Patent Publication Number: US-2009237955-A1

Title: Optical device and image exposure apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is related to an optical device equipped with a light emitting means in which a light emitting element is sealed in a sealing portion. The present invention is also related to an image exposure apparatus that employs the optical device. 
     2. Description of the Related Art 
     A conventional optical device that focuses a light beam emitted from a light source with an optical system and causes the light beam to enter an optical fiber is illustrated in  FIG. 16 . In this optical device, a transparent member  503 , of which the side facing a light source  501  is cut obliquely, is placed in the optical path of a light beam which is emitted from the light source  501  and focused by a focusing lens  502 . An optical contact is established between the side (light output surface) of the transparent member  503  which is not cut obliquely and an optical fiber  505 . This configuration reduces noise caused by light, which is reflected at light input surfaces of optical fibers, returning to light sources. 
     However, in the above conventional optical device, surfaces of the components provided in the optical path which are exposed to the atmosphere, for example, the light input surface  504  of the transparent member  503  illustrated in  FIG. 16 , become contaminated by matter such as dust being attached thereto. This causes a problem that the light output from an output end surface of the optical fiber  505  decreases. Particularly in cases that the wavelength of emitted light is less than or equal to 500 nm, the light energy is great, and easily influenced by contamination. In addition, the rate of decrease of the light output becomes greater as the light density of the light that passes through the contaminated surface increases. 
     The present inventors investigated to find to what degree the light density of light that passes through the light input surface  504  which is exposed to the atmosphere needed to be decreased in order to suppress the decrease in light output caused by contamination. As a result of the investigation, it was discovered that a linear correlative relationship exists between the degree of decrease in output and the light density of the light that passes through the light input surface  504  (refer to Japanese Patent Application No. 2007-121102). This relationship will be described below. 
     The present inventors focused a laser beam emitted from a laser  501  driven within a range of 50 mW to 400 mW with a lens  502  such that a predetermined power density was obtained. A transparent member  503  formed of glass was placed in the vicinity of the focal point of the laser beam, and the transmittance rate of the laser beam over time was measured. In addition, measurements were repeatedly performed while changing the light density at a light input surface  504 , by moving the transparent member  503  along the optical axis of the laser beam. 
     The results of the above experiment are illustrated in  FIG. 17 . The horizontal axis represents the light density (W/mm 2 ) of the laser beam at the light input surface  504  of the transparent member  503 . The vertical axis represents the degree of output decrease of light output due to contamination, that is, the rate of output decrease of the laser beam which has passed through the transparent member  503  per hour. Note that in  FIG. 17 , the circles indicate actual measured values, and the line illustrated in the graph was derived by the method of least squares. The following formula represents the line. 
       Log  R=− 6.5+0.9·Log( P/S )   (1) 
     Here, R is the rate of output decrease due to contamination of the light input surface of the transparent member  503  per hour (/hour), P is the output value (W) of the laser beam, and S is the transmittance area (mm 2 ) of the laser beam at the light input surface of the transparent member. 
     Here, the lifetime of a laser element is defined as the point in time at which the output of the laser element decreases from a predetermined output by 20%. In the case that a laser element having a lifetime of 10000 hours is utilized, it is desirable for the decrease in output due to contamination until the end of the element&#39;s life is 1/10 or less the decrease in the output of the laser element, that is, 2% or less. For this reason, the allowable rate of output decrease (/hour) is 0.02/10000=2.0·10 −6 . According to the graph of  FIG. 17 , the light density that corresponds to this value is 8 (W/mm 2 ). 
     Accordingly, in the configuration illustrated in  FIG. 16 , the decrease in light output caused by contamination can be suppressed by causing the light density at the light input surface  504  of the transparent member  503  to be 8 (W/mm ) or less. Specifically, factors such as the output value of the light source  501 , the magnification rate of the lens  502 , the length of the transparent member  503  in the direction of the main axis of the laser beam, and the refractive index of the transparent member  503  are set such that the light density at the light input surface  504  of the transparent member  503  becomes 8 (W/mm 2 ) or less. 
     Recently, developments in CAN package type light sources, in which light emitting elements that emit light having wavelengths of 500 nm or less are housed, are advancing. There are known light sources of this type which are capable of obtaining output of several hundred mW. The present inventors attempted to utilize a CAN package type light source as the light source of the aforementioned optical device. Transparent members are provided in windows of CAN package type light sources. At first, the present inventors assumed that the relationship between the light density at the window and the degree of deterioration of transmittance rates through the windows is substantially the same as the experimental results disclosed in Japanese Patent Application No. 2007-121102. A CAN package type light source, in which the light density is 4.5 (W/mm 2 ) at the window, was utilized and driven experimentally for 10000 hours. 
     However, when the CAN package type light source was driven for 10000 hours, it was found that the deterioration of transmission rate through the window was greater than expected. From this, it became clear that the light density of 4.5 (W/mm) at the window was too great. However, it was unclear to what degree the light density needed to be decreased in order to suppress the deterioration of transmittance rate through the window. Accordingly, there was a problem that the reliability of the optical device having this configuration would be adversely affected. 
     SUMMARY OF THE INVENTION 
     The present invention has been developed in view of the foregoing circumstances. It is an object of the present invention to provide an optical device equipped with a light emitting means, in which a light emitting element is sealed in a sealing portion, and a window, in which a transparent member is provided, which is capable of suppressing deterioration of the transmittance rate through the transparent member in the window even if driven for long periods of time. It is another object of the present invention to provide an image exposure apparatus that employs the optical device. 
     An optical device of the present invention comprises: 
     light emitting means constituted by: a light emitting element that emits a light beam having a wavelength within a range from 220 nm to 500 nm at an output of 230 mW or greater; a housing having a window that contains the light emitting element in a sealed state therein; and a first transparent member, which is transparent with respect to the light beam, that seals the window; and 
     a focusing optical system that focuses the light emitted by the light emitting element and output through the first transparent member; and is characterized by: 
     the light density of the light beam being 1.15 W/mm 2  or less at the light output surface of the first transparent member. 
     Note that here, “an output of 230 mW or greater” refers to the peak value of the output is 230 mW or greater, regardless of whether the light beam is emitted as pulses or a continuous wave. The “light emitting element that emits a light beam having a wavelength within a range from 220 nm to 500 nm” refers to the peak wavelength of the light beam emitted from the light emitting element being 220 nm or greater and 500 nm or less. The “light output surface of the first transparent member” is a facet of the first transparent member, from which the light beam is output to the exterior of the light emitting means. 
     The first transparent member may be fitted into the window, and protrude toward the exterior of the housing. 
     Alternatively, the first transparent member may abut the housing at the exterior thereof. 
     The optical device may further comprise: 
     an optical fiber provided such that the light which is focused by the focusing optical system enters thereinto. The optical fiber may be formed from quartz. 
     The optical device may further comprise: 
     a second transparent member, which is transparent with respect to the light beam, provided between a light input surface of the optical fiber and the focusing optical system. In this case, the optical fiber may be configured to be removably attached to the second transparent member, and optically positioned by abutting the second transparent member. 
     The optical device may further comprise: 
     a coupling preventing film formed by a fluoride material and having a thickness less than or equal to 1/12 the wavelength of the light beam, provided on one of the light output surface of the second transparent member and the light input surface of the optical fiber. 
     The light emitting element may be a semiconductor laser. The light emitting means may be a 9 mm diameter CAN package that houses the semiconductor laser. 
     The wavelength of the light beam emitted by the light emitting element may be within a range from 370 nm to 500 μm. Alternatively, wavelength of the light beam emitted by the light emitting element may be within a range from 400 nm to 410 nm. 
     An image exposure apparatus of the present invention is characterized by being equipped with the optical device of the present invention as an exposure light source. 
     The optical device of the present invention comprises: the light emitting means constituted by: the light emitting element that emits a light beam having a wavelength within a range from 220 nm to 500 nm at an output of 230 mW or greater (an output having peak values of 230 mW or greater regardless of whether the light beam is emitted as pulses or as a continuous wave); the housing having a window-that contains the light emitting element in a sealed state therein; and the first transparent member, which is transparent with respect to the light beam, that seals the window; and the focusing optical system that focuses the light emitted by the light emitting element and output through the first transparent member. The optical device of the present invention is characterized by the light density of the light beam being 1.15 W/mm 2  or less at the light output surface of the first transparent member. Therefore, deterioration in the transmittance rate at the light output surface of the first transparent member can be suppressed, even if the optical device is driven for a long period of time. 
     The first transparent member may be fitted into the window, and protrude toward the exterior of the housing. In this case, the distance from the light emitting element to the light output surface of the first transparent member increases. Therefore, the light density of the light beam at the light output surface of the first transparent member can be caused to be 1.15 W/mm or less, without increasing the size of the optical device. 
     Alternatively, the first transparent member may abut the housing at the exterior thereof. In this case, the distance from the light emitting element to the light output surface of the first transparent member increases. Therefore, the light density of the light beam at the light output surface of the first transparent member can be caused to be 1.15 W/mm 2  or less, without increasing the size of the optical device. In addition, the area of the light output surface of the first transparent member can easily be set to be greater than the area of the window. Therefore, the degree of freedom in designing the window is improved. 
     The optical device may further comprise: the optical fiber provided such that the light which is focused by the focusing optical system enters thereinto. In this case, the light beam emitted by the light emitting element can be efficiently propagated through the optical fiber. 
     The optical device may further comprise: the second transparent member, which is transparent with respect to the light beam, provided between a light input surface of the optical fiber and the focusing optical system, and the optical fiber may be configured to be removably attached to the second transparent member, and optically positioned by abutting the second transparent member. In this case, positioning of the optical fiber can be facilitated. 
     The optical device may further comprise: the coupling preventing film formed by a fluoride material and having a thickness less than or equal to 1/12 the wavelength of the light beam, provided on one of the light output surface of the second transparent member and the light input surface of the optical fiber. In this case, fusion at the surface where the second transparent member and the optical fiber abut each other can be prevented. 
     The image exposure apparatus of the present invention is characterized by being equipped with the optical device of the present invention as an exposure light source, which is capable of suppressing deterioration in the transmittance rate at the light output surface of the first transparent member, even if driven for a long period of time. Therefore, the reliability of the image exposure apparatus during use for a long period of time is improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional side view that illustrates the schematic structure of an optical device according to a first embodiment of the present invention. 
         FIG. 2  is a graph that illustrates the relationship between the degree of decrease in output and the light density of light that passes through the light input surface of a transparent member. 
         FIG. 3  is a sectional side view that illustrates the schematic structure of an optical device according to a second embodiment of the present invention. 
         FIG. 4  is a sectional side view that illustrates the schematic structure of an optical device according to a third embodiment of the present invention. 
         FIG. 5  is a sectional side view that illustrates the schematic structure of an optical device according to a fourth embodiment of the present invention. 
         FIG. 6  is a perspective view that illustrates the outer appearance of an image exposure apparatus according to an embodiment of the present invention. 
         FIG. 7  is a perspective view that illustrates the construction of a scanner of the image exposure apparatus of  FIG. 6 . 
         FIG. 8A  is a plan view that illustrates exposed regions, which are formed on a photosensitive material. 
         FIG. 8B  is a diagram that illustrates the arrangement of exposure areas exposed by exposure heads. 
         FIG. 9  is a perspective view that illustrates the schematic construction of an exposure head of the image exposure apparatus of  FIG. 6 . 
         FIG. 10  is a schematic sectional view that illustrates the exposure head of the image exposure apparatus of  FIG. 6 . 
         FIG. 11  is a partial magnified diagram that illustrates the construction of a digital micro mirror device (DMD). 
         FIG. 12A  is a diagram for explaining the operation of the DMD. 
         FIG. 12B  is a diagram for explaining the operation of the DMD. 
         FIG. 13A  is a plan view that illustrates the scanning trajectories of exposing beams in the case that the DMD is not inclined. 
         FIG. 13B  is a plan view that illustrates the scanning trajectories of the exposing beams in the case that the DMD is inclined. 
         FIG. 14A  is a perspective view that illustrates the construction of a fiber array light source. 
         FIG. 14B  is a front view that illustrates the arrangement of light emitting points of laser emitting portions of the fiber array light source. 
         FIG. 14C  is a diagram that illustrates the configuration of optical fibers. 
         FIG. 15  is a block diagram that illustrates the electrical configuration of the image exposure apparatus of  FIG. 6 . 
         FIG. 16  is a diagram that illustrates the schematic structure of a conventional optical device. 
         FIG. 17  is a graph that illustrates the relationship between the degree of decrease in output and the light density of light that passes through the light input surface of a transparent member. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter, an optical device  1  according to a first embodiment of the present invention will be described with reference to the attached drawings.  FIG. 1  is a sectional side view that illustrates the schematic structure of the optical device  1  of the first embodiment. 
     As illustrated in  FIG. 1 , the optical device  1  is constituted by: a CAN package  10  having a diameter of 5.6 mm, in which a GaN semiconductor laser LD having an output of 800 mW is hermetically sealed; a focusing lens  40  for focusing a laser beam B (light beam B) emitted by the GaN semiconductor laser LD; a cylindrical transparent member  42 , provided such that the laser beam B which has passed through the focusing lens  40  enters thereinto; an optical fiber  43 , into which the laser beam B which has passed through the transparent member  42  enters; and an optical fiber module  41  equipped with a sleeve  47  for holding the transparent member  42  and the optical fiber  43 . Note that the CAN package  10  functions as the light emitting means of the present invention. 
     The light emission shape of the semiconductor laser LD is 7·1 μm 2 . The horizontal radiation angle is 42 degrees, and the vertical radiation angle is 18 degrees. The semiconductor laser LD is fixed on a block  11  within the CAN package  10  by AuSn brazing material. The block  11  is fixed to a fixing member  12 . A metal case  14  having a circular window  13  is fixed to the fixing member  12  by resistance welding. The window  13  is sealed by a circular transparent plate member  15  and a cylindrical transparent member  16 . The circular transparent plate member  15  and the cylindrical transparent member  16  are formed by glass having Si and O as the main components thereof, such as quartz glass and borosilicate glass. The transparent member  15  is adhesively attached to the case  14  at the interior thereof. The diameter of the cylindrical transparent member  16  (the dimension in the vertical direction in  FIG. 1 ) is equal to the diameter of the window  13 , and the thickness thereof (the dimension in the horizontal direction in  FIG. 1 ) is 1 mm. Note that the distance from the semiconductor laser LD to a light output surface  15   b  of the transparent member  15  is 1 mm. Because the thickness of the transparent member  16  is 1 mm, the distance from the semiconductor laser LD to a light output surface  16   b  of the transparent member  16  is approximately 2 mm. The transmittance area of the laser beam B at the light output surface  16   b  of the transparent member  16  is approximately 0.70 mm 2  (1/e). 
     The end of the transparent member  16  toward a light input surface  16   a  thereof is inserted into the window  13  and abuts the light output surface  15   b  of the transparent member  15 . The end of the transparent member  16  toward the light output surface  16   b  protrudes toward the exterior of the case  14 . An anti reflective coating process is administered onto the light input surface  15   a  of the transparent member  15 , but not on the light output surface  15   b.  An anti reflective coating process is administered onto the light input surface  16   a  of the transparent member  15 , but not on the light output surface  16   b.  Wires  17  and the like for supplying drive current to the semiconductor laser LD are drawn out from the CAN package  10  through openings which are formed in the fixing member  12 . Note that the CAN package  10  is deaerated to remove volatile components, filled with an inert gas, then hermetically sealed. 
     Note that the fixing member  12  and the case  13  function as the housing of the present invention, and the transparent member  16  functions as the first transparent member of the present invention. 
     The lens  40  focuses the laser beam B output from the CAN package  10  onto a spot in the vicinity of the surface at which the transparent member  42  and the optical fiber  43  abut each other, at a predetermined magnification rate (4×, for example). Note that the focal position of the laser beam B is shifted from the abutment surface along the axis of the laser beam B, and is-either within the optical fiber  43  or within the transparent member  42 . 
     The optical fiber  43  is constituted by a core  44 , which is formed by quartz glass, for example, and a cladding  45  provided around the core  44 . Note that the transparent member  42  has an outer diameter greater than the beam diameter of the laser beam B that passes therethrough. That is, the transparent member  42  is configured such that the laser beam B is not obstructed. 
     The outer diameter of the transparent member  42  is equal to the outer diameter of the optical fiber  43 . A light input surface  42   a  of the transparent member  42  is cut obliquely such that an angle of 4 degrees is formed with respect to a direction that perpendicularly intersects the axis of the laser beam B. Thereby, the amount of light that returns toward the CAN package  10  can be reduced, and the coupling efficiency with respect to the optical fiber  43  can be improved. Alternatively, an anti reflective coating may be administered onto the light input surface  42   a,  instead of cutting the light input surface obliquely. This also can reduce the amount of light that returns toward the CAN package  10 . 
     A coupling preventing film  46  having a thickness less than or equal to 1/12 the wavelength of the laser beam B is provided on a light input surface  43   a  of the optical fiber  43 . The material of the coupling preventing film  46  is that which has high transparency with respect to short wavelengths of light (220 nm to 500 nm ). Fluoride materials such as YF 3 , LiF, MgF 2 , NaF, LaF 3 , BaF 2 , CaF 2 , and AlF 3  are examples of such materials. The coupling preventing film  46  is formed by IAD (Ion Assisted Deposition) coating. 
     The transparent member  42  and the optical fiber  43  are held by the cylindrical sleeve  47 . The transparent member  42  is fixed within the sleeve  47  by adhesive attachment. The optical fiber  43  is inserted into the sleeve  47  so as to abut the transparent member  42 . Note that the optical fiber  43  is removable from the sleeve  47 . However, the optical fiber  43  may also be fixed within the sleeve  47  if necessary. 
     As described previously, the present inventors found that that the deterioration of transmission rate through the window of a CAN package due to contamination when the CAN package type light source was driven for 10000 hours was greater than expected. The degree to which the light density needed to be decreased in order to suppress the deterioration of transmittance rate through the window was considered. A CAN package type light source having a semiconductor laser that emits a laser beam with a wavelength within a range from 400 nm to 410 nm housed therein, in which the light density is 4.5 (W/mm 2 ) at the window, was utilized to measure temporal changes in the transmittance rate of the laser beam at the window. 
     The results of the above experiment are illustrated in  FIG. 2 . The horizontal axis represents the light density (W/mm 2 ) of the laser beam at the light output surface of a transparent member. The vertical axis represents the degree of decrease in transmittance rate per hour (/hour). Note that the circles indicate actual measured values of the degree of light output decrease due to contamination of a transparent member provided to abut the light input surface of an optical fiber as disclosed in Japanese Patent Application No. 2007-121102. The line illustrated in the graph was derived by the method of least squares. The following formula represents the line. 
       Log  R=− 6.5+0.9·Log( P/S )   (1) 
     Here, R is the rate of output decrease due to contamination of the light input surface of the transparent member  503  per hour (/hour), P is the output value (W) of the laser beam, and S is the transmittance area (mm 2 ) of the laser beam at the light input surface of the transparent member. 
     In the case that foreign matter becomes attached to the window of a CAN package, reflection and scattering at the light output surface of the window increase. The deterioration of transmittance rate due to contamination is greater at the window of the CAN package, compared to the transparent member which is provided to abut the light input surface of the optical fiber. The inventors considered this phenomenon, and discovered several causes of the deterioration. One cause is that if foreign matter becomes attached to the light output surface of the window of a CAN package, the effects of an anti reflective coating cannot be sufficiently obtained, and the reflectance rate at the window increases. In addition, light scattering due to foreign matter attached to the window is another cause of the deterioration in transmittance rate. From these causes, it can be considered that a linear correlative relationship exists between the degree of deterioration of transmittance rate at the window of a CAN package and light density. 
     Accordingly, it is considered that the relationship between the light density and the degree of deterioration of transmittance rate at the window of a CAN package has the relationship indicated by the dotted line in the graph of  FIG. 2 , based on the linear relationship indicated by the solid line and measured values indicated by triangles. Note that in this case, the horizontal axis represents the light density (W/mm 2 ) of a laser beam at the light output surface of the window, and the vertical axis represents the degree of output decrease of light output due to contamination, that is, the rate of output decrease of the laser beam which has passed through the window per hour. Note that the following formula represents the dotted line. 
       Log  R′=− 5.76+0.9·Log( P′/S ′)   (2) 
     Here, R′ is the rate of decrease in transmittance rate per hour, and P is the output value of the laser beam. S′ is the transmittance area (mm 2 ) of the laser beam at the light output surface of the window. 
     As in the case described in Japanese Patent Application No. 2007-121102, the lifetime of a laser element is defined as the point in time at which the output of the laser element decreases from a predetermined output by 20%. In the case that a laser element having a lifetime of 10000 hours is utilized, it is desirable for the decrease in output due to contamination until the end of the element&#39;s life is 1/10 or less the decrease in the output of the laser element, that is, 2% or less. For this reason, the allowable rate of output decrease (/hour) is 0.02/10000=2.0·10 −6 . According to the graph of  FIG. 2 , the light density that corresponds to this value is 1.15 (W/mm 2 ). 
     In the present embodiment, the output of the semiconductor laser is 800 mW, and the transmittance area of the laser light at the light output surface  16   b  of the transparent member  16  is approximately 0.70 mm 2  (1/e). Therefore, the light density at the light output surface  16   b  of the transparent member  16  is 1.14 (W/mm 2 ). 
     The present inventors configured the optical device  1  such that the light density at the light output surface  16   b  of the transparent member  16  is 1.14 (W/mm 2 ) as described above, and performed measurements of transmittance rates. As a result, it was confirmed that deterioration of the transmittance rate was sufficiently suppressed. 
     Note that the transparent member  16  is fitted into the window  13  and adhesively attached. This simple structure increases the distance from the light emitting element to the light output surface of the first transparent member. Therefore, the light density of the laser beam at the light output surface  16   b  of the transparent member  16  can be caused to be 1.15 W/mm 2  or less. 
     In the case that the transparent member  16  is not provided, the laser beam will be output toward the exterior from the light output surface  15   b  of the transparent member  15 . In this case, the transmittance area of the laser beam through the light output surface  15   b  of the transparent member is approximately 0.18 mm 2  (1/e). Therefore, the light density at the light output surface  15   b  of the transparent member  15  will become 4.5 (W/mm 2 ), which is a great increase. 
     The optical device  1  according to the first embodiment is equipped with the transparent member  42  and the optical fiber  43 . Therefore, the light emitted by the semiconductor laser LD can be efficiently propagated. 
     The light input surface of the optical fiber  43  is configured to be removably attached to the transparent member  42 , and optically positioned by abutting the transparent member  42 . Therefore, positioning of the optical fiber  43  is facilitated. 
     The coupling preventing film  46  is provided on the light input surface  43   a  of the optical fiber  43 . Therefore, fusion at the surface where the transparent member  42  and the optical fiber  43  abut each other can be prevented. 
     Next, an optical device  2  according to a second embodiment of the present invention will be described.  FIG. 3  is a diagram that schematically illustrates the construction of the optical device  2  of the second embodiment. Note that in  FIG. 3 , elements of the optical device  2  which are the same as those of the optical device  1  are denoted by the same reference numerals, and detailed descriptions thereof will be omitted. 
     As illustrated in  FIG. 3 , the optical device  2  of the second embodiment is constituted by: a CAN package  20  having a diameter of 5.6 mm, in which a GaN semiconductor laser LD is hermetically sealed; a focusing lens  40  for focusing a laser beam B emitted by the GaN semiconductor laser LD; and an optical fiber module  41 , into which the laser beam B which has been focused by the focusing lens  40  enters. 
     A window  13  of the CAN package  20  is sealed by a transparent member  15  and a cylindrical transparent member  21 . The transparent member  15  and the cylindrical transparent member  21  are formed by glass having Si and O as the main components thereof, such as quartz glass and borosilicate glass. The transparent member  21  abuts and is adhesively attached to a case  14  at the exterior thereof. The thickness of the transparent member  21  is 1 mm. The distance from the semiconductor laser LD to a light output surface  21   b  of the transparent member  21  is approximately 2 mm. The transmittance area of the laser beam B through the light output surface  21   b  of the transparent member  21  is approximately 0.70 mm 2  (1/e). 
     In the second embodiment as well, the output of the semiconductor laser LD is 800 mW, and the transmittance area of the laser beam B through the light output surface  21   b  of the transparent member  21  is approximately 0.70 mm 2  (1/e). Therefore, the light density at the light output surface  21   b  of the transparent member  21  is 1.14 (W/mm 2 ). 
     The present inventors configured the optical device  2  such that the light density at the light output surface  21   b  of the transparent member  21  is 1.14 (W/mm 2 ) as described above, and performed measurements of transmittance rates. As a result, it was confirmed that deterioration of the transmittance rate was sufficiently suppressed. 
     Note that the transparent member  21  abuts and is adhesively attached to the case  14 . This simple structure increases the distance from the semiconductor laser LD to the light output surface  21   b  of the transparent member  21 . Therefore, the light density of the laser beam B at the light output surface  21   b  of the transparent member  21  can be caused to be 1.15 W/mm 2  or less. In addition, the area of the light output surface  21   b  of the transparent member  21  can easily be set to be greater than the area of the window  13 . Therefore, the degree of freedom in designing the window  13  is improved. 
     Next, an optical device  3  according to a third embodiment of the present invention will be described.  FIG. 4  is a diagram that schematically illustrates the construction of the optical device  3  of the third embodiment. Note that in  FIG. 4 , elements of the optical device  3  which are the same as those of the optical device  1  are denoted by the same reference numerals, and detailed descriptions thereof will be omitted. 
     As illustrated in  FIG. 4 , the optical device  3  of the third embodiment is constituted by: a CAN package  23  having a diameter of 5.6 mm, in which a GaN semiconductor laser LD is hermetically sealed; a focusing lens  40  for focusing a laser beam B emitted by the GaN semiconductor laser LD; and an optical fiber module  41 , into which the laser beam B which has been focused by the focusing lens  40  enters. 
     The CAN package  23  is the CAN package  20  of  FIG. 3 , from which the transparent member  15  has been removed. Therefore, a single transparent member  21  can be utilized to cause the light density of the laser beam B at the light output surface  21   b  of the transparent member  21  to be 1.15 W/mm 2  or less. The same advantageous effects as those obtained by the second embodiment can be obtained. 
     Next, an optical device  4  according to a fourth embodiment of the present invention will be described.  FIG. 5  is a diagram that schematically illustrates the construction of the optical device  4  of the fourth embodiment. Note that in  FIG. 5 , elements of the optical device  4  which are the same as those of the optical device  1  are denoted by the same reference numerals, and detailed descriptions thereof will be omitted. 
     As illustrated in  FIG. 5 , the optical device  4  of the fourth embodiment is constituted by: a CAN package  34  having a diameter of 9 mm, in which a GaN semiconductor laser LD is hermetically sealed; a focusing lens  40  for focusing a laser beam B emitted by the GaN semiconductor laser LD; and an optical fiber module  41 , into which the laser beam B which has been focused by the focusing lens  40  enters. 
     The semiconductor laser LD is fixed on a block  31  within the CAN package  30  by AuSn brazing material. The block  31  is fixed to a fixing member  32 . A metal case  34  having a circular window  33  is fixed to the fixing member  32  by resistance welding. 
     The window  33  is sealed by a circular transparent plate member  35 . The circular transparent plate member  15  is formed by glass having Si and O as the main components thereof, such as quartz glass and borosilicate glass. The transparent member  35  is adhesively attached to the case  14  at the interior thereof. Wires  37  and the like for supplying drive current to the semiconductor laser LD are drawn out from the CAN package  30  through openings which are formed in the fixing member  32 . 
     The distance from the semiconductor laser LD to a light output surface  35   b  of the transparent member  35  is approximately 2 mm. The transmittance area of the laser beam B at the light output surface  35   b  of the transparent member  35  is approximately 0.70 mm 2  (1/e). 
     In the fourth embodiment as well, the output of the semiconductor laser LD is 800 mW, and the transmittance area of the laser beam B through the light output surface  35   b  of the transparent member  35  is approximately 0.70 mm 2  (1/e). Therefore, the light density at the light output surface  21   b  of the transparent member  35  is 1.14 (W/mm 2 ). 
     The present inventors configured the optical device  4  such that the light density at the light output surface  35   b  of the transparent member  35  is 1.14 (W/mm 2 ) as described above, and performed measurements of transmittance rates. As a result, it was confirmed that deterioration of the transmittance rate was sufficiently suppressed. 
     The distance from the semiconductor laser LD to the light output surface  35   b  of the transparent member  35  is increased, simply by employing a case having a large diameter. By this simple structure, the light density at the light output surface  35   b  of the transparent member  35  is caused to be 1.14 (W/mm 2 ), and the optical device  4  can be produced as low cost. 
     Note that in the embodiments described above, GaN semiconductor lasers LD were employed as the light emitting elements. Alternatively, lasers that emit light at other wavelengths within the range from 220 nm to 500 nm may be employed. Because the amount of collected dust increases due to high energy within the wavelength range from 220 nm to 500 nm , application of the present invention is effective to prevent attachment of foreign matter. 
     The present inventors obtained similar results when the optical devices according to the above embodiments were produced using solid state ultraviolet lasers that emit light at a wavelength of 220 nm, using semiconductor lasers as excitation light sources. The wavelength range of the light emitting elements may be within a range from 370 nm to 500 nm, or from 400 nm to 410 nm. 
     Next, an image exposure apparatus which is equipped with the optical device of the present invention as exposure light sources will be described. 
     [Configuration of the Image Exposure Apparatus] 
     As illustrated in  FIG. 6 , the image exposure apparatus is equipped with a planar moving stage  152 , for holding sheets of photosensitive material  150  thereon by suction. A mounting base  156  is supported by four legs  154 . Two guides  158  that extend along the stage movement direction are provided on the upper surface of the mounting base  156 . The stage  152  is provided such that its longitudinal direction is aligned with the stage movement direction, and supported by the guides  158  so as to be movable reciprocally thereon. Note that the image exposure apparatus is also equipped with a stage driving apparatus  304  (refer to  FIG. 15 ), as a sub scanning means for driving the stage  152  along the guides  158 . 
     A C-shaped gate  160  is provided at the central portion of the mounting base so as to straddle the movement path of the stage  152 . The ends of the C-shaped gate  160  are fixed to side edges of the mounting base  156 . A scanner  162  is provided on a first side of the gate  160 , and a plurality (two, for example) of sensors  164  for detecting the leading and trailing ends of the photosensitive material  150  are provided on a second side of the gate  160 . The scanner  162  and the sensors  164  are individually mounted on the gate  160 , and fixed above the movement path of the stage  152 . Note that the scanner  162  and the sensors  164  are connected to a controller (not shown) for controlling the operations thereof. 
     The scanner  162  is equipped with a plurality ( 14 , for example) of exposure heads  166 , arranged in an approximate matrix having m rows and n columns (3 rows and 5 columns, for example), as illustrated in  FIG. 7  and  FIG. 8B . In this example, four exposure heads  166  are provided in the third row, due to constraints imposed by the width of the photosensitive material  150 . Note that an individual exposure head arranged in an m th  row and an n th  column will be denoted as an exposure head  166   mn . 
     An exposure area  168 , which is exposed by the exposure heads  166 , is a rectangular area having its short sides in the sub-scanning direction. Accordingly, band-like exposed regions  170  are formed on the photosensitive material  150  by each of the exposure heads  166 , accompanying the movement of the stage  152 . Note that an individual exposure area, exposed by an exposure head arranged in an m th  row and an n th  column will be denoted as an exposure area  168   m,n . 
     As illustrated in  FIG. 8A  and  FIG. 8B , each of the rows of the exposure heads  166  is provided staggered a predetermined interval (a natural number multiple of the long side of the exposure area,  2  times in the present embodiment) with respect to the other rows. This is to ensure that the band-like exposed regions  170  have no gaps therebetween in the direction perpendicular to the sub scanning direction. Therefore, the portion between an exposure area  168   11  and  168   12  of the first row, which cannot be exposed thereby, can be exposed by an exposure area  168   21  of the second row and an exposure area  168   31  of the third row. 
     Each of the exposure heads  166   11  through  168   mn  are equipped with a DMD  50  (Digital Micro mirror Device) by Texas Instruments (U.S.), for modulating light beams incident thereon according to each pixel of image data, as illustrated in  FIG. 9  and  FIG. 10 . The DMD&#39;s  50  are connected to a controller  302  to be described later (refer to  FIG. 15 ), comprising a data processing section and a mirror drive control section. The data processing section of the controller  302  generates control signals for controlling the drive of each micro mirror of the DMD  50  within a region that should be controlled for each exposure head  166 , based on input image data. Note that the “region that should be controlled” will be described later. The mirror drive control section controls the angle of a reflective surface of each micro mirror of the DMD  50  for each exposure head  166 , according to the control signals generated by the data processing section. Note that control of the angle of the reflective surface will be described later. 
     A fiber array light source  66 ; an optical system  67 ; and a mirror  69  are provided in this order, at the light input side of the DMD  50 . The fiber array light source  66  comprises a laser emitting section, constituted by a plurality of optical fibers having their light emitting ends (light emitting points) aligned in a direction corresponding to the longitudinal direction of the exposure area  168 . The optical system  67  corrects laser beams emitted from the fiber array light source  66  to condense them onto the DMD  50 . The mirror  69  reflects the laser beams, which have passed through the optical system  67 , toward the DMD  50 . Note that the optical system  67  is schematically illustrated in  FIG. 9 . 
     As illustrated in detail in  FIG. 10 , the optical system  67  comprises: a condensing lens  71 , for condensing the laser beams B emitted from the fiber array light source  66  as illuminating light; a rod shaped optical integrator  72  (hereinafter, referred to simply as “rod integrator  72 ”), which is inserted into the optical path of the light which has passed through the condensing lens  71 ; and a collimating lens  74 , provided downstream from the rod integrator  72 , that is, toward the side of the mirror  69 . The condensing lens  71 , the rod integrator  72  and the collimating lens  74  cause the laser beams emitted from the fiber array light source to enter the DMD  50  as a light beam which is close to collimated light and which has uniform beam intensity across its cross section. The shape and the operation of the rod integrator  72  will be described in detail later. 
     The laser beam B emitted through the optical system  67  is reflected by the mirror  69 , and is irradiated onto the DMD  50  via a TIR (Total Internal Reflection) prism  70 . Note that the TIR prism  70  is omitted from  FIG. 9 . 
     A focusing optical system  51 , for focusing the laser beam B reflected by the DMD  50  onto the photosensitive material  150 , is provided on the light reflecting side of the DMD  50 . The focusing optical system  51  is schematically illustrated in  FIG. 4 , but as illustrated in detail in  FIG. 10 , the focusing optical system  51  comprises: a first focusing optical system constituted by lens systems  52  and  54 ; a second focusing optical system constituted by lens systems  57  and  58 ; a micro lens array  55 ; and an aperture array  59 . The micro lens array  55  and the aperture array  59  are provided between the first focusing optical system and the second focusing optical system. 
     The micro lens array  55  is constituted by a great number of micro lenses  55   a,  which are arranged two dimensionally, corresponding to each pixel of the DMD  50 . In the present embodiment, only 1024×256 columns out of 1024×768 columns of micro mirrors of the DMD  50  are driven, as will be described later. Therefore, 1024×256 columns of micro lenses  55   a  are provided, corresponding thereto. The arrangement pitch of the micro lenses  55   a  is 41 μm in both the vertical and horizontal directions. The micro lenses  55   a  are formed by optical glass BK7, and have focal distances of 0.19 mm and NA&#39;s (Numerical Apertures) of 0.11, for example. Note that the shapes of the micro lenses  55   a  will be described in detail later. The beam diameter of each laser beams B at the position of each micro lens  55   a  is 41 μm. 
     The aperture array  59  has a great number of apertures  59   a  formed therethrough, corresponding to the micro lenses  55   a  of the micro lens array  55 . In the present embodiment, the diameter of the apertures  59   a  is 10 μm. 
     The first focusing optical system magnifies the images that propagate thereto from the DMD  50  by 3× and focuses the images on the micro lens array  55 . The second focusing optical system magnifies the images that have passed through the micro lens array  55  by 1.6×, and focuses the images onto the photosensitive material  150 . Accordingly, the images from the DMD  50  are magnified at 4.8× magnification and projected onto the photosensitive material  150 . 
     Note that in the present embodiment, a prism pair  73  is provided between the second focusing optical system and the photosensitive material  150 . The focus of the image on the photosensitive material  150  is adjustable, by moving the prism pair  73  in the vertical direction in  FIG. 10 . Note that in  FIG. 10 , the photosensitive material  150  is conveyed in the direction of arrow F to perform sub-scanning. 
     The DMD  50  is a mirror device having a great number (1024×768, for example) of micro mirrors  62 , each of which constitutes a pixel, arranged in a matrix on an SRAM cell  60  (memory cell). A micro mirror  62  supported by a support column is provided at the uppermost part of each pixel, and a material having high reflectivity, such as aluminum, is deposited on the surface of the micro mirror  62  by vapor deposition. Note that the reflectivity of the micro mirrors  62  is 90% or greater, and that the arrangement pitch of the micro mirrors  62  is 13.7 μm in both the vertical and horizontal directions. In addition, the CMOS SRAM cell  60  of a silicon gate, which is manufactured in a normal semiconductor memory manufacturing line, is provided beneath the micro mirrors  62 , via the support column, which includes a hinge and a yoke. The DMD  50  is of a monolithic structure. 
     When digital signals are written into the SRAM cell  60  of the DMD  50 , the micro mirrors  62  which are supported by the support columns are tilted within a range of ±α degrees (±12 degrees, for example) with respect to the substrate on which the DMD  50  is provided, with the diagonal line as the center of rotation.  FIG. 12A  illustrates a state in which a micro mirror  62  is tilted +α degrees in an ON state, and  FIG. 12B  illustrates a state in which a micro mirror  62  is tilted −α degrees in an OFF state. Accordingly, laser light beams incident on the DMD  50  are reflected toward the direction of inclination of each micro mirror  62 , by controlling the tilt of each micro mirror  62  that corresponds to a pixel of the DMD  50  according to image signals, as illustrated in  FIG. 11 . 
     Note that  FIG. 6  illustrates a magnified portion of a DMD  50  in which the micro mirrors  62  are controlled to be tilted at +α degrees and at −α degrees. The ON/OFF operation of each micro mirror  62  is performed by the controller  302 , which is connected to the DMD  50 . In addition, a light absorbing material (not shown) is provided in the direction toward which laser beams B reflected by micro mirrors  62  in the OFF state are reflected. The micro mirrors  62  of the present embodiment have distortions in their reflective surfaces. However, the distortions are omitted from  FIGS. 11 ,  12 A, and  12 B. 
     It is preferable for the DMD  50  to be provided such that its short side is inclined at a slight predetermined angle (0.1° to 5°, for example) with respect to the sub-scanning direction.  FIG. 8A  illustrates scanning trajectories of reflected light images  53  (exposing beams) of each micro mirror in the case that the DMD  50  is not inclined, and  FIG. 8B  illustrates the scanning trajectories of the exposing beams  53  in the case that the DMD  50  is inclined. 
     A great number (756, for example) of columns of rows of a great number (1024, for example) of micro mirrors aligned in the longitudinal direction, are provided in the lateral direction of the DMD  50 . As illustrated in  FIG. 13B , by inclining the DMD  50 , the pitch P 2  of the scanning trajectories (scanning lines) of the exposure beams  53  become narrower than the pitch P 1  of the scanning lines in the case that the DMD  50  is not inclined. Therefore, the resolution of the image can be greatly improved. Meanwhile, because the angle of inclination of the DMD  50  is slight, the scanning width W 2  in the case that the DMD  50  is inclined and the scanning width W 1  in the case that the DMD is not inclined are substantially the same. 
     In addition, the same scanning lines are repeatedly exposed (multiple exposure) by different micro mirror columns. By performing multiple exposure in this manner, it becomes possible to finely control exposure positions with respect to alignment marks, and to realize highly detailed exposure. Seams among the plurality of exposure heads, which are aligned in the main scanning direction, can be rendered virtually seamless by finely controlling the exposure positions. 
     Note that the micro mirror columns may be shifted by predetermined intervals in the direction perpendicular to the sub-scanning direction to be in a staggered formation instead of inclining the DMD  50 , to achieve the same effect. 
     As illustrated in  FIG. 14A , the fiber array light source  66  is equipped with a plurality ( 14 , for example) of the optical devices  1  of  FIG. 1 . The CAN package  10  is provided at the end of each of the optical fibers  43  via the focusing lens  40 , and second optical fibers  48  having the same core diameter as the optical fiber  43  and a cladding diameter smaller than that of the optical fiber  43 , is coupled to the other end of each of the optical fibers  43 . As illustrated in detail in  FIG. 14B , the second optical fibers  48  are arranged such that seven ends of the optical fibers  30  opposite the end at which they are coupled to the optical fibers  43  are aligned along the main scanning direction perpendicular to the sub scanning direction. Two rows of the seven second optical fibers  48  constitute a laser emitting section  68 . 
     As illustrated in  FIG. 14B , the laser emitting section  68 , constituted by the ends of the second optical fibers  48 , is fixed by being sandwiched between two support plates  65 , which have flat surfaces. It is desirable for a transparent protective plate, such as that made of glass, to be placed at the light emitting end surfaces of the second optical fibers  48 . The light emitting end surfaces of the second optical fibers  48  are likely to collect dust due to their high optical density and therefore likely to deteriorate. However, by placing the protective plate as described above, adhesion of dust to the end surfaces can be prevented, and deterioration can be slowed. 
     In the present embodiment, as illustrated in  FIG. 14C , the light output end surface of each optical fiber  43  having a large cladding diameter is coupled concentrically to the second optical fiber  48  having a small cladding diameter and a length of 1 cm to 30 cm. The optical fibers  43  and  48  are coupled such that the light input end surfaces of the second optical fibers  48  are fused to the light output end surfaces of the optical fibers  43  in a state that the core axes thereof are matched. As described above, the diameter of the cores  49  of the second optical fibers  48  are the same as the diameters of the cores  44  of the optical fibers  43 . 
     Next, the electric configuration of the image exposure apparatus of the present embodiment will be described with reference to  FIG. 15 . As illustrated in  FIG. 15 , a modulating circuit  301  is connected to a total control section  300 , and a controller  302  for controlling the DMD&#39;s  50  is connected to the modulating circuit  301 . An LD drive circuit  303  for driving the optical devices  1  is connected to the total control section  300 . Further, a stage driving apparatus  304  for driving the stage  152  is connected to the total control section  300 . 
     [Operation of the Image Exposure Apparatus] 
     Next, the operation of the image exposure apparatus described above will be described. IN each exposure head  166  of the scanner  162 , the laser beams B are emitted by each of the GaN semiconductor lasers LD of the CAN packages  10  (refer to  FIG. 1 ) that constitute the multiplex laser light source of the fiber array light source  66  in a diffuse state. The laser beams B are focused by the focusing lenses  40 , pass through the transparent members  42  and converge on the light input end surfaces of the cores  44  of the optical fibers  43 . The laser beams B that enter the cores  44  of the optical fibers  43  are output from the second optical fibers  48 , which are coupled to the light output end surfaces of the optical fibers  43 . 
     During image exposure, image data corresponding to an exposure pattern is input to the controller  302  of the DMD&#39;s  50  from the modulating circuit  301  of  FIG. 15 . The image data is temporarily stored in a frame memory of the controller  302 . The image data represents the density of each pixel that constitutes an image as binary data (dot to be recorded/dot not to be recorded). 
     The stage  152 , on the surface of which the photosensitive material  150  is fixed by suction, is conveyed along the guides  158  from the upstream side to the downstream side of the gate  160  by the stage driving apparatus  304  illustrated in  FIG. 15 . When the stage  152  passes under the gate  160 , the leading edge of the photosensitive material is detected by the sensors  164 , which are mounted on the gate  160 . Then, the image data recorded in the frame memory is sequentially read out a plurality of lines at a time. Control signals are generated by the signal processing section for each exposure head  166 , based on the read out image data. Thereafter, the mirror driving control section controls the ON/OFF states of each micro mirror of the DMD&#39;s  50  of each exposure head, based on the generated control signals. Note that in the present embodiment, the size of each micro mirror that corresponds to a single pixel is 14 μm×14 μm. 
     When the laser beams B are irradiated onto the DMD&#39;s  50  from the fiber array light source  66 , laser beams which are reflected by micro mirrors in the ON state are focused on the photosensitive material  150  by the lens systems  54  and  58 . The laser beams emitted from the fiber array light source  66  are turned ON/OFF for each pixel, and the photosensitive material  150  is exposed in pixel units (exposure areas  168 ) substantially equal to the number of pixels of the DMD&#39;s  50  in this manner. The photosensitive material  150  is conveyed with the stage  152  at the constant speed. Sub-scanning is performed in the direction opposite the stage moving direction by the scanner  162 , and band-shaped exposed regions  170  are formed on the photosensitive material  150  by each exposure head  166 . 
     When sub scanning of the photosensitive material  150  by the scanner  162  is completed and the trailing edge of the photosensitive material  150  is detected by the sensors  162 , the stage  152  is returned to its starting point at the most upstream side of the gate  160  along the guides  152  by the stage driving apparatus  304 . Then, the stage  152  is moved from the upstream side to the downstream side of the gate  160  at the constant speed again. 
     Next, an illuminating optical system for irradiating the laser beam B onto the DMD&#39;s  50 , comprising: the fiber array  66 , the condensing lens  71 , the rod integrator  72 , the collimating lens  74 , the mirror  69 , and the TIR prism  70  illustrated in  FIG. 10  will be described. The rod integrator  72  is a light transmissive rod, formed as a square column, for example. The laser beam B propagates through the interior of the rod integrator  72  while being totally reflected therein, and the intensity distribution within the cross section of the laser beam B is uniformized. Note that an anti-reflective film is coated on the light input surface and the light emitting surface of the rod integrator  72 , to increase the transmissivity thereof. By uniformizing the intensity distribution within the cross section of the laser beam B in this manner, unevenness in the intensity of the illuminating light can be eliminated, and highly detailed images can be exposed on the photosensitive material  150 .