CA1163094A

CA1163094A - Interferometer

Info

Publication number: CA1163094A
Application number: CA000385900A
Authority: CA
Inventors: Klaas Compaan
Original assignee: Philips Gloeilampenfabrieken NV
Current assignee: Koninklijke Philips NV
Priority date: 1980-09-22
Filing date: 1981-09-15
Publication date: 1984-03-06
Also published as: FR2490808B1; IT1139451B; SE8105526L; JPS63100626A; GB2084315B; JPS5786007A; DE3137211C2; DE3137211A1; IT8124043A0; GB2084315A; AU7546681A; FR2490808A1; AU551571B2; NL8005258A

Abstract

PHN.9846 15 1.9.1981 ABSTRACT:
"Interferometer".

An interferometer for various uses is described.
In the interferometer an interference line pattern is formed at the location of a multiple photo-cell, whose individual photo-diodes are consecutively connected to an electronic signal-processing circuit by an electronic switch. Changes in the measuring arm of the interferometer can be measured with high accuracy.

Description

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"~nterferometer"

The invention relates to an interferometer - comprising a radiation source, which produces a radiation beam, a beam splitter for deriving a first and a second subbeam from the radiation beam, which two subbeams, af-ter the first subbeam has been incident on the surface - of an object to be examined are made to coincide in the plane of a radiation-sensitive detection s~s-tem.
Such an interferometer, used as a displacement measuring device is known, for example -from "Philips l0 Technical Review", 30, No. 6/7, pages 160-l65. In the path of each of -the subbeams there is arranged a reflector.
One of these reflectors is arranged stationarily9 whilst ` the second one ls fi~edly connected to the object whose displacement is to be measured. Upon reflection of the two subbeams they are recombined by the beam splitter, said beams, which have covered substantially the same pathlengths, interfering with each other. The resulting intensity depends on the relative phase of the subbeams and will therefore vary periodically if the optical path-length of the second subbeam increases or decreases continuously owing to a displacement of the object. One period of the interference pattern corresponds to a path-length variation of the magnitude of a half wavelength of the light being used. By means of a radiation-sensitive detector a periodic electric signal can be obtained, whose number of periods, which is a measure of the dis-; placement of the object, can be counted.
The ar-ticle in "Philips Technical Review" 30, No. c>/7, pages 160-165 describes a special interferonleter with excellent properties, such as an accuracy down to a fraction Or 1/um, simple digital clisplay and discrimi-nation between forward and reverse displacements. 1-Iot~ever, .~

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said properties can only be ob-tained if use is macle of a special frequency-stabilised laser source, which produces a laser beam with two oppositely circularly polarised components of equal intensity bu-t different frequencles.
It is -the object of the present invention to provide an interferometer which is of substantially simpler construction, but which has substantially the same good properties. The interferometer according to the invention is characterized in that the radiation-sensitive detection system is constituted by a multiple photo-cell, comprising a linear array of pho-todiodes, which are sequentially connected to an electronic circuit for processing the signal produced in -the photodiodes by means of an electronic switch, so that the detection system functions as a moving grating-like de-tector~ the grating period of the multiple photo-cell corresponding to the period of the interference line pattern of the two superimposed subbeams.
The invention uti:Lizes the fact that in an interferometer a sinusoidal :interference pat-tern can be produced and that the intens:ity variation in the inter-ference pattern, which variation is caused by a variation in the pathlength of a subbeam, may be regarded as a dis-placement of said intarference pattern. The displacement of such a pattern of light and dark lines, which in its turn may be regarded as a grating, can be determined with the aid o~ a reference grating in the rorm of a multiple photo-cell.
By means of the electronic switch it is achieved that a reference grating apparently "travels"
over the surface of the multiple photo-cell. The radiat-ion-sensitive elements of the multiple photo-cell, the electronic switc:h and the electronic processing circuit 5 may be integra-ted on one chip of a semiconductor [naterial.
The said multiple photo-cell is described in United States Patent Specification No. 3,~73,11~. In 3 0 9 l~

accordance with said Paten-t Specification the displace-rnent of an object can be measured by projecting a first grating, which is connected to the object, on a reference grating constituted by the multiple photo-cell. ~he accuracy of this displacement measurement is determined by the grating period of the first grating. This grating period is for example 625/um. By suitable signal processing and interpolation within the signal period a displacement measurement with an accuracy down to in principle O.5/um is possible.
In the interferometer in accordance with the invention the periodicity in the electric signal depends on a periodicity of half the wavelength of the radia-tion used instead of on a periodic structure having a period of 635/um. Thus, using the same multiple photo-cell, a substantially higher accuracy can be obtained than attainable with the displacement meter in accordance with United States Patent Specification No. 31973~119.
The invention will now be described in more detail with reference to the drawing. In the drawing:
Figure 1 shows a first embodiment of an inter-feronieter in accordance with -the invention, Figure 2 shows the interference pa-ttern ~`ormed ; in said interferometer and a grating, Figures 3 and ~ sho-~ t-~o other embodiments o~
the interferometer, `~ ~igure 5 is a block diagram of the circuit arrangement employed in said in-terferometers, ~igure 6 shows a device for determining -the linearity of the movement of an object providecl with an interferometer in accordance with the invent:ion, and Figure 7 shows an optical writing apparatus provided with an interferometer in accordance ~ith -the invention.
In the interferometer shown in Figure 1 the radia-tion source 1 emits a beam Z. Depen~ling on the use of the interferometer, said beam has a srnaller or greater .

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coherence length. For measuring displacernents over longer distances the beam should have a greater coherence length.
The source 1 is then a laser, ~or example a helium-neon laser. ~y means o~ the beam splitter 3 a part of beam 2 is reflected to a stationary re~erence mirror 4 as a subbeam a. The subbeam b which is transmitted by beam splitter is incident on a second mirror 5, which is arranged on or secured to an objec-t 6, or is a part o~
said object whose displacement is to be determined.
Of the beam a which is reflected by the mirror 4 a part (a~) is transmitted by the beam splitter 3, whilst a part (b') of the beam b, which is re~lected by the mirror 5, is re~lected by the beam splitter. The beams a' and b~ then ~orm an inter~erence pattern I. The intensi-ty of the intarference pattern depends on the relative phase of the subbeams a~ and b'. Consequently, -this intensity ~; will vary periodically if the op-tical pathlength of the subbeam b increases or decreases owing to a displacement of the object 6.
In known interferometers the intensity of the ~-~ inter~erence pattern is measured locally by means of a radiation-sensitive detector, which is for example arranged on the optical axis of the system. I~ the object 6 moves, a periodic or pulse-shaped signal is produced on the output o~ the detector. By counting a number of pulses the magnitude of the displacement can be determined.
In the interferometer in accordance with the invention steps are taken to ensure that the subbeams a~ and b' make a small an~le with each other. This is possible, as is shown in Figure 1, by arranging -the mirror ` 5 at an angle relative to the beam b which differs slightly -from 9O. This results in an interference pattern I with a spatial intensity distribution, which pattern is schematically represented in Figure 1. The interfererlce lines are perpendicular to the plane of drawing in Figure 1.
Figure ~ again shows the interference pattern, but now in plan view.

A

If the beams a ancl b traverse equal optical pathlengths the intensity in a point c1, for example a point on the optical axis o~ the system, is maximum, and so is the intensity in points 2 and C3~ whilst S the intensity in points d1 and d2 is minimum. The inter-ference pattern is then as represented by the curve 11.
If the object 6 is displaced, the intensity in points C1, C2 and C3 will clecrease and that in points d1 and d2 will increase. If, for example, the object has moved over a 1n distance equal to a quarter wavelength of the beam ~, the intensity in points C1, c2 and C3 will be minimum and that in points d1 and d2 maximum. The interference pattern will then be as represented by the curve 12.
In the interferometer in acco-rdance with the lS invention use is made of the fact that the change of the intensity distribution may be regarded as "travelling"
of the interference pattern relative to point C1, c2, C3, d1~ d2- Furthermore, the interference pattern itself may be regarded as a grating with gradual transitions - 20 from the light to the clark grating stripes. In the inter-ferometer now proposed the displacement of the interfe-rence pattern, and -thus the displacement of the objact, is determined with the aid of a reference grating ~, which is arranged in the plane of the inter~erence pattern.
Use can -then be made of techniques employed in known measurement systems, in which two physical gratings move relative to each other. Since the period o~ a physical grating, which period is for example a few hundreds of /um, is now replaced by a period equal to hal~ a wavelengtll o~
30 the radia-tion used, ~or example 0.3164/um for a helium-neon laser, substantially smaller displacements can be measured than with a gra-ting measuremen-t system.
As is shown in l~igure 1, a moving reference grating can be formed with the aid of a mul-tiple photo-cell comprising a linear array of substantially identical photo-diodes, which are sequentially connec-tecl to an electronic processing circui-t by means o~ an electronic switch. This :

1 ~ 63~4 interferometer uses neither a reference grating nor any mo~ing parts ~or imparting a uni~orm motion to said grating, so that this interferometer i5 0~ a simple con-struction and is highly vibration-proof.
Figure 3 shows a second embodiment o~ an inter-ferometer in accordance with the invention. ~here the embodiment shown- in ~igure 1 employs a semitransparent mirror 3 as beam splitter, the embodiment shown in Fig. 3 employs a prism o~ special shape as beam splitter. Said lO prism may be thought o~ as comprising a normal semi-transparent prism, represented by dashed lines in Fig. 3, with a semi-reflecting sur~ace 15, but on which now a second ~ully re~lecting sur~ace 16 and a third semi-reflection sur~ace 17 are ground. By a suitable choice l5 of the angle between the sur~aces l6 and 17 it can be achieved that the re~lected subbeams a' and bt make a small angle with each other.
In this embodiment the reflecting elements are . .
so-called retro-reflectors 20 and 21. Such elemen-ts may 20 be cons-tituted by a prism having three re~lecting sur-faces which are perpendicular to each other, a so-called "corner-cube" prism. ~ beam which is consecuti~ely re~lected by said three surfaces has the same direction as the beam which enters the prism, regardless o~ the ; 25 angular position of the prisrn. Consequently, an ali~nment m of said angular position is not necessary. ~ similar e~ect can be obtained with a so-called "cats' eye"
mirror system, comprising a lens and a mirror arranged in the ~ocal plane o~ said lens.
Figure 4 shows an embodiment of -the inter~ero-meter in which a l~ollaston prisT~ 26 is empLoved in order to introduce a small angle between -the subbeams a' and b' which inter~ere with each other. In this embodiMen-t the beam splitter is a polarisation-sensitiYe splitting prism 22 with a polarisation-sensitive surface 23. The beam 2 now comprises two components which are polarised perpendicularly to each other, o~ which one component a - ~ 3 G30~

is reflected by the surface 23, whilst the o-ther com-ponent is transmitted by the surface 23 There may be pro-vided a polariser ~8 in order to adapt the direction of polarisation of the beam produced by the radia-tion source 1. The subbeam a passes through a ~/4 pla-te 24, is reflected by -the element 20 and -then traverses the ~/4 plate 24 again. The direction of polarisa-tion is then rotation through 90 in to-tal, so that the subbeam -- a~ is transmitted by the surface 23. The component of ` lO the beam 2 which is transmitted by the surface 23 tra-verses the ~/4 plate 25 two times and is then reflected by the surface 23. The coincident subbeams a~ and bl, which have directions of polarisation which are perpendicular to each other~ pass through a l~ollaston prism, which deflects lS the beam depending on their directions of polarisation.
The subbeams emerging from the prism then make a small angle with each other. After the subbeams have passed through an analyser, they can form an interference pattern with a spatial intensity-distribution at the location of ~` 20 the detector, the multiple photo-cell -~9.
In comparison with a semitransparent mirror, ~ a polarisation-sensitive beam splitter has the advantage ; that in principle no racliation is lost in the process of beam splitting and recomb:ining the subbeams. In the 25 interferometer shown in Figure 3 the surfaces 15 and 17 may alterna-tively be polarisation-sensitive surfaces. In that case ~/4 plates 18 and 19 should be arranged in the paths of the subbeams a and b and an analyser 27 bet~een the prism 14 and the detector 29.
l~hen a polarisation-sensitive beam splitter is employed steps must be taken to preclude additional polarisation changes during reflections on the surfaces of the reflecting prisms. For this purpose layers of silver may be provided on the reflecting surfaces of said prisms.
As stated previously, the interferometer in accordance with the invention employs a multiple photo-" ~ 1 630~1 cell as detection system.
Figure 5 shows a front view of the mul-tiple photo-cell 29 and a block diagram of the circuit. The photo-cell comprises a comparatively great number of photo-sensitive elements, such as photodiodes 30, ~hich are divided into a comparatively small number of groups.
~ach group consequently comprises a comparatively great number of photodiodes. Each photodiode of a group corresponds to a period, one light and one dark line, of the interference pattern I. As a result of this, a number of periods of the interference pa-ttern is scanned ; equal to the number of photodiodes in a group. The numbar of photodiodes per period of the interference pattern should be as great as possible for an optimum electrical reproduction of the optical signal. On the other hand, an as large as possible part of the interference pattern should be scanned.
In one version of the multiple photo-cell -the number of photodiodes was 200 and the length of each photodiode 1.8 mm. The width of each photodiode was 10/um, and -the spacing of the photodiodes also 10/um. The number of photodiodes per period of the interfe~ence pattern was 10, so that -the field of view covered 20 periods of the interference pattern. Correspo-nding photo-diodes o~ each set of 10 consecutive photodlodes ~ere interconnected, so that there were 10 groups of 20 photo-diodes each.
A stationary grating with a black-white ratio of 1 : 1 in the area of the multiple photo-cell 29 is simulated by activating five consecutive groups of photo-diodes (five groups of 20 photodiodes in the present em-bodiment). A travelling grating is obtained l~hen the set of five groups each time advances by one group.
In -the processing circuit, ~hich is block-schematically represented in ~igure 5, the clock pu:Lses 32, generated in the clock-pulse generator 31, are supplied -to a divider 33 and a clivider 34. The cli~icler 33 ~ IT ~ 3 ~ ~ ~

supplies pulses 35, which drive a ring coun-ter 36. The multiple photo-cell 29 ls activated by the ring counter 36 and supplies the measuring signal 37. The divider 3L~
supplies pulses 38 (generally of another repetition fre-quency than -the control pulses 35 from the divider 34), . which constitute the reference signal. In the buffer counter 39 the measuring signal 37 and -the reference pulses 38 are compared with each o-ther. The output pulses ; of -the buffer 39 are .for example applied to an indicator.
The ring counter 36 activates the consecutive groups of photodiodes of the multiple photo-cell 29, so that in effect a grating is obtained which travels ~1 over the sur~ace of the pholto-cell ~ with a constant :. velocity. The period of the grating is equal -to the period of the interference pa-ttern I. If the interference pattern is stationary relative to the photo-cell 29, the measuring signal has a constant ~requency. If the inter-ference pattern moves in the same direc-tion as the apparent grating activated by the ring counter 36, the frequency of the measuring signal 37 will decrease, ~ihilst if it travels in -the opposite direction the frequency of`
the measuring signal 37 will increase. Thus, -the direction and magnitude of the displacement o~ the interference pattern, and thus the displacement of the object 6, can be determined.
Within a range of one period of -the interference pattern I the position of the multiple photo-cell 29 re-lative to the interference pattern can be determined in an absolute manner by measuring the phase c1ifference bet~een the measuring signal 37 and the reset signa:l of the ring counter 36. The ring counter 36 is to be rese-t upon every start of a measurement in order to guarantee -that the counter 39 starts in a specific initial pOSitiOIl.
~Iowever, the circuit arrangement becomes simpler and more reliable if the ring counter 36 is reset upon each period. The reset signal is obtained by dividing the pulses 35 in -the divider 4O. The frequency of the `~ 3 ~309~

reset pulses is selected to be equal to the nominal frequency of the measuring signal 36.
lrhen the multiple photo-cell and processing circuit shown in Figure 5 are employed in a gra-ting measuring system in which the measuring grating has a period of 635/um, it is in principle possible to detect displacements of the measuring grating down to O.5/um.
By employing the multiple photo-cell and the circuit in accordance with the invention in an interferometer using ,lO a helium-neon laser beam with a wavelength of 0.6328/um ; it is in principle possible to detect the displacements down to ~ x O.5/um = 0.25 nm.
The displacement measuring device described in the foregoing can be used in all cases where small displacements have to be measured accurately, such as in machine tools, for example lathes, for measuring slide and shaft notions. An example of this is the numerically controlled lathe mentioned in "Optics Le-tters", ~ol. 14, no. 2, pages 7O-72, by means of which bi-aspherical objective lenses, that is lenses having two aspherical surfaces, can be manufacturecl.
The interferometer in accordance with the in-v~ntion may also be used for measuring the linearity of the movement of an object. For this use, as is shown in Figure 6, the radiation source l, the beam splitter 3, the reference mir~ror 4 and the radiation~sensitivity detection system ~ are all accommodated in one housing 41. This housing~ whose dimensions can be small, is moved wi-th the same velocity as the object 6 in the direction of the arrow 46. The movement of the object in this direction will not cause any change in the interference pattern. However, if the object moves obliquely relative to the arrow 46, the reflected subbeam b' will move rela-tive to the subbeam a~, so that the distribution within the interference pattern changes. The in-terference pattern -then begins to "travel" relative to the radiation-sensi--tive detection system. This movement can be measured 3~94 by counting the number of periods in the output signal of the detection system 29.
For a simultaneous movement of the housing 41 and the object 6 this housing may be mounted on the slide 43 by means of which the object is moved. I-t is alter-natively possible, as is shown in Figure 6, to provide the housing ~1 with separate drive mens 42 which via the connection 45 are energized by the motor 44 which drives the object slide.
In recent years there have been significant developments in the field of optically readable record carriers. On these record carriers a large amount of information, such as video- and/or audio information or digital information is stored, the information details having dimensions of the order of 1/um or smaller. In an apparatus for inscribing said record carriers an interferometer in accordance with the invention may be used for controlling the movement of the ~ri-te head transverse of the tracks, which movement may be very slo~
especially when audio information is recorded.
Figure 7 schematically represents such an apparatus. The record carrier 50 to be inscribed is placed on a table 51, which can be rotated by means of a motor 52. The write head 53 contains a laser L~4, ~hoje Z5 beam 61 is directed at the record carrier via the mirrors 55, 56 and 57, said beam being focused to a small write spot by means of an objective 58. The information to be recorded is applied to the terminals 60 of a modulator 59, which modulates the intensity of the beam in accordance with the information to be recorded. On the wall of the write head 53 a reflecting prism 6~ is arranged, which prism is accommodated in the measuring arm of an intcrfero-meter. This interferometer further comprises a splitting prism 62 and a second reflecting prlsrn 63. The beains a' and b' reflected by -the reflecting prisms have the same direc-tions as and are slightly shifted relative to the beams a and b incident on the prisrns. ~ ~edge 65 sliglltly ~ ~ 6309~

deflects the beam b~, so that the beams which emerge from the prism 6~ make a small angle with each o-ther.
Besides in displacemen-t measuring devices, and obviously also in velocity meters, in which the number of periods per unit of time of the measured signal is determined, the invention may be used in all cases where interferome-ters can be used. ~xamples of this are surface roughness meters, de-vices for measuring extremely small magnetostricti~e or electros-trictive effects, etc.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An interferometer, comprising a radiation source, which produces a radiation beam, a beam splitter for deriving a first and a second subbeam from the radiation beam, which two subbeams, after the first subbeam has been incident on the surface of an object to be examined, are made to coincide in the plane of a radiation-sensitive detection system, characterized in that the radiation-sensitive detection system is constituted by a multiple photo-cell, comprising a linear array of photo-diodes, which are sequentially connected to an electronic circuit for processing the signal produced in the photo-diodes by means of an electronic switch, so that the de-tection system functions as a moving grating-like de-tector, the grating period of the multiple photo-cell corresponding to the period of the interference line pattern of the two superimposed subbeams.

2. A device as claimed in Claim 1, characterized in that the photodiodes are divided into a plurality of groups and corresponding photo-cells in each group are electrically interconnected.

3. A device as claimed in Claim 1, characterized in that the groups of photo-cells are sequentially activated by a ring counter, which is controlled by clock pulses derived from a pulse generator, whilst a counting device receives both the signal produced in the photo-cells and pulse signals derived from the pulse generator, in which counting device said signals are compared with each other.

4. A device as claimed in Claim 3, characterized in that reset pulses are applied to the ring counter, which pulses are obtained by division of the clock pulses, said former pulses also being applied to the counting device.

5. A device for determining the linearity of the movement of an object, comprising an interferometer as claimed in Claim 1, 2 or 3, characterized in that the radiation source, the beam splitter, a reference reflector and the radiation-sensitive detection system are accommodated in a housing which is provided with means for moving said housing with the same velocity as the object.

6. An apparatus for recording information in a track-shaped information. structure on a record carrier with the aid of optical radiation, which apparatus com-prises a write head, which accommodates a radiation source and a modulator, a supporting system for the record carrier, the write head and the supporting system being arranged to be movable relative to each other, and an interferometer as claimed in Claim 1, 2 or 3, for determining the displacement of the write head in a direction transverse to the direction of the information tracks to be inscribed.