Abstract:
A method and apparatus involve an optical element having a passband with a center wavelength, and filtering radiation having first and second portions that arrive along a path of travel extending to the optical element. The first portion includes radiation inside the passband, and the second portion includes radiation above and below the passband. The optical element transmits one of the first and second portions of the radiation therethrough, and reflects the other of the first and second portions of the radiation therefrom. The optical element is supported for a range of movement relative to the path of travel. As the optical element moves through the range of movement, the center wavelength changes.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates in general to bandpass and notch filters and, more particularly, to optical bandpass and notch filters, including techniques for varying the center wavelength of optical bandpass and notch filters. 
       BACKGROUND 
       [0002]    In optical systems, it is often desirable to use an optical bandpass filter. Traditional optical bandpass filters are generally optimized to work over a restricted range of angles close to normal incidence. The effective bandwidth and center wavelength are essentially fixed during manufacture, and can only be tuned by a very small amount (always shorter and narrower), for example by tilting the filter relative to an incident beam. Moreover, at higher angles of incidence, the amplitude transmission deteriorates. In addition, it is often desirable to use the reflection beam from a bandpass filter. The reflection beam from a bandpass filter is a notch-filtered beam. However, the direction of travel of the notch-filtered beam changes with a change in the angle between the incident beam and the filter. Consequently, it can be difficult to align a notch-filtered beam from a traditional optical bandpass filter with other optical components of the optical system. 
         [0003]    The types of optical bandpass filters mentioned above, for transmitting bandpass-filtered beams and reflecting notch-filtered beams, have been generally adequate for their intended purposes. However, as noted in the foregoing discussion, they have not been satisfactory in all respects. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    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: 
           [0005]      FIG. 1  is a diagrammatic view of an optical filter apparatus that embodies aspects of the invention. 
           [0006]      FIG. 2  is a graph showing the transmittance of a filter in the apparatus of  FIG. 1  with respect to unpolarized radiation at a selected angle of incidence. 
           [0007]      FIG. 3  is a graph showing the reflectance of the filter of  FIG. 1  with respect to unpolarized radiation at a selected angle of incidence. 
           [0008]      FIG. 4  is a graph showing the transmittance of the filter of  FIG. 1  with respect to unpolarized radiation at selected angles of incidence. 
           [0009]      FIG. 5  is a graph showing the reflectance of the filter of  FIG. 1  with respect to unpolarized radiation at selected angles of incidence. 
           [0010]      FIG. 6  is a graph showing the transmittance of the filter of  FIG. 1  with respect to s-polarized radiation at selected angles of incidence. 
           [0011]      FIG. 7  is a graph showing the reflectance of the filter of  FIG. 1  with respect to s-polarized radiation at selected angles of incidence. 
           [0012]      FIG. 8  is a graph showing the transmittance of the filter of  FIG. 1  with respect to p-polarized radiation at selected angles of incidence. 
           [0013]      FIG. 9  is a graph showing the reflectance of the filter of  FIG. 1  with respect to p-polarized radiation at selected angles of incidence. 
           [0014]      FIG. 10  is a graph showing the transmittance of the filter of  FIG. 1  with respect to s and p polarized radiation at selected angles of incidence. 
           [0015]      FIG. 11  is a diagrammatic view of another optical filter apparatus that is an alternative embodiment of the optical filter apparatus shown in  FIG. 1 , and that embodies aspects of the invention. 
           [0016]      FIG. 12  is a graph showing the transmittance of a filter in the apparatus of  FIG. 11  with respect to unpolarized radiation at selected angles of incidence. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]      FIG. 1  is a diagrammatic view of an optical filter apparatus  10  that receives radiation as an input, filters the received radiation, and outputs respective portions of the filtered radiation along two paths of travel. In the disclosed embodiment the apparatus  10  is configured to have an operating range that is a selected portion of the spectrum between extreme ultraviolet radiation and long-wave infrared radiation. However, the apparatus  10  could be configured to have an operating range that includes some other portion of the electromagnetic spectrum. 
         [0018]    The optical filter apparatus  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. 
         [0019]    The optical filter apparatus  10  includes a filter  31  and a reflective element  32  that are each of a known type, and that each have one end fixedly secured to the member  12 . The filter  31  has a substrate  40  with a planar surface  41  thereon facing the reflective element  32 , and with another planar surface  42  parallel to and on a side opposite from the surface  41 . The filter  31  also includes a multi-layer filter coating  43  provided on the surface  41 . The multi-layer filter coating  43  has a planar outer surface  44 . In the disclosed embodiment, the filter  31  is a multi-cavity Fabry-Perot structure, but it could alternatively have some other suitable structure. The multi-layer filter coating  43  is transmissive to radiation inside a passband having a center wavelength, and reflective to radiation above and below the passband. Consequently, the radiation transmitted through the filter  31  is a bandpass-filtered beam and the radiation reflected from the filter  31  is a notch-filtered beam. The bandpass-filtered beam includes radiation inside the passband, and the notch-filtered beam includes radiation above and below the passband. 
         [0020]    The reflective element  32  has a substrate  50  with a planar surface  51  thereon that faces the filter  31 . The reflective element  32  also includes a mirror coating  52  provided on the surface  51 . In the disclosed embodiment, the mirror coating  52  is a multi-layer design including dielectric materials. Alternatively, however, the coating  52  could be made from any other suitable material or combination of materials, and could for example be made of a metallic material. The mirror coating  52  has a planar outer surface  53 . The multi-layer filter coating  43  and the mirror coating  52  are very thin but, for clarity, are shown with exaggerated thicknesses in  FIG. 1 . The filter  31  and the reflective element  32  are oriented so that the surfaces  41  and  51 , the coatings  43  and  52 , and the surfaces  44  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  41  and  51 . 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 . 
         [0021]    Radiation can travel along a path that includes three successive sections  71 ,  72 , and  73 . Also, radiation can travel along another path that includes successive sections  81  and  82 . The sections  71  and  82  intersect at the point  61 . A beam of radiation enters the optical filter apparatus  10  along the path section  71 . Assume for the sake of discussion that this beam is unfiltered, and includes radiation at wavelengths within the passband of the filter  31 , as well as wavelengths above the passband, and wavelengths below the passband. This unfiltered beam travels along section  71  of the path of travel, which passes through the point  61 , and eventually reaches the filter  31  at a location  83 . The section  71  of the path of travel forms an angle  86  with respect to a line  87  that is perpendicular to the surface  44  of the filter  31  at the location  83 . This angle  86  is referred to as the angle of incidence (AOI) of the radiation on the filter  31 . The AOI  86  can vary, as discussed later. When the member  12  is in the center position shown in  FIG. 1 , the AOI  86  is 22.5°. By optimizing the filter  31  for the center position of 22.5°, the filter  31  is more sensitive to angular movement, and thus more tunable. 
         [0022]    In the disclosed embodiment, wavelengths inside the passband of the filter  31  are transmitted through the filter  31  along the path section  72 . Refraction occurs as the transmitted radiation passes through the filter  31 , and causes the path section  72  to extend at an angle to the path section  71 . When this transmitted radiation passes through the surface  42  at a location  88 , the radiation refracts again such that the path section  73  is substantially parallel to the path section  71 . This transmitted radiation (the bandpass-filtered beam) then exits the filter  31  at the location  88  and travels along the path section  73 . For example,  FIG. 2  is a graph showing the transmittance of the filter  31  with respect to unpolarized radiation when the AOI  86  is 22.5°. When the AOI  86  is 22.5°, the member  12  is in its center position.  FIG. 2  shows that for an AOI of 22.5°, the passband of the filter  31  is between about 549 nm and 551 nm, and the center wavelength of the passband is at about 550 nm. Moreover,  FIG. 2  illustrates that the filter  31  is approximately 100% transmissive to radiation with wavelengths between 549 nm and 551 nm, and approximately 0% transmissive (or said another way, approximately 100% reflective) to radiation below 549 nm and above 551 nm. The ranges of wavelengths for which the filter  31  is approximately 0% transmissive are known as extinction bands. 
         [0023]    Wavelengths that are traveling along path section  71  and that are above and below the passband are reflected by the filter  31  at the location  83 , and then travel along the path section  81  of the other path of travel to a location  90  on the reflective element  32 . The path section  81  of the path of travel forms an AOI  91  with respect to a line  92  perpendicular to the surface  53  of the reflective element  32  at the location  90 .  FIG. 3  is a graph showing the reflectance of the filter  31  with respect to unpolarized radiation when the AOI  86  is 22.5°. The graph of  FIG. 3  is the inverse of the graph of  FIG. 2 . For example, at wavelengths having an approximately 100% transmittance through the filter  31 , the reflectance at the same angle of incidence is approximately 0%. Conversely, at wavelengths having an approximately 0% transmittance, the reflectance is approximately 100%. In further detail,  FIG. 3  shows that for an AOI  86  of 22.5°, the passband of the filter  31  is between about 549 nm and 551 nm, and the center wavelength of the passband is at about 550 nm. Moreover,  FIG. 3  shows that the filter  31  is approximately 100% reflective to radiation with wavelengths below 549 nm and above 551 nm, and approximately 0% reflective (or said another way, approximately 100% transmissive) to radiation between 549 nm and 551 nm. 
         [0024]    In the disclosed embodiment, the reflective element  32  is capable of reflecting all wavelengths within the operating range of the optical filter apparatus  10 . As discussed above, the apparatus  10  in the disclosed embodiment is configured to have an operating range that is a portion of the spectrum between extreme ultraviolet and long-wave infrared, depending on the materials used for the substrate  40 , and the coatings  43  and  52 . The filter  31  has already transmitted wavelengths that are inside the passband, and only wavelengths above and below the passband are reflected along the path section  81  to the reflective element  32 . Consequently, as a practical matter, the only radiation actually reflected by the reflective element  32  is radiation containing wavelengths that are above and below the passband of the filter  31 . These reflected wavelengths above and below the passband then travel along the path section  82 , which passes through the point  61 . This reflected radiation (the notch-filtered beam) then exits the optical filter apparatus  10  by continuing to propagate along the path section  82 . Although two different beams of radiation exit the disclosed apparatus (the bandpass-filtered beam at path section  73  and the notch-filtered beam at path section  82 ), it would alternatively be possible to modify the disclosed apparatus by adding a beam dump positioned to receive and absorb one of the two beams, so that only the other beam exits the apparatus. 
         [0025]    As discussed earlier, the pivot mechanism  14  can effect a few degrees of pivotal movement of the member  12 , the filter  31  and the reflective element  32  about the pivot axis  16 , in either of the directions  17  and  18 . As this pivotal movement occurs, the sections  71  and  82  of the paths 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  41  and  51 . Also, since the sections  71  and  82  of the paths of travel do not move as pivotal movement occurs, there is no need to effect optical realignment of the notch-filtered beam traveling along path section  82  in relation to other optical components. On the other hand, during pivotal movement of the member  12 , the filter  31 , and the reflective element  32 , the position of the section  81  of the path of travel will change slightly. 
         [0026]    As discussed earlier, the pivot mechanism  14  can effect a few degrees of pivotal movement of the member  12 , and the AOIs  86  and  91  will each change. In particular, if the member  12  with the filter  31  and the reflective element  32  is pivoted counterclockwise in the direction  17 , the AOI  86  will decrease, and the AOI  91  will increase. Conversely, if the member  12  with the filter  31  and the reflective element  32  is pivoted clockwise in the direction  18  about the axis  16 , the AOI  86  will increase and the AOI  91  will decrease. Due to these changes in the AOIs  86  and  91 , the passband and center wavelength of the filter  31  will change, as discussed in more detail below. 
         [0027]      FIG. 4  is a graph showing the transmittance of the filter  31  with respect to unpolarized radiation at selected different AOI  86 . It is an inherent characteristic of the multi-layer filter coating  43  that, as the AOI  86  varies, the passband of the filter  31  will shift.  FIG. 4  shows eleven curves that each represent the filtering characteristic of the filter  31  at a respective different AOI  86 . One of the curves shown in  FIG. 4  is labeled to indicate that it corresponds to an AOI  86  of 22.5°, when the member  12  is in the center position shown in  FIG. 1 . This curve is the same curve shown in  FIG. 2 . Other curves in  FIG. 4  show the transmissivity of the filter  31  at other AOIs. 
         [0028]      FIG. 4  shows that as the AOI  86  varies, the passband and extinction bands of the filter  31  will shift together within the optical spectrum. In particular, as the AOI  86  varies through a range of about 25°, the passband will shift up or down in the spectrum, such that the center wavelength of the passband of the filter  31  varies from a wavelength of about 530.5 nm up to a wavelength of about 562 nm. As an example, when the AOI  86  is 35°, the center wavelength of the passband of the curve  100  is about 530.5 nm. When the AOI  86  is 32.5°, the center wavelength of the passband of the curve  101  is about 535 nm. 
         [0029]      FIG. 5  is a graph showing the reflectance of the filter  31  with respect to unpolarized radiation at selected angles of incidence, and is the inverse of the graph in  FIG. 4  that shows the transmittance of the filter  31 . One of the curves shown in  FIG. 4  is labeled to indicate that it corresponds to an AOI  86  of 22.5°, when the member  12  is in the center position shown in  FIG. 1 . This curve is the same curve shown in  FIG. 3 . Other curves in  FIG. 5  show the reflectance of the filter  31  at other AOIs. 
         [0030]    As the center wavelength of the passband shifts for transmitted radiation traveling along path section  73 , the radiation reflected by the filter  31  along path sections  81  and  82  shifts in unison. Referring back to the previous examples given for the AOI  86 , when the AOI  86  is 35°, the passband shown in  FIG. 4  ranges from about 530 nm to 532 nm. Accordingly,  FIG. 5  shows 100% reflection of radiation below about 530 nm and above about 532 nm when the AOI  86  is 35°. Moreover, when the AOI  86  is  32 . 50 , the passband ranges from about 534 nm to 536 nm. Accordingly,  FIG. 5  shows 100% reflection of radiation below about 534 nm and above about 536 nm, and approximately 0% reflection between about 534 nm and 536 nm when the AOI  86  is 32.5°. 
         [0031]    When the AOI  86  is small, mixing of the s-polarized and p-polarized components of the transmitted radiation does not produce problems. However, as the AOI  86  becomes larger, the s-polarized and p-polarized components of the transmitted radiation begin to mix in a manner creating aberrations that can be seen in  FIGS. 4 and 5 . For example, when the AOI  86  is 35°,  FIG. 4  shows aberrations  110  and  111  that are a result of the mixing of the s-polarized and p-polarized components of the transmitted radiation. When the AOI  86  is 10°, such aberrations are practically absent from the transmitted radiation. 
         [0032]    Assume that the input radiation entering at  71  is s-polarized radiation rather than unpolarized radiation.  FIG. 6  is a graph showing the transmittance of the filter  31  with respect to s-polarized radiation at selected angles for the AOI  86 . The graph of  FIG. 6  is similar to the graph of  FIG. 4 , except that it shows the transmittance of s-polarized radiation instead of unpolarized radiation.  FIG. 7  is a graph showing the reflectance of the filter  31  with respect to s-polarized radiation at selected angles of incidence, and is the inverse of the graph in  FIG. 6  that shows the transmittance of the filter  31  with respect to s-polarized radiation. 
         [0033]    Now assume that the input radiation entering at  71  is p-polarized radiation rather than unpolarized radiation or s-polarized radiation.  FIG. 8  is a graph showing the transmittance of the filter  31  with respect to p-polarized radiation at selected different AOI  86 . The graph of  FIG. 8  is similar to the graphs of  FIGS. 4 and 6 , except that it shows the transmittance of p-polarized radiation instead of unpolarized radiation and s-polarized radiation, respectively.  FIG. 9  is a graph showing the reflectance of the filter  31  with respect to p-polarized radiation at selected angles of incidence, and is the inverse of the graph of  FIG. 8  that shows the transmittance of the filter  31  with respect to p-polarized radiation. 
         [0034]    It is an inherent characteristic of the multi-layer filter coating  43  that, at selected angles for the AOI  86 , the passband is wider for p-polarized radiation ( FIG. 8 ) than for s-polarized radiation ( FIG. 6 ). Thus, the width of the passband can also be varied by changing the polarization of the input radiation supplied to the apparatus  10  at  71 . The comparison of passband widths for s and p polarization is even more clearly shown in  FIG. 10 , discussed below. 
         [0035]      FIG. 10  is a graph showing the transmittance of the filter  31  with respect to s-polarized and p-polarized radiation at selected different AOI  86 .  FIG. 10  uses a logarithmic scale for the vertical axis, where the vertical axis represents transmittance. In particular, 0 dB represents 100% transmittance, −10 dB represents 10% transmittance, −20 dB represents 1% transmittance, −30 dB represents 0.1% transmittance, −40 db represents 0.01% transmittance, and so forth, all the way down to −100 dB which represents approximately 0% transmittance. Therefore, the portion of the graph in  FIG. 10  ranging from −10 db to −100 dB shows in an expanded scale the transmittance between 10% and approximately 0% on the linear transmittance scale in the graphs of  FIGS. 6 and 8 . Consequently,  FIG. 10  clearly illustrates that the passband is wider for p-polarized radiation transmitted by the filter  31  than for s-polarized radiation transmitted by the filter  31 . Moreover,  FIG. 10  also illustrates that the slope of the edges of the passband for s-polarized radiation is steeper than the slope of the edges of the passband for p-polarized radiation. 
         [0036]    The reflectivity of the filter  31  is represented by the inverse of the graph in  FIG. 10 . Therefore,  FIG. 10  shows that the spectrum of radiation reflected for s-polarized radiation is greater than the spectrum of radiation reflected for p-polarized radiation. 
         [0037]      FIG. 11  is a diagrammatic view of an optical filter apparatus  119  that is an alternative embodiment of the optical filter apparatus  10  shown in  FIG. 1 . Identical or equivalent elements are identified by the same reference numerals, and the following discussion focuses primarily on the differences. The optical filter apparatus  119  includes a filter  120  and a multi-layer filter coating  121  that respectively replace the filter  31  ( FIG. 1 ) and the multi-layer filter coating  43  ( FIG. 1 ). The filter  120  operates in a manner complementary to the filter  31  ( FIG. 1 ). The multi-layer filter coating  121  is reflective to radiation inside a passband having a center wavelength, and transmissive to radiation above and below the passband. Consequently, the radiation reflected from the filter  120  is a bandpass-filtered beam and the radiation transmitted through the filter  120  is a notch-filtered beam. 
         [0038]    In greater detail, the notch-filtered beam is transmitted through the filter  120  along the path section  72 . This transmitted notch-filtered beam then exits the filter  120  at the location  88  and travels along the path section  73 . In contrast, wavelengths inside the passband are reflected by the filter  120  at the location  83 , and travel along the section  81  of the other path of travel to the location  90  on the reflective element  32 . In the disclosed embodiment, the reflective element  32  is capable of reflecting all wavelengths within the operating range of the optical filter apparatus  119 . The filter  120  has already transmitted wavelengths that are above and below the passband, and only wavelengths inside the passband are reflected along the path section  81  to the reflective element  32 . Consequently, as a practical matter, the only radiation actually reflected by the reflective element  32  is radiation containing wavelengths that are inside the passband. These reflected wavelengths inside the passband then travel along the path section  82 , which passes through the point  61 . This reflected radiation (the bandpass-filtered beam) then exits the optical filter apparatus  119  by continuing to propagate along the path section  82 . 
         [0039]      FIG. 12  is a graph showing the transmittance of the filter  120  with respect to unpolarized radiation at selected different AOI  86 . It is an inherent characteristic of this type of filter  120  that, as the AOI  86  varies, the passband of the filter  120  will shift. In particular,  FIG. 12  shows that, as the AOI  86  varies through a range of about 25°, the center wavelength of the passband of the filter  120  will vary. 
         [0040]    As noted above, the graph of  FIG. 12  corresponds to a situation where the radiation entering the apparatus  119  at  71  is unpolarized radiation. By way of analogy to the discussion above of the embodiment of  FIGS. 1-10 , it will be recognized that if the radiation entering the apparatus  119  at  71  is polarized radiation, the polarized radiation can narrow or broaden the effective width of the passband. 
         [0041]    Although selected embodiments have 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.