DD254792A5 - MEASURING DEVICE AND MEASURING METHOD FOR DISTANCE MEASUREMENT BY POLARIZATION MODULATION - Google Patents

Abstract

In a distance measuring system, polarized light is projected from a main unit (8) to a target reflector (46) and reflected again. The polarization is modulated by a Pockels electro-optical crystal (20) located in the beam path of the projected and reflected light, with a variable light path (42, 44) being adjusted so that the photodetector (50) passes through after the return of the reflected light a polarization filter (18) is determined a zero value. In order for a single filter (18) to be sufficient for the polarization of the projected beam and for the filtering back of the beam, a relative phase delay of about one quarter of a wavelength can be introduced by a rhombus (38) into the projected and returned beam. In order to avoid the otherwise required orientation of the filter (18) with the axes of the crystal axes transverse to the beam, the rhombus must be placed in front of the crystal (20) in the projected beam such that the crystal generally receives circularly polarized light. To regulate the quality of the zero value achievable at the photodetector, the orientation of the rhombus can be readjusted. So that the zero value can be detected with greater accuracy, either the modulation wavelength, the length of the light path or the time delay of the light path can be wobbled. To derive measurements for a structure that are corrected for a reference temperature, the modulation wavelength can be set for a reference resonator (72) made of the same material as the structure and in contact with the crystal. Fig. 1

Description

Detection of the reflected part of the beam after the modulation; characterized by the step of introducing an adjustable unmodulated relative phase shift in the beam.

For this 4 pages drawings

<Field of the invention

The invention relates to a measuring device and a measuring method for the distance measurement using the polarization modulation of a beam.

A first aspect of the invention relates in particular to a measuring device consisting of:

- a device for the projection of a polarized beam, with the aid of which the beam can be reflected back (reflected) substantially along the same path;

- a device for the modulation of the polarization of the projected and reflected parts of the beam; and

- A device for the detection of the reflected part of the light beam after the modulation.

Furthermore, a second aspect of the invention relates to a corresponding measuring method comprising the following steps:

- Projection of a polarized beam;

- Reflection of the polarized beam substantially along the same path;

- modulation of the polarization of the projected and reflected parts of the beam; and

- Detection of the reflected part of the beam after modulation.

Characteristic of the known state of the art

Such a meter and method are described in GB patent 1172668. In the arrangement disclosed in this patent, the beam is polarized by a first polarizing filter and then modulated by at least one electro-optic crystal. The reflected part of the beam is modulated at least by another such crystal having a crystal structure whose X and Y axes are perpendicular to those of the first crystal, and then by a second polarizing filter whose polarization direction intersects with that of the first filter , filtered.

It is estimated that with two equal, precisely aligned crystals, with precisely aligned polarizing filters and eliminating scattering polarization effects or natural birefringence of the crystals, no radiation will pass through the second polarizing filter as the distance along the optical path from the first crystal to the second crystal enters is an integer multiple of the modulation wavelength. However, it is difficult, if not impossible, to design the measuring assembly to function perfectly.

Object of the invention

The aim of the invention is to reduce both the manufacturing costs and the adjustment effort.

Explanation of the essence of the invention

The invention has for its object to provide a measuring device and a measuring method for distance measurement by means of polarization modulation, which requires a smaller number of components and assemblies and in which all caused by the modulation device or by the other optical components unwanted polarization effects are compensated.

According to the first aspect of the invention, the meter is characterized by a phase shifting device for introducing an adjustable unmodulated relative phase shift in the beam. A corresponding measuring method according to the second aspect of the invention is characterized by the step of introducing an adjustable unmodulated relative phase shift in the beam. Thus, any unwanted polarization effects caused by the modulation device or by the other optical components of the system can be compensated. The phase shifting device may include a rhombus and means for adjusting the rhombus about an axis perpendicular to the plane of the rhombus so that the phase shift caused by the rhombus can be adjusted. It may also have means for adjusting the pivotal movement of the rhombus at least about an axis in the plane of the rhombus with which the major axes of the rhombus and the modulating device can be accurately aligned. On the other hand, if the radiation is specifically or additionally laser light, the phase shifting device may include a retardation plate and means for adjusting the plate about an axis perpendicular to the plate plane, which is for adjusting the required phase shift.

embodiments

Specific embodiments of the invention will now be described by way of example with reference to the accompanying drawings. In the drawing show:

Fig. 1: the schematic representation of an embodiment of the measuring device according to the invention; 2 a shows the output of the photodetector of the device according to FIG. 1 under different circumstances, while the length of the optical path up to 2 f: optical path of the light beam is varied by a modulation wavelength; 3 shows a reference resonator used in the measuring device according to FIG. 1;

Fig. 4 is a schematic representation of another embodiment of the measuring device according to certain aspects of the invention.

First, the structure of the optical arrangement of an embodiment of the present invention will be described. The measuring apparatus of Fig. 1 comprises a main unit 8 and a predetermined spaced reflector 46. In the main unit 8, a xenon discharge tube 12 generates high intensity white flash. This light is collimated by a diverging lens 14 and then reflected by a mirror 16 through a plane polarizer 18 and onto a Pockels crystal 20 which, as shown, is mounted in the resonator 22 with modulating cavity. The crystal 20 may be mounted in the cavity for the pivotal adjustment, and the resonator may be adjusted by rotating about its longitudinal axis so that the axis of the crystal structure is alignable with the rest of the gauge. The light exits the crystal 22, penetrates into an internally reflecting Porro prism 26 and from there to a second Porro prism 28 which is mounted on a spring 30 and is driven by an electromagnet 32. The light coming from the prism 28 enters a negative lens 34 of short focal length, which fills the transmission lens 36 with light. The light coming from the object 36 enters a totally internally reflecting glass rhombus 38 (TIR). The transmitted light emerging from the rhombus 38 crosses a variable length light path (VLP) including a fixed porro prism 40 and a porro prism 42 mounted on a movable scale 44. This movement can be measured by means of a vernier or by means of a conventional or digital micrometer. The light then leaves the main unit 8 and falls onto the target reflector 46 located outside the meter. The reflector 46 is preferably of the cat's eye type, which is a front silvered concave reflector located at the focal point of a diverging lens, the radius of the Reflector is the focal point of this lens. The main unit 8 and the reflector 46 have complementary reference surfaces 68 and 70, respectively. From the reflector 46, the light passes back through the system along its original path and enters the reflector 16 through a hole. The light is then focused through a lens 49 onto a pinhole 48 to minimize the passing daylight and incident on a photodetector 50, which is preferably a photomultiplier.

In Figure 1, the deflections of the beam are shown at right angles for the sake of simplicity in the plane of the drawing sheet. In practice, to avoid the introduction of undesirable relative phase shifts, any deflection in a plane must be compensated for by a corresponding deflection in a vertical plane, but it is not difficult to take this criterion into account.

Subsequently, the operation of the optical measuring device will be described. The polarization of the beam in the case of a perfect apparatus in which no scattering polarization is introduced by optical components in which no natural birefringence of the crystal 20 occurs and whose crystal modulation has a depth of, for example, 90 °, is as follows: The light leaving the flash discharge tube 12 is unpolarized, but after passing through polarizer 18 it is linearly polarized. The crystal 20 introduces a phase delay in the beam which varies sinusoidally with time between plus and minus 90 °. This is the case since a modulation M of 90 ° applies. Thus, the beam is elliptically modulated linearly polarized light, with the limiting cases of modulation being circularly polarized light of opposite direction. The rhombus 38 introduces a phase delay of 90 ° in the y component of the beam with respect to the x component, and thus the resulting phase lag alternates with time between 0 ° and -180 °. Thus, the beam is elliptically polarization modulated, circularly polarized light, with the limiting cases of the modulation being linearly polarized light of crossed direction. After being deflected by the particular spaced reflector 46, a phase delay of 180 ° can be introduced into both the x and y components without resulting in a relative phase change between the two components. As the beam returns through rhombus 38, another phase delay of 90 ° is introduced so that the transmitted beam is elliptically polarization modulated, linearly polarized light, although it must be noted that the direction of linear polarization is at right angles to the direction of polarization of the outgoing beam between the crystal 20 and the rhombus 38 runs. The crystal 20 adds another phase delay to the phase retardation 64 in the incident beam between the x and y directions which sinusoidally alternates with time between plus and minus 90 °. This phase delay occurs in phase with the phase delay added to the outgoing beam. However, the phasing of these two phase delays depends on the length of travel of the beam-measured by the crystal 20-out to the reflector 46 and back to the crystal. It should be noted that, with proper phase adjustment of these two phase delays, the sum of the phase delays is constant, producing linearly polarized light whose direction intersects that of the polarizer 18. In this way, no light passes through the polarizer and the photodetector 50 detects a light intensity of zero. The time T that the light takes to travel from the crystal 20 to the reflector 46 and back again is:

T = 2d / c

where d is the distance between crystal and reflector along the optical path and c is the speed of light. A proper phase adjustment between the phase delay of the returned beam entering the crystal and the phase delay caused by the crystal arises when

T = 2d / c = (η + 0.5) 1 / c; or T = 2d / c = n1 / c,

depending on the type of crystal and the method of use of the crystal, where 1 is the modulation wavelength and n is the number of complete wavelengths on the way back from the crystal and back to it. Therefore, the distance d is given by:

d = (0.5 n + 0.25) 1; or d = 0.5nl

Thus, at the photodetector 50, a phase difference of zero is determined when the beam path is an integer multiple of half the wavelength plus one quarter of the wavelength, or an integer multiple of half the wavelength, depending on the crystal.

An important feature of the arrangement of Figure 1 is that at zero modulation and proper values for d, the directions of polarization of the projected beam entering the crystal and the reflected beam leaving the crystal intersect. Thus, the use of a single crystal 20 and a single polarizer 18 is sufficient.

Another important feature of the measuring arrangement according to FIG. 1 is that scattering polarization effects can be easily compensated. If the crystal 20 has a natural birefringence that produces a phase lag of the y-component with respect to the x-component of the beam at zero modulation, then the phase delay caused by the remainder of the gauge will add twice the amount of delay. Thus, if there is an integer multiple of half the wavelengths (plus a quarter wavelength, depending on the crystal type) being traversed back and forth from the beam (which is the case when a zero is found on a perfect meter), then that becomes Crystal emerging elliptically polarized beams and has a phase delay between the x and y components of 180 ° (or half a wavelength) plus twice the delay, which is caused by the natural birefringence of the crystal. Therefore, the beam is not a linearly polarized light whose direction crosses the direction of the polarizer, and a "perfect" zero value is not observed, however, by adjusting the amount of phase delay caused by the rhombus 38 so that instead of delaying between the x and If, for example, a phase delay of 90 ° minus the delay caused by the natural birefringence of the crystal is produced on each passage through the rhombus 38, the effect of natural birefringence is removed and a zero value is observed However, this can be compensated in the same way by adjusting the amount of the phase delay caused by the rhombus 38. This amount of phase delay w is adjusted by turning the rhombus 38 in the direction A in FIG.

Yet another advantage of the measuring arrangement of Figure 1 is that the crystal can be oriented so that the Pockels effect (which is a linear birefringence-voltage effect) for the migration of light along the Z-axis of the crystal. and the Kerr effect (which is a square birefringence voltage effect) is brought into phase, so that the Kerr effect does not degrade the quality of the zero value to be achieved.

The rhombus 38 used need not be a perfectly designed Fresnel rhombus having a specific tip angle (or prism angle) of approximately 50 °. A rhombus may be used that has a point angle in the range of, for example, 10. Between 45 ° and 55 °, provided that devices are included which permit a slight adjustment by rotation of the rhombus in the direction A according to Figure 1 in order to adjust the phase delay caused by the rhombus 38.

The rhombus 38 is not only installed so that it can rotate in the direction A, it is also installed so that the pivoting movement of the rhombus about the axes in the plane of the drawing sheet can be adjusted by means of three adjusting screws 112. In this way, the axes of the rhombus 38 can be adjusted with respect to the axes of the crystal.

In a modified embodiment of the measuring arrangement according to FIG. 1, the rhombus 38 is firmly attached, and a broadband delay plate, such as a mica chip, is mounted in the light path and adjustable by turning and swiveling to achieve a zero value at the photomultiplier.

In another modified embodiment, the rhombus 38 and / or an additional retardation plate are positioned in front of the crystal 20 with respect to the direction of the projected beam portion. In this case a smaller rhombus can be used. Furthermore, it is important that the projected beam entering the crystal be substantially circularly polarized, and thus it is little or no problem to align the crystal axes perpendicular to the direction of the bundle of beams to the direction of polarization of the beam.

The movement of prism 42 results in an increase or decrease in the length of the beam path between crystal 20 and reflector 46 by twice the distance that the prism is moved. Thus, for example, if the modulation frequency f is approximately 1.5GHz, so that the modulation half wavelength c / 2f is exactly 100mm, the prism 42 must be movable a distance of at least approximately 50mm for a zero value to be achieved.

FIG. 2A shows how the output I of the photomultiplier 50 alternates when the reflector 46 is offset by a modulation wavelength, the depth of the modulation M caused by the crystal being 90 ° and the total constant phase shift P, caused by the meter is 180 °. It should be noted that the zero points are achieved at a quarter wavelength and three quarters of a wavelength, and that the photodetector output I has very steep gradients on either side of the zero point.

Figure 2 B shows the case where the depth of modulation M is greater than 90 °. It must be mentioned that even steeper gradients of the photomultiplier output I occur on both sides of all zero points. The modulation depth should not be increased too far beyond 90 °, since otherwise rounded zero points occur at zero and half wavelength points, or symmetrically around these points.

Fig. 2C shows the case where the depth of modulation M is smaller than 90 °. It should be noted that the curve is rounder near zero, and thus the sensitivity of the meter is reduced at this lower modulation depth.

FIG. 2 D corresponds to FIG. 2A except that the constant phase shift effected by the system is not set to 180 °, but an additional phase shift of 45 ° is introduced, so that the total constant phase shift is 225 °. It can be seen that a perfect zero can not be achieved.

Figures 2 E and 2 F correspond to Figures 2 B and 2 C, respectively, but they represent the case in which, similar to Figure 2 D, the total phase shift caused by the system is 225 ° instead of 180 °.

The basic steps for executing the distance measurement will now be described. First, the reflector 46 is laid flat with its reference surfaces 70 against the reference surfaces 68 of the main unit 8. The variable light path is then adjusted so that a zero point is generated at the photodetector 50. The position of the reference plane (normal fix point) of the variable light path can then be set to zero, or the measured value can be read off. Then, when the reflector is moved back a distance and the variable light path is adjusted by moving the prism 42 to the right (as shown in Fig. 1) so as to generate a zero point, the difference between the set position and the normal fixed point on Nonius 44 half of the distance between the reference surfaces 68 and 70 more than an integral multiple of the modulation half-wavelengths. For most equipment measurements, the required distance is roughly known, and with the aid of the variable light path, it is then possible to more accurately measure that distance. Assuming that the approximate distance between the reference surfaces 68 and 70 is 28.85 m and the movement of the prism 42 to the right (Figure 1) from the reference position to reach a zero point is 36.745 mm, then the shortening is the length of the variable one Light path 2 χ 36.745 = 73.49 mm and thus it is apparent that the exact distance is 28.87349m when the modulation half-wave is 100mm.

In the following, the control of the measuring arrangement according to FIG. 1 will be described. A pulse generator 10 provides the following outputs:

a) a high voltage pulse (ζ B. 1 000 V) with a duration of 30 microseconds and a repetition time of

10 milliseconds for the anode of a grounded lattice ceramic triode tube 24 connected in parallel with the modulation cavity 22;

b) a trigger pulse for a flash tube 12, which is timed to be approximately 12 microseconds after the individual pulses of the triode tube 24, so that when the flash tube 12 is triggered, a high steady power level is achieved in the modulation cavity 22. The flash generated by the tube has a duration of about 3 microseconds;

c) a 50 Hz synchronous square wave signal (firstly) to the electromagnet 32 via a condenser 80 for the prism 28, so that upon triggering of the flash tube 12, the prism 28 is alternately at one or the other end of its range of motion, and / or secondly to Varactor 82 (capacitance varying diode) incorporated in the modulation resonator 22;

d) a 50 Hz square wave reference signal for a synchronous detection device part 78; and

e) a 100 Hz square wave control signal for connecting (i.e., blocking) one or more of the dynodes of the photomultiplier 50 when no light is received.

The pulse applied to the triode 24 causes the modulation resonator 22 to resonate at a microwave frequency of approximately 1.5 GHz. The modulation resonator 22 is initially tuned to the reference resonator 72 by means of a tuning sweep 73. The reference resonator has a resonant frequency of 4.5 GHz and a Q-tube of between 4000 and 5000. A coupling loop 75 in the base of the modulation resonator 22 is inserted via a coaxial cable to a microwave diode of zero bias 74 in the reference resonator 72. A forward biased microwave diode 76, which is inserted in the reference resonator 72, is connected to the synchronous detector section 78 by means of a coaxial cable. An object of the synchronizer detector portion 78 is to detect the tuning of the modulating resonator 22 to the reference resonator 72 and display it on a symmetrical scale meter 84. The tuning screw 73 is adjusted until the meter 84 is set at the zero point in the middle of the scale and indicates that the modulation resonator 22 is tuned so that the reference resonator 72 resonates as the third harmonic of the modulation resonator, which means that the modulation resonator 22 at exactly 1, 5GHz resonates. In order to accurately determine the frequency of the reference resonator, the frequency of the modulation resonator is swept or oscillated by approximately Vsooo by means of a plunger coil 102 protruding from a spring 30 through an aperture in the modulation resonator 22 and vibrating at 50 Hz and / or by means of the varactor 82 is driven by the same signal as the solenoid 32. This frequency change of Vsooo causes the modulation frequency at each pulse to jump from one shoulder of the resonance resonator of the reference resonator to the other. In this way, the frequency of the modulation resonance pulse can be set more accurately, for example, to an accuracy of the order of one millionth, which can be achieved by tuning the frequency of the modulation resonator to the peak value of the resonance curve of the reference resonator. Once the modulation resonator 22 is tuned, it is phase locked to the reference resonator 72 by manual actuation of a selector switch 86 by means of a varactor 100 mounted in the modulation cavity. The. Phase lock can be accurately maintained by applying a DC bias to the solenoid 32 or to the varactor 82, with the capacitor 80 blocking the DC feedback to the phase generator 10. Variations in the measured distance (e.g., by thermal changes) may then be monitored by an error meter 110 in the phase locked loop to the solenoid 32 or to the varactor 82.

After adjusting the frequency of the modulation resonator 22, the detection device portion 78 is adjusted by manually operating the selector switch 86 to display the value of the photomultiplier output on the meter 96, and the orientation of the main unit 8 and reflector 46 can then be set to a maximum value achieved on the meter 96. The verifier section 78 includes an automatic gain control 98 and provides an (automatic) gain control signal (AGC) to the photomultiplier. The gain control can thus be set to an appropriate level. The object of the automatic gain control is to bring the light pulses perceived by the photomultiplier to an average constant level, so that the sensitivity of the device is greater, the smaller the deviation from a light minimum. For the detection of a zero point in a photomultiplier 50, the length of the light path by means of a vibrating prism 28 is wobbled or oscillated. It does not detect a zero point 104 (see Figure 3A), but detects the intensities 106 and 108 on either side of the zero point, with the synchronizer detector portion 78 taking action to detect the equality of these intensities. At full amplification of the photomultiplier results in a sensitivity of the measuring device of 0.01mm.

The wobble of the light path through the prism 28 and the wobble of the modulation frequency through the plunger coil or through the varactor 82 mutually reinforce each other and it may be that the wobble of the modulation frequency such as by the varactor 82 for the reference frequency detection and photomultiplier Zero point is sufficient.

A description will now be given of the reference resonator 22 with reference to FIG. 3.

The resonator 22 has a generally cylindrical body 88 having a cavity with a resonant frequency of one fourth the wavelength. The inner surface of the resonator is plated with copper or silver. A screw-threaded metal dipping core 90 is mounted in the cylindrical body and can be moved in the axial direction to adjust the resonant frequency. The inside of the resonator is vented to the atmosphere through a very fine hole 92, and silica gel granules 94 are introduced into the resonator. In this way, the air in the resonator is kept dry and at ambient temperature and pressure; this is at the same time a good measure of the compensation of atmospheric refraction changes. The body 88 of the resonator may be constructed of a material having a low coefficient of thermal expansion, such as quartz or 'invar' (36% nickel steel), so that it is not significantly affected by temperature changes. On the other hand, if the measurement of the structure of a known material is desired, the body of the resonator may be made of the same material as this structure or of a material having a similar coefficient of thermal expansion. In the latter case, the resonant frequency of the resonator may be calibrated at a certain reference temperature, such as 20 ^° C., and the resonator may be mounted on the structure to be measured. Due to the change of the resonant frequency with temperature, the meter will provide readings of the structure converted to the reference temperature and not the actual readings at the prevailing temperature.

Hereinafter, a further embodiment of a specific aspect of the invention will now be described with reference to FIG. In FIG. 4, the components corresponding in FIG. 1 have been identified by the same reference numerals.

While the measuring arrangement of Figure 1 is a coaxial system, that is, the projected and reflected beam portions travel along a common path, the measuring arrangement of Figure 4 is a quasi-coaxial system in which the projected and reflected beam portions travel in close proximity to each other in parallel. Instead of a single crystal, two crystals 20A and 20B are placed in the projected and reflected beam portion. The crystals may be arranged so that the Z-axes of the crystal structures are parallel to the light beam so that the X-axes of the crystal structures are perpendicular to each other. If the dimensions of the crystals 20A and 20B are equal in the beam direction within a few wavelengths of light, then this arrangement of crystal axes compensates for the natural birefringence of the crystals. To eliminate any difference between the natural birefringence of crystals 2OA and 2OB

For fine adjustment, a broadband phase shifter plate is inserted in the optical path of the polarized light beam which serves for adjustment by rotation or pivoting. In the case where white light is used, such a phase retardation plate may be of mica.

Two polarizing filters 18A and 18B are disposed in the projected and reflected beam portions, respectively. The directions of the polarizing filters cross each other.

For sweeping the effective length of the light path made for the purpose of obtaining a zero value in the photomultiplier 50, a rotatable arrangement for generating an alternating light path is provided. A glass block 114 and a relatively short glass block 116 are secured to radially opposite arms protruding from a shaft driven by a motor 118. A shaft position detector 120 is provided and the motor is driven in synchronism with the flashes of the flash tube 12 so that for alternate flashes, the long block 114 and the short block 116 are mounted in the projected portion of the beam. The light takes longer to pass through the long block 114 than the short block 116, causing the time required for the light to pass through the meter to be swept.

The measuring arrangement according to FIG. 4 works with an optical resonator 124 and not with a microwave resonator as the wavelength standard. The optical resonator 124 is a multiple reflection system having a pair of reflectors 126 and 128 which are separated by rods 130 of a heat-resistant material or of any suitable material, depending on the desired coefficient of expansion. A temperature sensor 132 is provided and the temperature of the optical resonator is displayed on a meter 134 so that any convenient temperature correction can be calculated.

A fan 136 assists the optical resonator in maintaining the ambient temperature. The optical resonator has an inlet port 138 and an outlet port 140. To tune the modulation cavity, the movable mirrors 142 and 144 in the projected and reflected beam portions are brought into the positions shown in Figure 4 so that the projected beam is transmitted through the movable mirror 142 and another mirror 146 is aligned with the entrance aperture 138 of the optical resonator body 124, and the beam from the exit aperture 140 is sent back to the modulating resonator 22 via a mirror 148 and via the movable mirror 144. The total optical path length thus achieved is 10 m, which can achieve sensitivity of up to one millionth. The tuning screw 73 is readjusted until the synchronous detector 78 indicates that a phase lock unit 150 has blocked the modulation half-wavelength to 100mm. As in the measuring device of FIG. 1 for detecting the modulation frequency, the frequency is swept by a varactor similar to the varactor 82 of FIG. 1 or by a movable reactive power device similar to that of the oscillating plunger coil 102 in FIG. 1. Once the modulation resonator 22 is tuned , the movable mirrors 142 and 144 are rotated so that the projected beam can be transmitted to the target reflector and the reflected beam is reflected back to the modulation resonator.

In the measuring arrangement according to FIG. 4, the entire main unit 8 is slidably mounted with respect to the scale 152, and a vernier scale 154 is likewise provided, so that a variable light path according to FIG. 1 can be dispensed with. In a modification of the measuring arrangement according to FIG. 4 by somewhat arranging the reflectors 142 and 144, the optical resonator can be integrated as a permanent component of the light transmission path. In this case, the main unit 8 can be calibrated by contacting the target reflector with the front of the main unit 8, whereby the reflectors 146 and 148 can be slightly adjusted in the lateral direction to ensure that the modulation half-wavelength is exactly 100mm when the phase lock is maintained. The advantage of this modification is that when the optical path length in the optical resonator z. 10m and the frequency change between the alternating pulses of the modulator sweep Vsooo is, then adequate sensitivity can be achieved without the use of the mechanical light path alternator utilizing the glass blocks 114 and 116.

The variable optical path, which is made possible by the measuring arrangement including the glass blocks 114 and 116, can be inserted into the measuring arrangement according to FIG. 1, so that the use of the oscillating prism 28 can be dispensed with. Furthermore, instead of the microwave cavity resonator 72 ', the optical resonator described with reference to FIG. 4 can also be used in the measuring arrangement according to FIG.

While various embodiments and modifications of the various aspects of the invention have been described above, it will be appreciated that other modifications and developments are possible within the scope of the invention.

Claims (5)

1. Measuring device for distance measurement by means of polarization modulation consisting of: a device for projecting a beam of polarized beams and thus for reflecting the beam along substantially the same path; a device for the modulation of the polarization of the projected part of the beam and the reflected part of the beam, characterized by a phase shifting device for the introduction of an adjustable unmodulated relative phase shift in the beam. 2. A meter according to claim 1, characterized in that the phase-shifting device comprises a rhombus (38) and a device for the adjustment of the rhombus (38) about an axis perpendicular to the plane of the rhombus (38). 3. A meter according to claim 2, characterized by a device for adjusting the pivoting movement of the rhombus (38) about at least one axis in the plane of the rhombus (38). 4. A meter according to any one of claims 1 to 3, characterized in that the phase-shifting device comprises a Phasenverzögerungsplättchen and a device for the adjustment of this plate about an axis perpendicular to the plane of the plate. 5. Measurement method for distance measurement by means of polarization modulation with the following steps: Projection of a bundle of polarized radiation; Returning the polarized beam along substantially the same beam path; Modulation of the polarization of the projected portion of the beam and the reflected portion of the beam;