Abstract:
Spatially-separated heterodyne interferometer architecture is combined with monolithic glass construction techniques to provide a monolithic, spatially-separated, common-path interferometer. The monolithic interferometer includes multiple optical components bonded together into a monolithic structure. The bonded components provide both optics and structure for the interferometer, thereby producing a small, compact, and light-weight interferometry system. Beam splitters and combiners are provided on the interfaces between the optical components to direct and combine signal measurement, signal reference and local oscillator beams used in the interferometry system. The spatially-separated architecture reduces cyclic error values below those of polarization-separated interferometers. In addition, the monolithic architecture of the interferometer minimizes the impact of mechanical, thermal, and optical variations within the system.

Description:
STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of [CONTRACT NO.] awarded by [AGENCY]. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to interferometry, and specifically to a monolithic, spatially-separated, common-path interferometer. 
   2. Description of the Related Art 
   Interferometry has applications in many fields where precise measurement of movement is necessary. Fields using interferometry include aerospace, semiconductors, and general metrology. 
   Traditionally, interferometers utilize polarizing beam splitters to separate a measurement beam from a reference beam. The measurement beam is then reflected off a measuring object and the reference beam off a fixed reference object. Subsequently, when the polarization separated beams are mixed together, the beams interfere with each other, creating an interference pattern or an interferogram. Such interferograms are extremely sensitive to relative changes in the paths of the measurement beam and the reference beam. Thus, a precise measurement of changes in distance can be made by measuring changes in the resulting interference pattern signals of an interferometer. 
   Polarization-separated interferometers, however, suffer from polarization leakage, which can introduce errors and limit the measurement resolution to approximately 1 nanometer. Existing interferometers are also large, complex devices that are cumbersome to deploy and difficult to align. In addition, they suffer from various mechanical, thermal, and optical variations within the many components that make up the interferometers. 
   SUMMARY OF THE INVENTION 
   The present invention addresses the problems noted above by providing a monolithic, spatially-separated, common-path interferometer. The optical components of the interferometer are bonded together, which allows the interferometer to be permanently aligned at the time of assembly. This further allows systems using the interferometer to be small, compact and light weight and simplifies deployment and maintenance of those systems. To reduce errors and improve resolution over polarization-separated interferometers, the interferometer of the invention is designed to use spatially separated signal measurement and reference beams. One example of this spatially-separated architecture uses the core of a signal beam as the signal measurement beam and the annulus of the signal beam as the signal reference beam. Finally, the interferometer is designed so that the signal measurement and reference beams travel on a common path through the optical components of the interferometer. In this manner, the effect of relative mechanical, thermal, and optical variations within the interferometer are minimized, resulting in a more accurate and stable interferometry system. 
   Furthermore, the present invention adds multi-axis capabilities to the interferometer. The signal beam is divided into multiple beams for measurement along multiple axes. Again, all of the essential optic and structural components are bonded into a monolithic structure. By combining spatially-separated interferometry axes into one unit, the size and mass of the metrology system is further reduced. 
   According to one aspect of the invention, a monolithic interferometer includes a first optic for receiving a signal measurement beam and a signal reference beam. The signal measurement and reference beams are spatially distinct components of a signal beam. A second optic is bonded to the first optic and a polarized beam splitter is provided on the interface between the first and second optics. The signal measurement and reference beams having a first polarization are transmitted from the first optic to the second optic through the polarized beam splitter. A quarter-wave plate, which is bonded to a reference reflector, is bonded to the second optic. The signal reference beam is transmitted from the second optic to the reference reflector through the quarter-wave plate and is reflected by the reference reflector back to the second optic through the quarter-wave plate. The signal measurement beam is transmitted from the second optic to a measurement reflector through the quarter-wave plate and a non-reflective portion of the reference reflector and is reflected back to the second optic by the measurement reflector through the non-reflective portion of the reference reflector and the quarter-wave plate. The polarized beam splitter deflects the reflected signal measurement and reference beams, which have a second polarization orthogonal to the first polarization. A third optic is bonded to the second optic and receives a local oscillator beam. A beam combiner is provided on the interface between the second and third optics. The beam combiner combines the local oscillator beam with the reflected signal measurement and reference beams deflected by the polarized beam splitter. 
   The monolithic interferometer is preferably part of a metrology gauge that includes a laser source for generating the signal and local oscillator beams, collimators for collimating the beams, and a detector for receiving the combined measurement and local oscillator beam and the combined reference and local oscillator beam and for detecting optical phase differences between the combined beams. 
   According to another aspect of the invention, a multi-axis monolithic interferometer includes a multiplexer for dividing signal measurement and reference beams into multiple measurement and reference beams. The signal measurement and reference beams are spatially distinct components of a signal beam. A first optic receives the multiple signal measurement and reference beams. A second optic is bonded to the first optic and a polarized beam splitter is provided on the interface between the first and second optics. The signal measurement and reference beams having a first polarization are transmitted from the first optic to the second optic through the polarized beam splitter. Quarter-wave plates, which are bonded to respective reference reflectors, are bonded to the second optic. The signal reference beams are transmitted from the second optic to respective reference reflectors through respective quarter-wave plates and are reflected by the respective reference reflectors back to the second optic through the respective quarter-wave plates. The signal measurement beams are transmitted from the second optic to respective measurement reflectors through respective quarter-wave plates and non-reflective portions of respective reference reflectors and are reflected back to the second optic by the respective measurement reflectors through the non-reflective portions of the respective reference reflectors and the respective quarter-wave plates. The polarized beam splitter deflects the reflected signal measurement and reference beams, which have a second polarization orthogonal to the first polarization. A third optic is bonded to the second optic and receives a local oscillator beam. A beam combiner is provided on the interface between the second and third optics. The beam combiner combines the local oscillator beam with the reflected signal measurement and reference beams deflected by the polarized beam splitter. 
   The multi-axis monolithic interferometer is preferably part of a metrology gauge that includes a laser source for generating the signal and local oscillator beams, collimators for collimating the beams, and a detector for receiving the combined signal measurement and local oscillator beam and the combined signal reference and local oscillator beam and for detecting optical phase differences between the combined beams. 
   Other and further objects and advantages of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims, and drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a block diagram of the primary components of a metrology gauge using a monolithic, spatially-separated, common-path interferometer according to one embodiment of the present invention. 
       FIG. 2  illustrates a schematic diagram depicting the components of a monolithic, spatially-separated, common-path interferometer according to one embodiment of the present invention. 
       FIG. 3  illustrates a block diagram of the primary components of a multi-axis metrology gauge using a multi-axis monolithic, spatially-separated, common-path interferometer according to one embodiment of the present invention. 
       FIG. 4  illustrates a schematic diagram depicting the components of a multi-axis, monolithic, spatially-separated, common-path interferometer according to one embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  is a block diagram depicting the primary components of a metrology gauge utilizing a monolithic, spatially-separated, common-path interferometer according to one embodiment of the invention. As shown in  FIG. 1 , the metrology gauge includes laser assembly  10 , signal beam collimator  11 , local oscillator (LO) beam collimator  12 , interferometer  13 , which is comprised of beam splitter and combiner optics  14  and reference optics  15 , measurement reflector  16  and detector system  17 . 
   Laser assembly  10  provides a signal beam  18 , which includes signal measurement beam  20  (depicted as a dotted line) and signal reference beam  24  (depicted as a solid line), and LO beam  19 , which is depicted as a dashed line, to signal beam collimator  11  and LO beam collimator  12 , respectively. Signal measurement beam  20  and signal reference beam  24  are spatially distinct beams, as described in more detail below. The beams are generated within laser assembly  10  using one or more laser sources together with frequency modulators to shift the frequency of LO beam  19  from the frequency of signal beam  18 . Typically, the generated beams are transmitted to the collimators using a fiber optic distribution network. One skilled in the art will recognize other systems and methods that can be used to generate and transmit signal beam  18  and LO beam  19  without departing from the scope of the invention. 
   Interferometer  13  receives the collimated beams from signal beam collimator  11  and LO beam collimator  12  and transmits signal measurement beam  20  to measurement reflector  16 . Measurement reflector  16  is attached to an object (not shown) whose change in position with respect to interferometer  13  is detected by the metrology gauge. Measurement reflector  16  is typically a corner cube reflector, however, other reflective devices can be used. Signal measurement beam  20  is reflected back to interferometer  13  by measurement reflector  16 . 
   Interferometer  13  then combines LO beam  19  with the reflected signal measurement beam  20  and signal reference beam  24  of signal beam  18  into a heterodyned combined beam  21 . Combined beam  21  includes heterodyne measurement beam  210  (depicted as a dashed/dotted line) and heterodyne reference beam  211  (depicted as an alternating dashed line). Heterodyne measurement beam  210  and heterodyne reference beam  211  are transmitted to detector system  17 . Using known interferometry techniques, detector system  17  detects changes in distance between interferometer  13  and measurement reflector  16  based on detected optical phase differences in heterodyne measurement beam  210  and heterodyne reference beam  211 . A more detailed explanation of the operation of interferometer  13  is provided below. 
     FIG. 2  is a schematic diagram depicting the components of interferometer  13  according to one embodiment of the invention. As mentioned above, interferometer  13  includes beam splitter and combiner optics  14  and reference optics  15 . As shown in  FIG. 2 , beam splitter and combiner optics  14  includes three optical components  14   a ,  14   b  and  14   c , which are bonded together. The optical components  14   a ,  14   b  and  14   c  are prisms or cubes preferably made of glass. However, other transparent media can be used to form optical components  14   a ,  14   b  and  14   c  in alternative embodiments of the invention. Optical components  14   a ,  14   b  and  14   c  are bonded together using traditional adhesives, optical contacting, and/or hydroxide-catalyzed bonding (e.g., potassium hydroxide solution). Other bonding techniques and materials known to those skilled in the art can be used without departing from the scope of the invention. 
   Reference optics  15  comprises quarter-wave plate  22  and reference reflector  23 . Quarter-wave plate  22  and reference reflector  23  are bonded together using any of the bonding techniques mentioned above. The bonded reference optics  15  is bonded to beam splitter and combiner optics  14  in a similar manner. 
   As mentioned above, signal beam  18  comprises two spatially distinct components: signal measurement beam  20  and signal reference beam  24 . According to one embodiment of the invention, these two components are spatially separated by using the core of signal beam  18  as signal measurement beam  20  and the annulus of signal beam  18  as signal reference beam  24 . In this manner, the two concentric beams are spatially separated and follow a common path within interferometer  13 . Using spatial separation removes sensitivity to polarization leak-though that limits the resolution of conventional polarization-separated interferometers. By employing the spatially separated interferometry architecture, a resolution of 10 picometers or better is possible, whereas conventional polarization-separated interferometers are typically limited to resolutions of 1 nanometer. Signal reference beam  24  is depicted as two solid lines parallel to the dotted line representing signal measurement beam  20 . Alternative embodiments may switch the portions of signal beam  18  used for each component, or may divide signal beam  18  in a different manner to obtain the two spatially distinct component beams. 
   As illustrated in  FIG. 2 , signal measurement beam  20  and signal reference beam  24  travel down an optical path through optical components  14   a  and  14   b  and through quarter-wave plate  22  to reference reflector  23 . Reference reflector  23  reflects the annulus of signal beam  18 , which is signal reference beam  24 , back into optical component  14   b . The core of signal beam  18 , which is signal measurement beam  20 , passes through a non-reflecting portion of reference reflector  23  and is transmitted to measurement reflector  16 . Measurement reflector  16  reflects signal measurement beam  20  back through reference reflector  23  and quarter-wave plate  22  into optical component  14   b . According to one embodiment of the invention, reference reflector  23  is an annulus mirror. Alternative embodiments of the invention can use other reflective devices such as a corner cube with the apex removed to allow signal measurement beam  20  to pass through. 
   The interface between optical components  14   a  and  14   b  is treated using known techniques to provide a polarized beam splitter  25 . Signal measurement beam  20  and signal reference beam  24  are initially linearly polarized with either a p-polarization or an s-polarization. The initial linear polarization allows signal measurement beam  20  and signal reference beam  24  to pass through polarized beam splitter  25 . When signal measurement beam  20  and signal reference beam  24  initially pass through quarter-wave plate  22 , the linear polarization is changed to a circular polarization. After being reflected by measurement reflector  16  and reference reflector  23 , signal measurement beam  20  and signal reference beam  24  pass back through quarter-wave plate  22 , which changes the polarization of the beams from a circular polarization to a linear polarization orthogonal to the initial linear polarization of the beams. The orthogonal linear polarization causes the reflected signal measurement beam  20  and signal reference beam  24  to be deflected by polarized beam splitter  25 . 
   The deflected signal measurement beam  20  and the deflected signal reference beam  24  travel through optical component  14   b  towards optical component  14   c . The interface between optical components  14   b  and  14   c  is treated using known techniques to provide beam splitter/combiner  26 . The collimated LO beam  19  transmitted from LO beam collimator  12  travels through optical component  14   c  and intersects signal measurement beam  20  and signal reference beam  24  at beam splitter/combiner  26 . Beam splitter/combiner  26  mixes LO beam  19  with signal measurement beam  20  and with signal reference beam  24  to form heterodyne beams. Beam splitter/combiner  26  splits the heterodyne beams into heterodyne measurement beams  210   a  and  210   b , which are depicted as dashed/dotted lines, and into heterodyne reference beams  211   a  and  211   b , which are depicted as alternating dashed lines. After being split by beam splitter/combiner  26 , heterodyne measurement beam  210   a  and heterodyne reference beam  211   a  are directed to detector system  17   a  and heterodyne measurement beam  210   b  and heterodyne reference beam  211   b  are directed to detector system  17   b.    
   Detector systems  17   a  and  17   b  detect changes in the position of the object attached to measurement reflector  16  based on interferogram signals created when LO beam  19  is combined with signal measurement beam  20  and signal reference beam  24 . The changes in position are determined using known interferometry techniques, which are discussed further herein. The use of two detector systems is an optional feature of the invention that allows the implementation of a primary detector system and a redundant detector system. Alternative embodiments of the invention can include a single detector system. The use of two detector systems also allows weak heterodyne signals detected by detector systems  17   a  and  17   b  to be combined for more accurate detection than could be obtained using a single weak signal. 
   As described above, interferometer  13  is a monolithic structure formed using its optical components as both optics and structure. In this manner, the ancillary metal components of traditional interferometers are not necessary. Hence, the size and mass of interferometer  13  are reduced, allowing smaller, lighter metrology systems and thus making possible more widespread use of the spatially-separated interferometer architecture. It is to be understood that the components and configuration depicted in  FIG. 2  represent a single embodiment of the invention. Other configurations and components can be included in alternative embodiments without departing from the scope of the invention. For example, signal beam collimator  11  and/or LO beam collimator  12  can be spaced apart from interferometer  13 , as shown in  FIG. 2 , or directly in contact and bonded with interferometer  13 . 
   A significant advantage of the present invention is the alignment of the optical components of interferometer  13 . Since the optical components are bonded together in a monolithic structure, alignment of the components is performed during assembly. The alignment of interferometer  13  is dependent upon the manufacturing tolerances of each of the optical components and is permanent once the optical components have been bonded together. This feature simplifies deployment and maintenance and provides additional stability to a metrology system using a monolithic interferometer according to the present invention. 
   The common-path architecture of the present invention, combined with the monolithic construction, provides additional advantages over conventional systems. For example, signal measurement beam  20  and signal reference beam  24  have a common path that passes through the same length of glass in the optical components of interferometer  13 . This feature reduces the sensitivity of the system to changes in thermal gradients, which effect the accuracy of the interferometer. Similarly, the beams in the interferometer are subject to the same mechanical, thermal, and optical variations, greatly reducing inaccuracies attributable to such variations. 
     FIG. 3  is a block diagram depicting the primary components of a multi-axis metrology gauge utilizing a monolithic, spatially-separated, common-path interferometer according to one embodiment of the invention. Components similar to the components depicted in  FIG. 1  have been given common reference numbers. As shown in  FIG. 3 , the metrology gauge includes laser assembly  10 , signal beam collimator  11 , local oscillator (LO) beam collimator  12 , interferometer  13 , which includes multiplexer  27 , beam splitter and combiner optics  14 , and reference optics  15 , measurement reflector  16  and detector system  17 . 
   Similar to the embodiment depicted in  FIG. 1 , laser assembly  10  provides signal beam  18 , which includes signal measurement beam  20  (depicted as a dotted line) and signal reference beam  24  (depicted as a dashed line), and LO beam  19  to signal beam collimator  11  and LO beam collimator  12 , respectively. Signal measurement beam  20  and signal reference beam  24  are spatially distinct beams, as described above. The beams are generated within laser assembly  10  using one or more laser sources together with frequency modulators to shift the frequency of LO beam  19  from the frequency of signal beam  18 . Typically, the generated beams are transmitted to the respective collimators using a fiber optic distribution network. One skilled in the art will recognize other systems and methods that can be used to generate and transmit signal beam  18  and LO beam  19  without departing from the scope of the invention. 
   Interferometer  13  receives the collimated beams from signal beam collimator  11  and LO beam collimator  12  and splits signal measurement beam  20  into three signal measurement beams  20   a ,  20   b  and  20   c , signal reference beam  24  into three signal reference beams  24   a ,  24   b  and  24   c , and LO beam  19  into three LO beams  19   a ,  19   b  and  19   c . The three sets of beams are used to detect changes in distance along three respective axes. It is to be understood, however, that interferometer  13  can split signal measurement beam  20 , signal reference beam  24  and LO beam  19  into a different number of beams to facilitate measurement along other numbers of axes without departing from the scope of the invention. 
   Interferometer  13  transmits signal measurement beams  20   a ,  20   b  and  20   c  to respective ones of measurement reflectors  16   a ,  16   b  and  16   c . Each of measurement reflectors  16   a ,  16   b  and  16   c  is attached to a respective object (not shown) whose change in position with respect to interferometer  13  is detected by the metrology gauge. Measurement reflectors  16   a ,  16   b  and  16   c  are typically corner cube reflectors, however, other reflective devices can be used. Signal measurement beams  20   a ,  20   b  and  20   c  are reflected back into interferometer  13  by measurement reflectors  16   a ,  16   b  and  16   c.    
   Interferometer  13  combines LO beams  19   a ,  19   b  and  19   c  with respective ones of signal measurement beams  20   a ,  20   b  and  20   c  and respective ones of signal reference beams  24   a ,  24   b  and  24   c  into heterodyned combined beams  21   a ,  21   b  and  21   c . Each of combined beams  21   a ,  21   b  and  21   c  include a corresponding heterodyne measurement beam (depicted as a dashed/dotted line) and a corresponding heterodyne reference beam (depicted as an alternating dashed line). Combined beams  21   a ,  21   b  and  21   c  are transmitted to detector system  17 , which detects changes in distance between interferometer  13  and measurement reflectors  16   a ,  16   b  and  16   c  using known interferometry techniques. Detector system  17  is depicted as a single system in  FIG. 3 . It is to be understood, however, that an individual detector system can be used for each combined beam or some other multiple of combined beams without departing from the scope of the invention. A more detailed explanation of interferometer  13  according to this multi-axis embodiment is provided below. 
     FIG. 4  is a schematic diagram depicting the components of interferometer  13  according to a multi-axis embodiment of the invention. Interferometer  13  includes multiplexers  27   a  and  27   b , beam splitter and combiner optics  14  and reference optics  15   a ,  15   b  and  15   c . Multiplexer  27   a  is an optical component that receives signal beam  18 , which includes a signal measurement beam and a signal reference beam, and splits the beam into multiple beams. In the embodiment depicted in  FIG. 4 , multiplexer  27   a  splits signal beam  18  into three signal measurement beams  20   a ,  20   b  and  20   c  (depicted as dotted lines) and three corresponding signal reference beams  24   a ,  24   b  and  24   c  (depicted as solid lines parallel to corresponding signal measurement beams). Multiplexer  27   a  is an optical component made of glass or another transparent medium with appropriate surface treatments known to those skilled in the art. According to one embodiment of the invention, multiplexer  27   a  is implemented as a shear plate for splitting the beam into the desired number of beams. Alternative embodiments of the invention can use an array of rhomboids (not shown) configured as beam splitters to split the beam into the desired number of beams. Other beam splitting optical components known to those skilled in the art can be used without departing from the scope of the invention. 
   Multiplexer  27   a  transmits signal measurement beams  20   a ,  20   b  and  20   c  and signal reference beams  24   a ,  24   b  and  24   c  into beam splitter and combiner optics  14 , which comprises optical components  14   a ,  14   b  and  14   c . Beam splitter and combiner optics  14  are implemented and operate in a similar manner as that described above with reference to  FIG. 2 . Each of signal measurement beams  20   a ,  20   b  and  20   c  and signal reference beams  24   a ,  24   b  and  24   c  travels through interferometer  13  along a path similar to the path of signal reference beam  24  and signal measurement beam  20  described above with reference to  FIG. 2 . Specifically, the beams travel through optical components  14   a  and  14   b  and a respective one of quarter-wave plates  22   a ,  22   b  and  22   c . The signal reference beams are reflected back by a respective one of reference reflectors  23   a ,  23   b  and  23   c , and the signal measurement beams are reflected back by a respective one of measurement reflectors  16   a ,  16   b  and  16   c . The signal measurement and reference beams are combined with respective ones of LO beams  19   a ,  19   b  and  19   c  into corresponding heterodyne combined beams  21   a ,  21   b  and  21   c  and are transmitted to detector systems  17   a  and  17   b  for detection. For details on the optical paths of the signal measurement and reference beams in interferometer  13 , please refer to the description of  FIG. 2  provided above. 
   As shown in  FIG. 4 , multiplexer  27   b  splits LO beam  19  into three LO beams  19   a ,  19   b  and  19   c , which are subsequently combined with the signal measurement and reference beams. Like multiplexer  27   a , multiplexer  27   b  is an optical component made of glass or another transparent medium with appropriate surface treatments known to those skilled in the art. According to one embodiment of the invention, multiplexer  27   b  is implemented as a shear plate for splitting the beam into the desired number of beams. An alternative embodiment of the invention uses an array of rhomboids (not depicted) configured as beam splitters to split the beam into the desired number of beams. While  FIG. 4  depicts two multiplexers  27   a  and  27   b , it is to be understood that a single multiplexer designed to split both signal beam  18  and LO beam  19  could be used in other embodiments. In addition, other types and configurations of beam splitting optical components known to those skilled in the art can be used without departing from the scope of the invention. Other alternative embodiments include using a single LO beam  19  having a beam diameter wide enough to mix with all the signal reference and measurement beams in interferometer  13 . 
   Detector systems  17   a  and  17   b  detect changes in the positions of measurement reflectors  16   a ,  16   b  and  16   c  based on interferogram signals created when the respective LO beams combine and interfere with the respective signal measurement and reference beams. The changes in positions are determined using known interferometry techniques which are described further herein. The use of two detector systems is an optional feature of the invention that provides a primary detector system and a redundant detector system. The redundant detector system can act as a backup to the primary detector system. Alternatively, the use of two detector systems allows weak heterodyne signals detected by detector systems  17   a  and  17   b  to be combined for more accurate detection than could be obtained using a single weak signal. Alternative embodiments of the invention can include a single detector system or various numbers of detector systems, with each detector system being used for detecting one or more combined beams  21 . 
   Interferometer  13  depicted in  FIG. 4  is a monolithic structure formed using its optical components as both optics and structures. The optical components, namely multiplexer  27 , beam splitter and combiner optics  14  and reference optics  15  are bonded together using the bonding techniques described above. In this manner, the ancillary metal components of traditional interferometers are not necessary. Hence the size and mass of interferometer  13  are reduced, allowing smaller, lighter metrology systems. It is to be understood that the components and configuration depicted in  FIG. 4  represent a single embodiment of the invention. Other configurations and components can be included in alternative embodiments of the invention without departing from the scope thereof. For example, signal beam collimator  11  and/or LO beam collimator  12  can be spaced apart from multiplexers  27   a  and  27   b , as shown in  FIG. 4 , or directly in contact and bonded with multiplexers  27   a  and  27   b.    
   As with the single-axis embodiment of the invention, the alignment of the optical components forming interferometer  13  provides a significant advantage over conventional interferometers. Since the optical components are bonded together in a monolithic structure, alignment of the components is performed during assembly of the device. The alignment of the components is therefore dependent upon the manufacturing tolerances of the individual components and is permanent once the interferometer has been bonded together. This advantage of the invention simplifies implementation and maintenance and provides improved axis-to-axis stability compared to conventional multi-axis systems. 
   Another significant advantage of the invention over conventional systems is the common-path architecture of interferometer  13 . Respective pairs of measurement beams  20   a ,  20   b  and  20   c  and reference beams  24   a ,  24   b  and  24   c  travel a common optical path through interferometer  13 . This feature reduces the sensitivity of the system to changes in thermal gradients, which affect the accuracy of the interferometer. Additionally, the respective pairs of beams are subject to the same mechanical, thermal and optical variations, greatly reducing inaccuracies attributable to such variations. 
   The foregoing description of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention not be limited by this detailed description, but by the claims and the equivalents to the claims appended hereto.