Abstract:
In the claimed invention for an improved AC type interferometric metrology apparatus, a change in position of an item disposed in a rotating reference frame is measured. The rotating reference frame accumulates a rotation angle over a time period with respect to a fixed reference frame. A quarter wave plate and an interferometer comprising a polarizing beam splitter, a reference path reflector, and a measurement path reflector are all mounted in the rotating reference frame. The measurement path reflector is mounted to item so that changes in its radial position along an axis orthogonal to the axis of rotation of the rotating reference system are measurable. In one preferred embodiment, the components in the fixed reference frame are a two-frequency laser light source, a stationary quarter wave plate, a receiver, for producing a measure signal containing information representing the change in optical path length between the measurement and reference paths of the rotating interferometer, the difference in the two frequencies of the beams from the laser source, and the rotation angle, and a signal processor. The signal processor receives the measure signal, a reference signal representing the difference frequency of the laser light source, and a signal from a rotary encoder containing information proportional to the rotation angle. The signal processor produces a signal representing the measurement of the change in position. In another embodiment, the rotary encoder is replaced by added optical components which automatically produce optical signals that contain information about the rotation angle and the difference in the two frequencies of the beams from the laser source.

Description:
FIELD OF THE INVENTION 
     This invention relates to a method and apparatus for using an interferometer to make measurements of changes in the location of an object. More particularly, this invention is for making such measurements while that object is in a rotating reference frame. 
     BACKGROUND FOR THE INVENTION 
     There is a need for measuring a change in position of an object, such as its linear or angular displacement, when such object is disposed in a rotating frame of reference. For example, in a CNC machine where a work piece is held stationary in a fixed reference frame and a cutting tool is rotated by a spindle, it would be very desirable to measure the radial position of the cutting tool as it is rotating and being used to machine the work piece into a finished shape. In the prior art, it is known to measure indirectly the radial position of the tool by measuring in the fixed reference frame the position of the mechanical linkages which control the radial position of the cutting tool. However, the accuracies of making such indirect measurements are limited by such problems as Abbe offset, thermal expansion, hysteresis, and backlash in the mechanical linkages. In other words, the point of measurement of radial position is not at or close to the rotating tool. Instead, the radial position is inferred from the amount of movement of a measurement scale which is mounted on a portion of a draw bar mechanically linked and located outside of the rotating spindle at a significant distance from the cutting tool. 
     AC type interferometers capable of measuring changes in linear or angular position are well known in the prior art. A typical example is described in U.S. Pat. No. 3,458,259, to Bagley, et al, and assigned to Hewlett-Packard Company (HP), the same assignee as the present patent application. HP also sells commercial products such as the HP 5517A laser transducer system which embodies an interferometer for making various types of displacement measurements. The HP system generates an electrical signals containing information representing the displacement measurements. The electrical signals are available for application to subsequent circuitry and devices for purposes such as displaying the measurements or controlling movement of the object. It would be desirable to use that interferometric metrology system for making a direct measurement of the radial position of the tool as it is rotating. The interferometric optics are relatively small and could be mounted to rotate with the spindle and used for directly measuring the radial position of the tool. However, the conventional AC type interferometer metrology system will not function properly if only the interferometric optics were mounted to rotate with the spindle. It is conceptually possible to mount the laser light source, interferometric optics, and the receiver for responding the optical signals so that they all rotate with the spindle. But, for many reasons, that arrangement is not practical for most of the CNC machines. For example, there are space limitations, problems of getting power to the electrical components of the metrology system, and complications of obtaining electrical signals out of the rotating reference frame. 
     SUMMARY 
     The problems and limitations of the prior art are overcome by an invention made in accordance with the teachings of the present invention. A new and improved AC type interferometric metrology apparatus is used to measure the position of a tool held by a rotating spindle located in a rotating reference frame. In a preferred embodiment, the rotating reference frame spins about an axis of rotation of the spindle and that axis is normal to a radial axis where the tool is positioned. A conventional two frequency laser light source mounted in a fixed frame of reference transmits twin beams each having a frequency which is different from one another by a prescribed amount. The beams from the source are converted from beams that have linear and orthogonal polarization states with respect to one another into input beams having right and left circular polarization states. The input beams are then sent to a quarter wave plate mounted in the rotating reference frame and converted into beams which have linear and orthogonal polarization states that rotate with the rotating reference frame. Mounted in the rotating reference frame, an interferometer includes a reference path reflector, a measurement path reflector, and a polarizing beam splitter with its polarization axis aligned at forty-five (45) degrees to the fast axis of the quarter wave plate. This arrangement ensures that the polarizing beam splitter divides the two beams so that one beam having one frequency is transmitted along a reference path, parallel to the axis of rotation, to the reference path reflector. The beam splitter reflects the other beam with the other frequency along a path parallel to the radial axis, to the measurement path reflector, which is connected to the tool. 
     The beams returning respectively from the reference and measurement path reflectors are transmitted back through the polarizing beam splitter and the quarter wave plate. Having right and left circular polarizations, the two emerging beams from the quarter wave plate are then sent to a second quarter wave plate mounted in the fixed frame and thereafter converted into beams having linear and orthogonal polarization states with respect to one another. A receiver mounted in the fixed frame of reference responds to the returning beams after they pass through the second quarter wave plate for generating an intermediate measure signal. However, the intermediate measure signal includes an unwanted component because the quarter wave plate in the rotating frame of reference introduces in the light beams passing back and forth through it a frequency shift proportional to the angular rotation of the reference frame. A signal processor is coupled to receive the intermediate measure signal, a reference signal from the light source, and a special signal representing a measurement of the angular rotation of the rotating quarter wave plate. The signal processor operates to remove the unwanted component and produces a signal representing the measure of the change in radial position of the measurement reflector. 
     In one preferred embodiment of the present invention, the special signal is produced by a rotary encoder disposed for measuring the amount of angular rotation of the rotating reference frame. In an alternate embodiment, the rotary encoder is replaced with an optical arrangement. Briefly described, this alternate embodiment requires the use of a second pair of input beams having linear and orthogonal polarization states with respect to one another and polarization axes that rotate with the rotating reference frame. A specially modified interferometer is arranged so that the second pair of input beams are both sent through the interferometer and bypass the polarizing beam splitter. After being transmitted to and reflected by the reference path reflector, the second pair of beams are passed back through the interferometer (bypassing the beam splitter) and through the quarter wave plate mounted in the rotating reference frame. Thereafter, those beams are sent into the fixed reference frame and pass through the second quarter wave plate for emerging as a second pair of returning beams having linear and orthogonal polarization states with respect to one another. A second receiver mixes the second pair of returning beams and generates a second receiver signal. The second receiver signal is combined with the intermediate measure signal to produce directly the signal representing the measurement of the change in position of the measurement reflector. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a preferred embodiment of the present invention; and 
     FIG. 2 is a partial view of an alternative embodiment of the present invention wherein a quarter wave plate is mounted in the rotating reference frame separated from the interferometer; 
     FIG. 3 is another preferred embodiment of the present invention wherein the frequency shift due to the rotation of a quarter wave plate is corrected via the use of optical beams; 
     FIG. 4 is a partial view of another alternate embodiment of the present invention wherein a quarter wave plate is mounted in the rotating reference frame separated from the interferometer. 
     FIG. 5 is another alternate embodiment of the present invention wherein the optical beams are coaxial; 
     FIG. 6 is a partial view of still another embodiment wherein a quarter wave plate is mounted in the rotating reference frame separated from the interferometer; 
     FIG. 7 is another embodiment of the present invention wherein the frequency shift due to the rotation of a quarter wave plate is corrected via the use of optical beams; 
     FIG. 8A is another embodiment of the present invention wherein the laser head generates twin beams having right and left circular polarizations with respect to one another; and 
     FIG. 8B is still another alternative embodiment wherein the laser head generates twin beams having right and left circular polarization states and the frequency shift due to the rotation of a quarter wave plate is corrected via the use of optical beams; where like reference numerals in each figure refer to common elements of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 depicts a preferred embodiment of an interferometric apparatus 10 made in accordance with the teachings of the present invention. A laser head 12 is mounted in a fixed reference frame and generates twin coincident light beams 14 &amp; 16 having stabilized frequencies f1 and f2, respectively. The two beams are produced so that one beam having frequencyf1 is linearly and orthogonally polarized with respect to the other beam having frequency f2. For ease of explanation, the beam 14 is assumed to have frequency f1 in a plane of polarization which is normal to the plane of FIG. 1 and is depicted as dots 17. The beam 16 is assumed to have frequency f2 in an orthogonal plane of polarization which is in the plane of the FIG. 1 and is depicted as vertical lines 19. The twin light beams 14 &amp; 16 are transmitted to a stationary quarter wave plate 18 which is mounted in the fixed reference frame and which converts the twin beams into beams 20 &amp; 22 having right and left hand circular polarizations (depicted as circular arrows 21 &amp; 23) with respect to one another. The twin beams 20 &amp; 22 are then sent to a quarter wave plate 24 mounted in a rotating reference frame 26. The quarter wave plate 24 converts those twinbeams to a pair of beams 25 &amp; 27 which have linear and orthogonal polarization states with respect to one another. The rotating frame 26 revolves about a rotation axis 28. Since the quarter wave plate 24 is mounted in and thus rotates in unison with the rotating reference frame 26, the axes of linear polarization of the beams 25 &amp; 27 also rotate with the rotating reference frame 26. For ease of explanation, the rotating reference frame is assumed to rotate with a rotational frequency of W revolutions/second, as indicated by a curved arrow 32, with respect to thestationary measurement frame where is mounted the light source 12. Since the beams 20 &amp; 22 are produced with opposite circular polarizations, the resulting beams 25 &amp; 27 emerging from the quarter wave plate 24 have theirfrequencies upshifted and downshifted respectively by an amount equal to W.It is also assumed for purposes of explanation that the beam 20 has frequency f1 and becomes the beam 25 having an upshifted frequency equal to f1+W. This arrangement also results in the beam 22 having frequency f2 which becomes the beam 27 having a downshifted frequency equal to f2-W. 
     A polarizing beam splitter 34 is mounted in and rotates with the rotating reference frame 26. The beam splitter 34 is disposed for receiving the twin beams 20 &amp; 22 transmitted from the quarter wave plate 24. The fast axis of the quarter wave plate 24 is aligned to be 45 degrees to a polarization axis of the polarizing beam splitter 34. This arrangement permits the linear and orthogonal beams 25 &amp; 27 to stay aligned to the polarizing beam splitter 34 as it rotates. The beam splitter 34 diverts the beam 25 with frequency f1+W upwards along a measurement path which is parallel to a radial axis 35, to a measurement reflector 36, which is preferably a conventional cube corner reflector. Although the measurement reflector 36 is movable back and forth along the radial axis 35, it is mounted in the rotating frame of reference 26 and thus rotates with respect the fixed reference frame. In the present description, the reflector 36 is assumed to have moved a distance D between the location where the reflector is depicted with solid lines and the location where the reflector is shown with dotted lines. The means for movably mounting the reflector 36 is not shown. The beam 25 after traveling out and back from the measurement reflector 36 returns to the beam splitter 34 as beam 40 with a Doppler shift of ±Δf. As is well known, the Doppler shift is positive or negative depending upon which frequency f1 or f2 is in the measurement path and the direction of movement of the measurement reflector 36. 
     The beam splitter 34 is arranged to pass beam 27 having frequency f2-W along a reference path having a fixed length to a reference reflector 38, which is also preferably a cube corner reflector. The reference path is parallel to the rotation axis 28 and the reflector 38 being mounted in therotating reference frame 26 rotates about the rotation axis 28. The beam 27after being reflected by the reflector 38 is returned to the beam splitter 34 as a beam 42. The beam 42 is not Doppler shifted. 
     The beam 40 is reflected by and the beam 42 is transmitted through the beamsplitter 34 and both are then sent through the quarter wave plate 24 and become beams 43 &amp; 47. Those beams are then passed through the stationary quarter wave plate 18 and emerge as returning beams 44 &amp; 46, each of whichis again frequency shifted up or down by a value equal to W. The beam 44 now has a frequency f1&#39;=f1±Δf+2W and the beam 46 has frequency f2&#39;=f2-2W. A receiver 48 mounted in the stationary measurement frame receives the beams 44 &amp; 46 after they have been redirected by a beam bender 49 and operates to mix them for producing a measurement signal 50 having a frequency f --  meas=f1&#39;-f2&#39;=f1-f2±Δf+4W. The terms of f --  meas are the split frequency, f1-f2, the Doppler shift, ±Δf, arising from the movement of the measurement reflector 36, and the frequency shift, 4W, arising from the quarter wave plate 24 as it rotates in unison with the rotating reference frame 26. A reference signal52 which is generated by the laser light source 12 has a frequency f -- ref=f1-f2, the split frequency between beams 14 &amp; 16. The measurement signal 50 and the reference signal 52 are transmitted to a compensator 54 which includes one portion 51 that operates by determining a frequency difference and integrating the result to produce an intermediate signal 56. The frequency difference, f --  diff, is calculated by the following equation: 
     
         f.sub.13 diff=f.sub.-- meas-f.sub.-- ref=±Δf+4W. 
    
     According to the Doppler effect, Δf=Fv/λ, where F is the interferometer fold factor, v is the velocity of the measurement reflector36, and λ is the wavelength of the laser light beam. Integrating f --  diff gives φ, which is the intermediate signal 56 and is the phase in fringes. In other words, 
     
         φ=∫f.sub.-- diff dt=F/λ∫vdt+4∫Wdt=F/λvt+4Wt. 
    
     Since vt=D, where D is the distance moved by the reflector 36, then φ=DF/λ+4Wt. So if D=0, W=1 revolution/sec, and t=1 second, then φ=4 fringes/revolution of the rotating reference frame. Thus, in orderto compensate for the frequency shift, occurring during the measurement of the change in displacement of the reflector 36 while the quarter wave plate 24 is rotating in unison with the rotating reference frame, a correction of 4 fringes/revolution of the rotating reference frame 26 is required to be subtracted. In the preferred embodiment, a conventional encoder 58 is coupled to the rotating reference frame 26 for detecting itsangular rotation and generating an encoder signal 60. 
     A second portion 53 of the compensator 54 receives the intermediate signal 56 and the encoder signal 60 and responds to produce a displacement signal62 representing the measurement of the change in position, D, of the reflector 36 along the radial axis 35. 
     As a result of the above arrangement, it can be understood that the presentinvention is adaptable for use in a CNC machine application where a spindlecontaining a radially movable cutting tool is rotating about the axis 28. By affixing the measurement reflector 36 to the slider holding the cuttingtool and arranging the rotational axis 28 to be coincident with the axis ofrotation of the spindle, the measurement of the displacement of the measurement reflector 36 is a direct measurement of the movement of the tool along a radial axis normal to the axis of rotation of the spindle. 
     In the preferred embodiment shown in FIG. 1, the quarter wave plate 24 is preferably affixed to the polarizing beam splitter 34 of an interferometer70. A partial view of an alternate embodiment is depicted in FIG. 2 in which a quarter wave plate 200 and interferometer 202 is used in place of the quarter wave plate 24 and interferometer 70 of FIG. 1. In this alternate embodiment, the quarter wave plate 200 is separated from the interferometer 202. The remaining elements of FIG. 2 (not shown) are the same as those in FIG. 1. 
     Depicted in FIG. 3 is another preferred embodiment of the present invention. In an interferometric apparatus 300, the laser head 12 mounted in a fixed reference frame generates twin beams 302 &amp; 304 having the frequencies f1 and f2, respectively, and linear and orthogonal polarizations. A non-polarizing beam splitter 305 divides the beams 302 &amp; 304 into the beams 14 &amp; 16 and another pair of beams 306 &amp; 308 which emerge from a beam bender 307. The beams 14 &amp; 16 have linear and orthogonal polarization states with respect to one another. The beams 306 &amp; 308 also have linear and orthogonal polarization states with respect to one another as depicted by the dot 17 and the line 19. A stationary quarter wave plate 309 converts the beams 14 &amp; 16 to the beams 20 &amp; 22 having right and left circular polarizations with respect to one another. The beams 306 &amp; 308 are also converted by the stationary quarter wave plate 309 into a pair of beams 310 &amp; 312 having right and left circular polarization states with respect to one another. A quarter wave plate 315 mounted in the rotating reference frame 26 converts the beams 14 &amp; 16 to the beams 25 &amp; 27 having linear and orthogonal polarization states to one another. The beams 310 &amp; 312 are also converted by the quarter wave plate 315 into a pair of beams 314 &amp; 316 having linear and orthogonal polarization states to one another. An interferometer 320 mounted in the rotating reference frame includes the polarizing beam splitter 34 which isformed as a central core portion of a cube 322 and a reference reflector 323. The reflector 323 is preferably a cube corner reflector. The beam splitter 34, the measurement reflector 36, and all other elements having the same reference numerals as in FIG. 1 are all arranged and operated in the same manner as described for those same elements. Accordingly, the beams 44 &amp; 46 are generated in the same manner as the embodiment of FIG. 1and are sent to the receiver 48 after being redirected by the beam bender 49. The receiver 48 generates the measurement signal 50. 
     The cube 322 includes an outer annular portion 324 which surrounds the polarizing beam splitter 34. The annular portion 324 transmits the beams 314 &amp; 316 to the reference reflector 38 which reflects those two beams back into the annular portion 324 as a pair of beams 326 &amp; 328. After being applied to the quarter wave plate 315, the beams 326 &amp; 328 emerge asa pair of beams 330 &amp; 332 having right and left circular polarization states to one another. The stationary quarter wave plate 309 thereafter converts the beams 330 &amp; 332 to a pair of beams 334 &amp; 336 having linear and orthogonal polarization states to one another. A beam bender 338 redirects the beams 334 &amp; 336 to a reference receiver 340 which is mountedin the fixed reference frame and which operates to generate a reference signal 342. A compensator 344 operating in response to the applied measurement signal 50 and the reference signal 342 generates the displacement signal 62 representing the change in position of the reflector 36 along the radial axis 35. Accordingly, this embodiment as depicted in FIG. 3 eliminates the need for the encoder 58 as well as the signal 52 from the laser head 12 as shown in FIG. 1. 
     In the embodiment shown in FIG. 3, the quarter wave plate 315 is preferablyaffixed to the cube 322. FIG. 4 depicts a partial view of another embodiment of the present invention wherein the quarter wave plate 315 is separated from the cube 322. The other elements (not depicted) are the same as those in FIG. 3. 
     FIG. 5 shows another alternate embodiment of an apparatus 500 made in accordance with the teachings of the present invention. As in the prior embodiments, the laser head 12 mounted in the fixed reference frame produces the twin beams 14 &amp; 16 which are transmitted through an isolator 502. The purpose of the isolator 502 will be explained in a later portion of this description. Next, the twin beams are passed through a non-polarizing beam splitter 504, the quarter wave plate 18 mounted in thefixed reference frame, and then to the quarter wave plate 24 mounted in therotating reference frame 26. The twin beams 20 &amp; 22 emerging from the quarter wave plate 18 have right and left circular polarizations with respect to one another. After emerging from the quarter wave plate 24, thetwin beams 25 &amp; 27 have linear and orthogonal polarization states but whichnow rotate with the rotating reference frame 26. The interferometer 70 operates the same as described in FIG. 1 so that the measurement reflector36 returns the beam 40 and the reference reflector 38 returns the beam 42. The beams 40 &amp; 42 after passing through the quarter wave plate 24 and the stationary quarter wave plate 18, are reflected by the non-polarizing beamsplitter 504 and emerge as the beams 44 &amp; 46 which are sent to the receiver48. The purpose of the isolator 502 is to block or significantly attenuate any of the returning beams from being transmitted back into and adversely affecting the operation of the laser head 12. The technology for designingand making suitable optical isolators is known. Optical attenuators are allwell known devices and many are suitable for use in the present invention. It should be pointed out that a disadvantage of using an attenuator is theresulting lower efficiencies due to lower beam intensities. 
     The other elements of FIG. 5 which have the same reference numerals as depicted in FIG. 1 operate in the same manner so that the apparatus 500 thereby produces the displacement signal 62. The apparatus 500 is very similar to the apparatus 10 (of FIG. 1), except that the beams entering and returning from the rotating reference frame 26 are coaxial in path instead of traveling along parallel paths. This embodiment is not preferred as that shown in FIG. 1 because the signal intensity of the beams is lower than used in apparatus 10. However, the interferometer 70 used in the apparatus 500 can be smaller than that for the apparatus 10, since the beams are coaxial. This embodiment is useful for those applications where space is very limited. 
     In the embodiment shown in FIG. 5, the quarter wave plate 24 is affixed to the interferometer 70. FIG. 6 depicts a partial view of another embodimentwherein the quarter wave plate 24 is separated from the interferometer 70. The other elements (not shown) are the same as those in FIG. 5. 
     The apparatus 500 of FIG. 5 still requires use of the encoder 58. In FIG. 7, another alternate embodiment of the present invention is shown wherein an apparatus 700 uses an optical arrangement similar to that of FIG. 3 in order to eliminate the need for an encoder and the reference signal from the laser head 12. In FIG. 7, the elements having the same numerals as those in FIGS. 3 &amp; 5 operate in like manner so that the apparatus 700 thereby generates the displacement signal 62. 
     It should be understood that the present invention requires the beams whichenter and return from the rotating reference frame must be along paths thatare parallel or coaxial to the rotation axis 28. After the beams pass through the quarter wave plate mounted in the rotating reference frame, the associated interferometer can be located in any suitable orientation, as long as the beams are directed to enter the interferometer properly. Inother words, in the embodiments of FIGS. 1-7, the measurement and referencepath reflectors are oriented on paths which are normal to the axis of rotation 28 and parallel (or coaxial) to the rotation axis, respectively. In the embodiments shown in FIGS. 4 &amp; 6 where the quarter wave plate in the rotating reference frame is separated, the associated interferometer can be oriented differently so that the measurement and reference reflectors are not along paths respectively normal and parallel (or coaxial) to the rotation axis. Orienting the interferometer differently isuseful for making other measurements in the rotating reference frame such as flatness or changes in angular position instead of just measuring radial displacement perpendicular to the rotation axis. 
     Although the above embodiments depict the reference reflector as being in afixed location and in contact with the beam splitter, such limitation is not needed for the operation of the present invention. In other words, thepresent invention can be used in so called differential interferometers where the reference reflector is movable, and the interferometric measurement is a differential measurement made between the locations of the reference and the measurement reflectors. 
     The laser head used in the above described embodiments preferably generate twin beams which are linear and orthogonal to one another. In FIG. 8A, an apparatus 800 is shown wherein a laser head 802 generates the twin beams 20 &amp; 22 having right and left circular polarizations with respect to one another. The beams 20 &amp; 22 are transmitted into the rotating reference frame 26 along a path parallel to the rotation axis 28. With the exceptionof a quarter wave plate 804, the other elements are the same as those in FIG. 1 or FIG. 2. As should be clear, the quarter wave plate 804 is not the same as the quarter wave plate 24 of FIG. 1 because the quarter wave plate 804 is only needed to generate the beams 44 &amp; 46. 
     In FIG. 8B, an apparatus 840 is shown wherein the laser head 802 is used for generating the beams 20, 22, 310, &amp; 312, with each respective pair having right and left circular polarizations states with respect to one another. With exception of a quarter wave plate 844, the other elements are the same as those in FIGS. 3 or 4. The quarter wave plate 844 is only needed to generate the beams 44, 46, 334, &amp; 336 and is thus not the same as the quarter wave plate 309 of FIG. 3. 
     While the present invention has been described and illustrated with reference to the specific embodiments, those skilled in the art will recognize that modifications and variations may be made without departing from the principles disclosed by the teachings of the present invention. For example, the stationary quarter wave plates of FIGS. 3, 4, 7, &amp; 8B which are each preferably monolithic in construction, can be divided into two portions, on portion for generating the beams 310, 312, 20, &amp; 22 and another portion for producing the beams 44, 46, 334, &amp; 336. The two portions need not be stacked on top of one another so long as they are in position for receiving the corresponding beams. It is also possible for having individual portions sized for receiving individual beams or varioussub-combinations of beams. 
     The quarter wave plates mounted in the rotation reference frame 26 of FIGS.3, 4, 7, &amp; 8B which are each preferably of monolithic construction, can be divided into an annular portion for producing the beams 314, 316, 330, &amp; 332 and a central core portion for generating the beams 25, 27, 43, &amp; 47. The two portions can be concentric to one another but the central portion does not have to be nested within the annular portion. 
     Moreover, the receivers 48 &amp; 340 of FIGS. 1-4, 7, 8A, &amp; 8B can be located online to receive the beams 44 &amp; 46 and 334 &amp; 336, respectively. In other words, the beam benders 49 &amp; 338 can eliminated if the size of the receivers are small enough so as not to interfere with the beams from the laser head or the parallel beam paths for the input and returning beams are separated with enough distance so that the receivers do not interfere with the input beams from laser head. 
     In the embodiments of FIGS. 5-6, flat mirrors can be used for the reflectors. The preferred devices are cube corner reflectors. 
     The isolator 502 of FIGS. 5-7, though preferred, is not always needed. Although its operation is not as efficient, the laser head can operate as a source of the twin beams without the use of an isolator even though somelaser light is returned to it. 
     Finally, the twin beams from the laser head are preferably beams having optical light frequencies. However, other non-optical frequencies can be used since the operation of the AC type of interferometer is not restricted to the optical frequencies of light.