Abstract:
The monochromator and the spectrometric method are disclosed wherein the measured beam converted into a parallel beam by a first collimator is diffracted by a plane diffraction grating, then the diffracted beam is returned so that the diffracted beam after the return is separated from that before the return along rulings of the plane diffraction grating, the diffracted beam is diffracted again by the plane diffraction grating, then the beam condensed by a second collimator is allowed to pass through an exit slit.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a monochromator and a spectrometric method for projecting a measured beam (a beam which is measured) on one and the same diffraction grating a plurality of times. 
     Conventionally, a spectroscope called “monochromator,” has been used as an instrument to measure wavelength characteristics of a measured beam. Particularly, a double monochromator is widely used to allow keeping a high resolution or a wide dynamic range by incidence of a beam into one or more diffraction gratings a plurality of times. 
     For example, a variety of monochromators have been Japanese Patent Laid-Open No. 8-145795. FIGS. 4 and 5 are perspective side views showing the configuration of representative monochromators disclosed in the publication. 
     The monochromator shown in FIG. 4 converts a beam emitted from an optical fiber  100  to a parallel beam by a collimator  102  followed by diffracting this parallel beam by a plane diffraction grating  104 . The diffracted beam is reflected by a plane mirror  106  having a reflecting surface perpendicular to the beam path, diffracted by the plane diffraction grating  104  followed by condensing by the collimator  102 , and finally passes through a slit  108 . A monochromator shown in FIG. 4 allows increasing the resolution of the wavelength λ of the diffracted beam passing through the exit slit  108 , because the measured beam is diffracted twice in the identical plane diffraction grating  104 . 
     In comparison with the structure shown in FIG. 4, the monochromator shown in FIG. 5 has a structure comprising an intermediate slit  110  and two mirrors  112  and  114 . In the monochromator shown in the FIG. 5, the diffracted beam returned by reflection by the collimator  102  is reflected 90° by one mirror  112 , passed through the intermediate slit  110  located in the condensing position of the diffracted beam, and reflected 90° by the other mirror  114  to return one more time through an optical system comprising the collimator  102 , the plane diffraction grating  104 , the plane mirror  106 . Thus, the monochromator shown in FIG. 5 allows the dynamic range of the beam to widen by passing through the intermediate slit  110  and the exit slit  108 . 
     As other conventional examples of the monochromator, those disclosed in U.S. Pat. Nos. 3,069,966 and 4,025,196 have been known. 
     Meanwhile, the conventional monochromator shown in FIG. 4 requires to locate both the optical fiber  100  used for incidence of the measured beam and the exit slit  108  in around the position of the focal point of the collimator  102  to make the structure around the focus position complex to disturb such work as assembling. Furthermore, the conventional monochromator shown in FIG. 5 requires to locate around the two mirrors  112  and  114  and the intermediate slit  110  in addition to the optical fiber  100  and the exit slit  108  around the position of the focal point of the collimator  102  to make the structure around the focus position more complex to disturb further such work as assembling. 
     SUMMARY OF THE INVENTION 
     The present invention created in consideration of such problems; the object is to provide a monochromator and a spectrometric method to allow such work as assembling by simplify the structure of the part where a measured beam is incoming and outgoing. 
     A monochromator of the present invention comprises a plane diffraction grating; a first collimator and a second collimator that are located in parallel to rulings of the plane diffraction grating; a first reflecting member that has at least two reflecting surfaces and returns a diffracted beam emitted from the plane diffraction grating so that an incident beam and an outgoing beam of the diffracted beam separate from each other along the rulings; and an exit slit located near a position of a focal point of the second collimator. By having the first reflecting member to separate and return an incident beam and an outgoing beam and the first and the second collimators for respective two separated rays, the exit slit may be located in the position of the focal point of the second collimator and other optical members may be located in the position of the focal point of the first collimator with a distance from each other. Therefore, the structures around respective positions of focus are simplified to improve such work as assembling. 
     More specifically, it is preferable that the incident member receiving the measured beam is located around the position of the focal point of the first collimator. Separating the incident member from exit slit with a distance simplifies respective fitting portions, increases a freedom of designing, and makes such work as mounting easy. Besides, improvement of resolution may become possible on the basis of that the identical plane diffraction grating carries out diffraction a plurality of times. 
     Alternatively, it is preferable that the exit slit and the incident member that receives the measured beam are located around the position of the focal point of the first collimator and that the intermediate slit and the second reflecting member, which is located in both outsides of the intermediate slit to reflect the emitted beam from the second collimator toward the second collimator, are located around the position of the focal point of the second collimator. Structures around the exit slit may be separated from the intermediate slit and the second reflecting member with a distance. Therefore, in comparison with that all these are located around the exit slit as conventional examples, respective parts maybe arranged more freely to allow freedom of designing and easy mounting work. Further, the dynamic range of the beam that passes through the exit slit may be widened by allowing to pass the measured beam through the intermediate slit in reflection of the measured beam by the second reflecting member. 
     Particularly, it is preferable that the direction of the intermediate slit in parallel to the rulings and that the two reflecting surfaces of the second reflecting member are located along the direction in which the beam emitted from the second collimator is swayed, when the plane diffraction grating is rotated about an axis which is parallel to the rulings of the grating. By such arrangement, an additive dispersion state may be realized to increase furthermore angular dispersion within the width of wavelength of the incident beam on the plane diffraction grating and also an increase in resolution becomes possible. 
     Alternatively, it is preferable that the intermediate slit is located in a direction that is perpendicular to the rulings and that the second reflecting member is located in a direction along the rulings. By such arrangement, a differential dispersion may be realized to reduce the angular dispersion within the width of wavelength of the incident beam on the plane diffraction grating. Under the condition of differential dispersion, the width of the exit slit need not change, even if the wavelength of the measured beam is changed, to make simplifying the structure possible. 
     The above described first reflecting member is preferable to emit the outgoing beam in a direction that is almost 180° opposite the direction of the incident beam. The exit slit may be easily disposed separately with a distance from other parts easily by locating the two collimators corresponding to these positions with the distance, because almost parallel reflected beam separated from the incident beam with the distance is returned. 
     Further, a spectrometric method of the present invention comprises the steps of: diffracting a measured beam converted into a parallel beam by a first collimator, by a plane diffraction grating; returning the diffracted beam so that the diffracted beam after the return is separated from and is almost parallel to that before the return along rulings of the plane diffraction grating; diffracting the diffracted beam again by the plane diffraction grating; condensing the diffracted beam by a second collimator; and allowing the diffracted beam to pass through an exit slit located in a position where the diffracted beam is condensed. The diffracted beam in the plane diffraction grating is returned to a separated position along the rulings and projected into the plane diffraction grating again in order to separate the focus positions of the two collimators, which have been installed to correspond to respective incident beam and outgoing beams, with a distance. Therefore, resolution may be improved and workability is also improved by simplifying the structure. 
     A spectrometric methods of the present invention comprises the steps of: diffracting a measured beam converted into a parallel beam by a first collimator, by a plane diffraction grating; returning the diffracted beam by a first reflecting member so that the diffracted beam after the return is separated from and is almost parallel to that before the return along rulings of the plane diffraction grating; diffracting the diffracted beam again by the plane diffraction grating; condensing the diffracted beam by a second collimator; returning the diffracted beam to almost the same beam path through an intermediate slit and a second reflecting member that are located in a position where the diffracted beam is condensed; and allowing the diffracted beam to pass through an exit slit located in the position where the diffracted beam is condensed by the first collimator. By such arrangement, the structure around the exit slit may be separated from the intermediate slit and the second reflecting member with a distance. Thus, respective parts may be arranged more freely to allow freedom of designing and easy mounting work. Further, the dynamic range of the beam that passes through the exit slit may be widen by allowing the measured beam to pass through the intermediate slit in reflection of the measured beam by the second reflecting member. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective side view showing the outlined structure of a monochromator of a first embodiment; 
     FIG. 2 is a perspective side view showing the outlined structure of a monochromator of a second embodiment; 
     FIG. 3 is a perspective side view showing the outlined structure of a monochromator of a third embodiment; 
     FIG. 4 is a perspective side view showing the outlined structure of a conventional monochromator; and 
     FIG. 5 is a perspective side view showing another outlined structure of the conventional monochromator. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Hereinafter, a monochromator of embodiments to which the present invention is applied will be described with the drawings that serve as a reference. 
     First Embodiment 
     FIG. 1 is a view showing the outlined structure of a monochromator of a first embodiment. As shown in FIG. 1, the monochromator of this embodiment comprises an incident fiber  10 , two parabolic mirrors  20  and  28 , a plane diffraction grating  22 , two plane mirrors  24  and  26 , an exit slit  30 , and an photodetector  32 . 
     The incident fiber  10  is used for emitting a measured beam toward one parabolic mirror  20  and of which position of an end for emission is set to correspond almost to the focus position of the parabolic mirror  20 . 
     The two parabolic mirrors  20  and  28  are located with a given distance in parallel to the direction of rulings of the plane diffraction grating  22 . The emission end of above described incident fiber  10  is located in the focus spot of one parabolic mirror  20 . The measured beam emitted radially from the emitting end of the incident fiber  10  is reflected by the parabolic mirror  20  to convert to a parallel beam. Meanwhile, the exit slit  30  is located in the focus spot of the other parabolic mirror  28 . The incident parallel beam on the parabolic mirror  28  is reflected and condensed by the exit slit  30 . By passing through this exit slit  30 , an unnecessary wavelength component is removed from the measured beam to project into the photodetector  32 . The photodetector  32  measures the intensity of the incident beam through the exit slit  30 . 
     The two plane mirrors  24  and  26  is located with a given distance in parallel to the direction of the rulings of the plane diffraction grating  22 . One plane mirror  24  reflects almost 90° the beam diffracted by the plane diffraction grating  22  in parallel to the direction of the rulings of the plane diffraction grating  22 . The other plane mirror  26  further reflects almost 90° the measured beam reflected by the one plane mirror  24 . These two plane mirrors  24  and  26  return the measured beam emitted from the plane diffraction grating  22  toward the plane diffraction grating  22  again. 
     The plane diffraction grating  22  has rulings formed in a given direction with a certain intervals and diffracts the incident beam from the parabolic mirror  20  or the plane mirror  26 . In FIG. 1, a plurality of grooves has been formed in parallel to a perpendicular direction. The width of the plane diffraction grating  22  along a direction that is normal to the direction of the rulings is assigned to W and the density of grooves is ρ [/mm]. A mechanism for rotating such as a motor (not illustrated) having a rotation axis in parallel to the rulings is mounted on the plane diffraction grating  22 . The rotating mechanism constitutes rotatably the plane diffraction grating  22  around the rotation axis as a center. 
     Above described incident fiber  10 , the two parabolic mirrors  20  and  28 , and the two plane mirrors  24  and  26  correspond to the incident member, the first and the second collimators, and the first reflecting member, respectively. 
     The monochromator of this embodiment has such structure. The action thereof is described below. 
     The incident beam  40  introduced from outside via the incident fiber  10  is reflected by one parabolic mirror  20 , converted to a parallel beam  41 , and projected on the plane diffraction grating  22 . The plane diffraction grating  22  diffracts the parallel incident beam  41  from the one parabolic mirror  20  to emit as a diffracted beam  42 . The diffracted beam  42  is projected on the one plane mirror  24  and reflected in a direction that is almost 180° opposite the direction in which it has been projected, by the two plane mirrors  24  and  26 . Furthermore, using the two plane mirrors  24  and  26  allows move the path of the diffracted beam  42  projected on these two plane mirrors  24  and  26  and the path of the diffracted beam  43  emitted from these two plane mirrors  24  and  26  toward a given distance in a direction of the rulings of the plane diffraction grating  22 . 
     The diffracted beam  43  reflected by the plane mirror  26  projected again into the plane diffraction grating  22 . The plane diffraction grating  22  diffracts again the diffracted beam  43  projected to emit as a diffracted beam  44 . The diffracted beam  44  is reflected by the other parabolic mirror  28  to form an image on the exit slit  30  located in the position of the focal point of this parabolic mirror  28 . 
     Meanwhile, Rotating slightly the plane diffraction grating  22  around the axis of rotation as a center changes an incident angle of the parallel beam  41  projected on the plane diffraction grating  22  from the parabolic mirror  20  and a diffraction angle of the diffracted beam  42  emitted toward the plane mirror  24  from the plane diffraction grating  22 . The same observation is yielded for the diffracted beam  43  projected on the plane diffraction grating  22  after reflection by the plane mirrors  26 . By such rotation the plane diffraction grating  22  allows changing the wavelength λ of the beam passing through the exit slit  30  because of change of the incident angle and the diffraction angle. 
     In this way, in the monochromator of this embodiment, the diffracted beam  42  emitted from the plane diffraction grating  22  is returned in a direction that is almost 180° opposite the direction in which it has been emitted and projected on the plane diffraction grating  22  by using the two plane mirrors  24  and  26 , and then the measured beam emitted from the incident fiber  10  is diffracted twice by the same plane diffraction grating  22  before it reaches the exit slit  30 . Therefore, it is possible to improve the resolution (=λ/Δλ=2Wρ) of the wavelength of the diffracted beam passing through the exit slit  30 . 
     Using the two plane mirrors  24  and  26  and the two parabolic mirrors  20  and  28  allows separating a path through which the measured beam projected from the incident fiber  10  reaches the one plane mirror  24  from a path through which the measured beam reflected by the other plane mirror  26  reaches the exit slit  30  with a distance along the rulings of the plane diffraction grating  22 . Thus, respective positions of the incident fiber  10  and the exit slit  30  may be separated with a distance to make the structure of respective fitting parts noncomplex, the freedom of designing higher, and work for mounting them easy. 
     Second Embodiment 
     FIG. 2 is a perspective side view showing the outlined structure of a monochromator of a second embodiment. As shown in FIG. 2, the monochromator of this embodiment comprises an incident fiber  10 , two parabolic mirrors  20  and  28 , the plane diffraction grating  22 , two plane mirrors  24  and  26 , the exit slit  30 , photodetector  32 , an intermediate slit  50 , and two tilted mirrors  52  and  54 . 
     The monochromator of this embodiment, as shown in FIG. 2, has differences in that the exit slit  30  and the photodetector  32  are located around the focus position of the one parabolic mirror  20  near the incident fiber  10  and that the intermediate slit  50  and the two tilted mirrors  52  and  54  are located around the focus position of the other parabolic mirror  28 , in comparison with the monochromator of the first embodiment shown in the FIG.  1 . Concerning basically the members same as those of the monochromator of the first embodiment shown in FIG.  1 , the identical symbols and used to omit a detailed description. 
     The two tilted mirrors  52  and  54  are used for reflection of the beam condensed by the parabolic mirror  28  toward the parabolic mirror  28  again. These two tilted mirrors  52  and  54  correspond to the second reflecting member. These two tilted mirrors  52  and  54  are lied in along the direction to which the beam emitted from the parabolic mirror  28  sways, when the plane diffraction grating  22  is rotated around the rotating axis as a center parallel to the rulings. The measured beam emitted from the parabolic mirror  28  is approximately 90° reflected by the one tilted mirror  52 , the beam that has passed through the intermediate slit  50  located in a position, in which the reflected beam is condensed, is approximately 90° reflected by the other tilted mirror  54 , and finally returned toward the parabolic mirror  28  again. 
     The monochromator of this embodiment has such structure. Action thereof will be described below. In the same way as the monochromator shown in FIG. 1, after a measured beam introduced from outside through the incident fiber  10  is reflected or diffracted by respective the one parabolic mirrors  20 , the plane diffraction grating  22 , the one plane mirrors  24 , the other plane mirrors  26 , and the plane diffraction grating  22 , it is condensed by the other parabolic mirrors  28 . As described above, the measured beam condensed by this parabolic mirrors  28  is reflected by the one tilted mirrors  52  to change 90° the course thereof, passed through the intermediate slit  50  located in the condensing position to reflect by the other tilted mirrors  54 , changed the course thereof approximately 90° to return toward the other parabolic mirrors  28  side. The measured beam projected on the parabolic mirrors  28  again by such steps goes back through the identical path so far traveled and passes through the exit slit  30  located around the focus position of the parabolic mirror  20  to reach the photodetector  32 . 
     In this way, in the monochromator of this embodiment, the diffracted beam emitted from the plane diffraction grating  22  is returned in a direction that is almost 180° opposite the direction in which it has been emitted and projected on the plane diffraction grating  22  by using the two plane mirrors  24  and  26 , and then the measured beam projected from the incident fiber  10  is diffracted twice by one and the same plane diffraction grating  22  before it reaches to the other parabolic mirrors  28 . Further, after these steps, the measured beam passes through the intermediate slit  50  located around the focus position of this parabolic mirror  28  to go back through the identical beam path and diffracted twice by one plane diffraction grating  22 . Thus, Passing through the intermediate slit  50  in addition to the exit slit  30  allows removal of an unnecessary wavelength component from the measured beam to widen the dynamic range of the beam. Besides, locating the two tilted mirrors  52  and  54  along a direction to which the measured beam sways by rotation of the plane diffraction grating  22  allows realization of the additive dispersion alignment. Therefore, it is possible to improve further the resolution (=λ/Δλ=4Wρ) of the wavelength of the diffracted beam passing through the exit slit  30 . 
     Using the two plane mirrors  24  and  26  and the two parabolic mirrors  20  and  28  allows separating a path of the measured beam traveling between the incident fiber  10  or the exit slit  30  and the one plane mirror  24  and a path of the measured beam traveling between the intermediate slit  50  and the other plane mirror  26 , with a distance along the rulings of the plane diffraction grating  22 . Thus, respective positions of the incident fiber  10  and the exit slit  30 , the two tilted mirrors  52  and  54 , and the intermediate slit  50  may be separated with a distance to make the structure of respective fitting parts noncomplex, the freedom of designing higher, and work for mounting them easy. 
     Separate positioning of the exit slit  30  and the intermediate slit  50  each other allows prevention of the phenomena so-called cross talk or stray phenomenon in which a part of the measured beam heading to the intermediate slit  50  from the parabolic mirror  28  travels into the exit slit  30 . Also, it improves the dynamic range. 
     Third Embodiment 
     FIG. 3 is a perspective side view showing the outlined structure of a monochromator of a third embodiment. As shown in FIG. 3, the monochromator of this embodiment comprises an incident fiber  10 , two parabolic mirrors  20  and  28 , the plane diffraction grating  22 , the two plane mirrors  24  and  26 , the exit slit  30 , the photodetector  32 , an intermediate slit  60 , and two tilted mirrors  62  and  64 . 
     The monochromator of this embodiment, as shown in FIG. 3, has differences in that the intermediate slit  50  and the two tilted mirrors  52  and  54  are replaced by the intermediate slit  60  and the two tilted mirrors  62  and  64  of different arrangement from the former combination, in comparison with the monochromator of the second embodiment, for carrying out the invention, shown in FIG.  2 . Concerning basically the members same as those of the monochromator shown in FIG.  1  and FIG. 2, the identical symbols are used to omit a detailed description. 
     The two tilted mirrors  62  and  64  are used for reflection of a beam condensed by the parabolic mirror  28  toward the parabolic mirror  28  again. These two tilted mirrors  62  and  64  correspond to the second reflecting member. These two tilted mirrors  62  and  64  are located side by side in a direction parallel to the rulings of the plane diffraction grating  22 . The measured beam emanated from the parabolic mirror  28  is approximately 90° reflected by the one tilted mirror  62 , the measured beam that has passed through the intermediate slit  60  located in a position, in which the reflected beam is condensed, is approximately 90° reflected by the other tilted mirror  64 , and finally returned toward the parabolic mirror  28  again. 
     The monochromator of the third embodiment has such structure. Action thereof will be described below. In the same way as the monochromator shown in FIG. 1, after a measured beam introduced from outside through the incident fiber  10  reflected or diffracted by respective the one parabolic mirrors  20 , the plane diffraction grating  22 , the one plane mirrors  24 , the other plane mirrors  26 , and the plane diffraction grating  22 , it is condensed by the other parabolic mirrors  28 . As described above, the measured beam condensed by this parabolic mirrors  28  is reflected by the one tilted mirrors  62  and shifted 90° from the direction of travel so as to be along the rulings of the plane diffraction grating  22 , passed through the intermediate slit  60  located in the condensing position to reflect by the other tilted mirrors  64 , changed the traveling direction thereof approximately 90° to return toward the other parabolic mirrors  28  side. The measured beam projected on the parabolic mirrors  28  again by such steps reverses the identical path so far traveled and passes through the exit slit  30  located around the focus position of the parabolic mirror  20  to reach the photodetector  32 . 
     As described above, in the monochromator of the third embodiment, passing through the intermediate slit  60  in addition to the exit slit  30  allows removal of an unnecessary wavelength component from the measured beam to widen the dynamic range of the beam. Meanwhile, the resolution (=λ/Δλ=2Wρ) of the wavelength λ of the diffracted beam passing through the exit slit  30  is the same as that of the monochromator of the first embodiment. However, locating the two tilted mirrors  62  and  64  in a direction parallel to the rulings of the plane diffraction grating  22  allows realization of differential dispersion alignment. The wavelength of the measured beam may be changed by rotating the plane diffraction grating  22  and by changing a slit width of the exit slit  30 . Changing a slit width of the intermediate slit  60  is unnecessary. Therefore, A wide dynamic range same as that of the monochromator, of the second embodiment, having the additive dispersion alignment can be realized with a relatively simple structure. 
     It is the same as that of the monochromator of the second embodiment that use of the two plane mirrors  24  and  26  and the two parabolic mirrors  20  and  28  allows separating a path of the measured beam traveling between the incident fiber  10  or the exit slit  30  and the one plane mirror  24  from a path of the measured beam traveling between the intermediate slit  60  and the other plane mirror  26 , with a distance along the rulings of the plane diffraction grating  22 . Thus, respective positions of the incident fiber  10  and the exit slit  30 , the two tilted mirrors  62  and  64 , and the intermediate slit  60  may be separated with a distance to make the structure of respective fitting parts noncomplex, the freedom of designing higher, and work for mounting them easy. 
     The present embodiment is to be considered in all respects as illustrative and not respective. Therefore, the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. For example, the two parabolic mirrors  20  and  28  were used as collimators in aforementioned mode for carrying out the present invention. However, condenser lenses may be used for this purpose. The traveling direction of incident beam is approximately 180° returned by using the two plane mirrors  24  and  26  and their beam paths were separated along the rulings of the plane diffraction grating  22  with a distance. However, three or more plane mirrors may be combined. Or, the incident beam may be reflected by using two reflecting surfaces that have been orthogonalized each other in the same member. 
     The monochromator may be prepared by combining the incident fiber  10 , the exit slit  30 , and the intermediate slit  50  and  60  of each mode for carrying out the present invention. For example, in the monochromator of the additional dispersion alignment shown in FIG. 2, the exit slit  30  may be replaced by the intermediate slit  60  and the tilted mirrors  62  and  64  shown in FIG.  3  and also the exit slit  30  and the photodetector  32  maybe moved around the intermediate slit  60 . In this case, a monochromator in combination of the additional dispersion alignment and a differential dispersion alignment can be realized. In any combinations, the incident fiber  10 , the exit slit  30 , and the intermediate slit  50  and  60 , which were intensively located in a single place so far, can be separately located in two places to make freedom of allocation of respective parts large, the structure simply, and improvement of such work as assembling possible.