GB2107079A - Improvements in or relating to interferometers - Google Patents

Abstract

An interferometer which is less susceptible to tilt than usual comprises a monochromatic light source (1) emitting a light beam (13) which passes through a first quarter wave plate (2) and is incident on a first beam-splitting means (5). The latter means (5) produces two partial beams, namely a reference beam (15) and a measuring beam (16). First and second beam reflecting means (6, 7) reflect the reference and measuring beams back to the beam-splitting means (5) where there interfere. The interfering measuring beam and reflecting beam, which constitute a first homologous beam couple, are directed by the first beam-splitting means (5) to a second beam-splitting means (8) which splits them into a second, deviated homologous beam couple (15'', 16'') and a third homologous beam couple (15', 16') which are detected by respective photodetectors (12, 11). The active photo-electric faces of the photodetector (12, 11) are aligned relative to the homologous beam couples (15'', 16'' and 15', 16') respectively and are displaceable. <IMAGE>

Description

SPECIFICATION Improvements in and relating to interferometers The present invention is concerned with interferometers and is particularly concerned with an interferometer which is unresponsive to tilt and which is particularly for use in all those cases where the technical and physical size to be measured effects a variation of the optical phase difference. This is the case with, for example, all length measurements; it is also relevant with measurements of the refractive index, of pressure, the composition of gases; or with the measurement of force, as far as its effect causes a variation of the geometric dimensions of a body.

In particular the invention permits an examination of components used in measuring operations which have well-reflecting surfaces, for example, all optical components, such as lenses, prisms, reflectors with respect of smoothness, or in which the measuring beam of the interferometer sweeps optically noncontacting and pointwise any other desired measuring component directly. With the development of laser techniques, many different interferometers have become known. These interferometers have in common that they include photo-electrical detectors and subsequent units which permit an automatic and correctly signed registration of variations of the optical phase differences.

This feature of the interferometer can be obtained in two ways: (1) The interferometer produces an interference image of small order of distance at the location of the photo-electric detectors, and the photodetectors are so arranged geometrically in this interference image that the electrical output signals delivered by them in the event of variations of the measuring size are 900 degree phase-shifted.

This phase-shift is necessary for a correctly signed automatic registration of the variations of the phase differences.

(2) The interferometer produces an interference image of wide order of distance at the photo-electric detectors and the 900 phaseshifted signals are produced by polarisation of optical and optically birefringent components.

Both types of interferometers have in common that the interference image selected has to be maintained unvaried during the entire measurement since otherwise interferences in the bi-directional counting operation will occur. In interferometers of the first mentioned type, mainly Fizeau interferences are produced between plane reflectors in a real or virtual wedge.

The order of distance depends on the angle between the reflectors. When the angle varies in the course of measurement then the order of distance varies too and, hence, the phase shift between the electrical output signals of the detectors. In order to prevent this, the reflectors are non-displaceably arranged and triple prisms are used as moveable reflecting members. When, however, plane reflectors are used as moveable reflecting members, the former require precision guides since any tilt by even minute angular amounts will cause considerable phase errors in the bi-directional counting operation.

In the second case, the reflectors which produce an intermediate image are in parallel to one another. When the reflectors have a position other than a parallel one, measuring errors result so that in this case also the irresponsiveness to tilt properties of triple prisms are exploited and triple prisms are used as moving reflecting members.

However, this arrangement is disadvantageous because, on the one hand, two planes have to be brought to one another into a defined (angular or parallel) position and, on the other hand, triple prisms have to be used, the production technology of which has to satisfy high requirements.

The admissible tolerances of the angles between the three reflecting faces of a triple prism used for interferometric purposes are a few angular seconds. When these tolerances are not adhered to, the triple prism functions like a glass wedge in the optical path and causes a variation of the order of distance in the interference image.

Furthermore, such an arrangement only permits one to sample the measuring component mechanically due to the use of triple prisms.

Therefore, the disadvantages of having a mechanical contact such as to cause deformation of the sample due to the measuring force exerted by the probe are involved in the measuring result, apart from variations of the measuring force due to friction and hysteresis.

A third known type of interferometers permits a smoothness test of very smooth optical surfaces. With these interferometers the surfaces to be tested are also optically and remotely sampled and usually an areal interference image is produced from the entire surface under test.

These interferometers have, however, the disadvantage that the interference images have to be evaluated visually and the automatic bidirectional counting method cannot be used.

It is an object of the present invention to obviate the above disadvantages.

It is a further object of the invention to provide an interferometer which permits small length measurements up to 100 mm, which empioys plane reflectors, eliminates the use of triple prisms and permits an angular movement of said reflectors within wide limits, which is sufficient for most technical applications.

It is a further object of the invention to provide a simple and inexpensive interferometer which can be produced without sophisticated optical and precision mechanical technologies.

It is a still further object of the invention to provide an interferometer for measuring objects having well-reflecting surfaces which can directly and pointedly be sampled without the necessity of precision adjustment of said measuring object relative to a reference face.

It is still a further object of the invention to provide an interferometer in which the phase positions of electrical signals emitted by the photoelectric detectors of said interferometer are constant and independent of the angle position of the reflecting faces which produce the interference, preferably said interferometer uses plane reflectors as measuring reflector and as reference reflector, provided that the optical centres of the detectors are only met by homologous pairs of beams.

In accordance with the present invention, there is provided an interferometer comprising a monochromatic light source which emits a light beam, a first and a second beam splitting means, a first and second beam reflecting means, a first and a second photodetecting means, said first and said second photodetecting means being displaceable and including an angle of substantially 900, said first beam splitting means producing from said light beam two partial beams, namely a reference beam and a measuring beam, said first beam reflecting means reflecting said reference beam back to said first beam splitting means, said measuring beam and said reference beam constituting a first homologous beam couple, said second reflecting means reflecting said measuring beam back to said first beam splitting means, where the reflected measuring beam and reference beam interfere, the interfering measuring beam and reference beam being directed by said first beam splitting means to said second beam splitting means which splits said first homologous beam couple into a second, deviated homologous beam couple and into a third transmitted homologous beam couple, said second homologous beam couple being detected by said first photodetecting means, said third homologous beam couple being detected by said second photodetecting means, and means for aligning said first and said second photodetecting means relative to said second homologous beam couple and to said third homologous beam couple respectively.

The partial beams which are produced when a beam impinges upon a beam splitting layer are referred to as homologous beam couples.

Thus, the first beam splitting layer produces the measuring and reference beam as a homologous beam couple from the beam originating from the light source.

The second beam splitting face produces two homologous beam couples. One results from the splitting of the measuring beam, the other from the splitting of the reference beam.

This is realised in that the photo-electric detectors are adjusted in a particular manner relative to the optical components of the interferometer and to the incident light beams.

The photo-electric detector can be so considered that its active photo-electric face is concentrated upon one point, namely the optical point of concentration. The optical points of concentration of the detectors are so adjusted that they are only met by homologous beam couples.

When the measuring reflector and the reference reflector are at right angles to one another, the homologous beam couples produced from the measuring beam and the reference beam coincide.

When the measuring beam takes an angular position other than the aforesaid the measuring beam and the reference beam produce homologous beam couples at the second beam splitting layer which include an angle with each other.

When the measuring reflector is moved in the direction of the measuring beam, the phase position between the electrical signals from the photo-electric detectors independent of the angular position between the two plane reflectors, is constant over the entire range of movement of the measuring reflector and the phase difference is zero. To permit forwardbackward-counting, a constant phase difference of 900 has to be produced between both detector signals. This is achieved by using optically birefringent and polarisation optical components.

The linear polarised light beam originating from the monochromatic light source impinges upon a quarter-wave plate made of optically birefringent material. Provided that the oscillation direction of the light beam has an angle of 450 relative to the two oscillation directions feasible in the quarter-wave plates, then the quarter-wave plate produces circularly polarized light from the arriving linearly polarized light. This is split at the first beam splitter into a reference beam and into a measuring beam in one of which a second quarterwave plate is arranged reversing the rotation sense of the circular polarisation of the respective beam due to the double passage.

The interference of right circularly and left circularly polarised light results in linearly polarised light after the first beam splitter, the oscillation plane of which light depends on the phase difference between the two interfering partial beams.

The beam resulting from interference is divided at the second beam splitter with an equal ratio of amplitudes. Each of the partial beams impinges upon a polarizer. The directions of transmission of both polarizers include an angle of 45 0. When moving the measuring reflector in the measuring direction, the signals delivered from the photoelectric detectors are constantly phase shifted by 900. Hereinbefore it was assumed that the photoelectrically active faces of the photo-electric detectors concentrate upon one point, the optical point of concentration. This theoretical assumption permits one to render the angle of tilt between the reference reflector and the moving reflector as large as desired and the forward/backward counting operation will not be out of phase when the homologous beam couples sweep the optical points of concentration of the detectors.

Though this theoretical assumption of a pointshaped photo-electric detector is considerably approximated due to recent detectors having active photo-electric faces of a few ,um2 produced by integration technologies, the influence of a small but finite detector face should be considered.

"Finite" means in this case that the photoelectric active detector face is < 1 mm2, square and circular, respectively, that is, not linear. When the measuring reflector and the reference reflector are at right angles to each other, the intensity distribution is constant on the photoelectric detectors.

When the measuring reflector is tilted in the course of a measuring movement for example due to inaccuracies in the guide, a sinoidal intensity distribution results which is detected by the detectors.

With small angles of tilt of a few angular seconds, the orders of distance will be great compared to the active photo-electric detector face and the sinoidal intensity distribution is point-wise swept by the detector to a certain extent.

With an increasing tilting angle to which the measuring reflector is subject, the order of distance becomes smaller and the detector starts to integrate via the sinoidal intensity distribution, in the course of which the amplitude of the electrical signal produced by the detector decreases. The amplitude is zero when the order of distance is equal to the active-photo-electric face. This case corresponds to a definite angular position of the measuring reflector relative to the reference reflector and must not be exceeded. The maximum angular range in which the measuring reflector is permitted to tilt is determined by the geometrical measurements of the active photoelectrical detector face.

Table 1 gives the admissible angles of tilt arelative to the edge lengths a of the photo-electric detector faces for 7r=633 nm.

Table 1 a+1'5" +2'11ff+10'52" +21'45" -+1040" a/mm 1 0.5 0.1 0.05 0.01 The 900 phase shift only depends on the adjustment of the photo-electric detecting faces relative to the incident beam.

In order that the invention may be more readily understood, reference is made hereinafter to the accompanying drawings which illustrate diagrammatically and by way of example only two embodiments of the present invention and wherein: Fig. 1 is a schematic view of an interferometer in the form of a two-beam interferometer; and Figure 2 is a schematic view of a multi-beam incident light interferometer.

According to Figure 1, a laser 1 emits a beam 1 3 of monochromatic light which passes a (quarter wave) plate 2 made of birefringent material and impinges upon a beam splitting cube 4 having a beam splitting layer 5.

The latter splits the light beam 13 with a 1 to 1 ratio into a reference beam 1 5 and into a measuring beam 1 6. The reference beam 1 5 is reflected at the beam-splitting layer 5 to the plane reference reflector 6 and from there back to the beam-splitting layer 5.

The measuring beam 1 6 passes through the beam-splitting layer 5, is reflected at a plane measuring reflector 7 and twice passes through a 7r/4 plate 3.

The beams 1 5 and 16 are brought to interference at the beam-splitting layer 5 and are split with a 1:1 ratio at a second beam-splitting face 8 and two homologous beam couples 15', 15", 1 6', 16" result from the beams 1 5 and 16.

The two partial beams 15', 16' pass through a polarising filter 10 and thereafter impinge upon a photo-electric detector 11, and in a similar manner, the two partial beams 15", 16" pass through a polarising filter 9 and subsequently impinge upon a second photo-electric detector 12.

The beam 13 is linearly polarized by the quarter wave plate 2 and the oscillation plane of its polarisation relative to the two feasible oscillation planes of the n/4 plate is so oriented that these include an angle of 450 so that the beam 14 is circularly polarized and, let it be assumed, dextropolarized.

The measuring beam 16, however, twice passes on its way back and forth to the measuring reflector 7 the 7r/4 plate 3, the orientation of which relative to incident beam is the same as that of the 7r/4 plate 2 relative to the beam 13.

Hence, the sense of rotation of the circular polarisation of the beam 1 6 returning to the beam-splitting layer 5 is changed. The beams 15, 1 6 subsequent to the beam splitting layer 5 interfere and the resulting interference light is of right and left circular polarisation.

The result of this interference is a beam the polarisation of which is linear, the oscillation plane, however, of the linear polarisation not being constant but being dependent on the phase difference between the reference beam 1 5 and the measuring beam 1 6. The polarizers 10 and 9 are so oriented relative to each other that the oscillation planes of the partial beams 1 5', 16' and 15", 1 5' include an angle of 450 after passage through the polarizers so that the electrical signals delivered from the photo-electric detectors 11, 12 are 900 phase shifted.

In order to enable special adjustment of the active photo-electric faces of the detectors 11, 12 relative to the homologous beam couples, an aperture 17 having a small opening which permits passage of the beam 13 and is removable after adjustment can be inserted into the light bundles emitted from the laser 1.

In Figure 1 the angular position between the measuring reflector 7 and the reference reflector 6 is perpendicular so that the partial beams 15, 1 6 originating from the beam 14 are at right angles to each other. Therefore the partial beams 1 5, 1 6 after their second passage through the beam-splitting layer 5 are shown coinciding in the drawing 1.

The homologous beam couple 15', 15" is produced from the beam 1 5 at the beam-splitting layer 8 and the homologous beam couple 16', 1 6" from the beam 1 6. The adjustment of the detectors is performed as follows: The photodetectors 11, 12 are displaced in (not shown) straight guides by means of (not shown) displacement means at right angles to the homologous beams 15', 15", for example, perpendicular to the drawing plane until the active photo-electric faces of the detectors 11, 12 are entirely illuminated by the homologous beams 1 5', 1 5", so that the detectors 11, 12 produce a maximum output voltage. This visual adjustment of the photo-electric detectors 11, 12 can be rendered more precise by employing oscillographic figures.To this end, the measuring reflector 7 is tilted from its perpendicular position so that the order of distance of the interference image is reduced until said order of distance corresponds to the edge length a of the active photo-electric face of the detectors 11, 12 and the latter integrate via the interference image so that the output voltage of the detectors 11, 12 becomes zero and the oscillographic figure is reduced to a point. It is, however, a condition that the circular shape of the oscillographic figure is maintained even when reduced to a point. When the oscillographic figure departs from the circular shape, one of the photodetectors 11, 12 has to be displaced until said circular shape is regained.The measuring reflector 7 is then returned to its original position and the aperture 1 7 is removed.

By this adjustment, a point shaped scanning of the interference image by the detectors 11, 12 is ensured.

One wedge 20 of a double wedge pair 1 9, 20 is non-displaceably arranged in the measuring beam 1 6 and the other wedge 19 is displaceably arranged at right angles to the measuring beam 1 6 so that a variation of the optical phase difference results between the reference beam 1 5 and the measuring beam 16 when the wedge 19 is displaced. It is also feasible to displace both wedges 19, 20 in this direction in countermovement. Due to the displacement of the wedges, a defined variation of the phase difference can be obtained for example, to perform interpolations or modulations.

In Figure 2 a further embodiment is shown in which multi-beam Fizeau interferences are produced with a real wedge. In Figure 1 and 2 like reference numbers refer to like components.

The linearly polarized beam 1 3 originating from the laser impinges under a 450 orientation upon the 7r/4 plate 2 which produces the right handed circularly polarized beam 14. This beam passes through the beam-splitting layer 5 and is split at the partially silvered layer 1 8 of the reference reflector 6 into a transmitted portion and into a reflected portion. The reflected portion 1 5 is the reference beam and is right hand circularly polarized. The transmited portion 1 6 is the measuring beam whose sense of rotation of the circular polarisation is reversed due to having twice passed through the r/4 plate 3.

Also the many fold reflections of the partial beams 1 6 between the reference reflector 6 and the measuring reflector 7 maintain the left hand circular polarisation since the number of reflections of the partial beam 1 6 from the partially silvered layer 1 8 and back is always an integer.

Claims (7)

Claims

1. An interferometer comprising a monochromatic light source which emits a light beam, a first and a second beam splitting means, a first and a second beam reflecting means, a first and a second photodetecting means, said first and said second photodetecting means being displaceable and including an angle of substantially 900, said first beam splitting means producing from said light beam two partial beams, namely a reference beam and a measuring beam, said first beam reflecting means reflecting said reference beam back to said first beam splitting means, said measuring beam and said reference beam constituting a first homologous beam couple, said second deflecting means reflecting said measuring beam back to said first beam splitting means, where the reflected measuring beam and reference beam interfere, the interfering measuring beam and reference beam being directed by said first beam splitting means to said second beam splitting means which splits said first homologous beam couple into a second, deviated homologous beam couple and into a third transmitted homologous beam couple, said second homologous beam couple being detected by said first photodetecting means, said third homologous beam couple being detected by said second photodetecting means, and means for aligning said first and said second photodetecting means relative to said second homologous-beam couple and to said third homologous beam couple respectively.

2. An interferometer as claimed in claim 1, wherein said first reflecting means and said second reflecting means include an angle of substantially 909.

3. An interferometer as claimed in claim 1 or 2, wherein a first 7r/4 plate made of optically birefringent material is arranged before the first beam-splitting means and a second 7r/4 plate is arranged in a partial beam subsequent to the first beam splitting means and wherein between the second beam splitting means and said first photodetecting means and said second photodetecting means a respective polarisation optical analyser is arranged in said second homologous beam couple and in said third homologous beam couple, the directions of transmission of said analysers including an angle with each other.

4. An interferometer as claimed in claim 1, wherein the first beam splitting means and the first beam reflecting means are formed on a single component member.

5. An interferometer as claimed in claim 1, wherein one beam reflecting means is embodied as a partially transmissive reflector and the beamreflecting means is arranged parallel to same, the n/4 plate being located between the two beam reflecting members.

6. An interferometer as claimed in claim 1, wherein a pair of double wedges is arranged in one of said two partial beams said interferometer.

7. An interferometer substantially as hereinbefore described with reference to and as illustrated in Figure 1 or in Figure 2 of the accompanying drawings.