JPS62228185A - Measuring device - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a measuring device and a measuring method using polarization modulation of a beam of radiation.

In particular, the first and third aspects of the invention provide means for projecting a beam of radiation polarized such that the beam can return along substantially the same path; The present invention relates to a measuring device comprising means for modulating the polarization of the returned and returned portions of the modulated beam and further means for detecting the returned portion of the modulated beam.

Additionally, second and fourth aspects of the invention provide the steps of projecting a beam of polarized radiation, returning the polarized beam along substantially the same path, and redirecting the projected portion of the beam. The present invention relates to a corresponding measurement method comprising the steps of modulating the polarization with the returned part and further detecting the returned part of the beam after modulation.

Such apparatus and method are covered by British Patent No. Section GB 1
It is known from No. 172668. In the device described therein, the beam is polarized by a first polarizing filter and then modulated by at least one electro-optic crystal. The returning portions of the beam are oriented perpendicularly to those of the first crystal.
and a y-axis, and then filtered by a second polarizing filter having a polarization direction that intersects that of the first filter.

With the matched crystals oriented correctly, the polarizing filters oriented correctly, and the absence of any stray polarization effects or natural birefringence of the crystals, any radiation will be in the path of light from the first crystal to the second crystal. It is recognized that when the distance along is an integer number of modulation wavelengths, it will not pass through the second polarizing filter. However, it is difficult, if not impossible, to set up a fully functional device.

According to a first aspect of the invention, the apparatus is characterized by means for introducing a phase shift into the beam such that at zero modulation the directions of polarization of the projected and returned parts of the beam intersect. do.

According to a second aspect of the invention, the method is characterized by the step of introducing a phase shift in the beam such that the directions of polarization of the projected and returned parts of the beam intersect at zero modulation. .

By introducing such a phase shift, it is possible to use a single modulation means to modulate the polarization of both the projected and returned parts of the beam. In this way, there is no need to use a matched pair of crystals and two
There is also no need to ensure correct relative orientation of the two crystals.

If polarization filtering means are used to polarize the projected portion of the beam, it is further noted that the same polarization filtering means can be used to filter the returned portion of the beam, introducing such a shift. This is an advantage.

Thus, the need for two filters is eliminated, and so is the need to ensure the correct relative orientation of two such filters.

Preferably, the phase shifting means includes elements arranged to cause a relative phase shift in the outgoing beam part and also a further phase shift in the returning beam part. If there is little natural birefringence in the modulation means and any other stray polarization effects are negligible, the phase shift created by the phase shift element will preferably be typically one quarter of a wavelength in each part of the beam. arranged to cause relative delay. however,
If a significant relative delay is caused by the natural birefringence of the modulating means or by other elements of the device,
The overall phase shift created by the phase shift element is
It is preferably arranged to cause a total relative delay of half a wavelength caused by the rest of the device. Conveniently, the phase shift element may be a parallelogram, which will produce a satisfactory effect when white light is used as radiation. If laser light is used, the phase shift element may conveniently be a retardation plate.

Preferably, the degree of phase shift caused by the phase shift element is adjustable. If a parallelogram is used as the phase shift element, the adjustment means may include means for mounting the parallelogram for beam-related movement.

The phase shift element may be arranged before the modulation means with respect to the direction of the projected beam portion. The device is characterized in that the modulating means is linear or Pockels (Pockels) such as potassium dihydrogen phosphate or lithium niobate or lithium tantalate.
ockels) exhibiting an electro-optical effect, for example of the 3m or 4bar2m type, is preferably provided by the crystal. A particularly advantageous effect of such a device is that the projected beam portion that enters the crystal is generally circularly polarized,
And because of this, it is unnecessary to align the direction of polarization with the structure of the crystal. A two-cut crystal means that the radiation is directed along two axes of the crystal. Alternatively, the phase shift means may be arranged after the modulation means with respect to the direction of the projected beam portion. This is preferred when the modulation means are provided by an X- or Y-cut crystal of lithium tantalate, for example. However, this device may also be used with the previously mentioned two-cut crystals, especially potassium dihydrogen phosphate.

The device may include reflector means for reflecting the projected beam portion to provide a return beam portion, and in this case the phase shifting means and the reflector means may together form the device.

According to a third aspect of the invention, the device features phase shift means for introducing an adjustable unmodulated relative phase shift in the beam. A corresponding measurement method according to a fourth aspect of the invention is characterized by introducing an adjustable unmodulated relative phase shift into the beam. In this way, compensation can be made for any undesired polarization effects caused by the modulation means or other optical elements of the system.

The phase shift means may include a parallelogram and means for adjusting the parallelogram about an axis perpendicular to the plane of the parallelogram, thus adjusting the phase shift imparted by the parallelogram. Means may also be provided for adjusting the inclination of the parallelogram about at least one linear axis in the plane of the parallelogram in order to correctly orient the main axes of the parallelogram and the modulation means. Alternatively, in particular if the radiation is laser light, or additionally, the phase shift means are arranged with a retardation plate and an axis perpendicular to the plane of the plate in order to adjust the phase shift provided. It may also include means for centering the plate.

A fifth aspect of the invention provides means for projecting a beam of radiation such that the beam can return to the apparatus, means for varying the length of the path traveled by the beam, and means for detecting a property of the returned portion of the beam that varies with the length of the path traveled by, and further when the rate of change of the detected property with respect to the length of the path is substantially zero; and means for detecting.

Furthermore, a sixth aspect of the invention relates to a corresponding measurement method.

Such a device or method is also known from British Patent Specification No. GB 1172668. In the apparatus described therein, the intensity of the light passing through the polarizing filter is detected and steps are taken to determine when the detected intensity is zero.

Fifth and sixth aspects of the invention relate to increasing the accuracy with which zeros can be detected. A fifth aspect of the invention provides means for introducing alternating deformations into the beam, and means for detecting when the detected characteristic is substantially equal to the alternating deformations. It is characterized by containing.

Thus, it can be determined when the intensities on either side of a zero are equal to achieve greater accuracy than simply detecting a zero.

A measurement method according to a sixth aspect of the invention introduces alternating deformations in the beam and determines the zero rate of change by determining when the detected property is substantially equal to the alternating deformations. characterized by corresponding stages.

Alternating deformations may be provided by alternating the length of the path traveled by the beam, for example by vibrating elements arranged in the path of the beam.

Additionally or alternatively, the time taken by the beam to travel along the path may be altered, for example, by alternately placing and removing elements in the beam path that reduce the speed of the beam. Additionally or alternatively, the characteristics of the beam may be alternately varied. For example, if the beam is modulated, the alternating means may be operable to alternate the modulation frequency.

A seventh aspect of the invention includes the steps of: projecting a beam of radiation from a first portion of the structure to a second portion of the structure; modulating the beam at a modulation wavelength defined with respect to a reference wavelength; Relating to a measurement method comprising the steps of: detecting a second portion of a structure after reaching it; and further determining a distance between the first portion and the second portion using the results of the detection step. The method of the seventh aspect of the invention includes the steps of defining a reference wavelength by means having a coefficient of thermal expansion substantially equal to the structure, and further adjusting the temperature of the wavelength defining means to be substantially equal to the structure. It is characterized by stages.

Thus, the wavelength defining means can be calibrated at a reference temperature and the method will self-correct to provide the dimensions of the structure at the reference temperature.

Particular embodiments of the invention will be described by way of example in conjunction with the accompanying drawings.

First, the configuration of an optical device according to an embodiment of the present invention will be described. Referring to FIG. 1, the apparatus includes a main unit 8 and a remote reflector 46. In the main device f8, a xenon flash tube 12 generates high intensity pulses of white light. This light is collimated by a convex lens 14 and then reflected by a mirror 16 through a plane polarizer 18 and onto a Pockels crystal 20, which is provided in a modulation cavity 22 as shown.

The crystal 20 may be mounted in a resonator for tilt adjustment, and the resonator may be rotatably adjusted about its long axis to align the axis of the crystal structure with the rest of the device. Good too. Light exits the crystal 22 and enters an internally reflecting (Forro) prism 26 and then a second Porro prism 28 mounted on a spring 30 and driven by an electromagnet 32. Light from prism 28 enters short focus negative lens 34, which serves to fill the conductive objective lens 36 with light. The light from the objective lens 36 undergoes total internal reflection (TIR) of the glass.
) enters parallelogram 38. The transmitted light emitted from the parallelogram 38 passes through a variable length optical path (V
LP). Movement of the working memory 44 can be measured by the illustrated vernier or by a conventional or digital micrometer. Light exits the main device 8 and enters the remote target reflector 46. The reflector 46 is preferably of the "cat's eye" type, employing a fully silvered concave reflector placed at the focal point of a convex lens, the radius of the reflector being equal to the focal point of the convex lens. Main device 8 and reflector 46 have complementary reference surfaces 68 and 70, respectively. From reflector 46, the light returns through the system along its original path and passes through the hole in reflector 16.

This light is then passed through a lens 49 to a pinhole stop 48.
enters a photodetector 50, which focuses on the light, minimizes sunlight leakage, and is preferably a light multiplier.

In FIG. 1, all of the beam deflections at right angles have been plotted for convenience on the paper. In practice, each deflection in one plane must be compensated by a corresponding deflection in the orthogonal plane to avoid introducing undesired relative phase shifts, but this standard is not difficult to meet.

The optical performance of this device will be described below. Without any stray polarization introduced by optical components, the crystal 2
The polarization of the beam in the case of the complete device without any natural birefringence of 0 and with a modulation depth of, say, 90 degrees for the crystal is as follows. The light exiting flash tube 12 is unpolarized, but after passing through polarizer 18 it becomes plane polarized. The crystal 2o introduces a phase delay in the beam, which varies sinusoidally with time between +90 degrees and -90 degrees given a modulation depth M of 90 degrees. In this way, the beam is elliptically modulated plane polarized light and the modulation limit is oppositely circularly polarized light. Parallelogram 38 introduces a 90 degree phase delay in the "y" component of the beam relative to the rxJ components, and the resulting delay therefore varies sinusoidally with time between 0 degrees and 1180 degrees. Therefore, the beam is elliptically modulated, circularly polarized light, and the modulation limit is plane polarized light with cross directions. Upon reflection by remote reflector 46, a 180 degree phase shift may be introduced in both the rxJ and "y" components, but it does not result in a relative phase change between the two components. When the beam returns through the parallelogram 38, an additional 90 degree phase delay is introduced so that the transmitted beam is elliptically modulated plane polarized light, but the direction of the plane polarization is towards the crystal 20.
and parallelogram 38 at right angles to the polarization direction of the exiting beam. The crystal 20 has a +
Add yet another phase delay that varies sinusoidally with time between 90 degrees and 190 degrees. This phase lag is in phase with the phase lag introduced into the outgoing beam.

However, the phase relationship of these two phase lags depends on the distance traveled by the beam from crystal 20 to remote reflector 46 and back to the crystal. It is noted that with the proper phase relationship of these two phase lags, the sum of these phase lags is constant and produces plane polarized light, the direction of which intersects that of polarizer 18. Therefore, none of the light passes through the polarizer and zero is detected by the photodetector 50.

The time T required for the light to travel from the crystal 20 to the reflector 40 and back again is as follows.

T-2d/c where d is the distance along the optical path from the crystal to the reflector and C is the speed of light. A suitable phase relationship is obtained when the relationship between the phase lag of the returning beam entering the crystal and the phase lag caused by the crystal is:

T-2d/c −(n + O-5) 1/C
or T-2d/c-n 1/c These depend on the crystal form and the method in which the crystal is used;
where 1 is the modulation wavelength and n is the number of complete wavelengths on the path from the crystal and back again. Therefore, the distance d is given by the following equation.

d - (0,5n+0.25) 1; or d-0
, 5nl Therefore, when this distance is an integer number of half wavelengths plus a quarter wavelength, or, depending on the crystal, an integer number of half wavelengths, a zero is obtained at photodetector 50.

An important feature of the arrangement of FIG. 1 is that at zero modulation and for appropriate values of d, the polarization directions of the incident beam entering the crystal and of the return beam leaving the crystal are crossed.

Therefore, one crystal 20 and one polarizer 18 can be used.

Another important feature of the arrangement of FIG. 1 is that it can easily compensate for stray polarization effects. If the crystal 20 has zero modulation and r
If it has natural birefringence that causes a delay in the yJ component,
Because of the two passes of the beam through the crystal, twice this amount of delay is added to the delay introduced by the rest of the device. Therefore, when the number of half-wavelengths that the beam travels from the crystal and back again is an integer number (plus a quarter wavelength due to the crystal) (i.e., if a perfect device produces a zero), the return beam leaving the crystal 20 is elliptically polarized and has a 180 degree rxJ component and ryJ component (
or half a wavelength) plus twice the retardation (or retardation) introduced by the natural birefringence of the crystal.

Therefore, the beam is not plane-polarized light that intersects the direction of the polarizer, and perfect zero is not observed.

However, rather than introducing a 90 degree delay between the X and X components on each pass through the parallelogram 38,
By adjusting the amount of retardation provided by parallelogram 38 to provide a retardation of 0 degrees minus the retardation due to the crystal's natural birefringence, the effect of natural birefringence is eliminated;
And zero can be observed. Similarly, distortions in the optical components of the device can cause stray phase changes between the X and X components of the beam, which can be achieved in the same way by adjusting the amount of delay provided by parallelogram 38 can be compensated with. The amount of this delay is adjusted by twisting parallelogram 38 in direction A in FIG.

Another advantage of the arrangement of Figure 1 is that for light traveling along the Z-axis of the crystal, the Pockels effect (which is a linear birefringent voltage effect) and the Kell effect (which is a second-order birefringent voltage effect) The effect is that the crystal can be oriented in such a way that the effects (which are the effects of this effect) are brought into phase and thus do not compromise the zero quality that the Kell effect can achieve.

The parallelogram 38 used need not be a fully designed Fresnel parallelogram with a specific vertex angle of approximately 50 degrees. In order to adjust the phase delay introduced by the parallelogram 38, it is possible to include means for slightly precise rotational adjustment of the parallelogram in the direction A shown in FIG. , a parallelogram with vertex angles may be used.

In addition to providing the parallelogram 38 for rotation in direction A, it is also provided such that the tilting of the parallelogram about the axis in the plane of the paper can be adjusted by means of three adjustment screws 122. Good too.

Therefore, the axis of the parallelogram 38 can be adjusted with respect to the axis of the crystal.

In a variant of the arrangement shown in FIG. 1, the parallelogram 38 is fixed and rotated and tilted in order to place a broadband retardation plate, for example made of mica, in the optical path and to achieve a zero in the optical multiplier. be made adjustable.

In yet other variations, a parallelogram 38 and/or such an additional retardation plate is placed in front of the crystal 20 with respect to the direction of the projection beam section. In this case, smaller parallelograms can be used. Additionally, and importantly, the projected beam entering the crystal is substantially circularly polarized, and therefore there is little difficulty in aligning the crystal axis perpendicular to the beam direction with either direction of the beam's polarization. Or it doesn't arrive in time at all.

Moving prism 42 results in the length of the beam path from crystal 20 to reflector 46 increasing or decreasing by as much as twice the distance the prism is moved. Therefore, if the modulation frequency f is, for example, about 1.5 GH
z, so the modulation half wavelength c/2f is exactly 100 m
m, then prism 42 must be movable over a distance of at least about 50 mm to be able to achieve zero.

FIG. 2A shows that as the reflector 46 is displaced through one modulation wavelength, if the modulation depth M provided by the crystal is 90 degrees and the total constant phase shift P provided by the device is 180 degrees. It shows how the output I from the optical multiplier 50 changes. It is noted that zeros are obtained at one-quarter and three-quarter wavelengths and that the photodetector output I exhibits a very steep slope on either side of zero.

FIG. 2B shows the case where the modulation depth M is greater than 90 degrees. It is noted that a steeper slope of the photomultiplier output occurs on either side of each zero.

The modulation depth should not increase too much beyond 90 degrees, otherwise even larger rounded zeros will occur at the zero or half-wavelength points or approximately symmetrically to these points.

FIG. 2C shows the case where the modulation depth M is less than 90 degrees.

It is noted that the curve is more rounded near each zero, and so the sensitivity of the device is reduced at this lower modulation depth.

Figure 2D corresponds to Figure 2A, except that the constant phase delay introduced by the twelve systems is not adjusted to 180 degrees, but rather an additional shift of 45 degrees is introduced so that the total constant phase shift is It is 225 degrees. It turns out that perfect zero cannot be achieved.

Figures 2E and 2F correspond to Figures 2B and 2C, respectively, but like Figure 2D, the system is
The case is shown where a total constant phase shift is provided by 225° rather than 0°.

The basic steps in performing telemetry are described. First, the reflector 46 is installed with the reference surface 7o flat with respect to the reference surface 68 of the main device 8. The variable optical path is then adjusted to produce a zero at photodetector 50. The data position of the variable optical path can then be set to zero or a data reading can be taken. If the reflector is then moved to a remote position and the variable optical path is adjusted to produce a zero by moving prism 42 to the right (as viewed in FIG. 1), then at the adjusted position and vernier 44. The difference between the data positions of the reference surfaces 68 and 70 is more than an integer multiple of the modulation half wavelength. For most mechano-optical measurements, the required distance is approximately known, and a variable optical path can then be used to more accurately measure the distance. If the approximate distance between reference surfaces 68 and 70 is 28.85 m,
and to the right of the data position of the prism 42 (as seen in Figure 1)
If the movement to obtain the zero to is considered to be 36.745mm, then the reduction in the length of the variable light path is 2X36.
745 = 73.49 mrn, and so if the modulation half wavelength is 100 mm, the exact distance is 28.8
It is clear that it is 7349m.

A manner of driving the apparatus shown in FIG. 1 will be explained. Pulse generator 10 provides an output as follows. (a) duration 30 microseconds and repetition time 10
A millisecond high voltage pulse (eg, 1000 V) is applied to the anode of a ground-grid ceramic tribridge valve 24 inserted across the modulation resonator 22 .

(b) Applying a trigger pulse to flash tube 12 at a timing of about 12 microseconds after each pulse to Mitsuhashi valve 24 so that a high steady-state power level is reached in modulating resonator 22 when flash tube 12 is triggered.

The flash produced by flash tube 12 has a duration of approximately 3 microseconds.

(c) (i) to an electromagnet 32 for the prism 28 so that the prism 28 is alternately at one end and the other end of its range of motion when the flash tube 12 is triggered;
and/or (11) the capacitor 80 to the varactor 82 (variable capacitance diode) attached to the modulation resonator 22;
provides a 50Hz synchronous square wave signal via the

(d) Applying a 50 Hz square wave reference signal to the synchronous detector unit 78.

(e) Applying a 100 Hz square wave gating signal to gate off one or more diodes of photomultiplier 50 when not receiving light.

The pulses applied to diode valve 24 cause modulating resonator 22 to resonate at a microwave frequency of approximately 1.5 GHz.

The modulating resonator 22 is first tuned with respect to the reference cavity 72 by the tuning screw 73 (;

The reference cavity has a resonant frequency of 4.5 GHz and a Q-valve between 4000 and 5000. A coupling loop 75 at the base of the modulating resonator 22 is connected via a coaxial cable to a zero biased microwave diode 74 attached to the reference resonator 72. A forward biased microwave diode 76 mounted on a reference resonator 72 is connected to a synchronous detector unit 78 via a coaxial cable. One of the functions of the synchronous detector unit 78 is that the reference resonator 72
Detecting the tuning of the modulation resonator 22 with respect to the tuning of the modulation resonator 22 and displaying it on a meter 84 with a zero at the center. Tuning screw 7
3 is adjusted until the meter 84 is centered and the reference resonator 72 is resonating as the third harmonic of the modulating resonator, i.e. the modulating resonator 22 is tuned at exactly 1.5 GHz.
This shows that it is tuned as if it were resonating.

To resolve the frequency of the reference resonator, the frequency of the modulating resonator is transmitted through the aperture of the modulating resonator 22 to the spring 30.
It is "enriched" or oscillated by a factor of about 5000 by a reaction plunger 102 that protrudes from and oscillates at 50 Hz and/or by a varactor 82 driven by the same signal as the electromagnet 32. This 5000
By changing the frequency by a factor of 1, the modulating frequency causes all pulses to jump from one shoulder of the reference oscillator's resonance curve to the other. Therefore, the frequency of the modulating resonator can be determined more precisely, for example with an accuracy on the order of parts per million, than can be achieved by tuning the modulating resonator frequency to the peak of the resonance curve of the reference resonator. Can be set to

Once the modulation resonator 22 is tuned, it is controlled by the selector switch 8 by a varactor 100 mounted in the modulation cavity.
6, the phase is locked with the reference resonator 72.

Phase lock can be maintained intact by applying a DC bias to electromagnet 32 or to varactor 82, with capacitor 80 serving to prevent DC feedback to pulse generator 10. Variations in the distance being measured (e.g., thermal changes) are detected by the error meter 110 of the phase-locked loop to the electromagnet 32 or varactor 82.
can be monitored by

After the frequency of the modulating resonator 22 has been set, the detector unit 78 is manually set by the selector switch 86 to display the intensity of the light multiplier output on the meter 96 and to The alignment of 46 can be adjusted to obtain the maximum value read on meter 96. Ejector unit 78 incorporates a gain control device 98 and also provides an automatic gain control (AGC) signal to the optical multiplier. In this way the gain control can be set to a convenient level. The action of the AGC is to bring the pulses of light seen by the light multiplier to an average constant level, such that the smaller the deviation from the light minimum, the greater the sensitivity of the device.

To resolve the zero in optical multiplier 50, the length of the optical path is "multiplied" or oscillated by oscillating prism 28. Thus, with reference to FIG. 2A, rather than detecting the zero point 104, the intensities 106, 108 on either side of the zero are detected and the synchronization detection unit 78 is operable to detect equality of intensities. be. Total optical multiplier gain, which is 0
．． This results in a sensitivity of the device of 0.01 mm.

The modulation frequency modulation by the prism 28 and the modulation frequency by the reaction plunger or by the varactor 82 increases each other, and the modulation frequency alone resolves both the reference frequency and the light multiplier zero, for example. It may happen that this needs to be done by varactor 82 for this purpose.

Now, reference resonator 22 will be explained with reference to FIG.

Resonator 22 has a generally cylindrical body 88 that provides a quarter wavelength resonant cavity. The inner surface of the resonator is plated with copper or silver. A threaded metal plunger 90 is mounted to the cylindrical body and is axially movable to adjust the resonant frequency.

The inside of the resonator is vented to the atmosphere by very fine holes 92, and silica gel granules 94 are disposed in the resonator. The air within the resonator is thus kept dry and at ambient temperature and pressure, thus providing an excellent measurement that compensates for changes in the refractive index of the atmosphere. The body 88 of the resonator is made of quartz or “invar” (
It may be made from a material with a low coefficient of thermal expansion, such as 36% nickel, so that it is virtually unaffected by temperature changes. Alternatively, if measurements of a known material structure are desired to be made, the body of the resonator may be constructed from the same material as that structure, or from a material with a similar coefficient of thermal expansion. In this latter case,
The resonant frequency of the resonator may be calibrated at a certain reference temperature, such as 20° C., and this resonator may be mounted on the structure to be measured. Because the resonant frequency changes with temperature, the device makes calibration measurements that are converted to a reference temperature rather than actual measurements at the actual temperature.

A further embodiment of this aspect of the invention will now be described with reference to FIG. In FIG. 4, elements similar to those shown in FIG. 1 are designated with the same reference numerals.

Whereas the device of FIG. 1 is a coaxial system, i.e. the projected beam portion and the returning beam portion travel along the same path, the device of FIG. It is a quasi-coaxial system in which the beam sections are closely parallel to each other.

Rather than using one crystal, two crystals 2OA,
20B are arranged in the projected beam portion and the returned beam portion, respectively. These crystals may be arranged such that the Z-axis of the crystal structure is parallel to the light beam and the X-axis of the crystal structure is orthogonal to each other.

If the size of the crystals 20A, 20B in the beam direction is equal to within 2 to 3 wavelengths of the light, the alignment of the crystal axes will compensate for the natural birefringence of the crystal. crystal 2OA
, 20B, a broadband retardation plate capable of rotation or tilt adjustment may be inserted in the optical path of the polarized beam to trim the difference between the natural birefringence of . If white light is used, such a retardation plate may be mica.

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

A rotating alternating light path configuration is provided to "enrich" the effective length of the light path so that the zero at light multiplier 50 can be resolved. A glass block 114 and a relatively short glass block 116 are mounted on arms that project radially in opposite directions from a shaft driven by motor 11B. A shaft position detector 120 is provided and the motor is driven synchronously with the flashes from the flash tube 12 so that for alternating flashes the long block 114 and the short block 11
6 is disposed in the area onto which the light beam is projected. The light takes a longer path through the longer block 114 than the shorter block 116, and therefore the effect is to increase the time it takes for the light to pass through the device.

- The device of Figure 4 uses an optical resonator 124 rather than a microwave resonator as a wavelength standard. The optical resonator 124 is a multiple reflection system comprising a pair of reflection units 126, 128 separated by a rod 130 of a thermally stable material or of any convenient material depending on the desired coefficient of expansion. . A temperature sensor 132 is provided to display the temperature of the optical resonator on a meter 134;
Thereby any suitable temperature corrections can be calibrated. Where possible, a fan 136 is provided to assist the optical resonator in reaching ambient temperature.

The optical resonator has an inlet aperture 138 and an outlet aperture 140. In order to tune the modulation cavity, the movable mirrors 142, 144 in the projected and returned beam portions are moved to the positions shown in FIG. The beam from the outlet 140 is directed via another mirror 146 to the inlet 138 of the optical resonator 124 and the beam from the outlet 140 is directed via the mirror 148 and movable mirror 144 back to the modulating resonator 22 . The total optical path length thus provided is 10 m, which makes it possible to approach sensitivities up to 1 in 1 million. The tuning screw 73 allows the synchronization detector unit 78 to halve the half wavelength that the phase lock unit 150 is modulating.
It is adjusted until it shows that it is locked at 00mm. 1st
As in the configuration shown, to resolve the modulation frequency, a varactor similar to varactor 82 of FIG.
Alternatively, it is "filled" by a movable reaction element similar to the oscillating reaction plunger 1o2 of FIG. Modulation resonator 2
2 is tuned, the movable mirror 14 is arranged so that the projected beam can be transmitted to the target reflector and the returned beam can be returned to the modulation resonator.
2,144 are rotated.

In the configuration of FIG. 4, rather than providing the variable optical path used in FIG.
52 and vernier scale 154
is provided.

By slight rearrangement of the reflectors 142, 144 in a modification of the configuration of FIG. 4, the optical resonator may be incorporated as a fixed part of the transmitted light path. in this case,
The main device 8 may be calibrated by contacting the target reflector with the front side of the main device 8 and the reflector 14
6.148 may be given a small lateral adjustment to ensure that the modulation half wavelength is exactly 100 mm when phase lock is obtained. The advantage of this modification is that
If the path length in the optical cavity is 10 m and the frequency change between the "n" alternating current pulses of the modulator is 1/5000, then a symbolic distance alternator using glass blocks 114, 116 can be used. It is possible to reach an appropriate sensitivity without using

The alternating light paths provided by the arrangement including glass blocks 114, 116 may be incorporated in the arrangement of FIG. 1, rather than using the oscillating prism 28.

Additionally, the optical resonator described with reference to FIG. 4 may also be used in the configuration of FIG. 1, rather than the microwave cavity 72.

While various embodiments and modifications of various aspects of the invention have been described, it will be understood that other modifications and developments may be made within the scope of the invention.

[Brief explanation of drawings]

FIG. 1 is an illustrative view of an embodiment of the 11P1 fixing device according to the present invention. Figures 2A-2F illustrate the output of the photodetector of the apparatus of Figure 1 in various situations when the optical path length of the light beam is varied through one modulation wavelength. FIG. 3 shows the reference resonator used in the device of FIG. FIG. 4 diagrammatically shows another embodiment of a measuring device according to aspects of the invention. In the figure, 8 is the main device, 12 is a xenon flash tube, 14 is a convex lens, 16 is a mirror, 18 is a plane deflector, 22
indicates a modulation cavity resonator, and 26 indicates a prism. Patent Applicant Rank Tiller Hobson Limited M<K7′ )'=lId(J″M
<90° F=225” February 1986 (1st 2, Name of Invention Measuring Device 3, Relationship with Amendment Person Case Patent Applicant Address UK, L.E. 47 G.K., Leicester 221 Star Road, 2 Name: Rank Deiter Hobson Limited Representative: Jiei Ri-O-Ning 4, Agent address: 2-8-1 Hirano-cho, Higashi-ku, Osaka City, Hirano-cho Yachiyo Building Telephone: Osaka ( 06) 222-0381 (Main)
Name Patent Attorney (6474) Fukami Kube 5, Date of Amendment Order
- Voluntary amendment 6, 3 of the application for amendment. Column for the representative of the patent applicant, all drawings, power of attorney and translation 7, and contents of the amendment as shown in the attached sheet. Please note that the content of the drawings remains unchanged. Patent Application No. 959 of 1986 dated March 2, 1988 before the above amendments, 2, name of invention measuring device 3, relationship with the case of the person making the amendment Patent Applicant Address L.E. 47, United Kingdom・JQ, Leicester New Star Road, 2 Name: Rank Tiller Hobson Limited Representative name: Jay Cronin 4, Agent address: 6, Hirano-cho Yachiyo Building, 2-8-1, Hirano-cho, Higashi-ku, Osaka, Number of inventions increased by amendment 67, Section 2 of the specification to be amended, Claims column and 3, Detailed explanation of the invention column 8, Contents of the invention (1) Add the Claims column to the attached sheet. Correct accordingly. (2) In the second line of page 2 of the specification, the phrase "radiated" is amended to "projected." (3) Page 9, lines 3, 5, 11, and 12 J
In lines 3 and 15, "transformation" is corrected to "'!i motion". (4) Page 9, line 16, IX, replace the phrase “to replace” with “
Correct to "alternate". 5) In the 5th line of page 10, the words ``alternate'' should be corrected to ``alternate''. (6) In the 5th line of page 24, the words "photomultiplier" are corrected to "light multiplier." 2. Claims (1) Means for projecting a beam of radiation polarized such that the beam can return along substantially the same path; the portion onto which the beam is projected; and the beam. means for modulating the polarization with the returned portion of the beam; further means for detecting the returned portion of the beam after modulation, the portion projected onto the beam with zero modulation and the returned portion; and a phase shift means for introducing a relative phase shift into the beam such that the directions of the polarizations of the beams intersect. (2) A single such modulation means is operable to modulate the polarization of both the projected and returned portions of the beam. equipment. (3) Polarization filtering means are provided for polarizing the projected portion of the beam, and similar polarization filtering means are arranged for filtering the returned portion of the beam, or Second
Equipment described in Section. (4) characterized in that the phase shift means comprises an element arranged to cause a relative phase shift in the projected beam portion and also to cause a relative phase shift in the returned beam portion; An apparatus according to any one of claims 1 to 3. 5. The apparatus of claim 4, wherein the phase shift created by the phase shift element causes a relative delay of generally one quarter of the wavelength of each portion of the beam. . (6) Claims characterized in that the total phase shift produced by the phase-shifting element causes a total relative delay of half a wavelength less a constant relative delay caused by the remainder of the device. Apparatus according to clause 4 or 5. (7) The device according to any one of claims 4 to 6, wherein the phase shift element is a parallelogram. (8) The apparatus according to any one of claims 4 to 6, wherein the projection means emits a laser beam and the phase shift element is a retardation plate. (9) The device according to any one of claims 4 to 8, characterized by means for adjusting the degree of phase shift caused by the phase shift element. (10) Claims characterized by means for adjusting the degree of phase shift caused by the phase shift element, the adjustment means including means for mounting a parallelogram for movement with respect to the beam Apparatus according to paragraph 7. (11) Device according to any one of claims 4 to 11, characterized in that the phase shift means are arranged in front of the modulation means with respect to the direction of the projected beam portion. 12. Device according to any of claims 4 to 10, characterized in that the phase shift means are arranged after the modulation means with respect to the direction of the projected beam portion. 13. Apparatus according to any one of claims 1 to 12, characterized by reflector means for reflecting a projected beam portion to provide a returned beam portion. 14. Device according to claims 12 and 13, characterized in that the reflector means and the phase shift means form a unit. (15) projecting a beam of polarized radiation; returning the polarized beam along substantially the same path; modulating the polarization of the projected portion of the beam and the returned portion of the beam; and detecting the returned portion of the beam after modulation, introducing a relative phase shift in the beam so that at zero modulation the direction of polarization of the projected and returned portions of the beam is A measurement method characterized by being crossed. (16) Modulation means are used to modulate the polarization of the projected part of the beam, and similar modulation means are used to modulate the polarization of the returned part of the beam. The method according to scope item 15. (17) Polarization filtering means are used to polarize the projected beam portion, and the same filtering means are used to filter the returned portion of the beam after modulation. or the method according to paragraph 16. (18) The phase shift step introduces a relative phase shift in the projected beam portion and a further relative phase shift in the returned beam portion. The method according to any one of Items 1 to 17. (19) The method according to claim 17, characterized by the step of adjusting the degree of phase shift introduced. (20) means for projecting a beam of polarized radiation such that the beam returns along substantially the same path; modulating the polarization of the projected portion of the beam and the returned portion of the beam; and means for introducing an adjustable unmodulated relative phase shift into the beam. (21) Claim 20, characterized in that the phase shift means comprises a parallelogram and means for adjusting the parallelogram about an axis perpendicular to the plane of the parallelogram.
Equipment described in Section. 22. Device according to claim 21, characterized by means for adjusting the inclination of the parallelogram about at least one axis of the planes of the parallelogram. (23) According to any one of claims 20 to 22, wherein the phase shift means includes means for adjusting the radiation plate and the plate about an axis perpendicular to the plane of the plate. equipment. (24) projecting a beam of polarized radiation; returning the polarized beam along substantially the same path; modulating the polarization of the projected portion of the beam and the returned portion of the beam; and further detecting a returned portion of the beam after modulation, and introducing an adjustable unmodulated relative phase shift in the beam. (25) means for projecting a beam of radiation such that the beam returns to the apparatus; means for varying the length of the path traveled by the beam; and the length of the path traveled by the beam. means for detecting a property of the returned portion of the beam that varies with the length of the beam; further means for determining when the rate of change of the detected property with respect to path length is substantially zero; A measuring device, provided for introducing an alternating variation into the beam; further characterized in that the determining means includes means for determining when the detected characteristic is substantially equal to the alternating variation. 26. The apparatus of claim 25, wherein the alternating variation means includes means for varying the length of the path traveled by the beam. 27. The apparatus of claim 26, wherein the means for alternating the length of the path comprises an element disposed in the beam path and means for varying the element. (28) The alternating variation means includes means for alternating the time required for the beam to travel along the path. equipment. (29) The means for alternating in time comprises elements for reducing the velocity of the beam and means for alternately placing and removing elements in the beam path. equipment. 30. A device according to any one of claims 25 to 30, characterized in that the alternating variation means include means for alternating the characteristics of the beam. 31. Apparatus as claimed in claim 30, including means for modulating the beam, the characteristic alternating means including means for alternating the modulation frequency. (32) Projecting the beam of radiation; Returning the beam of radiation; Changing the length of the path traveled by the beam; Returning the beam that changes with the length of the path traveled by the beam. and detecting when the rate of change of the detected characteristic with respect to the length of the path is substantially zero; introducing alternating fluctuations into the beam. and determining the rate of change of said zero by detecting when the detected characteristic is the same for alternating variations. 33. The method of claim 32, wherein the alternating variations are alternating variations in the length of the path traveled by the beam. (34) Claim 33, characterized in that the element is vibrated to provide an alternating variation in path length.
The method described in section. 35. A method according to any one of claims 32 to 34, characterized in that the alternating variations are alternating variations in the time required for the beam to travel along the path. 36. A method as claimed in claim 35, characterized in that elements reducing the velocity of the beam are alternately placed in and removed from the beam path. (37) Claims 32 to 3, characterized in that the alternating variations are alternating variations in beam characteristics.
How to describe in any of Section 6. 38. The method of claim 37, wherein the beam is modulated, alternating the modulation frequency. (39) projecting a beam of radiation from a first portion of the structure to a second portion of the structure; modulating the beam at a modulation wavelength defined with respect to a reference wavelength; and determining a distance between the first and second portions having a coefficient of thermal expansion substantially equal to that of the structure. A measurement method characterized by the steps of: defining a reference wavelength by means; and adjusting the temperature of the wavelength defining the means to be substantially equal to that of the structure. (40) A method according to claim 39, characterized in that the wavelength defining means are placed in contact with the structure.

Claims (1)

[Claims] (1) means for projecting a beam of radiation polarized such that the beam can return along substantially the same path; means for modulating the polarization; and further means for detecting a returned portion of the beam after modulation, such that at zero modulation the directions of polarization of the projected and returned portions of the beam intersect. A measuring device characterized by phase shift means for introducing a relative phase shift into the beam.