DE10224761B3 - Laser beam interferometer has radiation divider feeding measurement arm, reference branch, beam combiner superimposing individual beams from reference branch, measurement arm onto detector in pairs - Google Patents

Abstract

The device (1) has a laser source (5) supplying a measurement arm (3) and a reference branch (4), a radiation divider (7) that produces a fan with a number of individual beams in the measurement arm and reference branch from the beam output by the laser source and a beam combiner that superimposes the individual beams from the reference branch and the individual beams returned from the measurement arm onto a detector in pairs.

Description

The invention relates to a laser beam interterometer for contactless Distance measurement, which is a laser radiation source, which a measuring arm and feeds a reference branch.

Such laser beam interferometer offer for contactless Distance measurement with very high measuring accuracy, in particular if the measurement over a distance should be in the meter range. The exploited physical principle of action is based on the coherence of overlapping Laser beams. These overlaid Laser beams come from a source, but have after division different paths in a reference path or a measurement path kilometer. Laser interinterometers can this principle to both an incremental and an absolute Implement measurements. For an absolute measurement is made in a laser beam interterometer at at least two wavelengths needed.

This is possible in an incoherent length measuring method, in which a transmission beam power is modulated with respect to the transmission frequency, so that the measurement of a phase difference between the beam power going back and forth conveys length information about the object. This principle enables a coherently working, frequency-modulated continuous wave laser radar (also called FMCW laser radar or chirped laser radar), which determines the distance to the object using the signal propagation time by measuring the difference between the transmission and reception frequency. Such a laser radar is described in publication A., Slotwinski et al., "Utilizing GaAIAs Laser Diodes as a Source for Frequency Modulated Contineous Wave (FMCW) Coherent Laser Radars", Proc. SPIE Vol. 1043, Laser Diode Technology and Applications, 1989 , Pp. 245–251 DE 35 87 492 T2 , the DE 33 06 709 C2 , the DE 1960 0491 C1 and the DE 31 36 993 C2 describe laser-based distance measurement processes.

A frequency-modulated continuous wave laser beam interferometer can carry out a distance measurement, but it is a measurement of an object is not possible.

Such a contactless object measurement is but for many applications desirable for example, if the shape of an object is to be checked, as u. a. for wear measurements, e.g. B. of contact wires electrical railways, the case is.

The object of the invention is therefore based on a laser beam interferometer with a laser radiation source, the one measuring arm and feeds a reference branch, so that a non-contact Measurement of an object is possible.

According to the invention, this object is achieved by a laser beam interferometer which is a laser radiation source which a measuring arm and feeds a reference branch, a beam splitter, which from a beam emitted by the laser radiation source in measuring arm and one subject each in the reference branch generated with a plurality of individual beams, and a beam combining device comprises the individual beams from the reference branch and those from the object backscattered Single beams from the measuring arm superimposed in pairs on a detector device.

The concept according to the invention produces one in the measuring arm subjects with a multitude of single beams directed at the object become. The one from the object in the direction of the individual rays hitting it backscattered Radiant power is measured with the reference beam, which is also in a fan was divided from individual beams, brought to interference. By the division of the beam into subjects from individual beams, the structure according to the invention acts like a single beam independent Laser beam interferometer, as far as the measurement of the object is concerned is. Due to the large number of individual beams, there can be too many Points of the object's distance are measured, which automatically results information about the outline of the Object and thus a measurement of the object results.

Under the term "object" one understood structure to be measured, so it may well around an element of a larger object act.

In the laser beam interferometer according to the invention the individual beams from the reference branch are used to measure an object and measuring arm in pairs for overlay brought. It is to be understood by this term that the radiation in the measuring arm detected that is in the direction of the individual beams directed at the object backscattered becomes. Their intensity can be dependent of the backscattering capacity or vary the geometry of the surveying object greatly. from time to time will at all no radiant energy towards an irradiated individual measuring beam backscattered, for example because the object is completely absorbed or the one in question standing single jet at all does not hit the object. Even such cases with vanishingly backscattered radiation intensity are overlaid here in pairs with the associated Detected single beam of the reference branch.

The object distance can be determined for each individual beam. However, the laser beam interferometer according to the invention is not restricted to the application of the principle of a frequency-modulated continuous wave laser radar, son it can also be used in a known manner as a coherent interferometer, which enables simultaneous measurement of a large number of points of an object.

For example, the laser beam pulsed and the pulse transit time are recorded. Also, if appropriate Objects a coherent Summation of the radiation superimposed in the interference point, each after their division into reference branch and measuring arm different ways traveled has to be done. If only one wavelength is evaluated in the interferometer, so there is an incremental procedure. For a non-incremental, d. H. absolutely measuring laser beam interferometry is measurement information required for at least two different laser wavelengths. Then can the difference in length between measuring arm and reference branch clearly within half the synthetic wavelength of the two laser wavelengths can be determined, the synthetic wavelength the product of the laser wavelengths divided by the difference in magnitude. As an alternative to absolute laser beam interferometry Emission wavelength the laser radiation source from an initial value to an end value be tuned. With these measuring methods, however, the coherence in the measuring beam independently from the measurement site required, which is why these uses of the laser beam interferometer according to the invention only with the coherence not destructive, so-called cooperative object surface will offer.

In the case of technical, so-called non-cooperative surfaces On the other hand, the problem arises when measuring an object, that the interaction of a laser beam with the object destroys the spatial coherence, since such surfaces are scattered Light only has fixed phase relationships in neighboring spatial points having. Incremental laser beam interferometry is therefore mostly not possible on technical objects. You could this problem by mounting special reflectors on the surface to be measured Bypass object, however an adverse transition of non-contact to be touched Survey connected. In such cases, therefore mentioned Use of the interferometer as a frequency-modulated continuous wave laser radar by the object distance using the signal propagation time by measuring the difference between a transmit and a receive frequency is determined.

The laser beam interferometer according to the invention is particularly suitable for remote measurement of objects from large distances, d. H. if the object dimension is between a tenth and a Ten thousandths of the measuring distance lies, or resolutions between one hundredth and one millionth of the measuring distance should be achieved. Because in such cases the requirement is natural to one if possible great Signal / noise ratio consists in an advantageous development of the invention intended to use a laser radiation source, the two superimposed, linearly polarized light beams emits, the polarization directions of the light beams around Twisted 90 ° against each other and a detector device is used which has a pole divider and has two detectors. Propagate in the laser beam interferometer then two different directions of polarization, which ultimately results every single beam is doubled because it is in both polarization directions is evaluated. This training course is being built up two measuring interferometers realized who also make an object measurement, the Instrumental expenditure increases only insignificantly, especially not is doubled.

Is such a further developed Laser beam interferometer operated as a frequency-modulated continuous wave laser radar, can the frequency modulation of the superimposed, linearly polarized light beam opposite take place so that the two superimposed propagating in laser beam interferometers light beam not only through the 90 ° against each other twisted direction of polarization, but also in the current one Wavelength, or differentiate frequency. This means increased decoupling reached and a crosstalk largely avoided between the two polarization directions.

The length of the reference branch is effective themselves in the laser beam interferometer according to the invention directly on the interference phenomenon evaluated for object measurement out. The optical path length The reference branch should therefore be used under normal operating conditions preferably remain constant. This can be achieved particularly easily by using the Has a fiber optic link. Such an optical fiber link brings additional the advantage that the Length of Reference branch can be adjusted relatively easily to the length of the measuring arm, without doing it the size of the construction of the Laser beam interferometer would be significantly affected. It is therefore advantageous the fiber optic line on the length of the measuring arm vote so that the Reference branch essentially corresponds to the length of the measuring arm to be expected, which is directly affected by the distance to the object.

If the laser beam interferometer according to the invention is operated as a frequency-modulated continuous wave laser radar, the coherent overlay reception enables a considerable amplification of the useful signal even in the optical range, with which very large dynamic ranges and sensitivities can be achieved. However, considerable non-linearities with respect to parameters of the operating conditions sometimes occur, for example the tuning properties of laser radiation sources are regularly highly non-linear. Depending on the application, in particular the expected temperature range, calibration or correction can be carried out ture of the laser beam interferometer. In order to be able to maintain measuring operation, it is expedient to record the ambient conditions during the measurement and to carry out a corresponding correction, for example in accordance with a previously determined calibration curve.

In a particularly advantageous way in this regard Training, a correction interferometer arrangement is provided, the radiation from the laser radiation source is analyzed. It is it the correction goodness particularly beneficial, if the correction interferometer arrangement has as many components as possible common to the laser beam interferometer, i.e. radiation from decouples the actual interferometer for analysis. A particularly good one Correction can be achieved if the correction interferometer arrangement regarding the components that are not with the actual laser beam interterometer can be shared or become has a parallel structure.

If the reference branch is a fiber optic link it is expedient that the correction interferometer arrangement also includes a corresponding fiber optic link. Become two layered, linearly polarized light beams used in the laser beam interterometer, it has proven to be advantageous if they overlap Beams of light as well propagate in the correction interferometer arrangement and in two detectors acted upon by a pole divider. The detector signals of the correction interferometer arrangement can both for one Meßdatenkorrektur for as well a corresponding adjustment of the laser radiation source is used become.

For the use of the laser beam interferometer as a frequency modulated Laser radar, it is appropriate that the laser radiation source is tunable within certain limits. Of course one could Frequency modulation also take place through downstream elements, however, this is regularly structural consuming. Due to the small size, frequency modulators are particularly preferred Laser diodes.

The beam splitter generates the subjects with a multitude of in the laser beam interferometer Single beams with which the object is measured. in principle this splitting can take place anywhere in the laser beam interterometer as long as it is ensured that the object with the fan out Single beams are illuminated, and the correspondingly backscattered Radiation of the object with the correspondingly assigned individual beams of the reference branch for overlay brought. From the point of view that this overlap for the measurement result What is significant, it has proven to be particularly expedient, the subject production before dividing the beam path into measuring arm and reference branch, because then an identical divergence angle of the subjects is ensured, the a simple yet exact paired overlay of the individual beams allowed. It is therefore a further development of the laser beam interferometer preferred, in which the beam splitting device is a grating, that the subjects generated with the large number of individual beams, as well as a first Has beam splitter. If the beam splitter is then connected downstream of the grating, there is the described advantage of simple superimposition, since in the reference branch and the measuring arm right from the start a corresponding subject Single beams is injected.

Of course, the arrangement of Grid and beam splitter are also reversed, so that the first Beam splitter the beam generated by the laser radiation source in radiation for the measuring arm and radiation for splits the reference branch. The shares so divided can then by one or more grids fanned out become. This structure has the advantage that the length of the reference branch increases a big one Part of just one beam, not against it from a fan is traversed by single rays, which is from the point of view the beam guidance or less vulnerability for annoying beam attenuating effects is advantageous. In particular, there is the possibility to incorporate a glass fiber into the reference path that meets the requirements optical path length provides. BE REDUCED on the other hand, a fan already exists in the beam path at this point Single rays before, would a corresponding variety of fiber optic lines needs what represents a considerable expenditure on equipment.

The grid for generating the individual beams must be the required Ensure beam separation. A holographic is particularly preferred Grid because this is inexpensive producible and uncritical to Is misalignments.

In the variant mentioned, in which the division into the compartments with individual beams only takes place after the beam bundle has been separated into the measuring arm and reference branch, it is advantageous from the point of view of a possible precisely fitting superposition of the individual beams of the fan in front of the detector, the fanning out of the To effect beam bundles in the measuring arm and reference branch through one and the same grating, in particular if a glass fiber link is integrated in the reference branch. It is therefore preferred that in the above-mentioned development of the laser beam interferometer according to the invention with a grating and a first beam splitter, the first beam splitter divides the beam bundle emitted by the laser radiation source into a reference beam bundle fed into the glass fiber path of the reference branch and into a measuring beam bundle fed into the measuring arm, the grating being irradiated in parallel by the reference beam and by the measuring beam and the beam combining device then superimposing the individual beams of the subjects in pairs by means of a second beam splitter. With this approach will not only a very compact design is achieved, since only one glass fiber link is required to generate the required optical path length in the reference branch, and in particular when using a holographic grating, the use of only a single grating can ensure an identical fanning out of the two beam bundles , which is advantageous for a simple, yet exact, pair-wise superposition of the individual beams.

The laser beam interferometer according to the invention is especially for suitable for the measurement of objects from a great distance. The maximum measurement accuracy of course, when measuring an object, if the area over which the individual beams of the fan distribute are, if possible corresponds exactly to the object geometry to be measured, d. H. if possible all individual rays fall on the object area to be measured. To meet this requirement must the Divergence angle of the fan of rays be adapted to the distance to the object being measured and its size. For this purpose, it is conceivable for the properties of the beam splitting device to adapt, for example to use an appropriate grid. But then sometimes when adapting to different object or surveying geometries a considerable intervention in the optical structure of the interferometer would be necessary, it is advantageous in measuring arm arrange an optics that the divergence angle of the irradiation of the fan in the measuring arm adapts to the distance to the object and the object size. Even then can on itself while changes occurring during the measurement reacted by object distance or size become.

As mentioned earlier, the object should be through the in the measuring arm fanned out Single beams if possible Completed detected become. This requires an appropriate alignment of the laser beam interterometer to the object. However, it is often not possible to use the laser beam interferometer to be arranged in the required position. One example of this is surveying of contact wires electrical orbits where the location of a laser beam interferometer a measuring vehicle is specified within narrow limits so that the laser beam interferometer does not always the direction you want to the object. For such applications, it is appropriate to to provide the corresponding mirror in the measuring arm, the fan aimed at the object. change now the location of the object can be an otherwise inevitable Realignment of the laser beam interferometer to the object avoided if the mirror is designed as a rotating mirror, the is set so that the subjects in the measuring arm the object falls.

The intensity of the two superimposed in pairs Single rays, especially with a frequency-modulated continuous wave laser radar with one if possible low detection limit become. Corresponding detector fields are available here first for example arrays or rows. However, it can be considerable Bring effort to use detectors that have a low detection limit have to summarize in a detector field that the corresponding Has dimensions. For such cases it is advantageous to use individual detectors, each of which a fiber coupler is pre-shaded so that the inputs of the fiber coupler with the required geometric structure are arranged in which the superimposed Single beams fall onto the detector unit. This geometric The arrangement is regular Lined up along a straight line. The concept, fiber coupler to use offers a considerable economic Advantage because inexpensive, mass-produced but still highly sensitive single photo receivers used can be. Furthermore can such individual recipients also corrected relatively easily with regard to their detection characteristics become what at field-shaped Although detectors are also possible in principle, they are usually more complex or in terms of performance limited is. Gallium arsenide avalanche diodes, one of which are particularly suitable as detectors have a low detection limit and very good dynamic behavior.

The laser beam interferometer according to the invention can be varied be used where it is based on contactless remote measurement arrives at the geometry of an object. Since the laser beam interferometer according to the invention, especially when the principle of frequency modulated continuous wave laser radar tracked, enables a particularly fast measurement, can do it anywhere with special profit be used where short measuring times or a high measuring cycle are necessary. Such an application can be found, for example, in measurement the state of wear a contact wire of an electric train, since then with the laser beam interferometer according to the invention a route whose contact wire needs to be measured at high speed can be driven.

The invention is described below Reference to the drawing is explained in more detail by way of example. In the drawings show:

1 schematically a laser beam interferometer with which an object is measured,

2 the frequency profile of a modulated transmission laser beam and the frequency profile of a received laser beam in a laser beam interferometer operating as a frequency-modulated continuous wave laser radar,

3 a time course of the intensity of a signal with an intermediate frequency, which occurs when using a laser beam interferometer as a frequency-modulated continuous wave laser radar,

4 a schematic representation of a Laser beam interferometer with two correction interferometers,

5 a more detailed representation of a laser beam interferometer, similar to the structure of the 4 .

6 a further embodiment of a laser beam interferometer, similar to the structure of 4 .

7 a more detailed representation of a further embodiment of a laser beam interferometer,

8th a profile of a contact wire to be measured with a laser beam interferometer and

9 shows an arrangement of a laser beam interferometer on a measuring vehicle for detecting the state of wear of a contact wire of an electrical train.

In 1 is a laser radar 1 shown schematically to explain the principle of operation. It is used to measure an object 2 , in a measuring arm 3 of the interferometer setup of the laser radar 1 lies. The object 2 in the measuring arm 3 backscattered radiation into a reference arm 4 Coupled radiation brought to interference and from the evaluation of the interference phenomena on the distance to the object 2 closed.

The laser radar 1 has a laser diode 5 which generates monochromatic coherent light with a carrier frequency of 300 THz, ie the wavelength is of the order of 1 μm. The radiation from the laser diode 5 is tuned according to a scheme yet to be described, the modulation swing being in the order of magnitude of 300 GHz or 1 nm, and passes through an optical isolator 6 which is the laser diode 5 protects against possibly back-reflected radiation. Then the beam of light from a beam splitter 7 divided into two bundles. A reference beam is in the reference arm 4 directed and arrives at a mirror there 8th , The other bundle is called a measuring beam with a lens 9 on the object 2 focused. After reflection on the mirror 9 or the backscatter on the object 8th both beams hit the beam splitter again 7 and are coherently superimposed on the photodiode 10 directed. The signal from the photodiode 10 is from an amplifier 11 read out, amplified and via a connection (shown as a dashed line) to an evaluation unit 12 forwards. This in turn sends corresponding control signals to a modulator control 13 that tune the wavelength of the laser diode 5 controls.

The radiation of the reference arm and measuring arm is reproducibly coherent as long as the object is within the Rayleigh length of the focus of the optics 9 located.

2 shows the time course of the frequency of the transmission signal 14 as well as the received signal 15 plotted against time t. As can be seen, the frequency of the transmit signal 14 , ie the frequency of that of the laser diode 5 emitted radiation is adjusted sawtooth-like starting from a carrier frequency TF via a frequency deviation FH. The signal with the transmission frequency 14 lies in the form of radiation from the reference arm 4 on the photodiode 10 on. The receive signal 15 , which is shifted in time at the photodiode 10 arrives, however, comes from the measuring arm 3 , The reason for this time delay is a larger optical path length of the measuring arm 3 compared to the reference arm 4 , which is due to the previously unknown distance of the object 2 is conditional. The mixture of radiation from the measuring arm 3 and reference arm 4 , ie from the transmission signal 14 and reception signal 15 leads to an intermediate frequency IF, which in 2 is shown schematically.

3 shows the time course of the intensity of the signal at the photodiode 14 , in the form of an interference signal 16 , The period occurring in this signal is determined by the intermediate frequency IF and is therefore a measure of the distance of the object 2 ,

The intermediate frequency ZF or difference frequency ν _Diff is the difference in magnitude between the transmission frequency ν _s and the reception frequency ν _r at all times. It results as the distance sought (difference in the lengths of the measuring arm 4 and reference arm 3 ) R multiplied quotient of double frequency deviation Δν and product of modulation period T _m and speed of light c:

With knowledge of the intermediate frequency ZF, the distance R can be determined as the product of the difference frequency, modulation period and speed of light divided by the double frequency deviation:

You just have to use the interference signal 15 record the number of vibrations within a certain period of time. The evaluation unit 12 can thus be implemented as a simple counter. If, for simplification, only the number of intensity maxima is recorded within the modulation period, the distance can even be determined as the product of the number of intensity maxima and the speed of light divided by twice the difference frequency.

The resolution increases with the number of detected maxima. It follows that if the signal strength is sufficient, the resolution does not depend on geometric conditions, but only on the frequency swing (FH-TF). Of course, the measurement time is dependent on the number of recorded intensity maxima. The resolution is therefore also dependent on the measurement time, which of course is not surprising in the result.

4 shows a more detailed representation of an interferometer, which is essentially the structure of the 1 follows. The interferometer 1 of the 4 is used to measure contact wire 17 , Corresponding components are therefore given the same reference numerals; with regard to their description, if necessary, the description 1 directed.

A major difference in the construction of the 4 for schematic representation of the 1 however, is that next to the laser source 5 a second laser source 18 is provided. The radiation from both laser sources feeds the laser beam radar 1 , The two laser beam sources, as in 4 is shown schematically, tuned frequency-changing by push-pull; this is through appropriate sawtooth lines next to the laser diodes 5 and 18 respectively. Due to the push-pull control, one laser radiation source is changed in frequency upwards, while the other is tuned downwards. The radiation from the laser diodes 5 . 18 is polarized. This is the laser diode 18 a polarizing filter, the laser diode 5 a polarizing filter 2 downstream. A linear polarization takes place, the polarization directions of the two beams being rotated through 90 ° to one another, ie the polarization directions are perpendicular to one another.

The beams that are frequency-modified in this way and polarized perpendicular to one another are transmitted via a beam splitter 21 combines and then fall after passing through the optical isolator 6 , which is designed here as a spatial filter by means of optical fibers connected between two corresponding fiber couplers, onto a holographic grating 22 which separates the combined laser beam into seventeen individual laser beams of the same intensity. The fan formed in this way from seventeen individual beams is turned over a beam splitter 23 a single beam is divided and fed into a correction interferometer arrangement to be explained. The sixteen individual beams remaining in the fan are then passed through the beam splitter 7 either in the reference arm 4 or in the measuring arm 3 directed.

In the measuring arm 3 is a rotating mirror 26 provided the sixteen individual beams of the fan on the contact wire 17 directed. The objective 9 is designed so that the cross section of the fan in the object plane essentially the extension of the contact wire 17 equivalent. On the surface of the contact wire 17 the individual beams have a diameter of approximately 0.2 mm and a grid spacing of 1.7 mm. This achieves an optimum between resolution on the one hand and crosstalk between the beams on the other. The width of the compartment is about 25 mm. This is a measurement window in which a contact wire 17 can be measured. After reflection on the mirror 8th or backscattering on the contact wire 17 the radiation is returned in the two compartments, through the beam splitter 7 combined and passed through an optical system and thus fed coherently mixed to the detector device. To in the reference arm 4 To avoid unnecessary divergence of the fan is in the reference arm 4 an optic 24 arranged, which is essentially the lens 9 equivalent. The mean distance between the rotating mirror 26 and contact wire 17 is approximately three meters, the extreme values specifying the measuring range being between 2.75 and 3.25 m. The length of the reference arm 4 is therefore tuned to 2.75 m.

The detector device has a pole beam splitter 27 which divides the individual beams consisting of two parts polarized perpendicular to one another and a direction of polarization onto a detector line 28 the other on a detector line 29 passes. The detector lines 28 and 29 reproduce corresponding signals of the received coherently superimposed mixed light of the individual beams, whereby for each individual beam of the fan a separate signal corresponding to that in 3 interference signal shown 16 is produced. The distance for each individual beam can thus be determined or calculated according to the relationships given above. Based on the knowledge of the position of the individual beams or the spacing of the individual beams in the object plane, an exact profile of the measured contact wire can be obtained from this 17 win.

As already mentioned, the distance is determined, ie the geometry measurement of the object 17 the difference frequency of the radiation from the laser diodes 5 . 18 on. In order to be able to determine or set them as precisely as possible, a correction measuring arrangement is described with which the radiation from the laser diodes is used 5 and 18 is analyzed. For this, the one on the beam splitter 23 decoupled seventeenth beam from the grid 22 generated fan over a pole beam splitter 30 in two correction interferometers 31 and 32 initiated with the help of non-linearities and drift phenomena of the laser diodes 5 and 18 as well as the electrons are balanced. Furthermore, the path difference known in the correction interferometers between the two arms of the correction interferometer allows the difference frequency to be determined exactly. The correction interferometer analyzes this 31 the radiation from the laser diode 18 , the correction interferometer 32 the radiation from the laser diode 5 , The ratio of the lengths of the two interferometer arms of the correction interferometer 31 and 32 preferably corresponds to the length ratio of the measuring arm 3 to the reference arm 4 , because it makes it easier to correct the laser diodes or the detectors 29 and 28 emitted signals can be achieved.

The laser diode 5 is equipped with a frequency-selective resonator with a view to stable radiation of monomodal radiation. For the wavelength used in the infrared range in the present exemplary embodiment, one is used in the reso nator integrated structure used, which has the advantage that it can be controlled purely electrically without mechanical moving parts, whereby a high tuning speed of about 300 GHz is achieved in a time window of less than 0.2 ms. These high tuning speeds enable distance measurements over several meters. However, other tuning means can also be used for measurements over short distances, for example an external mechanical-optical resonator.

The detector lines 28 and 29 are silicon avalanche diode rows with sixteen elements designed as a detector array in the present embodiment. This type of diode provides a signal / noise ratio that is sufficient for the present measurement. Alternatively, indium germanium arsenide detectors could also be used.

The control of the laser diodes 5 and 18 through the in 4 The triangular signals shown schematically are such that an upward or downward ramp of the frequency is at least 512 periods in the signal of the correction interferometer 31 and 32 generated. After linearization of the detector lines 28 and 29 supplied measurement data using the signals of the correction interferometer 31 and 32 the measured values are digitized and a multiplication with one another necessary to eliminate the Doppler effect. Objects that have a component of motion in the direction of the measuring beam generate a speed-dependent intermediate frequency (Doppler effect), which adds or subtracts to the distance-dependent intermediate frequency, depending on whether the frequency of the laser is tuned up or down (up-chirp or down CHIRP). If the interference signals in the case of lasers are multiplied by up-chirp and down-chirp, the influence of the speed of the object is canceled out and the measurement signal (ie the intermediate frequency) only depends on the distance to be determined.

Then lie for each of the sixteen measuring locations 1024 Measured values in front of which the so-called Fast Fourier Transformation (hereinafter referred to as FFT) gives the intermediate frequency and thus the distance between the measuring locations on the contact wire 17 to the interferometer is determined. This FFT takes about 0.2 ms to complete. In order to achieve a measuring speed of 0.2 ms per profile section, the sixteen data streams are processed in sixteen specific FFT processors arranged in parallel. The sixteen result data then obtained in parallel are fed to a computer for profile calculation. This involves, among other things, averaging the profile data, averaging over approximately sixteen profiles. This results in a spatial resolution of 18 cm along the contact wire at a scanning speed of 80 km / h 17 reached.

Due to the use of two orthogonally linearly polarized beams in the interferometer and the multiplication of the interferometer signals mentioned, incorrect measurements due to Doppler shifts in the measuring frequency due to the relative movement of the contact wire 17 and interferometers can be avoided. That of the single beams in the fan of the measuring arm 3 included area (so-called measuring window) must of course change the position of the contact wire 17 follow, usually due to a slight zigzag laying of the contact wire 17 is the case over the rails. The wire position must therefore be detected separately to ensure that as many individual beams as possible in the measuring arm 3 on the contact wire 17 fall. For this purpose, a transit time laser radar (not shown) is provided, the laser beam of which is transverse to the longitudinal direction of the contact wire 17 scans to the position of the contact wire 17 capture. The detected position is then used to control the rotating mirror 26 evaluated.

5 shows a more detailed representation of a laser radar 1 which is essentially the same as in 4 schematically shown structure corresponds. The radiation from the laser diodes 5 and 18 is fed via polarization-maintaining monomode fibers. These fibers are in for the sake of clarity 5 provided with the reference numbers of the laser diodes.

After the radiation has been decoupled via appropriate fiber couplers, the polarizing filter is used 19 and 20 polarization of the radiation, and the partial beams are superimposed by means of a pole beam splitter PT1.

On a downstream first The resulting bundle becomes divisor T1 polarization-neutral in relation 19: 1 into a measuring and shared a reference bundle, each in a polarization-maintaining monomode fiber RF, RM be coupled.

The fiber RF traversed by the measuring bundle ensures that the beam geometry of the measuring bundle after emerging from the fiber is the same as the geometry of the reference bundle after emerging from the fiber RM. While the fiber RF thus only fulfills the function of a spatial filter, the fiber RM, as part of the reference arm, also generates the path difference required for the measurement; it is therefore part of the reference arm 4 ,

The measuring beam passes through the measuring arm 3 then a grating G, which generates sixteen individual bundles of the same intensity at a certain angle to one another; the fanning out is perpendicular to the display plane. The fan passes through a second divider T2 and is imaged by a lens L1 in such a way that a reduced image of the point grid required on the object is produced in a downstream aperture BI1. This grid of points is reflected on the mirror in Sp10 by lenses L2 and L3 (in 5 object not shown).

Part of the radiation reflected or backscattered on the object reaches the same Way back to the divider T2 and is deflected towards the aperture BI2. From the sixteen infinity bundles present there, passing through the aperture BI2, the lens L7 generates an image of the object point raster, which is divided by a pole divider PT2 according to the polarization planes into two receivers E1 and E2 designed as detector arrays.

The receivers E1 and E2 comprise sixteen Fibers (not shown), in the ends of which the sixteen infinity bundles of the lens L7 are coupled. Located on the other end of the fibers gallium indium arsenide photodiodes. The unit from the sixteen Photodiodes with the upstream fiber couplers act as receivers. The order the open ends of the fiber couplers correspond to the grid of points, d. H. the line of sixteen spots in the two image planes of the measuring interferometer.

In the reference arm 4 the fiber coupled out from the path length difference producing the radiation is directed to the grating G via a mirror Sp12 and a mirror Sp2. Before that, however, the reference bundle is divided again in a ratio of 19: 1 with a divider T3. The smaller divided part is used to operate a correction interferometer arrangement which has a divider T4, which comprises a reference arm with a mirror Sp7 and a branch corresponding to the measuring arm with a fiber FK and a mirror Sp9. The divider T4 superimposes the radiation from the measuring and reference arm of the correction interferometer. The superimposed radiation is then fed via a mirror Sp8 and a lens L5 to a pole beam splitter PT3, which separates the radiation according to the directions of polarization and feeds it to two receivers EK1 and EK2. The path length ratios of these two correction interferometers thus correspond exactly to those of the actual measuring interferometer with the nominal position of the measurement object.

The reference beam in the measuring interferometer radiates through the grating G parallel to the measuring beam and is also broken down into sixteen individual beams. In the same way as in the measuring arm 3 also generated in the reference arm 4 a lens L8 in a diaphragm BI3 an intermediate image, which is then imaged to infinity by a lens L9 and fed to the divider T2. To simplify the beam path, mirrors Sp3, Sp4 and Sp5 are also interposed.

In order to obtain interference at the receivers E1 and E2, the sixteen individual beams become measuring arms 3 and reference arm 4 merged at the divider T2 at the same height and at the same angle. The 1: 1 image through the lenses L8 and L9 in the reference arm 4 enables this.

In contrast to the Sciatic laser radar 1 of the 4 is in the variant of 5 that of the two laser diodes 5 and 18 generated beam bundles into measurement bundles and reference bundles not after but divided before passing through the grating G; the divider T1 is therefore in front of the grid and not after it. This has the advantage that most of the reference arm 4 is traversed by only one beam instead of all individual beams. The required path length difference, which in the embodiment of the 5 corresponds to a path length of 10.8 m in air, can be generated in a space-saving and inexpensive manner by the monomode fiber RM.

After passing through the fibers RF and RM, the two bundles pass the grating G spatially separated, ie laterally offset, as a result of which the sixteen individual beams of the reference arm 4 and the sixteen individual beams of the measuring arm 3 can be overlaid in a short way. This not only reduces the size of the laser radar 1 , but also facilitates the exact superposition of the individual bundles from the measuring arm 3 and reference arm 4 at the beam splitter T2.

The use of the three dividers T1, T2 and T3 further enables a distribution of the intensities of the measuring arm which is favorable for signal acquisition 3 and reference arm 4 and at the same time provides an additional bundle for operating the correction interferometer.

6 shows a further embodiment for a laser radar 1 , The embodiments of the 4 and 5 corresponding components are provided with the same reference numerals. The radiation from a laser diode 5 is fanned out into individual beams by means of a grating and via optics and an objective in a measuring arm 3 on the object 2 directed. Before that, however, using a beam splitter 43 the radiation for the reference arm is decoupled. Two mirrors 45 and 46 care together with the optics 24 and the mirror 8th for the corresponding path length difference in the reference arm 4 ,

The from the object 2 ' backscattered radiation is at a beam splitter 44 to a detector line 28 directed. Another beam splitter 47 combines the radiation from the measuring arm 3 and the reference arm 4 ,

The one between the grid 22 and the lens 9 lying lenses ensure that the focal plane of the separated individual bundles between the two, the grating 22 subordinate lenses, the object plane in which the object is 2 and the detector level in which the detector line is located 28 located, are all conjugated to each other. By the ratio of the focal lengths of the lens 9 and the optics 25 can widen the single beam fan on the object 2 the size of the detector line 28 be adjusted.

The 7 shows another design for a laser radar 1 , in which the radiation from two lasers, here diode lasers DL1 and DL2, are also combined by means of a pole splitter PTL, which is preceded by collimators K1 and K2. The pole divider PTL not only effects the superposition of the individual beams from the diode lasers DL1, DL2, but also the corresponding polarization. The radiation for the measuring interferometer (dividers T1 and T2) and the correction interferometer (T3 and downstream mirror) is divided via dividers T1, T2 and T3.

The split beam components are coupled into polarization-maintaining monomode fibers via fiber couplers FK1, FK3, FK5 and FK7. The fiber coupler FK1 feeds a fiber RM, which is part of the reference arm 4 of the interferometer. In contrast, the fiber RF connected via the fiber coupler FK3 has the function of the spatial filter already mentioned and corresponds to the fiber RF of 5 ,

After the radiation guided through the fiber RF has emerged at the fiber coupler FK4, the measurement beam on the grating G is separated into a fan (consisting of individual beams) (perpendicular to the plane of the representation). A lens L1, L2 and L3 and apertures BI1 and BI2 having optics focus the single beam on the object (not shown) in the measuring arm 3 , The radiation backscattered from the object is divided via a divider T5 located between the lenses L2 and L3 and fed via a mirror Sp2 through the lenses L2, the diaphragm BI1 and the lens L1 and further deflecting mirrors as well as a lens L7 and an aperture diaphragm ABI to a pole divider PTE , which directs the radiation in the two polarization directions separately to the receivers E1 and E2, each of which is preceded by a field diaphragm FBI1 and FBI2.

The radiation of the reference arm 4 After it has been coupled out of the fiber RM on a fiber coupler FK2, it is also separated from the grating G into a fan of individual beams. This fan then also passes through the lens L2, the diaphragm BI1 and the lens L2 and is deflected by a mirror Sp2 to a splitter T6 which emits the radiation from the reference arm 4 with the one in the measuring arm 3 object is superimposed on backscattered radiation.

The peculiarity of the structure of the 7 consists in that all emitted and returning rays pass through the same optics comprising the lenses L1, L2 and L3. This is achieved by a slight angular offset between the two compartments. Since also the fan of the reference arm 4 is passed through the lenses L2 and L1, overall a very symmetrical and thus insensitive to possible radiation attenuation is achieved.

The radiation for the correction interferometer goes through also one serving as an optical isolator between two fiber couplers FK5 and FK6 switched monomode fiber RF as well as the necessary distance difference producing fiber, also lying between two fiber couplers FK7 and FK8 RK. over Mirror and a divider T4, the radiation from the measuring arm and Reference arm of the correction interferometer superimposed on a divider T4 and over a lens L8 on two receivers EK1 and EK2 shown, with a lens L8 downstream Pole splitter PTK effects the separation according to the polarization planes.

8th shows the profile of a contact wire 17 how he with the laser radar 1 can be measured. Contact wires with a circular cross-section are common in the area of Deutsche Bahn. The contact wire 17 has a new height H, which corresponds to the diameter d. With increasing wear, however, only a residual height RH remains, as a proportion 41 from the contact wire 17 is ground during operation, leaving only a residual wire 42 remains. The residual height RH must be above a predetermined minimum value. If the diameter d is known, it can easily be determined from the mirror width a.

With a square or rectangular cross-section of a contact wire 17 As used, for example, in the area of Austrian railways, the residual height RH cannot, however, be calculated from a mirror width a, but must be measured from the side of the contact wire 17 be recorded.

9 shows a device for measuring a contact wire 17 that is a laser radar 1 , for example with the construction of the 5 used. The laser radar 1 is on a car roof 33 attached and in a housing 34 attached. The one from the laser radar 1 generated fan beams 35 is via a deflecting mirror 36 on the contact wire 17 directed. The contact wire 17 lies in the representation of the 9 not on the perpendicular to the middle of the car 37 , but is offset to the side. It is still at a contact wire height 38 over the top of the car roof 33 or above the top edge of the rails.

The one from the deflecting mirror 36 on the contact wire 17 targeted subjects 35 occurs at a window 40 out of the housing 34 out. The window 40 closes the case 34 that the laser radar 1 picks up so that the optical components of the laser radar 1 are protected against pollution. Here is the window 40 preferably provided with an automatic cleaning device, for example a windshield wiper or a steam jet. In addition, the window 40 can be darkened by a darkening device, for example a blind, around the interior of the housing 34 when the laser radar is not in operation 1 to protect against unnecessary heating by solar radiation. In addition, the darkening device can also effect the function of radiation protection by using the beam fan 35 exiting the case 34 is prevented.

The laser radar 1 is down to the rotating mirror 26 on a sled 39 attached, which is displaceable in the direction of an arrow (not specified) in order to adapt the laser radar 1 an un different contact wire heights 38 to be able to guarantee.

Because the length of a single contact wire in an electric train 17 is limited to approximately 1 km, overlaps occur at the respective ends between the preceding and the following contact wire 17 on. In the overlapping areas, two wires run over a distance of approximately 60 m in length at almost the same contact wire height 38 at a horizontal distance of 20 to 45 cm side by side. One contact wire sinks a little, the other rises. The height difference between ascending and descending wire is between 0 and 15 cm.

Since the elasticity of the contact wire system changes at the overlap areas compared to the rest of the route, wear on the contact wire can also occur there 17 be elevated. To ensure that such critical points are completely covered, are on the roof of the car 33 preferably in a housing 34 two independent laser radars 1 provided for each wire. To quickly find the contact wire that is newly added from the top to the side 17 To enable, the runtime laser radar used at the outset is used, whose upward laser beam continuously scans across the direction of travel and thus the height and lateral position of several contact wires 17 can detect, the measurement uncertainty is smaller than that of the beam fan 35 covered area.

If the contact wire 17 vertically above the laser radar 1 no residual height can be measured with a rectangular wire profile. This is particularly disadvantageous in the critical area in which two contact wires overlap. In the case of an overlap area that lies in the middle of the car, the remaining height of the contact wire is nevertheless required for a rectangular wire profile 17 To be able to record takes place as in 9 illustrated, the lighting with the fan of rays 35 over the deflecting mirror 36 from the side and not from the perpendicular to the middle of the car 37 out.

Claims (11)

Laser beam interferometer for the contactless measurement of an object ( 2 ), whereby - the laser beam interferometer ( 1 ) a laser radiation source ( 5 . 18 ) which has a measuring arm ( 3 ) and a reference branch ( 4 ) feeds, has, - a beam splitting device ( 7 . 22 ) is provided, which consists of a from the laser radiation source ( 5 . 18 ) emitted beam in the measuring arm ( 3 ) and in the reference branch ( 4 ) each creates a fan with a large number of individual beams, and - a beam combining device ( 7 ) is provided, the individual beams from the reference branch ( 4 ) and that of the object ( 2 ) backscattered individual beams from the measuring arm ( 3 ) on a detector device ( 10 ) superimposed in pairs. Laser beam interferometer according to claim 1, with a laser radiation source ( 5 . 18 ), which emits two superimposed, linearly polarized light bundles, the polarization directions of the light bundles being rotated by 90 ° relative to one another, and with a detector device which has a pole divider (PT2) and two detectors (E1, E2). Laser beam interferometer according to one of the above claims, the reference branch ( 4 ) one to the length of the measuring arm ( 3 ) has a coordinated fiber optic link (RK). Laser beam interferometer according to one of the above claims, comprising a correction interferometer arrangement ( 31 . 32 ), the radiation from the laser radiation source ( 5 . 18 ) analyzed. Laser beam interferometer according to one of the above claims, the laser radiation source ( 5 . 18 ) is tunable. Laser beam interferometer according to one of the above claims, the beam splitting device ( 7 . 22 ) has a grid (G) generating the compartments with the plurality of individual beams and a first beam splitter (T1). Laser beam interferometer according to claims 3 and 6, in which the first beam splitter (T1) that from the laser radiation source ( 7 . 22 ) emitted beams into a fiber optic link (RM) of the reference branch ( 4 ) fed reference beam and into a into the measuring arm ( 3 ) feeds in the measuring beam bundle, the grating (G) being irradiated in parallel by the reference beam bundle and by the measuring beam bundle and the beam combining device being superimposed on the individual beams of the subjects in pairs by means of a second beam splitter (T2). Laser beam interferometer according to one of the above claims, the measuring arm ( 3 ) has an optic (L2, L3) that detects the divergence angle of the individual beams of the fan in the measuring arm ( 3 ) to the distance to the object ( 2 . 17 ) and adapts to the object size. Laser beam interferometer according to one of the above claims, comprising a rotating mirror ( 26 ) which is in the measuring arm ( 3 ) the fan on the object ( 2 . 17 ) judges. Laser beam interterometer according to one of the above Claims, whose detector device several photodiodes with upstream Has fiber couplers, the inputs of which are lined up in a line and form an optical input of the detector device. Using a laser beam interferome ter according to one of the above claims for detecting the state of wear of contact wires of electrical railways.