Abstract:
The tilted relationship between the reference and test mirrors of a Fizeau interferometer is used to spatially separate the reflections from the two surfaces. The separate beams are filtered through a spatial polarization element that provides different states of polarization to the beams. The beams are subsequently recombined to form a substantially collinear beam that is processed using a spatial-phase-shift interferometer that permits quantitative phase measurement in a single video frame. Alternatively, two beams with orthogonal polarization are injected into the Fizeau cavity at different angles, such that after reflection from the reference and test optics they are substantially collinear. Unwanted reflections are blocked at the focal plane through the use of a circular aperture. Short coherence length light and a delay line may be used to mitigate stray reflections, reduce measurement integration times, and implement temporal phase averaging.

Description:
RELATED APPLICATIONS  
       [0001]     This application is based on and claims the priority of U.S. Provisional Application Ser. No. 60/498,522, filed Aug. 28, 2003. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The invention relates to measurement of electromagnetic wavefronts. In particular, it pertains to quantitative, instantaneous measurement of interference light beams produced by a Fizeau interferometer.  
         [0004]     2. Description of the Related Art  
         [0005]     Light-beam measurements and characterization are important in the manufacture of many optical components, such as data-storage laser heads, for example. Accordingly, many optical interferometric systems have been devised to increase the accuracy and reliability of the measurements. In general, a front-end interferometric device that produces a test and a reference beam is combined with a back-end optical device for resolving phase differences between the beams. This may be achieved simultaneously by encoding (or “marking”) the beams at the front end, such as by using different polarization states for the test and reference beams, and resolving them spatially at the back end. Alternatively, the optical path difference between the test and reference surfaces may be varied in time at the front end, such as by scanning one surface with respect to the other, and the phase differences are resolved temporally at the back end.  
         [0006]     One of the problems identified in the prior art has been the ability to properly encode the reference and test beams in the measurement portion of a Fizeau-type interferometer. In U.S. Pat. No. 4,872,755, Kuchel et al. addressed this shortcoming by adopting a different approach to both simultaneous and temporal phase measurement. By introducing an optical delay device in the measurement portion of the interferometer and judiciously selecting the coherence length of the light, the length of the delay path, and the length of the gap in the Fizeau cavity, two coherent test and reference beams as well as two incoherent beams are produced simultaneously. The delay device is used to vary the optical path difference between the coherent beams for temporal phase measurements. Alternatively, the test and reference beams may be polarized after they have been produced in the measurement portion of the device and introduced into a spatially-resolving receiver for simultaneous phase measurements.  
         [0007]     Thus, the Kuchel et al. approach requires fine adjustment of the length of the delay path, which is difficult and expensive to implement. In addition, the presence of the two incoherent light beams produces significant background light that may affect the measurements. Therefore, there is still a need for a phase measurement system based on a Fizeau interferometer that does not suffer from these shortcomings.  
         [0008]     In U.S. Pat. No. 6,304,330, Millerd et al. describe a back-end system wherein the test and reference wavefronts produced by an interferometer are collimated, divided into sub-wavefronts, phase-shifted, combined to produce interference, and detected along a common axis simultaneously on a single detector or a multiple detector array. The beams can also be detected sequentially on a single detector array, if desired. The Millerd optical system also requires encoded test and reference beams. Therefore, in combination with a front-end Fizeau configuration, the same encoding problems addressed by Kuchel et al. need to overcome. The present invention illustrates a novel approach whereby the output of a Fizeau cavity with a tilted reference mirror is combined with a polarizing element to produce coherent test and reference wavefronts suitable for simultaneous spatial phase measurements in a system as described in U.S. Pat. No. 6,304,330.  
       SUMMARY OF THE INVENTION  
       [0009]     The invention utilizes the tilted relationship between the reference and test mirrors of a Fizeau interferometer to spatially separate the reflections from the two surfaces. The separate beams are filtered through a spatial polarization element that provides different states of polarization to the beams, as required for simultaneous phase measurement. The beams are subsequently recombined to form a substantially collinear beam that is processed using a spatial-phase-shift interferometer that permits quantitative phase measurement in a single video frame.  
         [0010]     Alternatively, two beams with orthogonal polarization are injected into the Fizeau cavity at different angles, such that after reflection from the reference and test optics they are substantially collinear. Unwanted reflections are blocked at the focal plane through the use of a circular aperture. Short coherence length light and a delay line may be used, as taught by Kuchel et al., to mitigate stray reflections, reduce measurement integration times, and implement temporal phase averaging.  
         [0011]     Various other purposes and advantages of the invention will become clear from its description in the specification that follows and from the novel features particularly pointed out in the appended claims. Therefore, to the accomplishment of the objectives described above, this invention consists of the features hereinafter illustrated in the drawings, fully described in the detailed description of the preferred embodiment and particularly pointed out in the claims. However, such drawings and description disclose but one of the various ways in which the invention may be practiced.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1A  is a schematic view of a measurement apparatus according to the invention configured to produce a spatial separation between the test and reference beams using a tilt angle between the test and reference surfaces, and including a polarizing filter to produce orthogonally polarized test and reference beams.  
         [0013]      FIG. 1B  is a schematic illustration of a polarizing filter with adjacent orthogonally polarizing sections.  
         [0014]      FIG. 1C  is a schematic illustration of a polarizing filter with a first polarizing section contained in another, orthogonally polarizing section.  
         [0015]      FIG. 1D  is a schematic illustration of a polarizing filter wherein a dual-aperture mask is added to the polarizing filter of  FIG. 1B  in order to block additional beams caused by multiple reflections between the test and reference surfaces.  
         [0016]      FIG. 2  is a schematic representation of a generic spatial phase-shifting interferometer module suitable for combination with the optical device of the invention.  
         [0017]      FIG. 3  is a schematic view of a phase-shifting interferometer module wherein the orthogonally polarized reference and test beams are focused by a lens onto a beamsplitter that produces a plurality of sub-image beam pairs, which are then collimated and imaged by a lens onto a detector through a phase interference plate.  
         [0018]      FIG. 4  illustrates a spatial phase-shifting interferometer that employs a spatial-frequency carrier method of detecting the phase variations in the test wavefront.  
         [0019]      FIG. 5  is a schematic representation of the apparatus of  FIG. 1  with an optical delay line affecting the input light.  
         [0020]      FIG. 6A  is a schematic representation of another embodiment of the invention configured to produce a spatial separation between the test and reference beams using a polarizing beamsplitter operating on the input light beam, and including an aperture adapted to transmit on-axis beams and block off-axis beams directed toward the phase-shifting interferometer module.  
         [0021]      FIG. 6B  is a schematic view of an alternative to the polarizing beamsplitter of  FIG. 6A .  
         [0022]      FIG. 7  is a schematic representation of the apparatus of  FIG. 6A  with an optical delay line affecting the input light.  
         [0023]      FIG. 8  is a schematic view of another exemplary embodiment of the invention wherein the spatial separation between the test and reference beams is provided using a beamsplitter/mirror combination.  
         [0024]      FIG. 9  is a schematic view of another exemplary embodiment of the invention wherein the position of the polarizing beamsplitter and of the blocking aperture are relocated to other conjugate image planes in the imaging system.  
         [0025]      FIG. 10  is a schematic view of an optical device wherein the delay line and polarizing beamsplitter of  FIG. 5  are combined with the spatial phase-shifting interferometer of  FIG. 3  to characterize a test surface disposed in parallel to the reference surface in a Fizeau interferometer configuration.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]     In general, the invention lies in the idea of spatially separating the test and reference beams produced by a Fizeau type of interferometer and passing each beam through an encoding filter. By imparting orthogonal states of polarization to the test and reference beams, they may be recombined and processed in a spatial-phase-shift interferometer for simultaneous phase measurements.  
         [0027]     For the purposes of the invention, “tilt angle” refers to the angle between the test and reference surfaces in a Fizeau interferometer measured with respect to a perfect parallel condition. As such, tilt angles are used in the invention to provide fringes with an appropriate degree of resolution for the interferometric task at hand.  
         [0028]     The concept of the invention is exemplified by the interferometric device  10  of  FIG. 1A . A source  12  of collimated light L is expanded with an expansion lens  14 , reflected off a beamsplitter  16 , collimated by a collimation lens  18 , and directed toward an interferometer  20  in Fizeau configuration. Since the input beam L contains both horizontal and vertical polarizations, a half-wave plate  22  can be used to vary the ratio of light in each polarization state (vertical or horizontal). As is well understood in the art, the light in the interferometer is reflected from both the reference surface  24  and the test surface  26  to produce corresponding reference and test beams R and T, respectively. The reference and test surfaces of the interferometer are tilted with respect to one another, so as to produce reference and test beams R.T that are spatially separate, as illustrated in the figure (where the test surface  26  is normal to the incoming collimated beam and the test surface  24  is tilted toward it). As a result of this tilt, the light T reflected from the test surface  26  follows the optical path of the incoming light and, after passing through the beamsplitter  16 , is focused at a point  28  in the focal plane of the collimation lens  18 . The light R reflected from the back surface  24  of the reference optic is displaced by the tilt in the surface and is accordingly focused at a different point  30  of the focal plane of the collimation lens.  
         [0029]     According to the invention, a spatial polarization filter  32  is placed at the focal plane of the collimation lens  18 . As illustrated in  FIG. 1B , the polarization filter  32  includes two regions with different polarization components (preferably orthogonal to each other) positioned such that the test beam T and the reference beam R are transmitted through the different regions. Thus, as a result of interaction with the polarization elements, each beam emerges with orthogonal polarization. In the embodiment of  FIG. 1B , the polarization filter  32  consists of a first linear-polarizer region  34  and a second linear-polarizer region  36  that abut each other and have axes of polarization oriented orthogonally with respect to each other. In another embodiment  38 , shown in  FIG. 1C , the second polarizer region  36  is completely circumscribed by the first polarizer region  34 . Such a device can be manufactured, for example, as a patterned polarizer (available from Codixx of Barleben, Germany). In the preferred embodiment, illustrated in  FIG. 1D , a dual-aperture mask  33  is used in combination with the linear polarizer regions  34 , 36  of  FIG. 1B  to block the multiple reflections generated between the reference and test surfaces  24 , 26 .  
         [0030]     To permit operation of the interferometer over a wide acceptance angle, the thickness of the polarizers should preferably be less than the quantity 1.5λ(NA) 2 , where λ is the wavelength of the light and NA is the numerical aperture of the collimation lens  18 . As one skilled in the art would readily understand, greater thicknesses would require more tilt, which tends to introduce more aberrations through the optical system and would, accordingly, require greater calibration. It is understood that other, equivalent polarization means may be used instead of the filters illustrated in  FIGS. 1B-1D , such as two quarter waveplates with their respective axes oriented 90 degrees with respect to each other. Combinations of other birefringent and polarization elements are similarly possible and well understood in the art.  
         [0031]     At the back end of the interferometric device  10 , an imaging lens  40  is used to convert the spatial separation between the test and reference beams T,R into an angular separation. A polarization beamsplitter  42  is used to recombine the beams to produce substantially collinear and coextensive wavefronts. The beams are then processed by a spatial phase-shifting interferometer module  44 .  FIG. 2  illustrates generically a polarization phase-shifting interferometer  44  wherein the incoming wavefronts are subjected to four sequential processing steps while retaining their common path. The first step, occurring in a splitting/imaging section  46  of the interferometer, produces a plurality of copies T′,R′ of the test and reference wavefronts using refractive, diffractive and/or reflective optical splitters. The second step utilizes a phase-shifting section  48  to impart different relative phase shifts between the copies of the reference and test wavefronts. In the next step, carried out in an interference section  50 , the phase-shifted copies of the reference and test wavefronts are combined to produce interferograms through interaction with appropriate polarizing elements. Finally, in the final step, a detector section  52  with a plurality of photo-detectors is used to spatially sample the resulting interferograms.  
         [0032]     As described in copending U.S. Ser. No. 10/652,903, hereby incorporated by reference, spatial phase-shifting interferometer module  44  suitable for the invention can be implemented in various arrangements. For example,  FIG. 3  illustrates an embodiment  54  wherein the orthogonally polarized reference and test beams are focused by a lens  56  onto an appropriately positioned beamsplitter element  58 . The beamsplitter, through reflective, refractive or diffractive elements, produces a plurality of sub-image beam pairs (reference plus test) which are collimated and imaged by a lens  60  through a phase interference plate  62  onto a detector  64 . The plate  60  phase shifts and appropriately overlaps the collimated sub-image beams, thereby delivering phase-shifted interferograms  66  on the detector  64 . The plate  62  comprises substantially planar birefringent waveplates and polarizing elements arranged in parallel and/or adjoining layers, as is known in the art.  
         [0033]     The entrance region of the interferometer  54  preferably incorporates a field stop  68  that is conjugate with an input pupil image plane and the plane of detector  64 . The purpose of the field stop  68  is to limit overlap between sub-images on the detector. The detector  64  is typically a pixilated array that allows high-resolution digital sampling of the phase-shifted interferograms. The digitized interferograms are then processed by a computer in conventional manner to characterize the test surface using one of the many well known algorithms for phase determination.  
         [0034]     In another embodiment  70  shown in  FIG. 4 , the spatial phase-shifting interferometer employs a spatial-frequency carrier method of detecting the phase variations in the test wavefront. The reference and test beams are collimated as described above and directed to a polarization element  72  (which may be a birefringent crystal, such as a Wollaston prism, or any other refractive or diffractive component) inside the interferometer  70 . The element  72  acts as a polarization beamsplitter, thus introducing an angular separation between corresponding wavefronts. The waves are then interfered by a polarizer  74  and imaged on a single detector  76 . The contrast of the corresponding interferograms can be adjusted by rotating the polarizer  74  to compensate for arbitrary polarizations of the imaged reference and test waves. The digitized interferograms are further processed by computer to calculate phase and characterize the test surface.  
         [0035]     In another embodiment  80  of the invention illustrated in  FIG. 5 , an optical delay line  84  is used to generate two beams separated by an optical path delay AL, as taught by Kuchel et al. The input beam L is directed by a beamsplitter  86  toward two mirrors  88  and  90  along preferably orthogonal optical paths differing in length by the amount AL. The two reflected beams are then redirected by the beamsplitter and injected into the Fizeau interferometer described in  FIG. 1 , where they are both reflected from the reference surface  24  and the test surface  26 . At the focal plane of the primary lens  18 , the reflected beams are transmitted through the spatial polarization filter  32 , which transmits the test beam T and the reference beam R in selected orthogonal polarizations, as explained above. The test and reference beams may then be detected for temporal phase shifting, or phase shifted and processed as illustrated in  FIGS. 2-4 .  
         [0036]     The use of a short coherence light source (less than twice the optical length of the cavity) in the embodiment  80  of  FIG. 5  results in the suppression of reflections produced by optics other than the test and reference surfaces. The length of the delay line  84  is adjusted to produce the same path delay as in the Fizeau cavity  20 , so that the reference beam R and the test beam T are temporally coherent and yield high-contrast interference fringes in the spatial phase-shift interferometer  44 . Thus, spurious reflections from the imaging optics are greatly reduced. In addition, the broad-band source, because of its short coherence length, makes it possible to select different surfaces of the test optic for independent measurements, such as the front and back surfaces of a test platen. An additional advantage derived from the use of broad-band light is the fact that the rotating ground glass typically used in Fizeau interferometers to produce spatially incoherent beams may be eliminated, thereby generating higher light levels at the detector and correspondingly shorter integration-time requirements. Multiple measurements may be acquired and averaged with a random phase offset in order to reduce residual phase-dependent errors in the system, as taught in Ser. No. 10/652,903.  
         [0037]     The interferometric device  80  also provides advantages over the system disclosed by Kuchel et al. because only two beams remain to interfere after the filter  32  of the present invention, which yields higher contrast interference patterns. An entrance waveplate  92  may also be provided to adjust the contrast of the pattern to near unity for all combinations of reference and test object reflectivity. Finally, the delay line may be combined with a piezoelectric or other scanning element  94  to systematically introduce small phase-shifts in the delay line so that multiple phase maps can be averaged to reduce phase-dependent errors in the final phase map or to use conventional temporal phase shifting for applications utilizing large Fizeau interferometers, where piezo-shifting of the reference optic is not possible.  
         [0038]     It is noted that the beams produced by the delay line  84  may also be polarized to have orthogonal polarizations, although the feature is not necessary to practice the invention. To that end, the beamsplitter needs to be a polarizing beamsplitter and additional polarizing elements, such as waveplates  94  and  98 , are introduced in the optical paths of the two beams directed toward the mirrors  88  and  90 , respectively, of the delay line. As one skilled in the art would readily recognize, this configuration allows all of the light in the input beam L to be transmitted toward the Fizeau cavity, thereby improving energy efficiency and further reducing integration-time requirements.  
         [0039]     In another embodiment  100  of the invention shown in  FIG. 6A , a polarization beamsplitter  102  is placed in the input channel to generate two input beams  104  and  106  that have orthogonal polarizations and are spatially displaced from one another. The two beams are injected into the Fizeau cavity  20  with the tilted reference surface  24 . The separation of the two beams is judiciously selected so that the first beam  104  is reflected from the reference surface to form a beam  108  that is directed on-axis toward the spatial phase-shift interferometer  44 , while the second beam  106  is reflected to form a beam  110  that is directed off-axis toward the interferometer. Accordingly, an aperture  112  can be used to block the reflection of the second beam  106  (beam  110 ) and transmit the reflection of the first beam  116  (beam  108 ). The test surface  26  similarly reflects the first input beam  104  to form an off-axis beam  114  which is blocked by the aperture  112  and reflects the second input beam  106  to form an on-axis beam  116  that is transmitted by the aperture  112 . Beams  108  and  116  are substantially collinear and orthogonally polarized. Thus, they may be subsequently processed by the spatial phase-shifting interferometer module  44 . This embodiment has the advantage of not requiring the introduction of a polarizing beam splitter in the imaging section of the interferometer. A disadvantage lies in the loss of light in the beams blocked by the aperture element.  
         [0040]      FIG. 6B  shows another embodiment of the invention that is closely related to the one of  FIG. 6A . The polarization beamsplitter  102  at the input of the device is constructed using a diffraction grating  118 , the expansion lens  14 , and a polarization filter mask  120 . The combination of the these elements produces the orthogonally polarized input beams  104  and  108  that are then introduce into the tilted Fizeau cavity of  FIG. 6A .  
         [0041]      FIG. 7  shows another embodiment  130  closely related to the one illustrated in  FIG. 6A . The polarized input beams  104  and  106  are generated from a short coherence length source  82  that is used in combination with an optical delay line  84 . The input beam L is separated by a polarization beamsplitter  86  into two beams in the delay line as described above with reference to the embodiment  80  of  FIG. 5 . Thus, the two beams are encoded by orthogonal polarizations and beam  104  has an additional optical delay AL introduced in its path. In addition, the beamsplitter  86  is used also to impart a spatial separation on the beams  104  and  106  directed toward the Fizeau cavity. After reflection from the mirrors  88  and  90  in the delay line, the two beams are directed toward the Fizeau cavity by the beamsplitter  86  with an appropriate angle between the two beams to achieve the spatial separation required to block the unwanted reflection from each beam, as described above with reference to the embodiment  90  of  FIG. 6A . The expansion lens  14  is used to couple the beams into the Fizeau cavity. This embodiment has all the advantages of both embodiments  80  and  90  described above. The main disadvantage is loss of light and the additional complexity of the device.  
         [0042]     Yet another embodiment  140  of the invention is shown in  FIG. 8 , wherein the orthogonally polarized input beams  104  and  106  are generated using separate beam paths. The source light L is split by a polarizing beamsplitter  142  into the two orthogonally polarized beams  104 , 106  which are then directed toward the Fizeau cavity. A mirror  144  is used to provide the spatial separation necessary to practice the invention.  
         [0043]     Various other changes in the details, steps and components that have been described may be made by those skilled in the art within the principles and scope of the invention herein illustrated and defined in the appended claims. For example, the position of the polarization spatial filter  32  in  FIG. 1A , may be relocated by utilizing a series of transfer optics. A series of transfer optics can be used to relocate both the position of the polarizing beamsplitter  42  and the blocking aperture  112  of the embodiment of  FIG. 6A  to other conjugate image planes in the imaging system, as illustrated in  FIG. 9 . Similarly, as shown in  FIG. 10 , a the delay line and polarizing beamsplitter of  FIG. 5  may be combined with the spatial phase-shifting interferometer of  FIG. 3  to characterize a test surface disposed in parallel to the reference surface in a Fizeau interferometer configuration.  
         [0044]     Therefore, while the present invention has been shown and described herein in what is believed to be the most practical and preferred embodiments, it is recognized that departures can be made therefrom within the scope of the invention, which is not to be limited to the disclosed details but is to be accorded the full scope of the claims to embrace any and all equivalent methods and products.