Patent Publication Number: US-2009219958-A1

Title: Wavelength converting laser and image display

Description:
This application is entitled to the benefit of Provisional Patent Application No. 61/022,947, filed in United States Patent and Trademark Office on Jan. 23, 2008. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a wavelength converting laser capable of converting the wavelength of a fundamental wave and outputting a converted wave having a different wavelength from the fundamental wave, and an image display including the wavelength converting laser. 
     2. Description of the Background Art 
     Conventionally, there is a wavelength converting laser converting the wavelength of a fundamental wave into a converted wave such as a second harmonic, a sum frequency and a difference frequency by utilizing the non-linear optical phenomenon of a wavelength conversion element. 
       FIG. 17  is a schematic view showing a configuration of a conventional wavelength converting laser including, for example, a fundamental-wave laser light source  301 , a lens  302  concentrating a fundamental-wave laser beam emitted from the fundamental-wave laser light source  301 , a wavelength conversion element  303  generating a second harmonic from the concentrated fundamental-wave laser beam, and a dichroic mirror  304  splitting the fundamental-wave laser beam and the harmonic laser beam. 
     The wavelength conversion element  303  is made of a non-linear optical crystal and converts the wavelength of a fundamental wave by properly adjusting the crystal orientation, polarization inversion structure or the like in such a way that the phase of the fundamental wave matches with the phases of a converted wave. Particularly, a wavelength conversion element using the polarization inversion structure can conduct a wavelength conversion efficiently even with low power by quasi phase matching and conduct diverse wavelength conversions by design. The polarization inversion structure is a structure having a region in which the spontaneous polarization of a non-linear optical crystal is cyclically inverted. 
     A conversion efficiency η at which a fundamental wave is converted into a second harmonic is given by the following expression (1) if the interaction length of a wavelength conversion element is L, the power of a fundamental wave is P, the cross-section area of a beam in the wavelength conversion element is A and the gap from a phase matching condition is Δk. 
       η∝L 2 P/A×sinc 2 (ΔkL/2)  (1) 
     If a light-concentration condition is set to be suitable for the interaction length, the conversion efficiency η is given by the following expression (2). 
       η∝LP×sinc 2 (ΔkL/2)  (2) 
     It can be seen from the expression (2) that the conversion efficiency rises by extending the interaction length or increasing the fundamental-wave power. However, since the allowable range for the gap from a phase matching condition is inversely proportional to the interaction length, the conditions for temperature regulation and the fundamental wave become stricter as the interaction length becomes greater. Further, a rise in the fundamental-wave power may destroy the end faces of the wavelength conversion element or lower the conversion efficiency because of heat generated through optical absorption. 
     For example, Japanese Patent Laid-Open Publication No. 2004-125943 proposes a wavelength converter capable of conducting a wavelength conversion efficiently without any optical damage by including a light guiding means for guiding an incident laser beam to a plurality of optical paths on a mutually-different straight line, a wavelength converting means arranged on the plurality of optical paths, and a laser-beam extracting means for extracting the laser beam whose wavelength is converted by the wavelength converting means. 
     Furthermore, for example, Japanese Patent Laid-Open Publication No. 11-44897 proposes a wavelength converting laser capable of conducting a wavelength conversion efficiently by including a plurality of wavelength conversion elements arranged in sequence on an incident fundamental-wave laser-beam path, a plurality of light concentrating means for converging a laser beam passing through the plurality of wavelength conversion elements, and a beam splitter changing the path of the laser beam whose wavelength is converted by the plurality of wavelength conversion elements. 
     Moreover, for example, Japanese Patent Laid-Open Publication No. 2006-208629 proposes a wavelength conversion element having a higher wavelength-conversion efficiency by: reflecting a beam of light which is incident upon the incidence end of a polarization inversion element, is subjected to a wavelength conversion and reaches the other end thereof by a reflector arranged at the other end of the polarization inversion element to thereby change the optical path and lead the beam to be incident again upon the polarization inversion element and leading the beam again into passing into the polarization inversion element to thereby convert the wavelength thereof. 
     Although the above conventional proposals are capable of obtaining a high conversion efficiency even if a wavelength conversion element has a short interaction length, a plurality of beams are outputted, thereby requiring a plurality of optical parts for coordinating those beams. Further, the conventional proposals enlarge the effective light-source area of a converted wave, thereby making it hard to concentrate the converted wave. Still further, those proposals raise the problem of increasing the cost because a larger area is necessary for a wavelength conversion element. In addition, a wavelength converting laser needs a plurality of optical parts, thereby requiring looser regulations on the parts to bring the product onto the market. 
     SUMMARY OF THE INVENTION 
     In order to solve the above problems, it is an object of the present invention to provide a wavelength converting laser and an image display which are capable of obtaining a high conversion efficiency stably and being miniaturized. 
     A wavelength converting laser according to an aspect of the present invention includes: a light source emitting a fundamental wave; and a wavelength conversion element converting the fundamental wave emitted from the light source into a converted wave having a different wavelength from the fundamental wave, in which: a pair of fundamental-wave reflecting surfaces is arranged on both end sides of the wavelength conversion element in the directions of an optical axis thereof and reflects the fundamental wave to thereby pass the fundamental wave a plurality of times inside of the wavelength conversion element, and at least one of the fundamental-wave reflecting surfaces transmits the converted wave; and the pair of fundamental-wave reflecting surfaces allows the fundamental wave to cross inside of the wavelength conversion element and form a plurality of light-concentration points at places different from a cross point of the fundamental wave. 
     According to this configuration, the pair of fundamental-wave reflecting surfaces allows the fundamental wave to pass a plurality of times inside of the wavelength conversion element, cross inside of the wavelength conversion element and form a plurality of light-concentration points at places different from a cross point of the fundamental wave. 
     According to the present invention, the fundamental wave passes a plurality of times inside of the wavelength conversion element and forms a plurality of light-concentration points at places different from a cross point of the fundamental wave, thereby making it possible to obtain a high conversion efficiency stably and reduce the light-source area of a converted wave emitted as a plurality of beams, resulting in the whole apparatus being smaller. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view showing an exterior shape of a wavelength conversion element according to a first embodiment of the present invention. 
         FIG. 2A  is a schematic top view showing a configuration of a wavelength converting laser according to the first embodiment. 
         FIG. 2B  is a schematic side view showing a configuration of the wavelength converting laser according to the first embodiment. 
         FIG. 3  is a perspective view showing a configuration of a temperature regulator according to the first embodiment. 
         FIG. 4  is a schematic view showing an exterior shape of a wavelength conversion element according to a second embodiment of the present invention. 
         FIG. 5A  is a schematic top view showing a configuration of a wavelength converting laser according to the second embodiment. 
         FIG. 5B  is a schematic side view showing a configuration of the wavelength converting laser according to the second embodiment. 
         FIG. 6  is a schematic view showing a configuration of a multi-mode optical fiber connected to the wavelength converting laser of  FIGS. 5A and 5B . 
         FIG. 7  is schematic view showing a configuration of a wavelength converting laser according to a third embodiment of the present invention. 
         FIG. 8  is schematic top view showing a configuration of a wavelength converting laser according to a fourth embodiment of the present invention. 
         FIG. 9  is schematic top view showing a configuration of a wavelength converting laser according to a fifth embodiment of the present invention. 
         FIG. 10A  is schematic top view showing a configuration of a wavelength converting laser according to a sixth embodiment of the present invention. 
         FIG. 10B  is schematic side view showing a configuration of the wavelength converting laser according to the sixth embodiment. 
         FIG. 11A  is schematic top view showing a configuration of a wavelength converting laser according to a seventh embodiment of the present invention. 
         FIG. 11B  is schematic side view showing a configuration of the wavelength converting laser according to the seventh embodiment. 
         FIG. 12A  is schematic top view showing a configuration of a wavelength converting laser according to an eighth embodiment of the present invention. 
         FIG. 12B  is schematic side view showing a configuration of the wavelength converting laser according to the eighth embodiment. 
         FIG. 13  is schematic view showing a configuration of an image display including the wavelength converting laser of  FIGS. 12A and 12B . 
         FIG. 14  is schematic view showing a configuration of a wavelength converting laser according to a ninth embodiment of the present invention. 
         FIG. 15  is a schematic view showing an exterior shape of a wavelength conversion element according to a tenth embodiment of the present invention. 
         FIG. 16A  is schematic top view showing a configuration of a wavelength converting laser according to the tenth embodiment. 
         FIG. 16B  is schematic side view showing a configuration of the wavelength converting laser according to the tenth embodiment. 
         FIG. 17  is a schematic view showing a configuration of a conventional wavelength converting laser. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
     Embodiments of the present invention will be below described with reference to the attached drawings. The following embodiments, however, are merely specific examples, and thus, the scope of an art of the present invention is not supposed to be limited. 
     First Embodiment 
       FIG. 1  is a schematic view showing an exterior shape of a wavelength conversion element  10  according to a first embodiment of the present invention. The wavelength conversion element  10  is made of an MgO:LiNbO 3  crystal having a polarization inversion period structure and is shaped like a rod having a length of, for example, 10 mm and a width and a thickness of, for example, 1 mm, respectively. The wavelength conversion element  10  converts a fundamental wave into a converted wave having a different wavelength from the fundamental wave. One end face  12  of the wavelength conversion element  10  in the longitudinal directions is formed with a fundamental-wave inlet  11  for incidence of the fundamental wave. Both end faces of the rod-shaped wavelength conversion element  10  in the longitudinal directions are formed, except for the fundamental-wave inlet  11 , with a fundamental-wave reflective coat for reflecting the fundamental wave. 
     The other end face  13  in the longitudinal directions without the fundamental-wave inlet  11  is formed with the fundamental-wave reflective coat for reflecting the fundamental wave and a converted-wave transmission coat for transmitting the converted wave as a face for outputting the converted wave. The end face  12  is formed with a converted-wave reflective coat for reflecting the converted wave. Hence, the wavelength conversion element  10  includes the output face of the converted wave only in the other end face  13  in the longitudinal directions. 
     The fundamental-wave inlet  11  is shifted toward the lateral end from the center of the end face  12 , has a diameter of, for example, 100 μm and is formed with an AR (anti-reflective) coat for the fundamental wave. The end face  12  with the fundamental-wave inlet  11  has a convex cylindrical shape bent in the vertical directions of  FIG. 1  while the other end face  13  has a convex cylindrical shape bent in the lateral directions of  FIG. 1 . The curvature radii of both end faces  12  and  13  are each, for example, 13 mm. 
     The side faces of the wavelength conversion element  10  are coated with a resin clad  14  having a refractive index lower than the wavelength conversion element  10 , and via the resin clad  14 , the wavelength conversion element  10  is fixed on a holder and undergoes temperature regulation. The resin clad  14  coats the face other than the end faces  12  and  13  of the wavelength conversion element  10 . 
       FIG. 2A  is a schematic top view showing a configuration of a wavelength converting laser according to the first embodiment and  FIG. 2B  is a schematic side view showing a configuration of the wavelength converting laser according to the first embodiment.  FIGS. 2A and 2B  show the optical paths of a fundamental wave and a converted wave and are top and side views of the rod-shaped wavelength conversion element  10 , respectively. 
     A wavelength converting laser  100  includes a fundamental-wave laser light source  1 , a condensing lens  2 , the wavelength conversion element  10  and the resin clad  14 . 
     A fundamental wave emitted from the fundamental-wave laser light source  1  is concentrated into the fundamental-wave inlet  11  by the condensing lens  2  and incident upon the wavelength conversion element  10 , goes ahead in the longitudinal direction of the wavelength conversion element  10  and undergoes a wavelength conversion, and is reflected by the end face  13  and advances again inside of the wavelength conversion element  10 . Through the process, a converted wave is obtained and emitted from the end face  13 . The fundamental-wave inlet  11  is shifted from the rod center axis and the end face  13  has a curvature in the direction where the fundamental-wave inlet  11  is shifted from the rod center axis, thereby causing the fundamental wave to slant and reflect laterally in top view lest it should return to the fundamental-wave inlet  11 . 
     The end face  13  and the end face  12  are formed with the reflective coats and the side faces of the wavelength conversion element  10  are coated with the resin clad  14 . Accordingly, the fundamental wave is reflected by the end face  13  and the end face  12  and is totally reflected by the side-face resin clad  14 , and thereby, goes back and forth repeatedly in the longitudinal directions inside of the wavelength conversion element  10 . The end face  12  and the end face  13  function as a concave (cylindrical) mirror for enabling the fundamental wave to form a light-concentration point when going back and forth. 
     The fundamental wave going back and forth inside of the wavelength conversion element  10  crosses inside of the wavelength conversion element  10  and forms a light-concentration point Pb produced by the curvatures of the end face  12  and the end face  13  other than the light-concentration point formed by the condensing lens  2 . 
     At this time, a plurality of the light-concentration points Pb are formed at places different from a cross point Pa of the fundamental wave. In the first embodiment, the end face  12  and the end face  13  include cylindrical surfaces, thereby forming the light-concentration points Pb differing each other in the beam-diameter directions. 
     The converted wave is reflected by the end face  12  and the side faces of the wavelength conversion element  10 , led to the end face  13  and emitted as the flux of a plurality of beams from the end face  13 . The end face  13  has a rectangular shape whose sides are, for example, 1 mm and thus is an extremely small outlet, and the cylindrical shape thereof functions as a convex lens for the converted wave, thereby narrowing the divergence angle of a luminous flux spreading laterally in top view and emitting the luminous flux. 
     In the first embodiment, the end faces  12  and  13  of the wavelength conversion element  10  correspond to an example of the pair of fundamental-wave reflecting surfaces and the resin clad  14  corresponds to an example of the reflection portion. 
     In the first embodiment, the wavelength conversion element  10  includes the fundamental-wave reflecting surface on both sides in the longitudinal directions thereof, at least one fundamental-wave reflecting surface transmits the converted wave, the fundamental wave crosses inside of the wavelength conversion element  10 , and a light-concentration point is formed at a place different from a cross point. This makes it possible to enhance the conversion efficiency, simultaneously collect the converted wave emitted as a plurality of beams into one place to thereby reduce the light-source area thereof, and reduce the area necessary for the wavelength conversion element  10 . The fundamental wave going back and forth between the pair of fundamental-wave reflecting surfaces makes a plurality of passes inside of the wavelength conversion element  10 , and the fundamental wave going back and forth forms a plurality of light-concentration points, thereby making the conversion efficiency several times as high as the case where the fundamental wave passes only once inside of a wavelength conversion element. 
     On the other hand, if the fundamental wave does not converge while passing several times inside of the wavelength conversion element  10 , the effect of diffraction widens the beam diameter of the fundamental wave to lower the power density, thereby raising the conversion efficiency only a little. In the first embodiment, however, the beams passing inside of the wavelength conversion element  10  have the light-concentration points, thereby raising the conversion efficiency significantly without lowering the power density of the fundamental wave. Besides, when the fundamental wave goes back and forth between the fundamental-wave reflecting surfaces, the converted wave is outputted from at least one fundamental-wave reflecting surface, thereby reducing the interaction length for wavelength conversion to or below the length of one round trip of the wavelength conversion element  10 . This is useful for avoiding the problem of extending the interaction length. 
     In the first embodiment, the fundamental wave going back and forth in the longitudinal directions crosses inside of the wavelength conversion element  10 , thereby reducing the area in the width and thickness directions of the wavelength conversion element  10  which the fundamental wave passes through. 
     A part of the wavelength conversion element  10  through which the fundamental wave passes becomes a source generating the converted wave, and thus, the cross-section area in the width and thickness directions of the wavelength conversion element  10  is reduced, thereby reducing the light-source area. The cross-section area which the converted wave passes through is also made smaller, thereby enabling a simple optical part to control a plurality of beams. 
     In the first embodiment, there are the cross points and the light-concentration points of the fundamental wave inside of the wavelength conversion element  10 . At this time, if the cross points and the light-concentration points of the fundamental wave are concentrated, the power density of the fundamental wave becomes too high, thereby giving damage or optical absorption to the wavelength conversion element  10  to stagnate the wavelength conversion at the cross points and the light-concentration points. In the first embodiment, however, since there are the plurality of light-concentration points at places different from the cross points of the fundamental wave, the places where the power density is high and the wavelength conversion is intensely conducted can be dispersed, thereby obtaining a high conversion efficiency stably. In the first embodiment, the cross point of the fundamental wave indicates a point at which the fundamental-wave optical paths overlap in space except for an intersection formed by reflection. 
     In the first embodiment, a part of the fundamental wave incident upon the wavelength conversion element  10  is emitted from the fundamental-wave inlet  11 , and in order to prevent the fundamental wave from returning to the fundamental-wave laser light source  1 , preferably, an optical isolator or the like for may be employed. Alternatively, it may be appreciated that a shielding cover absorbing the fundamental wave emitted from the wavelength conversion element  10  is employed around the fundamental-wave inlet  11 . 
     In the first embodiment, it is preferable that the fundamental wave is reflected by not only the pair of fundamental-wave reflecting surfaces in the longitudinal directions of the wavelength conversion element  10  but also the side faces of the wavelength conversion element  10  to thereby return the fundamental wave into the wavelength conversion element  10 . Ordinarily, the area in the width and thickness directions of the wavelength conversion element  10  which the fundamental wave passes through becomes larger as the fundamental wave goes back and forth more times, and the fundamental wave equivalent to this increment in the area cannot be acquired. 
     In the first embodiment, however, the side faces of the wavelength conversion element  10  is formed with the resin clad (reflection portion)  14  reflecting the fundamental wave into the wavelength conversion element  10 , thereby keeping the area within a specified range which the fundamental wave passes through inside of the wavelength conversion element  10 . Besides, the side faces of the wavelength conversion element  10  reflect the fundamental wave, thereby limiting the fundamental-wave passage area and setting the converted-wave light-source area, so that the emitted converted wave can be easily controlled. In addition, the side faces of the wavelength conversion element  10  reflect the fundamental wave, thereby unifying the intensity distribution of the fundamental wave passing through the wavelength conversion element  10  to disperse the places having higher fundamental-wave power densities. It is preferable that the side faces of the wavelength conversion element  10  reflect the fundamental wave as well as the converted wave, thereby leading the converted wave to the end face  13  on the output side having a specified area and making the converted-wave intensity uniform. 
     In the first embodiment, it is preferable that the side faces of the wavelength conversion element  10  is coated with a material having a refractive index lower than the wavelength conversion element  10 . The side faces of the wavelength conversion element  10  coated with this material reflects the fundamental wave and the converted wave totally to thereby return the fundamental wave and the converted wave into the wavelength conversion element  10 . Besides, a coating portion (reflection portion) can be employed as a protective layer and a heat-insulating layer for the wavelength conversion element  10 . Particularly, the coating portion may preferably be a deformable and workable resin material. A non-linear crystal forming the wavelength conversion element  10  is hard and brittle and can be broken by an impact, but becomes stronger against a vibration or a deformation when coated with the resin material. Further, working the resin material makes it easier to join it to a holding portion holding the wavelength conversion element  10 . The resin material includes, for example, a UV-curing resin, a thermoset resin, a thermoplastic resin and the like. 
     The resin clad  14  is joined to a temperature regulator constantly regulating the temperature of the wavelength conversion element  10 .  FIG. 3  is a perspective view showing a configuration of a temperature regulator according to the first embodiment. A temperature regulator  15  includes a metal holder  16 , a Peltier element  17  and a radiation fin  18 . 
     The metal holder  16  is made of a rectangular, metal material and holds the wavelength conversion element  10  and the resin clad  14  so as to cover the side surface of the resin clad  14  over the full circumference. The cooling surface of the Peltier element  17  is joined to a side face of the metal holder  16  and absorbs heat from the metal holder  16 . 
     The radiation fin  18  is arranged on the side of the heat-radiating surface of the Peltier element  17  and radiates heat from the Peltier element  17 . The heat generated from the wavelength conversion element  10  is transferred to the resin clad  14  and the metal holder  16 , and the metal holder  16  is cooled by the Peltier element  17 . Then, the radiation fin  18  radiates the heat emitted from the Peltier element  17 . 
     In the first embodiment, it is preferable that the temperature regulator  15  is connected to the reflection portion (resin clad  14 ) coating the wavelength conversion element  10 . If the temperature regulator  15  is connected directly to the wavelength conversion element  10 , the connection part of the wavelength conversion element  10  and the temperature regulator  15  can absorb the fundamental wave going back and forth between the reflecting surfaces, thereby hindering precisely executing the function of regulating the temperature. 
     In the first embodiment, however, the reflection portion (resin clad  14 ) totally reflecting the fundamental wave and the converted wave is connected to the temperature regulator  15 , thereby preventing the fundamental wave and the converted wave from being absorbed into the temperature regulator  15 , so that precise temperature control can be executed. Besides, the reflection portion (resin clad  14 ) covers the side faces of the wavelength conversion element  10  over the full periphery, thereby also keeping the whole wavelength conversion element  10  at a fixed temperature. 
     The fundamental-wave laser light source  1  is formed by a fiber laser generating an oscillation having a wavelength of 1064 nm and having a linear polarization. In the wavelength converting laser  100 , polarization directions PD of the fundamental wave incident upon the wavelength conversion element  10  are the up-and-down directions in the side view of  FIG. 2B . The polarization directions PD of the fundamental wave corresponds to the z-axis directions of an MgO:LiNbO 3  crystal having a polarization inversion structure, thereby enabling an efficient wavelength conversion. 
     The sectional shape of a plane perpendicular to the optical axis of the wavelength conversion element  10  is a rectangle having sides parallel to the polarization directions PD and sides perpendicular thereto. In the first embodiment, it is preferable that the sectional shape of a plane perpendicular to the optical axis of the wavelength conversion element  10  is rectangular, at least one side is parallel to the polarization directions PD of the fundamental wave incident upon the wavelength conversion element  10  and the side faces of the wavelength conversion element  10  reflect the fundamental wave. 
     In the first embodiment, the fundamental wave is returned into the wavelength conversion element  10  using the reflection by the side faces of the wavelength conversion element  10 . If the polarization directions change at this time, the conversion efficiency lowers. In the first embodiment, however, the reflecting side faces are parallel or perpendicular to the polarization directions, thereby removing a change in the polarization directions to enable an efficient wavelength conversion even using the side-face reflection. Since the non-linear optical crystal has an optical axis, the polarization directions need to coincide with the optical axis for conducting a wavelength conversion. 
     In the first embodiment, it is preferable that the end faces of the wavelength conversion element  10  are the fundamental-wave reflecting surfaces and each have a convex shape. Furthermore, in the first embodiment, it is preferable that the pair of fundamental-wave reflecting surfaces is formed in both end faces of the wavelength conversion element  10 , respectively, in the optical-axis directions thereof, and at least one of both end faces of the wavelength conversion element  10  has a convex shape. 
     The wavelength conversion element  10  includes the fundamental-wave reflecting surfaces in both end faces in the longitudinal directions, and each end face is shaped like a convex cylinder whose axis is perpendicular to each other. The end faces of the wavelength conversion element  10  also serve as the fundamental-wave reflecting surfaces, thereby saving the process of coordinating the wavelength conversion element  10  and the fundamental-wave reflecting surfaces. Conventionally, if the fundamental wave passes several times inside of the non-linear optical crystal, there may occur a drawback that the number of coordination axes increases, the first embodiment realizes a compact configuration capable of decreasing the number of coordination axes and passing the fundamental wave to be concentrated a plurality of times inside of the wavelength conversion element  10 . 
     In addition, the fundamental wave goes back and forth inside of the wavelength conversion element  10 , and thus, there is no face transmitting the fundamental wave when passing through the wavelength conversion element  10 , thereby eliminating an optical loss. The convex end face of the wavelength conversion element  10  works as a concave mirror for the fundamental wave to be reflected to thereby form a light-concentration point inside of the wavelength conversion element  10 . On the other hand, the convex end face of the wavelength conversion element  10  reflecting the fundamental wave and transmitting the converted wave works as a convex lens for the converted wave to thereby narrow the divergence angle of the converted wave to be emitted. 
     Alternatively, it may be appreciated that only one of both end faces of the wavelength conversion element  10  is formed with a convex fundamental-wave reflecting surface, or the convex shape is not spherical but non-spherical. 
     In the first embodiment, preferably, at least one of both end faces of the wavelength conversion element  10  having the fundamental-wave reflecting surfaces may have a convex cylindrical shape. The fundamental-wave reflecting surface is a cylindrical surface to cause light-concentration points formed inside of the wavelength conversion element  10  to differ in the beam-diameter directions, thereby preventing the power density of the fundamental wave from concentrating. 
     Besides, the convex surface is cylindrical to decrease the number of coordination axes by one, compared with it is spherical, thereby facilitating the coordination process. 
     Further, the end faces of the wavelength conversion element  10  are also worked for a single axis, thereby enabling a reduction in the manufacturing cost. 
     Particularly, it is preferable that in the wavelength conversion element  10  having a rectangular shape in section, the axial directions of a cylindrical surface coincide with the sides of the rectangular cross section. This make it possible to prevent the fundamental wave from turning in the polarization direction when reflecting the side faces of the wavelength conversion element  10 . 
     It is preferable that both end faces of the wavelength conversion element  10  are convex-cylindrical fundamental-wave reflecting surfaces, and the axes of the cylindrical shapes are perpendicular to each other. The axes of the two reflecting surfaces capable of concentrating light cross at right angles, thereby causing light-concentration points formed inside of the wavelength conversion element  10  to differ in the directions perpendicular to each other. Besides, the axes of the cylindrical shapes are perpendicular to each other, and thereby, the two coordination axes of the wavelength conversion element  10  can be handled independent of each other, thereby facilitating the coordination. Further, it is separately worked for each axis, thereby enabling a reduction in the manufacturing cost including the easiness of coordination. 
     Particularly, it is preferable that the curvature radii of both cylindrical surfaces are equal to or more than the length of the wavelength conversion element  10 . The curvature radii are set to the above condition, thereby enabling a beam to go back and forth while securing the concentration characteristics thereof. Particularly, as shown in the side view of the wavelength converting laser  100  of  FIG. 2B , the optical path in the diametrical directions having a narrow positional gap between the optical axis and the fundamental-wave inlet  11  becomes a stable resonance condition, thereby bringing the beam diameter within a specified range even though the beam goes back and forth more times. 
     Preferably, the wavelength conversion element may have a thickness and a width of 1 mm or below. The thickness and width of the wavelength conversion element  10  is equivalent to the light-source area of the converted wave, and thus, the light-source area is within a range of 1 mm×1 mm, thereby collecting the converted wave within a range narrow enough. 
     In the first embodiment, a plurality of converted beams are outputted, and those converted beams are collected within a narrower range, thereby allowing each optical part to control beam shaping and propagation or the like, taking no account of the fact that there are several such converted beams. 
     The fundamental-wave laser light source  1  is a fiber laser, or another type of laser light source such as a semiconductor laser and a solid laser. The condensing lens  2  is used for leading a fundamental-wave laser beam to be incident through the fundamental-wave inlet  11  upon the fundamental-wave reflecting surfaces. In the first embodiment, various optical parts can be employed for leading the fundamental-wave laser beam to be incident upon the pair of fundamental-wave reflecting surfaces. The wavelength conversion element  10  is made of each kind of non-linear material—LBO, KTP, or LiNbO 3  or LiTaO 3  having a polarization inversion period structure. 
     In the first embodiment, as the fundamental-wave reflecting surfaces, curved surfaces capable of concentrating light are employed in such a way that the fundamental wave crosses inside of the wavelength conversion element  10  to thereby form a plurality of light-concentration points at places different from a cross point. In addition, the light-concentration points according to the first embodiment can be formed simply by concentrating beams incident upon the fundamental-wave reflecting surfaces. In the first embodiment, the fundamental-wave reflecting surfaces are convex cylindrical surfaces, the plurality of light-concentration points are formed at places different from a cross point, and the fundamental wave is crossed through reflection by the side faces of the wavelength conversion element  10  and reflection by the cylindrical surfaces. 
     The shape of the fundamental-wave inlet  11  is not especially limited, as long as it allows the fundamental wave to be incident between the pair of fundamental-wave reflecting surfaces. In the first embodiment, the end face  12  is circularly masked when the reflective coat thereof is formed, thereby designing only the fundamental-wave inlet  11  as a fundamental-wave transmission surface. Alternatively, it may be appreciated that a part of the fundamental-wave reflecting surface is worked into the fundamental-wave inlet  11 . In the first embodiment, the fundamental-wave inlet  11  is largely shifted laterally and slightly shifted longitudinally from the center of the end face  12  of the wavelength conversion element  10 . However, the position is the fundamental-wave inlet  11  is not especially limited. 
     Furthermore, in the first embodiment, the face for outputting the converted wave is only one end face of the wavelength conversion element  10 . However, the end face  12  may be covered with a transmission coat for the converted wave in such a way that the converted wave is outputted from both end faces. 
     Moreover, it is preferable that a light-concentration point formed for the first time by the fundamental wave inside of the wavelength conversion element  10  has an elliptic beam shape. In the first embodiment, first, the lens power of the condensing lens  2  concentrates the fundamental wave inside of the wavelength conversion element  10 . At this time, the condensing lens  2  causes the fundamental wave to have a effectively different NA (numerical aperture) in the two axial directions and be incident as an elliptic beam upon the wavelength conversion element  10 . Especially, the first light-concentration point tends to have a higher power density because the conversion has not yet progressed and the fundamental-wave power remains great. Accordingly, the beam shape of a light-concentration point formed for the first time by the fundamental wave inside of the wavelength conversion element  10  is set to an ellipse, thereby preventing the first light-concentration point from having a higher power density. 
     Second Embodiment 
       FIG. 4  is a schematic view showing an exterior shape of a wavelength conversion element  20  according to a second embodiment of the present invention.  FIG. 5A  is a schematic top view showing a configuration of a wavelength converting laser according to the second embodiment and  FIG. 5B  is a schematic side view showing a configuration of the wavelength converting laser according to the second embodiment. In the second embodiment, component elements are given the same reference characters and numerals as those of the first embodiment, as long as the former are identical to the latter, and thus, their description is omitted. 
     A wavelength converting laser  101  includes a fundamental-wave laser light source  1 , a condensing lens  2 , a wavelength conversion element  20  and a resin clad  14 . 
     The wavelength conversion element  20  is made of LiTaO 3  crystal having a polarization inversion period structure and is shaped like a rod having a length of, for example, 10 mm and a width and a thickness of, for example, 0.8 mm, respectively. The wavelength conversion element  20  converts a fundamental wave into a converted wave having a different wavelength from the fundamental wave. One end face  22  of the wavelength conversion element  20  in the longitudinal directions is formed with a fundamental-wave inlet  21  for incidence of the fundamental wave. Both end faces of the rod-shaped wavelength conversion element  20  in the longitudinal directions are formed, except for the fundamental-wave inlet  21 , with a fundamental-wave reflective coat for reflecting the fundamental wave. 
     The other end face  23  in the longitudinal directions without the fundamental-wave inlet  21  is formed with a fundamental-wave reflective coat for reflecting the fundamental wave and a converted-wave transmission coat for transmitting the converted wave as a face for outputting the converted wave. The end face  22  is formed with a converted-wave reflective coat for reflecting the converted wave. Hence, the wavelength conversion element  20  includes the output face of the converted wave only in the end face  23  in the longitudinal directions. 
     The fundamental-wave inlet  21  is shifted toward the lateral end from the center of the end face  22 , has a diameter of, for example, 90 μm and is formed with an AR coat for the fundamental wave. The one end face  22  with the fundamental-wave inlet  21  has a convex cylindrical shape bent in the lateral directions of  FIG. 4  while the other end face  23  has a convex spherical shape. The curvature radius of the end face  22  is, for example, 8 mm while the curvature radius of the end face  23  is, for example, 12 mm. 
     In the second embodiment, the end faces  22  and  23  of the wavelength conversion element  20  correspond to an example of the pair of fundamental-wave reflecting surfaces and the resin clad  14  corresponds to an example of the reflection portion. 
     A fundamental wave emitted from the fundamental-wave laser light source  1  is concentrated into the fundamental-wave inlet  21  by the condensing lens  2  and incident upon the wavelength conversion element  20 , goes ahead in the longitudinal direction of the wavelength conversion element  10  and undergoes a wavelength conversion, and is reflected by the end face  23  and advances again inside of the wavelength conversion element  20 . Through the process, a converted wave is obtained and emitted from the end face  23 . The end face  22  and the end face  23  function as a concave mirror for the fundamental wave, and the fundamental wave goes back and forth while forming a plurality of light-concentration points between the end face  22  and the end face  23 . The fundamental wave going back and forth crosses inside of the wavelength conversion element  10  and forms the plurality of light-concentration points at places different from a cross point. 
     The cylindrical surface forms the light-concentration points different in the beam-diameter directions, and the light-concentration points in the thickness directions of the wavelength conversion element  20  are formed near the end face  22 . The condensing lens  2  also forms a light-concentration point at a place different from a cross point. The converted wave is emitted as a plurality of beams from the end face  23  and can be handled as a luminous flux collected within the end face  23 . Further, the end face  23  functions as a convex lens for the converted wave and narrows the divergence angle of the converted wave. 
     In the second embodiment, the wavelength conversion element  20  includes the fundamental-wave reflecting surface on both sides in the longitudinal directions thereof, at least one fundamental-wave reflecting surface transmits the converted wave, the fundamental wave crosses inside of the wavelength conversion element  20 , and a light-concentration point is formed at a place different from a cross point. This makes it possible to enhance the conversion efficiency, simultaneously collect the converted wave emitted as a plurality of beams into one place to thereby reduce the light-source area thereof, and reduce the area necessary for the wavelength conversion element  20 . 
     In the second embodiment, it is preferable that the end faces of the wavelength conversion element  20  are the fundamental-wave reflecting surfaces and each have a convex shape. The end faces of the wavelength conversion element  20  have the convex fundamental-wave reflecting surfaces, thereby leading the fundamental wave going back and forth inside of the wavelength conversion element  20  to cross and form a light-concentration point inside of the wavelength conversion element  20 . In the second embodiment, the end faces of the wavelength conversion element  20  are the concave mirrors for the fundamental wave, thereby leading the fundamental wave to cross and concentrate. 
     In the wavelength converting laser  101 , preferably, one of the pair of fundamental-wave reflecting surfaces is a cylindrical surface and the other is a spherical surface. 
     At this time, preferably, the direction of the curvature of the cylindrical surface may coincide with the direction in which the fundamental-wave inlet  21  is formed with respect to the surface center thereof. In the second embodiment, the fundamental-wave inlet  21  is shifted laterally from the center of the end face  22  and thus the end face  22  is a cylindrical surface having a lateral curvature. The two end faces have the lateral curvatures, thereby leading the fundamental wave to pass several times and cross inside of the wavelength conversion element  20 . 
     Furthermore, only one of both end faces of the wavelength conversion element  20  is the cylindrical surface, thereby evading beam diffraction in the direction perpendicular to the direction from the curvature center of the end face  22  toward the position in which the fundamental-wave inlet  21  is formed, and preventing the beam diameter from widening while the fundamental wave goes back and forth between the pair of fundamental-wave reflecting surfaces. Particularly, the curvature radius of the spherical surface is greater than the wavelength-conversion element length, thereby becoming a stable resonance condition in the direction where the cylindrical lens has no lens power to keep the beam diameter constant even though the beam goes back and forth more times, so that the conversion efficiency becomes higher. 
     Moreover, one of both end faces of the wavelength conversion element  20  is designed as the cylindrical surface instead of the spherical surface, thereby reducing the number of coordination and working axes to cut down the laser production cost. Particularly, it is preferable that the total curvature radius of the cylindrical surface and the spherical surface is 1.8 to 2.2 times as long as the distance between the fundamental-wave reflecting surfaces. On this condition, the fundamental wave can go back and forth five or more times between the fundamental-wave reflecting surfaces even though not reflected by the side faces of the wavelength conversion element  20 . Unless the curvature radii of the cylindrical surface and the spherical surface meet the above condition, the fundamental wave may stop after going back and forth a couple of times between the fundamental-wave reflecting surfaces. 
       FIG. 6  is a schematic view showing a configuration of a multi-mode optical fiber  210  connected to the wavelength converting laser  101  of  FIGS. 5A and 5B . The multi-mode optical fiber  210  includes a core  211  having a diameter of, for example, 0.8 mm and made of pure quartz, and a clad  212  made of F-added quartz, and transmits a beam of light obtained from the wavelength converting laser  101 . The core  211  propagates the converted wave from the wavelength converting laser  101  and the clad  212  coats the core  211  and reflects the converted wave into the core  211 . 
     The wavelength conversion element  20  is connected directly to the core  211  and thereby the converted wave emitted from the end face  23  of the wavelength conversion element  20  is transmitted to the core  211 . The converted wave emitted from the wavelength conversion element  20  propagates through the core  211  while reflected by the core  211 . The connection surface of the core  211  of the multi-mode optical fiber  210  has a coating reflecting the fundamental wave and transmitting the converted wave. 
     The wavelength conversion element  20  is a rectangle having a thickness and a width of, for example, 0.8 mm, and emits the converted wave made up of a plurality of beams into a small area from the end face  23 . The end-face diameter of the wavelength conversion element  20  is substantially equal to the optical-fiber core diameter, thereby enabling the direct connection of the wavelength converting laser  101  and the multi-mode optical fiber  210 , though the converted wave is made up of the plurality of beams. The end face  23  has a convex shape to concentrate the converted wave, thereby enhancing the coupling efficiency to the multi-mode optical fiber  210 . 
     In the second embodiment, it is preferable that the end face  23  of the wavelength conversion element  20  is formed with a fundamental-wave reflecting surface reflecting the fundamental wave and transmitting the converted wave and is connected to the multi-mode optical fiber  210 . Although the wavelength converting laser  101  of the second embodiment outputs the plurality of converted-wave beams which can be difficult to handle, the plurality of converted-wave beams is emitted as a single luminous flux directly to the multi-mode optical fiber  210 , thereby easily transmitting the converted wave to various places. Besides, the wavelength conversion element  20  has a thickness and a width of 1 mm or below, thereby joining the plurality of converted-wave beams directly to the multi-mode optical fiber  210  having a core diameter making the bending easier. 
     Preferably, the end face  23  of the wavelength conversion element  20  may reflect the fundamental wave, transmit the converted wave and have a convex shape. In the wavelength converting laser  101  of the second embodiment, the thus configured end face  23  of the wavelength conversion element  20  leads the fundamental wave to go back and forth and cross inside of the wavelength conversion element  20  and form a light-concentration point at a plurality of places. In addition, the end face  23  of the wavelength conversion element  20  functions as a lens converging the plurality of outputted converted-wave beams, thereby enhancing the coupling efficiency to an optical part such as an optical fiber. Particularly, in the case where the wavelength converting laser  101  is directly joined to the multi-mode optical fiber  210 , since the end face  23  of the wavelength conversion element  20  is shaped like a convex, the coupling efficiency can be heightened even though there is an eccentricity. 
     In the second embodiment, it is preferable that the multi-mode optical fiber  210  is formed at an end face thereof with a coating reflecting the fundamental wave and transmitting the converted wave from the wavelength converting laser  101 . 
     In the case where the wavelength converting laser  101  is directly joined to the multi-mode optical fiber  210 , there can be the problem of separating the converted wave and the fundamental wave leaking from the end face  23  of the wavelength conversion element  20 . Taking this into account, the coating on the end face of the core  211  separates the fundamental wave from the wavelength converting laser  101  and the converted wave and thereby transfers only the converted wave. Further, the clad  212  prevents the fundamental wave leaking from the wavelength converting laser  101  from being outputted to the outside. 
     The core  211  and the clad  212  of the multi-mode optical fiber  210  can be made of quartz, as well as a flexible organic resin material, and the core  211  may be not only circular but also rectangular in section. 
     Third Embodiment 
       FIG. 7  is schematic view showing a configuration of a wavelength converting laser  102  according to a third embodiment of the present invention. In the third embodiment, component elements are given the same reference characters and numerals as those of the first and second embodiments, as long as the former are identical to the latter, and thus, their description is omitted. 
     The wavelength converting laser  102  includes a randomly-polarized fundamental-wave laser light source  39 , a condensing lens  2 , a wavelength conversion element  30  and a resin clad  14 . 
     The wavelength conversion element  30  is made of an MgO:LiNbO 3  crystal (PPMgLN) having a polarization inversion period structure and includes a first wavelength conversion element  35  and a second wavelength conversion element  36  which have a crystal axis perpendicular to each other and are joined together. In  FIG. 7 , the first wavelength conversion element  35  on the left side is made of PPMgLN↑ having a crystal z-axis in the upward direction of  FIG. 7  while the second wavelength conversion element  36  on the right side is made of PPMgLN← having a crystal z-axis in the depth direction of  FIG. 7 . The first wavelength conversion element  35  and the second wavelength conversion element  36  are in optical contact with each other. 
     The wavelength conversion element  30  is shaped like a cylinder having a length of, for example, 16 mm and a diameter of, for example, 1 mm. The wavelength conversion element  30  converts a fundamental wave into a converted wave having a different wavelength from the fundamental wave. One end face  32  of the wavelength conversion element  30  in the longitudinal directions is formed with a fundamental-wave inlet  31  for incidence of the fundamental wave. Both end faces  32  and  33  of the cylindrical wavelength conversion element  30  in the longitudinal directions are formed, except for the fundamental-wave inlet  31 , with a fundamental-wave reflective coat for reflecting the fundamental wave. 
     The end face  33  is formed with the fundamental-wave reflective coat and a converted-wave transmission coat for transmitting the converted wave as a face for outputting the converted wave. The fundamental-wave inlet  31  is near an arc of the cylindrical end face  32 , has a diameter of, for example, 100 μm and is formed with an AR coat for the fundamental wave. The end face  32  with the fundamental-wave inlet  31  has a plane shape while the other end face  33  in the longitudinal directions has a convex spherical shape. The curvature radius of the spherical end face  33  is, for example, 10 mm. 
     In the third embodiment, the end faces  32  and  33  of the wavelength conversion element  30  correspond to an example of the pair of fundamental-wave reflecting surfaces and the resin clad  14  corresponds to an example of the reflection portion. 
     The randomly-polarized fundamental-wave laser light source  39  emits a fundamental wave polarized at random. The fundamental wave emitted from the randomly-polarized fundamental-wave laser light source  39  is concentrated into the fundamental-wave inlet  31  by the condensing lens  2  and incident upon the wavelength conversion element  30  with inclined with respect to the axis of the cylindrical wavelength conversion element  30 . The incident fundamental wave goes ahead in the longitudinal direction of the wavelength conversion element  30 , and each polarization component thereof in the z-axis directions of PPMgLN undergoes a wavelength conversion in the first wavelength conversion element  35  and the second wavelength conversion element  36 , respectively. 
     The fundamental wave is reflected by the spherical end face  33 , thereafter reflected by the plane end face  32 , the end face  33  and the side surface of the wavelength conversion element  30  and goes back and forth in the longitudinal direction of the wavelength conversion element  30 . The fundamental wave is reflected by the spherical end face  33  and the side surface of the wavelength conversion element  30  and thereby crosses inside of the wavelength conversion element  30 . The spherical end face  33  functions as a concave mirror for the fundamental wave, and the fundamental wave going back and forth forms a plurality of light-concentration points other than cross points. 
     The end face  32  and the side surface of the wavelength conversion element  30  reflect the converted wave as well, and the converted wave subjected to a wavelength conversion is emitted from the end face  33 . The polarization direction of the fundamental wave changes through the reflection by the cylindrical side surface and the end face  33  of the wavelength conversion element  30 . The wavelength conversion element  30  is formed by the two non-linear materials (first wavelength conversion element  35  and second wavelength conversion element  36 ) which have a crystal axis perpendicular to each other and thereby conducts a wavelength conversion regardless of the polarization direction. Besides, the wavelength conversion element  30  can convert the wavelength of the fundamental wave even if the polarization direction thereof changes while going back and forth between the fundamental-wave reflecting surfaces. 
     In the third embodiment, it is preferable that the wavelength conversion element  30  is formed by the two sections (first wavelength conversion element  35  and second wavelength conversion element  36 ) which have a crystal axis perpendicular to each other. The wavelength conversion element has the pair of fundamental-wave reflecting surfaces, the fundamental wave passes several times inside of the wavelength conversion element, and the polarization direction of the fundamental wave can be changed as it passes repeatedly. In the third embodiment, however, the fundamental wave can be certainly converted, though the polarization direction thereof changes while going back and forth between the fundamental-wave reflecting surfaces. 
     The configuration according to the third embodiment utilizing reflections by the curved surfaces is especially effective because the polarization is occasionally changed. 
     Further, in the case of a fundamental-wave laser light source emitting a beam of light polarized at random, the first wavelength conversion element  35  and the second wavelength conversion element  36  having a crystal axis perpendicular to each other are indispensable for enhancing the conversion efficiency. 
     Fourth Embodiment 
       FIG. 8  is schematic top view showing a configuration of a wavelength converting laser  103  according to a fourth embodiment of the present invention. In the fourth embodiment, component elements are given the same reference characters and numerals as those of the first to third embodiments, as long as the former are identical to the latter, and thus, their description is omitted. 
     The wavelength converting laser  103  includes a fundamental-wave laser light source  1 , a condensing lens  2  and a wavelength conversion element  40 . 
     The wavelength conversion element  40  is made of an MgO:LiNbO 3  crystal having a polarization inversion period structure and is shaped like a rod having a length of, for example, 10 mm and a width and a thickness of, for example, 0.8 mm, respectively. The wavelength conversion element  40  includes two kinds of wavelength conversion elements (first wavelength conversion element  45  and second wavelength conversion element  46 ) which have a polarization inversion period different from each other. The polarization inversion period of the first wavelength conversion element  45  having an end face  42  is a double-wave generation period for generating a double wave and the polarization inversion period of the second wavelength conversion element  46  having an end face  43  is a triple-wave generation period for generating a triple wave. The polarization inversion period of the first wavelength conversion element  45  is designed so as to come into a quasi-phase matching condition for generating a double wave of the fundamental wave. The polarization inversion period of the second wavelength conversion element  46  is designed so as to come into a quasi-phase matching condition for generating a triple wave equivalent to the sum frequency of the fundamental wave and the double wave. 
     The wavelength conversion element  40  converts the fundamental wave into a converted wave (double wave and triple wave) having a different wavelength from the fundamental wave. The end face  42  of the wavelength conversion element  40  in the longitudinal directions is formed with a fundamental-wave inlet  21  for incidence of the fundamental wave. 
     The end face  42  of the rod-shaped wavelength conversion element  40  in the longitudinal directions is formed with a reflective coat for reflecting the fundamental wave and the double wave. The end face  43  is formed with a reflective coat for reflecting the fundamental wave and a transmission coat for transmitting the double wave and the triple wave as a face for outputting the double wave and the triple wave as the converted wave. The fundamental-wave inlet  21  is shifted toward the lateral end from the center of the end face  42 , has a diameter of, for example, 90 μm and is formed with an AR coat for the fundamental wave. The shapes of the end face  42  and the end face  43  are the same as the end face  22  and the end face  23  according to the second embodiment. 
     The fundamental wave goes back and forth inside of the wavelength conversion element  40  in the same way as the second embodiment, crosses inside of the wavelength conversion element  40  and forms a plurality of light-concentration points at places different from a cross point of the fundamental wave. 
     The wavelength converting laser  103  is a wavelength converting laser outputting the double wave and the triple wave. The fundamental wave incident upon the fundamental-wave inlet  21  goes ahead in the longitudinal direction of the wavelength conversion element  40 . The fundamental wave advancing through the first wavelength conversion element  45  is converted into a double wave, and the double wave obtained in the first wavelength conversion element  45  is accompanied by the fundamental wave, goes inside of the first wavelength conversion element  45  and is incident upon the second wavelength conversion element  46 . The fundamental wave and the double wave incident upon the second wavelength conversion element  46  is converted into a triple wave, and the thus obtained double wave and triple wave are outputted from the end face  43 . The fundamental wave is reflected by the spherical end face  43  goes ahead again inside of the wavelength conversion element  40 . 
     The end face  42  and the end face  43  works as a concave mirror for the fundamental wave. The fundamental wave goes back and forth between the end face  42  and the end face  43  while forming a plurality of light-concentration points, and the fundamental wave going back and forth crosses inside of the wavelength conversion element  40 , and however, also forms a plurality of light-concentration points at places different from a cross point. A double wave is generated when the fundamental wave goes ahead inside of the first wavelength conversion element  45 , and a triple wave is generated when the fundamental wave together with the generated double wave goes through the second wavelength conversion element  46 . The fundamental wave passes several times inside of the wavelength conversion element  40  to thereby generate the double wave and the triple wave repeatedly. 
     In the fourth embodiment, the end faces  42  and  43  of the wavelength conversion element  40  correspond to an example of the pair of fundamental-wave reflecting surfaces, and in the fourth embodiment, the side surface of the wavelength conversion element  40  may be coated with a resin clad. 
     In the fourth embodiment, it is preferable that a plurality of wavelength conversion elements having a mutually different phase matching period generate higher-order converted waves while the fundamental wave goes back and forth between the fundamental-wave reflecting surfaces. A conventional wavelength conversion into higher-order converted waves (such as triple to five-times waves) is extremely inefficient and requires a complex configuration. 
     In contrast, the wavelength conversion element  40  according to the fourth embodiment is capable of generating higher-order converted waves efficiently by generating a higher-order converted wave using a quasi-phase matching period when the fundamental wave and the converted wave make several passes inside thereof. Particularly, in the wavelength conversion element  40  according to the fourth embodiment, light-concentration points are dispersed to thereby disperse places where higher-order converted waves are generated, so that the higher-order converted waves can be prevented from causing optical absorption to thereby deteriorate the conversion efficiency and damage the wavelength conversion element  40 . 
     In the fourth embodiment, the spherical end face  43  transmits the double wave and the triple wave, however it may be formed with a reflective coat reflecting the double wave in such a way that only the triple wave is transmitted. 
     The wavelength conversion element  40  leads the double wave to go back and forth between the pair of reflecting surfaces, thereby raising the power of the double wave and improving the efficiency of conversion into the triple wave. 
     Fifth Embodiment 
       FIG. 9  is schematic top view showing a configuration of a wavelength converting laser  104  according to a fifth embodiment of the present invention. In the fifth embodiment, component elements are given the same reference characters and numerals as those of the first to fourth embodiments, as long as the former are identical to the latter, and thus, their description is omitted. 
     The wavelength converting laser  104  includes a fundamental-wave laser light source  1 , a wavelength conversion element  50 , a concave mirror  53  and a collimating lens  54 . 
     The wavelength conversion element  50  is made of an MgO:LiNbO 3  crystal having a polarization inversion period structure and is shaped like a rectangular parallelepiped having a length of, for example, 10 mm, a width of, for example, 2 mm and a thickness of, for example, 1 mm. One end face  52  of the wavelength conversion element  50  is formed with a reflective coat for reflecting the fundamental wave and the converted wave and the other end face  51  in the longitudinal directions of the wavelength conversion element  50  is formed with a transmission coat for transmitting the fundamental wave and the converted wave. The concave mirror  53  is a spherical mirror having a curvature radius of 10 mm and is formed with a reflective coat reflecting the fundamental wave and a transmission coat for transmitting the converted wave. The concave mirror  53  is an output mirror for outputting the converted wave, and the end face  52  and the concave mirror  53  constitute a pair of fundamental-wave reflecting surfaces in the longitudinal directions of the wavelength conversion element  50 . 
     A fundamental wave emitted from the fundamental-wave laser light source  1  is collimated by the collimating lens  54 , thereafter reflected by the concave mirror  53  and incident upon the wavelength conversion element  50 . The incident fundamental wave is reflected by the end face  52 , the side faces of the wavelength conversion element  50  and the concave mirror  53  and passes a plurality of times inside of the wavelength conversion element  50 . The fundamental wave passing inside of the wavelength conversion element  50  is converted into a converted wave and the obtained converted wave is outputted from the concave mirror  53 . The concave mirror  53  with the above curvature concentrates the fundamental wave going back and forth between the reflecting surfaces to form a light-concentration point. Further, the fundamental wave is reflected by the side faces in the width directions of the wavelength conversion element  50  and thereby crosses inside of the wavelength conversion element  50 . 
     In the fifth embodiment, the end face  52  of the wavelength conversion element  50  and the concave mirror  53  correspond to an example of the pair of fundamental-wave reflecting surfaces, and in the fifth embodiment, the side faces of the wavelength conversion element  50  may be coated with a resin clad. 
     In the fifth embodiment, using reflection by the concave mirror  53  and reflection by the side faces of the wavelength conversion element  50 , the fundamental wave crosses inside of the wavelength conversion element  50  and forms a plurality of light-concentration points at places different from a cross point. This makes it possible to obtain a higher conversion efficiency while dispersing places where the power densities of the fundamental wave and the converted wave become higher and collect sections for emitting a plurality of beams into a single small section. 
     In the wavelength conversion element  50 , a plurality of light-concentration points are formed near the end face  52  which is a reflecting surface with no curvature. The reflective coat of the end face  52  for reflecting the fundamental wave and the converted wave is formed by a laminated dielectric film in nine layers of MgF 2  and TiO 2  from the side of the wavelength conversion element  50  and a metal film made of aluminum and having a thickness of 200 nm evaporated onto the laminated dielectric film. 
     In the fifth embodiment, it is preferable that at least one of the pair of fundamental-wave reflecting surfaces includes a reflective film for reflecting the fundamental wave and the converted wave, the plurality of light-concentration points are formed near the reflective film, and the reflective film includes a metal film having a thickness of 100 nm or above. In the wavelength conversion element  50 , the plurality of light-concentration points are formed near the end face  52 , and the end face  52  has the reflective coat includes a metal film having a thickness of 100 nm or above which reflects the fundamental wave and the converted wave. The light-concentration points cause intense optical absorption and local heat generation, and the metal film near the light-concentration points functions as a heat transfer route and thereby suppresses a local rise in the temperature of the wavelength conversion element  50 . 
     Accordingly, the reflective film with the metal film is useful for avoiding element destruction and fall in the conversion efficiency which can be caused when the temperature of the wavelength conversion element  50  goes up. 
     The metal film functions as a heat transfer route and thus requires a thickness of 100 nm or above. Preferably, the metal film may be directly connected to a metal heat sink, thereby securing a heat transfer route. 
     Sixth Embodiment 
       FIG. 10A  is schematic top view showing a configuration of a wavelength converting laser  105  according to a sixth embodiment of the present invention and  FIG. 10B  is schematic side view showing a configuration of the wavelength converting laser  105  according to the sixth embodiment of the present invention. In the sixth embodiment, component elements are given the same reference characters and numerals as those of the first to fifth embodiments, as long as the former are identical to the latter, and thus, their description is omitted. 
     The wavelength converting laser  105  includes a fundamental-wave laser light source  1 , a condensing lens  2 , a wavelength conversion element  60 , a cylindrical mirror  62  and a concave mirror  63 . 
     The wavelength conversion element  60  is made of an MgO:LiNbO 3  crystal having a polarization inversion period structure and is shaped like a rectangular parallelepiped having a length of, for example, 25 mm, a width of, for example, 4 mm and a thickness of, for example, 1 mm. Both end faces in the longitudinal directions of the wavelength conversion element  60  is formed with an AR coat for the fundamental wave and the converted wave. 
     The wavelength conversion element  60  converts the fundamental wave into a converted wave having a different wavelength from the fundamental wave. One end face of the wavelength conversion element  60  in the longitudinal directions is formed with a fundamental-wave inlet  61  for incidence of the fundamental wave. 
     The cylindrical mirror  62  partly cut so as to correspond to the position of the fundamental-wave inlet  61  of the wavelength conversion element  60  is arranged near the end face in the longitudinal directions of the wavelength conversion element  60  on the side of the fundamental-wave laser light source  1 . The cylindrical mirror  62  has a reflective coat for reflecting the fundamental wave and the converted wave and has a curvature in the width directions of the wavelength conversion element  60  whose curvature radius is, for example, 20 mm. In order for the fundamental wave to be incident upon the fundamental-wave inlet  61  located at the end of the wavelength conversion element  60  in the width directions, the section of the cylindrical mirror  62  corresponding to the incidence optical path of the fundamental wave is cut off. 
     On the other hand, the spherical concave mirror  63  is arranged near the other end face in the longitudinal directions of the wavelength conversion element  60 . The concave mirror  63  has a curvature radius of, for example, 22 mm and has a reflective coat for reflecting the fundamental wave and a transmission coat for transmitting the converted wave. The concave mirror  63  is an output mirror for outputting the converted wave, and the cylindrical mirror  62  and the concave mirror  63  constitute a pair of fundamental-wave reflecting surfaces. The distance between the fundamental-wave reflecting surfaces is approximately 21 mm in air-reduced length. 
     A fundamental wave emitted from the fundamental-wave laser light source  1  is concentrated by the condensing lens  2 , incident from the fundamental-wave inlet  61  upon the wavelength conversion element  60 , concentrated inside of the wavelength conversion element  60 , thereafter reflected by the concave mirror  63  and again incident upon the wavelength conversion element  60 . The fundamental wave which has passed through the wavelength conversion element  60  is reflected by the cylindrical mirror  62  and again incident upon the wavelength conversion element  60 . The fundamental wave goes back and forth a plurality of times between the cylindrical mirror  62  and the concave mirror  63  and is converted into a converted wave when passing through the wavelength conversion element  60 , and the converted wave is outputted from the concave mirror  63 . 
     The concave mirror  63  and the cylindrical mirror  62  refract the fundamental wave and lead it to cross inside of the wavelength conversion element  60 , and the condensing lens  2 , the concave mirror  63  and the cylindrical mirror  62  allows it to form a plurality of light-concentration points. 
     The cylindrical mirror  62  causes the fundamental wave to form the light-concentration points different from each other in the beam-diameter directions. At this time, the beam diameter in the thickness directions of the wavelength conversion element  60  becomes a stable resonance condition, thereby keeping the beam diameter constant even though the beam goes back and forth repeatedly. The condensing lens  2 , the concave mirror  63  and the cylindrical mirror  62  lead the fundamental wave to form the plurality of light-concentration points at places different from a cross point of the fundamental wave. 
     In the sixth embodiment, the cylindrical mirror  62  and the concave mirror  63  correspond to an example of the pair of fundamental-wave reflecting surfaces, and in the sixth embodiment, the side faces of the wavelength conversion element  60  may be coated with a resin clad. 
     In the sixth embodiment, the fundamental wave passes several times through the wavelength conversion element  60 , crosses inside of the wavelength conversion element  60  and forms the plurality of light-concentration points at places different from a cross point. This makes it possible to obtain a higher conversion efficiency while dispersing places where the power densities of the fundamental wave and the converted wave become higher and collect sections for emitting a plurality of beams into a single small section. 
     In the sixth embodiment, it is preferable that one of the pair of fundamental-wave reflecting surfaces is a cylindrical surface and the other is a spherical surface. 
     Since the one fundamental-wave reflecting surface is a cylindrical surface, both fundamental-wave reflecting surfaces are capable of concentrating light and the different light-concentration points in the beam-diameter directions are formed, thereby dispersing places where the power densities of the fundamental wave and the converted wave become higher. 
     Further, since the cylindrical surface is employed, the beam diameter in the one direction becomes a stable resonance condition, thereby preventing the beam diameter from widening because of diffraction when the fundamental wave goes back and forth. This makes it possible to suppress an increase in the beam diameter and thereby a decline in the conversion efficiency as the fundamental wave goes back and forth more times. 
     Seventh Embodiment 
       FIG. 11A  is schematic top view showing a configuration of a wavelength converting laser  106  according to a seventh embodiment of the present invention and  FIG. 11B  is schematic side view showing a configuration of the wavelength converting laser  106  according to a seventh embodiment of the present invention. In the seventh embodiment, component elements are given the same reference characters and numerals as those of the first to sixth embodiments, as long as the former are identical to the latter, and thus, their description is omitted. 
     The wavelength converting laser  106  includes a fundamental-wave laser light source  1 , a condensing lens  2 , a wavelength conversion element  60 , a cylindrical mirror  62  and a concave mirror  73 . 
     The wavelength converting laser  106  is configured by the same component elements as the wavelength converting laser  105  according to the sixth embodiment, except for the concave mirror  73 . The concave mirror  73  includes a converted-wave transmission portion (transmission region)  74  formed only within a diameter of 1 mm in the middle thereof and having a coat for reflecting the fundamental wave and transmitting the converted wave, and a converted-wave reflection portion (reflection region)  75  formed in the periphery part of the converted-wave transmission portion  74  and having a coat for reflecting both the fundamental wave and the converted wave. The converted wave generated when the fundamental wave passes inside of the wavelength conversion element  60  is outputted outside only from the converted-wave transmission portion  74 . 
     In the seventh embodiment, the cylindrical mirror  62  and the concave mirror  73  correspond to an example of the pair of fundamental-wave reflecting surfaces, and in the seventh embodiment, the side faces of the wavelength conversion element  60  may be coated with a resin clad. 
     In the seventh embodiment, it is preferable that the section of a fundamental-wave reflecting surface which transmits the converted wave is only one region of the fundamental-wave reflecting surface, and the fundamental wave and the converted wave are reflected in the other region. 
     In the seventh embodiment, the fundamental-wave reflecting surfaces reflect the converted wave to thereby incline the optical path thereof, and the converted wave undergoes a change in the optical path every time it is reflected. The transmission section transmitting the converted wave is the single region of the fundamental-wave reflecting surface, thereby outputting the converted wave only when reaching the transmission section. Since the converted wave is emitted only from the transmission region, a plurality of converted-wave beams are emitted from the limited transmission region, thereby significantly reducing the area of the converted-wave emission region, so that a plurality of converted-wave beams can be handled as a single fine luminous flux. 
     Eighth Embodiment 
       FIG. 12A  is schematic top view showing a configuration of a wavelength converting laser  107  according to an eighth embodiment of the present invention and  FIG. 12B  is schematic side view showing a configuration of the wavelength converting laser  107  according to an eighth embodiment of the present invention. In the eighth embodiment, component elements are given the same reference characters and numerals as those of the first to seventh embodiments, as long as the former are identical to the latter, and thus, their description is omitted. 
     The wavelength converting laser  107  includes a fundamental-wave laser light source  1 , a condensing lens  2  and a wavelength conversion element  80 . 
     The wavelength conversion element  80  is made of an MgO:LiTaO 3  crystal having a polarization inversion period structure and is shaped like a pillar in which the area of an end face  82  for incidence of the fundamental wave is larger than the area of an end face  83  for emission of the converted wave on the opposite side and the side faces have a trapezoidal shape in section. The wavelength conversion element  80  has a length of, for example, 10 mm, the end face  82  is shaped like a rectangle having a width of, for example, 4 mm and a thickness of, for example, 2 mm and the end face  83  is shaped like a rectangle having a width of, for example, 1 mm and a thickness of, for example, 0.75 mm. 
     The end face  82  is a convex spherical surface, has a curvature radius of, for example, 24 mm and is formed, except for a fundamental-wave inlet  81 , with a reflective coat for reflecting the fundamental wave and the converted wave. The end face  83  is a plane surface and is formed with a reflective coat for reflecting the fundamental wave and a transmission coat for transmitting the converted wave. The side faces of the wavelength conversion element  80  reflect the fundamental wave and the converted wave totally. The fundamental-wave inlet  81  is formed with a transmission coat for transmitting the fundamental wave, has a diameter of, for example, 200 μm and is shifted widthwise, for example, by 1.2 mm from the center of the end face  82 . The spherical end face  82  and the plane end face  83  in the longitudinal directions of the wavelength conversion element  80  are a pair of fundamental-wave reflecting surfaces. The converted wave is emitted with a plurality of beams thereof overlapping each other from the end face  83 . 
     A fundamental wave emitted from the fundamental-wave laser light source  1  is concentrated into the fundamental-wave inlet  81  by the condensing lens  2  and incident upon the wavelength conversion element  80 , goes ahead in the longitudinal direction of the wavelength conversion element  80 , is reflected by the side faces, the end face  83  and the end face  82 , and thereby goes back and forth between the end face  82  and the end face  83 . The fundamental wave going back and forth crosses at several places, and the capabilities of the condensing lens  2  and the spherical end face  82  to concentrate light lead the fundamental wave to form a plurality of light-concentration points. 
     At this time, the wavelength conversion element  80  forms a plurality of light-concentration points at places different from a cross point of the fundamental wave and generates a converted wave from the fundamental wave going ahead inside thereof. A plurality of converted-wave beams are outputted with overlapping each other from the plane end face  83 . Since the area of the end face  83  on one side for the output is smaller than the area of the end face  82  on the other side, a large number of converted-wave beams are emitted from the end face  83  after reflected by the side faces of the wavelength conversion element  80 . The thus outputted converted wave has a uniform intensity distribution. 
     In the eighth embodiment, the end faces  82  and  83  of the wavelength conversion element  80  correspond to an example of the pair of fundamental-wave reflecting surfaces, and in the eighth embodiment, the side faces of the wavelength conversion element  80  may be coated with a resin clad. 
     In the eighth embodiment, it is preferable that the end face  83  on one side of the wavelength conversion element  80  is formed with the coats for reflecting the fundamental wave and for transmitting the converted wave, and the area of the end face  83  on one side is smaller than the area of the end face  82  on the other side. Since the area of the end face  83  for emission of the converted wave is smaller than the area of the end face  82  for incidence of the fundamental wave, the converted wave is outputted with a plurality of beams thereof overlapping each other when emitted. The outputted converted-wave beams are superimposed on each other, thereby unifying the intensity distribution to enable the wavelength converting laser  107  to serve directly in the field of machining, illumination or the like. Besides, the smaller converted-wave emission area is useful in miniaturizing an optical part employed for the converted wave. 
       FIG. 13  is schematic view showing a configuration of an image display  200  including the wavelength converting laser  107  of  FIGS. 12A and 12B . The image display  200  includes the wavelength converting laser  107 , an image-casting optical system  85 , a spatial modulation element  86 , a projection optical system  87  and a display surface  88 . 
     The converted wave emitted from the end face  83  of the wavelength converting laser  107  is rectangular and has a uniform intensity distribution. The image-casting optical system  85  enlarges and projects the converted wave emitted from the end face  83  onto the spatial modulation element  86 . The spatial modulation element  86  has a rectangular shape analogous to the end face  83  having a width-height ratio of 4:3. The spatial modulation element  86  is formed, for example, by a transmission-type liquid crystal and a deflecting plate, modulates a laser beam of each color and emits the laser beam modulated into two dimensions. The projection optical system  87  projects the laser beam modulated by the spatial modulation element  86  onto the display surface  88 . 
     In the eighth embodiment, it is preferable that an image of the end face  83  transmitting the converted wave of both end faces of the wavelength conversion element  80  in the wavelength converting laser  107  is projected on the spatial modulation element  86  modulating the converted wave. 
     In the eighth embodiment, the converted wave made up of a plurality of beams is shaped according to the shape of the end face  83  of the wavelength conversion element  80  in the wavelength converting laser  107 , and the plurality of converted-wave beams overlaps each other, thereby unifying the intensity distribution. In accordance with the characteristics of the wavelength converting laser  107 , the image of the end face  83  of the wavelength conversion element  80  is projected on the spatial modulation element  86 , thereby making the converted wave efficiently usable. Since there is no need to provide any optical part for beam shaping, a loss caused by beam shaping can be suppressed and the number of necessary optical parts reduced. The image-casting optical system  85  may be further provided, in addition to a lens, with a diffusion plate for adjusting the intensity distribution or the like. 
     Preferably, the image display  200  may include the wavelength converting laser and a modulation element modulating the converted wave emitted from the wavelength converting laser. The wavelength converting laser emits a plurality of wavelength-converted beams within a specified angle from end face of a small area, thereby leading the converted wave extremely efficiently to the modulation element. 
     This makes it possible to realize an image display capable of utilizing light efficiently and thereby reduce the power consumption of the whole image display  200 . Particularly, it can be effectively used as an image display making a display having a width across-corner of 30 inches or above whose electric power is mostly consumed by a light source thereof. 
     In addition to a spatial modulation element such as a transmission-type or reflection-type liquid-crystal element, the modulation element includes an element such as a scanning mirror which scans a beam of light to thereby modulate a place where the beam is to be displayed. 
     The image display  200  can be applied to a projector, a liquid-crystal display, a head-up display and the like. 
     Furthermore, the image display  200  is provided with the wavelength converting laser  107  according to the eighth embodiment, but the present invention is not limited especially to this, and thus, the wavelength converting laser  107  may be replaced with the wavelength converting lasers  100  to  106  according to the first to seventh embodiments and wavelength converting lasers  108  and  109  according to ninth and tenth embodiments of the present invention described later. 
     Ninth Embodiment 
       FIG. 14  is schematic view showing a configuration of a wavelength converting laser  108  according to a ninth embodiment of the present invention. In the ninth embodiment, component elements are given the same reference characters and numerals as those of the first to eighth embodiments, as long as the former are identical to the latter, and thus, their description is omitted. 
     The wavelength converting laser  108  includes a fundamental-wave laser light source  1 , a condensing lens  2 , a wavelength conversion element  10 , a resin clad  14  and a vibration mechanism  91 . 
     The wavelength converting laser  108  is configured by attaching the vibration mechanism  91  operating the wavelength conversion element  10  during the emission of a laser beam to the wavelength converting laser  100  according to the first embodiment. The vibration mechanism  91  turns and vibrates the wavelength conversion element  10  in lateral directions Y 1  around a turning axis R 1  intersecting the incidence direction of a fundamental wave upon a fundamental-wave inlet  11 . The vibration mechanism  91  is attached to the resin clad  14 , formed by, for example, an electro-magnetic coil and swings an end face  13  emitting a converted wave at a wavelength of 0.2 mm and a frequency of 200 Hz. 
     The wavelength conversion element  10  generates the converted wave from the fundamental wave going ahead inside thereof, and the quantity of the converted wave generated through a one-way optical path between fundamental-wave reflecting surfaces is determined based on the beam intensity and the gap from a phase matching condition. The wavelength conversion element  10  moves slightly, thereby varying the angle of each optical path of the fundamental wave as time elapses to change the gap from a phase matching condition. 
     A plurality of converted-wave beams generated through each optical path are superimposed on each other and emitted from the emission end face  13 . 
     The intensity distribution of the emitted converted wave varies as time passes because of variation in the quantity of the converted wave generated through each optical path, thereby changing the interference condition of the emitted converted wave as well along with the elapse of time. This means that the interference pattern changes as time passes, and thus, a time integral is executed to thereby unify and reduce the interference noise, particularly, a speckle noise caused in the field of display and illumination. Although the converted-wave intensity distribution changes, each optical path is related so as to compensate for a conversion efficiency, thereby evading a significant variation in the total output of the converted wave. 
     In the ninth embodiment, it is preferable that the wavelength conversion element  10  is vibrated during emission of the converted wave. The wavelength conversion element  10  moves slightly during the emission, thereby reducing the interference noise of the outputted converted wave. In the ninth embodiment, although the converted wave made up of a plurality of beams generated through each optical path are superimposed and outputted, the converted-wave intensity distribution is changed as time elapses, thereby reducing the interference noise. In the ninth embodiment, each fundamental-wave optical path compensates a decline in the conversion efficiency, thereby evading a sharp variation in the total output of the converted wave, though the intensity distribution thereof varies. 
     Tenth Embodiment 
       FIG. 15  is a schematic view showing an exterior shape of a wavelength conversion element  110  according to a tenth embodiment of the present invention.  FIG. 16A  is a schematic top view showing a configuration of a wavelength converting laser  109  according to the tenth embodiment of the present invention and  FIG. 16B  is a schematic side view showing a configuration of the wavelength converting laser  109  according to the tenth embodiment of the present invention. In the tenth embodiment, component elements are given the same reference characters and numerals as those of the first to ninth embodiments, as long as the former are identical to the latter, and thus, their description is omitted. 
     The wavelength converting laser  109  includes a fundamental-wave laser light source  1 , the wavelength conversion element  110 , a resin clad  114 , a metal holder  115 , and a condensing lens  117 . The wavelength conversion element  110  converts a fundamental wave into a converted wave having a different wavelength from the fundamental wave. 
     One end face  112  of the wavelength conversion element  110  in the longitudinal directions is formed with a fundamental-wave inlet  111  for incidence of the fundamental wave. 
     The wavelength conversion element  110  is made of MgO:LiNbO 3  crystal having a polarization inversion period structure and is shaped like a flat plate having a length of, for example, 10 mm, a width of, for example, 5 mm and a thickness of, for example, 20 μm. The wavelength conversion element  110  is covered in the thickness directions with the resin clad  114  and functions as a multi-mode slab optical waveguide. Both end faces of the wavelength conversion element  110  in the longitudinal directions are formed, except for the fundamental-wave inlet  111 , with a reflective coat for reflecting the fundamental wave. 
     The other end face  113  without the fundamental-wave inlet  111  is formed with a reflective coat for reflecting the fundamental wave and a transmission coat for transmitting the converted wave as a face for outputting the converted wave. The end face  112  for incidence of the fundamental wave is formed with a reflective coat for reflecting the converted wave. Hence, the wavelength converting laser  109  includes the output face only in the end face  23 . The fundamental-wave inlet  111  is shifted laterally from the center of the end face  112  having a plane shape, has a size of, for example, 100 μm×20 μm and is formed with an AR coat for the fundamental wave. 
     The one end face  112  with the fundamental-wave inlet  111  has a plane shape while the other end face  113  has a convex cylindrical shape bent in the lateral directions of  FIG. 15  and a curvature radius of, for example, 200 mm. The wavelength conversion element  110  is fixed via the resin clad  114  on the metal holder  115  and radiates heat through the metal holder  115 . The condensing lens  117  concentrates a beam of light in such a way that the beam is incident upon the fundamental-wave inlet  111 . 
     The wavelength conversion element  110  as the slab optical waveguide guides the fundamental wave, and leads the fundamental wave to reflect at the end face  112  and the end face  113 , go back and forth repeatedly and change the optical path, and form a light-concentration point and cross. 
     The converted wave converted from the fundamental wave inside of the wavelength conversion element  110  is emitted from the end face  113 . 
     In the tenth embodiment, the end faces  112  and  113  of the wavelength conversion element  110  correspond to an example of the pair of fundamental-wave reflecting surfaces. 
     In the wavelength converting laser  109 , preferably, the wavelength conversion element  110  may be a slab optical waveguide reflecting the fundamental wave and the converted wave totally at the side faces thereof. In the tenth embodiment, specifically, it is preferable that the wavelength conversion element  110  is shaped like a flat plate having a predetermined thickness, and the resin clad  114  is arranged on two faces having the largest area and facing each other in the flat plate wavelength conversion element  110 . The fact that the wavelength conversion element  110  is a slab optical waveguide makes it possible to keep a fundamental-wave beam from spreading in the thickness directions, thereby maintaining the light intensity at a high level even if the fundamental wave reflects repeatedly inside of the wavelength conversion element  110 . 
     Therefore, the wavelength conversion efficiency can be enhanced for any optical paths of the fundamental wave. 
     Particularly, in the tenth embodiment, preferably, the wavelength conversion element  110  may have the function of a multi-mode slab optical waveguide. In the tenth embodiment, most of the fundamental wave incident upon the wavelength conversion element  110  is converted while being repeatedly reflected, and hence, it is important to heighten the beam coupling efficiency of the wavelength conversion element  110  and thereby equip the wavelength conversion element  110  with the multi-mode optical waveguide function capable of easily improving the beam coupling efficiency. Further, the multi-mode optical waveguide function is useful in expanding the allowable temperature range of the wavelength conversion element  110  because of the difference in phase matching condition according to the mode. 
     The resin clad  114  between the wavelength conversion element  110  and the metal holder  115  has a thickness of, for example, 5 μm, and preferably, 10 μm or below. The thinner the resin clad  114  becomes, the lower the thermal resistance becomes and the more heat generated from the wavelength conversion element  110  the metal holder  115  can radiate. Particularly, if the fundamental wave and the converted wave have a high power, the heat of the wavelength conversion element  110  can be more effectively radiated. If the allowable temperature range of the wavelength conversion element  110  is wide, there is no need to control the temperature especially using a Peltier element or the like, and hence, the radiation mechanism of the metal holder  115  is enough. 
     The present invention is not limited to the above first to tenth embodiments, variations can be suitably expected without departing from the scope of the present invention. 
     It is a matter of course that a combination can be employed of each first to tenth embodiment according to the present invention. 
     In the first to tenth embodiments, a part of light-concentration points of fundamental wave formed inside of the wavelength conversion element may overlap a cross point of the fundamental wave. As far as most of the light-concentration points of the fundamental wave do not coincide with the cross point of the fundamental wave, any arrangement may be used. 
     Herein, the above specific embodiments mainly include inventions having configurations as follows. 
     A wavelength converting laser according to an aspect of the present invention includes: a light source emitting a fundamental wave; and a wavelength conversion element converting the fundamental wave emitted from the light source into a converted wave having a different wavelength from the fundamental wave, in which: a pair of fundamental-wave reflecting surfaces is arranged on both end sides of the wavelength conversion element in the directions of an optical axis thereof and reflects the fundamental wave to thereby pass the fundamental wave a plurality of times inside of the wavelength conversion element, and at least one of the fundamental-wave reflecting surfaces transmits the converted wave; and the pair of fundamental-wave reflecting surfaces allows the fundamental wave to cross inside of the wavelength conversion element and form a plurality of light-concentration points at places different from a cross point of the fundamental wave. 
     According to this configuration, the pair of fundamental-wave reflecting surfaces allows the fundamental wave to pass a plurality of times inside of the wavelength conversion element, cross inside of the wavelength conversion element and form a plurality of light-concentration points at places different from a cross point of the fundamental wave. 
     Therefore, the fundamental wave passes a plurality of times inside of the wavelength conversion element and forms a plurality of light-concentration points at places different from a cross point of the fundamental wave, thereby making it possible to obtain a high conversion efficiency stably and reduce the light-source area of a converted wave emitted as a plurality of beams, resulting in the whole apparatus being smaller. 
     In the above wavelength converting laser, it is preferable that the side faces of the wavelength conversion element reflect the fundamental wave into the wavelength conversion element. 
     According to this configuration, the side faces of the wavelength conversion element reflect the fundamental wave into the wavelength conversion element. This makes it possible to keep the area within a specified range which the fundamental wave passes inside of the wavelength conversion element through and unify the intensity distribution of the fundamental wave passing through the wavelength conversion element to thereby disperse the places having higher fundamental-wave power densities. 
     Furthermore, preferably, the above wavelength converting laser may further include a reflection portion made of a material having a refractive index lower than the wavelength conversion element and coating the side faces of the wavelength conversion element. 
     According to this configuration, the side faces of the wavelength conversion element are coated with a reflection portion made of a material having a refractive index lower than the wavelength conversion element. Therefore, the fundamental wave and the converted wave can be totally reflected by the side faces of the wavelength conversion element and thereby returned inside of the wavelength conversion element. 
     Moreover, preferably, the above wavelength converting laser may further include a temperature regulator regulating the temperature of the wavelength conversion element via the reflection portion. 
     According to this configuration, the temperature of the wavelength conversion element can be regulated via the reflection portion, thereby preventing the fundamental wave and the converted wave from being absorbed into the temperature regulator and hence executing precise temperature control. 
     In addition, in the above wavelength converting laser, it is preferable that: the wavelength conversion element has a rectangular shape in a section crossing the optical axis thereof; and the direction of a polarization of the fundamental wave is parallel to a side of the section. 
     According to this configuration, the side faces of the wavelength conversion element reflecting the fundamental wave are parallel or perpendicular to the polarization directions, thereby removing a change in the polarization directions caused by the reflection to make the wavelength conversion efficient. 
     Furthermore, in the above wavelength converting laser, it is preferable that: the pair of fundamental-wave reflecting surfaces is formed in both end faces of the wavelength conversion element, respectively, in the optical-axis directions thereof; and at least one of both end faces of the wavelength conversion element has a convex shape. 
     According to this configuration, the convex end face of the wavelength conversion element works as a concave mirror for the fundamental wave to be reflected to thereby form a light-concentration point inside of the wavelength conversion element. On the other hand, the convex end face of the wavelength conversion element reflecting the fundamental wave and transmitting the converted wave works as a convex lens for the converted wave to thereby narrow the divergence angle of the converted wave to be emitted. 
     Moreover, in the above wavelength converting laser, preferably, at least one of both end faces of the wavelength conversion element may have a convex cylindrical shape. 
     This configuration causes light-concentration points formed inside of the wavelength conversion element to differ in the beam-diameter directions, thereby preventing the power density of the fundamental wave from concentrating. 
     In addition, in the above wavelength converting laser, it is preferable that one of the pair of fundamental-wave reflecting surfaces includes a cylindrical surface and the other includes a spherical surface. 
     According to this configuration, one of both end faces of the wavelength conversion element is a cylindrical surface, thereby evading beam diffraction and preventing the beam diameter from widening while the fundamental wave goes back and forth between the pair of fundamental-wave reflecting surfaces. 
     Furthermore, in the above wavelength converting laser, it is preferable that: the pair of fundamental-wave reflecting surfaces is formed in both end faces of the wavelength conversion element, respectively, in the optical-axis directions thereof; and one end face reflecting the fundamental wave and transmitting the converted wave of both end faces of the wavelength conversion element has an area smaller than the other end face. 
     According to this configuration, one end face reflecting the fundamental wave and transmitting the converted wave of both end faces of the wavelength conversion element has an area smaller than the other end face. This makes it possible to output the converted wave with a plurality of beams thereof overlapping each other, thereby unifying the intensity distribution. 
     Moreover, in the above wavelength converting laser, preferably, the wavelength conversion element may have a thickness and a width of 1 mm or below. 
     According to this configuration, the wavelength conversion element may have a thickness and a width of 1 mm or below and the light-source area of the converted wave is within a range of 1 mm×1 mm, thereby collecting the converted wave within a range narrow enough. 
     In addition, in the above wavelength converting laser, it is preferable that: the wavelength conversion element is a flat plate having a predetermined thickness; and the reflection portion is formed in two largest-area faces facing each other of the wavelength conversion element shaped like the flat plate. 
     This configuration makes it possible to keep a fundamental-wave beam from spreading in the thickness directions, thereby maintaining the light intensity at a high level even if the fundamental wave reflects repeatedly inside of the wavelength conversion element. 
     Furthermore, in the above wavelength converting laser, it is preferable that: the pair of fundamental-wave reflecting surfaces is formed in both end faces of the wavelength conversion element, respectively, in the optical-axis directions thereof; and one end face of both end faces of the wavelength conversion element reflects the fundamental wave and transmits the converted wave, and is connected to a multi-mode optical fiber propagating the converted wave. 
     According to this configuration, although a plurality of converted-wave beams are emitted from the wavelength conversion element, the plurality of converted-wave beams are incident as a single luminous flux directly to the multi-mode optical fiber, thereby easily transmitting the converted wave to various places. 
     Moreover, in the above wavelength converting laser, preferably, the connection end face of the multi-mode optical fiber to the wavelength conversion element may reflect the fundamental wave and transmit the converted wave. 
     This configuration makes it possible to separate the fundamental wave leaking from the end face of the wavelength conversion element and the converted wave and thereby transfer only the converted wave. 
     In addition, in the above wavelength converting laser, preferably, the fundamental-wave reflecting surface transmitting the converted wave may include a transmission region for transmitting the converted wave and a reflection region for reflecting both the fundamental wave and the converted wave. 
     According to this configuration, since the converted wave is emitted only from the transmission region, a plurality of converted-wave beams are emitted from the limited transmission region, thereby significantly reducing the area of the converted-wave emission region, so that a plurality of converted-wave beams can be handled as a single fine luminous flux. 
     Furthermore, preferably, the above wavelength converting laser may further include a vibration mechanism vibrating the wavelength conversion element when the converted wave is emitted. 
     According to this configuration, the wavelength conversion element vibrates during the emission of the converted wave, thereby reducing the interference noise of the outputted converted wave. 
     Moreover, in the above wavelength converting laser, preferably, an image of an end face transmitting the converted wave of both end faces of the wavelength conversion element may be projected on a modulation element modulating the converted wave. 
     This configuration makes it possible to shape a plurality of converted-wave beams according to the shape of the end face of the wavelength conversion element and overlap the plurality of converted-wave beams to thereby unify the intensity distribution. Besides, since there is no need to provide any optical part for beam shaping, a loss caused by beam shaping can be suppressed and the number of necessary optical parts reduced. 
     In addition, in the above wavelength converting laser, it is preferable that: at least one of the pair of fundamental-wave reflecting surfaces includes a reflective film for reflecting the fundamental wave and the converted wave; the plurality of light-concentration points are formed near the reflective film; and the reflective film includes a metal film having a thickness of 100 nm or above. 
     According to this configuration, the metal film having a thickness of 100 nm or above functions as a heat transfer route and thereby suppresses a local rise in the temperature of the wavelength conversion element caused by concentrating the fundamental wave. 
     An image display according to another aspect of the present invention includes: the wavelength converting laser according to any of the above; and a modulation element modulating the converted wave emitted from the wavelength converting laser. 
     In this image display, the fundamental wave passes a plurality of times inside of the wavelength conversion element and forms a plurality of light-concentration points at places different from a cross point of the fundamental wave, thereby making it possible to obtain a high conversion efficiency stably and reduce the light-source area of a converted wave emitted as a plurality of beams, resulting in the whole apparatus being smaller. 
     The wavelength converting laser and the image display according to the present invention are capable of obtaining a high conversion efficiency stably and being miniaturized and are useful as a wavelength converting laser capable of converting the wavelength of a fundamental wave and outputting a converted wave having a different wavelength from the fundamental wave and an image display including the wavelength converting laser. 
     Herein, the specific implementation or embodiments given in the section of Detailed Description of the Preferred Embodiments of the Invention merely clarify the contents of an art according to the present invention, and hence, without being limited only to the specific examples and interpreted in a narrow sense, numerous variations can be implemented within the scope of the spirit of the present invention and the following claims.