Abstract:
An object of the invention is to provide a mirror fixing method capable of reducing stress distortion of a surface of a mirror which constitutes an optical system, with hardly deforming a surface shape of the mirror. To this end, the present mirror fixing method is characterized in that a mirror part including a base plate formed with a mirror on one face thereof, is provided with a boss on the other face of the base plate opposite to the face on which the mirror is formed, and only the boss is fixed, so that the mirror part excluding the boss is not in contact with other members.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    (1) Field of the Invention  
           [0002]    The present invention relates to a mounting technique for fixing mirrors that are used for constructing an optical system. In particular, the invention relates to a mirror fixing method for preventing stress distortion of mirror surface due to the fixing, and to an optical apparatus using the method.  
           [0003]    (2) Related Art  
           [0004]    Normally, in order to construct an optical system, members such as a light source, lenses and mirrors are used to change an optical path or to collect lights. For example, as shown in FIG. 8, a mirror part  100  is sometimes used for changing a propagation direction of light emitted from a light source by 90°. In this mirror part  100 , typically a mirror  101  with a mirror surface formed by vapor deposition of metal or the like on the surface of a base material made of glass, plastic, or the like, is fixed on a jig  102  for use. As a conventional method of fixing the mirror  101  on the jig  102 , there is known a method in which an adhesive is applied on the back of the mirror  101 , which is then adhered at a desired position of the jig  102 . Moreover, for example, as shown in FIG. 10, there is also applied a method where the mirror  101  is inserted into the jig  102  with a threaded type fixing ring  103 .  
           [0005]    The mirror  101  fixed to the jig  102  as described above, is used as a part of an optical system such as a variable wavelength dispersion compensator (to be referred to as a VIPA-VDC) which uses a VIPA (abbreviation of Virtually Imaged Phased Array, a device that branches for each wavelength, an optical signal that is a combination of more than one wavelength) as disclosed in Japanese National Publication Nos. 2000-511655 and 2002-514323 of PCT Applications previously filed by the present applicant. FIG. 11 is a perspective view showing an outline of the abovementioned VIPA-VDC. This VIPA-VDC has a configuration where, for example, a mirror assembly  110  in which an aspherical mirror  112  fixed to an L-shaped jig  111  is installed on a movable stage  113 , is combined with a VIPA assembly  120  in which an optical fiber  121 , collimator lenses  122  and  123 , a VIPA  124 , and a collimator lens  125  are arranged in that order. In this VIPA-VDC, light emitted from the optical fiber  121  is input to the VIPA  124  via the collimator lenses  122  and  123 . In the VIPA  124 , the incident light is subjected to multiple reflections, and is branched for each of different wavelengths to be emitted. The emitted light is sent to the mirror assembly  110  via the collimator lens  125 . The light branched for each wavelength in the VIPA assembly  120 , is reflected by the aspherical mirror  112  in the mirror assembly  110 , to be again input to the VIPA assembly  120 , and is propagated through the VIPA assembly  120  in a direction opposite to the above propagation direction is collected by the optical fiber  121 . The VIPA-VDC of such a construction has the feature in that a compensation amount for wavelength dispersion can be changed by moving the aspherical mirror  112  according to a dispersion amount. The reflecting surface of the aspherical mirror  112  is prepared with high accurately so that a desired compensation amount can be obtained.  
           [0006]    However, if the mirror  101  is fixed to the jig  102  by adhesive or mechanical fixing as with the above described conventional technique, there is a problem of stress distortion developing on the mirror surface due to the fixing.  
           [0007]    More specifically, when for example, as shown in FIG. 12, the mirror  101  is fixed to the jig  102  with adhesive  104 , then considering the case of a change in temperature, the shape of the mirror surface is deformed to become concave or convex as shown at the right of FIG. 12, since the material used for the jig  102  has the coefficient of thermal expansion different from that of the mirror  101  (plastic for instance). Moreover, the surface shape of the mirror  101  is sometimes deformed due to shrinkage stress of the adhesive, other than the temperature change. If such distortion develops on the surface of the mirror  101 , the surface of the mirror  101  has a curved surface shape different from the designed value and hence a desired compensation amount cannot be obtained in devices such as the aforementioned VIPA-VDC illustrated in FIG. 11. As another drawback of the fixing method using an adhesive, there is a possibility that the mirror  101  comes away from the jig  102  when the adhesive is deteriorated. Especially, for the various types of devices used in optical communication systems, such as the VIPA-VDC, long-term reliability with a product life of 25 years or so is required. Therefore, there is a need to realize a stable fixing method without the use of an adhesive.  
           [0008]    In the case where a mechanical fixing method is applied, then for example as shown in FIG. 13, a pressing force acts on a part “b” where the mirror  101  and a member  102 B that forces the mirror  101  to a member  102 A being a part of the jig, are in contact with each other. However, for a part “a” where there is not contact, since a pressing force does not act (refer to the cross-section A-A in the lower part of FIG. 13), then if the temperature rises, the part “a” of the mirror  101  expands, whereas the part “b” does not expand so much. Consequently, the surface of the mirror  101  is distorted in a convex shape.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention addresses the above problems, with an object of providing a mirror fixing method capable of reducing stress distortion of a surface of a mirror which constitutes an optical system, by practically avoiding deformation in the shape of the mirror surface, and an optical apparatus using the method.  
           [0010]    To achieve the aforesaid object, a mirror fixing method of the present invention, is characterized in that a mirror part including a base plate formed with a mirror which constitutes an optical system on one face thereof, is provided with a boss on the other face of the base plate opposite to the face on which the mirror is formed, and only the boss is fixed, so that the mirror part excluding the boss is not in contact with other members.  
           [0011]    Moreover, an optical apparatus of the present invention having an optical system constructed using a mirror, comprises: a mirror part including a base plate formed with the mirror on one face thereof, and a boss provided on the other face of the base plate opposite to the face on which the mirror is formed; and a first fixture fixing only the boss, so that the mirror part excluding the boss is not in contact with other members.  
           [0012]    According to the abovementioned mirror fixing method and optical apparatus, the stress at the time of fixing has little effect on the mirror. Moreover, since restriction by other members, of thermal expansion due to a temperature change or the like of the base plate on which the mirror is formed is also avoided, it becomes possible to reduce stress distortion of a mirror surface, developing due to the fixing.  
           [0013]    Moreover, as a specific aspect of the abovementioned optical apparatus, it is preferable that the first fixture includes a receiving plate which is formed with an opening capable of inserting the boss therein and provided with a screw hole perpendicular to a side wall of the opening, and the boss which is inserted in the opening of the receiving plate is fixed with a screw from the side using the screw hole. As a result, the mirror part is screw fixed to the receiving plate by the first fixture, and the stress at the time of fixing acts only from the side of the boss.  
           [0014]    In addition, in the above-mentioned optical apparatus, the construction may be such that a movable stage and a second fixture mountable on the movable stage are provided, and the second fixture and the first fixture are coupled with each other to constitute a mirror module, and the mirror module is installed on the movable stage via the second fixture. As a result, the mirror module comprising the mirror part, the first fixture and the second fixture, is installed on the movable stage, so that the mirror constituting an optical system is fixed in a movable manner.  
           [0015]    Furthermore, for the mirror part in the above-mentioned optical apparatus, an aspherical surface mirror used in a variable wavelength dispersion compensator may be formed on the one face of the base plate. As a result, a possibility that the surface shape of the aspherical mirror constituting the optical system of the variable wavelength dispersion compensator is deformed due to the stress at the time of fixing is considerably reduced. Therefore, wavelength dispersion compensation can be performed stably with high accuracy.  
           [0016]    Other objects, features and advantages of this invention will become apparent from the following description of embodiments in conjunction with the appended drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0017]    [0017]FIG. 1 is an exploded perspective view showing components in one embodiment of an optical apparatus of the present invention.  
         [0018]    [0018]FIG. 2 is a perspective view showing the appearance with the components of FIG. 1 assembled.  
         [0019]    [0019]FIG. 3 is a sectional view for explaining a method of fixing a mirror part to a first fixture in the embodiment.  
         [0020]    [0020]FIG. 4 is a diagram showing an example of a method of installing a mirror module on a movable stage in the embodiment.  
         [0021]    [0021]FIG. 5 is a plan view of the mirror module installed on the movable stage of FIG. 4, seen from above.  
         [0022]    [0022]FIG. 6 is a plan view illustrating a preferable shape of a receiving plate of a second fixture in the embodiment.  
         [0023]    [0023]FIG. 7 is a block diagram showing an example of when the present invention is applied to an interference optical system for a surface shape measuring device, in conjunction with the embodiment.  
         [0024]    [0024]FIG. 8 shows an example of an optical system constructed using a typical mirror.  
         [0025]    [0025]FIG. 9 is a diagram for explaining a conventional mirror fixing method using an adhesive.  
         [0026]    [0026]FIG. 10 is a diagram for explaining a conventional mirror fixing method using mechanical fixing.  
         [0027]    [0027]FIG. 11 is a perspective view showing the appearance of a known VIPA-VDC.  
         [0028]    [0028]FIG. 12 is a diagram for explaining problems of a conventional mirror fixing method using an adhesive.  
         [0029]    [0029]FIG. 13 is a diagram for explaining problems of a conventional mirror fixing method using mechanical fixing. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0030]    Hereunder is a description of embodiments of the present invention based on the drawings.  
         [0031]    [0031]FIG. 1 is a perspective view of one embodiment of an optical apparatus to which a mirror fixing method of the present invention is applied, showing respective components in exploded form. FIG. 2 is a perspective view showing the appearance with the components of FIG. 1 assembled.  
         [0032]    As shown in FIG. 1 and FIG. 2, the present optical apparatus comprises, for example, a mirror part  1 , a first fixture  2  and a second fixture  3 .  
         [0033]    The mirror part  1  includes, for example, a base plate  1 A formed using a material such as glass, plastics or the like, a mirror  1 B formed by vacuum deposition of a metal or the like on one face (upper face in the figure) of the base plate  1 A, and a protruding type boss  1 C formed on the other face (lower face in FIG. 1) of the base plate  1 A opposite the face on which the mirror  1 B is formed. The mirror  1 B is made with high accuracy so that its surface achieves a desired shape (for example, a plane surface, a spherical surface or an aspherical surface etc.). The boss  1 C here is a cylindrical shape protrusion formed integrally with the base plate  1 A. However, the boss  1 C is not limited to have the cylindrical shape alone, and as described later, this can be of any arbitrary shape capable to be fixed by the first fixture  2 . Furthermore, the base plate  1 A and boss  1 C need not be formed integrally, and may be formed by securing together separate members.  
         [0034]    The first fixture  2  includes a receiving plate  2 A fixing the mirror part  1  at a predetermined position, and this receiving plate  2 A is formed with a boss receiving V-groove part  2 B serving as an opening and screw holes  2 C,  2 D and  2 E. The boss receiving V-groove part  2 B comprises a hole formed in an approximately central part of the receiving plate  2 A to pass from the upper face to the lower face of the receiving plate  2 A. The boss  1 C of the mirror part  1  is inserted in this hole to be held therein. Furthermore, for the boss receiving V-groove part  2 B, here a part of the side wall of the hole is made a V-groove structure to increase the accuracy of positioning the mirror part  1 . However, the hole (opening) that receives the boss  1 C is not limited to the aforementioned V-groove structure, and can be a cylindrical shape or the like corresponding to the shape of the boss  1 C. The screw holes  2 C and  2 D are formed in opposite lengthwise edge portions of the receiving plate  2 A. These are sites for fixing the second fixture  3  to the lower face of the receiving plate  2 A by means of receiving plate fixing screws  5 A and  5 B. The screw hole  2 E is formed in a perpendicular direction to the side wall of the receiving plate  2 A opposite the V-groove of the boss receiving V-groove part  2 B, and as described later, is a site to which is attached a boss fixing screw  4  for fixing the boss  1 C at a predetermined position of the V-groove.  
         [0035]    The second fixture  3  includes a receiving plate  3 A for further mounting the first fixture  2  with the mirror part  1  secured thereto, on for example a movable stage or the like. An angle adjusting boss  3 B as well as countersunk holes  3 C and  3 D are respectively formed on the receiving plate  3 A. The angle adjusting boss  3 B, is a cylindrical shape protrusion formed on the surface (lower face in the figure) of the receiving plate  3 A opposite the face contacting the first fixture  2 . As described later, the angle adjusting boss  3 B mounts the mirror part  1  on a movable stage or the like such that an angle of the mirror part  1  can be adjusted. The countersunk holes  3 C and  3 D are formed in opposite lengthwise edge portions of the receiving plate  3 A, and are sites in which are inserted the receiving plate fixing screws  5 A and  5 B to be screwed in the screw holes  2 C and  2 D in the first fixture.  
         [0036]    Here, the method of fixing the mirror part  1  to the first fixture  2 , which is a main characteristic of the mirror fixing method according to the present invention, will be described in detail with reference to a sectional view of FIG. 3. Note, the sectional view of FIG. 3 shows a cross-section of the optical apparatus shown in FIG. 2, when cut along the Y-Z plane through the center of the boss  1 C. However, here three-dimensional coordinates X-Y-Z shown in the upper left of FIG. 2, that is, a coordinate system with the upper face of the base plate  1 A on which the mirror  1 B is formed, as the X-Y plane, and a direction orthogonal to the X-Y plane as the Z axis, is set.  
         [0037]    As shown in FIG. 3, the boss  1 C is inserted in the boss receiving V-groove  2 B of the first fixture  2 , and this boss  1 C is screw fixed from the side by means of the boss fixing screw  4 , so that the mirror part  1  is fixed at a predetermined position on the first fixture  2 . In such a fixing method, the mirror part  1  receives a force in the Y-axis direction from the boss fixing screw  4 , but does not receive a force in the Z-axis direction. Therefore, a possibility that a surface shape of the mirror  1 B formed on the upper face (X-Y plane) of the base plate  1 A is deformed due to an influence by forces at the time of the fixing is extremely low. Furthermore, since there is no influence by shrinkage stress of an adhesive as with conventional fixing methods using adhesive, the surface shape of the mirror  1 B is stable. Therefore, it becomes possible to fix the mirror part  1  at a desired position with causing hardly any distortion on the surface of the mirror  1 B.  
         [0038]    Furthermore, the mirror part  1  is fixed so that the mirror part  1  excluding the boss  1 C is not in contact with the receiving plate  2 A of the first fixture  2 , in other words, the mirror part  1  is fixed in a condition with a required gap G ensured between the lower face of the base plate  1 A of the mirror part  1  and the upper face of the first fixture  2 . The gap G is previously set to have a space such that thermal expansion of the mirror part  1  is not hindered. By providing the gap G, it becomes possible to considerably reduce a possibility that the shape of the surface of mirror  1 B is deformed due to a temperature change.  
         [0039]    Furthermore, at the time of screw fixing the boss  1 C as mentioned above, it is preferable to install a boss plate  6  as a cushion member between the side face of the boss  1 C and the tip portion of the boss fixing screw  4 , as shown at the center of FIG. 3. By providing the boss plate  6  in this way, for example, even if there is a burr or the like at the tip portion of the boss fixing screw  4 , biting of this burr into the boss  1 C is avoided and hence the mirror part  1  can be more stably fixed to the first fixture  2 .  
         [0040]    In this way, in this embodiment, the mirror  1 B is stably fixed at the predetermined position on the first fixture  2 , with hardly distortion to the surface of the mirror  1 B. Furthermore, here the first fixture  2  on which the mirror part  1  is fixed is used for example as the optical system comprising the mirror assembly of the VIPA-VDC as shown in FIG. 11 described above. Therefore, the second fixture  3  is screw fixed to the first fixture  2  by means of the receiving plate fixing screws  5 A and  5 B, to be mounted at a required position on the movable stage.  
         [0041]    Here is a description of a specific method of installing the structure in which the mirror part  1 , first fixture  2  and second fixture  3  are assembled (hereunder called a mirror module) on the movable stage.  
         [0042]    [0042]FIG. 4 shows an enlarged movable stage used in the VIPA-VDC, illustrating a method of installing the mirror module on the movable stage.  
         [0043]    In FIG. 4, an L shaped jig  8  is installed on a movable stage  7 . A hole  8 A is formed as an opening having a shape corresponding to the angle adjusting boss  3 B of the second fixture  3 , on the face perpendicular to the face on which the movable stage  7  is installed, in the L shaped jig  8 . The mirror module is installed on the movable stage  7  by inserting the angle adjusting boss  3 B in the hole  8 A.  
         [0044]    The movable stage  7  and the L shaped jig  8 , shown in FIG. 4, correspond to the movable stage  113  and the L-shaped jig  111  shown in FIG. 11 described above.  
         [0045]    The mirror module installed on the movable stage  7 , as shown in a plan view, seen from above in FIG. 5 (the Z-axis direction), becomes rotatable in the X-Y plane about the angle adjusting boss  3 B. In this way, by making the mirror module rotatable in the X-Y plane, it becomes possible to easily perform adjustment for positioning the mirror  1 B at an optimum angle relative to a travel shaft  7 A (refer to FIG. 4) of the movable stage  7 . That is to say, the mirror used in the optical system of the VIPA-VDC has an aspherical surface shape, as also shown in FIG. 11 described above, and the central axis being the design basis exists on the aspherical mirror. With the VIPA-VDC, in view of the characteristic thereof, it is necessary to position the central axis of the aspherical mirror and the travel shaft of the movable stage in parallel with each other. Therefore, by making the mirror module rotatable in the X-Y plane in the abovementioned manner, the central axis of the mirror  1 B can be easily adjusted to be parallel with the travel shaft  7 A of the movable stage  7 .  
         [0046]    Regarding the method of adjusting the central axis of the aspherical mirror in the VIPA-VDC, the present applicant has proposed a specific technique for adjusting the central axis of the aspherical surface and the travel shaft of the movable stage, based on reflected light obtained by irradiating a parallel light from the Z axis direction onto the aspherical mirror and the movable stage (refer to Japanese Patent Application No. 2002-000449). By applying the technique disclosed in this earlier patent application, it is possible to perform parallel adjustment of the mirror central axis in the present embodiment.  
         [0047]    The mirror module in the present embodiment has a structure in which the center of the boss  1 C of the mirror part  1 , and the center of the angle adjusting boss  3 B of the second fixture  3  are positioned on the central axis of the aspherical mirror. By adopting such a structure, an influence by any rotational deviation about the X-axis is minimized. Therefore, it becomes possible to more easily perform parallel adjustment of the mirror central axis relative to the running shaft  7 A.  
         [0048]    When the abovementioned adjustment of the central axis of the mirror  1 B is completed, the mirror module is fixed to the L shaped jig  8  by welding or the like. This fixing by welding can be performed, for example on the outer peripheral portion at the lengthwise opposite edges of the receiving plate  3 A. In this case, as illustrated in a plan view, seen from above in FIG. 6 (the Z-axis direction), it is preferable that the lengthwise opposite edge portions of the receiving plate  3 A (and the receiving plate  2 A) are of a shape so as to be positioned on the circumference of radius R centered on the boss  1 C. By having this shape, the position of the outer peripheral portion of the receiving plate  3 A is not deviated from being on the circumference of radius R, even if the mirror module is rotationally adjusted in the X-Y plane. Therefore, the welding position of the receiving plate  3 A and the L shaped jig  8  can be set on the same circumference. As a result, it is possible to efficiently perform the operation of fixing by welding the mirror module to the L shaped jig  8 .  
         [0049]    The mirror module fixed by welding to the L shaped jig  8  in the above manner is fixed in a movable state at a desired position along the travel shaft  7 A of the movable stage  7 , and as a result, the abovementioned mirror assembly  110  of the VIPA-VDC as shown in FIG. 11 is constructed. Then, by installing this mirror assembly  110  on the VIPA assembly  120  as with the conventional case, a VIPA-VDC is constructed, in which a compensation amount for wavelength dispersion is changed in accordance with the position of the mirror module. The compensation operation for wavelength dispersion in the VIPA-VDC constructed in this manner is the same as for the conventional case, and hence description thereof is omitted here.  
         [0050]    According to the VIPA-VDC to which the mirror fixing method of the present invention is applied as in the above manner, a possibility that the surface shape of the mirror is deformed due to temperature change or the like is extremely low. Therefore, wavelength dispersion compensation can be performed stably with high accuracy.  
         [0051]    In the abovementioned embodiment, the description has been made for the example where the mirror fixing method of the present invention is applied to the mirror assembly of the known VIPA-VDC. However the applicable scope of the present invention is not limited to this one example, and the present invention can be widely applied to well known optical systems using mirrors.  
         [0052]    For example, it is also effective to apply the present invention to a mirror part constituting an interference optical system for a surface shape measuring device as shown in FIG. 7. More specifically, in the interference optical system of FIG. 7, in order to measure the surface shape of a detection object  10 , a light emitted from an input optical path  11  passes through a lens  12  and is branched into two by a beam splitter  13 , and then, branched lights are respectively irradiated onto the detection object  10  and the mirror  1 B that is fixed by applying the present invention. Then, the light reflected from the target  10  and the light reflected from the mirror  1 B pass through the beam splitter  13  to be combined, and then sent to a CCD camera  15  via a lens  14 . Interference fringes which are caused by interference of the reflected lights, are detected by the CCD camera  15 , and the surface shape of the detection object  10  is measured from the distribution of the fringes. In this case, since the mirror  1 B becomes the standard for measurement, it is preferable that the surface shape thereof is as flat as possible without waviness or the like. The mirror  1 B of the optical apparatus according to the present invention is practically unaffected by thermal expansion due to temperature change as mentioned above. Therefore, the surface shape of the mirror  1 B can be kept flat, enabling prevention of deterioration in measurement accuracy.