WO1992006353A1

WO1992006353A1 - Interferometer

Info

Publication number: WO1992006353A1
Application number: PCT/AT1991/000106
Authority: WO
Inventors: Gerhard Leuchs
Original assignee: Tabarelli, Werner
Priority date: 1990-10-01
Filing date: 1991-10-01
Publication date: 1992-04-16

Abstract

An interferometer for measuring the distance or the displacement path of a movable component has a source (1) of laser light and a beam divider (B) with an asymmetric division ratio that divides the light emitted by the source of laser light into a reference beam and a measurement beam. The measurement beam is guided along a measurement section that extends partially in the air and through a movable measurement mirror (8). In a recombining device (A) the reference beam guided along a reference section (5) and the measurement beam returning from the measurement section interfere. A highly sensitive detector device (12a-d, 19) analyzes the interference signals outputted by the recombining device (A). The measurement beam returning from the measurement section is guided through a tridimensional filter (9, 10, 11; 30) in order to eliminate wave front distorsions. An appropriate selection of the division ratio of the beam divider (B) allows background noise to be eliminated from the interference signal.

Description

Interferometer

The invention relates to an interferometer, in particular for determining the distance or displacement path of a movable component, with a laser light source, with an asymmetrical beam splitter for the intensity-uneven division of the light emitted by the laser light source into a reference beam and one Measuring beam, which is guided over a measuring path running at least partially in a gaseous ambient medium and leading over a movable measuring mirror, with a recombination device on which the reference beam guided over a reference path and the measuring beam returning from the measuring mirror interfere, and with a detector device for analyzing at least one optical interference signal emerging from the recombination device.

In principle, interferometers are suitable for determining the distance or determining the displacement path and thus for detecting the position or changes in position of movable components, for example machine components.

A practically unavoidable wavefront distortion of the measuring beam coming back from the measurement section is a problem. Such wavefront distortions result from air streaks and, above all, from contaminations (oil vapors, etc.) of the air occurring in industrial use in which the measurement is located ¬ beam propagates, as well as from inaccuracies of the optical components used. The latter inaccuracies of the optical components could at best be avoided by correspondingly expensive components. But this makes the entire Internet more expensive Ferometer arrangement essential and wavefront distortions of the measuring beam due to the air conditions occurring in practice are still not avoided. In known single-frequency homodyne interferometers, the wavefront distortions mentioned lead to a change in the observed spatial interference pattern and thus to a falsification of the measurement result. However, the known wavefront distortions also have an unfavorable effect in known two-frequency heterodyne interferometers, in which the beat frequency between the measuring and reference beams is observed at a slightly different frequency. This is because, like already slight deviations from the ideally exact adjustment, they reduce the quality of the interference, that is to say the modulation deviation of the oscillating interference signal to be evaluated in relation to the signal background. This leads to problems.

The signal from the photodetector must be electrically amplified for the evaluation. In addition, measurements should also be carried out very quickly, so that the mirror speed can be up to a few meters per second. Particularly in the case of a homodyne-operated interferometer, the generally difficult task is to amplify a small signal which is seated on a large background. This is particularly a problem if the surface is not completely stable over time and therefore cannot be easily subtracted electronically. Detection systems which have the required amplification at high bandwidth are highly sensitive, low-capacity detectors, e.g. B. avalanche diodes, combined with very low-noise electronic amplifier circuits, or secondary electron multipliers (photomultiplier), which the photo- Combine electrical detection and electrical amplification in a high-voltage vacuum tube. These detector systems cannot be used in harsh environmental conditions or wavefront distortions, since the signal background caused by the wavefront aberration drives the detection amplifier into saturation.

It is now known that in some cases a constant, i.e. H. interference signal background which does not vary with the optical path length difference in the measuring and reference branch can be excluded by an asymmetrical beam splitter ratio, d. H. that the asymmetrical beam splitter distributes the light intensity unevenly between the measuring beam (e.g. 80%) and the reference beam (e.g. 20%). However, this does not apply in the event that the loss of contrast is caused by wave front distortions.

In the case of wavefront distortions, when using an asymmetrical beam splitter, the signal component which varies with the optical path difference between the reference beam and the measuring beam becomes smaller relative to the background, and the amplifier problem only becomes greater. An asymmetrical beam control is therefore not a solution to the problem of a relatively large background or amplifier saturation.

The object of the invention is to provide a compact interferometer with which measurements can be carried out quickly even under harsh environmental conditions and which is therefore particularly suitable for industrial use. This is achieved according to the invention in that a spatial filter device is arranged between the measuring mirror and the recombination device, through which the measuring beam returning from the measuring mirror is guided.

It is known that by coupling the measuring beam into a single-mode waveguide, a spatial filter effect is achieved by which wavefront distortions of the measuring beam returning from the measuring section are eliminated or greatly reduced. Such wavefront distortions lead in the focal plane of the coupling lens to intensities outside the axis (outside the diameter of the diffraction-limited focal point), where the entry point of the waveguide, for example a glass fiber, lies and does not get into the waveguide, so it is not Room filter hidden. After the room filter, i.e. after the entry surface of the single-mode waveguide, one has a measuring beam "cleaned" of welien-front distortions with an almost ideal wavefront. The measuring beam thus becomes weaker, but can interfere perfectly with the full reference beam. So only the measuring beam is influenced.

However, when using a spatial filter alone (without an asymmetrical beam splitter), the ratio of the desired signal component to the background in the photoelectric interference signal is still at a maximum when the loss of measuring beam power generated in the spatial filter is zero. So the spatial filter alone cannot solve the problem of a too large background or amplifier saturation. Only the combination according to the invention of asymmetrical beam splitter and a spatial filter device in the measuring beam allows the elimination of the signal background and thus a highly amplifiable output signal for the use of highly sensitive, fast detection and amplifier systems.

Let it be the electric field amplitude of the light in the reference beam e R and in the measuring beam a (l-R). The loss of measuring beam power generated in the spatial filter and on the measuring section is (1-σ), with 0 <σ <l.

The photoelectric signal is then: I - <^{' R ^« ac ^ββ <i £ t) ♦ ^ σ - ^ (lR) -a» cos <j £, t * kL >> ^a > time average - R * a ^a / 2 * σ- (lR) .a ^a / 2 + «fσ-« (1-R> .a ^a -cos <kL), where w is the angular frequency, k is the wave number and L is the optical path difference between the reference and Are measuring beam. The angle brackets "<> time average" indicate that averaging takes place over a light period, so that the zero oscillating threshing floor quickly falls out.

The absolute size of the interference signal is reduced, but the constant background can be brought to zero by a suitable choice of the division ratio on the beam splitter. The condition for R is:

R = σ / (l + σ)

The losses on the measuring section and in the spatial filter (1- σ) are compensated for by the fact that more asymmetric light (1-R) is sent onto the measuring section due to the asymmetrical ratio at the beam splitter:

1-R • = l / {2- (l-σ)} As a result, the constant signal background is reduced to zero and the most sensitive amplifiers can be used.

Since the ratio of the "constant" background to the signal swing U / S, which varies with the optical path length difference between the measuring and reference beams, has a minimum when R = σ / σ + l, the ratio U / S changes from small fluctuations σ, i.e. with small fluctuations in losses on the measuring section and in the spatial filter, not.

According to a preferred embodiment of the interferometer according to the invention it is provided that the spatial filter device is arranged at the end of the part of the measuring section running in the gaseous ambient medium. This completely eliminates the wavefront distortions that occur mainly on the free measuring section running in the surrounding medium. Between the spatial filter and the recombination device there is hardly any risk of renewed wavefront distortions of the "cleaned" measuring beam. If the spatial filter is formed by a coupling-in lens and the entry end of a single-mode waveguide, the measuring beam, which is justified in the wavefront, can be guided without further interference with a well-defined wavefront to the recombination device and there can interfere perfectly with the reference beam . A beam splitter with a symmetrical division ratio of 1: 1 can advantageously be used as the recombination device. Unlike many conventional interferometers, where the beam splitter for division into reference and measuring beams and the recombination device are formed by one and the same beam controller, the asymmetrical beam splitter for division into measuring and reference beams and the recombining device are separate elements, which may, however, be formed on one and the same carrier substrate (wafer). The division ratio of the asymmetrical beam splitter is preferably adjustable in order to allow a perfect adaptation to the respective circumstances.

In this embodiment, the measuring beam at the measuring mirror is not thrown back into itself, but is coupled into a second single-mode waveguide with a beam offset by means of a separate coupling lens (or a coupling lens arrangement combined from several to form a lens system). This additional constructive effort ensures that no interfering light is fed back from the interferometer into the laser light source and thus brings decisive advantages, particularly in the case of laser diodes which sensitively change their emission when the light is fed back. The retroreflector used (ie a reflector which reflects rays lying in a certain angular range at least in a certain plane of incidence regardless of the angle of incidence parallel ~ u itself with beam offset uin) does not represent a disadvantage in comparison with plane mirrors. It has been shown that the beam offset change occurring when the retroreflector is laterally shifted can be easily mastered with a suitable choice of the coupling and decoupling lenses, that is to say that the meeting of the second single-mode waveguide or coupling lens is relatively uncritical when the waveguide returns from the measuring path - is table. In addition, the retroreflector has preferred have a triple mirror or triple prism, the property of the tilt invariance (even when the retroreflector is turned, the reflected beam remains parallel to the incident beam), which has an advantage over longer mirrors compared to flat mirrors. A triple mirror reflects incident light rays parallel to itself in a specific solid angle range.

Compared to conventional interferometers without the use of light guides, the adjustment effort in the area of the measuring beam is considerably reduced. The interferometer is then correctly adjusted (assuming a relatively simple pre-adjustment of the other optical components) if sufficient light is coupled into the second glass fiber. As already mentioned above, this is not particularly critical due to the property of the retroreflector to always reflect rays back in parallel, and due to suitable lens dimensions. The interferometer will advantageously be designed such that the single-mode waveguide from which the measuring beam is coupled out and the second single-mode waveguide lie parallel in their end region facing the measuring section. In order to achieve a precise relative adjustment of the separate coupling-in and coupling-out lens and waveguide, it can preferably be provided that at least the end regions of the single-mode waveguide facing the measuring section, from which the measuring beam is coupled out, and of the second single-mode waveguide a common carrier are fixed, the coupling and decoupling lenses being attached to this carrier. If a flat measuring mirror is nevertheless to be used, an optical attachment can ensure that the returning beam enters the second waveguide is coupled. If it is a single-mode waveguide, the wavefront in the waveguide is almost ideal. Wavefront distortions along the measurement path only worsen the coupling into the waveguide. Such a waveguide can expediently be designed as a waveguide path which is diffused onto a wafer. It is particularly advantageous if the single-mode waveguide of the interferometer, the beam controller and the recombination device are integrated on the same wafer, and the reference beam and the interference signal originating from the recombination device are also guided in single-mode waveguide paths. which are diffused into the same wafer. This integrated design means that most of the optical components are pre-adjusted and you get ideal interference between the reference beam and the measurement beam "cleaned" of wavefront distortions. Due to the almost exclusive use of integrated optical components, a considerable price advantage can be achieved compared to known interferometers. The use of waveguide tracks diffused into a wafer for an interferometer is indeed known per se. In the known integrated interferometer, however, the measurement beam emerging from the waveguide path is partially thrown back into the same waveguide by a plane mirror arranged at a small distance from the exit point, which leads to undesired feedback into the light source.

Of course, it is also possible for the beam splitter and the recombination device to be separate optical elements. As with the optically integrated variant described above, it is also Here, in view of a simple, well-defined adjustment, it is advantageous if the light originating from the light source is present in the entire interferometer arrangement except for the measuring section running through the retroreflector in the surrounding medium between the coupling and decoupling lens (and optionally) is guided into the area behind the recombination device in single-mode waveguides, the beam splitter and the recombination device being formed by waveguide couplers.

Further advantages and details of the invention are explained in more detail with reference to exemplary embodiments in the following description of the figures.

Figures 1 to 3 show schematic representations of exemplary embodiments of the interferometer arrangement according to the invention.

The interferometer arrangement shown in FIG. 1 has a laser light source 1 (preferably a laser diode), the light of which is coupled into a single-mode waveguide track 2, which is diffused onto a wafer 3 made of lithium niobate. In a beam splitter B with an asymmetrical division ratio in favor of a higher measuring beam line, the division into the measuring beam takes place, which is initially continued in the waveguide path 4 and into the reference beam, which is also integrated in the reference branch 5 in the wafer 3 via a mirror 6 to the recombina ¬ tion device A leads. The measuring beam emerges from the waveguide 4 and is collimated with a decoupling lens 7. The measuring section now leads through the gaseous ambient medium (usually air) and via a retroreflector 8 (triple mirror), which, for example is attached to a tool slide (not shown). This retroreflector 8 sends the measurement beam back parallel offset, with a lateral displacement of the retroreflector or its tilting the direction of the retroreflective

Measurement beam does not change, but only its parallel offset to an uncritical extent.

The measuring beam returning from the retroreflector 8 is coupled into a second single-mode waveguide 11 by a separate coupling lens 9. This prevents feedback of disruptive laser light into the laser diode 1.

The spatial filter device is essentially formed by the lens 9 arranged at the end of the free measuring section and the single-mode waveguide 11, that is to say in the exemplary embodiment shown in FIG. 1 the surface 10 arranged in the focal plane of the lens 9 acts around the focal point in FIG Wafer 3 diffused single-mode waveguide 9 as a spatial filter, which fades out intensities caused by wavefront distortions lying outside the focal point region. Since it is a single-mode waveguide, the wavefront of the measurement beam coupled into the waveguide 11 via the lens 9 is almost ideal and can therefore interfere perfectly with the reference beam. Wavefront distortions along the measurement path via the retroreflector 8 only lead to a deterioration in the coupling. To adjust the interferometer arrangement according to the invention, only a sufficient amount of light needs to get into the waveguide path 11 diffused into the wafer 3. However, this is not critical since the rays also coming back in parallel from the retro reflector 8 len are collected by the lens 9 at the same point in the focal plane and then all hit the waveguide 11. In the exemplary embodiment shown, coupling into the second waveguide is typically carried out for those beams which impinge on the coupling lens in the millimeter range around the lens axis, which is not a particularly critical requirement.

The measuring beam returning from the measuring section is brought into interference in the recombination device designed as beam splitter A with a symmetrical division ratio with the reference beam guided in the reference branch 5. The interference signals are then detected by detectors 12a-d and evaluated in an electronic evaluation circuit, not shown.

The division ratio R of the asymmetrical beam splitter B is now selected so that the power losses (1-σ) on the measurement section and in the spatial filter 9, 10, 11 are compensated by correspondingly more light (1-R) on the measurement section is sent. Typical values of R are between 10% and 30%. With R = σ / (l + σ) the constant signal background can be reduced to zero and highly sensitive photoelectric detectors suitable for the detection of fewer photons can be used. The typical light output on one of the detectors is approximately 2 μW. In order to be able to measure even at high displacement speeds (2-10 m / s), the measuring time interval is 10-50 ns. During this time, only 60,000 to 300,000 photons arrive at the detector. With these numbers, a photo-to-noise ratio of 1: 1 can only be achieved with photo diodes. With the low-noise and high-amplification photo multiply you get a signal-to-noise ratio of «100: 1. The detectors 12a-d are only shown schematically and can be, for example, secondary electron multipliers or avalanche diodes with an electronic amplifier circuit.

The exemplary embodiment shown is a single-frequency interferometer, in which two different polarization directions of the light emitted by the light source 1 are used in order to obtain information about the direction of movement of the retroreflector 8. For this purpose, a polarization-dependent phase retarder 13 is integrated in front of the recombination device, which causes, for example, a relative phase difference of 90 * between the two directions of polarization. The two complementary outputs of the recombination device A are connected by way of waveguide tracks 14 and 15 diffused into the wafer 3 with polarizing beams 16 and 17, the outputs of which lead to the photo detectors 12a-d mentioned. In their phase position, these photodetectors 12a-d receive interference signals which are shifted relative to one another, from which not only the displacement path of the retroreflector 8 but also its displacement direction can be clearly determined. Furthermore, from the detection of the intensities of both complementary outputs 14 and 15 of the recombination device A, influencing factors (thermal drifts of the mechanics and electronics, fluctuations in intensity of the light source, fluctuations in intensity due to slight misadjustments), which otherwise lead to measurement errors in homodyne operation can eliminate, so that it can be determined whether changes in intensity only due to the change of influencing factors or actually from a movement of the retroreflector. After combining the measurement and reference beam, one not only uses an interference signal beam, but also uses the second output of the beam splitter, which delivers a complementary interference signal. Now everyone

Influences which change the intensity in a disturbing manner act on both complementary signals in the same way, while only one actual method of the slide leads to a change in the interference pattern and thus to relative intensity shifts of the two complementary signals, is the object of this approach ¬ in the case of the homodyne laser interferometer, as in the case of the dual-frequency device (heterodyne method) described below, the possibility of reliably detecting a standstill of the mechanical system. The coupling of the measurement beam into a second single-mode waveguide and the associated spatial filter effect and the ideal wave fronts in the single-electrode waveguides ensure that complementary interference signals are actually well-defined even under industrial conditions which can then be reliably evaluated by the electronics.

In principle, the interferometer arrangement according to the invention can also be operated heterodyne, two light frequencies having slightly different frequencies being used. The lithium niobate material used has the advantage that acousto-optical modulators can also be integrated, these being vaporized in order to obtain a light frequency shifted by the acoustic frequency. Such an acousto-optic modulator would then have to be installed shortly before the measuring beam emerges from the wafer 3. it would be recommendable to install a second acousto-optic modulator with a modulation frequency different from the first modulator in the reference beam in order to avoid direct crosstalk into the detection electronics. In the heterodyne variant of the interferer, only one detector is then required instead of the four photo elements 12a-d. The polarization-dependent phase retarder and the polarization beam splitters 16 and 17 can also be omitted.

The integrated design, in which the light in the wafer is guided in single-mode waveguide tracks, achieves a compact design and excellent interference between the reference beam and measuring beam, since both have ideal wavefronts in the single-mode waveguides. In addition, a large part of the optical components can already be pre-adjusted.

As a result of the measure that the reference path 5 running in the wafer and the sum of the parts 4 and 11 of the measuring path running in the wafer are of equal length, the interferometer is also largely independent of the temperature of the wafer 3. Particularly suitable as light sources Laser diodes 1 because they take up little space and are inexpensive.

Since the measurement result of an interferometer is available in units of the air wavelength present on the measurement section, which in turn depends on the frequency of the light and the refractive index of the surrounding medium (mostly air), it is advantageous to continuously determine this air wavelength using your own device. If the frequency of the light source is known, a refractive index determination is sufficient to know the air wavelength. In the simplest case, the refractive index can be determined the so-called parameter method can be determined by determining the air temperature, the air humidity and the air pressure. If you want to determine the wavelength of light in the air directly, you can compare it with a measuring standard (etalon) in which the same environmental conditions prevail as on the measuring section. In such a case, it is necessary to divert light originating from the light source from the actual interferometer. In the simplest case, this is done using a beam controller C, which is arranged immediately after the light source in the beam path.

The measure of leading the measurement beam in front of the measurement section in the surrounding medium in a single-mode waveguide and then guiding it via a decoupling lens to the measurement section achieves excellent constancy and reproducibility of the optical beam guidance.

In addition to the lithium niobate mentioned, glass and gallium arsenide, for example, are also suitable as wafer materials.

In addition to the possibility of integrating the single-mode waveguides on a wafer, there is also the possibility of using optical fibers, in particular glass fibers, as is the case with the exemplary embodiment shown in FIG. 2. The interferometer shown in FIG. 2 is used to record the displacement of a movable component (not shown) on which the triple mirror 8 is favorably attached directly. The light originating from a laser is coupled into the glass fiber 2 and then reaches the coupler B, which has an asymmetrical see division of the light into the glass fiber 4 belonging to the measuring branch and the glass fiber 5 belonging to the reference branch. Via a decoupling lens 7, the light from the glass fiber 4 reaches the measurement section actually running in the ambient medium (air) and is reflected back parallel to itself with a beam offset by the triple mirror 8. A special coupling-in lens 9 couples the back-reflected measuring beam into a second single-mode waveguide (glass fiber 11), the spatial filter effect again being used and wavefront distortions being "straightened".

In the exemplary embodiment shown in FIG. 2, the end regions of the glass fibers 4 and 11 facing the measuring section are advantageously parallel, which means that there is always a good coupling of the measuring beam returning from the measuring section into the glass fiber without complex deflection devices when the triple mirror 8 is displaced 11 is given. In order to achieve a precise, relative alignment of the ends of the glass fibers 4 and 11, these can be fastened on a common carrier 18, which advantageously also carries the decoupling lens 7 and the coupling lens 9, which also means their relative Adjustment to the glass fibers 4 and 11 is ensured.

The coupling lens 9 and the coupling lens 7 are advantageously of the same design and in particular have the same focal length. Gradient index lenses, which can be attached directly to the carrier 18 via their flat connection surface, are particularly suitable as the coupling-in and coupling-out lenses. The light returned from the measuring section in the glass fiber 11 and the light guided in the glass fiber 5 of the reference branch are recombined in the symmetrical coupler A. The preferably 90 ° phase shift between the two polarizations for detecting the direction of movement of the measuring carriage can be achieved in a known manner via total reflection in the triple prism or with a birefringent quarter wave plate (not shown in FIG. 2) in the measuring beam. The interference signals at the two outputs from coupler A are detected separately with a polarization beam splitter or with a beam splitter and polarizer according to the polarization states which are perpendicular to one another (schematically illustrated detectors 12a, b, c, d). According to the exemplary embodiment in FIG. 2, the polarization-state-dependent division is carried out over two beam splitters AI * and A2 'and four polarization filters 26a-d arranged in front of each detector 12a-d. If polarization beam splitters are used instead of the beam splitters AI ¹ and A2 ', the polarization filters 26a-d can be omitted.

The polarization-dependent signal division can alternatively also take place as shown in FIGS. 2a and 2b (in the variants of these figures, the arrangement in front of the recombination device A is the same as in FIG. 2). According to FIG. 2a, a lens 27 is provided, which is arranged just before the two exit points of the recombination device A. The two beams imaged by the lens 27 reach a polarization beam splitter A3 and from there to the four detectors 12a-d. In the variant according to FIG. 2b, compared to the variant shown in FIG. 2, the beam splitter is used AI * and A2 'are dispensed with and the divergent radiation cones 28, 29 from the two exit fibers of the recombining device A are used in order to achieve a spatial and ultimately the polarization state-dependent signal distribution via the polarization filters 26a-d. The detectors 12a-d are connected to an electronic evaluation circuit 19 which, for example, calculates and displays the position of the measuring mirror 8.

As can be seen from FIG. 2, the light originating from the light source 1 is guided in single-mode waveguides (glass fibers) up to the measuring section running in the ambient medium, which largely eliminates the influence of stray light and during recombination in the coupler A there are well-defined wavefronts in the measuring branch glass fiber 11 and in the reference branch glass fiber 5, which guarantee an excellent interference signal. The spatial filter effect which occurs when the measuring beam returning from the measuring section is coupled into the glass fiber 11 eliminates any wavefront distortions that may be present. The higher losses on the measurement section and in the spatial filter 9, 11 can be compensated for by the asymmetrical beam splitter B.

In many known interferometers, the beam expensive for dividing the light in the measuring beam and reference beam is also used as a recombination device at which the measuring beam and the reference beam are brought into interference. According to a preferred embodiment of the invention, it is provided that the beam splitter (coupler B) and the recombination device (coupler A) have separate op- Table components are to reliably exclude the effects of laser light on the light source.

The sum of the glass paths in the glass fibers 4 and 11 and the path in the glass fiber 5 are preferably of the same length in order not to have any relative changes in length between the reference and measuring branches in the event of temperature fluctuations.

When dimensioning the coupling and uncoupling lenses (especially their focal length), there are several points to consider: the achievable accuracy in the angular adjustment of the optical axes of the coupling and decoupling lenses, the diameter and thus the diffraction-limited divergence of the light beam on the measurement section and the Angular accuracy of the retroreflector. It has been found that with the typical core diameters of a single-mode waveguide (e.g. monomode glass fiber) in the micrometer range and with the typical opening angles of the radiation cone from the waveguide, lens focal lengths in the range from 2 mm to 20 mm, preferably between 5 and 10 mm are particularly favorable if one wants to record displacement distances in the meter range.

The spatial filter device 30 can also be implemented by a lens-pinhole-lens combination, as shown in FIG. 3. Such a combination could also be retrofitted with existing interferometers with discrete optical components. The measurement beam 31 returning from the measurement section is focused by a converging lens 32 onto the opening of the perforated diaphragm 33 and then collimated by a collimation lens 34. The pinhole 33 dazzles higher spatial frequencies originating from wavefront distortions zen out, since these are located radially outside in the focal point region of the lens 32, where the opening of the pinhole 33 is arranged.

The cleaned measuring beam 31 'and the reference beam

35 can interfere perfectly with the recombination device designed as a 1: 1 beam splitter plate A ¹ . The interference signal 36, which contains no signal background due to a suitable choice of the division ratio of the asymmetrical beam splitter (not shown) for division into the measurement and reference beam, passes through a lens 37 to a highly sensitive amplifier 38.

Claims

Patent claims:

1. Interferometer, in particular for determining the distance or displacement path of a movable component, with a laser light source, with an asymmetrical beam splitter for the intensity-wise non-uniform division of the light emitted by the laser light source into a reference beam and a measuring beam, the a measuring path running at least partially in a gaseous ambient medium and passing over a movable measuring mirror is guided, with a recombination device, on which the reference beam guided over a reference path and the measuring beam returning from the measuring mirror interfere, and with ei¬ ner detector device for analyzing at least one optical interference signal emerging from the recombination device, characterized in that a spatial filter device (9, 10, 11; 30) is arranged between the measuring mirror (8) and the recombination device (A), through which the from the measuring mirror (8) to returning measuring beam is guided.

2. Interferometer according to claim 1, characterized in that the spatial filter device (9, 10, 11; 30) is arranged at the end of the part of the measuring section running in the gaseous ambient medium.

3. Interferometer according to claim 1 or 2, characterized in that the spatial filter device (30) has a perforated diaphragm (33) arranged at a distance behind a lens (32), through which the measuring beam (31) is guided and behind which a further lens (34) is preferably arranged at a distance.

4. Interferometer according to claim 1 or 2, characterized ge indicates that the spatial filter device has a waveguide (11) and a coupling lens (9) for coupling the measuring beam returning from the measuring mirror (8) into the waveguide (11).

5. Interferometer according to claim 4, characterized in that the waveguide (11) is a single-mode waveguide.

6. Interferometer according to claim 5, characterized in that the single-mode waveguide (11) is an onomode glass fiber.

7. Interferometer according to claim 4 or 5, characterized in that the waveguide (11) is formed as a waveguide path diffused into a wafer.

8. Interferometer according to one of claims 4 to 7, characterized in that the measuring beam after the asymmetrical beam splitter (B) is initially guided in a single-mode waveguide (4) and from this one via a collimating coupling-out lens (7) to the im Gaseous part of the measuring section running in the gaseous medium passes, and that a second one-electrode waveguide (11) and a separate coupling lens (9) are provided, which reflect the measuring beam (8) which is reflected by the beam mirror as a retroreflector (8) couples the second single-mode waveguide (11).

9. Interferometer according to one of claims 1 to 8, characterized in that the light coming from the laser light source (1) in the entire interferometer except for the coupling and decoupling The lens is guided over the part of the measuring section that extends over the measuring mirror and optionally onto the area behind the recombination device (A) in single-mode waveguides, the beam splitter (B) and the recombination device (A) being formed by waveguide couplers.

10. Interferometer according to one of claims 1 to 9, characterized in that the beam controller (B) and the recombination device (A) are separate optical elements.

11. Interferometer according to one of claims 1 to 10, characterized in that the sum of the distances between the beam splitter (B) and the recombination device (A) in a waveguide (4, 11) extending the measurement beam and in a waveguide (5) running reference distance are of equal length.

12. Interferometer according to one of claims 1 to 11, characterized in that a waveguide (2), preferably an optical fiber or a waveguide path diffused into a carrier substrate, is provided for supplying light to the beam splitter (B) and the asymmetrical beam splitter ( B) is designed as a fiber coupler or waveguide coupler.

13. Interferometer according to one of claims 1 to 12, characterized in that the division ratio of the asymmetrical beam splitter (B) is chosen so that the constant signal background of the interference signal intensity detected by the detector device (12a-d, 19; 38) Reduced to zero.

14. Interferometer according to claim 13, characterized in that the division ratio of the asymmetrical beam splitter (B) is essentially equal to R = σ / σ + l, with σ being based on the measuring beam power directly after the asymmetrical beam splitter (B ) related portion of the measuring beam power that is still present after the spatial filter device (9, 10, 11; 30), and the electrical field amplitude of the light after the asymmetrical beam splitter (B) in the reference beam a75 and in the measuring beam a _ / (lR ), where a is a positive number.

15. Interferometer according to one of claims 1 to 14, characterized in that the division ratio of the asymmetrical beam splitter (B) is adjustable.

16. Interferometer according to one of claims 1 to 15, characterized in that the division ratio of the beam splitter (A) used as a recombination device is approximately 1: 1.

17. Interferometer according to one of claims 1 to 16, characterized in that the photoelectric

Detector device comprises a highly sensitive combination of photodetectors and amplifiers suitable for the detection of fewer photons.

18. Interferometer according to one of claims 1 to 17, characterized in that the photoelectric detector device has at least one low-capacitance photodetector, preferably an avalanche diode, combined with an electronic amplifier circuit.

19. Interferometer according to one of claims 1 to 17, characterized in that the photoelectric detector device has at least one secondary electron multiplier (photomultiplier).

20. Interferometer according to one of claims 1 to 19, characterized in that it only works with a light frequency emitted from the laser light source (homodyne operation) and that at least two photodetectors (12a, 12c or 12b, 12d) are provided are which detect the signal intensities of two phase-shifted interference signals from the recombination device (A).

21. Interferometer according to claim 20, characterized in that light with two, preferably mutually perpendicular polarization components is used, that a device (13) is provided in the measuring and / or reference branch, which before recombination a relative of the measurement and reference beam

Phase shift, preferably 90 ", between the two polarization components, and that in at least one output branch (14 or 15) of the recombination device (A) a polarization splitter device (16 or 17) is arranged, its partial beam intensities via photodetectors (12a, 12b or 12c, 12d) are detectable.