Abstract:
A path of travel for radiation extends to one optical element, then to another optical element, and then away from the latter. One of the optical elements is respectively reflective and non-reflective to radiation above and below a first wavelength, and the other is respectively reflective and non-reflective to radiation below and above a second wavelength. According to a different aspect, a path of travel for radiation extends to one of first and second optical elements, then to the other optical element, and then away from the latter. The first optical element is reflective and non-reflective to radiation on respective sides of a first wavelength, and the second optical element is reflective and non-reflective to radiation on respective sides of a second wavelength. The first optical element can tilt in relation to the path of travel to change the first wavelength.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates in general to bandpass filters and, more particularly, to optical bandpass filters, including techniques for varying the bandwidth of an optical bandpass filter. 
       BACKGROUND 
       [0002]    In optical systems, it is often desirable to use a bandpass filter. One traditional form of optical bandpass filter involves a substrate with a coating containing multiple layers of optical materials. However, the number of layers needed in the coating can often exceed 100-150 layers, resulting in a very high cost. Moreover, the effective bandwidth and the center wavelength are essentially fixed during manufacture, and can only be tuned by a very small amount (always shorter and narrower), in particular by tilting the filter relative to an incident beam. However, at higher angles of incidence, the amplitude transmission deteriorates. Also, due to the transmissive nature of the filter, it can be difficult to design a coating that provides a passband for certain wavelengths ranges. For example, the substrate and/or coating materials may tend to absorb radiation in the ultra violet range. 
         [0003]    Another consideration is that optical alignment problems can result from deviation imparted to the beam as the beam passes through the substrate. Still another consideration is that the relatively large number of coating layers can induce substrate curvature, due to tensile and/or compressive stresses stacking up in the coating. This can cause wavefront distortion and/or beam deviation, resulting in optical misalignment problems in sensitive optical systems. 
         [0004]    According to a different approach, a beam is routed successively through two separate edge filters, one of which passes longer wavelengths, and the other of which passes shorter wavelengths. Each of these filters has a gradient-thickness coating provided on a plane-parallel substrate. In other words, each has a substrate of uniform thickness, with a coating that progressively increases or decreases in thickness along the substrate. The coating may be a multi-layer coating, where each layer progressively increases or decreases in thickness along the substrate. The two filters can be moved in a lengthwise direction with respect to each other, or in other words approximately perpendicular to the direction of travel of radiation. As a result of this relative movement, the width of the passband increases or decreases. However, the manufacture of gradient-thickness coated filters is complex and expensive. Also, due to the transmissive nature of the filters, absorption and beam deviation can be problems. 
         [0005]    The types of optical bandpass filters mentioned above have been generally adequate for their intended purposes but, as noted in the foregoing discussion, they have not been satisfactory in all respects. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    A better understanding of the present invention will be realized from the detailed description that follows, taken in conjunction with the accompanying drawing, in which: 
           [0007]      FIG. 1  is a diagrammatic view of an optical bandpass filter that has two edge filters and a variable bandwidth, and that embodies aspects of the invention. 
           [0008]      FIG. 2  is a graph relating to one of the edge filters of  FIG. 1 , showing the reflectivity of that edge filter over a selected range of wavelengths. 
           [0009]      FIG. 3  is a graph relating to the other of the edge filters of  FIG. 1 , showing the reflectivity of that edge filter over a selected range of wavelengths. 
           [0010]      FIG. 4  is a graph showing the combined effect of the two edge filters, representing the overall passband of the optical bandpass filter of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0011]      FIG. 1  is a diagrammatic view of an optical bandpass filter  10  that has a variable bandwidth, and that embodies aspects of the invention. The bandpass filter  10  includes a support member  12 , and a pivot mechanism that is shown diagrammatically at  14 . The pivot mechanism  14  supports the member  12  for limited pivotal movement about a pivot axis  16  that extends perpendicular to the plane of the drawing. In  FIG. 1 , the member  12  is shown in a center position. The pivot mechanism  14  can selectively pivot the member  12  a few degrees away from the illustrated center position about the axis  16 , in either of two opposite directions  17  and  18 . The pivot mechanism  14  can also releasably maintain the member  12  in any angular position. 
         [0012]    The bandpass filter  10  includes two edge filters  31  and  32  of a known type that each have one end fixedly secured to the member  12 . The edge filter  31  has a substrate  41  with a planar surface  40  thereon that faces the other edge filter  32 . The edge filter  31  also includes a multi-layer filter coating  42  provided on the surface  40 . The filter coating  42  has a planar outer surface  43 . Similarly, the edge filter  32  has a substrate  51  with a planar surface  50  thereon that faces the other edge filter  31 . The edge filter  32  also includes a multi-layer filter coating  52  provided on the surface  50 . The filter coating  52  has a planar outer surface  53 . The filter coatings  42  and  52  are very thin but, for clarity, are shown with exaggerated thicknesses in  FIG. 1 . The edge filters  31  and  32  are oriented so that the surfaces  40  and  50 , the coatings  42  and  52 , and the surfaces  43  and  53 , form a 45° angle  58  with respect to each other. The pivot axis  16  is positioned at a location corresponding to an intersection of the surfaces  40  and  50 . When the member  12  is in the center position shown in  FIG. 1 , a not-illustrated imaginary line that bisects the 45° angle  58  would intersect the pivot axis  16 , and also a point  61 . 
         [0013]    Radiation can travel along a path that includes three successive portions  71 ,  72  and  73 . The portions  71  and  73  intersect at the point  61 . An unfiltered beam of radiation enters the bandpass filter  10  along the portion  71  of the path of travel. Assume for the sake of discussion that this unfiltered beam includes radiation at wavelengths within the passband of the filter  10 , as well as wavelengths above the passband, and wavelengths below the passband. This unfiltered beam travels along the path of travel  71 , which passes through the point  61 , and eventually reaches the edge filter  32 . The portion  71  of the path of travel forms an angle  76  with respect to a line  77  that is perpendicular to the surface  53  of the edge filter  32 . This angle  76  is referred to as the angle of incidence (AOI) of the radiation on the edge filter  32 . The AOI  76  can vary, as discussed later. When the member  12  is in the center position shown in  FIG. 1 , the AOI  76  is 22.5°. 
         [0014]    In the disclosed embodiment, the edge filter  32  functions as a short wavelength reflection filter. In particular, wavelengths above the passband of the filter  10  are transmitted through the edge filter  32  along a path  81 , and are discarded. For example, they may be absorbed by a beam dump  82 . The beam dump  82  is shown diagrammatically in broken lines in  FIG. 1 , because it is optional, and is an arrangement of a known type. In contrast, wavelengths within and below the passband of the filter  10  are reflected by the edge filter  32 , and travel along the portion  72  of the path of travel to the edge filter  31 . The portion  72  of the path of travel forms an AOI  86  with respect to a line  87  perpendicular to the surface  43  of the edge filter  31 . When the member  12  is in its center position, the AOI  86  is 22.5°. 
         [0015]    The edge filter  31  functions as a long wavelength reflection filter. Wavelengths below the bandpass of the filter  10  are transmitted through the edge filter  32  along a path  91 , and are discarded. For example, these wavelengths may be absorbed by a beam dump  92 . The beam dump  92  is shown diagrammatically in broken lines in  FIG. 1 , because it is optional, and is an arrangement of a known type. The edge filter  31  reflects wavelengths that are within and above the passband of the filter  10 . Of course, as a practical matter, the filter  32  has already removed wavelengths that are above the passband. Consequently, as a practical matter, the only radiation actually reflected by the filter  32  is radiation containing wavelengths that are within the passband. These reflected wavelengths in the passband then travel along the portion  73  of the path of travel, which passes through the pivot axis  16 . This radiation then exits the filter apparatus  10  by continuing to propagate along the portion  73  of the path of travel. 
         [0016]    As discussed earlier, the pivot mechanism  14  can effect a few degrees of pivotal movement of the member  12  and the edge filters  31  and  32  about the pivot axis  16 , in either of the directions  17  and  18 . As this pivotal movement occurs, the portions  71  and  73  of the path of travel will remain in the same positions shown in  FIG. 1 , in part because the pivot axis  16  has intentionally been located at a position corresponding to an intersection of the surfaces  40  and  50 . Since the portions  71  and  73  of the path of travel do not move, there is no need to effect optical realignment in relation to other optical components as the width of the passband is adjusted. 
         [0017]    On the other hand, during pivotal movement of the member  12  and edge filters  31  and  32 , the position of the portion  72  of the path of travel will change slightly, and the AOIs  76  and  86  will each change. In particular, if the member  12  with filters  31  and  32  is pivoted counterclockwise in the direction  17 , the AOI  76  will decrease, and the AOI  86  will increase. Conversely, if the member  12  with filters  31  and  32  is pivoted clockwise in the direction  18  about the axis  16 , the AOI  76  will increase and the AOI  86  will decrease. Due to these changes in the AOIs  76  and  86 , the width of the passband of the filter  10  will change, as discussed in more detail below. 
         [0018]    As mentioned earlier, the edge filter  32  functions as a short wavelength reflection filter, based on the wavelengths that it reflects.  FIG. 2  is a graph showing the reflectivity of the edge filter  32  with respect to a selected range of wavelengths. It is an inherent characteristic of this type of edge filter that, as the AOI  76  varies, the wavelength of the “edge” of the filter  32  will change. In particular,  FIG. 2  shows that, as the AOI  76  varies through a range of about 15°, the “edge” of the filter  32  will vary from a wavelength of about 530 nm up to a wavelength of about 540 nm. 
         [0019]    Similarly, as mentioned earlier, the edge filter  31  functions as a long wavelength reflection filter, based on the wavelengths that it reflects.  FIG. 3  is a graph showing the reflectivity of the edge filter  31  with respect to a selected range of wavelengths.  FIG. 3  shows that, as the AOI  86  varies through a range of about 15°, the “edge” of the filter  31  will vary from a wavelength of about 485 nm up to a wavelength of about 503 nm. 
         [0020]    At the left side of  FIG. 2 , it will be noted that the edge filter  32  exhibits some aberrations in the range of approximately 400 nm to 410 nm. However, this does not matter in the apparatus  10  of  FIG. 1 , because the other edge filter  31  strips off and discards wavelengths in this range. Similarly, at the right side of  FIG. 3 , it will be noted that the edge filter  31  exhibits some aberrations for wavelengths above 650 nm. But again this does not matter, because the edge filter  32  removes and discards radiation in this range. 
         [0021]      FIG. 4  is a graph showing the combined effect of the two edge filters  31  and  32 , or in other words the overall passband defined by the optical bandpass filter  10  of  FIG. 1 . It will be noted in  FIG. 4  that, when the member  12  with edge filters  31  and  32  has been rotated 7.5° clockwise in the direction  18  from its center position, the width of the bandpass of the filter  10  will be approximately 26 nm (from about 503 nm to about 529 nm), where FWHM in  FIG. 4  means full width at half maximum. If the member  12  with edge filters  31  and  32  is then rotated counterclockwise in the direction  17 , the width of the passband will progressively increase in a continuous manner. For example, when the member  12  with edge filters  31  and  32  is in the center position of  FIG. 1 , the passband will be approximately 40 nm (from about 495 nm to about 535 nm). If the member  12  with edge filters  31  and  32  is then rotated another 7.5° counterclockwise in the direction  17 , the passband will further increase to a width of approximately 54 nm (from about 485 nm to about 539 nm). 
         [0022]    When the member  12  with the edge filters  31  and  32  is in its center position, the AOIs  76  and  86  are each 22.5°. In this position, the two coatings  42  and  52  are oriented so that they each have the greatest sensitivity to angular movement, with little adverse influence from the Brewster&#39;s-angle effect. In other words, for randomly polarized light, the two edges of the passband can be kept as sharp as possible, without having the edge shape degraded by polarization splitting. If the input beam is fully polarized, there is no adverse change in edge shape caused by polarization splitting. 
         [0023]    The coatings  42  and  52  are simple edge filters of a type known in the art, and are relatively easy and cheap to manufacture. Also, the filters  31  and  32  work in reflection, thereby reducing potential problems of material and substrate absorption. The substrates  41  and  51  can be made sufficiently thick to reduce or eliminate stress-related beam deviation or wavefront distortion. Moreover, the geometry of the filter  10  ensures that the output beam does not move relative to the input beam as the bandwidth is adjusted, thereby ensuring that beam pointing and boresight alignment do not vary as a result of bandpass tuning. 
         [0024]    In the bandpass filter  10  of  FIG. 1 , the positions of the edge filters  31  and  32  could be swapped, so that radiation first encounters and is reflected by a long wavelength reflection filter, and then encounters and is reflected by a short wavelength reflection filter. 
         [0025]    Although a selected embodiment has been illustrated and described in detail, it should be understood that a variety of substitutions and alterations are possible without departing from the spirit and scope of the present invention, as defined by the claims that follow.