FR2490808A1 - INTERFEROMETER AND DEVICE COMPRISING THIS INTERFEROMETER - Google Patents

Abstract

THIS IS AN INTERFEROMETER FOR MULTIPLE APPLICATIONS. IN THE INTERFEROMETER, AN INTERFERENCE FIGURE IS SHAPED ON A PHOTOCELL, THE SEPARATE PHOTODIODES OF WHICH ARE SUCCESSIVELY CONNECTED BY AN ELECTRONIC SWITCH TO AN ELECTRONIC SIGNAL PROCESSING CIRCUIT. VARIATIONS OCCURRING IN THE MEASURING ARMS OF THE INTERFEROMETER CAN BE MEASURED PRECISELY. APPLICATION TO THE MEASUREMENT OF VERY SMALL DISPLACEMENTS.

Description

"Interferometer and device comprising this interferometer"

  The invention relates to a combined interferometer

  carrying a radiation source providing a beam

  of beams, a beam splitter serving to form a first

  first partial beam and a second partial beam from the beam of rays, these two partial beams

  being brought into coincidence on the radiation-sensitive surface

  detection system, after the first

  partial seal has been in contact with a face of the object to be examined. Such an interferometer, which is used for displacement measurement, is known, among other things, for

  "Philips' Technical Review" 30 N 6/7, pages 160 to 165.

  In the path of each of the partial beams is inserted a reflector. One of these reflectors is fixedly arranged, while the second is rigidly connected to the object, whose displacement must be measured. After reflection of the two partial beams, they are joined by the beam splitter so that these beams, which have traveled paths of approximately the same length interfere with each other. The intensity thus obtained is a function of the relative phase of the partial beams and therefore varies periodically when the length of the optical path of the

  second partial beam increases or decreases con-

  nude as a result of moving the object. A period of the

  figure of interference corresponds to a variation in the length

  distance of a magnitude equal to half the length

  the wave of light used. A radiation sensitive detector provides a periodic electrical signal. The number of periods of this signal, which constitute a measure

  of displacement, of the object can be counted.

  The article in "Philips' Technical Review" Ne 6/7, pages 160 to 165 discloses a special interferometer having very good performance, such as a precision of a fraction of 1 μm, a simple digital indication and

  the distinction between moving ahead and

  Rière. However, these performances can be obtained only in the case of using a special stabilized frequency laser source providing a laser beam comprising two components of opposite circular polarization,

  of the same intensity, but of different frequencies.

  The present invention aims to provide a

  terferometer, whose structure is essentially

  simple, but which has about the same performance.

  The interferometer according to the invention is characterized in that the radiation-sensitive detection system is a multiple photocell constituted by a row of linear photodiodes, which are successively connected by an electronic switch, to an electronic circuit designed to process the signal. provided by the photodiodes,

  so that the detection system functions as a de-

  frame-shaped mobile detector, the period of the frame of the multiple photocell corresponding to the period of

  the line-shaped interference figure of the two

overlapping partial seals.

  The invention is based on the fact that

  sinusoidal interference pattern to be formed in a

  interferometer and that the variation of intensity occurs

  as a result of the variation in the length of the partial beam path in the interference pattern may be

  regarded as a displacement of this figure of inter-

  ference. The displacement of such a clear and dark figure, which can be considered in turn as a

  frame, can be determined using a reference frame

  this in the form of a multipole photocell.

  The electronic switch makes it possible to obtain that a reference frame moves, so to speak, on the surface of the multiple photocell. Elements sensitive to the radiation of the multiple photocell, the circuit

  electronics and the electronic processing circuit can

  can be integrated on a plate of semiconduct-

  tor. The aforesaid multiple photocell is described

  in U.S. Patent No. 3,973,119.

  According to this patent, the displacement of an object can be measured by projection of a first frame, connected to the object, on a reference frame constituted by the multiphotocell.

  tiple. The accuracy of this displacement measurement is de-

  terminated by the period of the first frame. This period

  is for example 635 / um. The processing of an appropriate signal

  and interpolation in the signal period allows a measurement of the displacement with, in principle, a precision

0.5 μm.

  In the interferometer according to the invention, the periodicity of the electrical signal is determined, not by

  a structure with a period of 635 / um, but with a structure

  half of the wavelength

  of the radiation used. As a result, in the case of

  of the same multiple photocell, it is possible to

  to dye a precision significantly higher than that obtained

  bare with the device to measure displacement

  according to U.S. Patent No. 3,973,119.

  The description below, with reference to

  drawings, all given as a non-limitative example

  tative, will make clear how the invention can be realized. Figure 1 shows a first form of

  embodiment of an interferometer according to the invention.

  Figure 2 shows the interference figure

  formed by this interferometer, as well as a frame.

  Figures 3 and 4 show two more

  embodiments of the interferometer.

  Figure 5 is a block diagram of the

  cooked used in these interferometers.

  FIG. 6 shows a device for determining the linearity of the displacement of an object and provided with an interferometer according to the invention, and FIG. 7 a recording device.

  optical equipped with an interferometer according to the invention.

  In the interferometer according to FIG.

  radiation source 1 emits a beam 2. Depending on the

  cation of the interferometer, this beam has a smaller or greater length of coherence. In order to measure displacements over greater distances, the beam must have a great length of coherence. In this case, the source 1 is a laser, for example

  a helium-neon laser. Part of the beam 2 is reflected

  The partial beam b transmitted by the beam splitter reaches a second reflector 5, which is applied or fixed to an object 6 or a beam.

  part of that object, the movement of which must be determined

  mine. A part (a1) of the beam reflected by the reflector 4 is transmitted by the beam splitter 3,

  while a part (b ') of the beam b reflected by the re-

  flector 5 is reflected by the beam splitter. The beams a 'and b' thus form an interference figure I. The intensity of the interferenced figure is a function of the relative phase of the partial beams al and b '. This

  intensity varies periodically when the length of the

  optical beam of the partial beam b increases or decreases

following the movement of the object 6.

  In known interferometers, the intensity of the interference pattern is measured locally by means of a radiation sensitive detector 9 which is arranged for example on the optical axis of the system. During a displacement of the object 6, a periodic or pulse-shaped signal is produced at the output of the detector. Counting the number of pulses makes it possible to determine the magnitude of the displacement. In the interferometer according to the invention, it is arranged that the partial beams a 'and b' make a small angle between them. As shown in Figure 1, this is done by ensuring that the reflector 5 makes an angle differing only slightly 90 'of the beam b. So, he

  occurs an interference figure I presenting a re-

  partition of spatial intensity, which figure is schematically represented in FIG. 1. The interference lines are perpendicular to the plane of the drawing of FIG. 1. FIG. 2 represents the interference figure a second

times, now in plan.

  If the beam a and the beam b travel on an optical path of the same length, the intensity, measured

  at a point c1, for example located on the optical axis of the

  is maximal, just like the intensity at points c2 and

  C3, while the intensity at points dl and d2 is minimal.

  The interference pattern is represented by curve 11.

  When the object 6 moves, the intensity at the points c1, c2 and c3 decreases and that at the points d1 and d2 increases. Thus, when the object moves over a distance equal to one quarter of the wavelength of the beam 2, the intensity at points c1, c2 and C3 is minimal and that at points d1 and d2 maximum. Thus, the interference figure presents

the allue indicated by curve 12.

  In the interferometer according to the invention, advantage is taken of the fact that the variation of the intensity distribution can be considered as a "displacement" of the interference pattern with respect to the points c1, c2P C3, d1, d2. Moreover, the interference pattern itself can be considered as a frame presenting transitions

  regular between the light and dark stripes. In the in-

  proposed terferometer, the displacement of the

  and, as a result, the displacement of the object 6, is

  determined using a reference frame 8, which is

  plicated to the plane of the interference pattern. We can shoot

  of the techniques applied to the measuring systems con-

  naked, in which two material frames move relative to each other. Since at present, the period of a hardware frame, which is for example a few hundreds of / um, is replaced by a period of a magnitude equal to half the wavelength of the radiation used, for example 0, 3164 / unl for a helium-neon laser, it is possible to measure displacements significantly smaller than those which can be measured with

  a measurement system with two hardware frames.

  As shown in Figure 1, a frame of

  mobile reference can be done with a photo-

  multiple cell constituted by a row of substantially identical linear photodiodes, which are successively connected by an electronic switch to an electronic processing circuit. In this interferometer are omitted the reference frame and the moving parts used to communicate a uniform movement to the frame, so that this interferometer has a simple structure and resists

i suitably to vibrations.

  Figure 3 shows a second form of

  production of an interferometer according to the invention.

  Whereas, in the embodiment

  1, a semitransparent reflector 3 is used.

  As a beam splitter, the embodiment se-

  Figure 3 uses a special shaped prism as a beam splitter. This prism can be considered as

  formed from a normal semitransparent prism 14 indi-

  dotted in FIG. 3, having a semi-reflective surface 15, prism on which are polished a completely reflective second face 16 and a third semi-reflective face 17. A rigorous choice of the included angle of the faces 16 and 17 allows get the partial beams reflected a 'and b' do a little

angle between them.

  In this embodiment, the reflector elements are retroreflectors 20 and 21. Such elements may be constituted by a prism having

  three reflective faces perpendicular to each other,

  peeled triple reflectors (in English "corner cube" prism).

  A beam reflected successively by these three faces has the same direction as the beam entering the

  prism, regardless of the angular position of the prism.

  An adjustment of this angular position is therefore not necessary. The same effect can be obtained with a reflective system called cat's eye, in English "cat's eye", consisting of a lens and a reflector arranged in the

focal plane of the latter.

  Figure 4 represents a form of realization

  interferometer, in which a Wollaston prism 26 is used to introduce a small angle between the partial beams a 'and bt which interfere. In this embodiment, the beam splitter is a polarization-sensitive divider prism 22 having a polarization-sensitive separation face 23. The

  beam 2 has two components polarized perpendicularly

  between them, one of which is reflected by the

  face 23, while the other is transmitted by the face 23.

  Optionally, a polarizer 28 is provided for adapting

  tation of the beam polarization direction provided

  by the radiation source 1. The partial beam has

  pours a * V4 blade 24, is reflected by the element 20 and

  then crosses again the blade N / 4 24. Thus, the direction

  The polarization portion is wholly rotated by 900, so that the partial beam a 'is transmitted by the face 23. The component of the beam 2 transmitted by the face 23 passes through a blade twice and is reflected. over there

  23. The coinciding partial beams a 'and b', pre-

  Sensing directions of polarization perpendicular to each other, pass through a Wollaston prism which deflects the beams in their direction of polarization. The partial beams leaving the prism thus make a small angle between them. After passing through an analyzer, the partial beams can form an interference pattern

  with a spatial intensity distribution at the location

  of the detector the multiple photocell 29.

  Compared to a divider reflector

  semitransparent, a beam splitter that is sensitive to

  larization has the advantage that no radiation is lost in principle during beam splitting and during

  meeting of partial beams. In the interferometer se-

  In FIG. 3, the faces 15 and 17 may also be polarization-sensitive separation faces. In this case, blades at 18 and 19 must be inserted in the path of the partial beams a and b and an analyzer 27.

  between the prism and the detector 20.

  In the case of using a polarization-sensitive beam splitter, care must be taken

  that there is no change in polarization ad-

  during the reflection by the faces of the reflective prisms. For this purpose, films can be applied

  of silver on the reflective faces of these prisms.

  As already mentioned above, the

  terferometer according to the invention comprises a photocel-

  multiple lule as detection system.

  Figure 5 shows a front view of the multiple photocell 29, as well as a block diagram of the circuit. The photocell is composed of a large number of photosensitive elements, such as photodiodes 30,

  which are arranged in a small number of groups.

  Each group therefore has a fairly large number of photo-

  diodes. Each photodiode in a group corresponds to one

  riode, a clear line and a dark line, of the figure

  Therefore, the number of periods explo-

  of the interference figure is equal to that of the photo-

  diodes in a group. The number of photodiodes per period

  the interference figure must be as high as pos-

  sible to obtain the most accurate electrical reproduction of the optical signal. On the other hand, as much as possible of the interference figure must

to be explored.

  In one embodiment of the photo-

  multiple cell, the number of photodiodes was 200 and

  the length of each photodiode was 1.8 mm. The

  The size of each photodiode was 10 μm and the photodiode spacing was also 10 μm. The number of photodiodes per period of the interference pattern was 10,

  so that the total field consisted of 20 periods of

  interference. Photodiodes corresponding to

  each set of 10 successive photodiodes were intercon-

  nected, which implies the presence of 10 groups

each having 20 photodiodes.

  A fixed frame with a black ratio

  white 1: 1 at the surface of the multiple photocell 30

  is simulated by activation of 5 successive groups of

  todiodes (five groups of 20 photodiodes in the exemplary embodiment). A mobile frame is formed when the set

  five groups advance one position each time.

  In the illustrated processing circuit

  in the form of the block diagram in Figure 5, the im-

  clock pulses 32 generated by the pulse generator

  31 are fed to a divider 33 and a divider 34.

  The divider 33 provides pulses 35, which control a

  cylinder counter 36. The multiple photocell 20 is ac-

  by the cylinder counter 36 and produces the measurement signal 37. The divider 34 provides pulses 38 (most often of a different repetition rate than the

  control pulses 35 from the divider 33 which forms

  the reference signal). In the buffer counter 39 are compared the measurement signal 37 and the reference pulses 38. Thus, output pulses of the

  counter 39 are brought for example to an indicator.

  The cylinder counter 36 ensures the activation

  successive groups of photodiodes in the cell

  photoelectric array 29, so that a frame

  place so to speak at constant speed on the surface of the photocell 20. The period of the frame is equal to

  that of the interference figure I. At the end of the fi

  In view of interference with photocell 29, the measurement signal has a constant frequency. If the interference pattern moves in the same direction as the apparent frame activated by the cylinder counter 36, the frequency of the measurement signal 37 decreases, whereas in the case of a displacement in the opposite direction, the frequency of the signal measurement 37 increases. Thus, it is possible to

  determine the direction and magnitude of the movement of the

  interference and, as a result, of

the object 6.

  In a period of the interference pattern

  I, the position of the multiple photocell 29 can be determined

  completed in absolute terms in relation to the figure of inter-

  by measuring the phase difference between the measurement signal

  37 and the reset signal of the & cylinder counter 36.

  In fact, the cylinder counter 36 must be re-established for each beginning of the measurements in order to ensure that the counter

  39 starts counting from a defined initial state.

  The circuit is however simpler and more reliable and the cylinder counter 36 is re-established after each

  period. The recovery signal is produced by division

  pulses 35 in the divider 40. The frequency of the recovery pulses is chosen equal to the

  nominal frequency of the measurement signal 37.

  In the case of using the multiple photocell and the processing circuit shown in FIG. 5 in a frame measuring system, whose frame has a period of 635 μm, it is in principle possible to detect the displacement of the measuring frame up to 0.5 / um. However, the application according to the invention of the multiple photocell and the circuit in an interferometer using a helium-neon laser beam of wavelength

  of 0.6328 / um in principle allows the detection of

o03164

up to x 0.5 μm = 0.25 nm.

  The aforesaid device for measuring the displacement can be used everywhere where it is necessary to strictly measure small displacements, such as towers for measuring movements of carts and shafts. On this subject, one can think of the digital control lathe mentioned in "Optic Letters", Volume 14 N 2,

  pages 70 to 72, which makes it possible to obtain lenses of

  bi-aspheric lenses, which means lenses that

  feeling two non-spherical surfaces.

  The interferometer according to the invention can

  It should also be used to measure the linearity of

  an object. As shown in FIG. 6, for this application, the radiation source 1, the beam splitter 3, the reference reflector 4 and the radiation sensitive detection system 7 are all arranged in a single box 41. This box, whose dimensions may be small, is moved at the same speed as the object 6 in the direction indicated by the arrow 46. The displacement

  in this direction of the object will not result in

  interference pattern. However, when the

  The jet travels obliquely with respect to the arrow 46, the reflected partial beam b 'moves relative to the partial beam a', so that the intensity distribution in the interference pattern undergoes variations. The interference pattern moves relative to the radiation sensitive detection system. This displacement can be measured by measuring the number of periods in the signal of

  output of the detection system 29.

  For a simultaneous movement of the box 41 with the object 6, this box can be mounted on the carriage 43 with which the object is moved. Moreover, as shown in FIG. 6, it is possible to provide the housing 41 with own drive means 42, which are excited by the motor 44 driving the object carriage by

through the link 45.

  In recent years there have been

  important developments in the field of

  optically readable records. In these carriers

  registration is applied a large amount of information

  such as video and / or audio information or information

  digital information, the information elements presenting

  dimensions of the order of 1 or less. In the provisions

  designed for the recording of information in these

  registrars, may be interfered with

  meter according to the invention for the control of

  the recording head perpendicular to the tracks,

  which may be slow, particularly in the case of

recordings of audio information.

  Figure 7 shows schematically such a device. The recording carrier 50, on which information is to be written, rests on a table 51, which is rotated by means of a motor 52. The recording head 53 comprises a laser 54, the beam of which 61 is directed to the recording carrier using the reflectors 55, 56 and 57, this beam being focused by a lens 58 so as to obtain a small recording spot. The information to be recorded is brought to the terminals 60 of a modulator 59, so that the intensity of the beam is modulated in accordance with the information to be recorded. On the wall of the recording head 53 is applied a reflective prism 64, which is inserted into the measuring arm of an interferometer. This interferometer further comprises a divider prism 62 and a second reflector prism 63. The beams a 'and b' reflected by the reflector prisms have the same direction as the

  beams a and b that reach these prisms and they are

  heavily offset from them. A wedge 65 provides a small deflection of the beam b 'so that the beams coming out of the prism 62 make a small angle between them.

  In addition to the devices for measuring

  Of course, the speed counters in which the number of periods per unit of time of the measured signal are determined, the invention can be applied wherever interferometers are used. On this subject, we are thinking

  devices for measuring the roughness of

  devices for measuring magneto-magnetic effects.

  extremely stringent or electro-restrictive, etc.

Claims (6)

CLAIMS:   1. An interferometer comprising a radiation source providing a beam of beams, a beam splitter for forming a first partial beam and a second partial beam from the beam of beams, said two partial beams being brought into coincidence on the radiation-sensitive face of a detection system after the first partial beam has been in contact with a face of the object to be examined, characterized in that the   radiation-sensitive detection system is a photo-   multiple cell (29) consisting of a row of   linear diodes (30), which are successively connected by an electronic switch, to an electronic circuit adapted to process the signal provided by the photodiodes of   so that the detection system works like a de--   frame-shaped mobile detector, the period of the frame of the multiple photocell corresponding to the period of   the line-shaped interference figure of the two overlapping partial seals.   2. Interferometer according to claim 1,   characterized in that the photodiodes (3o) are arranged in   several groups-and the corresponding photocells   to each group are interconnected.   3. Interferometer according to claim 1,   characterized in that the groups of photocells are succes-   activated by a cylinder counter (36) which is controlled by clock pulses (32) taken from a pulse generator (31) while the signal (37) provided by the photocells (30), as well as the pulse signals from the pulse generator are fed to a counter device (39), in which these signals are compared with each other.   4. Interferometer according to claim 3,   characterized in that at the cylinder counter (36) are fed 1 4   recovery impulses, which are formed by   view of the clock pulses (32), these clock pulses   being brought after division, to the counter device (39). 5. Device for determining linearity   movement of an object, including an interferometer   one of the claims 1 to 4, characterized in that   the radiation source (1), the beam splitter (3), a reference reflector (4) and the radiation-sensitive detection system (7) are combined in a housing (41) which is provided with means (42) to move this   boot at the same speed as the object (6).   6. Device for recording information   by means of optical radiation in a structure   information in the form of a track in a carrier of   registration, which includes a recording head   (53) in which are incorporated a radiation source (54) and a modulator (59), a carrier system   (51) for the recording wearer (50), the head d2en-   registration and carrier systems being   in a manner that is movable with respect to each other and   a radioometer according to one of claims 1 to 5, in order to   determining the displacement of the recording head (53) in a direction perpendicular to the direction of the tracks to provide information.