Abstract:
A light source apparatus equipped with a GaN type semiconductor laser, wherein deformation of the shape of the light spot due to fluctuations in the drive current of the light emitting element is prevented, is provided. A light source apparatus equipped with a GaN type semiconductor laser is provided with a slit panel or other spatial filter for eliminating stray light, which amounts to 20% or less of the total output occurring when the GaN type semiconductor laser is driven at maximum output, from the light emitted from the GaN type semiconductor laser.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a light source apparatus equipped with a GaN type semiconductor light emitting element, and more particularly to a light source apparatus equipped with a GaN type semiconductor light emitting element that has a stray light eliminating function. 
     Further, the present invention relates to a method of eliminating the aforementioned stray light. 
     Still further, the present invention relates to an image forming apparatus for scanning a photosensitive material with light which has been modulated based on image data and forming an image borne by said image data. 
     2. Description of the Related Art 
     Presently, GaN type semiconductor lasers, which comprise an active layer formed of InGaN, InGaNAs, or GaNAs, that emit blue light are nearing practical application. Further, a so-called SLD (Super Luminescent Diode), which is a light emitting diode provided with an active layer of a striped structure formed of a GaN type semiconductor has also been disclosed, as described in Japanese Unexamined Patent Publication No. 11(1999)-74559, for example. Although this SLD does not generate laser oscillation, because the emission region is controlled by the striped configuration, this SLD can emit a green or blue light beam having a narrow emission angle and a microscopic emission radius. 
     These GaN type semiconductor lasers can be employed advantageously in apparatuses, especially color image forming apparatuses, for forming an image borne by image data by scanning a photosensitive material with a light modulated based on said image data, for example, as a light source that emits blue light. 
     However, according to these GaN type semiconductor light emitting elements (including both a semiconductor laser and a light emitting diode), the stray light characteristically emitted by the semiconductor material is more easily generated. Hereinafter, this phenomenon will be explained in detail. 
     With regard to an SLD or a semiconductor laser of a configuration comprising a layer of AlGaInP, AlGaAs, InGaAsP or the like formed on a GaAs substrate, the GaAs forming the substrate is a material which is absorptive of the wavelengths of the emitted light; further, the contact layer formed under the electrode on the side opposite from the substrate is also formed of an emission absorptive material such as InGaAs or GaAs. Therefore, even if unnecessary stray light not contained within the width of the emissions wavelength range, normally on the order of several μm, is generated, this stray light becomes absorbed by the substrate and poses no particular problems with respect to practical applications. 
     As opposed to this, with regard to a GaN type semiconductor light emitting element, a material transparent to light contained within the emission wavelengths, such as sapphire or SiC, is used for the substrate. As a result, a problem has been encountered wherein stray light travels to the terminal end of the substrate side or the opposing electrode side, is reflected and returned to the vicinity of the emission region and a variety of stray light patterns are formed by a plurality of reflections. 
       FIG. 9  shows a comparative example of the characteristics of the output of the drive current of a GaN type semiconductor laser and a semiconductor laser formed of AlGaInP. As shown in  FIG. 9 , the intensity of the naturally emitted light below the oscillation threshold value is markedly stronger for the GaN type semiconductor laser. 
     For cases in which this type of semiconductor light emitting element is driven by a current larger than the laser oscillation threshold value, because the intensity of the light emitted by the laser oscillation is of a higher magnitude in comparison to the intensity of the naturally emitted light, which becomes the origin of the stray light, this stray light normally does not cause the problem described above. However, in the case that the GaN type semiconductor light emitting element is employed as a recording light source for recording a gradation image, and driven in a low current range with direct modulation in order to make it capable of recording a high gradation image, this stray light comes to pose problems in practical application. 
     That is to say, if the aforementioned semiconductor light emitting element is driven by a low level drive current as described above, the generation of the aforementioned stray light becomes more likely, and in extreme cases, a light emission pattern occurs not only at the stripe portions but over the entirety of the element. The light generated in this way from the portions outside of the stripe portions cause deformation of the spot formed by focusing the recording light, which brings about a degradation of the coupling efficiency of the recording light and the optical system. If such a state is produced, it becomes difficult to accurately control the quantity of the recording light (the exposure light quantity) when a high gradation image is to be recorded, and the image quality of the recorded image is deteriorated. 
     SUMMARY OF THE INVENTION 
     The present invention has been developed in view of the foregoing circumstances, and it is a primary object of the present invention to prevent the changes to the spot shape of the recording light due to fluctuations in the drive current of a light emitting element occurring in a light source apparatus equipped with a GaN type semiconductor light emitting element. 
     Further, another object of the present invention is to prevent deterioration of the image quality, due to changes in the spot shape of the recording light, of an image formed by an image forming apparatus, which comprises a GaN type semiconductor laser as the light source thereof, for forming an image borne by image data by scanning a photosensitive material with a light modulated based on said image data. 
     The light source apparatus equipped with a GaN type semiconductor light emitting element according to the present invention is provided with a spatial filter for eliminating stray light (e.g., the stray light generated when the drive current driving a GaN type semiconductor light emitting element is less than the laser oscillation threshold value) from the light emitted from GaN type semiconductor light emitting element; said stray light amounting to 20% or less of the total output of the light emitted from said GaN type semiconductor light emitting element when said GaN type semiconductor light emitting element is driven at the maximum output thereof. 
     Note that for cases in which a focusing optical system for focusing the light emitted from the GaN type semiconductor light emitting element has been provided, a slit panel or a pinhole panel disposed adjacent to the convergence position of this focusing optical system or, alternatively, a partially reflective mirror that reflects a portion of the focused light near the convergence position, can be used as the spatial filter. 
     Further, a polarization element that eliminates light components other than the TE mode component (a polarized light component having an electrical field vector parallel to the pn junction plane of the GaN type semiconductor light emitting element) of the light emitted from the GaN type semiconductor light emitting element can also be employed as the spatial filter. 
     Meanwhile, the stray light eliminating method according to the present invention comprises the step of eliminating, by use of a spatial filter, stray light from the light emitted from the light source apparatus equipped with a GaN type semiconductor light emitting element; wherein, said stray light amounting to 20% or less of the total output of the light emitted from said GaN type semiconductor light emitting element when said GaN type semiconductor laser is driven at the maximum output thereof. 
     Further, the image forming apparatus according to the present invention is an image forming apparatus for scanning a photosensitive material with a light modulated based on image data to form the image borne by said image data; wherein, the above-described light source apparatus according to the present invention is employed as the light source apparatus thereof. 
     Note that the referents of “photosensitive material” include not only materials in which changes in the concentration thereof occur upon the absorption of light thereby (including cases in which a temporary latent image is formed and the concentration changes are brought about by performing a subsequent development process), but include also photo sensitive heat sensitive materials such as those described in Japanese Unexamined Patent Publication No. 2000-132642. 
     Further, it is desirable that the image forming apparatus according to the present invention be configured so as to modulate the intensity of the light to be used for scanning the photosensitive material to form a concentration gradation image thereon. 
     For cases in which modulated recording light is utilized to record a high gradation image, such as when a photographic image is recorded on a silver halide sensitized medium, in general, a recording light intensity dynamic range capable of expressing at least 256 gradations, and particularly for higher image quality images, a dynamic range of approximately 1:1000 is required. Accordingly, in this case, the light source emitting the recording light becomes utilized for emitting light within a range having a maximum light intensity of {fraction (1/256)} to a minimum output on the order of {fraction (1/1000)}. The percentage of the total output occupied by the intensity of the stray light, which has a larger component of naturally emitted light than induced emission laser or super radiance light, becomes relatively smaller as the output of the light emitting element becomes larger. Therefore, in order to obtain a higher quality image, the light source can be run up to as high an output range as possible to relatively reduce the ratio of the stray light component. 
     However, because there is a limit to the performance capacity of the light emitting element, there is a corresponding limit to how high the output capacity thereof can be improved. Further, there is a problem in that the more the light emitting element is operated at the high output range, the lower the reliability thereof becomes. In investigations about the use of silver halide sensitized media and photographic images, the applicants of the present application have determined that in order to record a high image quality concentration gradation image, the intensity of the stray light must be less than or equal to 20% of the intensity of the total light outputted when the light element output is at its highest. Hereinafter, this point will be described in detail. 
       FIG. 10  is a model drawing of an image exposure apparatus of the type employing a GaN type semiconductor laser, such as that described above, that scans a silver halide sensitized media with a spot beam; the polygon mirror (rotating multi-faced mirror) and other components of the scanning optical system have been omitted from FIG.  10 . In this  FIG. 10 , the laser beam  71  emitted in a dispersed state from the stripe portions of the GaN type laser  70  is converged by the focusing lens  72  so as to be converged onto the silver halide sensitized medium  73  in a microscopic spot  74 . At this time, although naturally emitted light (hereinafter referred to as EL light)  75  of which the emission position as well as the emission direction are random is also emitted from the GaN type semiconductor laser  70 , this EL light  75  is not focused into a spot, and becomes stray light that causes a blurred pattern  76  on the silver halide sensitized medium  73 . 
     Moreover, the fact that these undesirable types of stray light cause problems of a higher degree in silver halide exposure systems, which are capable of recording extraordinarily high sensitivity, high quality images in comparison to electron photographic systems or the like, has been elucidated by the research of the applicants of the present application and others. 
     That is to say, when the silver halide sensitized medium  73  is scanned by the microscopic spot  74  to form a pattern such as the stripe pattern  77  shown in  FIG. 11 , which has a line width approximately the same as the diameter of the microscopic spot  74 , there are cases in which low concentration blurred portions  78  occur between the stripes forming the stripe pattern  77 , as shown in  FIG. 12 , and the originally desired pattern shown in  FIG. 11  is not realized. Because of this, the sharpness of the image becomes reduced, and the image quality of the obtained photographic image is remarkably deteriorated. In performing a detailed evaluation relating to the image quality of this type of photographic image, it has been discerned that the photographic image obtained if the intensity of the stray light present when the gradation image is exposed exceeds 20% of the maximum intensity of the exposure light is an image that can in no way be used as a high image quality image. 
     When intensity modulating the recording light and recording a gradation image, there are also cases in which a low output range, which is 10% or less of the maximum intensity of the recording light, is used; the ratio of the EL light component contained in the recording light becomes larger as the output range of the recording light range used is lowered.  FIG. 13  shows the results of the measurement of the drive current and output characteristics of each of a polarization component of which the polarization directionality is parallel to the pn junction plane (a horizontal polarization component), and a polarization component of which the polarization directionality is perpendicular to the pn junction plane (a perpendicular polarization component) of the light that has been emitted from the GaN type semiconductor laser and split by use of a Glan-Thompson Prism. As shown in  FIG. 13 , because the perpendicular polarization component is formed only of the EL light component, which is not laser oscillation induced, the emission efficiency of this perpendicular component does not change. In contrast to this, the emission efficiency of the horizontal polarization component, which is formed of laser light, increases if the drive current is greater than or equal to the oscillation threshold value (in other words, if the drive current is in the range less than the oscillation threshold value, the emission efficiency is reduced). Therefore, because there is a difference between the emission efficiency of the laser light and the EL light, as the intensity of the light becomes smaller, the percentage of stray light contained in the entire quantity of light is relatively increased, and the negative effect thereof on the image quality is correspondingly increased. Note that this stray light refers to the randomly polarized light emitted from the portions of the semiconductor light emitting element other than the stripe portions of the active layer thereof. Further, this includes light that leaks from the stripe portions of the active layer of the semiconductor light emitting element to the portions other than the stripe portions, and reflected within the interior of the semiconductor light emitting element and emitted outside the element. 
     Based on the forgoing information, according to the light source apparatus equipped with a GaN type semiconductor light emitting element and the method of eliminating stray light of the present invention: because the stray light, which is 20% or less of the total light output when the GaN type semiconductor light emitting element is driven at the maximum output thereof, is eliminated from the light emitted from said GaN type semiconductor light emitting element by use of a spatial filter as described above, the stray light (this stray light has the characteristics of the type described above if the GaN type semiconductor light emitting element is driven at the maximum output thereof), which is mainly generated when the GaN type semiconductor light emitting element is driven by a drive current in the range less than the laser oscillation threshold value, for example, is eliminated by the spatial filter. Accordingly, the changes caused to the spot shape of the laser beam by this stray light can be prevented. 
     Therefore, the light source apparatus according to the present invention is capable of accurately controlling the quantity of recording light (exposure light quantity), and can be employed advantageously in the printing, photography, and medical imaging fields wherein high image quality gradation exposures are sought. 
     For example, the image forming apparatus according to the present invention, which employs the light source apparatus according to the present invention, is capable of preventing the spot shape of the laser beam from becoming a blurred pattern, whereby the image quality can be improved. 
     In particular, for cases in which a configuration of the image forming apparatus wherein the recording light thereof is intensity modulated and a concentration gradation image is formed on a photosensitive medium is presumed, as explained with reference to  FIG. 12 , the formation of a blurred pattern by stray light on the portions on which the original image does not appear does not occur, whereby it becomes possible to form a high image quality concentration gradation image having a high degree of sharpness. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic plan view of the light source apparatus according to the first embodiment of the present invention, 
         FIG. 2  is a vertical cross-sectional view of the GaN type semiconductor light emitting element employed in the light source apparatus shown in  FIG. 1 , 
         FIG. 3  is a schematic plan view of a light source which is provided as a comparative example to that of the present invention, 
         FIG. 4  is a graph showing the characteristic relation between the semiconductor drive current and the emission output characteristics occurring in the light source apparatus shown in  FIG. 1  along with that of a comparative example, 
         FIG. 5  is a schematic plan view of the second embodiment of the light source apparatus according to the present invention, 
         FIG. 6  is a graph showing the characteristic relation between the semiconductor drive current and the emission output characteristics occurring in the light source apparatus shown in  FIG. 5  along with that of a comparative example, 
         FIG. 7  is a schematic plan view of a light source apparatus according to the third embodiment of the present invention, 
         FIG. 8  is a schematic plan view of the light source apparatus according to the fourth embodiment of the present invention, 
         FIG. 9  is a graph showing comparative examples of the characteristic relation between the semiconductor drive current and the emission output characteristics of a GaN type semiconductor laser and those occurring in another type of semiconductor laser, 
         FIG. 10  is a model drawing of a type of image exposure apparatus employing a GaN type semiconductor laser, wherein a silver halide sensitized material is scanned with a spot beam recording light, 
         FIG. 11  is a schematic drawing of an example of an exposure pattern produced by the image exposure apparatus shown in  FIG. 10 , 
         FIG. 12  is a schematic drawing of another example of an exposure pattern produced by the image exposure apparatus shown in  FIG. 10 , 
         FIG. 13  is a graph showing the relation between the semiconductor drive current and the emission output characteristics for each polarization component occurring in the GaN type semiconductor laser, 
         FIG. 14  is a block diagram of an image forming apparatus according to an embodiment of the present invention, 
         FIG. 15  is a perspective view of the exterior of the image forming apparatus shown in  FIG. 14 , 
         FIG. 16  is a perspective view of a portion of the optical system employed in the image forming apparatus shown in  FIG. 14 , and 
         FIG. 17  is a block drawing of the control portion of the image apparatus shown in FIG.  14 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter the preferred embodiments of the present invention will be explained with reference to the attached drawings.  FIG. 1  is a schematic plan view of the light source apparatus equipped with a GaN type semiconductor laser according to the first embodiment of the present invention, and  FIG. 2  is a model drawing of a vertical cross-section of the semiconductor laser  20  employed in the light source apparatus shown in FIG.  1 . 
     First, the semiconductor laser  20  will be explained in detail with reference to FIG.  2 . This semiconductor laser  20  comprises: a double hetero configuration consisting of an active layer  7  sandwiched between a clad layer  6  and a clad layer  8 ; and a stripe shaped current injection opening (a portion of cap layer  10 ) for containing the light; wherein the oscillation wavelength thereof is 400 nm. Further, the cleavage plane of the element serves as a reflective surface, whereby an optically reflective configuration is formed. 
     Hereinafter, a simple explanation of the manufacturing method of the layer configuration of the semiconductor laser  20  will be provided. After a low temperature n-GaN buffer layer  2  has been formed on a sapphire c surface substrate  1  by use of an MOCVD method, a stripe shaped SiO 2  mask  14  is formed. Next, a n-GaN buffer layer  3  (Si doped, 5 μm), an n-In 0.05 Ga 0.95 N buffer layer  4  (Si doped, 0.1 μm), an undoped active layer  7 , a p-GaN light guiding layer  8  (Mg doped, 0.1 μm), a p-Al 0.1 Ga 0.9 N clad layer  9  (Mg doped 0.5 μm), and a p-GaN cap layer  10  are formed sequentially thereon. Then, a p-type impurity is activated by use of a heat process in a nitrogen gas atmosphere. 
     Note that the active layer  7  is of a triple quantum well configuration formed of: undoped In 0.05 Ga 0.95 N (10 nm); an undoped In 0.28 Ga 0.72 N quantum well layer (2.5 nm, wavelength 488 nm); undoped In 0.05 Ga 0.95 N (5 nm); an undoped In 0.28 Ga 0.72 N quantum well layer (2.5 nm); undoped In 0.05 Ga 0.95 N (5 nm); an undoped In 0.28 Ga 0.72 N quantum well layer (2.5 nm); undoped In 0.05 Ga 0.95 N (5 nm); and undoped Al 0.1 Ga 0.9 N (10 nm). 
     Next, in order to form a ridge stripe 6 μm in width, the epitaxial layer other than the ridge stripe portion from the cap layer  10  to midway through the clad layer  9  is removed by RIBE (reactive ion beam etching) utilizing chlorine ions. Next, a SiN film  11  is formed, by use of a plasma-activated CVD method, on the exposure surface including the ridge stripe portions. Then, in order to form the n side electrode, the epitaxial layer other than the portion of the light emitting region including the ridge stripe portions is eliminated by use of an etching process employing photo lithography and RIBE utilizing chlorine ions until the n-GaN buffer layer  3  is exposed. Note that at this time a resonator end face is formed. 
     Then, a stripe shaped opening (10 μm in width) into which electrical current is injected is formed on the Si film  11  on the upper surface of the ridge portion, and after Ni/Al has been applied as a p side electrode  12  by use of a vacuum deposition method so as to cover said stripe shaped opening and Ti/Al has been applied to the exposed portion of the n-GaN buffer layer  3  as an n side electrode  13  by use of a vacuum deposition method, an ohmic electrode is formed by annealing within nitrogen. 
     Note that the following is an example of the dimensions of the semiconductor laser  20  shown in FIG.  2 : W 1 =2 μm; W 2 =300 μm; H 1 =0.5-1 μm; H 2 =3-5 μm; and H 3 =100 μm. 
     Next, the light source apparatus shown in FIG.  1  and equipped with this semiconductor laser  20  will be explained in detail. As shown in  FIG. 1 , this light source apparatus comprises: the semiconductor laser  20 ; a focusing lens  22  for focusing the 400 nm laser beam  21  emitted in a dispersed state from the semiconductor laser  20 ; and a slit panel  23  disposed at the convergence position of the laser beam  21  focused by the focusing lens  22 . Note that photodetector  30  shown in  FIG. 1  is a photodetector for detecting the quantity of light of the laser beam  21 . 
     The semiconductor laser  20  shown in  FIG. 1  is disposed so that the pn junction plane is parallel to the surface of the drawing sheet. On the other hand, the slit panel  23  is disposed so that elongated slit  23   a  extends in the direction perpendicular to the surface of the drawing sheet. Further, as to the focusing lens  22 , that having an opening number of NA=0.75 can be used therefor, and the optical loss occurring due to the insertion of the lens is controlled to approximately 10%. In order to confirm the efficacy of the slit panel  23 , as shown in  FIG. 3 , a system has been built wherein the laser beam  21  emitted from the semiconductor laser  20  in a dispersed state is received directly by the photodetector  30 . Accordingly, the system shown in FIG.  3  and the light source apparatus shown in  FIG. 1  each change the drive current of the semiconductor laser  20 , and the accompanying change in the light output is measured by the photodetector  30 . The result of this measurement is shown in FIG.  4 . Note that in  FIG. 4 , curve a shows the measurement result for the case in which there is no slit (the configuration shown in FIG.  3 ), and curves b and c show the measurement results for cases in which the width of the slit  23   a  shown in  FIG. 1  is 1 mm and 0.7 mm, respectively. 
     In the example shown in  FIG. 4  the laser oscillation threshold value current is approximately 38 mA. The value of the output occurring in the range larger than this threshold current, that is, the output in the laser oscillation range, is almost unchanged whether or not there is a slit panel  23  present. The difference therebetween lies in the difference in the degree of EL light outputted in the range below the oscillation threshold value; for example, the difference between the light output shown by the curves a and c occurring at 40 mA is approximately 0.1 mW. That is to say, regarding the oscillation light emitted from the stripe portions of the active layer  7  of the semiconductor laser  20 , it can be stated that the slit panel  23  causes almost no loss in light output. 
     In contrast to this, in the range below the aforementioned threshold current, that is, in the naturally emitted light range, the light output for the case in which a slit panel  23  has been provided is reduced to approximately ½ of that occurring in the case in which the slit panel  23  is not provided. That is to say, it can be stated that in the naturally emitted light range the stray light emitted from the portions other than the stripe portions of the active layer  7  of the semiconductor laser  20  is cutoff by the slit panel  23 . 
     It can be clearly seen in  FIG. 4  that in the naturally emitted light range, approximately  ½ of the light quantity of the light emitted from the semiconductor laser 20 is stray light. If the quantity of stray light is large in this way, when this stray light becomes mixed with the laser beam 21, the spot shape of the laser beam 21 becomes deformed; therefore, for cases in which said light source apparatus is employed in a high gradation image recording apparatus, it becomes difficult to accurately control the light quantity of the recording light (exposure light), and the image quality of the recorded images becomes deteriorated. However, if stray light of this type can be cutoff by the slit panel 23, it is possible to avoid these types of problems.    
     Note that if the width of a slit  23   a  of the slit panel  23  is made to be very near the width of the emission, it becomes difficult to modulate the optical system; although the permissible degree of mechanical vibration is reduced, even if the width of the slit panel  23   a  is made comparatively large, that is, 1 mm or 0.7 mm as described above, a result wherein there is a remarkable reduction in the stray light is obtained. In general, if this slit width is less than or equal to twice the spot diameter of the light at the convergence portion, a clear result showing that the stray light has been eliminated can be obtained. Note that for the case of the configuration shown in  FIG. 1 , if the width of the slit  23   a  is made to be 0.5 mm or less, the quantity of transmitted light is dramatically reduced. 
     According to the explanation provided above regarding the configuration shown in  FIG. 1 , the stray light expanding in the direction perpendicular to the pn conjunction surface (the direction perpendicular to the surface of the drawing sheet) of the semiconductor laser  20  cannot be eliminated by the slit panel  23 . In order to eliminate that type of stray light, a pin hole panel can be used instead of the slit panel  23 . 
     Similar effects can also be obtained by employing a partially reflective mirror for partially reflecting the laser beam  21  in the vicinity of the convergence position thereof. 
     Next, another embodiment of the present invention will be explained.  FIG. 5  is a schematic plan view of the second embodiment of the light source apparatus equipped with a GaN type semiconductor laser according to the second embodiment of the present invention. Note that elements included in  FIG. 5  that are the same as those shown in  FIG. 1  are likewise labeled, and in so far as it is not particularly required, further explanation thereof has been omitted (the same applies to all embodiments hereinafter). 
     According to the second embodiment of the present invention, the 400 nm wavelength laser beam  21  emitted from the semiconductor laser  20  is collimated by a collimator lens  40 , and is then passed through a Glan-Thompson prism  41 . Then, the laser beam  21  that has passed through the Glan-Thompson prism  41  is focused by a focusing lens  42  and received by a photodetector  30 . 
     The semiconductor laser  20  shown in  FIG. 5  is diposed so that the pn junction plane thereof is parallel to the surface of the drawing sheet. Meanwhile, the Glan-Thompson prism  41 , which serves as the polarization element, is disposed at an angle determined so as to transmit only the TE mode component of the laser beam  21  (the polarization component having an electric field vector parallel to that of the pn junction plane), and so that the other polarization components are eliminated. 
     In order to confirm the efficacy of the Glan-Thompson prism  41 , the drive current of the semiconductor laser  20  in each of the light source apparatus shown in FIG.  5  and the system shown in  FIG. 3  described above were changed, and the accompanying change in the light output was measured by the photodetector  30 . The result of this measurement is shown in FIG.  6 . Note that in  FIG. 6 , curve a shows the measurement result for the case in which there is no Glan-Thompson prism  41  and no slit panel  23  (the configuration shown in FIG.  3 ), and curve d show the measurement results for the case in which a Glan-Thompson prism  41  has been provided (the configuration shown in FIG.  5 ). Further, for the sake of reference, the characteristics for the case in which the width of the slit  23   a  occurring in the configuration shown in  FIG. 1  is 0.7 mm is shown by the curve c. 
     In the example shown in  FIG. 6 , the laser oscillation threshold value current is also approximately 38 mA. The output occurring above this threshold current range, that is, in the laser oscillation range, is of a value that is almost unchanged whether the Glan-Thompson prism  41  has been provided or not. As shown in the enlarged view within  FIG. 6 , the difference in light output after the laser oscillation threshold value has been reached substantially matches the output difference of the EL light occurring before the laser oscillation value has been reached; in this example, the light output difference between curves a and d, as well as the light output difference of the curves d and c is approximately 0.4 mW. That is to say, regarding the TE mode oscillation light emitted from the stripe portions of the active layer  7  of the semiconductor laser  20 , there is almost no loss incurred thereof due to the Glan-Thompson prism  41 . 
     In contrast, for cases in which the Glan-Thompson prism  41  has been provided, the light output occurring in the naturally emitted light range, that is, in the range below the oscillation threshold value current, is reduced markedly compared to the case in which the Glan-Thompson prism  41  has not been provided. That is to say, it can be considered that in this naturally emitted light range, the randomly polarized stray light emitted from portions other than the stripe portions of the active layer  7  of the semiconductor laser  20  is by and large cutoff by the Glan-Thompson prism  41 . 
     Note that in the example shown in  FIG. 6 , although the stray light eliminating efficacy for the case in which a slit panel  23  has been inserted is higher compared to the case in which a Glan-Thompson prism  41  has been inserted, this improvement in the efficacy regarding eliminating stray light lies in the structure or characteristics of each individual element. Accordingly, it is possible to optimize the efficacy of the present invention by selecting and matching the elements to be employed for eliminating stray light. For cases in which a slit panel is employed, a focusing optical system is required for converging the laser beam, and accurate optical adjustments are also required; however, for cases in which a polarizing element is employed, the optical adjustments can be completed with less stringent accuracy and a high degree of freedom is attained in regards to the insertion position of the element. 
     Although the two embodiments explained above have been equipped only with the basic structure formed of the core portion of the light source apparatus, it is possible to provide the light source apparatus according to the present invention with a scanning optical system formed by utilizing a polygon mirror (a rotatable mirror) or a galvano mirror for scanning or the like. In this case, lenses and other required optical elements can be combined appropriately to form an optical system such as one of those shown in  FIGS. 7 and 8 . 
     In addition to the configuration shown in  FIG. 1 , the third embodiment shown in  FIG. 7  comprises an optical system provided with a focusing lens  50  for focusing the laser beam  21  that has passed through the slit panel  23 , and a cylindrical lens  51  for focusing the laser beam  21  only in the direction perpendicular to the surface of the drawing sheet. 
     Further, the forth embodiment shown in  FIG. 8  comprises an optical system provided with a collimator lens  40  and a focusing lens  42  of the same type as those employed in the configuration shown in  FIG. 5 , in addition to a focusing lens  60  for converging the laser beam  21  that has been collimated by the collimator lens  40  onto the position of the slit panel  23 , and a collimator lens  61  for collimating the laser beam  21  that has passed through the slit panel  23 . 
     Next, an embodiment of the image forming apparatus according to the present invention will be explained with reference to  FIGS. 14  to  17 . Note that a digital lab system is proffered as an example of the image forming apparatus according to the present invention. 
     First, a general explanation of the entire system will be provided.  FIG. 14  is a schematic drawing of the digital lab system  110 , and  FIG. 15  is an exterior view of the digital lab system shown in FIG.  14 . As shown in  FIG. 14 , the lab system  110  comprises: a line CCD scanner  114 ; a an image processing portion  116 ; a laser printer portion  118 , which is an image forming apparatus according to the current embodiment; and a processor portion  120 ; wherein, the line CCD scanner  114  and the image processing portion  116  are provided in an integrated form as the input portion  126  shown in  FIG. 126 , and the laser printer portion  118  and the processor portion  120  are provided in an integrated form as the output portion  128  shown in FIG.  15 . 
     The line CCD scanner  114  is a means for reading out a film image (a positive or a negative image obtained by developing a photographed image) that has been recorded on a photosensitive medium such as a negative film or a reversal film (hereinafter referred to simply as a photographic film) the line CCD scanner  114  is capable of reading out a photographic image from, for example; a 135 size photographic film, a 110 size photographic film; a photographic film on which a transparent magnetic layer has been formed (240 size photographic film: so-called APS film); and 120 and 220 size (blowny size) photographic film. The line CCD scanner  114  reads out the subject film image by a three-line color CCD, and outputs image data spanning each color data: R (red), G (green), and B (blue). 
     As shown in  FIG. 15 , the line CCD scanner  114  is installed on the operations table  130 . The image processing portion  116  is housed within the housing portion  132  formed on the bottom side of the operations table  130 , and an opening and closing door  134  is provided at the opening of the housing portion  132 . The interior portion of the housing portion  132  is normally in the covered state wherein it is concealed by the opening and closing door  134 ; if the opening and closing door is rotated the interior portion is exposed, and it becomes possible to remove the image processing portion  116 . 
     Further, a display  164  is provided towards the rear of the operations table  130  and two types of keyboards,  166   a  and  166   b  are jointly provided. The keyboard  166   a  is provided as a unit built into the operations table  130 . On the other hand, the keyboard  166   b  is provided so as to be able to be stored within a drawer  136  of the operations table  130  when not in use; when the keyboard  166   b  is to be used, it is removed from the drawer  136  and stacked on the keyboard  166   a . When the keyboard  166   b  is to be used, by connecting the connector (not shown), which is provided on the distal end of the cord extending from the keyboard  166 , to the jack  137  provided on the operations table  130 , the keyboard  1666   b  becomes electrically connected to the image processing portion  116  via the jack  137 . 
     Further, a mouse  140  is provided on the operations surface  130   u  of the operations table  130 . The cord of the mouse  140  extends through a hole  142  provided on the operations table  130  to the interior of the housing portion  132 , wherein it is connected to the image processing portion  116 . The mouse  140  is stored in the mouse holder  140   a  when not in use; when the mouse  140  is to be used it is removed from the mouse holder  140   a  and placed on the operations surface  130   u.    
     The image processing portion  116  inputs the image data outputted from the line CCD  114 , that is, the scanner image data, and is also configured so as to be able to input image data obtained from a digital camera, image data obtained by scanning and reading out an original film image other than a reflection original or the like, image data formed by a computer or the like (hereinafter referred to as file image data) from an external portion. This input is obtained by way of a recording medium such as a memory card or the like, or by way of a communications circuit. 
     The image processing portion  116  performs various types of image processes, such as a correction process or the like, on the inputted image data, and inputs the processed image data obtained thereby into a laser printer portion  118  as recording image data. Further, the image processing portion  116  is configured so as to be capable of outputting processed image data to an external portion as an image data file. This output is recorded on a data recording medium such as a memory card or the like, or is transmitted to other image processing devices over a communications circuit, etc. 
     The laser printer portion  118  is equipped with R, G, and B laser light sources, and irradiates onto printing paper laser light modulated according to the recording image data inputted from the image processing portion  116  to record the image (latent image) onto the printing paper by use of scanning exposure light. Further, the processor portion  120  performs each type of process, such as color development, bleaching, washing, drying, and the like, on the printing paper on which the latent image has been formed by the scanning exposure light. The image is formed on the printing paper in this manner. 
     Next, the configuration of the laser printer portion  118  will be explained in detail.  FIG. 16  shows the optical system of the laser printer  118 . As shown in  FIG. 16 , the laser printer portion  118  comprises three laser light sources: laser light source  211 R,  210 G, and  211 B. The laser light source  211 R is formed of an LD (semiconductor laser) that emits laser light in the red range (hereinafter referred to as R laser light) of, for example, a 685 nm wavelength. Further, the laser light source  210 G comprises: an LD  210 L, which serves as a light emitting means, and a wavelength converting element (SHG element)  210 S for converting the laser light emitted from said LD  210 L to laser light of half the wavelength thereof. An oscillation wavelength of 1064 nm, for example, is employed for the LD  210 L, whereby laser light in the green range (hereinafter referred to as G laser light) having a wavelength of 532 nm is emitted from the SHG element  210 S. 
     Further, the laser light source  211 B is formed of a light source apparatus that emits laser light in the blue range (hereinafter referred to as B laser light) of, for example, a 440 nm wavelength. According to the current embodiment, a light source apparatus equipped with a GaN type semiconductor laser such as that shown in  FIG. 1  is employed as the aforementioned light source apparatus. 
     A collimator lens  212  and an AOM (acoustic optical modulator)  214 G, which serves as an external modulating means, are disposed sequentially along the optical path of the laser light emitted from the laser light source  210 G. The AOM  214 G is disposed so that the light inputted thereto passes through an acoustic optical modulating medium, and is connected to an AOM driver (not shown). When a high frequency signal is inputted from the AOM driver, an ultrasonic frequency corresponding to the high frequency signal is propagated within the acoustic optical modulating material, and the laser light passing through the acoustic optical modulating material is refracted by the effect of the acoustic optical modulation; a refracted laser light of an intensity corresponding to the oscillation width of the high frequency signal is thereby emitted from the AOM  214 G. 
     A flat mirror  215  is disposed along the light path of the light emitted from the AOM  214 G; a spherical lens  216 , a cylindrical lens  217 , and a polygon mirror (rotatable multi-faced mirror)  218  are disposed sequentially along the light path of the light reflected by said flat mirror  215 . The G laser light emitted from the AOM  214 G is reflected by the flat mirror  215 , passes through the spherical lens  216  and the cylindrical lens  217 , and then impinges on a predetermined position of the reflection surface of the polygon mirror  218 , whereby it is reflected and deflected by said polygon mirror  218 . 
     Meanwhile, a collimator lens  213  and a cylindrical lens  217  are disposed sequentially on the laser light emitting side of the laser light source  211 R and the laser light source  211 B; the laser beams emitted from the laser light sources  211 R and  211 B, respectively, are collimated by the collimator lens  213 , pass through the cylindrical lens  217  to impinge upon substantially the same predetermined position of the reflection surface of the polygon mirror as that described above, and are reflected and deflected by said polygon mirror  218 . 
     The three laser beams R, G, and B reflected and deflected by the polygon mirror  218  pass through an fθ lens  220  and a cylindrical lens  221  sequentially, and after being reflected by a cylindrical mirror  222 , are projected onto the printing paper  224  through an aperture portion  226  after being reflected in a substantially vertical downward direction by a return mirror  223 . Note that the return mirror  223  can be omitted and the laser light may be reflected by the cylindrical mirror  222  directly in a substantially vertical downward direction and projected onto the printing paper  224 . 
     Meanwhile, a scanning start detecting sensor (hereinafter referred to as a SOS detecting sensor)  228  for detecting the R laser light that has arrived thereat through the aperture portion  226  is disposed adjacent to the scanning exposure light starting position. Note that the reason the laser light detected by the SOS detecting sensor is the R laser light is that because the light sensitivity of the printing paper is lowest with respect to the R laser light, the light quantity of the R laser light is the largest and is therefore capable of being detected accurately, and the R laser light is reflected by the rotation of the polygon mirror  218  so that said R laser light reaches the SOS detecting sensor  228  fastest. Further, the SOS detecting sensor  228  according to the current embodiment is configured so that the output signal thereof (hereinafter referred to as a sensor output signal) is normally a low level signal, and only when R laser light has been detected does the signal become a high level signal. 
     The image forming apparatus according to the current embodiment is provided with the control portion shown in FIG.  17 . This control portion has a control circuit  180  including a micro computer. The control circuit  180  is connected to a bus  188 ; image memories  174 ,  176 , and  178  are connected to said bus  188 . That is to say, the image memories  174 ,  176 , and  178  are provided as the recording memory that records the image data for recording an image onto the printing paper  224 . The image data memory  174  is a memory for recording the R image data; in the same manner, the image data memory  176  is a memory for recording the G image data, and the image data memory  178  is a memory for recording the B image data. 
     Further, the bus  188  comprises: an R-LD drive circuit  196  for driving the R laser light source; a G-LD drive circuit  198  for driving the G laser light source; and a B-LD drive circuit  100  for driving the B laser light source; wherein the R-use LD drive circuit  196  and the B-use LD drive circuit  100  are connected via modulation circuits  190  and  192 , respectively. That is to say, the modulation circuits  190  and  192  form modulation signals based on the image data inputted thereto, and by superimposing the drive current of the LD drive circuits  196  and  100  on these modulation signals, the intensity of each LD forming the laser light source  211 R and the laser light source  211 B is directly modulated. 
     Further, an AOM drive current circuit  194  is connected to the bus  188 ; the driving of the AOM  214 G is controlled, and the G laser light emitted from the laser light source  214 G is modulated by this AOM  214 G. 
     Still further, the bus  188  is also connected to a polygon motor drive circuit  182  for driving the polygon motor  183  that drives the rotation of the polygon mirror  218 , and a printing paper conveyance motor drive circuit  184  for driving the printing paper conveyance motor  186  that conveys at a uniform speed the printing paper  224 ; each of these drive circuits is controlled by the control circuit  180 . 
     Hereinafter, the operation of the laser printer portion  118  will be explained. When an image is to be recorded onto the printing paper  224 , the control circuit  180  of the control portion shown in  FIG. 17  performs, based on the image recording parameters inputted from the image processing portion  116  shown in  FIG. 14 , various types of correction processes on the recording image data to form a scanning exposure light image data in order to record onto the printing paper  224  by the scanning exposure light the image represented by the recording image data inputted from the image processing portion  116 ; said scanning exposure light image data is recorded in the image data memories  174 ,  176 , and  178 . 
     Then, the control circuit  180  is drives the polygon motor  183  so as to rotate the polygon mirror  218  in the direction indicated by the arrow mark A shown in  FIG. 16 , and supplies drive current to the semiconductor lasers of the laser light sources  211 R,  210 G, and  211 B, whereby each respective color of laser light is emitted. Further, the control circuit  180  forms a modulation signal based on the scanning exposure light image data; the amplitude of the high frequency signal supplied, according to the level of the modulation signal, to the AOM  214 G is changed, and the G laser light emitted from the AOM  214 G is modulated thereby. Accordingly, this G laser light is intensity modulated in accordance with the concentration of the image to be recorded on the printing paper  224 . This G laser light is irradiated onto the printing paper  224  by way of the flat mirror  215 , the spherical lens  216 , the cylindrical lens  217 , the polygon mirror  218 , the fθ lens  220 , the cylindrical lens  221 , the cylindrical mirror  222 , and the return mirror  223 . 
     Further, by modulating the drive current value applied to the laser light sources  211 R and  211 B, the control circuit  180  intensity modulates the laser light emitted therefrom. Accordingly, the laser light sources  211 R and  211 B emit R laser light and B laser light, respectively, that has been intensity modulated in accordance with the concentration of the image to be recorded on the printing paper  224 . These R and B laser beams are each projected onto the printing paper  224  by way of the collimator lens  213 , the cylindrical lens  217 , the polygon mirror  218 , the fθ lens  220 , the cylindrical lens  221 , the cylindrical mirror  222 , and the return mirror  223 . 
     Then, the spot of each of the R, G, and B laser light, which have been deflected with the rotation of the polygon mirror  218 , is moved in the direction indicated by the arrow mark B shown in FIG.  16  and scanned across the printing paper  224 , and the printing paper  24  is conveyed at a uniform speed in the direction indicated by the arrow mark C shown in  FIG. 16 , whereby the widthwise scanning of each laser light is performed, and a two-dimensional image (a latent image) is formed on the printing paper  224  by this scanning exposure light. 
     The printing paper  224  on which an image has been formed by said scanning exposure light is sent into the processor portion  120 , wherein each type of process, such as color development, bleaching, washing, drying, and the like, is performed thereon. In this manner, the photographic latent image formed on the printing paper is developed. 
     Note that the modulation of the timing of the laser light or of the timing of the conveyance of the printing paper is determined based on the output signal of the SOS detecting sensor  228 . 
     Here, the laser printer  118  according to the current embodiment, because a light source apparatus utilizing a GaN type semiconductor laser is employed as laser light source  211 B for emitting B laser light, the above-described stray light is emitted therefrom concurrently with the B laser light. However, according to this light source apparatus shown in  FIG. 1 , because the stray light on a path toward the printing paper  224  is cutoff by the slit panel  23  as described above, a reduction in the sharpness of the image recorded on the printing paper  224  due to this stray light can be prevented, and it is possible to record a high image quality concentration gradation image. 
     Note that the light source apparatus according to the present invention and employed in an image forming apparatus is not limited to the particular apparatus of the embodiment shown in  FIG. 1 ; the light source apparatuses according to other embodiments as well are capable of being readily employed.