JP2022027890A - Displacement detector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a displacement amount detector capable of detecting a displacement amount stably and highly accurately.

SOLUTION: As a polarization holding fiber for transmitting light from a light source to a displacement detector, a length not becoming the same as a length determined by dividing a product between an even-numbered time of a length obtained by multiplying a double length of a resonator by a refractive index of the resonator and a beat length determined from a difference of a propagation constant between two polarization modes, by a wavelength of the light source, is selected from in the range containing the same length.

SELECTED DRAWING: Figure 1

COPYRIGHT: (C)2022,JPO&INPIT

Description

The present invention relates to a displacement detection device that detects displacement of a surface to be measured by using light emitted from a light source.

Conventionally, a displacement detection device using light has been widely used as a device for measuring the displacement of the surface to be measured in a non-contact manner. There are various methods for the specific configuration of the displacement detection device, which will be described later in the embodiment. In any of the methods, the displacement detection device uses optical fiber to transfer the light from the light source to the displacement detector. It is a configuration that leads. Then, the displacement detecting device detects the displacement amount of the measured surface by changing the phase of the light from the light source based on the displacement of the measured surface and detecting the changed state of the phase of the light.

Japanese Unexamined Patent Publication No. 7-332957 Japanese Unexamined Patent Publication No. 2006-194855

When detecting the displacement of the surface to be measured by the displacement detector, it is important that the phase of the light reaching the displacement detector from the light source is stable. That is, unnecessary light is applied due to the application of stress such as a change in the bending state of the optical fiber that guides the light from the light source, the fitting state of the optical fiber connector, and the bonding state between the ferrule connecting the optical fibers and the optical fiber. Phase fluctuation may occur. When the phase of the light fluctuates unnecessarily, the stability of the displacement detection of the surface to be measured by the displacement detector is lowered, and the displacement detection accuracy is lowered.

As a method in which the unnecessary phase fluctuation of the light in the optical fiber does not affect the displacement detection accuracy in the displacement detector, for example, after canceling the polarization of the light emitted from the optical fiber, the light is polarized by the splitter. After that, there is a method of taking out arbitrary polarization and injecting it into the displacement detector (see Patent Document 1).
Further, as another method, there is a method of using a polarization-retaining fiber as an optical fiber and increasing the extinction ratio as the light incident on the polarization-retaining fiber to reduce the light of an unnecessary polarization component (patented patent). See Document 2).

In the method of depolarizing the light emitted from the optical fiber and then polarizing it with a splitter, the light is incident on the displacement detector because the polarization is once depolarized and then arbitrary polarization is taken out and incident on the displacement detector. There is a problem that the amount of light is reduced.
In addition, the method of increasing the extinction ratio of the light incident on the polarization-retaining fiber does not reduce the amount of light, but when adding an optical connector or the like, it is necessary to precisely align the optical axis at each mating portion. There was a problem that the manufacturing cost was high.
Further, in recent years, as the resolution of the A / D converter provided in the displacement detection device has increased, leakage of the polarized beam having a component orthogonal to the optical axis of the polarization holding fiber has become a problem, and interference due to unnecessary light has become a problem. The amplitude needs to be within the quantization error of the A / D converter.

The present invention eliminates the need for measures to supply the displacement detector with a beam having a high extinction ratio as in the past, and attenuates interference with unnecessary beams without causing light loss. It is an object of the present invention to provide a displacement amount detection device capable of detecting a displacement amount with stable and high accuracy.

The displacement detection device of the present invention has a low coherence laser light source having a plurality of oscillation spectra accompanying the length of the resonator that emits light, a lens that collects the light from the laser light source, and light emitted from the laser light source. Optical delay of light via the X polarization mode axis and light via the Y polarization mode axis at the exit portion when the light is incident so as to be divided into the X polarization mode axis and the Y polarization mode axis of the polarization holding fiber. Via the X polarization mode axis and the Y polarization mode axis in the range where the peaks of multiple interference intensities with distance are -20dB or more with respect to the interference intensity of optical delay distance 0 and the area excluding the peak of interference intensity. The polarization-retaining fiber having a length such that the interference intensity of the light of the above is -20 dB or less with respect to the interference intensity of the optical delay distance of 0 and the light transmitted by the polarization-retaining fiber are divided into two, and each light is divided into two. The light emitted from the polarization-retaining fiber includes a polarization detector that converts each light into an interference signal by interfering with each other in order to use the multiplied phase change as the amount of displacement. It was made to be incident on the displacement detector without going through the canceling element.

According to the present invention, by configuring the polarization-retaining fiber with an arbitrary length according to the oscillation characteristics of the light source, it is possible to attenuate the interference with an unnecessary beam. This makes it possible to minimize the components that cause phase fluctuations in the displacement detection signal of the displacement detection device, limit the amplitude fluctuations of the interference signal, and detect displacements that are not easily affected by external stress on the polarization holding fiber. become. According to the present invention, since the amount of light is not lost, the efficiency of using the light from the light source is good, and it is not necessary to take measures to supply the beam having a high extinction ratio to the displacement detector.

It is a block diagram which shows the example (the example which displaces a diffractive lattice) of the displacement detection apparatus to which this invention is applied. It is a block diagram which shows the example (Michelson interferometer type example) of the displacement detection apparatus to which this invention is applied. It is a block diagram which shows the example of the displacement detection apparatus to which this invention is applied (the example which the target mirror and the reference mirror are arranged in parallel). It is sectional drawing which shows the example of the polarization holding fiber by one Embodiment of this invention. It is a characteristic diagram which shows the example of the multimode laser by the example of one Embodiment of this invention. It is a block diagram which shows the structural example which measures the coherent distance of the light source which has a wide band with poor coherence according to one Embodiment of this invention. It is a characteristic diagram which shows the example of the coherence distance by the example of one Embodiment of this invention. It is a block diagram which shows the example which connected the plurality of polarization holding fibers by one Embodiment of this invention.

Hereinafter, the displacement detection device of one embodiment of the present invention (hereinafter referred to as “this example”) will be described with reference to the accompanying drawings. First, three examples of the overall configuration of the displacement detection device to which the present invention is applied will be described with reference to FIGS. 1 to 3.

[1. Example of overall configuration of displacement detection device (example of displacement of diffractive grid)]
FIG. 1 shows a configuration example of a displacement detection device 100 that displaces the diffraction grid in conjunction with the displacement of the surface to be measured.
The displacement detection device 100 includes a light source 101. As the light source 101, a laser, an LED, or the like can be used. When a laser is used as the light source 101, a multimode semiconductor laser diode having low coherence may be used. Semiconductor lasers include self-pulsation laser diodes and superluminescence diodes.
Further, the solid-state laser used as the light source 101 includes a YAG laser, an Nd laser, a titanium sapphire laser, a fiber laser and the like. Further, the laser used as the light source 101 includes a helium cadmium laser (metal laser) and a dye laser (liquid laser). It is desirable that the light source 101 has a range of oscillation wavelengths in the longitudinal mode due to continuous oscillation and has low coherence.
Alternatively, as the light source 101, a single-mode semiconductor laser whose coherence is lowered by high-frequency superposition may be used.

The beam emitted from the light source 101 is focused by the first lens 102 and incident on the polarization-retaining fiber 150. The polarization-retaining fiber 150 is an optical fiber having different refractive index distributions in the vertical direction and the horizontal direction of the cross section of the core, and the details will be described later. The polarization optical axis of the light source 101 is preferably incident on the optical axis at which the polarization retention of the core cross section of the polarization retention fiber 150 is established, but it does not have to be strictly aligned. The length of the polarization holding fiber 150 is determined by the conditions described later.

The beam transmitted by the polarization holding fiber 150 is incident on the displacement detector 110.
The beam incident on the displacement detector 110 is incident on the first polarizing beam splitter (PBS) 112 via the second lens 111. The first polarization beam splitter 112 splits the beam into an S-polarized beam and a P-polarized beam. For example, the S-polarized beam is reflected by the first polarization beam splitter 112, and the P-polarized beam is transmitted through the first polarization beam splitter 112.

The S-polarized beam reflected by the first polarizing beam splitter 112 is incident on the diffraction grid 115 via the first mirror 113. Further, the P-polarized beam transmitted through the first polarizing beam splitter 112 is incident on the diffraction grid 115 via the second mirror 114.
The diffraction grid 115 functions as a scale for detecting the amount of displacement of the surface to be measured, and moves as shown by the arrow X1 in conjunction with the displacement of the surface to be measured.
The diffracted light of each beam incident on the diffraction grid 115 is incident on the third mirror 119 via the third lens 116 or the fourth lens 117 and the first phase plate 118, and is reflected by the third mirror 119. The beam is returned to the diffraction grid 115. The first phase plate 118 is made of, for example, a 1/4 wave plate.

The diffracted light of the beam returned to the diffractive grid 115 is returned to the first polarizing beam splitter 112 via the first mirror 113 or the second mirror 114. Here, since each beam passes through the first phase plate 118 twice, the S polarization becomes P polarization and returns, and the P polarization returns to S polarization.
Therefore, the two beams returned from the diffraction grid 115 are emitted in a superposed state from a surface different from the plane of incidence of the beam from the light source 101 on the first polarizing beam splitter 112, and are incident on the light receiving unit 130. To.

The beam incident on the light receiving unit 130 is incident on the beam splitter 131 and split into two. One of the beams divided into two is incident on the second polarization beam splitter 133 via the second phase plate 132. The beam incident on the second polarizing beam splitter 133 is split into each of the polarization components, and then incident on the first light receiving element 141 and the second light receiving element 142. Further, the other beam divided into two is incident on the third polarizing beam splitter 134, divided for each polarization component, and then incident on the third light receiving element 143 and the fourth light receiving element 144.

Next, the operation of the displacement detection device 100 to detect the displacement of the diffraction grid 115 will be described.
The beam emitted from the light source 101 is focused and incident on the end face of the polarization-retaining fiber 150 by the first lens 102. The beam emitted from the polarization-retaining fiber 150 is collimated by the second lens 111. The collimated beam is incident on the first polarization beam splitter 112 in a polarized state such that the beam is split 1: 1 by the first polarization beam splitter 112. The beam of one of the S polarization components distributed by the first polarization beam splitter 112 passes through the first mirror 113 and is incident on the position P1 of the diffraction grating 115. The beam vertically diffracted by the diffraction grating 115 is circularly polarized by the first phase plate 118, is reflected by the third mirror 119, and is folded back to the diffraction grating 115 side. At this time, by passing through the first phase plate 118 again, it becomes a beam of the P polarization component, becomes light diffracted twice by the diffraction grating 115, and heads toward the first polarization beam splitter 112.

The beam of the P polarization component distributed by the first polarization beam splitter 112 passes through the second mirror 114 and is incident on the position P2 of the diffraction grating 115. The beam vertically diffracted by the diffraction grating 115 is circularly polarized by the first phase plate 118, is reflected by the third mirror 119, and is folded back. At this time, by passing through the first phase plate 118 again, it becomes a beam of the S polarization component, becomes light diffracted twice by the diffraction grating 115, and heads toward the first polarization beam splitter 112.

The two beams whose polarization components are interchanged with each other are superposed by the first polarization beam splitter 112 and directed toward the light receiving unit 130. This superposed beam is composed of a linearly polarized S-polarized component and a P-polarized component. This beam is split into two by the beam splitter 131, one of which is incident on the second polarizing beam splitter 133 via the second phase plate 132.
When passing through the second phase plate 132, the linearly polarized light that is orthogonal to each other becomes a superposition of circularly polarized light that rotates in the opposite direction to each other. Then, the polarization direction of the linearly polarized light incident on the second polarization beam splitter 133 rotates twice when the diffraction grating 115 moves by one wavelength in the moving direction. Therefore, in the first light receiving element 141 and the second light receiving element 142, an interference signal of Acos (4Kx + δ') can be obtained. Here, A is the amplitude of the interference signal, and K is the wave number represented by 2π / Λ. Further, x indicates the amount of movement of the diffraction grating, and δ'is the initial phase. Λ is the spacing of the gratings in the diffraction grating.

Further, the other beam split into two by the beam splitter 131 is incident on the third polarization beam splitter 134. At this time, since the third polarization beam splitter 134 is rotated with respect to the second polarization beam splitter 133 with respect to the optical axis of 45 °, the third light receiving element 143 and the fourth light receiving element 144 are the first light receiving element 141. And the second light receiving element 142, the phase of the signal to be photoelectrically converted is different by 90 degrees. As a result, the moving direction of the diffraction grating can be recognized.

As described above, according to the displacement detection device 100, the interference signal can be detected as the displacement amount in the X1 direction of the diffraction grating 115 from the light receiving state of the first to fourth light receiving elements 141 to 144.
The interference signals obtained by the four light receiving elements 141 to 144 do not contain a component related to the wavelength of the light source 101. Therefore, even if the wavelength of the light source fluctuates due to changes in atmospheric pressure, humidity, and temperature, the interference intensity is not affected.

[2. Example of overall configuration of displacement detector (Michelson interferometer type example)]
FIG. 2 shows a configuration example of a Michelson interferometer type displacement detection device 200 that moves the target mirror 214 in conjunction with the displacement of the surface to be measured.
The displacement detection device 200 includes a light source 201. The conditions applicable to the light source 201 are the same as the conditions of the light source 101 described in the displacement detection device 100 shown in FIG. 1. For example, a light source having low coherence, a multimode semiconductor laser, or the like is used.

The beam emitted from the light source 201 is focused by the first lens 202 and incident on the polarization-retaining fiber 250. The polarization-retaining fiber 250 is an optical fiber having different refractive index distributions in the vertical direction and the horizontal direction of the cross section of the core, and the length of the polarization-retaining fiber 250 is determined by the conditions described later.

The beam transmitted by the polarization holding fiber 250 is incident on the displacement detector 210.
The beam incident on the displacement detector 210 is incident on the first polarization beam splitter 212 through the second lens 211. The P-polarized beam transmitted through the first polarizing beam splitter 212 is incident on the target mirror 214 via the first phase plate 213. The target mirror 214 moves in the X2 direction in conjunction with the displacement of the surface to be measured. Further, the S-polarized beam reflected by the first polarizing beam splitter 212 is incident on the reference mirror 216 via the second phase plate 215. The first phase plate 213 and the second phase plate 215 are made of, for example, a 1/4 wave plate.

When the target mirror 214 is the reference position, the optical path length from the first polarizing beam splitter 212 to the target mirror 214 and the optical path length from the first polarizing beam splitter 212 to the reference mirror 216 are set to be equal.
Then, the beams returned from the target mirror 214 and the reference mirror 216 to the first polarization beam splitter 212 become an superimposed interference signal, and are incident on the light receiving unit 230.

The beam incident on the light receiving unit 230 is incident on the beam splitter 231 and split into two. One of the beams divided into two is incident on the second polarization beam splitter 233 via the third phase plate 232. The beam incident on the second polarization beam splitter 233 is split into each of the polarization components and then incident on the first light receiving element 241 and the second light receiving element 242. Further, the other beam divided into two is incident on the third polarization beam splitter 234, divided for each polarization component, and then incident on the third light receiving element 243 and the fourth light receiving element 244.

With this configuration, the optical path is constant for the component reflected by the reference mirror 216 in the beam supplied from the first polarizing beam splitter 212 to the light receiving unit 230. On the other hand, for the component reflected by the target mirror 214, the optical path changes in conjunction with the displacement of the measured surface.
Therefore, the displacement detection device 200 can detect the interference signal as the displacement amount in the X2 direction of the target mirror 214 from the light receiving states of the first to fourth light receiving elements 241 to 244.

[3. Example of overall configuration of displacement detector (example of target mirror and reference mirror arranged in parallel)]
FIG. 3 shows a configuration example of the displacement detection device 300 in which the target mirror 315 and the reference mirror 316 are arranged in parallel.
The displacement detection device 300 includes a light source 301. The conditions applicable to the light source 301 are the same as the conditions of the light source 101 described in the displacement detection device 100 shown in FIG. 1. For example, a light source having low coherence, a multimode semiconductor laser, or the like is used.

The beam emitted from the light source 301 is focused by the first lens 302 and incident on the polarization-retaining fiber 350. The polarization-retaining fiber 350 is an optical fiber having different refractive index distributions in the vertical direction and the horizontal direction of the cross section of the core, and the length of the polarization-retaining fiber 350 is determined by the conditions described later.

The beam transmitted by the polarization holding fiber 350 is incident on the displacement detector 310.
The beam incident on the displacement detector 310 is incident on the first polarization beam splitter 312 via the second lens 311. The P-polarized beam transmitted through the first polarizing beam splitter 312 is incident on the target mirror 315 via the first phase plate 313. The first phase plate 313 is made of, for example, a 1/4 wave plate. Here, the beam incident on the target mirror 315 is incident on the target mirror 315 at an angle of 45 °. The target mirror 315 moves in the X3 direction in conjunction with the displacement of the surface to be measured.

The beam reflected by the target mirror 315 is incident on the transmission type diffraction grating 317, and the diffracted light transmitted through the transmission type diffraction grating 317 is incident on the target mirror 315. Then, the diffracted light reflected by the target mirror 315 is incident on the mirror 318. The incident angle of the diffracted light with respect to the mirror 318 is 90 °, and the mirror 318 reflects the incident diffracted light in the incident direction.

The S-polarized beam reflected by the first polarizing beam splitter 312 is incident on the reference mirror 316 via the second phase plate 314. The second phase plate 314 is made of, for example, a 1/4 wave plate. Here, the beam incident on the reference mirror 316 is incident on the reference mirror 316 at an angle of 45 °.

The beam reflected by the reference mirror 316 is incident on the transmission type diffraction grating 317, and the diffracted light transmitted through the transmission type diffraction grating 317 is incident on the reference mirror 316. Then, the diffracted light reflected by the reference mirror 316 is incident on the mirror 319. The incident angle of the diffracted light with respect to the mirror 319 is 90 °, and the mirror 319 reflects the incident diffracted light in the incident direction. The first phase plate 313 and the second phase plate 314 are made of, for example, a 1/4 wave plate.

The beam reflected by the mirrors 318 and 319 follows a path opposite to that at the time of incident, and is incident on the first polarizing beam splitter 312. Then, the two beams returned to the first polarization beam splitter 312 are superimposed to form an interference signal, which is incident on the light receiving unit 330.
The beam incident on the light receiving unit 330 is incident on the beam splitter 331 and split into two. One of the beams divided into two is incident on the second polarization beam splitter 333 via the third phase plate 332. The beam incident on the second polarization beam splitter 333 is split into each of the polarization components and then incident on the first light receiving element 341 and the second light receiving element 342. Further, the other beam divided into two is incident on the third polarizing beam splitter 334, divided for each polarization component, and then incident on the third light receiving element 343 and the fourth light receiving element 344.

With this configuration, among the beams supplied from the first polarizing beam splitter 312 to the light receiving unit 330, the components reflected by the reference mirror 316 pass through the same portion of the transmission type diffraction grating 317. become. On the other hand, the component reflected by the target mirror 315 becomes a beam that has passed through different points of the transmission type diffraction grating 317 due to the movement of the target mirror 315 in the X3 direction. Therefore, depending on the position of the target mirror 315, the interference state of the interference signal superimposed by the first polarizing beam splitter 312 changes, and the displacement detection device 300 changes from the light receiving state of the first to fourth light receiving elements 341 to 344. The phase component of the diffraction grating can be detected as the amount of displacement of the target mirror 314 in the X3 direction.
In the case of the configuration shown in FIG. 3, the optical path length does not change even if the position of the target mirror 315 changes.

[4. Description of polarization-retaining fiber]
The displacement detectors 100, 200, and 300 all use polarization-retaining fibers 150, 250, and 350 for beam transmission between the light sources 101, 102, 103 and the displacement detectors 110, 210, 310.
FIG. 4 shows a cross-sectional view of a configuration example of the polarization-retaining fiber 150. The other polarization-retaining fibers 250 and 350 have the same configuration as the polarization-retaining fiber 150.
The polarization-retaining fiber 150 includes stress-applying materials 152 and 153 above and below the core 151 that transmits the beam. Then, the stress applying materials 152 and 153 are configured to cover the entire core 151 with the clad 154 in a state where stress is applied from above and below to the core 151.
In this example, the vertical direction in which stress is applied to the core 151 is defined as the X polarization mode axis, and the direction orthogonal to the X polarization mode axis is defined as the Y polarization mode axis. At this time, the X polarization mode axis becomes the optical axis at which the polarization retention is established.

By the way, in the beam passing through the polarization holding fiber 150, a component of the optical axis (X polarization mode) in which the polarization holding of the polarization holding fiber 150 is established and a component orthogonal to the component (Y polarization mode) are contained. May be included. Generally, the extinction ratio of a semiconductor laser is said to be around 20 dB, and it is not completely linearly polarized. In addition, there are variations in the angle adjustment of the optical axis when incident on the polarization holding fiber 150.
By using the polarization holding fiber 150, the polarization is held for the components of the X polarization mode axis and the Y polarization mode axis. However, since the refractive indexes of the polarization components of the X polarization mode axis and the Y polarization mode axis in the core 151 are different, when stress is applied to the polarization holding fiber 150, the beam passes through the X polarization mode axis. A phase difference occurs with the beam passing through the Y polarization mode axis.

Here, if the amount of light of the beam via the Y polarization mode axis is not sufficiently small with respect to the beam via the X polarization mode axis, the beams via the X polarization mode axis and the beam via the Y polarization mode axis interfere with each other. Will end up. As a result, when stress is applied to the polarization-retaining fiber 150, a slight phase change is detected, and the displacement detection accuracy deteriorates.
Therefore, in this example, the coherence of the light source and the length of the polarization-retaining fiber 150 are defined, and the influence thereof is suppressed so as to be minimized.

[5. Example of using a multi-mode laser as a light source]
Next, the conditions for determining the lengths of the polarization-retaining fibers 150, 250, and 350 will be described. The conditions for determining the length of the polarization-retaining fibers 150, 250, 350 include the conditions when a multimode laser is used as the light sources 101, 201, 301 and the band having poor coherence as the light sources 101, 201, 301. There are two conditions when using a wide light source.
First, the conditions when using the multimode laser will be described.

Typical laser light sources having a plurality of oscillation spectra depending on the resonator length and having low coherence include a multimode semiconductor laser and a single mode semiconductor laser to which high frequency superimposition is applied. The resonator length here is, for example, in the case of a semiconductor laser, the distance between one end face (mirror surface) and the other end face, and the wavelength λ of the laser beam is determined by the resonator length.
The distance Lperiod that causes interference is given by the equation [Equation 1], where one cycle is from the interference state where the optical path length difference is 0 to the time when the interference intensity rises again and reaches the peak.

Here, n _eff is the refractive index of the resonator, and L _cav is the length of the resonator.
That is, it means that the peak of the interference intensity appears in the beam at an integer m times the length obtained by multiplying the length of the resonator by the refractive index of the resonator.

Next, the optical delay distance D at the fiber output end between the components of the X polarization mode axis and the Y polarization mode axis propagating in the polarization holding fiber 150 is given by the equation [Equation 2].

とすると、［数３］式で表せる。 Here, λ indicates the wavelength of the light source, Lfiber indicates the optical fiber length (polarization holding fiber length), and Lbeat indicates the beat length.
Here, the beat length is the difference between the propagation constants (βx, βy) indicating the speed of light between the X polarization mode axis and the Y polarization mode axis propagating in the polarization holding fiber.

Then, it can be expressed by the formula [Equation 3].

That is, when the optical delay distance D caused by the difference in propagation constant and the distance causing interference match, the beam via the X polarization mode axis and the beam via the Y polarization mode axis interfere with each other.
Therefore, the length of the polarization-holding fiber 150 is an integral multiple of the length obtained by multiplying the resonator length of the light source by the refractive index of the resonator, and the propagation constants of the X and Y polarization modes of the polarization-holding fiber. The length is obtained by dividing the wavelength of the light source from the product of the beat lengths obtained from the difference. By doing so, it is possible to avoid interference between the beam via the X polarization mode axis and the beam via the Y polarization mode axis, and stable displacement detection with respect to the stress of the polarization holding fiber 150 becomes possible. ..
The following equation [Equation 4] shows an equation for obtaining the optical fiber length L _fiber .

In the case of a multi-mode semiconductor laser, the interference intensities of the beam via the X polarization mode axis and the beam via the Y polarization mode axis are as shown in FIG. 5, for example, and the larger the optical delay distance, the less likely it is to interfere with each other. .. That is, when the interference intensity when the optical delay distance is 0 (the position of the peak P11) is 1, the interference intensity gradually weakens as the optical delay distance deviates from 0 by a certain distance, and the peaks P12, P13, P14, ... Exists, and after a certain optical delay distance, the interference intensity becomes almost zero.
Here, setting the length so that the beam via the X polarization mode axis and the beam via the Y polarization mode axis do not interfere with each other means that, for example, the interference intensity between the peak P11 and the peak P12 becomes almost 0. The length should have an optical delay distance. It may be a length having an optical delay distance at which the interference intensity between peaks P11 and other peaks other than peak P12 becomes almost zero.

[6. Example of using a light source with poor coherence and a wide band]
As a light source having a coherent distance and having a low coherence, there is also a light source which does not have a plurality of oscillation spectra due to the above-mentioned resonator length but has a specific wavelength width and whose intensity changes broadly with respect to a wavelength. For example, there are superluminescence diodes and LEDs (light emitting diodes). Alternatively, examples of the solid-state laser include a titanium sapphire laser and the like.

When the light sources of these methods are used as the light sources 101, 201, 301, the coherent distance L _low of the light source is measured in advance.
FIG. 6 shows a configuration example for measuring the coherent distance L _low of the light source. Note that FIG. 6 shows the principle of measuring the coherent distance L _low , and omits a configuration unnecessary for explaining the basic principle of measurement.
Here, the light source 401 is the light source to be measured. The beam output by the light source 401 is incident on the polarization beam splitter 402 and is distributed to one and the other polarization components. One polarization component is reflected by the fixed mirror 403 and returns to the polarization beam splitter 402, and the other polarization component is reflected by the movable mirror 404 and returns to the polarization beam splitter 402.
The position of the movable mirror 404 moves in the X4 direction.
Then, the two beams returning to the polarization beam splitter 402 are incident on the light receiving element 405 in a superposed state.

FIG. 7 shows the interference intensity detected by the light receiving element 405.
Here, the optical path length (reciprocating length) between the polarizing beam splitter 402 and the movable mirror 404 is La, and the optical path length (reciprocating length) Lb between the polarizing beam splitter 402 and the fixed mirror 403 is defined as La. .. The horizontal axis of FIG. 7 is the optical path length difference (La-Lb).

As can be seen from FIG. 7, when the optical path length difference (La-Lb) is 0, the interference intensity becomes the maximum value 1, and thereafter, as the optical path length difference becomes longer, the interference intensity becomes almost 0. For example, the optical path length difference from the maximum value 1 to 1/1000 of the interference intensity is defined as the coherent distance L _low of the light source.

Then, the product of the beat lengths obtained from the difference between the coherent distance L _low of the light source thus obtained and the propagation constants of the X and Y polarization modes of the polarization holding fiber 150 is obtained, and the product of the beat lengths is obtained. , The length of the polarization-retaining fiber 150 is made larger than the length divided by the wavelength of the light source 101. The displacement detection device 100 using the polarization holding fiber 150 having such a length can prevent the beam via the X polarization mode axis and the beam via the Y polarization mode axis from interfering with each other, and is polarized. Stable displacement detection is possible with respect to the stress of the holding fiber 150.
In general, the above-mentioned light source having low coherence has a wider wavelength width than a multimode semiconductor laser and has a very short coherence distance L _low , so that the optical fiber length L _fiber is unlikely to become very long.
[Equation 5] is a mathematical expression showing the conditions for determining the length L _fiber of the above-mentioned polarization holding fiber 150.

As described above, regardless of which light source is used, the beam via the X polarization mode axis and the beam via the Y polarization mode axis are mutually arranged by determining the length of the polarization holding fiber under the above-mentioned conditions. It is possible to avoid interference. Therefore, according to the displacement detection device of this example, stable displacement detection is possible against stress such as bending applied to the optical fiber, and the displacement detection accuracy is improved. Further, it is preferable that the optical axis of the beam polarization emitted from the light source is incident on the optical axis in which the polarization retention of the core cross section of the polarization retention fiber is established, but in the case of this example, the polarization retention fiber is used. The accuracy can be maintained even if the optical axis of the end face of the is not exactly aligned.
In the description so far, the condition of the length of the polarization holding fiber 150 included in the displacement detecting device 100 shown in FIG. 1 is shown, but the polarization provided by the displacement detecting devices 200 and 300 shown in FIGS. 2 and 3 is shown. The length conditions of the holding fibers 250 and 350 can be determined in the same manner.

[7. Example of combining multiple fibers]
The polarization holding fibers 150, 250, 350 included in the displacement detecting devices 100, 200, 300 may be connected to a plurality of polarization holding fibers.
For example, as shown in FIG. 8, the beam from the light source 101 may be incident on the beam connected to the three polarization-retaining fibers 150a, 150b, 150c via the first lens 102. good. The beams transmitted through the three polarization-retaining fibers 150a, 150b, and 150c are incident on the displacement detector 110.
Here, the fitting of the polarization-retaining fibers 150a, 150b, 150c means a state in which the end faces of the polarization-retaining fibers 150a, 150b, 150c are connected without a gap.
Such a fitting state can be realized by using, for example, a connector (not shown) attached to a polarization holding fiber.

The lengths of the respective polarization-retaining fibers 150a, 150b, and 150c need to be set in consideration of the coherence of the light source according to the type of the light source already described, but the following conditions are further added. That is, from the leakage of the beam of the X polarization mode axis and the Y polarization mode axis generated at the fitting portion of each of the polarization holding fibers 150a, 150b, 150c, the beam via the X polarization mode axis and the Y polarization mode It is necessary to take measures against the interference of the beam via the axis.
For example, if the polarization-holding fibers having the same length are fitted together, the beam via the X + Y polarization mode axis and the beam via the Y + X polarization mode axis may interfere with each other due to leakage of the fitting portion.

When there are a plurality of mating portions, the connection order of the polarization-retaining fibers is the last-stage polarization-retaining fiber (for example, the polarization-retaining fiber 150a in FIG. 8) and the polarization-retaining fiber excluding the polarization-retaining fiber (for example, FIG. 8). The total length of the polarization-retaining fiber with any combination of the polarization-retaining fibers 150b, 150c) in 8 is an even multiple of the length obtained by multiplying the resonator length by the refractive index of the resonator and the polarization-retaining fiber X, It is necessary to avoid being the same as the product of the beat lengths obtained from the difference in the propagation constants of the Y polarization mode divided by the wavelength of the light source. By setting such a condition, in the interference between lights having different propagation phase differences emitted from the last-stage polarization holding fiber generated by the mating portion, the combination of interference having a large influence on the interference of the main component is reduced. It becomes possible. Further, the respective interferences in the components other than the combination of the interferences having a large influence on the main interferences are originally sufficiently small and can be ignored.
For example, in the case of a polarization holding fiber in which light is propagated with the X polarization mode as the main component, if the main component is combined with the X polarization mode of the polarization holding fiber in the next stage, the component leaks to the Y polarization mode. There is. Here, by having a plurality of fitting portions, various leakage components are generated, but the interference between the Y polarization modes is sufficiently small and can be ignored.

When a light source having a wide band with poor coherence is used, the length of each of the polarization holding fibers 150a, 150b, 150c is 2 m × L (m is an integer of 0 or more, L is the shortest polarization). It is necessary that the length of the holding fiber) is used and the length of each polarization holding fiber does not have the same combination.
For example, each polarization holding fiber length needs to be L1 ≠ L2 ≠ L3, such as L1 = 2 × L3 and L2 = 4 × L3.

[8. Modification example]
The displacement detection device to which the present invention is applied may detect displacement by a configuration and principle different from those of the three displacement detection devices 100, 200, and 300 shown in FIGS. 1, 2, and 3. That is, any displacement detection device having a configuration in which the light from the light source is guided to the displacement detector via the polarization holding fiber can be applied to other configurations.

Further, in the example of FIG. 8 using a plurality of polarization-holding fibers, an example applied to the displacement detection device 100 of FIG. 1 is shown, but a plurality of displacement detection devices 200 and 300 shown in FIGS. 2 and 3 are used. The same can be applied to the condition of the length of each fiber when the polarization holding fiber of the above is used. Further, in FIG. 8, an example in which three polarization holding fibers 150a, 150b, and 150c are fitted is taken, but when two or four or more polarization holding fibers are fitted, the same conditions as in the above-mentioned example are used. Is applicable.

100 ... displacement detector, 101 ... light source, 102 ... first lens, 110 ... displacement detector, 111 ... second lens, 112 ... first polarization beam splitter, 113 ... first mirror, 114 ... second mirror, 115 ... Diffraction grid, 116 ... 3rd lens, 117 ... 4th lens, 118 ... 1st phase plate, 119 ... 3rd mirror, 130 ... light receiving unit, 131 ... beam splitter, 132 ... 2nd phase plate, 133 ... 2nd Polarized beam splitter, 134 ... 3rd polarized beam splitter, 141 ... 1st light receiving element, 142 ... 2nd light receiving element, 143 ... 3rd light receiving element, 144 ... 4th light receiving element, 150, 150a, 150b, 150c ... Holding fiber, 151 ... core, 152, 153 ... stress applying member, 154 ... clad, 200 ... displacement detector, 201 ... light source, 202 ... first lens, 210 ... displacement detector, 211 ... second lens, 212 ... 1 Polarized beam splitter, 213 ... 1st phase plate, 214 ... Target mirror, 215 ... 2nd phase plate, 216 ... Reference mirror, 230 ... Light receiving unit, 231 ... Beam splitter, 232 ... 3rd phase plate, 233 ... 2nd Polarized beam splitter, 234 ... 3rd polarized beam splitter, 241 ... 1st light receiving element, 242 ... 2nd light receiving element, 243 ... 3rd light receiving element, 244 ... 4th light receiving element, 250 ... Polarization holding fiber, 300 ... Displacement Detection device, 301 ... light source, 302 ... first lens, 310 ... displacement detector, 311 ... second lens, 312 ... first polarization beam splitter, 313 ... first phase plate, 314 ... second phase plate, 315 ... target Mirror, 316 ... Reference mirror, 317 ... Transmission diffractive lattice, 318, 319 ... Mirror, 330 ... Light receiving part, 331 ... Beam splitter, 332 ... Third phase plate, 333 ... Second polarization beam splitter, 334 ... Third. Polarized beam splitter, 341 ... first light receiving element, 342 ... second light receiving element, 343 ... third light receiving element, 344 ... fourth light receiving element, 350 ... polarization holding fiber, 401 ... light source, 402 ... polarized beam splitter, 403. ... Fixed mirror, 404 ... Movable mirror, 405 ... Light receiving element

The displacement detection device of the present invention has a plurality of oscillation spectra depending on the length of the resonator that emits a light beam, and has a plurality of interference intensity peaks and a low coherence laser light source, and condenses the light from the laser light source. When the light emitted from the lens and the laser light source is incident so as to be divided into the X polarization mode axis and the Y polarization mode axis of the polarization holding fiber, the beam and the Y deviation via the X polarization mode axis at the exit portion. L _fiber = (2 mn) in the range in which the interference intensity peak when the interference intensity peak of the beam passing through the wave mode axis is the vertical axis includes a length that does not become almost 0 when the optical delay distance is the horizontal axis. _eff L _CAV L _beat ) / λ The interference intensity between m and (m + 1) of the L _fiber is almost 0. The light is focused by a lens selected from the length that can be set to the optical delay distance. It includes a polarization holding fiber that transmits light, and a light receiving element that receives light of all wavelengths in order to use light of all wavelengths of a laser light source propagating through the polarization holding fiber for detecting a displacement amount. Here, m is a positive integer of 0 or more, n _eff is the effective refractive index of the resonator of the laser light source, L _CAV is the resonator length of the laser light source, L _beat is the beat length of the polarization holding fiber, and λ is the light source. The wavelength.

Claims (1)

A laser light source with low coherence, which has multiple oscillation spectra depending on the length of the resonator that emits light,
A lens that collects light from the laser light source and
When the light emitted from the laser light source is incident so as to be divided into the X polarization mode axis and the Y polarization mode axis of the polarization holding fiber, the light via the X polarization mode axis and the Y polarization mode in the emission portion The X polarization in the range where the peaks of a plurality of interference intensities associated with the optical delay distance of the light passing through the axis are -20 dB or more with respect to the interference intensity of the optical delay distance of 0 and the region excluding the peak of the interference intensity. A polarization-maintaining fiber having a length such that the interference intensity of the light passing through the mode axis and the Y polarization mode axis is -20 dB or less with respect to the interference intensity of the optical delay distance of 0.
With a displacement detector that divides the light transmitted by the polarization holding fiber into two and converts each light into an interference signal by interfering with each other in order to use the change in phase multiplied by the light as the displacement amount. , Including
A displacement detection device in which light emitted from the polarization holding fiber is incident on the displacement detector without passing through a depolarization element that eliminates polarization.

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