Abstract:
An adjustable holder ( 1 ) for a concave holographic grating ( 2 ) for enabling the correct positioning of the grating vis-à-vis a light entrance port ( 15 ) and a light exit port ( 20 ), characterized in, that it comprises a grating hinge ( 10 ) fixed to said grating, said grating hinge ( 10 ) enabling rotational adjustment of the grating position with respect to said holder ( 1 ), the center point of the rotational adjustment being located at the vertex of said grating ( 2 ).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a filing under 35 U.S.C. §371 and claims priority to international patent application number PCT/SE2010/050342 filed Mar. 29, 2010, published on Oct. 7, 2010 as WO 2010/114466, which claims priority to application number 0900417-7 filed in Sweden on Mar. 30, 2009. 
     TECHNICAL FIELD 
     The present invention pertains to the field of holders for holographic gratings. More specifically it pertains to the field of adjustable holders for concave holographic gratings for use e.g. in optical units where monochromatic light is to be generated. 
     BACKGROUND OF THE INVENTION 
     Concave holographic gratings are commonly used in tuneable monochromators for obtaining monochromatic light of one or several wave-lengths. A typical application of such a grating is the use in the optical unit of e.g. a chemical analysis instrument such as an HPLC (High Performance Liquid Chromatography) system wherein high intensity monochromatic light via an optical fiber impinges on a flow cell through which the liquid to be analyzed, usually containing a protein, is flowing. When mounting the concave holographic grating in the optical unit it is crucial that the grating is aligned in a correct position vis-à-vis an entrance port from which the light derives and an output port constituting the entrance of the optical fiber. In order to achieve a consistent output, the vertex of the grating should be held essentially at the center of revolving of the holder, i.e. to the center of the axis around which the grating is revolved with high precision when the grating is used to generate different wave-lengths. Previously this adjustment has been performed e.g. by a “tripod” arrangement with three non-orthogonal screws which implies that the adjustment is not pure vertical or pure horizontal but gives a tilt. Furthermore the adjustment implies that the position of the vertex of the concave surface of the grating may be moved away from the center of revolving of the holder. Thus there is a need for an improved holder for a concave holographic grating where the adjustment is less cumbersome. 
     SUMMARY OF THE INVENTION 
     In a first aspect it is an object of the present invention to provide for an adjustable holder for a concave holographic grating for enabling the correct positioning of the grating vis-à-vis a light entrance port and a light exit port. A holder meeting this aspect is defined in claim  1  of the appended claims. 
     Further scope and applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that a detailed description and specific examples while indicating preferred embodiments of the invention are given by way of illustration only. There are changes and modifications within the spirit and scope of the invention which will become apparent to those skilled in the art from the detailed description below. 
     Specifically it should be noted that the use of the holder of the invention is illustrated for an optical unit in an HPLC system. However, it is useful in many other fields of application where there is a need for a holder for a concave holographic grating where the position of the grating could easily be adjusted with a high precision. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top schematic view of an optical unit for an HPLC system; 
         FIG. 2  is a perspective view of a holder for a concave holographic grating according to the invention; 
         FIG. 3  is a cross-sectional view of the holder according to  FIG. 2  along the line A-A and 
         FIG. 4  is a cross-sectional view of the holder according to  FIG. 2  along the line B-B. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In  FIG. 1 , showing a schematic view of an optical unit for an HPLC system, reference  17  denotes a flash lamp, e.g. a xenon flash lamp, the light flashes of which via a condenser and a block filter  18  and an entrance port  15  impinges on a concave holographic grating  2 . The grating  2  is used for impinging monochromatic light of various wave-lengths onto an output port  20  coupled to an optical fiber  21 . The grating  2  causes a diffraction of the light and thus different and desired wave-lengths of light can be directed towards the port  20  by revolving the grating around a pivot point P. In order to obtain the exact desired wave-length at the output port  20  it is important that the revolving of the grating is made with an extremely high precision. An example of how this can be achieved is described e.g. in the U.S. Pat. No. 6,226,084 and does not form part of the present invention. The monochromatic light from the output port  20  is via the optical fiber  21  directed through a flow cell  22  for the liquid to be analyzed to a sample detector  16  as well as to a reference detector  14  so as to make possible to measure the absorbance of light in the flow cell. To make the optical unit function in an optimal way it is important that the grating  2  is correctly positioned with a very high accuracy vis-a-vis the entrance slit  8  and the output port  20 . Thus the vertex of the concave grating has to be located at the center of rotation. 
     In  FIGS. 2 ,  3  and  4 , where  FIG. 2  is a perspective view of an embodiment of a grating holder according to the invention,  FIG. 3  is a cross-sectional view of the holder according to  FIG. 2  along the line A-A and  FIG. 4  is a cross-sectional view of the holder according to  FIG. 2  along the line B-B, reference  1  denotes a grating holder which can be revolved around a revolution axis when choosing different wave-lengths for the optical unit. In the disclosed embodiment, the revolution axis is defined by a shaft  30 , arranged to be rotatably attached to a chassis or the like of the monochromator. In the drawings a grating  2  is solidly attached to a grating hinge  10  having a spherical surface and being tiltable in a seat of the grating holder. The spherical surface of the hinge is kept in contact with the seat by means of a grating hinge spring  7  and a locking washer  3 . The grating is solidly attached to the grating hinge  10  by means of e.g. gluing and the spherical surface is shaped so as to have its center point located at the vertex of the holographic grating. Hence the position of the vertex of the holographic grating will be kept essentially static with respect to the revolution axis when the grating hinge is adjusted. The grating hinge spring  7  is resting on a cylindrical surface having its center axis running through the vertex of the holographic grating. As is best viewed in  FIGS. 3 and 4 , the grating hinge  10  comprises a cylindrical section  31  extending from the spherical surface through the holder main body  33  and which comprises a lock groove  32  receiving the locking washer  3 . The holder main body  33  comprises a hinge seat  34  and is formed with a through hole  35  for receiving the cylindrical section  31  of the grating hinge  10 . The through hole  35  is essentially coaxial with the hinge seat  34 , but as can be seen in  FIGS. 3 and 4  in combination, it is of non circular cross section in order to allow displacement of the cylindrical section  31  in the A-A plane but essentially not in the B-B plane. An adjustment screw  4  is working on an adjustment sleeve  6  towards a reverse sleeve  9 , a reverse spring  5  and a reverse screw  8 . Thus the position of the cylindrical section  31  in the A-A plane may be adjusted by operating the screw  4 , whereby the grating can be tilted upwards and downwards. The resulting movement of the grating  2  is a pure rotation about a first adjustment axis transverse to the revolution axis of the holder  1 , and to the normal of the vertex of the grating. According to one embodiment, the grating hinge  10  is arranged to enable rotational adjustment of the grating position with respect to the holder  1 , the center point of the rotational adjustment being located at the vertex of the grating  2 . 
     The distal end of the cylindrical section  31  is provided with an adjustment head  13  for rotational adjustment of the grating hinge  10  to align the grooved grating lines with the revolution axis of the grating holder  1 , so as to achieve a maximum light yield. The upon rotational adjustment by turning the adjustment head  13 , the grating hinge  10  is prevented from displacement in the B-B plane my the side walls of the through hole  35 , and in the A-A plane by the adjustment sleeve  6  and the reverse sleeve  9 . As the rotational adjustment is performed about a second adjustment axis that is orthogonal with respect to the first adjustment axis, any adjustments about either axis can be made independent of the other. When all settings are made the grating hinge  10  is locked in position by means of a locking screw  12  operating on a locking sleeve  11  that acts on the cylindrical section  31  in the B-B plane in order to avoid disturbance of the adjusted position. The locking screw has a conical shape at the tip so as to insert its hit at the center. By doing so any rotation of the grating is prevented when setting the screw. 
     All the above adjustments and calibration of the correct position are done with the help of a microscope according to a procedure well known by the person skilled in the art. This is thus not part of the invention and will not be described in detail. 
     The invention as described above by way of an example could obviously be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention and all such modification as will be obvious to one skilled in the art are intended to be included in the scope of the following claims.