Diffraction apparatus with correcting grating and method of making
TECHNICAL FIELD 
     The present invention concerns a diffraction apparatus, such as a spectrograph or monochromator, of the type that employs two spherical concave mirrors and a flat grating with increased aberration correction. It also concerns a method of holographically recording the plane grating employed in the apparatus. 
     BACKGROUND OF THE INVENTION 
     The inventive apparatus is utilized, for example, in Raman spectroscopy. The light that is to be analyzed is introduced through an entry slit positioned at an off-axis focal point of a collimator that consists of a spherical mirror. The mirror reflects a parallel pencil of light to a plane diffraction grating. The grating diffracts the light and reflects it in several parallel pencils to another spherical concave mirror, which acts as a focusing mirror. The result is separate images of the entry slit for each wavelength of incoming light. It is then possible, when the apparatus is employed as a spectrograph, to record or observe the whole resulting spectrum or, when the apparatus is employed as a monochromator, to isolate a single wavelength at one exit slit. 
     The collimating and focusing mirrors in an apparatus of this type are employed off axis and produce aberrations, especially spherically aberrations, which may also be accompanied by coma and astigmatism. 
     The object of the present invention is to reduce these aberrations as much as possible by correcting them by means of a particular arrangement of the lines on a corrected plane grating in such away that the lines will exhibit very slight deviations in parallelism and pitch. The corrected grating is a holographic grating, and another object of the invention is accordingly the method employed to holographically record it. 
     DISCLOSURE OF THE INVENTION 
     The method of holographically recording a corrected plane grating in accordance with the invention comprises the two successive stages of 
     A. Creating an auxiliary spherical grating by holographically recording the interference fringes of two monochromatic pencils of the same wavelength within the correction band, namely 
     a parallel pencil generated by one of the spherical mirrors in the apparatus from a source located at the associated operational focal point subsequent to reflection onto a plane mirror located at the grating, and 
     a divergent pencil arriving from another source located at the center of the spherical surface, 
     on a spherical concave surface, and 
     B. creating the final plane grating by holographically recording the interference fringes of two monochromatic pencils of the same wavelength employed in the initial stage, namely 
     a parallel pencil generated by the other spherical mirror in the apparatus from a source located at the associated operational focal point, and 
     a parallel pencil generated by reflection onto the auxiliary grating from another source located at the center of the surface that the grating is recorded on, 
     onto a plane surface positioned where the grating usually is within the apparatus. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described with reference to the attached drawings, wherein 
     FIG. 1 is an optical diagram illustrating both the state of the art and apparatus in accordance with the invention, the only difference being in the arrangement of the lines on the plane grating, and 
     FIGS. 2a and 2b illustrate the two successive stages of the method of recording the plane grating in accordance with the invention. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     FIG. 1 illustrates the usual elements of a conventional diffraction apparatus, which is employed in this case as a monochromator. The light to be analyzed is introduced through an entry slit 1 located at an off-axis focal point of a spherical mirror 2, which accordingly relfects it as a parallel pencil 3 of light to a plane diffraction grating 4. Grating 4 reflects the light as a plurality of parallel pencils, 5 and 6 for instance, each one of a different wavelength, to another spherical mirror 7, which acts as a focusing mirror. Thus, different images 8 and 9 of entry slit 1 are obtained for each wavelength of the light introduced at the slit, and it is possible to isolate a desired wavelength at an exit slit 10. 
     The diffraction apparatus created in accordance with the invention employs exactly the same optical configuration except that the particular position and shape of lines on flat grating 4 allows the aberrations introduced by spherical mirrors 2 and 7 to be corrected. 
     The particular method of holographically recording flat diffraction grating 4 involves an initial stage of provisionally creating an auxiliary concave holographic grating and a subsequent stage of creating plane grating 4. As will be evident from FIGS. 2a and 2b, both spherical mirrors 2 and 7 and slits 1 and 10 remain at the points they are located at in the operation illustrated in FIG. 1. 
     In the initial stage (FIG. 2a) the flat grating is replaced with a plane mirror 12. Slit 10 is employed in the capacity of an entry slit for monochromatic light of a wavelength 1 o , preferably within the range that will subsequently be employed in the apparatus. Spherical mirror 7 now reflects a parallel pencil 13 of light that mirror 12 reflects in the form of another parallel pencil 14. 
     A spherical surface 15, coated with a photosensitive resin as is conventional for recording holographic gratings, is positioned in the path of parallel pencil 14. Spherical surface 15 is simultaneously illuminated with a divergent pencil 16 of monochromatic light of the same wavelength 1 o  as parallel pencil 14, coming from a slit 17 located at the center of spherical surface 15. Once the interference fringes of pencils 14 and 16 has been imprinted on spherical surface 15, the surface is treated in the conventional manner to obtain a concave holographic grating 25 (FIG. 2b) with lines that represent the intersection of the complex interference surfaces and spherical surface 15. 
     In the second stage of the operation (FIG. 2b) the auxiliary concave grating 25 obtained in the initial stage is positioned at the same point as surface 15 and plane mirror 12 is replaced with a plane surface 22 coated, again as conventional for recording holographic gratings, with a photosensitive resin. Surface 22 is then illuminated with an initial parallel monchromatic pencil 23 of wavelength 1 o  deriving from slit 1 and reflected by mirror 2. Surface 22 is simultaneously illuminated with another parallel monochromatic pencil 24 of the same wavelength 1 o  deriving from slit 17 and reflected by grating 25. It will be evident that, since grating 25 was recorded by the interferences of parallel pencil 14 and divergent pencil 16 deriving from slit 17, it is necessary only to illuminate grating 25 with a single pencil from slit 17 for that pencil to be reflected on the surface in the form of a pencil 24 with the same property as that of recording pencil 14, parallel and aimed at the surface 22 that replaces mirror 12. 
     Once the interference fringes of pencils 23 and 24 have been exposed on the coating on surface 22, the surface is treated in the conventional manner to obtain a plane holographic grating with lines that represent the intersection of surface 22 and the interference surfaces of parallel pencils 23 and 24. It is this plane grating that is employed as the grating 4 in the apparatus illustrated in FIG. 1. 
     The interference surfaces of the plane waves of parallel pencils 23 and 24 are generally equidistant parallel planes, and their intersection with plane 22 results in a plane grating with parallel and equidistant lines. The aberrations introduced by spherical mirror 2, however, translate into aberrations in the planarity of the waves of pencil 23. The planarity of the waves of pencil 24 (FIG. 2a) is also subject to aberrations introduced by mirror 7, and the same planar aberrations are found in pencil 24 (FIG. 2b) subsequent to reflection onto grating 25, which has been recorded with pencil 14. The result is that the generally parallel and equidistant lines of grating 4 also have very small aberrations in parallelism and in the regularity of their pitch. 
     Most importantly, the aberrations thus intentionally created in the arrangement of grating 4 allows a definite correction in the aberrations introduced parasitically by spherical mirrors 2 and 7. 
     EXAMPLE 
     A monochromator of the type illustrated in FIG. 1 has been created for an operation wavelength of 4880 Å. Mirrors 2 and 7 have focal distances of 639.699 and 639.226 mm respectively. The plane grating has 1200 lines per millimeter. The angle of the axis of the incident pencil deriving from slit 1 to the axis at the apex of mirror 2 is 4.29421° . Grating 4 receives parallel pencil 3 at an incidence of 8.130° and reflects wavelength 4880 Åat an angle of 26.370° . The reflected pencil is at an angle of 4.82568° to the normal at the apex of mirror 7. A spherical surface 15 with a diameter of 938 mm and a recording wavelength of 4880 Å is employed for auxiliary grating 25. 
     Subject to these conditions the quality of the image of the entry slit is much better. Astigmatism in particular has been decreased, and the results indicated by the table are obtained for entry and exit slits 1 mm high and 0.005 mm wide. 
     
                       TABLE 1                                                     
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Wavelength        Uncorrected                                             
                             Corrected                                    
Å             grating    grating                                      
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Height of                                                                 
        3131          1.4        0.5                                      
astigmatism                                                               
        4880          1.4        0                                        
(mm)    5460          1.4        0.15                                     
Resolution                                                                
        3131          0.24       0.12                                     
(Å) 4880          0.20       0.08                                     
        5460          0.20       0.08                                     
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