Abstract:
A laser-diode pumped solid-state laser apparatus comprises at least one laser diode producing a pumping laser light, and at least one laser light generator including a monocrystalline substance doped with a dopant element and pumped with the pumping laser light from at least one laser diode, the monocrystalline substance containing the dopant element with a concentration profile such that the dopant element increases a concentration thereof in a direction perpendicular to a laser oscillation direction gently in the form of a slope from a near zero concentration.

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to laser-diode pumped solid-state laser apparatuses, optical scanning apparatuses, image forming apparatuses and display apparatuses. More particularly, the present invention relates to a laser-diode pumped solid-state laser apparatus having a solid-state laser crystal in which pumping is caused by laser diode. Further, the present invention relates to any of an optical scanning apparatus, an image forming apparatus or a display apparatus that uses such a laser-diode pumped solid-state laser apparatus. 
     These days, lasers are used in various fields including laser printers and laser measuring instruments. Further, aiming practical application in future, investigation and development are being made with regard to laser display apparatus or the like. In such apparatuses that use laser light, there is an increasing demand for downsizing of laser source and improvement of laser light quality. 
     Various proposals have been made with regard to so-called solid-state lasers (see Patent References 1-11, for example). 
     For example, Patent Reference 1 discloses a solid-state laser oscillation apparatus that uses a laser medium having a doped part and undoped part. Further, Patent Reference 2 discloses a laser apparatus having a gain medium and a waveguide of pumping light, while Patent Reference 3 discloses a laser apparatus having a laser medium and a waveguide optical system. 
     Further, Patent Reference 4 discloses a method of manufacturing an oxide monocrystal (single crystal) having a core part and a cladding part, while Patent Reference 5 discloses an oxide monocrystal of fiber shape for optical applications formed of a fiber body of an oxide monocrystal and a liquid-phase epitaxial layer of an oxide monocrystal formed so as to cover the surface of the foregoing fiber body. Further, Patent Reference 6 discloses a solid-state laser crystal in which doping concentration of laser-activating ions is increased continuously or stepwise from an end surface where excitation is caused toward an end surface where cooling is made. 
     Further, Patent Reference 7 discloses a composite laser device having a transparent crystal body including therein a region where laser oscillation can take place and a second crystal body jointed to the transparent crystal body, wherein at least one of the transparent crystal body and the second crystal body is formed of a polycrystalline material. Further, Patent Reference 8 discloses a solid-state laser oscillator having a polycrystalline ceramic composite laser medium in which a polycrystalline transparent ceramic not containing active element and a polycrystalline transparent ceramic doped with an active element are jointed. 
     Further, Parent References 11 and Non-Patent Reference 1 disclose a composite monocrystal that has regions formed by so-called dual-die EFG (edge-defined film-fed growth) process with different compositions or components. 
     Patent Reference 1 
     Japanese Patent 3,503,588 
     Patent Reference 2 
     Japanese Laid-Open Patent Application 2004-356479 
     Patent Reference 3 
     Japanese Laid-Open Patent Application 2004-152817 
     Patent Reference 4 
     Japanese Patent 3759807 
     Patent Reference 5 
     Japanese Laid-Open Patent Application 8-278419 
     Patent Reference 6 
     Japanese Patent 3266071 
     Patent Reference 7 
     Japanese Laid-Open Patent Application 2005-327997 
     Patent Reference 8 
     Japanese Laid-Open Patent Application 2002-57388 
     Patent Reference 9 
     Japanese Laid-Open Patent Application 6-128089 
     Patent Reference 10 
     Japanese Laid-Open Patent Application 6-128076 
     Patent Reference 11 
     Japanese Laid-Open Patent Application 6-128078 
     Non-Patent Reference 
     P. Rudolph, K. Shimamura and T. Fukuda, “The Radial Selectivity of In-situ Core-doped Crystal Rods Grown by the Double Die EFG Method, Cryst. Res. Technol. vol. 29, 1994, No. 6, pp. 801-807. 
     SUMMARY OF THE INVENTION 
     However, with conventional laser optical source, it has been difficult to obtain a high-quality laser light without inviting increase of size of the laser optical source. 
     Thus, the present invention has been made in view of the foregoing problems and it is a first object of the present invention to provide a laser-diode pumped solid-state laser apparatus capable of providing high-quality laser output without inviting increase of size. 
     A second object of the present invention is to provide an optical scanning apparatus capable of scanning a surface with high precision. 
     A third object of the present invention is to provide an image forming apparatus capable of forming high-quality images. 
     A fourth object of the present invention is to provide a display apparatus capable of displaying information with high display quality. 
     In a first aspect, the present invention provides a laser-diode pumped solid-state laser apparatus, comprising: 
     at least one laser diode producing a pumping laser light; and 
     at least one laser light generator comprising a monocrystalline substance doped with a dopant element (optical emission center: rare earth element or transitional metal element) and pumped with said pumping laser light from said at least one laser diode, 
     said monocrystalline substance containing said dopant element with a concentration profile such that said dopant element increases a concentration thereof in a direction perpendicular to a laser oscillation direction gently in the form of a slope from a near zero concentration. 
     According to the present invention, in which at least one laser light generator used therein comprises the monocrystalline substance doped with the dopant element (optical emission center: rare earth element or transitional metal element) excited by the pumping laser light from the at least one laser diode, with the concentration profile such that the dopant element increases a concentration thereof from near zero concentration gently in the form of a slope in the direction perpendicular to the laser oscillation direction, it is easily possible to attain the desired distribution of optical absorption in the monocrystalline substance. Further, there is no need with the present invention to shape the excitation laser light, and it becomes possible to provide a laser output of high beam quality, without inviting increase of size of the solid-state laser apparatus. 
     In a second aspect, the present invention provides an optical scanning apparatus scanning a surface by an optical beam, wherein the optical scanning apparatus comprises at least one laser-diode pumped solid-state laser apparatus of the present invention for producing the optical beam. 
     According to the present invention, in which the optical scanning apparatus comprises at least one laser-diode pumped solid-state laser apparatus as set forth above, it becomes possible to scan the surface with the optical beam with high precision as a result. 
     In a third aspect, the present invention provides an image forming apparatus forming an image on an object by using a laser light, wherein the image forming apparatus comprises at least one laser-diode pumped solid-state laser apparatus as set forth above for producing the laser light. 
     According to the present invention, in which the image forming apparatus comprises at least one laser-diode pumped solid-state laser apparatus as set forth above, it becomes possible to form high-quality images as a result. 
     In a fourth aspect, the present invention provides a display apparatus displaying information by using laser light, wherein the display apparatus comprises at least one laser-diode pumped solid-state laser apparatus as set forth above for producing the laser light. 
     According to the present invention, in which the display apparatus comprises at least one laser-diode pumped solid-state laser apparatus as set forth above, it becomes possible to attain high-quality display of information as a result. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are diagrams for explaining a laser-diode pumped solid-state laser apparatus according to a first embodiment of the present invention; 
         FIGS. 2A and 2B  are diagrams for explaining the solid-state laser crystal of  FIGS. 1A and 1B ; 
         FIG. 3  is a diagram for explaining a cavity in  FIG. 1A ; 
         FIG. 4  is a diagram for explaining the absorption coefficient and absorption amount in the solid-state laser crystal of  FIGS. 2A and 2B ; 
         FIG. 5  is a diagram for explaining the absorption amount in the solid-state laser crystal of  FIGS. 2A and 2B  in a three-dimensional plot; 
         FIG. 6  is a diagram for explaining the absorption coefficient and absorption amount for the case of uniform Nd concentration in a core part; 
         FIG. 7  is a diagram for explaining the absorption amount in a three-dimensional plot for the case of uniform Nd concentration in a core part; 
         FIGS. 8A and 8B  are diagrams for explaining a laser-diode pumped solid-state laser apparatus according to a second embodiment of the present invention; 
         FIGS. 9A and 9B  are diagrams for explaining a laser-diode pumped solid-state laser apparatus according to a third embodiment of the present invention; 
         FIGS. 10A and 10B  are diagrams for explaining a laser-diode pumped solid-state laser apparatus according to a fourth embodiment of the present invention; 
         FIGS. 11A and 11B  are diagrams for explaining a laser-diode pumped solid-state laser apparatus according to a fifth embodiment of the present invention; 
         FIG. 12  is a diagram for explaining a laser-diode pumped solid-state laser apparatus according to a sixth embodiment of the present invention; 
         FIGS. 13A and 13B  are diagrams for explaining a solid-state laser apparatus  600 R of  FIG. 12 ; 
         FIGS. 14A and 14B  are diagrams for explaining a solid-state laser apparatus  600 B of  FIG. 12 ; 
         FIGS. 15A and 15B  are diagrams for explaining a solid-state laser apparatus  600 G of  FIG. 12 ; 
         FIGS. 16A and 16B  are diagrams for explaining a laser-diode pumped solid-state laser apparatus according to a seventh embodiment of the present invention; 
         FIGS. 17A and 17B  are diagrams for explaining a modification of the solid-state laser crystal; 
         FIG. 18  is a diagram for explaining the schematic construction of a laser printer according to an embodiment of the present invention; 
         FIG. 19  is a diagram explaining the schematic construction of an optical scanning apparatus used in the laser printer of  FIG. 18 ; 
         FIG. 20  is a diagram for explaining a laser printer according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     &lt;&lt;Laser-Diode Pumped Solid-State Laser Apparatus&gt;&gt; 
     First Embodiment 
     Hereinafter, a laser-diode pumped solid-state laser apparatus according to a first embodiment of the present invention will be described. 
       FIGS. 1A and 1B  show the schematic construction of a laser-diode pumped solid-state laser apparatus  100  according to a first embodiment of the present invention. In the present invention, explanation will be made based on the coordinate system in which Z-axis is chosen in the laser oscillation direction and X and Y-axes are chosen in a plane perpendicular to the Z-axis with mutually perpendicular relationship. 
     Referring to  FIGS. 1A and 1B , the laser-diode pumped solid-state laser apparatus  100  of the present embodiment is an apparatus of so-called side-pumped type and includes two laser diode array elements LDa and LDb for the purpose of pumping, a solid-state laser crystal  10 , an output mirror  40 , and a heat sink  30 . 
     The two laser diode array elements LDa and LDb are disposed on the +Z-side of the heat sink  30  in a manner so as to face each other in the Y-axis direction, wherein each of the laser diode array elements LDa and LDb produces a pumping laser light of the wavelength of 808 nm with the output power of 40 W. In the illustrated example, the laser diode array LDa emits the pumping laser light in the +Y direction while the laser diode array LDb produces the pumping laser light in the −Y direction. 
     Further, there are provided two optical systems  20   a  and  20   b  respectively in correspondence to the laser diodes LDa and LDb, wherein each of the optical systems  20   a  and  20   b  is formed by a combination of plural lenses and is disposed on the +Z side of the heat sink  30 . Thereby, it becomes possible to form an optical spot on the incident surface of the solid-state laser crystal  100  with a size of 100 μm (in the thickness direction of the solid-state laser crystal  10 : Z-axis direction)×1000 μm (X-axis direction). Here, the optical system  20   a  is deposed at the +Y side of the laser diode array LDa and focuses the pumping laser light therefrom. Further, the optical system  20   b  is deposed at the −Y side of the laser diode array LDb and focuses the pumping laser light therefrom. 
     The solid-state laser crystal  10  is disposed on the +Z side of the heat sink  30  at the +Y side of the optical system  20   a  and at the −Y side of the optical system  20   b.    
     As shown in  FIGS. 2A and 2B , the solid-state laser crystal  10  may be a uniaxial monocrystal of gadolinium vanadate (GdVO 4 ) having a disk shape (or chip shape), wherein it should be noted that the solid-state laser crystal  10  is doped with neodymium (Nd) as the dopant element (emission center) excited by the pumping laser light from the laser-diode array elements. 
     As represented in the example of  FIG. 2A , the concentration profile of Nd has a sloped shape in which the Nd concentration increases gradually from near zero concentration in the direction perpendicular to the direction of laser oscillation (Z-axis direction in the present case). Hereinafter, the part contributing to laser oscillation will be designated as “core part  10   a ” and the part scarcely contributing to laser oscillation will be designated as “cladding part  10   b ”. Thus, the solid-state laser crystal  10  is integrally formed of the core part  10   a  and the cladding part  10   b.    
     In the present example, the core part  10   a  is a circular part of a diameter Da located at a central part of the solid-state laser crystal  10 , while the cladding part  10   b  is a donuts shape part surrounding the core part  10   a.    
     As shown in the example of  FIG. 2A , the concentration of Nd in the core part  10   a  becomes maximum near the center of the core part  10   a  and decreases gradually toward the cladding part  10   b . In one example, the Nd concentration near the center of the core part  10   a  is about 0.5 at %. Thereby, the Nd concentration profile has a shape resembling a Gauss distribution profile. 
     In one example, the solid-state laser crystal  10  is formed by slicing a rod-shaped monocrystal (columnar crystal) ingot manufactured by the dual-die EFG process explained before or by a μPD process and may have a thickness t of 0.5 mm, a diameter Db of 5 mm and a diameter Da of 1 mm. It should be noted that the solid-state laser crystal  10  is a crystal that is designed to emit a laser light of linear polarization upon excitation by optical pumping. 
     Further, as shown in  FIG. 3 , the surface of the solid-state laser crystal  10  at the side where the heat sink  30  is provided (the surface at −Z side, designated as “A surface” for the sake of convenience) is provided with a coating providing a reflectance of 99.9% for the light of a wavelength of 1063 nm. Further, as shown in  FIG. 3 , the surface of the solid-state laser crystal  10  at the side opposite to the side where the heat sink  30  is provided (the surface at +Z side, designated as “B surface” for the sake of convenience) is provided with a coating providing a transmittance of 99.9% for the light of the wavelength of 1063 nm. 
     On the surface of the coating at the A surface, there is formed a metal layer  32  of Cr/Ni/Au laminated structure, wherein the metal layer  32  is jointed to an Au—Sn alloy layer  31  formed on the heat sink  30 . 
     Further, there is disposed an output mirror  40  at the +Z side of the solid-state laser crystal  10 . This output mirror  40  has a radius of curvature of 5000 mm at the −Z side surface and has a transmittance of 5% to the light of the wavelength of 1063 nm. 
     Thereby, there is formed a cavity  35  by the A surface of the solid-state laser crystal  10  and the output mirror  40  as shown in  FIG. 3 . 
     In the illustrated example, the distance between the A surface of the solid-state laser crystal  10  and surface of the output mirror  40  at the −Z side is set to 100 mm. Further, the laser light forms a beam of the beam diameter of 0.5 mm inside the solid-state laser crystal  10 . 
     It should be noted that the output mirror  40  may have a so-called microchip construction integrated with the solid-state laser crystal  10 . 
     Next, the operation of the laser-diode pumped solid-state laser apparatus  100  of the foregoing construction will be explained briefly. 
     Referring to  FIG. 1 , the laser diode array LDa emits a pumping laser light with the wavelength of 808 nm, wherein the emitted pumping laser light is injected into the solid-state laser crystal  10  at a side surface thereof after passing through the optical system  20   a . Similarly, the laser diode array LDb emits a pumping laser light with the wavelength of 808 nm, wherein the emitted pumping laser light is injected into the solid-state laser crystal  10  at a side surface thereof after passing through the optical system  20   b.    
     Thereby, the Nd dopant in the solid-state laser crystal  10  undergoes excitation by the pumping laser light and there is caused laser oscillation by the cavity  35  formed by the A surface of the solid-state laser crystal  10  and the output mirror  40  (see  FIG. 3 ) with the wavelength of 1063 nm. That laser light of the wavelength of 1063 nm is then emitted after passing through the output mirror  40 . 
     As explained above, the laser-diode pumped solid-state laser apparatus  100  of the first embodiment is thus provided with the solid-state laser crystal  10  that includes a uniaxial monocrystal of gadolinium vanadate (GdVO 4 ) doped with Nd, which undergoes excitation by the pumping laser lights from the two pumping laser diode array elements LDa and LDb, wherein Nd is doped with such a concentration profile that the concentration of Nd increases gradually in the sloped shape from near zero concentration in the direction perpendicular to the laser oscillation direction (Z-axis direction in the present example). As a result, it is easily attained the absorption profile such as those shown in  FIGS. 4 and 5 , in which there appears a peak of absorption at the central part of the solid-state laser crystal  10 . With this, a lateral mode of excellent Gaussian distribution is attained for the laser light output therefrom (designated hereinafter as “output laser light” for the sake of convenience). 
     Meanwhile, in the case a solid state laser crystal having a uniform Nd concentration profile for the core part, and thus having a sharp increase of Nd concentration from near zero concentration, is used for the laser-diode pumped solid-state laser apparatus of side pumping construction, there occurs strong and sharp absorption for the pumping laser light injected from the side surface as shown in  FIGS. 6 and 7 , and it becomes difficult to attain an output laser light of excellent beam quality. Thus, in this case, it is advisable to provide a correction mechanism for correcting the beam quality, while such correction mechanism invites increase of size and cost of the laser apparatus. In addition, such a construction requires adjustment of the correction mechanism. 
     Further, according to the laser-diode pumped solid-state laser apparatus  100  of the first embodiment, the heat generated in the solid-state laser crystal  10 , originating from the energy difference between the pumping laser light and the output laser light, is radiated directly via the A surface, and it becomes possible to suppress the temperature rise of the solid-state laser crystal  10 . As a result, it becomes possible to achieve high power operation of the laser-diode pumped solid-state laser apparatus  100 . 
     It should be noted that, in the event there is a uniform Nd concentration in the core part, it should be noted that there arises a heat distribution profile similar to that of the absorption profile in the solid-state laser crystal, while such heat distribution profile induces a change of refractive index (thermal lens effect), leading to further deterioration of the beam quality. 
     With the laser-diode pumped solid-state laser apparatus  100  of the first embodiment, it is possible to optimize the heat distribution profile by adjusting the concentration profile of Nd in the solid-state laser crystal  10 . Thus, with the laser-diode pumped solid-state laser apparatus  100  of the first embodiment of the present invention, it becomes possible to provide an output laser light of excellent beam quality without providing a correction mechanism for the change of refractive index (thermal lens effect). 
     Thus, according to the laser-diode pumped solid-state laser apparatus  100  of the first embodiment, it becomes possible to output a laser light of the wavelength of 1063 nm with excellent beam quality and with high output power. 
     While the first embodiment has been explained for the case of the output laser light has the wavelength of 1063 nm, the present invention is not limited to such a particular construction. For example, by appropriately choosing the specification of coating of the solid-state laser crystal  10  and the optical properties of the output mirror  40 , it is also possible to obtain an output laser light of the wavelength of 912 nm or 1340 nm. 
     Second Embodiment 
     Hereinafter, a laser-diode pumped solid-state laser apparatus according to a second embodiment of the present invention will be described with reference to  FIGS. 8A and 8B . 
       FIG. 8A  shows the schematic construction of a laser-diode pumped solid-state laser apparatus  200  according to a second embodiment of the present invention. 
     Referring to  FIG. 8A , the laser-diode pumped solid-state laser apparatus  200  has a construction similar to that of the laser-diode pumped solid-state laser apparatus  100  of the first embodiment, except that there is disposed a non-linear optic crystal  50  on the optical path of the laser light between the solid-state layer crystal  10  and the output mirror  40  and that the optical properties of the output mirror  40  is different and the specification of the coating of the solid-state laser crystal  10  is different. Otherwise, the construction of the present embodiment is the same as the first embodiment. 
     Hereinafter, explanation will be made mainly on the difference over the first embodiment. Thereby, it should be noted that the same reference numerals are used for the parts identical to or equivalent to the parts of the first embodiment and the description thereof will be simplified or omitted. 
     The non-linear optic crystal  50  converts the laser light of the wavelength of 1063 nm of the fundamental mode to a laser light of the wavelength of 531.5 nm forming a second harmonic mode. For this non-linear optic crystal  50 , a PPMgLN device (a device of LiNbO 3  having a periodically inversed polarization structure doped with MgO) may be used. This non-linear optic crystal  50  has a length of 5 mm (in the X-axis direction and Y-axis direction) and a thickness of 2 mm (in the Z-axis direction). Thereby, the non-linear optic crystal  50  is disposed with such an orientation that the crystal axis thereof points the direction in which there is attained a pseudo phase matching condition with the polarization direction of the laser light inside the cavity. On both end surfaces of the non-linear optic crystal  50 , there are provided a coating providing a transmittance of 99.5% or higher for both the fundamental mode light of the wavelength of 1063 nm and the second harmonics light of the wavelength of 531.5 nm. In the explanation below, the surface at the −Z side of the non-linear optic crystal  50  will be designated as C surface, while the surface at the +Z side will be designated as D surface. 
     On the surface (A surface) of the solid-state laser crystal  10  jointed to the heat sink  30 , there is provided a coating having a reflectance of 99.9% with regard to the light of the wavelength of 1063 nm as shown in  FIG. 8B . Further, as shown in  FIG. 8B , there is provided a coating on the surface (B surface) opposite to the A surface of the solid-state laser crystal  10 , such that the coating provides the transmittance of 99.9% for the light of the wavelength of 1063 nm and the reflectance of 99% for the light of the wavelength of 531.5 nm. With this, it is possible to suppress the incidence of the second harmonic wave generated inside the cavity to penetrate into the solid-state layer crystal  10 . 
     The output mirror  40  has a radius of curvature of 5000 mm at the −Z side surface and has a reflectance of 99.9% to the light of the wavelength of 1063 nm and the transmittance of 99% to the light of the wavelength of 531.5 nm. 
     Thereby, there is formed a cavity  35 A by the A surface of the solid-state laser crystal  10  and the output mirror  40  as shown in  FIG. 8B . 
     Next, the operation of the laser-diode pumped solid-state laser apparatus  200  will be explained briefly. 
     Referring to  FIG. 8A , the laser diode array LDa emits a pumping laser light with the wavelength of 808 nm, wherein the emitted pumping laser light is injected into the solid-state laser crystal  10  at a side surface thereof after passing through the optical system  20   a . Similarly, the laser diode array LDb emits a pumping laser light with the wavelength of 808 nm, wherein the emitted pumping laser light is injected into the solid-state laser crystal  10  at a side surface thereof after passing through the optical system  20   b.    
     Thereby, the Nd dopant element in the solid-state laser crystal  10  undergoes excitation by the pumping laser light and there is caused laser oscillation by the cavity  35 A formed by the A surface of the solid-state laser crystal  10  and the output mirror  40  (see  FIG. 8B ) with the wavelength of 1063 nm. Thereby, the laser light of the wavelength of 1063 nm is confined inside the cavity  35 A and forms the fundamental mode. Thereby, because the non-linear optic element  50  is disposed inside the cavity  35 A, the confined fundamental mode wave undergoes wavelength transition, resulting in generation of the second order harmonics, and the laser light of the wavelength of 531.5 nm is formed as the second order harmonics, wherein this laser light of the wavelength of 531.5 nm is outputted through the output mirror  40 . 
     As explained above, the laser-diode pumped solid-state laser apparatus  200  of the second embodiment is thus provided with the solid-state laser crystal  10  that includes a uniaxial monocrystal of gadolinium vanadate (GdVO 4 ) doped with Nd, which undergoes excitation by the pumping laser lights from the two pumping laser diode array elements LDa and LDb, wherein Nd is doped with such a concentration profile that the concentration of Nd increases gently in the sloped shape from near zero concentration in the direction perpendicular to the laser oscillation direction (Z-axis direction in the present example). As a result, it is easily attained the desired absorption profile in which there appears a peak of absorption at the central part of the solid-state laser crystal  10 . With this, a lateral mode of excellent Gaussian distribution is obtained for the laser light from the solid-state laser crystal  10 . Thus, the laser light of the wavelength of 1063 nm of high power and excellent beam quality is injected into the non-linear optic element  50 . 
     Meanwhile, it is known that, in non-linear optic elements, the output of the second harmonics is proportional to the square of the optical power of the incident light. This means that there occurs increase of output in proportion with the beam quality. 
     According to the laser-diode pumped solid-state laser apparatus  200  of the second embodiment, in which it is possible to inject a high power laser light of excellent beam quality into the non-linear optic element  50 , there is attained improvement of efficiency of conversion in the non-linear optic element  50 . 
     Thus, according to the laser-diode pumped solid-state laser apparatus  200  of the second embodiment, it becomes possible to output a laser light of the wavelength of 531.5 nm with excellent beam quality and with high output power, without inviting increase of size of the apparatus. 
     In the laser-diode pumped solid-state laser apparatus  200  according to the second embodiment explained above, the non-linear optic element  50  may have a length of 5 mm and a thickness of 2 nm, while the present invention is by no means limited to such a specific example. Thus, other construction may be used as long as the laser light of the fundamental mode having the wavelength of 1063 nm is converted to the laser light of the second harmonics of the wavelength of 531.5 nm with desired conversion efficiency. 
     Third Embodiment 
     Hereinafter, a laser-diode pumped solid-state laser apparatus according to a third embodiment of the present invention will be described with reference to  FIGS. 9A and 9B . 
       FIG. 9A  shows the schematic construction of a laser-diode pumped solid-state laser apparatus  300  according to a third embodiment of the present invention. 
     Referring to  FIG. 9A , the laser-diode pumped solid-state laser apparatus  300  has a construction similar to that of the laser-diode pumped solid-state laser apparatus  200  of the second embodiment, except that the non-linear optic crystal  50  is disposed a the +Z side of the output mirror  40 . Otherwise, the construction of the present embodiment is same to that of the second embodiment. Hereinafter, explanation will be made mainly on the difference over the second embodiment. Thereby, it should be noted that the same reference numerals are used for the parts identical to or equivalent to the parts of the second embodiment and the description thereof will be simplified or omitted. 
     Referring to  FIG. 9A , the non-linear optic element  50  is disposed on the optical path of the laser light passed through the output mirror  40 . This non-linear optic crystal  50  may have a thickness (length in the Z-axis direction) of 10 mm, for example. It should be noted that, in the case the non-linear optic crystal  50  is disposed inside the optical cavity as in the case of the second embodiment, the non-linear optic crystal  50  may have a small thickness in view of large optical intensity, while in the case of the third embodiment in which the non-linear optic element  50  is disposed outside the optical cavity, it is necessary to achieve the wavelength conversion with one pass of the laser light through the non-linear optic crystal  50 , and thus, there is a need of increasing the thickness of the non-linear optic crystal  50  for attain the conversion efficiency comparable to that of the second embodiment. 
     According t the laser-diode pumped solid-state laser apparatus  300  of the third embodiment, in which the laser light of the wavelength of 1063 nm of high power and excellent beam quality is injected into the non-linear optic element  50 , it becomes possible to emit a laser light of the wavelength of 531.5 nm with high beam quality and high output without inviting increase of size of the apparatus, similarly to the laser-diode pumped solid-state laser apparatus  200  according to the second embodiment of the present invention. 
     In the laser-diode pumped solid-state laser apparatus  300  according to the third embodiment explained above, the non-linear optic element  50  may have a length of 5 mm and a thickness of 10 nm, while the present invention is by no means limited to such a specific example. Thus, other construction may be used as long as the laser light of the fundamental mode having the wavelength of 1063 nm is converted to the laser light of the second harmonics of the wavelength of 531.5 nm with desired conversion efficiency. 
     Further, with the laser-diode pumped solid-state laser apparatus  300  of the third embodiment, it is also possible to dispose a lens between the output mirror  40  and the non-linear optic crystal  50 . With this, it becomes possible to focus the fundamental mode wave incident to the non-linear optic element  50 , resulting in improvement of beam strength of the fundamental wave, and the conversion efficiency in the non-linear optic crystal  50  is improved further as a result. 
     Fourth Embodiment 
     Hereinafter, a laser-diode pumped solid-state laser apparatus according to a fourth embodiment of the present invention will be described with reference to  FIGS. 10A and 10B . 
       FIG. 10A  shows the schematic construction of a laser-diode pumped solid-state laser apparatus  400  according to a fourth embodiment of the present invention. 
     Referring to  FIG. 10A , the laser-diode pumped solid-state laser apparatus  400  has a construction similar to that of the laser-diode pumped solid-state laser apparatus  200  of the second embodiment, except that the specification is changed for the coating on the D surface of the non-linear optic crystal  50  and that the output mirror  40  is eliminated. Otherwise, the construction of the present embodiment is same to that of the second embodiment. Hereinafter, explanation will be made mainly on the difference over the second embodiment. Thereby, it should be noted that the same reference numerals are used for the parts identical to or equivalent to the parts of the second embodiment and the description thereof will be simplified or omitted. 
     As shown in  FIG. 10B , there is provided a coating on the D surface of the non-linear optic crystal  50 , wherein the coating thus provided has a reflectance of 99.9% for fundamental mode light of the wavelength of 1063 nm and the transmittance of 99.5% for the second harmonics light of the wavelength of 531.5 nm. Further, on the C surface of the non-linear optic crystal  50 , there is provided a coating providing a transmittance of 99.5% or higher for both the fundamental mode light of the wavelength of 1063 nm and the second harmonics light of the wavelength of 531.5 nm. 
     Thereby, there is formed a cavity  35 A by the A surface of the solid-state laser crystal  10  and the D surface of the non-linear optic crystal  50  as shown in  FIG. 10B . 
     Thereby, the solid-state laser crystal  10  and the non-linear optic element  50  are fixed with each other with simple contact or with a photo-resistive adhesive. Thereby, the solid-state laser crystal  10  and the non-linear optic crystal  50  may make an optical contact free from coating. 
     As explained heretofore, according t the laser-diode pumped solid-state laser apparatus  400  of the fourth embodiment, in which the laser light of the wavelength of 1063 nm of high power and excellent beam quality is injected into the non-linear optic element  50 , it becomes possible to emit a laser light of the wavelength of 531.5 nm with high beam quality and high output without inviting increase of size of the apparatus, similarly to the laser-diode pumped solid-state laser apparatus  200  according to the second embodiment of the present invention. 
     Further, according to the laser-diode pumped solid-state laser apparatus  400  according to the fourth embodiment of the present invention, there is no need of providing the output mirror  40  used with the laser-diode pumped solid-state laser apparatus  200 , and it becomes possible to facilitate size reduction further. 
     Fifth Embodiment 
     Hereinafter, a laser-diode pumped solid-state laser apparatus excited according to a fifth embodiment of the present invention will be described with reference to  FIGS. 11A and 11B . 
       FIG. 11A  shows the schematic construction of a laser-diode pumped solid-state laser apparatus  500  according to a fifth embodiment of the present invention. 
     Referring to  FIG. 11A , the laser-diode pumped solid-state laser apparatus  500  has a construction similar to that of the laser-diode pumped solid-state laser apparatus  300  of the second embodiment, except that the specification is changed for the solid-state laser crystal  10  and the non-linear optic crystal  50  and that the output mirror  40  is eliminated. Otherwise, the construction of the present embodiment is same to that of the third embodiment. Hereinafter, explanation will be made mainly on the difference over the third embodiment. Thereby, it should be noted that the same reference numerals are used for the parts identical to or equivalent to the parts of the third embodiment and the description thereof will be simplified or omitted. 
     On the A surface of the solid-state laser crystal  10 , there is provided a coating having a reflectance of 99.9% with regard to the light of the wavelength of 1063 nm as shown in  FIG. 11B . Further, as shown in  FIG. 11B , there is provided a coating having a transmittance of 5% with regard to the light of the wavelength of 1063 nm on the B surface of the solid-state laser crystal  10 . 
     Thus, there is formed a cavity  35 A by the A surface and the B surface of the solid-state laser crystal  10  as shown in  FIG. 11B . 
     Further, on both edge surfaces of the non-linear optic crystal  50 , there are provided a coating having a transmittance of 99.5% or more for the second harmonics light of the wavelength of 531.5 nm. 
     Thereby, the solid-state laser crystal  10  and the non-linear optic element  50  are fixed with each other with simple contact or with a photo-resistive adhesive. Thereby, the solid-state laser crystal  10  and the non-linear optic crystal  50  may make an optical contact free from coating. 
     As explained heretofore, according t the laser-diode pumped solid-state laser apparatus  500  of the fifth embodiment, in which the laser light of the wavelength of 1063 nm of high power and excellent beam quality is injected into the non-linear optic element  50 , it becomes possible to emit a laser light of the wavelength of 531.5 nm with high beam quality and high output without inviting increase of size of the apparatus, similarly to the laser-diode pumped solid-state laser apparatus  300  according to the third embodiment of the present invention. 
     Further, according to the laser-diode pumped solid-state laser apparatus  500  according to the fifth embodiment of the present invention, there is no need of providing the output mirror  40  used with the laser-diode pumped solid-state laser apparatus  300 , and it becomes possible to facilitate size reduction further. 
     While the second through fifth embodiments have been explained for the case of the output laser light has the wavelength of 131.5 nm, the present invention is not limited to such a particular construction. Thus, by choosing the pitch of polarization reversal or coating specification of the non-linear optic crystal  50  appropriately, it is also possible to obtain an output laser light of the wavelength of 670 nm or 456 nm. 
     Sixth Embodiment 
     Hereinafter, a solid-state laser apparatus excited by laser diode according to a sixth embodiment of the present invention will be described with reference to  FIG. 12  and  FIGS. 13A and 13B ,  14 A and  14 B and  15 A and  15 B. 
       FIG. 12  shows the schematic construction of a laser-diode pumped solid-state laser apparatus  600  according to a sixth embodiment of the present invention. 
     The laser-diode pumped solid-state laser apparatus  600  includes: a first solid-state laser apparatus  600 R emitting an output laser light of red color with the wavelength of 670 nm; a second solid-state laser apparatus  600 B emitting an output laser light of blue color with the wavelength of 456 nm; and a third solid-state laser apparatus  600 G emitting an output laser light of green color with the wavelength of 531.5 nm. 
     As shown in  FIG. 13A , the first solid-state laser apparatus  600 R has a construction similar to the laser-diode pumped solid-state laser apparatus  500  according to the fifth embodiment explained before. In one example, there is provided a coating having a reflectance of 99.9% with regard to the light of the wavelength of 1063 nm as shown in  FIG. 11B  on the A surface of the solid-state laser crystal  10 . Further, as shown in  FIG. 13B , there is provided a coating having a transmittance of 5% with regard to the light of the wavelength of 1340 nm and the transmittance of 99.9% with regard to the light of the wavelength of 1063 nm on the B surface of the solid-state laser crystal  10 . With regard to the material of the solid-state laser crystal  10 , the same material as in the case of the laser-diode pumped solid-state laser apparatus  500  of the fifth embodiment may be used. The non-linear optic crystal  50  is a PPMgLN device having a different pitch for the polarization reversal and carries a coating on the respective end surfaces thereof such that a transmittance of 99.5% or more is attained for the light of the wavelength of 1340 nm and the light of the wavelength of 670 nm. 
     As shown in  FIG. 14A , the second solid-state laser apparatus  600 B has a construction similar to the laser-diode pumped solid-state laser apparatus  500  according to the fifth embodiment explained before. Thus, in one example, there is provided a coating having a reflectance of 99.9% with regard to the light of the wavelength of 912 nm on the A surface of the solid-state laser crystal  10  as shown in  FIG. 14B . Further, as shown in  FIG. 14B , there is provided a coating having a transmittance of 3% with regard to the light of the wavelength of 912 nm and the transmittance of 99.9% with regard to the light of the wavelength of 1063 nm on the B surface of the solid-state laser crystal  10 . With regard to the material of the solid-state laser crystal  10 , the same material as in the case of the laser-diode pumped solid-state laser apparatus  500  of the fifth embodiment may be used. The non-linear optic crystal  50  is a PPMgLN device having a different pitch for the polarization reversal and carries a coating on the respective end surfaces thereof such that a transmittance of 99.5% or more is attained for the light of the wavelength of 912 nm and the light of the wavelength of 456 nm. 
     As shown in  FIGS. 15A and 15B , the third solid-state laser apparatus  600 G has a construction similar to the laser-diode pumped solid-state laser apparatus  500  according to the fifth embodiment explained before. 
     As explained heretofore, it becomes possible, with the laser-diode pumped solid-state laser apparatus  600  of the sixth embodiment including therein a plurality of solid-state laser apparatuses each having a solid-state laser crystal  10 , to provide a plurality of laser output lights of high output power with excellent beam quality, without inviting increase of size of the laser apparatus  600 . 
     In the sixth embodiment, it is possible that each of the solid-state laser apparatuses emits the laser light with the same wavelength. 
     Further, while explanation has been made for the case of the laser apparatus includes three solid-state laser apparatuses, the present invention is by no means limited to such a construction. Thus, the laser-diode pumped solid-state laser apparatus  600  may include two solid-state laser apparatuses or four or more solid-state laser apparatuses. 
     While the second through sixth embodiment has been explained for the case of using the PPMgLN device, it should be noted that the present invention is not limited to such a specific construction. Thus, any non-linear optic crystal having a function equivalent to the PPMgLN device may be used. 
     Seventh Embodiment 
     Hereinafter, a laser-diode pumped solid-state laser apparatus according to a seventh embodiment of the present invention will be described with reference to  FIGS. 16A and 16B . 
       FIG. 16A  shows the schematic construction of a laser-diode pumped solid-state laser apparatus  700  according to a seventh embodiment of the present invention. 
     Referring to  FIG. 16A , the laser-diode pumped solid-state laser apparatus  700  is an apparatus of the edge pumping structure and includes a laser diode LD for pumping, a lens  20 , the solid-state laser crystal  10  and the output mirror  40 . 
     The laser-diode LD is a device of single-stripe structure and can produce a laser light of the wavelength of 808 nm with an output power of 2 W. In the illustrated example, the laser diode LD emits a laser light in the +Z direction. 
     The lens  20  is disposed at the +Z side of the laser diode LD and focuses the pumping laser light from the laser diode LD upon the solid-state laser crystal  10 . For example, the lens  20  may be the element capable of focusing the pumping laser light to form a beam with a beam diameter of about 1 mm. 
     The solid-state laser crystal  10  is disposed at the +Z side of the lens  20 . Further, as shown in  FIG. 16B , the surface of the solid-state laser crystal  10  at the side where the lens  20  is provided (the surface at −Z side, designated as “E surface” for the sake of convenience) is provided with a coating providing a reflectance of 99.9% for the light of the wavelength of 808 nm and a transmittance of 0.1% for the light of the wavelength of 1063 nm. Further, as shown in  FIG. 16B , the surface of the solid-state laser crystal  10  at the side opposite to the E surface (the surface at +Z side, designated as “F surface” for the sake of convenience) is provided with a coating providing a transmittance of 99.9% for the light of the wavelength of 1063 nm. 
     Further, there is disposed an output mirror  40  at the +Z side of the solid-state laser crystal  10 . This output mirror  40  has a radius of curvature of 5000 mm at the −Z side surface and has a transmittance of 5% to the light of the wavelength of 1063 nm. 
     Thereby, there is formed a cavity  35 C by the E surface of the solid-state laser crystal  10  and the output mirror  40  as shown in  FIG. 16B . 
     In the illustrated example, the distance between the E surface of the solid-state laser crystal  10  and surface of the output mirror  40  at the −Z surface is set to 100 mm. Further, the laser light forms a beam of the beam diameter of 0.5 mm inside the solid-state laser crystal  10 . 
     It should be noted that the output mirror  40  may have a so-called microchip construction integrated with the solid-state laser crystal  10 . 
     Next, the operation of the laser-diode pumped solid-state laser apparatus  700  of the foregoing construction will be explained briefly. 
     The laser light (pumping laser light) of the wavelength of 808 nm emitted from the laser diode LD passes through the lens  20  and enters into the solid-state crystal  10 . Thereby, the Nd dopant element in the solid-state laser crystal  10  undergoes excitation by the pumping laser light and there is caused laser oscillation by the cavity  35 C formed by the E surface of the solid-state laser crystal  10  and the output mirror  40  with the wavelength of 1063 nm. That laser light of the wavelength of 1063 nm is then emitted after passing through the output mirror  40 . 
     Meanwhile, with a laser-diode pumped solid-state laser apparatus of the edge pumping structure, it is generally known that the beam shape (spot shape) of the focused pumping laser light provides a profound influence on the beam quality of the laser light (output laser light) emitted from the laser-diode pumped solid-state laser apparatus. 
     According to the laser-diode pumped solid-state laser apparatus  700  of the seventh embodiment thus provided with the solid-state laser crystal  10  that includes a uniaxial monocrystal of gadolinium vanadate (GdVO 4 ) doped with Nd, which undergoes excitation by the pumping laser lights from the pumping laser diode LD, wherein Nd is doped with such a concentration profile that the concentration of Nd increases gently in the sloped shape from near zero concentration in the direction perpendicular to the laser oscillation direction (X-axis direction in the present example). As a result, it becomes possible to obtain a high-quality laser output without shaping the spot shape or optical intensity distribution for the pumping laser light. Thus, it becomes possible to provide a laser light output of high beam quality, without inviting increase of size of the solid-state laser apparatus. 
     While explanation has been made in the foregoing embodiments with regard to the case of using a disc-shaped (or chip-shaped) crystal for the solid-state laser crystal  10 , the present invention is by no means limited to such a specific example, and thus, the solid-state laser crystal  10  may also be the one having a rectangular plate shape as shown in  FIGS. 17A and 17B , wherein the solid-state laser crystal  10  of  FIGS. 17A and 17B  may be manufactured by cutting a periphery of a rod-shaped crystal ingot produced by a dual-die EFG process or μPD process, followed by a slicing process. Further, the solid-state laser crystal  10  may also have a polygonal shape. 
     While explanation has been made in the embodiments heretofore with regard to the case of using GdVO 4  for the material of the solid-state laser crystal  10 , the present invention is not limited to such a specific example and it is also possible to use a crystal of yttrium vanadate (YVO 4 ) or other crystal. 
     Further, while explanation has been made in the embodiments heretofore with regard to the case of using Nd for the dopant element of the solid-state laser crystal  10 , the present invention is not limited to such a specific example and it is also possible to use other rare earth element or metal ions. Further, the concentration of the dopant element is not limited to 0.5 at %. 
     Further, while explanation has been made in each of the foregoing embodiments of using a uniaxial monocrystal for the solid-state laser crystal  10 , the present invention is not limited to such a specific construction and it is also possible to use a biaxial monocrystal. 
     Further, while explanation has been made in the foregoing embodiments for the ease of the solid-state laser crystal  10  has a thickness t of 0.5 mm, the diameter Da of 5 mm and the diameter db of 1 mm, the present invention is not limited to such a specific construction. The dimensions of the solid-state laser crystal  10  may be changed as needed according to the beam quality demanded for the output laser light. 
     &lt;&lt;Laser Printer&gt;&gt; 
       FIG. 18  shows a schematic construction of a laser printer  1000  as an image forming apparatus according to an embodiment of the present invention. 
     Referring to  FIG. 18 , the laser printer  1000  comprises an optical scanning apparatus  900 , a photosensitive drum  901 , an electrostatic charger  902 , a developing roller  903 , a toner cartridge  904 , a cleaning blade  905 , a sheet feed tray  906 , a sheet feed roller  907 , resist roller pairs  908 , a transfer charger  911 , discharging unit  914 , a fixing roller  909 , a sheet discharging roller  912 , a sheet discharging tray  910 , and the like. 
     The electrostatic charger  902 , the developing roller  903 , the transfer charger  911 , the discharging unit  914  and the cleaning blade  905  are disposed in the vicinity of the photosensitive drum  901 . Thereby, the electrostatic charger  902 , the developing roller  903 , the transfer charger  911 , the discharging unit  914  and the cleaning blade  905  are disposed in the order of: electrostatic charger  902 →developing roller  903 →transfer charger  911 →discharging unit  914 →cleaning blade  905 , along the rotating direction of the photosensitive drum  901 . 
     The photosensitive drum  901  carries thereon a photosensitive layer. In the present example, the photosensitive drum  901  rotates in the clockwise direction (arrow direction) within the plane of  FIG. 18 . 
     The electrostatic charger  902  charges the surface of the photosensitive drum  901  uniformly. 
     The optical scanning apparatus  900  irradiates a modulated light upon the surface of the photosensitive drum  901  charged with the electric charger  902  with modulation based upon the image information from upper hierarchy apparatus such as personal computer. With this, there is formed a latent image corresponding to the image information on the surface of the photosensitive drum  901 . The latent image thus formed is moved in the direction of the developing roller  903  with rotation of the photosensitive drum  905 . It should be noted that the elongating direction of the photosensitive drum  901  (direction along the rotational axis) is called “main scanning direction” and the rotational direction of the photosensitive drum  901  is called “sub-scanning direction”. This construction of this optical scanning apparatus  900  will be explained later. 
     The toner cartridge  904  holds toners, and the toners are supplied therefrom to the developing roller  903 . 
     Thus, the developing roller  903  causes the toners supplied from the toner cartridge  904  to adhere to the latent image formed on the surface of the photosensitive drum  901 , and with this, development of the image information is attained. The latent image thus formed is moved in the direction of the transfer charger  911  with rotation of the photosensitive drum  901 . 
     The sheet feed tray  906  accommodates therein recording sheets  913 . Further, there is disposed a sheet feed roller  907  in the vicinity of the sheet feed tray  906 , and the sheet feed roller  907  picks up the recording sheet  913  one by one from the sheet feed tray  906  and supplies the same to the resist roller pair  908 . The resist roller pair  908  is disposed in the vicinity of the transfer roller  911  and holds the recording sheet  913  picked up by the sheet feed roller  907  temporarily and supplies the recording sheet to the gap between the photosensitive drum  901  and the transfer charger  911  in synchronization with the rotation of the photosensitive drum  901 . 
     Thereby, the transfer charger  911  is applied with a voltage of reverse polarity to the toners for attracting the toners on the surface of the photosensitive drum  901  to the recording sheet  913  electrically. With this voltage, the toner image on the surface of the photosensitive drum  901  is transferred to the recording sheet  913 . The recording sheet  913  thus transferred with the toner image is then forwarded to the fixing roller  909 . 
     With this fixing roller  909 , heat and pressure is applied to the recording sheet  913  and the toner image is fixed upon the recording sheet  913 . The recording sheet  913  thus fixed with the toner image is forwarded to the sheet discharge tray  901  via the sheet discharging roller  912  and is stuck upon the sheet discharge tray  910  one by one. 
     The discharging unit  914  discharges the surface of the photosensitive drum  901 . 
     The cleaning blade  905  removes the toner (residual toner) remaining on the surface of the photosensitive drum  901 . The residual toners thus removed are used again. After removal of the residual toners, the photosensitive drum  901  returns to the position of the electrostatic charger  902 . 
     &lt;&lt;Optical Scanning Apparatus&gt;&gt; 
     Next, the construction and function of the optical scanning apparatus  900  will be explained with reference to  FIG. 19 . 
     The optical scanning apparatus comprises an optical source  11 , a coupling lens  12 , a modulator  17 , a cylindrical lens  13 , a polygonal mirror  14 , a fθ lens  15  a toroidal lens and a main controller not illustrated but used for controlling the foregoing various parts, wherein the optical source  11  includes a laser apparatus equivalent to any of the laser-diode pumped solid-state laser apparatus  100 - 500  and  700  explained previously. 
     The coupling lens  12  shapes the optical beam emitted from the optical source  11  to form a generally parallel light. 
     The modulator  17  turns on and off the optical beam passed through the coupling lens  12 . 
     The cylindrical lens  13  focuses the optical beam passed through the modulator  17  upon a reflection surface of the polygonal mirror  14 . 
     The polygonal mirror  14  has a right hexagonal pillar member of low profile and carries six deflection surfaces on the lateral side thereof. Further, the polygonal mirror  14  is rotated at a constant angular velocity in the direction of arrow indicated in  FIG. 19 . Thus, the optical beam emitted from the optical source  11  and is focused upon the deflection surface of the polygonal mirror  14  by the cylindrical lens  13  undergoes deflection with a constant angular velocity with rotation of the polygonal mirror  14 . 
     The fθ lens  15  has an image height proportional to the incident angle of the optical beam from the polygonal mirror  14  and causes the image plane of the optical beam deflected by the polygonal mirror  14  with the constant angular velocity with an equal speed in the main scanning direction. 
     The toroidal lens  16  focuses the optical beam passed through the fθ lens  15  on the surface of the photosensitive drum  901 . 
     As explained heretofore, according to the optical scanning apparatus  900  of the present embodiment, it becomes possible to scan the surface of the photosensitive drum  901  with high precision in view of the fact that the optical scanning apparatus  900  includes, for the optical source  11  thereof, a laser apparatus equivalent to any of the laser-diode pumped solid-state laser apparatuses  100 - 500  and  700 . 
     Further, according to the laser printer  1000  of the present embodiment, it becomes possible to form high-quality images in view of the fact that the laser printer  1000  includes the optical scanning apparatus  900 , which in turn includes a laser-diode pumped solid laser apparatus equivalent to any of the laser-diode pumped solid laser apparatuses  100 - 500  or  700  noted before. 
     Further, in the optical scanning apparatus  900  of the foregoing embodiment, the optical source  11  may include the foregoing laser apparatus in plural numbers. In such a case, it becomes possible to carry out plural scanning simultaneously, and as a result, it becomes possible to form images with high speed with the laser printer  1000 . 
     Further, with the foregoing embodiment, it is possible to use a MEMS (micro elector mechanical systems) mirror in place of the polygonal mirror  14 . In this case, the deflection direction of the optical beam is controlled by controlling the deflection angle of the MEMS mirror. 
     Further, while the foregoing embodiment has been explained for the case in which the image forming apparatus in the laser printer  1000 , the present invention is by no means limited to this specific application. In summary, it becomes possible to form high-quality images with stability by using the laser apparatus equivalent to any of the laser-diode pumped solid-state laser apparatuses  100 - 500  or  700  for the image forming apparatus. 
     Further, the image forming apparatus may be the one that includes a laser apparatus equivalent to any of the laser-diode pumped solid-state laser apparatuses  100 - 700  and irradiates the laser beam directly to the medium such as a sheet that shows coloring with laser irradiation. 
     &lt;&lt;Display Apparatus&gt;&gt; 
       FIG. 20  shows a schematic construction of a laser display apparatus  2000  as a display apparatus according to an embodiment of the present invention. 
     The laser display apparatus  2000  comprises an optical source  101 , an optical system  103  including a mirror and directing a laser light from the optical source  101  to a screen  104 , and a control apparatus  105  for controlling the optical source  101  and the optical system  103 , wherein the optical source  101  includes a laser apparatus equivalent to any of the laser-diode pumped solid-state laser apparatuses  100 - 700 . 
     Thus, with the laser display apparatus  2000  of the present embodiment, it becomes possible to draw pictures or characters on the screen  104  with high quality as a result of use of any of the laser-diode pumped solid-state laser apparatuses  100 - 700  for the optical source of the laser light. 
     Further, with the use of the present invention also for the optical source  101 , it is possible to improve the display effect with the laser display apparatus that performs image display by laser lights penetrating through the space. 
     INDUSTRIAL APPLICABILITY 
     As explained heretofore, the laser-diode pumped solid-state laser apparatus can provide the laser light of excellent beam quality without inviting increase of size of the laser apparatus. Further, according to the optical scanning apparatus of the present invention, it becomes possible to scan a surface with high precision. Further, according to the image forming apparatus of the present invention, it becomes possible to form high-quality images. Further, according to the display apparatus of the present invention, it becomes possible to display information with high display quality. 
     Further, the present invention is by no means limited to the embodiments described heretofore, but various variations and modifications may be made without departing from the scope of the invention. 
     The present invention is based on Japanese priority application No. 2006-178884 filed on Jun. 29, 2006, the entirety of which are incorporated herein as reference.