DE102015203697B3 - Laser length measuring system - Google Patents

Abstract

The invention relates to a laser length measuring system (54) comprising a beam generating device (10) comprising (a) a first laser light source (12) for first laser light (L1) of a first fundamental frequency (νg1) and third laser light (L3 ) having a third fundamental frequency (νg3) corresponding to a multiple, in particular twice, the first fundamental frequency (νg1), at least one second laser light source (14) for a second laser light (L2) of a second fundamental frequency Frequency (νg2) and fourth laser light (L4) having a fourth fundamental frequency (νg4) corresponding to a multiple, in particular twice, the second fundamental frequency (νg2), at least one frequency shifter (AOM1, AOM2, AOM3) for shifting frequencies of laser light (L1, L2, L3, L4) by a first high frequency (fAOM1), a second high frequency (fAOM2) and at least a third high frequency (fAOM3), the high frequencies (fAOM1, fAOM2 , fAOM3) are chosen so that by superimposing laser light to at least two different beat frequencies can be generated, an interferometer (56) for superimposing laser light (L1, L2, L3, L4), so that the beat frequencies (fSchweb) arise and an evaluation unit (58), which is set up to automatically calculate the absolute , refractive index compensated optical path length from the phases of the beat frequencies (fSchweb).

Description

The invention relates to a laser length measuring system.

From the EP 2 620 742 is an absolute measuring laser length measuring system is known in which by means of a frequency comb, a series of synthetic wavelengths is generated, by means of which the distance of a retroreflector can be measured absolutely.

In addition, interferometrically measuring laser length measuring systems are known in which a retroreflector with several interferometers is targeted. If the retroreflector is moved, it can be concluded from the changes in the distances to the absolute position of the retroreflector relative to the interferometers.

From the DE 10 2010 032 407 B3 and the US 5,838,485 In each case a laser length measuring system is known in which two laser beams of different frequencies are used to calculate the actual path length from the measured optical path lengths. The disadvantage of such a system is that it can not measure absolutely.

From the US 5,694,216 is a heterodyne interferometer is known in which the light beam of a laser is split into two spatially separated sub-beams. Each of the two sub-beams is then transformed into a beam scanning an object and a reference beam. With such a system, no measurement of the absolute distance is possible.

From the EP 2 789 969 A1 is an absolute measuring, refractive power compensated multi-wavelength heterodyne interferometer known using three different wavelengths, each stabilized on an absorption cell. A disadvantage of this system is the comparatively complex structure.

In the DE 103 34 350 B3 is a refractive power compensated multi-wavelength heterodyne interferometer that does not allow absolute length measurement.

The article "Scanning differential-heterodyne-interferometer with acousto-optical reflectors", Optics Communications 123 (1996) 34-40 by Tiziani et al describes a heterodyne interferometer, which is designed as a profilometer. An absolute distance measurement is therefore not possible.

The article "Refractive Index Determination in Length Measurement by Two-Color Interferometry", Measurement Science and Technology 19 (2008) 084004 (5pp) by Meiners-Hagen and Abou-Zeid describes the determination of the refractive index by means of two-wavelength interferometry. An absolute distance measurement is therefore not possible.

The invention is based on the object to enable an absolute, refractive index compensated distance measurement with simpler means.

The invention solves the problem by a laser length measuring system with the features of claim 1.

Such a laser length measuring system is able to measure absolutely while at the same time largely eliminating temperature-related errors. This leads to a high measuring accuracy and a high measuring speed.

It is an advantage that an interruption of one or more of the light beams does not mean that the length measuring system has to be re-measured.

In the context of the present description, the feature that at least one frequency shifter for shifting frequencies of laser light is present, in particular means that laser light is shifted from a laser light source in its frequency. It does not matter which frequency, in particular which fundamental frequency is shifted by which high frequency, as long as the resulting synthetic wavelengths are suitable for determining the optical path length.

The laser light is understood to be the light emitted by a laser light source and possibly shifted by a high frequency. In other words, even after shifting the frequency, the laser light is regarded as the same laser light to achieve a simple nomenclature.

The feature that the path length is calculated from the beat frequencies is understood in particular to mean that the phases of the beat frequencies between laser light in a reference branch of the interferometer and a measuring branch of the interferometer are evaluated. The laser light in the measuring branch has preferably traveled to a retroreflector, the laser light in the reference branch has preferably not run to this retroreflector.

By the feature that the path length is determined from the phases of the beat frequencies, it is to be understood that the path length is determined from the phases of the electromagnetic waves having the respective beat frequency.

It is possible, and is a preferred embodiment, for the beam generating device to output first laser light having a first fundamental frequency that differs by at least 20 GHz from the second fundamental frequency. For example, the third laser light has twice the first fundamental frequency.

In the interferometer, in particular such laser light, which has traveled in its preceding light path a running distance to a retroreflector, superimposed with laser light, which has traveled a fixed distance. Thus, the distance of the retroreflector relative to a fixed point of the laser length measuring system can be calculated.

It is quite favorable if frequency shifters are present which, in addition to the generation of the heterodyne frequencies, provide further synthetic wavelengths. These synthetic wavelengths are preferably chosen such that they have uniqueness intervals which lie relative to one another such that an absolute optical path length can be determined.

It is favorable if the second high frequency differs from the first high frequency. In addition, the third high frequency preferably differs from both the first high frequency and the second high frequency.

It is advantageous if the laser length measuring system has a retroreflector for reflecting a measuring laser beam and a laser beam tracking device, which is designed to track the measuring laser beam in the retroreflector.

According to a preferred embodiment, the laser length measuring system comprises (a) an at least partially convex spherical reflector having a radius of curvature center and a radius of curvature radius, (b) a focusing device for focusing laser light, in particular the first laser light, (c) a retroreflector for reflecting the first measuring laser beam to form a first reflected laser beam, and (d) a laser beam tracking device pivotable at least about the circle of curvature center is and is adapted to track the measuring laser beam to the retroreflector. The retroreflector is attached, for example, to the object whose position is to be determined. It can then be determined the position of the object with high accuracy relative to the circle of curvature center. According to a preferred embodiment, the first laser light source and the second laser light source are frequency-stabilized. The measurement uncertainty of the length measurement measured with a synthetic wavelength is larger than the measurement uncertainty of the length measurement with an optical wavelength by the factor synthetic wavelength as a numerator to the optical wavelength as a denominator. For short synthetic wavelengths, therefore, a second laser light source is advantageous whose frequency is well known relative to the first laser light source.

The feature that a laser light source is frequency-stabilized means, in particular, that the fluctuation of the frequency is so small that the respective measurement uncertainty is so small that the determination of the interference order of the staggered synthetic wavelengths is possible. In most cases it is sufficient if Δν / ν is at most 10 ^-10 , where ν is the frequency.

Preferably, at least one of the laser light sources is frequency-stabilized on an absorption cell. For example, a laser light source is frequency stabilized to a doppler-free iodine absorption line. In this way, a high accuracy of measurement is achieved.

It is favorable if the first laser light source is frequency stabilized on a first absorption cell and the second laser light source is frequency stabilized on a second absorption cell, the absorption lines of the two absorption cells differing. Preferably, the absorption lines differ by at least 0.5 THz, in particular at least 5 THz, with at least 15 THz being particularly favorable.

Alternatively, it is possible that the first laser light source and the second laser light source are synchronized via a beat frequency, which is preferably at least 1 GHz, in particular 10 GHz. Preferably, the beat frequency is at most 100 GHz. If one of the laser light sources is frequency-stabilized, the basic beat frequency of the second laser light source is also frequency-stabilized by the constant beat frequency. This results in a particularly high measurement accuracy. If several beat frequencies exist, for example because the laser light sources are frequency-multiplied, the specified range always refers to the smallest beat frequency.

Preferably, the high frequencies are chosen so that at least one synthetic wavelength is formed which is at least one meter, in particular at least 10 meters. It is particularly favorable if the high frequencies are chosen such that at least one synthetic wavelength is at least 30 meters. In this way distances can be determined absolutely, as long as, in particular on the basis of other measurements, to half the synthetic wavelength is known, how large is the absolute distance to be measured.

Preferably, the evaluation unit is configured to automatically perform a method comprising the steps of (i) determining a first optical path length based on the first fundamental frequency and / or the second fundamental frequency, (ii) determining a second optical path length based on the third fundamental frequency and / or the fourth fundamental frequency, and (iii) calculating a distance between the laser length measuring system and the retroreflector from the first optical path length and the second optical path length by compensating the influence of the air refractive index over the dispersion.

In the following the invention will be explained in more detail with reference to the accompanying drawings. Showing:

1 a beam generating device of a laser length measuring system according to the invention and

2 an inventive laser length measuring system with the beam generating device according to 1 ,

1 shows a jet generating device 10 which is a first laser light source 12 , in the present case in the form of a frequency-doubled Nd: YAG laser. The first laser light source 12 outputs first laser light L1 having a first fundamental frequency v _g1 and a third laser light L3 having a third fundamental frequency v _g3 = 2v _g1 .

The ray generating device 10 also includes a second laser light source 14 , in the present case also in the form of a frequency-doubled Nd: YAG laser, via a frequency coupler 16 with the first laser light source 12 is coupled. The frequency coupler 16 synchronizes the laser light sources 12 . 14 over a beat frequency f _s . The beat frequency f _s1 between the first fundamental frequency ν _g1 and the second fundamental frequency ν _g2 is f _s = 20.01 GHz. A second beat frequency f _s2 between the third fundamental frequency ν _g3 and the fourth fundamental frequency ν _g4 is therefore f _s2 = 40.02 GHz in the embodiment shown here.

The first laser light source 12 is with its third fundamental wavelength at 532 nm on an absorption cell 18 frequency-stabilized, in the present case on a doppler-free iodine absorption line. In this way, both are laser light sources 12 . 14 both relative to each other and absolutely high frequency stable.

The second laser light source 14 emits a second laser light L2 having a second fundamental frequency ν _g2 , which in the present case corresponds to a vacuum wavelength of 1064 nm. In addition, the second laser light source emits 14 a fourth laser light L4 having a fourth fundamental frequency ν _G4 corresponding to a vacuum wavelength of 532 nm.

The jet generating device 10 has a first frequency shifter AOM1 for shifting the frequency ν _G3 to a first high frequency f _AOM1 , which in the _exemplary embodiment f _AOM1 = 92.5 MHz. The resulting laser light is coupled into a polarization-preserving singlemode glass fiber G1. On the right are the frequencies of the coupled-in laser light.

The light beam which passes the first frequency shifter AOM1 unshifted, with the sub-beam which has passed the second frequency shifter AOM2 unshifted, in a third frequency shifter AOM3, the third high frequency f _AOM3 in the embodiment shown f _AOM3 = 100 MHz. The resulting laser light is coupled into a second polarization-maintaining singlemode glass fiber G2. These beams are used in the below described interferometer as a local oscillator. They result in the present case heterodyne signals of 5 MHz and 7.5 MHz.

That of the second laser light source 14 The next laser light L4 passes through a second frequency shifter AOM2 whose high frequency f _AOM2 in the embodiment shown is f _AOM2 = 95 MHz. The resulting laser beam is in a beam splitter 20 associated with the laser beam coming from the first laser light source 12 and shifted from the first frequency shifter AOM1 by the first high frequency. The laser beam thus obtained is also coupled into the glass fiber G1 after it has passed through a λ / 2 plate and a polarizing filter.

The portion of laser light L4, which has passed through the second frequency shifter AOM2 unchanged, is in a second beam splitter 22 the laser light L1 combined and coupled into the glass fiber G2. Before the glass fiber G2, the respective light also passes through a λ / 2 plate and a polarizing filter, which is marked with Pol.

The laser light L1 becomes a third, polarizing beam splitter 24 fed, which diverts a portion of the laser light in a fourth frequency shifter AOM4. The frequency shifted beam passes through a fourth beam splitter and is from a mirror 28 back to the fourth frequency shifter AOM4 reflected, so that a light beam is produced, which has the frequency ν = ν _G1 + 2f _AOM4 . This one is about a mirror 30 and a fifth beam splitter 32 a polarization-maintaining singlemode glass fiber R2 supplied.

The light L1 is also on mirror 34 . 36 , Beam splitter 38 and more mirrors 40 . 42 a polarization-maintaining singlemode glass fiber R1 supplied.

The light L2 is transmitted through a seventh, polarizing beam splitter 44 partially fed to a frequency shifter AOM5 and behind an eighth beam splitter 46 reflected back to the frequency shifter AOM5, resulting in a double frequency shift. The thus frequency-shifted light is supplied to the glass fiber R2.

Light which has passed through the frequency shifter AOM5 only once is passed through a ninth beam splitter 48 fed to a seventh frequency shifter AOM7 and frequency-shifted also coupled into the second optical fiber R2. The second light L2 becomes partially after passing through the seventh beam splitter 44 and a tenth beam splitter 50 via an eleventh, polarizing beam splitter 52 a sixth frequency shifter AOM6 supplied and coupled into the glass fiber R1 after passing twice. Both in front of the glass fiber R1 and in front of the glass fiber R2, a λ / 2 plate and a polarizing filter are arranged in each case.

By the specified components, the frequencies that are adjacent to the glass fibers G1, G2, R1, R2 are given.

2 shows a laser length measuring system according to the invention 54 , in addition to the jet-generating device 10 , which is schematically drawn, an interferometer 56 and an evaluation unit 58 includes. The interferometer 56 also includes a spherical reflector 60 and a focusing device 62 for focusing laser light of multiple frequencies on the reflector 60 ,

The glass fibers G1 and R2 provide the light for a measuring laser beam 63 looking for a retro reflector 64 is directed. From the retro reflector 64 comes a reflected laser beam 65 back. The distance l of the retroreflector 64 from a circle of curvature center M of the reflector 60 can with the laser length measuring system 54 be determined.

The light from the fiberglass G1 is transmitted through a twelfth, polarizing beam splitter 66 and in this case a Fresnel rhombus 68 to the focusing device 62 directed. The light beam is at the reflector 60 reflected on itself and passes as part of the measuring laser beam 63 to the retro reflector 64 and is from there part of the reflected laser beam 65 ,

The reflected laser beam 65 is via a thirteenth, polarizing beam splitter 70 on a fourteenth beam splitter 72 directed. There, the beam interferes with the light from the glass fiber G2, which is added via a fifteenth beam splitter. There are beating frequencies f _Schweb , by a first detector 76 be recorded. In front of the detector 76 is a filter 78 arranged, which transmits the light from the glass fibers G1 and G2, but not the light from the glass fibers R1 and R2.

In a sixteenth beam splitter 80 the light rays originating from the glass fibers G1 and G2 are combined, which are not the retroreflector 64 have gone. It creates beat frequencies that serve as a reference.

On the fourteenth beam splitter 72 In addition, the light rays originating from the glass fibers R1 and R2 and passing through the same paths as the light from G1 and G2 also interfere. The generation of the beat signals is from a second detector 82 detected, before a second filter 84 is arranged. The second filter 84 lets the light from the fibers R1 and R2 through, the light from G1 and G2 not.

The light rays that are supplied from the glass fibers R1 and R2 and not the retroreflector 64 have run, interfere with the beam splitter 80 and are from a third detector 86 captured, before a filter 88 is arranged, the filter 84 equivalent. A fourth detector 90 detects the beat signals resulting from the superposition of the light rays from the glass fibers G1 and G2. A beam path in front of the fourth detector 90 arranged filter 92 corresponds to the filter 78 ,

The optical components surrounded by a dashed box are on a simplified laser beam tracking device 79 fixed, which is pivotable about the circle of curvature center M and is adapted to the measuring laser beam 63 the retro reflector 64 track automatically. Such a laser beam tracking device 79 belongs to the prior art and is therefore not described here.

The evaluation unit 58 is with the four detectors 76 . 82 . 86 . 90 connected. The calculation of and computes a first optical path length w _{1 from} the first fundamental frequency and the second fundamental frequency and a second optical path length w ₂ from the third fundamental frequency and the fourth fundamental frequency will be described below.

In the interferometer 56 two laser beams with different frequencies are brought into interference. A beam passes through the route to be measured. The interference is generated on the detectors 76 . 90 an alternating voltage signal with the beat frequency in the form of the difference frequency of both laser beams. The phase Φ of this beat frequency is proportional to the length l of the continuous path (n - refractive index of the air, λ - wavelength of the light): Φ = 4π / λnl (1)

The length is thus: l = Φ / πλ / n (2)

mit

The phase Φ is determined, for example, from a cosine and sine signal by means of the arctangent function. If the length l is measured with light of two wavelengths, two phases result. The difference is:

With

Equation 3 is similar to equation 1, wherein the synthetic wavelength Λ is greater than the two used optical wavelengths λ ₁ , λ ₂ and wherein the refractive index n is changed. An absolute Distance measurement is thus possible within half the synthetic wavelength. The distance interval within which the absolute distance measurement is possible is referred to as uniqueness interval.

Although the uniqueness interval can be set arbitrarily large by the choice of the synthetic wavelength, the measurement uncertainty in the ratio of synthetic to optical wavelength increases.

If the uncertainty is not too great, the interference order N of an optical wavelength can be determined with the length measured from the synthetic wavelength. The "more accurate" length then results from the phase of the optical wavelength according to equation 2 and N times the half optical wavelength: l = N λ / 2n + Φ / 4πλ / n (4)

The scaling of the measurement uncertainty limits the uniquely measurable length. For longer distances, appropriate additional optical wavelengths can be used to provide a chain of synthetic wavelengths. The longest synthetic wavelength determines the unique measurement range and the others must be staggered so that their interference orders can be determined.

The synthetic wavelengths are preferably chosen such that the interference order of an optical wavelength can be determined so that the measurement uncertainty of a counting interferometer results with the uniqueness range of the absolute measurement.

Refractive index compensation

In the above equations, the refractive index n in air is included in the calculation of the length. The refractive index depends on the wavelength of the light, the air temperature, the air pressure, the humidity and the CO ₂ content of the air. The parameters of the air must be measured to precisely determine the length.

The optical path length n1 is measured by the method described above (see equation 1). If the same distance is measured with two different wavelengths resulting from the dispersion of two different optical path lengths w ₁ = n ₁ l and w ₂ = n ₂ L wherein n ₁ and n ₂ are the refractive indices for the two wavelengths.

3 shows the dispersion, ie the dependence of the refractive index on the wavelength.

und g(λ) = 10^–10(3.802 – 0.0384/(λ²/μm)²) (7) und

From the two optical path lengths of w _1, w _2, the influence of the refractive index can be compensated to the measured length. The refractive index can be represented by an equation of the following structure (t - temperature, p - air pressure, x - CO ₂ content of the air, pw - water vapor partial pressure in the air): n (λ, t, p, x, p _w ) - 1 = K (λ) · D (t, p, x) - p _w · g (λ) (5) With

and g (λ) = 10 ^-10 (3,802 - 0.0384 / (λ ² / μm) ² ) (7) and

If the optical path lengths n ₁ l and n ₂ l, designated w ₁ and w ₂ , can be given for dry air (pw = 0) the following relationship for the true distance l: l = w ₁ -A (w ₂ -w ₁ ) (9) With

The parameters of the air in function D are shortened in factor A, so that the length can be determined without measuring these parameters and the factor is given only by the wavelengths.

The factor A is relatively large. For the wavelengths 532 nm and 1064 nm mentioned in the exemplary embodiment, ≈65 applies. The measurement uncertainty in the difference between the two optical path lengths w ₁ and w ₂ increases by this factor.

At humidity, the terms with the functions D and g do not shorten, so that there is a dependency on the air parameters. For moist air, the following solution for the length l can be specified:

The calculated length l thus depends only on the partial pressure of water vapor in the air and not on temperature, pressure and CO ₂ content. Although in Eq. If no explicit factor A emerges, the effect of increasing the measurement uncertainty by this factor remains.

Table 1 below illustrates the effect of changes in air on the measured length, unless compensated by a corresponding measurement. Interferometry with only one wavelength shows the dependence of the refractive index on the air parameters. In the case of the compensated method, the validity of the above-mentioned approximation formulas leaves no dependence on temperature, pressure and CO ₂ content. size default compensated Temperature Δt = +1 ° C -1 x 10 ^-6 - Pressure Δp = +1 hPa +2.7 × 10 ^-7 - Relative humidity ΔRH = +1% -1 × 10 ^-8 -2.4 × 10 ^-8

From the beat frequency f _s between the first fundamental frequency and the third fundamental frequency results in a synthetic wavelength of 15 mm. For the third and fourth fundamental frequency gives a synthetic wavelength of 7.5 mm. The determination of the interference order of the optical wavelengths is not possible here because the uncertainties are too large. To increase the uniqueness interval, further wavelengths are generated by means of acousto-optical frequency shifters in the infrared laser light.

For the laser light with the first and second fundamental frequency, in the present case, the infrared light, in principle a same structure could be used. But since more wavelengths are to be generated with the AOMs, the in 1 used construction shown below.

The beat frequencies are therefore f _Schweb1 = 5 MHz, f _Schweb2 = 7.5 MHz, f _Schweb3 = 8 MHz and f _Schweb4 = 13 MHz.

The four beating frequencies in the measuring beam result in six combinations for synthetic wavelengths. In Table 2 below, all combinations are listed. The frequencies mentioned in the first column relate to the frequency of the first laser light source 12 ,

This results in four synthetic wavelengths Λ of about 15 mm and two synthetic wavelengths Λ of about 1.5 m. The uniqueness interval of the measurement is 0.75 m. However, it is possible to derive further, indirect synthetic wavelengths from four phases:

One of these other wavelengths is about 40 m and provides a uniqueness interval of 20 m for the absolute distance measurement. Frequency / MHz Δ ₁ / MHz Λ ₁ / m Δ ₂ / MHz Λ ₂ / m Δ ₃ / MHz Λ ₃ / m -5 185 190 1,578 20,002.5 20,007.5 0.014984 19,817.5 0.015128 20200 20205 0.014838 20015 0.014978 197.5 1,518 Table 2: Frequency combinations and synthetic wavelengths for the laser light with the first and the third fundamental frequency (here: 1064 nm)

Due to the different heterodyne frequencies, ie the frequencies by which the fundamental frequencies were effectively shifted, there are two beat frequencies at the detectors for the green light 67 . 86 on, for the infrared four beats at the detectors 83 . 92 , Using lock-in technology, all six associated phase values are measured simultaneously.

For evaluation, first the length l with the 40 m long synthetic wavelength is determined and from this the interference order of a 1.5 m long synthetic wavelength. The combination of the order of interference and the phase of the 1.5 m long synthetic wavelength results in a more accurate determination of the length. This finally calculates the interference order of a 15 mm long synthetic wavelength and gives the absolute optical path length for the light at 1064 nm.

The evaluation unit determines from this length 58 the interference order of the 7.5 mm long synthetic wavelength at 532 nm. To avoid errors in the determination of the order by the dispersion, the optical length is converted in the infrared with standard values for the refractive index in the optical length in the green.

Finally, the refractive-index-compensated distance can be determined from both optical lengths. It should be noted that the factor A is significantly smaller for the synthetic wavelengths than for the optical ones. According to Equation 3, the group refractive index is effective for the synthetic wavelengths. Although this differs only on the order of 10 ^-6 of the phase refractive index, but has a stronger dispersion. The factor A is about 20 instead of 65 at the wavelengths used.

Based on the absolute distance measurement, the length can be further determined by counting the interference orders. Since the interference orders of the optical wavelengths can not be derived from the absolute length, a greater uncertainty results for the starting value. The differences to this value, however, can be measured with a smaller uncertainty.

LIST OF REFERENCE NUMBERS

10

 Ray generating device

12

 first laser light source

14

 second laser light source

16

 frequency coupler

18

 absorption cell

20

 beamsplitter

22

 second beam splitter

24

 third, polarizing beam splitter

26

 fourth beam splitter

28

 mirror

30

 mirror

32

 fifth beam splitter

34

 mirror

36

 mirror

38

 sixth beam splitter

40

 mirror

42

 mirror

44

 seventh beam splitter

46

 eighth beam splitter

48

 ninth beam splitter

50

 tenth beam splitter

52

 Eleventh, polarizing beam splitter

54

 Laser length measuring system

56

 interferometer

58

 evaluation

60

 reflector

62

 focusing

63

 Measuring laser beam

64

 retroreflector

65

 reflected laser beam

66

 twelfth, polarizing beam splitter

68

 Fresnel rhombus

70

 Thirteenth, polarizing beam splitter

72

 Fourteenth, polarizing beam splitter

74

 fifteenth, polarizing beam splitter

76

 first detector

78

 filter

80

 beamsplitter

82

 second detector

84

 second filter

86

 third detector

88

 third filter

90

 fourth detector

92

 filter

AOM

 frequency shifter

G1

 glass fiber

G2

 glass fiber

R1

 glass fiber

R2

 glass fiber

f _s

 Beat Frequency

f _hover

 Beat frequencies

l

 distance

M

 Circle of curvature center point

n

 refractive index

w ₁

 first optical path length

w ₂

 second optical path length

λ

 wavelength

Φ

 phase

Claims (10)

Laser length measuring system ( 54 ) with (i) a jet generating device ( 10 ) which (a) is a first laser light source ( 12 ) for - first laser light (L1) of a first fundamental frequency (ν _g1 ) and - third laser light (L3) having a third fundamental frequency (ν _g3 ) which is a multiple of the first fundamental frequency (L1) ν _g1 ), (b) at least one second laser light source ( 14 ) for - second laser light (L2) of a second fundamental frequency (ν _g2 ) and - fourth laser light (L4) having a fourth fundamental frequency (ν _g4 ) which is a multiple of the second fundamental frequency (L2) ν _g2 ), (c) at least one frequency shifter (AOM1, AOM2, AOM3) for shifting frequencies of laser light (L1, L2, L3, L4) by - a first high frequency (f _AOM1), - a second radio frequency (f _AOM2), and - at least a third high frequency (f _AOM3) comprises, (d) wherein the high frequencies (f _AOM1, f _AOM2, f _AOM3) are selected such that at least two different beat frequencies (f _hover ) can be generated by superimposing laser light, (ii) an interferometer ( 56 ) for superimposing laser light (L1, L2, L3, L4) so that the beat frequencies (f _hover ) arise and (iii) an evaluation unit ( 58 ) arranged to automatically calculate the absolute refractive index compensated optical path length from the phases of the beat frequencies (f _hover ). Laser length measuring system ( 54 ) According to claim 1, characterized in that the third fundamental frequency (ν _g3) corresponding to twice the first fundamental frequency (ν _g1). Laser length measuring system ( 54 ) according to one of the preceding claims, characterized by (a) an at least partially convex spherical reflector ( 60 ) having a circle of curvature center (M) and a radius of curvature radius (R), (b) a focusing device (FIG. 62 ) for focusing laser light, in particular of the first laser light (L1), onto the reflector ( 60 ), so that a first measuring laser beam ( 63 ), (c) a retroreflector ( 64 ) for reflecting the first measuring laser beam ( 63 ), so that a reflected laser beam ( 65 ), and (d) a laser beam tracking device ( 79 ), which is pivotable at least about the circle of curvature center (M) and is adapted to the measuring laser beam ( 63 ) the retroreflector ( 64 ) automatically track. Laser length measuring system ( 54 ) according to one of the preceding claims, characterized in that at least one of the laser light sources, in particular an absorption cell ( 18 ), frequency stabilized. Laser length measuring system ( 54 ) according to one of the preceding claims, characterized in that the first laser light source ( 12 ) and the second laser light source ( 14 ) are synchronized. Laser length measuring system ( 54 ) according to claim 5, characterized in that the first laser light source ( 12 ) and the second laser light source ( 14 ) are frequency stabilized. Laser length measuring system ( 54 ) according to claim 5, characterized in that the first laser light source ( 12 ) and the second laser light source ( 14 ) are synchronized over at least one beat frequency (f _S ) which is at least 20 GHz. Laser length measuring system ( 54 ) according to one of the preceding claims, characterized in that the high frequencies (f _AOM1 , f _AOM2 , f _AOM3 , f _AOM4 ) are chosen so that further synthetic wavelengths arise, which are staggered so that half the longest wavelength covers a predetermined measuring range and the measurement uncertainty of the length measurement with one of these synthetic wavelengths is sufficient to successively determine the interference orders of shorter synthetic wavelengths. Laser length measuring system ( 54 ) according to one of the preceding claims, characterized in that the interferometer ( 56 ) is a heterodyne interferometer. Laser length measuring system ( 54 ) according to one of the preceding claims, characterized in that the evaluation unit ( 58 ) is arranged to automatically perform a method comprising the steps of: (i) determining a first optical path length (W ₁ ) based on the first fundamental frequency (ν _g1 ) and / or the second fundamental frequency (ν _g2 ), (ii) Determining a second optical path length (W ₂ ) on the basis of the third fundamental frequency (ν _g3 ) and / or the fourth fundamental frequency (ν _g4 ) and (iii) calculating a distance (d) between the laser length measuring system ( 54 ) and the retroreflector ( 64 ) From the first optical path length (W ₁₎ and the second optical path length (W ₂₎ by compensating the influence of the refractive index of air over the dispersion.

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