GB1564781A

GB1564781A - Distance measuring devices

Info

Publication number: GB1564781A
Application number: GB436077A
Authority: GB
Original assignee: Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1977-02-02
Filing date: 1977-02-02
Publication date: 1980-04-16
Also published as: DE2715052C2; DE2715052B1

Description

(54) DISTANCE MEASURING DEVICES (71) We, THE MARCONI COMPANY LIMITED. a British Company of Marconi House, hlew Street, Chelmsford, Essex CMI I PL, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to measuring devices, and is concerned in particular with such devices which are capable of measuring lengths to an accuracy of the order of nano-metres over a measurement range of the order of 10 cm. Such devices are required for the accurate positioning of photo-lithographic masks used in microcircuit manufacture, and for the accurate measurement of machined parts.

Known devices for this purpose do not readily lend themselves to be calibrated for a specified, desired accuracy, and it is an object of this invention to provide new measuring devices wherein this disadvantage is at least partially mitigated.

Accordingly, in one aspect this invention provides a length measuring device comprising: 1) arranged sequentially along a light path a) a monochromatic light source, b) an acousto-optical modulator acting as a diffraction grating and arranged to give both an undiffracted beam and at least one beam which is diffracted by a predetermined angle (and which is therefore frequency-modulated by the modulator's generator frequency), c) a mechanical diffraction grating aligned with the acousto-optical modulator, this mechanical grating having a pitch arranged to diffract by the predetermined angle light incident thereon substantially normal to the plane thereof, and being movable in a direction across the light path perpendicular to Its grating lines (so as to phise-shift ans beam diffracted thereby by an amount proportional to the amount of movement). and d) focussing means whereby both diffracted and undiffracted light from the mechanical grating is focussed on to e) a photodetector the output of which is proportional to its input (and is thus an amplitude-modulated signal having a frequency the same as, but a phase which may be different to that of, the acoustooptical modulator's generator frequency and phase respectively) together with 2) phase comparator means for measuring and comparing the frequency and phase of the photodetector's output with respect to the acousto-optical modulator's generator frequency and phase, the phase difference being proportional to and indicative of the mechanical grating's amount of movement.

It will be appreciated that the use of a grating and an acousto-optical modulator (involving selection of the period of the grating lines, and of the frequency of the acousto-optical modulator's generator) allows the required accuracy to be achieved.

Since the ideal of a pure monochromatic light source is not currently practicable, the term "monochromatic", as used herein means those light sources which tend towards monochromaticity, and are generally regarded as being monochromatic. The light source conveniently is a gas discharge lamp, a lightemitting diode, or a laser.

The acousto-optical modulator's generator is conveniently an RF-that is, radio frequency-generator.

The mechanical grating, which is conveniently supported by a suitable linear bearing, may be manufactured by the steps of splitting a parallel beam of light from a laser into two substantially equal parts, focussing each part onto a photosensitive coating on a homogeneous, transparent, optical member so that an interference pattern is exposed upon the photosensitive coating and developing the coating to pro(lu. c the required grating lines. By utilising such a method, the period of the grating may easily be adjusted simply by altering the angle at which the median rays of the two beams strike the light sensitive coating. The focussing means is preferably a lens with an aperture at the principle focus thereof dimensioned to reject those light beams not travelling at the appropriate predetermined angle.

When the displacement of the mechanical grating is greater than the period of the grating lines, the change in phase measured by the comparator means is in excess of 2x radians. The phase change in radians may be represented by the expression N(2X)+0m, where N is the number of whole grating periods moved and 5im is the remainder.

Because the effect of a change of this magnitude is the same no matter what the value of N, and in order to avoid the possibility of ambiguity in the measurement, means are preferably provided for establishing the number of complete grating line periods moved by displacement of the mechanical grating (the value of N). One such means conveniently comprises an up/down counter for determining the displacement of the grating about a datum.

Another such means involves using an additional acousto-optical modulator using an additional RF generator of a different frequency to that of the first modulator, together with an associated additional mechanical diffraction grating and an associated focussing means, photodetector and phasemeter; additional comparison means are then provided to compare the relative changes in the phasemeter outputs-and, by suitably selecting the two generator frequencies, there is achieved a non-ambiguous result for the mechanical grating movement. Very preferably, however, there is employed only one acousto-optical modulator using two generators of different frequencies, and the two mechanical gratings are in fact most conveniently a single grating support upon which both series of grating lines are superposed.

One embodiment of the length measurement device of -the present invention may be used to compare the profiles of two objects; in such an embodiment there are employed two independently movable mechanical gratings each of which is arranged to transmit diffracted light to respective focussing means, both focussing means having an associated photodetector. In operation the outputs from the two photodetectors are applied separately to a phase comparator which produces a signal representative of the difference in displacement of the two mechanical gratings.

The invention will now be described, though by way of illustration only, with reference to the drawings accompanying the Provisional Specification, in which drawings: Figure 1 shows in diagrammatic form a side view of one embodiment of a measuring device in accordance with this invention Figure 2 shows in diagrammatic form a side view of another embodiment, in which means are provided for establishing the number of complete grating line periods moved by displacement of the grating; Figure 3 shows in diagrammatic form a plan view of a device for comparing the profiles of two objects; Figure 4 shows in diagrammatic form a detail of a side view of the probes for use in the embodiment of Figure 3; and Figure 5 shows in diagrammatic form an apparatus for producing a mechanical grating suitable for use in the invention.

In the Figures, like reference numerals denote like parts.

The measuring device shown in Figure 1 has a monochromatic light source in the form of a laser 1 producing a light beam which is converted into a parallel beam by a system of lenses represented by a single lens 2. The plane parallel beam is modulated by an R.F. energy source 3 generating an angular frequency Q)m in an acousto-optical light modulator 4 of the Raman-Nath type.

The modulator 4 comprises a rectangular transparent block made of glass or fused quartz and having optical polished plane opposing faces perpendicular to the light beam; in this block a travelling longitudinal (or shear) acoustic wave, denoted by the lines 5, is set up by an acoustic transducer 6 attached to one end of the modulator and to which the R.F. energy source 3 is connected. The acoustic wave energy in the modulator is absorbed by an absorber 7 attached to the opposite end of the modulator from the transducer 6. The light beam incident on the modulator is perpendicular to the direction of propagation of the acoustic wave, and is partially diffracted by the acoustic wave in dependence upon the power of the R.F.

source 3; the power of the R.F. source 3 is arranged to be sufficiently small so that only first order diffracted beams have significant intensity, the two first order diffracted beams (8, 9 respectively) emerging from the modulator 4 travelling at angles +o to the undiffracted light beam 10 (where: ###1 #s if 0 l; A, is the wavelength of light in air; and As is the wavelength of sound in the modulator block at angular frequency 4)m).

The diffracted beam 9 is up-shifted in frequency by an amount m (and the beam 8 is down-shifted by Cl)m) so that the three beams emerging from the modulator have angular frequencies (sz)o+Ca)m), w, and (oa'm) (where w, is the angular frequency of the incident light). The beams 8, 9, 10, of which only beams 8 and 10 are further utilised, are subsequently passed through a movable mechanical diffraction grating 11 positioned adjacent to the modulator 4.

The grating 11 consists of a coating of exposed photo-resist, on a glass or quartz plate 12, formed into a sinusoidallymodulated absorption or phase grating 13 having a uniform pitch approximately equal to the acoustic wavelength As (produced by a method to be subsequently explained); two first order diffracted beams are produced travelling respectively at angles +0 and -0 to the undiffracted light. The mechanical grating 11 is co-planar with the polished plane faces of the modulator 4. The grating lines are arranged on the surface of the grating 11 removed from the modulator 4, and are parallel to the acoustic wavefronts 5 in the modulator 4.The grating 11 is constrained by linear bearings (not shown in Figure 1) to move in a direction parallel to the plane surfaces of the grating 11 and perpendicular to the grating lines 13, as indicated by double-arrow-headed line A.

As stated above, for each of the three beams (8, 9, 10) incident on the mechanical grating 11, two additional first order diffracted beams are produced in the light transmitted therefrom, these travelling respectively at angles +0 and -0 to the undiffracted light. However, since only the diffracted beam produced by the incident light beam 10 normal to the grating is utilised, for clarity only the diffracted beam 14 caused thereby is shown. Thus, two beams of relevance emerge from the mechanical grating 11 travelling at an angle 8 to the incident beam 10, these beams being the undiffracted beam 8 and the first order diffracted beam 14. Both of the beams 8 and 14 are collected by a lens 15 which focusses them through an aperture 16 in a window 17.The aperture 16 is positioned at the principle focus of the lens 15, and the window is dimensioried to reject those beams which do not travel at an angle 0 to the incident beam 10. The beam passing through the aperture 16 is focussed onto a photodetector 18 which produces a photo current having an R.F. component at frequency Q)m- It can be shown that the beam 14 is phase shifted by movement of the grating in the direction indicated by the double arrow headed line A by an amount + (where sss equals 2#x #s where X is the displacement of the grating).

The amplitudes E8 and E14 of the two beams 8 and 14 respectively incident on the photo-detector can be expressed as the real parts of E8=E8o exp .I(a)0a)m)t E14=E140 exp j(w,t+) (where E80 and E,40 represent peak amplitudes of the beams 8 and 14 respectively, and where constant phase factors independent of the grating motion have been neglected). The combined intensity on the photo-detector is I where loci E8+E,4 1 2=(E8+E,4)(E8*+E14*) (where * denotes the complex conjugate).

Therefore:- I E8E8*+E14E14*+E8E14*--E14E8* E802+E1402+E8OE140[exp j(w,t+i) +exp 3(wrnt+)] OCE802+E1402+2E80E140@cos(#mt+#) The time-dependent component of the photo current is thus proportional to cos(#mt+#).

A phase meter 19 is connected to receive output from the photodetector 18 and the R.F. energy source 3 so that the photocurrent is compared with the reference signal from the source 3. An output signal proportional to sJ is obtained, and this is, of course, proportional to the displacement of the grating.

When the displacement of the grating is greater than the period of the grating lines, the change in phase measured by the phase meter 19 will be greater than 2w radians.

The phase change is, therefore, of the form fm+27rN, where Çsm is the phase meter reading and N is an integer equal to the number of complete grating periods moved by the grating 11. Thus, to provide an unambiguous measure of N, an up/down counter 20 is connected to the output of the phase meter so that each time m exceeds 27r the count of counter 20 is either increased by 1 (if the change of phase is positive) or reduced by 1 (if the change of phase is negative).

An alternative embodiment for avoiding ambiguity in the phase measurement is shown in Figure 2. Here, an additional R.F.

energy source 3' is connected to the modulator 4 and is arranged to oscillate at a different frequency from that of the R.F.

energy source 3. The incident light on the modulator is therefore modulated at two different frequencies, and light emerging from the modulator 4 is diffracted to a first order by 0 in one case and by 0' in another; these diffracted beams are denoted by the reference numerals 8 and 8' respectively.

The mechanical grating 11 has two series of grating lines superposed thereon, one series being arranged to diffract a light beam which arrives normal to the plane of the grating by 0, the other by 0' (these beams are referenced 14 and 14' respectively). The beams 8 and 14 are focussed by the lens 15 and window 17 onto the photodetector 18, and the beams 8' and 14' are focussed onto a further photodetector 18' by a lens 15' and window 17' having an aperture 16'.The output from each photodetector 18, 18' is fed to a respective phase meter 19, 19' where comparison is made with the phase of the reference R.F. energy sources 3, 3' to derive fm and '. The outputs of the phase meters 19, 19' are then applied to a comparator 21 to compare the relative changes in phase between fm and fmt and hence to provide an overall indication of the distance moved by the grating 11.

The present invention may also be utilised to compare the profiles of machined parts: a device for performing this is shown in Figures 3 and 4. The mechanical grating 11 is divided into two identical, independently, movable structures 111 and 211 mounted between linear bearings 130 and 230 respectively with the plane of movement of the gratings 111 and 211 being the same as for the grating 11 shown in Figures 1 and 2.

The gratings 111 and 211 have rigid probes 131 and 231 respectively connected to their undersides (as shown in Figure 4) which rest under gravity on one of two surfaces 132 and 232 respectively to be compared. The operation of the Figure 3 embodiment is basically similar to that shown in Figure 1; the diffracted beams emerging from each grating at an angle 0 to the normal are for grating 111 denoted as 108 and 114; and for grating 211 denoted 208 and 214. These pairs of beams are collected separately by similar lenses 115, 215 and passed through respective :perturbs 116 and 216 onto photodetectors 118 and 218 respectively.

The photocurrents produced by the photodetectors 118 and 218 have components at frequency t m produced by the R.F. energy source 3 which have phase shifts I, and 02 caused by movement of the gratings 111 and 211 respectively. These photo currents are compared in a phase meter 119 which consequently produces an output proportional to 0 2. and.

therefore, proportional to the difference in movement of the two gratings Such an arrangement has the advantage that temperature effects in the acousto optic modulator arising from the temperature dependence of acoustic velocity in the modulation material are common to both sets of beams on the respective photodetectors, and so have no effect on the differential output of the phasemeter 119.

The two gratings are moved across the surfaces 132, 232 in the direction indicated by double-arrow-headed line B, and the probes may rest on the surfaces either by gravity (as described above) or by some externally applied force.

The diffraction grating utilised in the embodiment of Figure 1 is manufactured employing the apparatus shown in Figure 5.

A laser 1 emits a parallel beam of light which is split into two substantially equal parts by a beam splitter 50. The split beams are reflected from plane mirrors 51, 52 respectively, and are expanded by substantially identical lenses 53, 54 respectively. The expanded beams 55, 56 are positioned to illuminate the same area of a recording plate 57 consisting of a homogenous, transparent, optical member (an optically flat plate of glass) which is coated with a light-sensitive photo-resist layer 58 on the illuminated side thereof. The plate 57 is equidistant from the lenses 53, 54, and oriented so that the median rays of the beams 55, 56 are symmetrically positioned about the normal to the surface of the plate 57. The combination of the beams 55, 56 forms a static interference pattern to which the photo-sensitive surface is exposed, and which is in the form of the required grating.

The period of the grating is adjusted by altering the angle between the median rays of the beams 55, 56; to make the grating period substantially linear over the surface of the plate 57, the projection distance measured perpendicularly from the surface of the plate 57 to the lenses 53, 54 is made large. After exposure, the plate 57 is developed in suitable chemicals to produce the required absorption or phase grating. To produce the grating required in the embodiment of Figure 2 (where two series of grating lines are required), a third beam is prolected onto the plate 57 so that two interference patterns are formed thereon.

WHAT WE CLAIM IS: 1. A length measuring device comprising: 1) arranged sequentially along a light path a) a monochromatic light source, b) an acousto-optical modulator acting as a diffraction grating and arranged to give both an undiffracted beam and at least one beam which is diffracted bv a predetermined angle (and whi h is therefore frequencv-modulated by

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. modulator is therefore modulated at two different frequencies, and light emerging from the modulator 4 is diffracted to a first order by 0 in one case and by 0' in another; these diffracted beams are denoted by the reference numerals 8 and 8' respectively. The mechanical grating 11 has two series of grating lines superposed thereon, one series being arranged to diffract a light beam which arrives normal to the plane of the grating by 0, the other by 0' (these beams are referenced 14 and 14' respectively). The beams 8 and 14 are focussed by the lens 15 and window 17 onto the photodetector 18, and the beams 8' and 14' are focussed onto a further photodetector 18' by a lens 15' and window 17' having an aperture 16'.The output from each photodetector 18, 18' is fed to a respective phase meter 19, 19' where comparison is made with the phase of the reference R.F. energy sources 3, 3' to derive fm and '. The outputs of the phase meters 19, 19' are then applied to a comparator 21 to compare the relative changes in phase between fm and fmt and hence to provide an overall indication of the distance moved by the grating 11. The present invention may also be utilised to compare the profiles of machined parts: a device for performing this is shown in Figures 3 and 4. The mechanical grating 11 is divided into two identical, independently, movable structures 111 and 211 mounted between linear bearings 130 and 230 respectively with the plane of movement of the gratings 111 and 211 being the same as for the grating 11 shown in Figures 1 and 2. The gratings 111 and 211 have rigid probes 131 and 231 respectively connected to their undersides (as shown in Figure 4) which rest under gravity on one of two surfaces 132 and 232 respectively to be compared. The operation of the Figure 3 embodiment is basically similar to that shown in Figure 1; the diffracted beams emerging from each grating at an angle 0 to the normal are for grating 111 denoted as 108 and 114; and for grating 211 denoted 208 and 214. These pairs of beams are collected separately by similar lenses 115, 215 and passed through respective :perturbs 116 and 216 onto photodetectors 118 and 218 respectively. The photocurrents produced by the photodetectors 118 and 218 have components at frequency t m produced by the R.F. energy source 3 which have phase shifts I, and 02 caused by movement of the gratings 111 and 211 respectively. These photo currents are compared in a phase meter 119 which consequently produces an output proportional to 0 2. and. therefore, proportional to the difference in movement of the two gratings Such an arrangement has the advantage that temperature effects in the acousto optic modulator arising from the temperature dependence of acoustic velocity in the modulation material are common to both sets of beams on the respective photodetectors, and so have no effect on the differential output of the phasemeter 119. The two gratings are moved across the surfaces 132, 232 in the direction indicated by double-arrow-headed line B, and the probes may rest on the surfaces either by gravity (as described above) or by some externally applied force. The diffraction grating utilised in the embodiment of Figure 1 is manufactured employing the apparatus shown in Figure 5. A laser 1 emits a parallel beam of light which is split into two substantially equal parts by a beam splitter 50. The split beams are reflected from plane mirrors 51, 52 respectively, and are expanded by substantially identical lenses 53, 54 respectively. The expanded beams 55, 56 are positioned to illuminate the same area of a recording plate 57 consisting of a homogenous, transparent, optical member (an optically flat plate of glass) which is coated with a light-sensitive photo-resist layer 58 on the illuminated side thereof. The plate 57 is equidistant from the lenses 53, 54, and oriented so that the median rays of the beams 55, 56 are symmetrically positioned about the normal to the surface of the plate 57. The combination of the beams 55, 56 forms a static interference pattern to which the photo-sensitive surface is exposed, and which is in the form of the required grating. The period of the grating is adjusted by altering the angle between the median rays of the beams 55, 56; to make the grating period substantially linear over the surface of the plate 57, the projection distance measured perpendicularly from the surface of the plate 57 to the lenses 53, 54 is made large. After exposure, the plate 57 is developed in suitable chemicals to produce the required absorption or phase grating. To produce the grating required in the embodiment of Figure 2 (where two series of grating lines are required), a third beam is prolected onto the plate 57 so that two interference patterns are formed thereon. WHAT WE CLAIM IS:

1. A length measuring device comprising: 1) arranged sequentially along a light path a) a monochromatic light source, b) an acousto-optical modulator acting as a diffraction grating and arranged to give both an undiffracted beam and at least one beam which is diffracted bv a predetermined angle (and whi h is therefore frequencv-modulated by

the modulator's generator frequency), c) a mechanical diffraction grating aligned with the aeousto-optlcal modulator. this mechanicai grating having a pitch arranged t & diffract by the predetermined angle light incident thereon substantially normal to the plane thereof, and being movable in a direction across the light path perpendicular to its grating lines (so as to phase-shift any beam diffracted thereby by an amount proportional to the amount of movement), and d) focussing means whereby both diffracted and undiffracted light from the mechanical grating is focussed on to e) a photodetector the output of which is proportional to its input (and is thus an amplitude-modulated signal having a frequency the same as, but a phase which may be different to that of, the acousto-optical modulator's generator frequency and phase respectively), together with 2) phase comparator means for measuring and comparing the frequency and phase of the photodetector's output with respect to the acousto-optical modulator's generator frequency and phase, the phase difference being proportional to and indicative of the mechanical grating's amount of movement.

2. A measuring device as claimed in claim I, wherein the light source is a gas discharge lamp, a light-emitting diode, or a laser.

7 A measuring device as claimed in either of the preceding claims, wherein the generator for the acousto-optical modulator is a radio frequency generator.

4. A measuring device as claimed in any of the preceding claims, wherein the mechanical grating is an interference grating.

5. A measuring device as claimed in any of the preceding claims, wherein the focussing means is a lens with an aperture at the principle focus thereof dimensioned to reject those light beams not travelling at the appropriate predetermined angle.

6 A measuring device as claimed in any of the preceding claims, wherein there is provided an up/down counter for determining the number of complete grating line periods moved by displacement of the mechanical grating.

7 A measuring device as claimed in any of claims I to 5. which Rddltionalh comprises, arranged sequentially along the light path. a second awousto-optical modulator of A different generator frequency to that of the first modulator together with an associated additional mechanical diffraction grating and an associated focussing means, photodetector and phasemeter, the device including additional comparison means provided to compare the relative changes in the phasemeter outputs so that, by suitably selecting the two generator frequencies, there is achieved a non-ambiguous result for the mechanical grating movement.

8. A measuring device as claimed in claim 7, wherein there is employed only one acousto-optical modulator using two generators of different frequencies, and the two mechanical gratings are a single grating support upon which both series of grating lines are superposed.

9. A measurement device as claimed in any of the preceding claims and for use in comparing the profiles of two objects; wherein there are two independentlymovable mechanical gratings each of which is arranged to transmit diffracted light to respective focussing means, both focussing means having an associated photodetector, such that in operation the outputs from the two photodetectors are applied separately to a phase comparator which produces a signal representative of the difference in displacement of the two mechanical gratings.

10. A distance measuring device substantially as illustrated in and described with reference to Figure 1 of the drawings accompanying the Provisional Specification.

I 1. A distance measuring device substantially as illustrated in and described with reference to Figure 2 of the drawings accompanying the Provisional Specification.

12. A distance measuring device substantially as illustrated in and described with reference to Figures 3 and 4 of the drawings accompanying the Provisional Specification.