent light beam from the first light source and the second coherent light beam from the second light source; wherein the first light source emits the first coherent light beam in a predetermined energy requiring for processing for a predetermined time; the wavelength converter emits a light beam having a one-fifth wavelength of the first coherent light beam. In accordance with another embodiment of the present invention, said single-crystal lithium tetraborate (Li 2 B 4 O 7 ) contains an element for activating laser oscillation. Preferably, the element for activating laser oscillation is a rare earth element, and the content ranges from 0.01 to 10 weight percent. BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects, features and advantages of the present invention will be apparent from the following detailed description of the preferred embodiments of the invention in conjunction with the accompanying drawings, in which: FIG. 1 is a cross-sectional view illustrating a pull-up equipment for making a single-crystal lithium tetraborate (Li 2 B 4 O 7 : LBO) in accordance with the present invention; FIG. 2 is a schematic illustration of a definition of a phase matching angle in the single-crystal LBO; FIG. 3 is a schematic diagram illustrating a correlation between the cut face of the single-crystal LBO in accordance with an embodiment of the present invention and the phase matching angle; FIG. 4 is a graph illustrating a correlation between the incident angle of the laser and the fundamental wavelength in the phase matching using a single-crystal LBO in an embodiment in accordance with the present invention; FIG. 5 is a schematic diagram illustrating a correlation between the phase matching angle and the light incident angle when a single-crystal LBO in accordance with an embodiment of the present invention is used as a wavelength converter device for the second harmonic; FIG. 6 a schematic diagram illustrating a laser apparatus using a single-crystal LBO as a wavelength converter device for the fifth harmonic in Example 3 in accordance with the present invention; FIG. 7 is a schematic diagram illustrating a laser apparatus using a single-crystal LBO as a wavelength converter device for the fifth harmonic in Example 4 in accordance with the present invention; FIG. 8 is a single-pulse-type laser apparatus using a single-crystal LBO as a wavelength converter device in Example 5 in accordance with the present invention ; FIG. 9 is a single-pulse-type laser apparatus using a single-crystal LBO as a wavelength converter device in Example 6 in accordance with the present invention ; FIG. 10 is an optical parametric oscillator using a single-crystal LBO as a wavelength converter device in Example 7 in accordance with the present invention ; FIG. 11 is a laser apparatus for lithography using a single-crystal LBO as a wavelength converter device in Example 8 in accordance with the present invention ; FIG. 12 is a short wavelength solid-state laser apparatus for processing using a single-crystal LBO as a wavelength converter device in Example 9 in accordance with the present invention ; FIG. 13 is a laser-diode-excitation-type laser apparatus comprising a single-crystal LBO as a non-linear optical material in Example 10 in accordance with the present invention ; FIG. 14 is a flash-lamp-excitation-type laser apparatus comprising a single-crystal LBO as a non-linear optical material in Example 11 in accordance with the present invention ; FIG. 15 is a laser-diode-excitation-type laser apparatus comprising a single-crystal LBO as a non-linear optical material in Example 12 in accordance with the present invention ; FIG. 16A is a block diagram illustrating a laser apparatus using a single-crystal LBO as a non-linear optical material in Example 13 in accordance with the present invention ; and FIG. 16B is an outlined view of the single-crystal LBO used in the apparatus in FIG. 16A. DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, examples are described in which as Li 2 B 4 O 7 crystal is irradiated with one or two beams and a single output beam is generated. It should be noted that only these example in which said crystal is irradiated with 2 beams constitute embodiment of the invention. The present inventors have investigated single-crystal Lithium tetraborate (abbreviated as single-crystal Li 2 B 4 O 7 or single-crystal LBO), and have found that single-crystal LBO has a wide transparent wavelength range to an incident laser, high resistivity to laser damage, easy production of high quality and large size crystal, excellent processability, low deliquescent property, and easy handling characteristics, and that it is useful as an optical wavelength converter device. Preferred embodiments of single-crystal LBO in accordance with the present invention and optical converter devices using the single-crystal LBO will now been illustrated in detail. "Single-crystal LBO in accordance with the present invention (or single-crystal LBO in accordance with Example(s))" refers to single-crystal LBO produced by the method set forth below, and "general single-crystal LBO" refers to conventional single-crystal LBO used for SAW device and the like. Mere "single-crystal LBO" refers to the both cases. Basic Items on single-crystal LBO Basic items on single-crystal LBO as an optical converter device in accordance with the present invention will be explained. Method of Production Preferred method for producing (or growing) single-crystal LBO in accordance with the present invention is as follows: Single-crystal LBO in accordance with the present invention is produced from polycrystalline LBO using pull-up equipment 110 of single-crystal LBO based on the Czochralski method as shown in FIG. 1. The pull-up equipment 110 includes a platinum crucible 101 containing melted LBO. Since Li 2 B 4 O 7 has a lower melting point among oxides, the single-crystal can be grown up in the platinum crucible. The platinum crucible 101 is surrounded by a heater 104, e.g. a resistance heating furnace, to dissolve polycrystalline LBO through heat insulation materials 102, 103. Heat insulator walls 105, 106 are provided double above the platinum crucible 101, and a pull shaft 107 for mounting a seed crystal is provided so as to pass through the heat insulator walls 105, 106. A single-crystal LBO rod is prepared from polycrystalline LBO using the pull-up equipment 110 so that the single-crystal LBO exhibits desired characteristics as an optical converter device. The single-crystal LBO rod is cut at a predetermined angle to the optical axis depending to the use. An embodiment of the growing conditions of single-crystal LBO will be explained. A predetermined amount of polycrystalline LBO is filled in the platinum crucible 101, it is melted by the heater 104, and then single-crystal LBO is pulled up in the (110) pull-up direction. The pull-up conditions are as follows; the temperature gradient between the melt surface and a position of just 10 mm above the melt is 30 to 200 °c/cm and preferably 50 to 150 °c/cm. and the temperature gradient above the position is 5 to 50 °C/cm and preferably 10 to 40 °C/cm and the pull-up speed of the single-crystal LBO body is 0.1 to 2 mm/hour and preferably 0.3 to 1 mm/hour. The ground that the temperature gradient between the melt surface and a position of just 10 mm above the melt is set to be 30 to 200 °C/cm. will be explained. When the temperature gradient is less than 30 °C/cm, polycrystalline LBO is readily produced. On the other hand, when the temperature gradient exceeds 200 °C/cm, any transition readily forms in the pulled single-crystal LBO due to thermal strain. The ground that the temperature gradient at a point more than 10 mm above the melt is set to be 5 to 50 °C/cm will be explained. A smaller temperature gradient can reduce cracks due to the difference of the thermal expansions at the final growing stage. The ground that the pull-up speed of the single-crystal LBO body is set to be 0.1 to 2 mm/hour will be explained. When the pull-up speed is higher than the upper limit, white turbidity (nebula), or so called "nest" readily forms in the crystal, like general LBO crystal used for surface acoustic wave devices. The production method and properties of the general LBO will be explained below. A single-crystal block having a diameter of more than 1 inch can be prepared by the growing method of single-crystal LEO in accordance with the present invention. In a series of experiments, a single-crystal block having a diameter of 4 inches was able to be prepared. Etch-pit Density It is found that general single-crystal LBO also exhibits excellent properties as an optical converter device compared with KTP or BBO, as shown in experimental results below. However, such general single-crystal LBO has some defects, such as transition and impurity inclusion, which cause the decrease in the wavelength converting efficiency of the wavelength converter device due to the absorption and scattering of the laser light, i.e., the decrease in the quality of the emitted laser beam. When the general single-crystal LBO is used for a substrate material of SAW devices, such defects causes no problem. The single-crystal LBO in accordance with the present invention produced by the above-mentioned conditions has a low etch-pit density. As a result of the investigation, it is found that when the single-crystal LBO in accordance with the present invention having a low etch-pit density is used as an optical converter device, it less absorbs or scatters laser light compared. with the general LBO and thus it exhibits excellent optical characteristics. When the single-crystal LBO in accordance with the present invention having a low etch-pit density is used for a wavelength converter device, the absorption and scattering of laser light can decrease and thus the converting efficiency or the quality of emitted light beam is enhanced. When the single-crystal LBO in accordance with the present invention is grown up from polycrystalline LBO by the method and conditions set forth above, the etch-pit density can be suppressed to a low value, i.e., 1×10 3 /cm 2 or less. Further, the refractive index variation is 10 -5 /mm or less. The etch-pit density can be evaluated by the etched pit observation after etching or X-ray topography, for example. The etch-pit density of the single-crystal LBO in accordance with the present invention is preferably 1×10 3 /cm 2 or less when using for an optical wavelength converter. General single-crystal LBO used for a substrate material of a SAW device has an etch-pit density higher than 1×10 3 /cm 2 , and often has white turbidity or nests, although such single-crystal LBO exhibits excellent optical converter device characteristics compared with BBO or KTP. Optical Characteristics It is found that wavelength converting can be achieved when a laser light beam having a predetermined wavelength is irradiated to single-crystal LBO at a specified angle to the optical axis (C axis) of the single-crystal, as shown in FIG. 2. As the standard angle, the phase matching angle  m is used. When a fundamental wave is irradiated at an angle  m to the optical axis, its second harmonic wave is generated with phase matching in the same direction of the fundamental wave. This angle  m is called the phase matching angle. Single-crystal LBO is a negative uniaxial crystal having Type I phase matching condition. Therefore, only the phase matching angle  m among phase matching conditions should be taken into account. Features 1. Since single-crystal LBO has a homogenous composition, the upper and lower portions of the crystal have the same composition. 2. Single-crystal LBO exhibits low deliquescence and is stable to handle compared with other conventional crystals used for optical converter devices. When a deliquescent crystal is used, the optical converter device must be contained in a closed vessel with temperature control, resulting in the complicated large apparatus. In contrast, single-crystal LBO is free from such problems. 3. Single-crystal LBO has excellent processability in polishing, cutting and the like because of the crystal hardness similar to that of quartz. 4. Single-crystal LBO can be produced at lower production costs compared with other crystals used for wavelength converter devices. 5. Single-crystal LBO does not require single polarization treatment Further, incident light can be converted with a high efficiency by using twinning crystal because of the appearance of a quasi-phase matching (QPM) phenomenon. In this case, a wavelength converting method in which the C + axis is rotated by 180 degrees is called wavelength converting by QPM. The QPM has an advantage that the phase matching wavelength can be freely determined by the periodic length. Further, since single-crystal LBO can form a plurality of periods by means of a twinning crystal, phase matching wavelength region can be expanded. Moreover, single-crystal LBO has a temperature range allowable to phase matching twice or more times wider than those in conventional crystals. single-crystal LBO can be used in a bulk or light waveguide state. Additionally, single-crystal LBO can use the nonlinear optical constant d(33). Because the QPM is achieved by rotating the C + axis by 180 degrees with a twinning crystal, it has been thought that it is applicable to only ferroelectric materials. The present inventors, however, have found that it is applicable to the single-crystal LBO in accordance with the present invention which is a paraelectric material. Features set forth above are significantly important to utilize the single-crystal LBO as an optical converter device. Uses of Single-Crystal LBO 1. Wavelength converter device The present inventors have found as the result of the investigation on the optical characteristics of the single-crystal LBO in accordance with the present invention that it can perform wavelength conversion equivalent to or more KTP or BBO. For example, by a wavelength converter device using single-crystal LBO, the Nd:YAG laser (wavelength: 1064 nm) can generates highly coherent fourth harmonic (266 nm) and fifth harmonic (213 nm) light. If fourth or fifth harmonic light can be generated, laser light of ultraviolet and its neighboring regions can be readily generated from an infrared laser by an existing high-output apparatus. It enables applications in various fields, such as marking, lithography, semiconductor processing, and medical applications, as well as finer processing. The present inventors have found that the single-crystal LBO is practical to the use for wavelength converter devices because the transparent wavelength region to the incident light is wider than those in other crystals. Thus, the wavelength converter device in accordance with the pressent invention is applicable to a wide wavelength region. Although a wavelength converter device using the single-crystal LBO has a small nonlinear optical constant, the conversion efficiency can be improved by a large size crystal which can be readily prepared. (At the present stage, a single-crystal LBO having a diameter of 4 inches can be prepared.) The laser output is proportional to the square of the incident light intensity and the square of the crystal length. A wavelength converter device using the single-crystal LBO has a significantly large threshold to a laser damage compared with other crystals, resulting in the applicability as the wavelength converter device. A small threshold to a laser damage causes crystal damages by a smaller laser energy and the decrease in durability. Since the wavelength converter device in accordance with the present invention has a large threshold, it enables the higher laser output as well as excellent durability. 2. Various laser apparatuses In accordance with the wavelength converting method of the present invention, the wavelength of a laser light is converted by passing through the single-crystal LBO. Further, it has been found that when two or more kinds of laser light beams are irradiated to the wavelength converter device using the single-crystal LBO at the same time, a laser light of sum frequency generation or difference frequency mixing of wavelengths of input laser light beams can be obtained. 3. Optical parametric oscillator A wavelength converter device using the single-crystal LBO in accordance with the present invention can also perform optical parametric oscillation. EXAMPLES Preferred EXAMPLES of the single-crystal LBO in accordance with the present invention will be explained with reference to optical apparatuses below. EXAMPLE 1 A single-crystal Li 2 B 4 O 7 (single-crystal LBO) used for a second harmonic device (wavelength converter device) will be explained. Single-crystal LBO Production (Growing) in Accordance with the Present Invention Using a pull-up apparatus shown in FIG. 1, polycrystalline LBO having a stoichiometric composition and a purity of 99.99 weight percent was filled into a 90-mm diameter, 100-mm high platinum crucible 101, and single-crystal LBO was grown by the Czochralski method. As explained above, FIG. 2 is a graph illustrating the definition of the phase matching angle of single-crystal LBO. Processing of the single-crystal LBO The single-crystal LBO was cut so that the angle of the cut face 32 is 45 degrees to the optical axis (C axis), and the cut face 32 was optically polished to prepare a 15-mm by 15-mm by 10-mm optical converter device 30A of single-crystal LBO. When a coherent light beam i is irradiated to the optical converter device 30A, the optical converter device emits light o having a half wavelength of the incident light, that is, the optical converter device 30A acts as an optical converter device (wavelength converter device) for the second harmonic generation (SHG). For example, when a light having a wavelength of 532 nm (0.532 µm) is irradiated to the wavelength converter device 30A for SHG, a light having a wavelength of 266 nm (0.266 µm) is emitted. Refractive Index Variation The refractive index variation of the single-crystal LBO in this example measured with a Mach-Zender interferometer was 10 -6 /mm. Experiment 1 A laser light having a wavelength range of 486 to 1265 nm from an optical parametric oscillator set forth below was irradiated to the wavelength converter device 30A, and a second harmonic coherent light having a wavelength range of 243 to 633 nm was emitted. No strain of the interference fringes was observed at the passing wavefront. Phase Matching Angle FIG. 4 demonstrates the phase-matched incident angle of the laser light vs. the wavelength of the laser light emitted from the optical parametric oscillator, wherein the incident angle of the laser light in the direction perpendicular to the 45°-cut face 32 in FIG. 3 is 0 degree, the single-crystal LBO is rotated as plus toward the C axis and minus toward the (110) direction. The phase matching angle  m corresponding to each wavelength can be determined by the incident angle. FIG. 4 demonstrates that when a coherent light having a predetermined wavelength is irradiated to the single-crystal LBO of this example so as to satisfy the phase matching condition, a light having a wavelength of the half of that is emitted. In the following EXAMPLES AND COMPARATIVE EXAMPLES, a coherent light having a wavelength of 1064 from YAG laser is mostly used as a typical light beam. However, FIG. 4 demonstrates that the single-crystal LBO in accordance with the present invention can emit the light having a half wavelength of the incident light within the incidental light range shown in the figure. The single-crystal LBO in accordance with the present invention can convert a coherent light having an any wavelength into a light having a half wavelength of the coherent light. In apparatuses using the single-crystal LBO, such as a wavelength converter and laser apparatus, set forth below, the use of the single-crystal LBO prepared by the preferred conditions will be exemplified. Various single-crystal LBOs in accordance with the present invention other than the single-crystal LBO set forth above can also be used. When laser light is irradiated to the crystal from a predetermined angle, the laser light passes through the crystal while refracting as a function of the refractive index according to the Snell's law. The phase matching angle m is equal to the angle between the refracted laser light after the second harmonic is generated and the C axis, when the single-crystal LBO is used as a wavelength converter device for SHG. The graph in FIG. 4 evidently demonstrates that the incident angle is approximately 19 degrees when a laser light having a wavelength 1,064 nm is irradiated to the face 32 of the single-crystal LBO. In other words, when the fundamental wave (1,064 nm) of YAG laser is irradiated, the phase matching is achieved at an incident angle of approximately plus 19 degrees. Since the refractive index of the single-crystal LBO is approximately 1.6, the refractive angle is approximately 12 degrees. Thus, the angle with the C axis, i.e., phase matching angle, is approximately 33 degrees. A phase matching angle at an appropriate wavelength can be determined in a similar manner. The correlation between the wavelength and the phase matching angle is summarized in Table 1. Wavelength vs. Phase Matching Angle Incident Wavelength(nm) Emitted Wavelength(nm) Refractive Index Phase Matching Angle 1,064.0 532.0 1.6009 33.22 709.3 354.7 1.6054 44.41 643.8 321.9 1.6083 50.09 578.1 289.1 1.6107 58.18 532.0 266.0 1.6132 67.17 508.6 254.3 1.6170 74.43 Table 1 shows the refractive index of the single-crystal LBO and the phase matching angle when a light having a given wavelength is converted into a light having the half wavelength. As the incident light, a coherent light from the optical parametric apparatus was used. Further, the light of 1,064.0 nm is that from a YAG laser, and the light of 532.0 nm is obtained by converting the light from the YAG laser to the half wavelength with KTP as a SHG. Phase matching methods include phase matching by anglemethod using birefringence which is applied in the present invention and phase matching by temperature control using the dependency of the refractive index on the temperature. In the present invention, the latter matching also is applicable. Discussion of Table 1 and FIG. 1 The graph in FIG. 4 demonstrates that when a light is irradiated to the single-crystal LBO in accordance with this example at an incident angle shown in FIG. 4, a light having a half wavelength of the incident light can be emitted, as well as the converting of the incident light of 1064 nm into the half wavelength light of 532 mm as set forth above. Converting of the light having a wavelength other than 1,064 nm has not been achieved. Table 1 illustrates actual examples of the graph shown in FIG. 4. In Table 1, the phase matching angle is used instead of the incidental angle in FIG. 4. The Sellmeier's equation can be obtained by measuring the refractive indices at various wavelengths and adjusting the operation process. Alternately, the refractive indices at various wavelengths can be calculated by solving the Sellmeier's equation, and thus the phase matching angle can be determined. FIG. 5 is a geometrical view illustrating the correlation the phase matching angle  m and the incident angle when the single-crystal LBO set forth above is used as a wavelength converter device and an incident wave of 532 nm is converted into the half wave of 266 nm. Wavelength Converting Function KTP or BBO other than single-crystal LBO also can act as SHG. The single-crystal LBO can emit the light of the one-third or one-fifth of the incident wavelength 1,064 nm from a YAG laser by adding two coherent light beams having different wavelengths (hereinafter merely referred to as "sum frequency generation"), as set forth below. In contrast, KTP does not have such a wavelength converting function. Although BBO can emit the light of the one-third or one-fifth of the incident wavelength 1,064 nm from a YAG laser by adding or subtracting two coherent light beams having different wavelengths like single-crystal LBO, a large size of crystal cannot be used because of a large walk-off angle, not resulting in the improvement in converting efficiency. When the single-crystal LBO having a low etch-pit density in accordance with the present invention is used, a light of the one-third or one-fifth wavelength of the incident coherent light can be emitted with a high efficiency. Typically, the single-crystal LBO can stably emit a light of the one-third or one-fifth wavelength of the incident light (1,064 nm) from the YAG laser. Resistivity to Laser Damage The resistivity of the wavelength converter device comprising the single-crystal LBO of this example to laser damage was determined by the following method and the results were summarized in Table 2. Table 2 demonstrates that the resistivity of the single-crystal LBO to laser damage is five times or more larger than that in BBO (compare the resistivity to laser damage of Example 1 (a few dozen GW/cm 2 ) with Comparative Example 2 (a few GW/cm 2 )). The resistivity to laser damage was determined by the following steps; the output of the laser irradiated to the crystal was gradually increased; the output of the laser was measured when the damage of the crystal can be visualized; the laser beam radius was measured at a predetermined position of the crystal; and the output per unit area was calculated. Transparency The transparent wavelength region of the wavelength converter device 30A using the single-crystal LBO of this example to the incident laser light ranges from 170 to 3500 nm as shown in Table 2. Thus the wavelength converter device 30A has a wide transparent region. Handling and Processability The wavelength converter device 30A comprising the single-crystal LBO of this example exhibits extremely slight deliquescence and excellent polishing characteristics, resulting in excellent handling characteristics, as shown in Table 2. Size of Single-Crystal It was confirmed that a large single-crystal LBO having a diameter of 4 inches or more can be formed. Durability The wavelength converter device 30A comprising the single-crystal LBO of this example is less affected by ultraviolet light and thus has a longer life compared with BBO. REFERENCE 1 The single-crystal LBO of EXAMPLE 1 was compared with a conventional single-crystal LBO. A conventional single-crystal LBO was grown up using the pull-up equipment 110 shown in FIG. 1, wherein the grow-up conditions were changed. In REFERENCE 1, the temperature gradient between the melt surface and a position of just 10 mm above the melt was 250 °C/cm, the temperature gradient above the position was 70 °C/cm, and the pull-up speed of the single-crystal body was 2.5 mm/hour. That is, the temperature gradients and the pull-up speed were increased in REFERENCE 1, compared with those in EXAMPLE 1. Other conditions were the same as EXAMPLE 1. The resulting single-crystal LBO was cut as shown in FIG. 3 like EXAMPLE 1, and the incident and emitting faces of the single-crystal LBO were optically polished. The resulting crystal has mostly the same size as that in EXAMPLE 1. When a light of 532 nm from the second harmonic of a YAG laser was irradiated to the conventional single-crystal LBO, a light having a half wavelength of the incident light was irradiated like the single-crystal LBO in accordance with the present invention. Thus, it is confirmed that the conventional LBO also has optical converting characteristics like the single-crystal LBO in accordance with the present invention. However, in the conventional single-crystal LBO, white turbidity can be observed and thus the intensity of the emitted light is lower than that in the single-crystal LBO in accordance with the present invention. The refractive index variation of the conventional single-crystal LBO measure by the Mach-Zender interferometer was 10 -4 /mm, and was higher than 10 -6 /mm in EXAMPLE 1. As set forth above, in REFERENCE 1, when a coherent light of wavelength of 1,000 nm or less is irradiated at a predetermined angle to the light incident face of the conventional single-crystal lithium tetraborate (Li 2 B 4 O 7 ) which is cut at a predetermined plane to the optical axis so as to satisfy a predetermined phase matching angle, light having a wavelength of approximately the one-second of that of the incident light can be emitted. EXAMPLE 2 A single-crystal LBO is used as a wavelength converter device for the second harmonic in EXAMPLE 2. Production Method of the Single-Crystal LBO The single-crystal LBO was prepared with the pull-up equipment 110 shown in FIG. 1. A 90-mm diameter, 100-mm high platinum crucible 101 was filled with 1300 g of polycrystalline LBO having a stoichiometric composition and a purity of 99.99%. After the LBO was melted with a heater, a single-crystal LBO having a diameter of 2 inches was pulled up with the pull-up direction (110). The pull-up conditions in EXAMPLE 2 were as follows; the temperature gradient between the melt surface and a position just 10 mm above the melt was 80 °C/cm, the temperature gradient above the position was 30 °C/cm, and the pull-up speed of the single-crystal LBO body was 0.5 mm/hour. The etch-pit density of the grown single-crystal LBO determined by an etching method was sufficiently small, i.e., 5×10 /cm 2 . Processing The resulting single-crystal LBO was cut so as to satisfy the phase matching angle  m of approximately 67° in order to evaluate the second harmonic characteristics, and the incident and emitting faces were optically polished. The size of the crystal was 10 nm in diameter and 25 mm in length. Laser light having an output power of 100 mJ and a wavelength of 532 mm was irradiated to this single-crystal, and the transparent light was evaluated. A emitted light having a wavelength of 266 nm was observed, illustrating that the single-crystal LBO obtained in EXAMPLE 2 is a material exhibiting excellent wavelength converting characteristics. The converting efficiency calculated from the laser incident output power and the emitted output power was approximately 15%. REFERENCE 2 A single-crystal LBO was prepared according to a conventional process of single-crystal LBO used for SAW devices. The conventional single-crystal LBO was grown using the pull-up equipment shown in FIG. 1, but the grow-up conditions were changed as follows; the temperature gradient between the melt surface and a position of just 10 mm above the melt was 250 °C/cm, the temperature gradient above the position was 70 °C/cm and the pull-up speed of the single-crystal body was 2.5 mm/hour. The temperature gradients and the pull-up speed in REFERENCE 2 were increased compared with EXAMPLE 2. Other conditions were the same as EXAMPLE 2. The etch-pit density of the resulting single-crystal LBO determined by the etching method was 2×10 4 /cm 2 and turbidity was observed in the single-crystal LBO. Thus, the quality of this conventional single-crystal LBO prepared by the conditions of REFERENCE 2 is low as the optical converter device. This single-crystal LBO was cut according to EXAMPLE 2 and the incident and emitting faces were optically polished in order to evaluate second harmonic characteristics. The crystal has the same size as that in EXAMPLE 2. A laser light having an output power of 100 mJ and a wavelength of 532 nm was irradiated to this single-crystal and the transparent light was observed to evaluate. Although light of 266 nm wavelength was observed, the converting efficiency was lower, i.e., approximately 9%, compared with that in EXAMPLE 2. In other words, when the single-crystal LBO of EXAMPLE 2 is used as an optical converter device, it exhibits a high converting efficiency. EXAMPLE 3 A single-crystal LBO was used for a wavelength converter device for fifth harmonic in EXAMPLE 3. Single-Crystal LBO Production Using the pull-up equipment 110 shown in Fig. 1, 1,300 g of polycrystalline LBO having a stoichiometric composition and a purity of 99.99 weight percent was placed into a 90-mm diameter, 100-mm high platinum crucible, and a single-crystal LBO was grown up by the Czochralski method. The pull-up conditions were the same as those in EXAMPLE 1 or 2. Processing of Single-Crystal LBO The grow-up single-crystal LBO was cut into (110) plane and (001) plane, and the incident face or the (110) plane was optically polished to prepare a wavelength converter device 30B comprising 15-mm by 15-mm by 15-mm single-crystal LBO. Experiment 2 Laser light (output power 1 J) of fundamental frequency ω (incident wavelength λ 2 = 1,064 nm) and auxiliary laser Light (output power 250 mJ) of the fourth harmonic wave light 4 co of the fundamental frequency (λ 1 = 1/4·λ 2 = 266 nm) were irradiated to 5 co of the incident the wavelength converter device 30B at the same time. Light (output power 150 mJ) of the fifth harmonic wave light frequency (wavelength = 1/5·λ2 = 213 nm) was generated by sum frequency generation of two waves. The phase matching angle determined by the method according to Experiment 1 was approximately 79 degrees, the shortest wavelength by sum frequency generation was 209 nm, and the phase matching angle at the shortest wavelength was approximately 90 degrees. The correlation between various incident angles and their respective phase matching angles will be shown in Table 3. Wavelength vs. Phase Matching Angle Incident λ 1 (nm) Wavelength λ 2 (nm) Emitted Wavelength (nm) Phase Matching Angle (Degree) 266 1,064 213 79 266 981 209 90 Table 3 demonstrates that when obtaining an emitted light by sum frequency generation, for example obtaining an emitted light of wavelength x = 209 nm, the wavelength of the first coherent light is an appropriate w (nm), for example, 981 nm, and that of the second coherent light is 1/[(1/x)-[1/w)] (nm), for example 266 nm. Further, when a first coherent light of the first wavelength and a second coherent light of the one-second or one-fourth of the wavelength of the first coherent light were irradiated to the incident face of the single-crystal Li 2 B 4 O 7 , at least coherent light having a wavelength of the one-third or one-fifth of the first coherent light can be emitted. The phase matching angle in Table 3 calculated at the refractive index of 1.6 may be not accurate, because the refractive index of the crystal varies with temperature and the wavelength of the incident light. However, the accurate phase matching angle is probably within ±10 degrees. When the refractive index is accurately determined, the accurate phase matching angle can be readily determined by the method shown in Experiment 1. The phase matching was carried out by angle control utilizing birefringence. The phase matching by temperature control utilizing the dependency of the refractive index on temperature also is applicable. The refractive indices of single-crystal LBO were measured at various wavelengths and the operation process was matched to determine the Sellmeier equation. Alternately, the refractive indices at various wavelengths can be calculated by solving the Sellmeier equation to determine the phase matching angle. Reference 3 The conventional single-crystal LBO explained in References 1 and 2 was used for one-fifth wavelength converting. The phase matching angle was approximately 79 degrees, the shortest wavelength by sum frequency generation was 209 nm, and the output power at the shortest wavelength was 50 to 120 mJ. Turbidity was found in the conventional single-crystal LBO and the refractive index variation determined by a Mach-Zender interferometer was 10-4/mm. In contrast, when the one-fifth wavelength converting was carried out using the single-crystal LBO in EXAMPLE 3 having no turbidity and a retractive index variation of 10 -6 /mm the fifth harmonic wave was stably obtained. Accordingly, although the fifth harmonic light beam can be obtained by using the conventional single-crystal LBO, it can be more stably obtained with a high converting efficiency by using the single-crystal LBO of EXAMPLE 3. COMPARATIVE EXAMPLE 1 KTP instead of the single-crystal LBO in accordance with the present invention was used as a wavelength converter device 30B. The converting wavelength characteristics, resistivity to laser damage, transparent region, deliquescence, availability of a high quality, and large crystal were investigated like Experiment 2. Results are shown in Table 2. Table 2 illustrates that the wavelength converter device 30B using the single-crystal LBO in accordance with the present invention is excellent compared to that in KTP. COMPARATIVE EXAMPLE 2 BBO instead of the single-crystal LBO in accordance with the present invention was used as a wavelength converter device 30B. The converting wavelength characteristics, resistivity to laser damage, transparent region, deliquescence, availability of a high quality, and large crystal were investigated like Experiment 1. Results are shown in Table 2. Table 2 illustrates that the wavelength converter device 30B using the single-crystal LBO in accordance with the present invention is excellent compared to that in BBO. Since BBO was grown up by a flux method, it readily includes impurities and the yield is low, whereas the single-crystal LBO in accordance with the present invention has a high yield. Further, after irradiating ultraviolet light to BBO, the crystal center is colored because of crystal deterioration, whereas the wavelength converter device using the single-crystal LBO In accordance with the present invention does not undergo such deterioration. COMPARATIVE EXAMPLE 3 KDP instead of the single-crystal LBO in accordance with the present invention was used as a wavelength converter device 30B. The converting wavelength characteristics, resistivity to laser damage, transparent region, deliquescence, availability of a high quality, and large crystal were investigated like Experiment 1. Results are shown in Table 2. Table 2 illustrates that the wavelength converter device 30B using the single-crystal LBO in accordance with the present invention is excellent compared to that in KDP. As set forth above, when obtaining an emitted light by sum frequency generation, for example obtaining an emitted light of wavelength x = 209 nm, the wavelength of the first coherent light is an appropriate w (nm), for example, 981 nm, and that of the second coherent light is 1/[(1/x)-(1/w)] (nm), for example 266 nm. Further, when a first coherent light having a first wavelength and a second coherent light having the one-second or one-fourth wavelength of the first coherent light are irradiated to the incident face the single-crystal Li 2 B 4 O 7 at a predetermined angle, at least coherent light having one-third or one-fifth wavelength of the first coherent light can be emitted. single -crystal lithium tetraborate (Li 2 B 4 O 7 ) is cut at a predetermined plane to the optical axis, has a refractive index variation of 10 -5 /mm or less and/or an etch-pit density of 1×10 3 /cm 2 or less, wherein coherent light having a predetermined wavelength is irradiated to the light incident face at a predetermined angle so as to satisfy a predetermined phase matching angle, and light having the one-second wavelength of the incident light is emitted. EXAMPLE 4 Utilizing the one-fourth or one-fifth wavelength converting function of single-crystal LBO, a laser apparatus for emitting ultraviolet light from the incident infrared light by using the single-crystal LBO as a wavelength converter device was realized shown in FIG. 6. The laser apparatus 100 in FIG. 6 comprises a Nd:YAG laser 10 for emitting coherent light having a infrared wavelength of 1,064 nm, a secondary light source 20 for emitting coherent light having a wavelength of 266 nm, a wavelength converter device 30B comprising single-crystal LBO in accordance with the present invention, and a half mirror 40. The secondary light source 20 receives laser light having a wavelength of 1,064 nm from a Nd:YAG laser, for example, and emit laser light having the one-fourth wavelength (266 nm) of the fundamental light by passing through twice a wavelength converter device, for example, a β-BaB 2 O 4 (BBO) single-crystal wavelength converter device for converting the incident light into the one-second wavelength, and a wavelength converter device comprising single-crystal LBO for SHG in accordance with the present invention. Alternately, the secondary source 20 can convert the incident light having a wavelength of 1,064 nm into laser light having the one-fourth wavelength (266 nm) of the incident light by using a wavelength converter device comprising single-crystal LBO for SHG. At a first optical path, laser light having a wavelength of 1,064 nm emitted from the Nd:YAG laser 10 is irradiated to the wavelength converter device 30B through the half mirror 40. At a second optical path, laser light having a wavelength of 266 nm from the secondary light source 20 is reflected with the half mirror 40 and is irradiated to the wavelength converter device 30B with the laser light from the Nd:YAG laser 10. Laser light beams which are irradiated to the wavelength converter device 30B from the first and second optical paths are converted into short wavelength laser light of one-fifth wavelength (213 nm) of the laser light (1,064 nm) from the Nd:YAG laser 10 by sum frequency generation in the wavelength converter device 30. As set forth above, this laser apparatus can convert infrared light (1,064 nm) into ultraviolet light (213 nm) having the one-fifth wavelength of the infrared light by means of the combination of the Nd:YAG laser 10, the secondary light source 20, and the wavelength converter device 30B. The Nd:YAG laser 10 comprises a transparent crystal YAG matrix and Nd as additive The Nd:YAG laser 10 continuously emits a strong infrared light in the region of 1.05 to 1.12 µm and in particular 1.064 µm. Further, single-crystal LBO has a high transparency to a wide range of wavelength and is less damaged by laser light. Moreover, high quality, large size of crystal can be readily produced. Additionally, single-crystal LBO has excellent processability, is less deliquescent and thus can readily handle. single-crystal LBO has a long life. The laser apparatus 100 in accordance with the present invention is compact and stably works for a long time. Further, this laser apparatus can emit high energy, short wavelength, ultraviolet light (213 nm, for example) at room temperature. This laser apparatus can substitute for an excimer laser. Thus, the laser apparatus 100 may he applicable to various fields, such as printing, reproduction photography, optical measurement, and lithography in ultra LSI. EXAMPLE 5 Another laser apparatus using the single-crystal LBO in accordance with the present invention as a wavelength transducer device for the fifth harmonic will be explained. Single-Crystal Production A single-crystal LBO was grown up with the pull-up equipment shown in FIG. 1 by Czochralski method. When the single-crystal LBO was pulled up in the C axis direction many bubbles were included, whereas when it was pulled up in the (100) or (110) direction perpendicular to the C axis, small bubbles were included. Processing A (100), (001) face sample (approximately 30 mm by 30 mm by 40 mm) was prepared from the grown crystal. Operation Tests A laser apparatus shown in FIG. 7, similar to the apparatus in FIG. 6, was used for experiments. The fundamental wave ω from a Nd:YAG laser (not shown in the figure) was irradiated to a 2nd & 4th harmonics generating unit 42, and was irradiated to a wavelength transducer device 30C using the single-crystal LBO in accordance with the present invention to generate the fifth harmonic light beam from the wavelength converter device 30C. The conditions were as follows; the output power of the fundamental wave of the Nd:YAG laser was 400 mJ; the output power of the fourth harmonic wave was 110 mJ; these were irradiated at a phase matching angle of 79 degrees, 10 Hz and 10 nsec. to obtain the fifth harmonic wave of 70 mJ. Light from an optoparametric oscillator was irradiated to the single-crystal LBO while rotating the crystal in the (001) direction, in which the wavelength of the irradiated light was changed gradually and the transduced wavelength was measured. The minimum wavelength was 209 nm when the light is irradiated almost perpendicularly to the (100) plane sample. As set forth above, laser oscillation which is variable in a wide wavelength range was observed in this laser apparatus using the single-crystal LBO in accordance with the present invention as the wavelength transducer device 30C. EXAMPLE 6 A further laser apparatus will be explained using the single-crystal LBO in accordance with the present invention as a wavelength transducer device for the fifth harmonic. Single-Crystal Production A single-crystal LBO was grown up with the pull-up equipment shown in FIG. 1 by Czochralski method. When the sc was pulled up in the C axis direction many bubbles were included, whereas when it was pulled up in the (100) or (110) direction perpendicular to the C axis, small bubbles were included. Processing A grown crystal block was cut into a single-crystal LBO with a light incident face having the tilt of 79 degrees from the (001) plane. The size of the single-crystal was 20 mm by 20 mm by 45 mm. Operation Test The single-crystal sample was provided as a wavelength transducer device 30D between filters 60, 62 as shown in FIG. 8, a fundamental wave having a wavelength ω was emitted from a YAG laser 10a comprising a YAG rod 50, a Q switch 52, and mirrors 54, 55, the fundamental wave was passed through SHG crystals 56, 58, and the transduced light was irradiated to the light incident face of the wavelength transducer device 30D. SHG crystals 56, 58 are composed of single-crystal LBO or BBO, for example. The single-crystal sample 30D cut at 79 degrees was placed on a stage and was rotated in the C axis direction as shown in FIG. 2. The generation of the fifth harmonic wave (5ω) was observed at 79 ± 10 degrees. The maximum intensity of the emitted light was 120 mJ when using a YAG laser (1.5 J, 10 Hz). EXAMPLE 7 An optical-resonator-type laser apparatus using the single-crystal LBO in accordance with the present invention as a wavelength transducer device is explained with reference to FIG. 9. Single-Crystal Production A single-crystal LBO was grown up with the pull-up equipment shown in FIG. 1 by Czochralski method. When the sc was pulled up in the C axis direction many bubbles were included, whereas when it was pulled up in the (100) or (110) direction perpendicular to the C axis, small bubbles were included. Processing A grown crystal block was cut into a single-crystal LBO with a light incident face having the tilt of 79 degrees from the (001) plane. The size of the single-crystal was 20 mm by 20 mm by 45 mm. Operation Test The resulting single-crystal samples were provided between mirrors 64, 66 and between mirrors 68, 70 as wavelength transducer device 30E1, 30E2, as shown in FIG. 9. The fundamental wave ω of 1,064 nm emitted from the half mirror 64 was transduced into the second harmonic wave 2ω (532 nm) with the first wavelength transducer device 30E1, the second harmonic wave 2ω was reflected with mirrors 66, 70 and was irradiated to the second wavelength transducer device 30E2 having a cut face for transducing the 532-nm wave in order to transduce it into the forth harmonic wave 4ω, the fourth harmonic wave 4ω was reflected with mirror 68, 64 so that the wave is again irradiated to the wavelength transducer device 30E1 in order to transduce it into the fifth harmonic wave 5ω. The resulting fifth harmonic wave 5ω was emitted through the half mirror 66. The fundamental wave and the fourth harmonic wave were set to be always reflected with mirrors 64, 66, 68, 70. The wavelength transducer device 30E1 of single-crystal LBO with a 79-degree cut face was placed on a stage and rotated in the C axis. The generation of the fifth harmonic wave was observed at 79 ± 10 degrees. The maximum intensity of the emitted light was 200 to 300 mJ when using a YAG laser (1.5 J, 10 Hz) as an incident light source. EXAMPLE 8 An optoparametric oscillator using the single-crystal LBO in accordance with the present invention as a wavelength transducer device is explained with reference to FIG. 10. Single-Crystal Production and Processing A single-crystal LBO block produced with the pull-up equipment in FIG. 1 was cut so as to satisfy the phase matching angle for optoparametric oscillation, and the incident and emitting faces were optically polished to prepare a single-crystal LBO in accordance with the present invention having a size of 10 mm by 10 mm by 40 mm. Construction FIG. 10 is a block diagram illustrating a basic construction of an optoparametric oscillator using the single-crystal in accordance with the present invention. The optoparametric oscillator comprises an optical transducer device 30F provided with an input mirror 80 and an output mirror 82 at the both sides. Operation Test Light having a pumping frequency ω p from a laser oscillator 84 comprising a YAG laser was irradiated to the input mirror 80 through a collector 86. In the optoparametric oscillator, mixing occurs between three kinds of waves, i.e., a wave having the pumping frequency ω p , a signal wave having the fundamental wavelength ω s , and an idler wave having a incident frequency ω i = ω p - ω s , the latter two waves are amplified in the single-crystal LBO media. Although the optoparametric oscillation is a kind of light mixing phenomenon, it essentially differs from general light mixing in that only one kind of incident wave is used in the optoparametric oscillation, while two kinds of incident waves are used in the general color mixing. Further, in the optoparametric oscillation, a threshold exists because of the oscillation phenomenon. The fundamental frequency ω s and the incident frequency ω i were determined by external factors, such as a resonator and phase matching, regardless the inherent frequency of the material. When the wavelength transducer device 30F of the single-crystal LBO in accordance with the present invention is placed in the signal or idler wave of light wavelength region, or an optical resonator resonant with both waves, and when the intensity of the pumping wave reaches a given threshold, the signal and idler waves are generated at the same time by the gain of the optoparametric amplification. The threshold corresponds to a value at which the optoparametric gain equals to the losses of the signal and idler waves. Laser light having a continuously variable wavelength in a wide wavelength range can be obtained as follows; the incident angle to the wavelength transducer device 30F of single-crystal LBO is changed by adjusting the optoparametric oscillator; alternately, the temperature of the wavelength transducer device 30F is changed. Thus, this optoparametric oscillator using the single-crystal LBO as a wavelength transducer device has a practical significance. Light of fourth harmonic (4ω = 266 nm) of the YAG with a pulse width 10 nsec., which was emitted from the laser oscillator 84 comprising the YAG laser, was irradiated to the input mirror 80 through the collector 86. As a result, a mixing phenomenon occurred between three waves, i.e., the fourth harmonic waves (266 nm) of the YAG laser, the signal wave of 355 nm, and the idler wave of 1,061 nm, where the signal and idler waves are amplified in the optical transducer device 30F. No color center was observed in the optical transducer device 30F of the single-crystal LBO after 100-hours operation. The color center represents transparent spot defects inside the crystal which is detected by the generation of the absorbance band in the single-crystal. Signal and idler waves which have continuously variable wavelengths were obtained by changing the incident angle to the optical transducer device 30F or by changing the temperature of the single-crystal LBO of the optical transducer device 30F. The optoparametric oscillator using the single-crystal LBO in EXAMPLE 8 has a higher resistivity to lager damage compared with that using BBO. Thus this optoparametric oscillator can receive the high power YAG laser and can emit high power, continuously variable, short-wavelength laser. Further, since the single-crystal LBO has an excellent durability to ultraviolet light, the optical transducer device using the single-crystal LOB has a longer life. EXAMPLE 9 Another optoparametric oscillator using the single-crystal LBO in accordance with the present invention as a wavelength transducer device will explained. Optoparametric oscillation was carried out in the same manner as EXAMPLE 8 illustrated in FIG. 10, except that the fifth harmonic wave (5ω: 213 nm) of the YAG laser was used as light irradiated to the optoparametric oscillator. As a result, a mixing phenomenon occurred between three waves, i.e., the fifth harmonic waves (213 nm) of the YAG laser, the signal wave of 300 nm, and the idler wave of 734 nm, where the signal and idler waves are amplified in the single-crystal LBO medium. No color center was observed in the optical transducer device of the single-crystal LBO after 100-hours operation. Signal and idler waves which have continuously variable wavelengths were obtained by changing the incident angle to the single-crystal LBO (by means of the adjustment of the optoparametric oscillation) or by changing the temperature of the crystal. COMPARATIVE EXAMPLE 4 Optoparametric oscillation was carried out in the same manner as EXAMPLE 8, except that BBO single-crystal instead of the single-crystal LBO was used as the optical transducer device 30F. The color center as the result of the crystal deterioration was observed after the 100-hours operation. Although a single-crystal LBO having a small transition density was used in EXAMPLEs 8 and 9, a conventional single-crystal LBO used for SAW also does not have a color center. Of course, the single-crystal LBO in accordance with the present invention having a low refractive index variation and a low transition density is preferably used compared with the conventional single-crystal LBO. EXAMPLE 10 A laser apparatus for lithography will be explained with reference to FIG. 11, to which the wavelength transducer device using the single-crystal LBO in accordance with the present invention is applied. Single-Crystal Production A single-crystal LBO was grown up in the (110) pull-up direction with the pull-up equipment 110 shown in FIG. 1. The grow-up conditions were as follows; the temperature gradient between the melt surface and a position of just 10 mm above the melt was 80 °C/mm, the temperature gradient above the position was 30 °C/mm, the pull-up speed of the single-crystal LBO body was 0.5 mm/hour, and the rotation of the seed crystal was 2 rpm. A single-crystal LBO rod having a diameter of 2 inches and a length of 120 mm was obtained. Processing The resulting single-crystal LBO rod was cut into a block of 10-mm by 10-mm by 30-mm while tilting by 79 degrees from the C axis, and both the incident and emitting face were optically polished. Configuration of Apparatus A laser apparatus for lithography comprises a wavelength transducer device 221 of the formed single-crystal LBO block provided with mirrors 222A, 222B at its both sides, and a lens system 224 provided in front of the input side mirror 222A, and a filter 226 and integrator 228 provided at the rear of the output side mirror 222B. The filter 226 is used for separating light other than those having desirable wavelengths. The integrator 228 may include; for example, that in which small convex or concave lenses are uniformly formed on the one or both side(s) of the optical lens; that in which small convex or concave lenses are uniformly formed on the one or both side(s) of the optical glass; and that comprising the combination of lenses such as a dragonfly eye. The integrator 228 can be omitted in the laser apparatus in accordance with the present invention. Operation Tests When a light beam of a fourth harmonic wave 4ω (266 nm) of the light (1,064 nm) from a YAG laser (500 Hz, 100 mJ) and a light beam having a frequency ω from a YAG laser (400 mJ) were irradiated to te wavelength transducer device 221 (30G) at the same time through the lens system 224, an ultraviolet beam (30 mJ) of fifth harmonic wave 5ω of the light emitted from the YAG laser was emitted by mixing of the two light beams i.e., sum frequency mixing. After light having frequencies other than the desired wavelength was removed with the filter 226, the intensity distribution of the desired light was smoothed with the integrator 228. No color center formed in the wavelength transducer device of LBO crystal after this state was maintained for 100 hours or more. The color center represents transparent spot defects inside the crystal which is detected by the generation of the absorbance band in the single-crystal. Examples of the light sources for lithography, other than the optoparametric oscillator, include Nd:YAG, Nd:YVO 4 , Cr:LiSAF, ruby laser, glass laser, alexandrite laser, garnet laser, sapphire laser, and semiconductor laser. COMPARATIVE EXAMPLE 5 By using BBO block of 5 mm by 5 mm by 5 mm as a wavelength transducer device, a laser apparatus as shown in FIG. 11 was constructed and subjected to the durability tests like EXAMPLE 10. Ultraviolet light of a fifth harmonic wave 5ω of the light emitted from a YAG laser (25 mJ) was observed in the laser apparatus using BBO. However, the allowable ranges of the phase matching angle and temperature are small. Further BBO is heated because of the ultraviolet light absorption. Thus, it is extremely difficult to generate stably high power fourth harmonic waves (4ω) and fifth harmonic waves (5w) for a long time. The deterioration of the crystal as a color center was observed after 100 hours operation. Accordingly, it was confirmed that BBO is unsuitable for the laser apparatus for lithography requiring a long term stability. Further, BBO readily includes impurities and the yield is law because of flux growing, resulting in the increase in the production cost. EXAMPLE 11 A high power short wavelength solid laser apparatus for processing will be explained with reference to FIG. 12, as an example of the application of the wavelength transducer device using the single-crystal LBO in accordance with the present invention. The single-crystal LOB can generate a highly coherent fourth harmonic wave (266 nm) and fifth harmonic wave (213 nm) from the light (1,064 nm) emitted from a Nd:YAG laser, according to the present inventors' investigation. The fifth harmonic wave (213 nm) is shorter than the KrF excimer laser (248 nm). By generating the fourth or fifth harmonic wave from infrared laser emitted from an existing high power apparatus, laser light of ultraviolet and its neighboring regions can be readily obtained. Abrasion processing can be achieved by using the laser light. Two or more kinds of laser light beams also can be irradiated to the single-crystal LBO at the same time to generate a laser light beam of the sum or difference frequency of these light beams. Single-Crystal Production A single-crystal LBO having sizes of 2 inches or more in the (001), (110) and (100) directions were grown up with the pull-up equipment shown in FIG. 1. The grow-up conditions were as follows; the temperature gradient between the melt surface and a position of just 10 mm above the melt was 80 °C/mm, the temperature gradient above the position was 30 °C/mm, the pull-up speed of the single-crystal LBO body was 0.5 mm/hour, and the rotation of the seed crystal was 2 rpm. Processing The resulting single-crystal LBO was cut into a block of 20-mm by 20-mm by 50-mm while tilting by 79 degrees from the C axis, and both the incident and emitting face were optically polished. Apparatus Configuration As shown in FIG. 12, a wavelength transducer device 321 (30H) comprising the resulting block single-crystal LBO was provided in front of a YAG laser 322 (1.5 J, 1 KHz, 10 nsec.) and a 2nd & 4th harmonics generation unit 323. Operation Tests A 90-mJ light beam having a frequency 4ω (266 nm) and a 850-mJ light beam having a frequency ω (1,064 nm) are generated by passing a 1.5-J laser light of 1,064 nm from the YAG laser through the 2nd & 4th harmonics generation unit 323. When these light beams passed through the wavelength transducer device 321 comprising the single-crystal LBO, a 20-mJ ultraviolet light beam of 5ω (213 nm) was generated by the sum frequency mixing of the two light beams. Undesired light was eliminated with a filter such as a prism to irradiate only the desired light (5ω) to a 1-mm thick glass plate 324. The abrasion of the glass was observed by the bloom at the section of the glass which was irradiated with the fifth harmonic wave light. After two or three minutes, The fifth harmonic wave light formed a through-hole. The wavelength transducer device 321 worked without problem or failure during the operation. In EXAMPLE 11, non-reflective coating was not applied to the polished face of the single-crystal LBO. The output power of the fifth harmonic wave will further increase by non-reflective coating. Examples of the light sources for processing, other than the example set forth above, include Nd:YVO 4 , Cr:LiSAF, ruby laser, glass laser, alexandrite laser, garnet laser, sapphire laser, and semiconductor laser. COMPARATIVE EXAMPLE 6 By using BBO block of 5 mm by 5 mm by 5 mm as a wavelength transducer device, a laser apparatus as shown in FIG. 12 was constructed and subjected to the abrasion working like EXAMPLE 11. Ultraviolet light (15 mJ) of a fifth harmonic wave 5ω was observed in the laser apparatus using BBO. However, the allowable ranges of the phase matching angle and temperature are small. Further BBO is heated because of the ultraviolet light absorption. Thus, it is extremely difficult to generate stably high power fourth harmonic waves (4ω) and fifth harmonic waves (5w) for a long time. The deterioration of the crystal as a color center was observed after 100 hours operation. Accordingly, it was confirmed that BBO is unsuitable for the laser apparatus for processing requiring a long term stability. Further, BBO readily includes impurities and the yield is law because of flux growing, resulting in the increase in the production cost. The single-crystal LBO in accordance with the present invention can solve such problems. The single-crystal LBO is inferior to the BBO single-crystal in the non-linear optical constant. On the other hand, a large size single-crystal LBO can be grown up. Since the wavelength transducing efficiency is proportional to the square of the input power and the square of the crystal length, the single-crystal LBO, of which a larger single-crystal is available, has a higher wavelength transducing efficiency, as a result. The resistivity to laser damage of the single-crystal LBO is at least 10 times higher than that of BBO, thus a high power laser can be irradiated to the single-crystal LBO. Further, no color center occurs in the single-crystal LBO by the long term irradiation of ultraviolet rays. Accordingly, the laser apparatus using the wavelength transducer device comprising this single-crystal LBO can be preferably used as a short wavelength solid laser apparatus for processing. EXAMPLE 12 An optical transducer device comprising single-crystal LBO (Li 2 B 4 O 7 ) containing a laser-oscillation-activating device will be explained. Features An optical apparatus in EXAMPLE 12 uses a nonlinear optical device which acts as both a wavelength transducing material and a laser oscillating material, resulting in the miniaturization of the apparatus. The nonlinear optical device has a high wavelength transducing efficiency and a high resistivity to laser damage, and a large size single-crystal can be grown up. The present inventors have found that the single-crystal Li 2 B 4 O 7 as the wavelength transducer device preferably contains a laser-oscillation-activating device. Examples of the laser-oscillation-activating element include rare earth elements, such as Nd, Tm and Er. A desired material characteristic or crystal characteristic can be achieved by changing the kind and/or amount of the laser-oscillation-activating device. According to the experimental results, the amount to be added is preferably 0.01 to 10 weight percent. Since the laser-oscillation-activating device in the single-crystal LBO enhances the laser oscillation function of this single-crystal, a compact laser apparatus will be readily achieved. Further, laser oscillation and wavelength transducing proceeds in the entire crystal, and thus the single-crystal LBO acts as a resonator, resulting in the improvement in the wavelength transducing efficiency. Moreover, the phase matching can be readily adjusted. The non-linear optical material in accordance with the present invention can emit a laser light beam having a novel wavelength by mixing two light beams among an exciting light beam (wavelength λ 1 ), an oscillating light beam (wavelength λ 2 ) emitted by the excitation, and a second harmonic generation light beam (wavelength λ 3 ). The wavelengths of the exciting and oscillating light beams varies with the added dopant as an activator of laser oscillation, such as Nd, Tm, Er, Pr, or U. The wavelength of the emitted laser light can be appropriately determined. For example, a novel laser light beam can be generated by mixing the following three light beams; a semiconductor laser light beam (wavelength 804 nm) as excitation light, a fundamental light beam (wavelength 1,064 nm) emitted from a Nd-containing single-crystal Li 2 B 4 O 7 (Nd:LBO) excited by the semiconductor laser; and a SHG light beam (wavelength 532 nm). By adding the semiconductor laser (wavelength 804 nm) and the fundamental laser (wavelength 1,064 run), a laser light beam of 458 nm is obtained (sum frequency mixing: 1/λ 1 + 1/λ 2 ). Alternately, a laser light beam of 1573 nm is obtained by differential frequency mixing of the semiconductor laser of 804 nm and the YAG·SHG light of 532 nm. The single-crystal LBO is inferior to the BBO single-crystal in the non-linear optical constant. On the other hand, a large size single-crystal LBO can be grown up. Since the wavelength transducing efficiency is proportional to the square of the input power and the square of the crystal length, the single-crystal LBO, of which a larger single-crystal is available, has a higher wavelength transducing efficiency, as a result. The resistivity to laser damage of the single-crystal LBO is at least 10 times higher than that of BBO, thus a high power laser can be irradiated to the single-crystal LBO. Further, no color center occurs in the single-crystal LBO by the long term irradiation of ultraviolet rays. Single-Crystal Production A single-crystal LBO containing a laser-oscillation-activating device was grown up from melted polycrystal LBO containing a laser-oscillation-activating device in the platinum crucible 101 by using the pull-up equipment shown in FIG. 1. The grow-up conditions were as follows; the temperature gradient between the melt surface and a position of just 1 cm above the melt was 30 to 200 °C/mm, the temperature gradient above the position was 10 to 50 °C/mm and the pull-up speed of the single-crystal LBO body was 1 to 2 mm/hour. Examples of dopants as activators of laser oscillation include rare earth elements, such as Nd, Tm and Er. The amount of such an device in the single-crystal is preferably 0.01 to 10 weight percent. The weight percent in the added device is to 100 percent of the stoichiometric composition of the crystal. EXAMPLE 12 - NO. 1 A laser-diode-excitation-type apparatus in accordance with the present invention will be explained with reference to FIG. 13. A single-crystal LBO containing 1-weight-percent Nd as an activator was grown up using the pull-up equipment shown in FIG. 1. The platinum crucible 101 was filled with 1300 g of polycrystal LBO with 1 weight percent of Nd, the content was melt, and the single-crystal LBO was pulled up in the (110) pull-up direction. The grow-up conditions were as follows; the temperature gradient between the melt surface and a position of just 10 mm above the melt was 80 °C/mm, the temperature gradient above the position was 30 °C/mm, the pull-up speed of the single-crystal LBO body was 0.5 mm/hour, and the rotation of the seed crystal was 2 rpm. A single-crystal LBO having a diameter of 2 inches and a length of 120 mm was obtained, as a result. The resulting single-crystal LBO was cut into a block so as to satisfy a phase matching angle  m of approximately 33 degrees,and both the incident and emitting face were optically polished. The laser block has a diameter of 10 mm and a length of 100 mm. The both end faces are coated to form a resonator. As shown in FIG. 13, a laser-diode-excitation-type laser apparatus 420 comprises the laser block 421 (301) comprising the shaped single-crystal LBO. When a laser light beam (output power 2 W) from a laser diode 422 which emits 0.80-µm light was irradiated to the laser block 421 through a focusing lens 423, the laser block 421 was resonated by oscillation of 1.06-µm light, and a green second harmonic wave of 0.53 µm (530 nm) was emitted. The output power of the emitted light was 0.008 W, and thus the transducing efficiency of the laser block was 0.4%. EXAMPLE 12 - NO. 2 Another laser-diode-excitation-type apparatus in accordance with the present invention will be explained with reference to FIG. 13. A single-crystal LBO containing 0.5-weight-percent Er as an activator was grown up using the pull-up equipment shown in FIG. 1, wherein platinum crucible 101 was filled with 1300 g of polycrystal LBO with 0.5 weight percent of Er. As shown in FIG. 13, a laser-diode-excitation-type laser apparatus 420 comprises the laser block 421 (30I) comprising the shaped single-crystal LBO containing Er. When a laser light beam from a laser diode 422 was irradiated to the laser block 421 in the same condition as EXAMPLE 12 NO. 1, the laser block 421 was resonated by oscillation of 1.06-µm light, and a green second harmonic wave of 0.53 µm (530 nm) was emitted. The output power of the emitted light was 0.006 W, and thus the transducing efficiency of the laser block was 0.3%. EXAMPLE 12 - NO. 3 A flash-lamp-excitation-type apparatus in accordance with the present' invention will be explained with reference to FIG. 14. A single-crystal LBO containing 1-weight-percent Nd and 1-weight-percent Cr as activators was grown up from polycrystal LBO containing Nd and Cr, using the pull-up equipment shown in FIG, 1. As shown in FIG. 14, a flash-lamp-excitation-type laser apparatus 430 comprises the laser block 431 (30J) comprising the shaped single-crystal LBO containing Nd and Cr. The shape and size of the laser block 431 are the same as EXAMPLE 12 NO. 1. When a laser light beam (1,000 W) from a flash-lamp 432 was irradiated to the laser block 431, the laser is oscillated by excitation with the flash lamp, a light of 1.06-µm (1,060 nm) was resonated with resonating mirrors 433, and a green second harmonic wave of 0.53 µm (530 nm) was emitted. The output power of the emitted light was 1 W, and thus the transducing efficiency of the laser block 431 was 0.1%. EXAMPLE 12 - NO. 4 Another flash-lamp-excitation-type apparatus in accordance with the present invention will be explained with reference to FIG. 14. A single-crystal LBO was grown up from polycrystal LBO containing Er and Cr, using the pull-up equipment shown in FIG. 1. As shown in PIG. 14, a flash-lamp-excitation-type laser apparatus 430 comprises the laser block 431 (30L) comprising the shaped single-crystal LBO containing Er and Cr. When a laser light beam (1,000 W) from a flash-lamp 432 was irradiated to the laser block 431 (30L), the laser is oscillated by excitation with the flash lamp, a light of 1.06-µm (1,060 nm) was resonated with resonating mirrors 433, and a green second harmonic wave of 0.53 µm (530 nm) was emitted. The output power of the emitted light was 0.8 W, and thus the transducing efficiency of the laser block 431 was 0.8%. EXAMPLE 12 - NO. 5 A flash-lamp-excitation-type apparatus in accordance with the present invention will be explained with reference to FIG. 15. In this example, a novel light beam is obtained by mixing three light beams; the excitation light having a wavelength λ 1 , a light having a wavelength λ 2 generated by the excitation, and a SHG light (a wavelength λ 1 ) of the light having a wavelength λ 2 . The excitation wavelength and oscillation wavelength vary with the added activation device for the laser oscillation, e.g. Nd, Er, Pr, and U. For example, among three light beams of a light beam having a wavelength of 804 nm from the semiconductor laser, a fundamental wave 1,064 nm and a SHG light beam (532 nm) of the fundamental wave, which both waves are generated in a Nd:Li 2 B 4 O 7 by the semiconductor laser excitation, the SHG light beam (532 nm) is removed with a filter 440. The residual two beams are irradiated to a laser block 421b (30N) comprising single-crystal LBO. A novel light beam of 458 nm is obtained by sum frequency mixing of these two light beams (sum frequency mixing: 1/λ 1 + 1/λ 2 = 1/λ 3 . Similarly, a light beam of 1,573 nm is obtained by differential frequency mixing of the light of 804 nm from the semiconductor laser and the SHG light of 532 nm from the YAG. EXAMPLE 13 An example, in which the single-crystal Li 2 B 4 O 7 (single-crystal LBO) in accordance with the present invention is used for another laser apparatus, will be explained with reference to FIGs. 16A and 16B. Principle The present inventors have found that a twinning crystal of the single-crystal LBO (not ferroelectric material) in accordance with the present invention causes a phenomenon like quasi-phase matching (QPM) and thus can transduce an incident light with a high efficiency. The QPM represents a wavelength transducing method in which the C + axis is rotated by 180 degrees by the twinning crystal of the single-crystal LBO. The QPM has some advantages. First, the phase matching wavelength can be freely determined by varying the periodic length. Further, the phase matching wavelength region can be extended by setting a plurality of periods by means of the twinning crystal. Moreover, the allowable temperature range requiring for the phase matching can be extended to twice or more by using the twinning crystal of the single-crystal LBO. The twinning crystal can be used either in a balk state or in a wave guide state. Additionally, a nonlinear optical constant d (33) can be used. Since the QPM is prepared by rotating the C+ axis direction by 180 degrees by means of the twinning crystal, it has been thought that this is applicable to only ferroelectric materials. However, the present inventors have found that this is also applicable to a paraelectric material of single-crystal LBO. The nonlinear optical device using the single-crystal LBO in accordance with the present invention has a monolithic structure in which right-hand crystals and left-hand crystals are alternately piled up, wherein both crystals have different rotation directions of spiral axes in the C axis direction. Each of such crystals are a single-crystal, for example. In piezoelectric crystals having a spiral axis in the C axis direction, so-called right-hand crystal and left-hand crystal exist by the rotation direction of the spiral. These right-hand and left-hand crystals often form a twinning crystal with composite planes other than the C axis. Only the C+ direction rotates by 180 degrees at the composite plane boundaries of these crystals. Thus, each crystal in the twinning crystal has a mutually reversed nonlinear optical constant. By preparing alternately right-hand crystals and left-hand crystals, the sign of the nonlinear optical constant can be reversed. Thus, the QPM structure like the polarization reversion in the ferroelectric crystal can be formed with a paraelectric crystal. Such a paraelectric twinning crystal can be prepared by the following methods; thin film growing or bulk growing with a special skill; and piling up of thin crystals. Such a twinning crystal has the basic features of the QPM as follows: The phase matching wavelength can be freely determined by varying the periodic length. Further, the phase matching wavelength region can be extended by setting a plurality of periods by means of the twinning crystal. Moreover, the allowable temperature range requiring for the phase matching can be extended to twice or more by using the twinning crystal of the single-crystal LBO. The twinning crystal can be used either in a balk state or in a wave guide state. Additionally, a nonlinear optical constant d (33) can be used. As set forth above, the nonlinear optical device in accordance with the present invention generates a quasi-QPM phenomenon and the wavelength of the incident light can transduce with a high efficiency. In particular, the single-crystal LBO can generate a highly coherent fourth harmonic wave (266 nm) and fifth harmonic wave (213 nm) from a Nd:YAG laser (1,064 nm), for example. The fifth harmonic wave is shorter than that in the excimer laser (248 nm). By generating the fourth or fifth harmonic wave from infrared laser emitted from an existing high power apparatus, laser light of ultraviolet and its neighboring regions can be readily obtained. It can be applied to various fields, such as marking, lithography, semiconductor processing, and medical applications. In particular, a high power laser apparatus for transducing the incident light with a high efficiency can be obtained by providing alternately twinning crystals in the single-crystal LBO and by generating a quasi-QPM phenomenon. Since a large size of single-crystal LBO can be grown up, a large size of a nonlinear optical device is readily produced. Further, high power laser can be irradiated to the single-crystal LBO because of the high resistivity to laser damage. Moreover, the device has a prolonged life due to a decreased deterioration by ultraviolet rays. Single-Crystal Production Using the pull-up equipment shown in FIG. 1, a single-crystal LBO containing alternately twinning crystals. Polycrystal LBO (1,300 g) was placed and melted in the platinum crucible 101, and then a single-crystal LBO having a diameter of 2 inches or more was pulled up in the (001), (110) and (100) directions. The grow-up conditions were as follows; the temperature gradient between the melt surface and a position of just 10 mm above the melt was 80 °C/mm, the temperature gradient above the position was 30 °C/mm, the pull-up speed of the single-crystal LBO body was 0.5 mm/hour, and the rotation of the seed crystal was 2 rpm. A seed crystal formed by alternately piling up right-hand crystals and left-hand crystals was used. Processing The resulting single-crystal LBO was cut into a block (20 mm by 20 mm by 50 mm) by tilting by 79 degrees so that the pile-up direction of the right-hand crystals and the left-hand crystals agrees with the longitudinal direction, and both the incident and emitting face were optically polished. In such a manner, a nonlinear optical device 521 comprising a single-crystal LBO block, in which right-hand crystals 521a and left-hand crystals 521b are alternately piled up, was obtained. The distance between the composite planes in the twinning crystal is preferably approximately a few µm to a few dozen µm. Apparatus Configuration and Operation The nonlinear optical device 521 (300) comprising the resulting single-crystal LBO block was provided in front of a YAG laser 522 (output power: 1.5 J, 10 Hz, 10 nsec.) and a 2nd & 4th harmonics generating unit 523. When a laser light beam having a wavelength of 1,064 nm and an output power of 1.5 J generated by a YAG laser passed through the 2nd & 4th wave generating unit 523, a light beam having a wavelength 4ω (266 nm, output power: 110 mJ) and a light beam of wavelength ω (1,064 nm, output power: 400 mJ) were generated. When these light beams passed through the nonlinear optical device 521 comprising the single-crystal LBO, an ultraviolet light beam of wavelength 5ω (213 nm, output power: 150 mJ) were generated by sum frequency mixing of these two light beams. No extraordinary state was observed in the nonlinear optical device 521 comprising the single-crystal LBO during the operation. In EXAMPLE 13, non-reflective coating was not applied to the polished face of the single-crystal LBO. The output power of the fifth harmonic wave will further increase by non-reflective coating. The present invention is not limited EXAMPLES set forth above, and thus various modification can be available within the scope of the present invention. For example, the light sources, other than the example set forth above, include Nd:YVO 4 , Cr:LiSAF, ruby laser, glass laser, alexandrite laser, garnet laser, sapphire laser, and semiconductor laser. Although the nonlinear optical device 521 comprises a bulk single-crystal LBO containing twinning crystals in which right-hand crystals and left-hand crystals are alternately piled up in EXAMPLE 13, an optical waveguide single-crystal LBO also can be used as the nonlinear optical device 521. Further paraelectric crystals, e.g. quartz and the like, as well as the single-crystal LBO, can be used in the present invention. REFERENCE 4 Sum frequency mixing was carried out in the same manner as EXAMPLE 13, except that a block single-crystal LBO not containing twinning crystals (20 mm by 20 mm by 50 mm) was used as a nonlinear optical device. An ultraviolet light beam of 5ω (213 nm) was generated in an output power of 70 mJ. COMPARATIVE EXAMPLE 7 Sum frequency mixing was carried out in the same manner as EXAMPLE 13, except that a cubic BBO (β-BaB 2 O 4 ) having a side of 5 mm was used for the laser apparatus shown in FIG. 16B. Although an ultraviolet light beam of 5ω having an output power of 100 mJ was observed, the BBO has a restricted clearance of the phase matching angle and temperature and a high ultraviolet absorbance, resulting in heating during operation. Thus, high power 4ω and 5ω light beams cannot be generated stably for a long time. Further, the deterioration of the crystal was confirmed by the color center formation after 100 hours operation. The color center represents the point defect in the transparent single crystal which is detected by the absorbance band generation. Further, BBO readily includes impurities and the yield is law because of flux growing, resulting in the increase in the production cost. The single-crystal LBO in accordance with the present invention can solve such problems.

Claims: A method for converting the wavelength of light comprising the steps of: a) irradiating a first and second coherent light beam (10, 20; 40, 42; 10a, 56, 58, 60; 224; 322, 323; 522, 523) each having a different wavelength to the light incident face of a single-crystal lithium tetraborate (Li 2 B 4 O 7 ) (30B ;30C ;30D ;221 ;321 ;521) at a predetermined angle, which is cut at a predetermined plane to the optical axis so as to satisfy a predetermined phase matching angle; and b) emitting a light beam having a wavelength of sum frequency mixing of the wavelengths of these two coherent light beams; wherein c) the first coherent light beam and the second coherent light beam which has a one-second or one-fourth wavelength of the first coherent light beam are irradiated to the incident face of said single-crystal Li 2 B 4 O 7 at a predetermined angle, and at least a coherent light beam having one-third or one-fifth wavelength of said first coherent light beam is emitted. A method for converting the wavelength of light according to claim 1, wherein the wavelength of the generated emitted light is within the range of ultraviolet light (e.g. 213 nm). A method for converting the light wavelength according to claim 1, wherein said light beams are irradiated to said single-crystal lithium tetraborate (Li 2 B 4 O 7 ) having a refractive index variation of 10 -5 /nm or less and an etch-pit density of 1x10 3 /cm 2 or less. An optical wavelength converter, comprising: a) a single-crystal lithium tetraborate (Li 2 B 4 O 7 ) (30B ;30C ;30D ;221 ;321 ;521); b1) a first light source (10; 40; 10a; 224; 322; 522) for emitting a first coherent light beam having a first wavelength; b2) a second light source (20; 42; 56, 58, 60; 323; 523) for emitting a second coherent light beam having a second wavelength; wherein b3) said first and second incident coherent light beams are irradiated by said first and second light sources to the light incident face of said single-crystal Li 2 B 4 O 7 (30B ; 30C ;30D ; 221 ; 321 ;521) at a predetermined angle, which is cut at a predetermined plane to the optical axis so as to satisfy a predetermined phase matching angle; d) said single-crystal Li 2 B 4 O 7 (30B ;30C ;30D ;221 ;321 ;521) emitting a light beam having a wavelength of sum frequency generation of said first and second incident coherent light beams; and e) said first coherent light beam having a predetermined wavelength and said second coherent light beam having a one-second or one-fourth wavelength of said first coherent light beam are irradiated to said single-crystal Li 2 B 4 O 7 , and a light beam having a one-third or one-fifth wavelength of said first coherent light beam is emitted. An optical wavelength converter according to claim 4, wherein said single-crystal lithium tetraborate (Li 2 B 4 O 7 ) has a refractive index variation of 10 -5 /nm or less and an etch-pit density of 1x10 3 /cm 2 or less. An optical wavelength converter according to claim 4, wherein the wavelength of the generated emitted light is within the range of ultraviolet light (e.g. 213 nm). A laser apparatus, comprising: a) the optical wavelength converter set forth in claim 4; b) an optical device (40; 42) for introducing said first and second coherent light beams to said wavelength converter; c) wherein said first and second light sources emit laser light beams; and d) wherein said laser apparatus emits a light beam having said wavelength x of sum frequency generation of the wavelengths of said first and second coherent light beams. A laser apparatus according to claim 7, wherein said second light source has the wavelength converter set forth in claim 12; and the first coherent light beam emitted from said first light source is irradiated to said wavelength converter and a light beam having a half wavelength of the incident light is emitted from said wavelength converter. An optical-resonance-type laser apparatus, comprising: a) a first wavelength converter (30, E1) set forth in claim 4; b) a second wavelength converter (30, E2) set forth in claim 4 provided at a position parallel to the first wavelength converter (30, E1); c) a first half mirror (64), provided at the incident side of the first wavelength converter, for irradiating the first coherent light beam to the first wavelength converter; d) a second half mirror (66) for receiving a light beam emitted from the second wavelength converter to deflect it by a predetermined angle; e) a third mirror (70) for deflecting the light beam emitted from the second half mirror to the incident face of the second wavelength converter; and f) a fourth mirror (68) for deflecting the light beam emitted from the second wavelength converter to the first half mirror to irradiate the light beam to the first wavelength converter; g) wherein the first coherent light beam having a predetermined wavelength is irradiated to the first half mirror (64) and a coherent light beam having a one-fifth wavelength of the first coherent light beam is emitted from the second half mirror (66). An optical parametric oscillator, comprising: a) a light source (84) for emitting a coherent light beam having a pumping frequency of ω p ; and b) an optical parametric oscillating section (80, 30F, 82) having the wavelength converter (30F) set forth in claim 4 and first and second plano-concave half mirrors (80, 82) for receiving the coherent light beam from the light source to achieve optical parametric oscillation in front of and behind the wavelength converter. A laser apparatus for lithography, comprising: a) a light incident means (224) for receiving a first coherent light beam having a first wavelength and a second coherent light beam having a one-second or one-fourth wavelength of the first coherent light beam; b) a wavelength converter set forth in claim 4; c) a first plano-concave half mirror (222A) provided at the light incident side of said wavelength converter; d) a second plano-concave half mirror (222B) provided at the light emitting side of said wavelength converter; and e) an optical filter (226) provided behind said second half mirror (222B). A laser apparatus for lithography according to claim 11; wherein the first coherent light beam irradiating to said light incident means is a laser beam from a light source selected from Nd:YAG laser; Nd-YVO 4 laser, Cr: LiSAF laser, ruby laser, glass laser, alexandrite laser, garnet laser, sapphire laser, and semiconductor laser. A laser apparatus for lithography according to claim 11, wherein the laser apparatus for lithography further comprises an integrator (228) for smoothing the intensity of the light beam, provided behind said optical filter (226). A short wavelength solid-state laser apparatus for processing, comprising: a) a first light source (322) emitting a first coherent light beam having a first wavelength; b) a second light source for emitting a second coherent light beam having a wavelength of one-second or one-fourth of the first coherent light beam from the first light source (322); and c) the wavelength converter (321, 30H) set forth in claim 4 for receiving the first coherent light beam from said first light source (322) and the second coherent light beam from said second light source; d) wherein said first light source (322) emits the first coherent light beam in a predetermined energy for processing for a predetermined time; and e) said wavelength converter emits a light beam having a one-fifth wavelength of said first coherent light beam. An optical wavelength converter according to claim 4, characterized in that said single-crystal lithium tetraborate (Li 2 B 4 O 7 ) contains an element for activating laser oscillation. An optical wavelength converter according to claim 4, characterized in that said element for activating laser oscillation is a rare earth element, and the content ranges from 0.01 to 10 weight percent.

Inventors: KOMATSU RYUICHI, SUGAWARA TAMOTSU, TAKAHASHI, KOMATSU, RYUICHI, SUGAWARA, TAMOTSU, Komatsu, Ryuichi, c/o Mitsubishi Mat. Corp., Sugawara, Tamotsu, c/o Mitsubishi Mat. Corp., Takahashi, Masakuni, c/o Mitsubishi Mat. Corp.
Applicants: MITSUBISHI MATERIALS CORP, MITSUBISHI MATERIALS CORPORATION
Application year: 1996
Main classification: G02F1
IPC codes: G02F1, H01S3