Abstract:
A monolithic frame for optics used in interferometers where the material of the monolithic frame may have a substantially different coefficient of thermal expansion from the beamsplitter and compensator without warping, bending or distorting the optics. This is accomplished through providing a securing apparatus holding the optics in place while isolating the expansion thereof from the expansion of the frame. Stability in optical alignment is therefore achieved without requiring a single material or materials of essentially identical coefficients of thermal expansion. The present invention provides stability in situations where it is not possible to utilize a single material for every component of the interferometer.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to provisional U.S. application Ser. No. 61/086,603, filed on Aug. 6, 2008, the entirety of which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention is in the field of monolithic interferometers. Such monolithic interferometers provide stability in optical alignment by, among other factors, using a single material. The present invention provides stability in situations where it is not possible to utilize a single material for every component of the interferometer. 
       BACKGROUND OF THE INVENTION 
       [0003]    Fourier transform infrared (“FTIR”) spectrometers are well known in the art. Michelson interferometers function by splitting a beam of electromagnetic radiation into two separate beams via a beam splitter. Each beam travels along its own path, e.g. a reference path of fixed length and a measurement path of variable length. A reflecting element, such as a retroreflector, is placed in the path of each beam and returns them both to the beam splitter. The beams are there recombined into a single exit beam. The variable path length causes the combined exit beam to be amplitude modulated due to interference between the fixed and variable length beams. By analyzing the output beam, the spectrum, which is the intensity of the input beam as a function of frequency, may be derived after suitable calibration. 
         [0004]    When the above interferometer is employed in a FTIR spectrometer, the exit beam is focused upon a detector. If a sample is placed such that the modulated beam passes through it prior to impinging upon the detector, the analysis performed can determine the absorption spectrum of the sample. The sample may also be placed otherwise in the arrangement to obtain other characteristics. 
         [0005]    Because Michelson interferometers rely upon the interference from recombination of the two beams, a quality factor of such a device is the degree to which the optical elements remain aligned. The beam splitter and mirror-supporting structures must be isolated to the greatest possible degree from extraneous forces which would tend to produce distortions of the structure. Such forces and resultant distortions introduce inaccuracies into the optical measurements. The forces may arise from vibrational effects from the environment and can be rotational or translational in nature. A similarly pervasive issue concerns distortions due to changes in the thermal environment. Needless to say, considerations of weight, size, facility of use, efficiency, manufacturing cost and feasibility are also of primary importance. 
         [0006]    Prior art optical assemblies used in the construction of standard Michelson interferometers, and other type interferometers, have consisted primarily of structures having parts which are in need of high accuracy alignment. For example, the arrangement of the two reflecting assemblies and the beamsplitter must be highly accurate in the perpendicular and reflecting arrangements in order to avoid errors introduced due to any such misalignment. The trouble with these prior art interferometers and optical assemblies arises from the costs involved in meticulously aligning the optical elements, the necessity for active subsystems to maintain the alignment, and subsequent costs to service and readjust the interferometer if shocks and vibrations have introduced uncompensated misalignment. 
         [0007]    U.S. Pat. Nos. 5,949,543 and 6,141,101 to Bleier and Vishnia addressed the above issues with a monolithic interferometer constructed from a single material, preferably a material having a low coefficient of thermal expansion. However, it is not always possible to utilize a monolithic interferometer made out of a single material because materials having reflectance/transmittance properties appropriate to a necessary wavelength of light may not technically or economically lend themselves to elements of the monolithic interferometer other than the optical elements. 
         [0008]    Accordingly, it would be desirable to provide a monolithic interferometer with optical elements of a different material than the remainder of the interferometer that, nevertheless, provides high accuracy measurements. Such an interferometer would facilitate easy and cost effective maintenance by replacement of the entire optical assembly, which optical assembly is not subject to misalignment from shocks, vibrations, or temperature changes due to the monolithic structure of the assembly. It would be further desirable to provide an optical assembly which allows for use of multiple wavelength light sources to achieve a “fringe” result in a spectrometry application. 
       SUMMARY OF THE INVENTION 
       [0009]    Accordingly, it is a broad object of the invention to provide an optical assembly for use with a precision instrument comprising a frame assembly having a top plate; a bottom plate; a first support member bonded between a first portion of the top plate and a first portion of the bottom plate; and a second support member bonded between a second portion of the top plate and a second portion of the bottom plate. The top plate, bottom plate and support members all formed of one of the same material or a materials having substantially the same coefficient of thermal expansion and defining a frame interior space. The frame having a beamsplitter inside the interior space; the beamsplitter extending between the top plate and bottom plate and having a first face and a second face. A mirror is attached to the frame assembly, the mirror having a reflecting surface in a reflecting relation with the beamsplitter. The beamsplitter is attached to the frame by a securing apparatus having spring arms and mounting buttons, the spring arms each having a free end and an end attached to one of either the top plate or the bottom plate and the mounting buttons each attached to one of either the top plate or bottom plate, the mounting buttons engaging the beamsplitter first face and the spring arm free end engaging the beamsplitter second face. Wherein the optical assembly is substantially stable regarding the reflective relationship between the mirror and the beamsplitter and the beamsplitter first and second face having limited exposure to bending and warping. 
         [0010]    The optical assembly securing apparatus may comprise pressure plates attached to the beamsplitter at the point of engagement between the spring arm free end and the beamsplitter second face. 
         [0011]    The optical assembly may also have a compensator disposed between the mirror and the beamsplitter and the compensator may have its own securing apparatus having spring arms and mounting buttons, the compensator securing spring arms each having a free end and an end attached to one of either the top plate or the bottom plate and the compensator securing buttons each attached to one of either the top plate or bottom plate, the compensator securing buttons engaging a compensator first face and the compensator securing spring arm free end engaging a compensator second face. 
         [0012]    The spring arm attached end may comprise a cylinder and the top plate and bottom plate further comprising securing apparatus holes sized to receive the spring arm cylinder. 
         [0013]    The optical assembly may also have a second mirror attached to the frame assembly having a reflecting surface facing away from the frame interior space. 
         [0014]    The securing apparatus may comprise a three-point mounting having two sets of buttons and springs on one of the top plate or bottom plate and one button and spring set on the other of the top plate or bottom plate. 
         [0015]    In an alternative embodiment of the present invention, an interferometer is disclosed comprising a radiation source and a monolithic frame comprising a top plate and a bottom plate rigidly held in relation to one another by a first support member bonded to the top plate and the bottom plate and a second support member bonded to the top plate and the bottom plate, the plates and supports of the frame being of the same material and defining an interior space. A beamsplitter extends between the top plate and the bottom plate. A first mirror is attached to the frame assembly and has a reflecting surface in a first direct reflecting relation with the beamsplitter. A second mirror is attached to the frame assembly and has a reflecting surface in a second direct reflecting relation with a retroreflector. The retroreflector is external to the interior space, moveable relative to the frame and has a retroreflection relation with both the beamsplitter and the second mirror. The beamsplitter is attached to the frame by a beamsplitter securing apparatus having spring arms and mounting buttons, the spring arms each having a free end and an end attached to one of either the top plate or the bottom plate and the mounting buttons each attached to one of either the top plate or bottom plate. The mounting buttons engage a beamsplitter first face and the spring arm free ends engage a beamsplitter second face. Wherein the optical assembly is substantially stable regarding the reflective relationships and the beamsplitter first and second face having limited exposure to bending and warping. The described interferometer may have the same optional components as the optical assembly described above. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a diagram showing how radiation is reflected in a prior art Michelson interferometer; 
           [0017]      FIG. 2  is a perspective view of an interferometer having the monolithic optical assembly of the invention; 
           [0018]      FIG. 3  is a perspective view of a monolithic optical assembly of the prior art; 
           [0019]      FIG. 4  is a top view of a monolithic optical assembly of the prior art; 
           [0020]      FIG. 5  is a perspective exploded view of the optical assembly of the present invention; 
           [0021]      FIG. 6  is a top view of the optical assembly of  FIG. 5  with all non-optical elements removed; 
           [0022]      FIG. 7  is a top view of the optical assembly of  FIG. 5 ; 
           [0023]      FIG. 7B  is a side sectional view of the assembly of  FIG. 5 , the section taken along line B-B of  FIG. 7 ; 
           [0024]      FIG. 7C  is a detail sectional view at circle C-C of  FIG. 7 ; 
           [0025]      FIG. 8  is a perspective detail of a portion of the optical assembly of  FIG. 5  with the top plate removed to show interior details; 
           [0026]      FIG. 9  is a perspective view of the optical assembly of  FIG. 5  with all elements removed other than the beamsplitter  11 , compensator  8  and their associated securing apparatus; and 
           [0027]      FIG. 10  is a perspective view of spring  9 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0028]    Referring to  FIG. 1 , the general principals of a standard Michelson interferometer are shown. The Michelson interferometer has a radiation source  10  which sends a single radiation beam  20  towards beamsplitter  30  which is situated at an angle to two mirrors, a fixed mirror  40  and a movable mirror  50 . Radiation beam  20  is partially reflected toward fixed mirror  40  in the form of radiation beam  22 , and is partially transmitted through beamsplitter  30  towards movable mirror  50  as radiation beam  24 . Beam  22  is then reflected off of fixed mirror  40 , back towards beamsplitter  30 , where it is once again partially split, sending some radiation  25  back towards source  10 , and some radiation  26  toward detector  60 . Similarly, beam  24  reflects off of movable mirror  50  and is reflected back toward beamsplitter  30 . Here also, beam  24  is again split, sending some radiation back to source  10  and other radiation  26  toward detector  60 . 
         [0029]    Detector  60  measures the interference between the two radiation beams emanating from the single radiation source. These beams have, by design, traveled different distances (optical path lengths), which creates a fringe effect which is measurable by detector  60 . 
         [0030]      FIG. 2  shows the lay out and component structure of a Michelson interferometer of the prior art, e.g. U.S. Pat. No. 6,141,101 to Bleier, herein incorporated by reference.  FIG. 2  shows interferometer  100 , and includes a radiation source  110 , a beamsplitter  130 , a movable reflecting assembly  150 , a fixed reflecting assembly  140  and a detector  142 . Radiation source  110  is mounted in a secure position by mounting assembly  112 . With radiation source  110  in mounting assembly  112 , radiation beam  120  is alignable along a path which will fix the direction of the beam at the appropriate angle to beamsplitter  130 . 
         [0031]    Radiation source  110  can be collimated white light for general interferometry applications, such as optical surface profiling, or even a single collimated radiation intensity laser light source, for accurate distance measurements. 
         [0032]    Movable reflecting assembly  150  may utilize a hollow corner-cube retroreflector  152 . The hollow corner-cube retroreflector  152  could be made in accordance with the disclosure of U.S. Pat. No. 3,663,084 to Lipkins, herein incorporated by reference. 
         [0033]    Retroreflector  152  is mounted to a movable base assembly  154 , which assembly allows for adjustment of the location of retroreflector  152  in a line along the path of beam  120 . The displacement of assembly  154  is adjustable; e.g., through use of adjusting knob  146 . Other means of moving assembly  154  are also anticipated by the invention, including such means that might allow for continuous, uniform movement of assembly  154 . For example, movement of assembly  154  might be accomplished in accordance with the structure described in U.S. Pat. No. 5,335,111 to Bleier, herein incorporated by reference, or by co-pending application Ser. No. 12/505,279 filed on Jul. 17, 2009. 
         [0034]    The use of retroreflector  152  as movable reflecting assembly  150  allows for any angular orientation of retroreflector  152  as long as edge portions of the retroreflector mirrors do not clip a portion of beam  120 . 
         [0035]    From the foregoing, the length of the light paths  20 ,  22  and  26  are fixed and known while the length of light path  24  may be varied. The variation of the length of light path  26  is, of course, critical to the operation of the interferometer, as is knowing the length as precisely as possible. 
         [0036]    A monolithic optical assembly  200 , as seen in  FIG. 3 , comprises a beamsplitter  130  and reflecting assembly  140  mounted within a top plate  260 , a bottom plate  270  and at least first and second support members  210  and  220 , respectively. As an add-on for some additional structural stability, which stability is not essential, third support member  230  can also be used. Support member  210  has an edge  214 . A portion of edge  214  is bonded to a portion of edge  262  of top plate  260 , while another portion of edge  214  of support member  210  is bonded to a portion of an edge surface of bottom plate  270 . 
         [0037]    Continuing with  FIG. 4 , around the corner from support member  210 , is second support member  220 . Second support member  220  is bonded to top and bottom plates  260  and  270  along different portions of a surface  222  thereof. The portions of surface  222  of support member  220  are bonded to portions of an edge surface  264  of top plate  260  and edge surface  274  of bottom plate  270 . 
         [0038]    Beamsplitter  130  may comprise of two panels bonded to each other along a common surface. The common surface is an optically flat reflecting surface having a beamsplitter coating thereon. Beamsplitter  130  is bonded along portions of top edges  137  to portions of bottom surface  267  of top plate  260 , and along portions of bottom edges  138  to portions of top surface  278  of bottom plate  270 . One panel of beamsplitter  130  is a compensating member.  40  The purpose of the compensating panel is to equate the material portions of the optical path difference of the two beams created by the beamsplitter. Without the compensating panel, the beam transmitted through the beamsplitter would travel through the optical material of the beamsplitter twice, while the reflected beam would travel through optical material zero times. By adding a compensating panel, ideally of the same thickness, wedge, and material as the beamsplitter, both beams travel twice through equal portions of optical material before being recombined at the beamsplitter surface, thereby equating any differences they may have experienced in that portion of their optical path length through material. 
         [0039]    The support combination of first support member  210 , second support member  220  and beamsplitter  130  between top plate  260  and bottom plate  270  creates a monolithic structure. As early discussed, it is also possible to have third support member  230  situated between portions of third edge surfaces  266  and  276  of top and bottom plates  260  and  270 , respectively, as seen in the figures 
         [0040]    To complete the required reflecting elements of a Michelson interferometer, it is seen in the figures that a mirror panel  140  is bonded to a portion of top surface  278  of bottom plate  270 , and to a second edge surface  214  of support member  210 . Mirror panel  140  is slightly over hanging top surface  278  of bottom plate  270  by a portion of a bottom edge surface of mirror panel  140 , and is bonded between these touching surfaces. Bonding also takes effect between the side edge surface of mirror panel  140  that touches edge surface  214  of support member  210 . Bonding must avoid distorting the optically flat nature of the reflecting surface  142  of mirror panel  140 . 
         [0041]    Since mirror panel  140  is fixedly attached to assembly  200 , as has just been discussed, there is no necessity for panel  140  to be other than a single, flat paneled mirror; for example, panel  140  does not need to be a retroreflector. One of the benefits of using a retroreflector (as has been discussed earlier regarding movable reflecting assembly  150 ) in a structure is that the orientation of the retroreflector is unimportant. In the subject invention, the secured mounting of panel  140  to the monolithic structure assures that the orientation of panel  140  will not fluctuate due to vibration and shock, and therefore, a retroreflector is unnecessary (although a retroreflector could of course be utilized). 
         [0042]    The portion of beam  120  that passes through beam splitter  130  and interacts with retroreflector  152  may also be returned via a second mirror panel, similar to mirror panel  140 . This second mirror panel may be made integral with second support member  220  or be a separate panel supported by one or all of the second support member  220 , edge  264  of top plate  260  and bottom plate  270 . 
         [0043]    Assembly  200  can also have a fourth support member  240 . While the main purpose of fourth support member  240  is not to help stabilize the monolithic structure of assembly  200 , it is nevertheless called a support member herein. Instead, fourth support member  240  is positioned in relation to the path traveled by beam  120  so as to allow beam  120  to pass through opening  242  in member  240 , to travel between beamsplitter  130  and movable reflecting assembly  150 . One or both of elements  244 ,  246  can comprise reflecting elements for returning beam  120  to retroreflector  252 . 
         [0044]    All members  210 ,  220 ,  230 ,  240 ,  260 ,  270 ,  130  and  140 , of assembly  200 , may be made of the same material. The material preferably being fused quartz or annealed Pyrex. The use of identical materials allows the coefficients of expansion of the materials to be identical, so that any temperature changes experienced by assembly  200  is experienced equally throughout each member to allow assembly  200  to expand and contract uniformly, thereby removing the possibility of distortions in the reflecting surfaces of beamsplitter  130  and mirror panel  140 . 
         [0045]    The monolithic construction discussed above has the benefit of high thermal stability in its optical alignment. This stability derives from the construction of the unit from a single, low expansion material such as Pyrex glass, fused silica, Zerodur or Cervit. However, in the application of infrared Fourier transform spectroscopy, often called FTIR, it may not be possible to fabricate the beamsplitter and compensating plate from the same material as the assembly. This may occur when the need for high transmission in the infrared (“IR”) is not consistent with available low expansion structural materials. In particular, the high IR transmission optical material may have a much higher thermal expansion coefficient. 
         [0046]    Attaching optical elements having a thermal expansion coefficient different from the expansion coefficient of the remainder of the assembly could introduce wavefront distortion in the interfering optical beams or even result in mechanical failure under temperature changes. In order to take advantage of the permanent optical alignment afforded by a monolithic assembly, the connection between optical elements, e.g. beamsplitter and compensating plate, and the rest of the monolithic assembly should transmit minimal stress from this assembly to the optical elements under temperature changes. 
         [0047]      FIG. 5  shows an improved monolithic interferometer  200 . The basic monolithic assembly is formed top plate  1 , bottom plate  2 , first support member  3   a , second support member  3   b  and third support member  4 . The outward facing surface  17  of third support member  4  serves as a mirror, reflecting beam  16  from retroreflector  152  and returning beam  16  to retroreflector  152 . Mirror  5  is inwardly facing and has a mirror post  12   a  which is bonded to top plate  1  at post hole  12   b . Mounting post  6  allows the monolithic interferometer  200  to be attached to the interferometer assembly  100  of which it is part. 
         [0048]    In a two-beam interferometer, two beams are created from a single incident ray  13  striking beamsplitter  11 . The two beams are the reflected beam  14  and the transmitted beam  15 . Beam  14  is reflected by beamsplitter  11  towards mirror  5 . Beam  14  passes through compensator plate  8  on the way to mirror  5  and returning to beamsplitter  11 . In a well aligned interferometer, beam  14  is exactly perpendicular to fixed mirror  5 . The transmitted beam  15 , after exiting beamsplitter  11 , proceeds to retroreflector  152  which reflects beam  16  back toward mirror  17  on third support member  4 . In a well aligned interferometer, beam  15  is exactly parallel to beam  16 , which is perpendicular to mirror  17 .  FIG. 2  is a plan view showing only the optical elements, less retroreflector  152 , and beams of the present invention. 
         [0049]    It is the aim of the monolithic interferometer that once alignment is achieved during assembly, final assembly permanently and rigidly locks this alignment into the structure. 
         [0050]    Beamsplitter  11  and compensator  8  must be of material transmissive to the light being processed by the interferometer, often IR light. It must also be wedged to prevent interference effects from the front and back surfaces from creating ghost beams that can interfere with the main beams in the application. Compensator  8  is made of the same material as beamsplitter  11  with substantially and ideally the same thickness and wedge angle to compensate the optical path  13 - 14  with the optical path  15 - 16 . Obviously, any means of improved mounting of beamsplitter  11  must be repeated for compensator  8 . 
         [0051]    Springs  9 , mounting buttons  7  and pressure plates  18  are used to mount beamsplitter  11  and compensator  8  in monolithic assembly  200 .  FIGS. 7 ,  7 B and  7 C show details of the assembly  200 . Section B-B is taken along a direction perpendicular to the vertical edges of beamsplitter  11  and compensator  8 . Top plate  1  is in cross-section to illustrate insertion of spring body  21  into spring hole  28 . Spring arm  32  presses against pressure plate  18  attached to the beamsplitter  11  and compensator  8 . This pressure exerted on beamsplitter  11  and compensator  8  through pressure plates  18  urges beamsplitter  11  and compensator  8  against mounting buttons  7 . A total of three buttons  7 , springs  9 , and pressure plates  10  are used to achieve 3-point mounting of the beamsplitter  11  and compensator  8 . 
         [0052]    Each spring body  21  may be bonded in hole  28 . This arrangement is further shown in perspective in  FIG. 8 .  FIG. 9  shows further detail of the mounting scheme, in a perspective view with assembly members not shown. In this embodiment, two spring-pad-button arrangements per optical element are utilized in engaging the top plate  1  and one spring-pad-button arrangement is utilized per optical element in engaging the bottom plate  2 . The outside surface of the spring body  21  is bonded to the inner surface of the hole  28 . 
         [0053]    Pressure pads  10  are bonded to beamsplitter  11  and compensator  8 . The pressure pads prevent damage of optical element surfaces by spring arm  32 . Alternatively pads  7  may be fused directly to those members. 
         [0054]    Fixturing must be done to assure that the planes formed by the contacting cylindrical surfaces of buttons  7  are properly oriented before the monolithic assembly is fixed, to facilitate the alignment of the interferometer. Once fixturing is accomplished, all buttons  7  and spring arms  32  may be bonded to the respective beamsplitter  11  or compensator  8 . When beamsplitter  11  and compensator  8  expand, they will expand only laterally. This preserves their optical surfaces against bending and warping. The bonding method used must be sufficiently flexible so as to not interfere with this slight expansion, yet strong enough to guarantee resistance of the assembly to shocks. 
         [0055]      FIG. 10  shows a detailed view of the springs  9 . Body  21  is formed with diameter slightly larger than the holes  28 . This facilitates the frictional holding of these springs in holes  28 . Gap  23  facilitates the insertion of the springs  9  into their respective holes  28 . Holes  36  permit oozing of liquid bonding material into the interior area of the spring during curing. The liquid bonding material will tend to form plugs in holes  24 , helping to resist the rotation of the assembly. Hole  35  on the end of the spring arm  32  aids in assembly, by permitting the assembler to grab and pull back the flex arm using a hook or other tool temporarily inserted into this hole. In this manner, the flex arm  32  may be retracted to clear the beamsplitter  11  or compensator  8  while it is being installed. 
         [0056]    It is noted that the foregoing examples have been provided merely for the purpose of explanation and are in no way to be construed as limiting of the present invention. While the invention has been described with reference to various embodiments, it is understood that the words which have been used herein are words of description and illustration, rather than words of limitations. Further, although the invention has been described herein with reference to particular means, materials and embodiments, the invention is not intended to be limited to the particulars disclosed herein; rather, the invention extends to all functionally equivalent structures, methods and uses, such as are within the scope of the appended claims. Those skilled in the art, having the benefit of the teachings of this specification, may achieve numerous modifications thereto and changes may be made without departing from the scope and spirit of the invention in its aspects.