Procedure and device for absolute measurements with a laser-interferometer.

The invention relates to an interferometric measuring method for absolute measurements, in particular for absolute distance measurements, using a laser that can be tuned without mode jump in at least one wavelength range, and to a laser interferometer arrangement suitable for the measuring method according to one of the preceding claims, with a laser that can be tuned without mode jump in at least one wavelength range.

From DE-PS 36 08 075 a device for measuring the distance of an object using a laser beam is known, which is composed of a measuring interferometer and a reference interferometer. The beam from the same laser is applied to both interferometers. By frequency modulation of the laser beam, a so-called reference beat wave is generated from two interfering partial beams, the number of waves in the measurement beat wave or the reference beat wave being determined by means of a measuring device and determining the measurement distance from the ratio between the number of these waves and the known distance of the reference path becomes.

On the one hand, this measuring method has the disadvantage that the phase of the reference beat wave can be determined at most with a resolution of approximately 1/10 of the total wavelength of the beat wave. This resolution is nowhere near the resolution that can be achieved with other methods. On the other hand, this method is unsuitable for measuring sections that vary in an extreme range. However, these requirements are often relevant, for example, for system controls in manufacturing processes.

From these disadvantages, it is an object of the present invention to increase both the measuring accuracy and the detectable measuring range in a method or a device for measuring the distance of the type described in the introduction.

This object is achieved in the measuring method according to the invention in that two measuring interferometers acted upon with the laser beam of the same laser are used with respective measuring sections, the reference section L formed from the arithmetic sum or difference of the two measuring sections L 1 and L 2_Ref is kept constant that at least one interference signal measured by means of photodetectors is generated from two interfering partial beams, that the air wavelength of the laser beam is continuously tuned within a mode-free wavelength range between the values ​​λ₁ and λ₂ and that the integral phase changes Δ die₁ and ΔΦ₂ of the respective interference signals are detected during the wavelength modulation and the length of one of the measuring sections according to the formula<maths id="math0001" num=""><img file="EP0623801A2_D0001.tif" /></maths> is determined.

This measuring method has the advantage over the known method that the length of the reference path can be scaled with the length of the two measuring paths, ie the size of the measuring distance to be determined. This ensures the overall achievable measurement resolution even with very different measurement distances.

In the measuring method according to the invention it can further be provided that the wavelength modulation is carried out via the operating current of the laser.

In order to keep the time for a measurement cycle as short as possible and thus to minimize the influence of errors, it can be advantageous to carry out the wavelength change only by means of current modulation, but not, for example, by a temperature change in the heat sink of the laser, preferably a semiconductor laser. Although the continuous tuning range for a temperature change is about a factor 5 larger than that for a current change, this process takes several seconds as opposed to a few milliseconds for a current modulation.

The achievable measurement resolution can be further increased according to the invention by determining the respective remaining phases in the range of the wavelength values ​​λ₁ and λ₂. The phase values, which are fractional in the end regions of the entire wavelength interval, are referred to as residual phases.

A further increase in resolution in the phase measurement can be achieved according to the invention in that the phase change is determined on the basis of the elliptical Lissajous figure formed from two interference signals phase-shifted by 90 °.

The evaluation of the Lissajous ellipse allows an extremely precise determination of the phases of the interference signals, in particular the remaining phases. In order to be able to carry out this evaluation expediently, however, it must be ensured that the light intensities of the interference signals registered by the photodetectors come to lie on a circular Lissajous curve in a two-axis representation in a biaxial representation. As a rule, however, the measured phase values ​​do not lie on a circle, but on an ellipse, which is not closed due to the performance characteristics of a current modulation, but rather widens in a spiral. Mainly responsible for the ellipse shape are an inexact 90 ° phase shift, unequal amplification factors of the two interference signals phase-shifted by 90 ° and the occurrence of an offset in only one of the two interference signals. A known method according to 'Heydemann' offers an opportunity to correct the measured phase values, in which an ellipticity of the Lissajous curve is converted into a circle by a major axis transformation. Using a Lissajous curve corrected in this way, angular resolutions of approximately 1 ° can be achieved.

In addition to absolute interferometric measurements, relative distance measurements can also be carried out and the direction of the relative movements of the object to be measured can be determined by introducing a lambda / 4 plate into at least one of the measuring sections.

In the measuring method according to the invention it can further be provided that the phase changes ΔΦ₁ and ΔΦ₂ are determined by means of the same trigger signal by oversampling.

The integral phase change is generally measured in the form of a step-shaped digitization curve. The number of digitization levels in absolute interferometry with semiconductor lasers that are commercially available today is generally extremely small and by the factor<maths id="math0002" num=""><img file="EP0623801A2_D0002.tif" /></maths> less than in conventional interferometry. In the case of digital data acquisition, the phase resolution can be increased by oversampling the measuring electronics at a constant sampling rate. The increase in resolution is proportional to the ratio of the step length to the length of a sampling interval. The wavelength end ranges within which the oversampling takes place correspond to a maximum of 1/2 of the total wavelength interval.

In the measuring method according to the invention it can further be provided that at least in one of the measuring interferometers from at least two interfering partial beams, interference signals which are phase-shifted with respect to one another are generated, which are each measured by means of a photodetector.

As is known, the generation of interference signals phase-shifted by 90 ° can be carried out by introducing a lambda / 4 plate into the beam path. On the one hand, this procedure offers the advantage that relative distance measurements can also be carried out in addition to absolute interferometric measurements, it being known that the direction of the relative movements of the object to be measured can also be determined by introducing a lambda / 4 plate into at least one of the measuring sections.

In the measuring method according to the invention it can finally be provided that a second laser, a laser beam with at least one further discrete wavelength λ₃ emitting laser or a multi-wavelength laser is used, the laser beam of which is also coupled into the respective interferometer, the laser wavelengths being operated in succession in alternation, and wherein the wavelength value λ₃ is matched to the wavelength values ​​λ₁ and λ₂, that a synthetic beat with a certain beat wavelength is formed by a subsequent superimposition of the interference signals by means of electronics or a computer.

These measures can further increase the length resolution in the measuring method according to the invention. Due to the additional discrete wavelength, for example, a beat wavelength can be generated which is greater than the measurement uncertainty due to the method of continuously tuning the wavelength.

The object on which the invention is based is achieved in a laser interferometer arrangement suitable for the measuring method according to the invention in that two measuring interferometers each having a measuring section are provided, in which the reference section formed from the arithmetic sum or difference of the two measuring sections has a constant length and each has a beam splitter , have a spatially fixed reflector and a movable reflector and which are acted upon by means of a primary beam splitter and at least one further reflector or optical waveguide with the beam of the same laser, an interference signal being formed in each case, which is detected by means of a photodetector.

The beam splitters of the two interferometers can determine the ends of the reference path, the respective measuring reflectors being movable. However, an embodiment is also possible in which the beam splitters of the two interferometers are movable, while the respective measuring reflectors determine the ends of the reference path.

In the laser interferometer arrangement according to the invention it can further be provided that the laser is a semiconductor laser.

In the laser arrangement according to the invention, provision can further be made for the measuring interferometer to have preamplifiers for the interference signals, the amplification factor of which is inversely proportional to the laser power. This means that after the preamplifiers, the signal levels are independent of the laser power and thus of the wavelength,

In the laser interferometer arrangement according to the invention it can further be provided that at least one of the measuring interferometers has two interference signals which are phase-shifted by 90 ° and which are each detected by means of photodetectors.

On the one hand, this measure enables the phase evaluation to be carried out in accordance with the measuring method on the basis of the Lissajous curves to be made possible at all. On the other hand, as is known, the direction of the relative movements of the movable reflector (s) can also be determined by introducing a lambda / 4 plate into one of the measurement sections.

The laser interferometer arrangement according to the invention can also be designed such that the measuring sections are arranged in alignment on one axis, the movable reflectors being rigidly connected to one another on the back and arranged linearly movable between the beam splitters of the two measuring interferometers along the measuring sections and the arithmetic sum of the reference path formed in both measuring sections has a constant length.

The laser interferometer arrangement according to the invention can also be designed such that the measuring sections are arranged along axes running in parallel, the movable retroreflectors being rigidly connected and arranged to be linearly movable, and the reference section formed from the arithmetic difference of the two measuring sections has a constant length.

In the laser interferometer arrangement according to the invention it can further be provided that the measuring sections are arranged in alignment on one axis, a reflector being linearly movable along the measuring sections being arranged between the beam splitters of the two measuring interferometers and the reference section formed from the arithmetic sum of the two measuring sections being of constant length having. The interferometer arms are arranged so that a common reflector can be used.

In the laser interferometer arrangement according to the invention it can further be provided that a counter or counter electronics as well as evaluation electronics or a computer are provided to automate the phase evaluation.

As a result, all steps in the measuring method can be automated and therefore successive successive distance measurements can be carried out.

In the laser interferometer arrangement according to the invention, it can finally be provided that at least part of the arrangement is designed as integrated optics and parts of the light path as optical waveguides.

Miniaturization is only possible through the exclusive use of standard components known in integrated optics and opens up an even more extensive field of application for the laser interferometer arrangement according to the invention.

In the following part, the measuring method according to the invention is explained in more detail using an exemplary embodiment of the laser interferometer arrangement according to the invention.

In detail show:<dl id="dl0001"><dt>Fig. 1</dt><dd>an embodiment of the laser interferometer arrangement according to the invention, in which the reference path is formed from the difference between the two measuring paths;</dd><dt>Fig. 2</dt><dd>an embodiment of the laser interferometer arrangement according to FIG 1, but in which the reference path is formed from the sum of the two measuring paths.</dd><dt>Fig. 3</dt><dd>an example of the measuring method with a representation of the wavelength modulation and two resulting phase profiles of the interference signals occurring in the two measuring interferometers;</dd><dt>Fig. 4</dt><dd>a representation of the Lissajous ellipse of two interference signals phase-shifted by 90 ° and</dd><dt>Fig. 5</dt><dd>a signal curve for phase determination according to the method of varying the laser wavelength in the area of ​​the remaining phases.</dd></dl>

The absolute measuring laser interferometer arrangement 1 shown in FIG. 1 has a semiconductor laser 2 which can be tuned at least in one wavelength range without a jump, and which is modulated by means of an operating power supply (not shown) in a wavelength range of its characteristic curve without a jump. Two measuring interferometers 3, 4 are also provided, in which the interferometer arms 5, 6 form the actual measuring sections (5, 6). In this example, the reference path L formed from the arithmetic difference between the two measuring paths L 1 and L 2 5,6_Ref a constant length. To generate at least two interfering partial beams 7, 8 and 9, 10, beam splitters 11, 12 and a displaceable retroreflector assembly 13 formed from two retroreflectors 13 1 and 13 2 as well as two fixed retroreflectors 14, 15 are provided. The respective interfering partial beams 7, 8 or 9, 10 are detected by means of photodetectors 16, 17 or 18, 19. Furthermore, the photodetectors 16, 17, 18, 19 are coupled to counter electronics, not shown here, and the semiconductor laser 2 is coupled to a control device for the laser wavelength, also not shown. The two measuring interferometers 3, 4 are subjected to the beam of the same semiconductor laser 2 by means of a primary beam splitter 20 and a reflector or mirror 21. The measuring sections 5, 6 are otherwise arranged along axes running in parallel, the movable retroreflector 13 being arranged to be linearly movable parallel to the measuring sections 5, 6. When the retroreflector 13 moves, the reference path formed from the arithmetic difference of the two measuring paths 5, 6 has a constant length. Due to the exclusive use of standard components, the entire laser interferometer arrangement 30 can be designed as integrated optics.

The absolute measuring laser interferometer arrangement 30 shown in FIG. 2 has a semiconductor laser 31 which can be tuned at least in one wavelength range without a jump, and which is modulated by means of an operating power supply (not shown) in a wavelength range of its characteristic curve without a jump. Furthermore, two measuring interferometers 32, 33 are provided, in which the interferometer arms 34, 37 form the actual measuring sections (34, 37). In this example, the reference path formed from the arithmetic sum of the two measuring paths 34, 37 has a constant length. Beam splitters 40, 41 and two retroreflectors 42, 43 and 44, 45 are provided to generate at least two inter-interfering partial beams 35, 36 and 38, 39, respectively. The respective interfering partial beams 35, 36 and 38, 39 are detected by means of photodetectors 46, 47 and 48, 49. Furthermore, the photodetectors 46, 47, 48, 49 are coupled to counter electronics, not shown here, and the semiconductor laser 31 is coupled to a control device for the laser wavelength, also not shown. The beam of the same semiconductor laser 31 is applied to the two measuring interferometers 32, 33 by means of a primary beam splitter 50 and three reflectors or mirrors 51, 52, 53. The measuring sections 34, 37 are otherwise arranged on axially running axes, the movable retroreflectors 42, 44 being connected to a rigid unit and being arranged to be linearly movable along the measuring sections 34, 37, the arithmetic sum of the two measuring sections 34, 37 formed reference path has a constant length. Due to the exclusive use of standard components, this laser interferometer arrangement 30 can also be designed as integrated optics.

In the upper part of FIG. 3, a linearly modulated wavelength curve 61 is shown, in which a measuring cycle begins at time t 1 at a wavelength value λ 1, which ends at time t 2 at a wavelength value λ 2. These two wavelength values ​​are selected, inter alia, in such a way that the wavelength interval between them lies within a range of the characteristic curve of the semiconductor laser 31 that is free of mode jumps. The phase changes 62, 63 of the interference signals 35, 36, 38, 39 that occur in the measuring interferometers 32, 33 during the wavelength modulation are shown in the lower part of the figure in FIG. 3. The sawtooth profile of these curves 62, 63 results from the fact that when the laser wavelength is tuned, the phase always changes linearly until a new interference order is jumped over. The different number of phases passed through the interference signals measured in the two measuring interferometers 32, 33 is a consequence of the different measuring path lengths of the measuring interferometers 32, 33. The integral phase change ΔΦ is now obtained by simply counting the sawtooth peaks and adding the remaining phases determined at the end points of the measurement curves 62, 63. The measuring section 34 (or 37) is finally determined from the integral phase changes measured in the two measuring interferometers 32, 33 and the known length of the reference section according to the formula mentioned in claim 1. The determination of the respective remaining phases will be discussed below.

FIG. 4 shows a typical Lissajous ellipse, which was determined by analytical adaptation to two interference signals phase-shifted by 90 ° and detected by means of the photodetectors 46, 47 or 48, 49. The inner Lissajous curve 71 represents the phase profile of the two interference signals of a wave period at the beginning of a measuring cycle, the outer curve 72, however, the phase profile at the end of a measuring cycle. The different radii are consequently a consequence of the power characteristic of the semiconductor laser 31 increasing with the wavelength. The remaining phases φ₁ and φ₂ each lie on one of these curves and can be determined extremely precisely therefrom. However, it will generally be expedient to transform the ellipse into a circle beforehand using the known method previously cited.

Finally, FIG. 5 shows a signal curve for phase determination according to the method of varying the laser wavelength in the range of the wavelength end values. In addition to the phase values ​​determined statically there, the phases of a complete period are recorded. The diagram in FIG. 5 shows this for phase data 81, 82 of the interference signals phase-shifted by 90 °, recorded at the beginning of a measuring cycle, and data 83, 84 measured accordingly at the end. These curves 81, 82, 83, 84 are recorded by varying the operating current of the semiconductor laser 31, which causes a phase change over at least one period. A Lissajous ellipse is adapted to the data determined in this way and can be evaluated therein in accordance with the method according to FIG. 4.