JP7578769B2 - Displacement Detection Device - Google Patents

Description

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

Conventionally, optical displacement detection devices have been widely used as devices for non-contact measurement of the displacement of a surface to be measured. There are various types of specific configurations of the displacement detection device, which will be described later in the embodiments, but in any type, the displacement detection device is configured to guide light from a light source to a displacement detector via an optical fiber. The displacement detection device changes the phase of the light from the light source based on the displacement of the surface to be measured, and detects the change in the phase of the light to detect the amount of displacement of the surface to be measured.

Japanese Patent Application Publication No. 7-332957 JP 2006-194855 A

When detecting the displacement of a surface to be measured using a displacement detection device, it is important that the phase of the light reaching the displacement detector from the light source is stable. In other words, there is a risk of unwanted fluctuations in the phase of the light occurring due to the application of stress, such as changes in the bending state of the optical fiber that guides the light from the light source, the fitting state of the optical fiber connector, or the adhesive state between the optical fiber and the ferrule that connects the optical fibers. If unwanted fluctuations in the phase of the light occur, the stability of the detection of the displacement of the surface to be measured by the displacement detector decreases, reducing the accuracy of the displacement detection.

As a method for preventing unwanted phase fluctuations of light in the optical fiber from affecting the displacement detection accuracy of the displacement detector, for example, there is a method in which the light emitted from the optical fiber is depolarized, then polarized by a polarizer, and any polarized light is extracted and input to the displacement detector (see Patent Document 1).
Another method is to use a polarization-maintaining fiber as the optical fiber, and increase the extinction ratio of the light incident on the polarization-maintaining fiber to reduce unnecessary polarized light components (see Patent Document 2).

In the method of depolarizing the light emitted from the optical fiber and then polarizing it with a polarizer, the light is first depolarized and then any polarized light is extracted and made incident on the displacement detector, which has the problem of reducing the amount of light incident on the displacement detector.
In addition, the method of increasing the extinction ratio of light incident on a polarization-maintaining fiber does not reduce the amount of light, but when adding an optical connector, etc., it is necessary to precisely align the optical axis at each mating part, which increases manufacturing costs.
Furthermore, as the resolution of A/D converters in displacement detection devices has increased in recent years, leakage of polarized beam components orthogonal to the optical axis of the polarization-maintaining fiber has become a problem, and it has become necessary to keep the amplitude of interference due to unwanted light within the quantization error of the A/D converter.

The present invention aims to provide a displacement detection device that can detect displacement with high stability and accuracy by eliminating the need to provide a beam with a high extinction ratio, as has been required up until now, and by attenuating interference with unnecessary beams without causing loss of light.

A displacement detection device of a first invention comprises a low-coherence multimode laser light source having multiple interference intensity peaks caused by an optical path length difference, a lens for collecting light from the laser light source, a beam splitter for branching the light collected by the lens and transmitted through a polarization-maintaining fiber, a diffraction grating onto which the branched light is irradiated, and a light-receiving element for receiving interference light produced by superimposing and interfering with diffracted light from the diffraction grating, and detects the amount of displacement of the diffraction grating from a change in intensity of the interference light, and the fiber length L of the polarization-maintaining fiber is set to a length at which the intensity of the interference light due to light passing through two orthogonal propagation modes peaks, and to a length at which the interference intensity between (2mn _{eff LCAV} L _beat )/λ and (2(m+1)n _{eff LCAV} L _beat )/λ is approximately 0 (where m is a positive integer greater than or equal to 0, n _eff is the effective refractive index of the resonator of the laser light source, _LCAV is the resonator length of the laser light source, L _beat is the beat length of the polarization-maintaining fiber, and λ is the wavelength of the laser light source).

A displacement detection device of a second invention includes a lens that collects light from a laser light source, a beam splitter that splits the light transmitted by the polarization-maintaining fiber, a target mirror that moves in conjunction with the movement of the surface to be measured and reflects the split light, and a light-receiving element that receives interference light caused by overlapping and interfering with the light reflected from the target mirror, and detects the displacement of the target mirror from changes in intensity of the interference light. In this displacement detection device, the fiber length L of the polarization-maintaining fiber is set to a length at which the intensity of the interference light caused by light passing through two orthogonal propagation modes peaks, and to a length at which the interference intensity between (2mn _{eff LCAV} L _beat )/λ and (2(m+1)n _{eff LCAV} L _beat )/λ is approximately 0 (where m is a positive integer greater than or equal to 0, n _eff is the effective refractive index of the resonator of the laser light source, _LCAV is the resonator length of the laser light source, L _beat is the beat length of the polarization-maintaining fiber, and λ is the wavelength of the laser light source).
A displacement detection device of a third invention includes a low-coherence light source having a coherence length that emits light, a lens that collects the light from the laser light source, a beam splitter that branches the light collected by the lens and transmitted through a polarization-maintaining fiber, a diffraction grating onto which the branched light is irradiated, and a light-receiving element that receives interference light produced by superimposing and interfering with the diffracted light from the diffraction grating, and detects the amount of displacement of the diffraction grating from a change in intensity of the interference light. In this displacement detection device, when light emitted from the laser light source is incident on the polarization-maintaining fiber, the length L of the polarization-maintaining fiber is longer than the product of the coherence length _L and _{the beat} length L obtained from the difference between the propagation constants of the two polarization modes, divided by the wavelength λ of the light source, and transmits the light collected by the lens such that the intensity of the interference light due to light passing through the two orthogonal propagation modes of the polarization-maintaining fiber is approximately zero.
A displacement detection device of a fourth invention includes a lens that collects light from a laser light source, a beam splitter that branches the light collected by the lens and transmitted by the polarization-maintaining fiber, a target mirror that moves in conjunction with the movement of the surface to be measured and reflects the branched light, and a light-receiving element that receives interference light caused by overlapping and interfering with the light reflected from the target mirror, and detects the displacement of the target mirror from a change in intensity of the interference light. In this displacement detection device, when light emitted from the laser light source is incident on the polarization-maintaining fiber, the length L of the polarization-maintaining fiber is longer than the product of the coherence length _L and the _beat length L obtained from the difference between the propagation constants of the two polarization modes, divided by the wavelength λ of the laser light source, and transmits the light collected by the lens such that the intensity of the interference light caused by the light passing through the two orthogonal propagation modes of the polarization-maintaining fiber is approximately zero.

According to the present invention, by configuring the polarization-maintaining fiber to any length in accordance with the oscillation characteristics of the light source, it is possible to attenuate interference with unnecessary beams. This makes it possible to minimize the components that cause phase fluctuations in the displacement detection signal of the displacement detection device, limiting it to amplitude fluctuations in the interference signal, and enables displacement detection that is less susceptible to the effects of external stress on the polarization-maintaining fiber. According to the present invention, there is no loss of light, so the light from the light source is used efficiently, and furthermore, measures to supply a beam with a high extinction ratio to the displacement detector are not required.

1 is a configuration diagram showing an example of a displacement detection device to which the present invention is applied (an example in which a diffraction grating is displaced). 1 is a configuration diagram showing an example of a displacement detection device (an example of a Michelson interferometer type) to which the present invention is applied; 1 is a configuration diagram showing an example of a displacement detection device to which the present invention is applied (an example in which a target mirror and a reference mirror are arranged in parallel). 1 is a cross-sectional view showing an example of a polarization-maintaining fiber according to an embodiment of the present invention. 1 is a characteristic diagram showing an example of a multimode laser according to an embodiment of the present invention. FIG. 1 is a diagram showing an example of a configuration for measuring the coherence length of a light source having poor coherence in a wide band according to an embodiment of the present invention. FIG. 4 is a characteristic diagram showing an example of a coherence length according to an embodiment of the present invention. 1 is a configuration diagram showing an example in which a plurality of polarization-maintaining fibers are connected together according to an embodiment of the present invention;

A displacement detection device according to one embodiment of the present invention (hereinafter referred to as "this example") will be described below with reference to the attached drawings. First, three examples of the overall configuration of a displacement detection device to which the present invention is applied will be described with reference to Figs. 1 to 3.

[1. Example of the overall configuration of a displacement detection device (example of displacing a diffraction grating)]
FIG. 1 shows an example of the configuration of a displacement detection device 100 that displaces a diffraction grating in conjunction with the displacement of a surface to be measured.
The displacement detection device 100 includes a light source 101. A laser, an LED, or the like can be used as the light source 101. When a laser is used as the light source 101, a multimode semiconductor laser diode with low coherence may be used. Semiconductor lasers include self-pulsation laser diodes and superluminescence diodes.
In addition, the solid-state laser used as the light source 101 includes a YAG laser, a Nd laser, a titanium sapphire laser, a fiber laser, etc. Furthermore, the laser used as the light source 101 includes a helium cadmium laser (metal laser) and a dye laser (liquid laser). Note that it is preferable that the light source 101 is a continuous wave laser having a wide range of oscillation wavelengths in the longitudinal mode and low coherence.
Alternatively, the light source 101 may be a single-mode semiconductor laser whose coherence is reduced by high-frequency superposition.

The beam emitted from the light source 101 is focused by the first lens 102 and enters the polarization-maintaining fiber 150. The polarization-maintaining fiber 150 is an optical fiber with different refractive index distributions in the vertical and horizontal directions of the core cross section, and will be described in detail later. Note that it is preferable for the optical axis of the polarized light from the light source 101 to be incident in alignment with the optical axis that maintains polarization in the core cross section of the polarization-maintaining fiber 150, but it does not have to be aligned precisely. The length of the polarization-maintaining fiber 150 is determined by the conditions described later.

The beam transmitted by the polarization maintaining fiber 150 is incident on the displacement detector 110 .
The beam incident on the displacement detector 110 is incident on a first polarizing beam splitter (PBS) 112 via a second lens 111. The first polarizing 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 polarizing beam splitter 112, and the P-polarized beam is transmitted through the first polarizing beam splitter 112.

The S-polarized light beam reflected by first polarizing beam splitter 112 is incident on diffraction grating 115 via first mirror 113. In addition, the P-polarized light beam transmitted through first polarizing beam splitter 112 is incident on diffraction grating 115 via second mirror 114.
The diffraction grating 115 functions as a scale for detecting the amount of displacement of the surface to be measured, and moves as indicated 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 grating 115 is incident on a third mirror 119 via a third lens 116 or a fourth lens 117 and a first phase plate 118, and each beam reflected by the third mirror 119 is returned to the diffraction grating 115. The first phase plate 118 is formed of, for example, a quarter-wave plate.

The diffracted light of the beams returning to the diffraction grating 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-polarized light returns as P-polarized light, and the P-polarized light returns as S-polarized light.
Therefore, the two beams returning from the diffraction grating 115 are emitted in a superimposed state from a surface different from the surface of incidence of the beam from the light source 101 onto the first polarizing beam splitter 112, and are incident on the light receiving section 130.

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

Next, an operation of the displacement detection device 100 for detecting the displacement of the diffraction grating 115 will be described.
The beam emitted from the light source 101 is focused and incident on the end face of the polarization-maintaining fiber 150 by the first lens 102. The beam emitted from the polarization-maintaining fiber 150 is collimated by the second lens 111. The collimated beam is incident on the first polarization beam splitter 112 in a polarization state such that the first polarization beam splitter 112 splits the beam into two beams at a ratio of 1:1. One of the beams split by the first polarization beam splitter 112, which is an S-polarized component, passes through the first mirror 113 and is incident on the diffraction grating 115 at position P1. The beam diffracted vertically by the diffraction grating 115 becomes circularly polarized light by the first phase plate 118, is reflected by the third mirror 119, and is folded back toward the diffraction grating 115. At this time, the beam passes through the first phase plate 118 again to become a P-polarized component beam, which becomes light diffracted twice by the diffraction grating 115 and heads toward the first polarization beam splitter 112.

The P-polarized component beam split by the first polarizing beam splitter 112 passes through the second mirror 114 and is incident on the diffraction grating 115 at position P2. The beam diffracted vertically by the diffraction grating 115 becomes circularly polarized light by the first phase plate 118 and is reflected by the third mirror 119 and folded back. At this time, the beam passes through the first phase plate 118 again to become an S-polarized component beam, which is diffracted twice by the diffraction grating 115 and heads toward the first polarizing beam splitter 112.

The two beams, whose polarization components have been swapped, are superimposed by the first polarizing beam splitter 112 and proceed toward the light receiving unit 130. This superimposed beam is made up of an S-polarized component and a P-polarized component of linear polarization. This beam is split into two by the beam splitter 131, and one of the beams passes through the second phase plate 132 and enters the second polarizing beam splitter 133.
When passing through the second phase plate 132, the linearly polarized lights perpendicular to each other become a superposition of circularly polarized lights in opposite directions. The polarization direction of the linearly polarized light incident on the second polarizing beam splitter 133 rotates twice when the diffraction grating 115 moves by one wavelength in the moving direction. Therefore, an interference signal of Acos(4Kx+δ') is obtained at the first light receiving element 141 and the second light receiving element 142. Here, A is the amplitude of the interference signal, and K is the wave number represented by 2π/Λ. Furthermore, x indicates the amount of movement of the diffraction grating, and δ' is the initial phase. Λ is the grating spacing in the diffraction grating.

The other beam split into two by the beam splitter 131 is incident on the third polarizing beam splitter 134. At this time, the third polarizing beam splitter 134 is rotated 45° around the optical axis relative to the second polarizing beam splitter 133, so that the phase of the photoelectrically converted signal differs by 90° between the third light receiving element 143 and the fourth light receiving element 144 compared to the first light receiving element 141 and the second light receiving element 142. This makes it possible to recognize the direction of movement of the diffraction grating.

In this manner, the displacement detection device 100 can detect the interference signal as the amount of displacement of the diffraction grating 115 in the X1 direction from the light receiving states 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 components related to the wavelength of the light source 101. Therefore, even if the wavelength of the light source fluctuates due to changes in air pressure, humidity, or temperature, the interference intensity is not affected.

[2. Example of the overall configuration of a displacement detection device (example of a Michelson interferometer type)]
FIG. 2 shows an example of the configuration of a Michelson interferometer type displacement detection device 200 that moves a target mirror 214 in conjunction with the displacement of a 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 those of the light source 101 described in the displacement detection device 100 shown in Fig. 1, and for example, a light source with 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 enters the polarization-maintaining fiber 250. The polarization-maintaining fiber 250 is an optical fiber with different refractive index distributions in the vertical and horizontal directions of the core cross section, and the length of this polarization-maintaining fiber 250 is determined by the conditions described below.

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

When the target mirror 214 is in a 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 returning from the target mirror 214 and the reference mirror 216 to the first polarizing beam splitter 212 are superimposed to become an interference signal, which is incident on the light receiving section 230 .

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

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

[3. Example of the overall configuration of the displacement detection device (example in which the target mirror and the reference mirror are arranged in parallel)]
FIG. 3 shows an example of the configuration of a displacement detection device 300 in which a target mirror 315 and a 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 those of the light source 101 described in the displacement detection device 100 shown in Fig. 1, and for example, a light source with 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 enters the polarization-maintaining fiber 350. The polarization-maintaining fiber 350 is an optical fiber whose core cross section has different refractive index distributions in the vertical and horizontal directions, and the length of the polarization-maintaining fiber 350 is determined by the conditions described below.

The beam transmitted by the polarization maintaining fiber 350 is incident on the displacement detector 310 .
The beam incident on the displacement detector 310 is incident on a first polarizing beam splitter 312 via a second lens 311. The P-polarized beam transmitted through the first polarizing beam splitter 312 is incident on a target mirror 315 via a first phase plate 313. The first phase plate 313 is made of, for example, a quarter-wave plate. Here, the beam incident on the target mirror 315 is incident at an angle of 45° with respect to the target mirror 315. 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. The diffracted light reflected by the target mirror 315 is then incident on the mirror 318. The angle of incidence of the diffracted light with respect to the mirror 318 is 90°, and the mirror 318 reflects the incident diffracted light in the direction of incidence.

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 quarter-wave plate. Here, the beam incident on the reference mirror 316 is incident at an angle of 45° with respect to the reference mirror 316.

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. The diffracted light reflected by the reference mirror 316 is then incident on the mirror 319. The angle of incidence of the diffracted light with respect to the mirror 319 is set to 90°, and the mirror 319 reflects the incident diffracted light in the direction of incidence. The first phase plate 313 and the second phase plate 314 are, for example, quarter-wave plates.

The beams reflected by the mirrors 318 and 319 travel along a path opposite to that of the incident beam, and are incident on the first polarizing beam splitter 312. Then, the two beams returning to the first polarizing beam splitter 312 are superimposed to become 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 split beams is incident on the second polarizing beam splitter 333 via the third phase plate 332. The beam incident on the second polarizing beam splitter 333 is split into its polarization components and then incident on the first light receiving element 341 and the second light receiving element 342. The other of the split beams is incident on the third polarizing beam splitter 334 and split into its polarization components 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 become beams that have passed through the same location of the transmission type diffraction grating 317. On the other hand, the components reflected by the target mirror 315 become beams that have passed through different locations of the transmission type diffraction grating 317 due to the movement of the target mirror 315 in the X3 direction. Therefore, the interference state of the interference signals superimposed by the first polarizing beam splitter 312 changes depending on the position of the target mirror 315, and the displacement detection device 300 can detect the phase component of the diffraction grating as the displacement amount of the target mirror 314 in the X3 direction from the light receiving states of the first to fourth light receiving elements 341 to 344.
In the configuration shown in FIG. 3, even if the position of the target mirror 315 changes, the optical path length does not change.

4. Description of Polarization-Maintaining Fiber
The displacement detection devices 100 , 200 , 300 all use polarization maintaining fibers 150 , 250 , 350 for transmitting beams between the light sources 101 , 102 , 103 and the displacement detectors 110 , 210 , 310 .
4 shows a cross section of an example of the configuration of the polarization-maintaining fiber 150. The other polarization-maintaining fibers 250 and 350 have the same configuration as the polarization-maintaining fiber 150.
The polarization-maintaining fiber 150 includes stress-applying members 152, 153 above and below a core 151 that transmits a beam. The stress-applying members 152, 153 apply stress to the core 151 from above and below, and the core 151 is entirely covered with a cladding 154.
In this example, the vertical direction in which stress is applied to the core 151 is defined as the X-polarized mode axis, and the direction perpendicular to the X-polarized mode axis is defined as the Y-polarized mode axis. In this case, the X-polarized mode axis becomes the optical axis along which polarization maintenance is achieved.

Incidentally, the beam passing through the polarization-maintaining fiber 150 may contain a component of the optical axis that maintains the polarization of the polarization-maintaining fiber 150 (X-polarization mode) and a component orthogonal thereto (Y-polarization mode). In general, the extinction ratio of a semiconductor laser is said to be around 20 dB, and is not completely linearly polarized. Also, there is variation in the angle adjustment of the optical axis when entering the polarization-maintaining fiber 150.
By using the polarization-maintaining fiber 150, the polarization is maintained for the components of the X-polarization mode axis and the Y-polarization mode axis. However, since the refractive indexes of the polarized 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-maintaining fiber 150, a phase difference occurs between the beam that has passed through the X-polarization mode axis and the beam that has passed through the Y-polarization mode axis.

If the amount of light of the beam via the Y polarization mode axis is not sufficiently small relative to the beam via the X polarization mode axis, the beams via the X polarization mode axis and the Y polarization mode axis will interfere with each other. As a result, when stress is applied to the polarization-maintaining fiber 150, a slight phase change will be detected, and the detection accuracy of the displacement will deteriorate.
Therefore, in this embodiment, the coherence of the light source and the length of the polarization maintaining fiber 150 are specified to suppress the influence to a minimum.

[5. Example of using a multimode laser as a light source]
Next, we will explain the conditions for determining the length of the polarization-maintaining fibers 150, 250, and 350. There are two conditions for determining the length of the polarization-maintaining fibers 150, 250, and 350: a condition when a multimode laser is used as the light source 101, 201, or 301, and a condition when a light source with a wide band and poor coherence is used as the light source 101, 201, or 301.
First, the conditions for using a multimode laser will be described.

Representative laser light sources with low coherence and multiple oscillation spectra according to the cavity length include multimode semiconductor lasers and single-mode semiconductor lasers with high frequency superposition. The cavity length here is the distance between one end face (mirror surface) and the other end face in the case of a semiconductor laser, for example, and the wavelength λ of the laser light is determined by the cavity length.
The distance L _period at which interference occurs is given by equation 1, where one period is the time from an interference state where the optical path difference is 0 until the interference intensity rises again and reaches a peak.

Here, _neff is the refractive index of the cavity, and _Lcav is the cavity length.
This means that a peak of the interference intensity appears in the beam at an integer m times the length of twice the resonator length multiplied 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 through the polarization-maintaining fiber 150 is given by [Equation 2].

Here, λ denotes the wavelength of the light source, Lfiber denotes the optical fiber length (polarization-maintaining fiber length), and Lbeat denotes the beat length.
Here, the beat length is the difference between the propagation constants (βx, βy) that indicate the speed of light propagating in the polarization-maintaining fiber in the X-polarized mode axis and the Y-polarized mode axis, Δβ = βx - βy.
Then, it can be expressed by the formula [3].

In other words, when the optical delay distance D resulting from the difference in propagation constant coincides with the distance at which interference occurs, the beam passing through the X polarization mode axis and the beam passing through the Y polarization mode axis interfere with each other.
Therefore, the length of the polarization-maintaining fiber 150 is set to a length that avoids an integer multiple of twice the resonator length of the light source multiplied by the refractive index of the resonator, and a length obtained by dividing the product of the beat length calculated from the difference in the propagation constants of the X- and Y-polarization modes of the polarization-maintaining fiber by the wavelength of the light source. In this way, interference between the beam via the X-polarization mode axis and the beam via the Y-polarization mode axis can be avoided, and stable displacement detection against stress on the polarization-maintaining fiber 150 becomes possible.
The following formula [4] shows a formula for calculating the optical fiber length L _fiber .

In the case of a multimode semiconductor laser, the interference intensity of the beam via the X-polarization mode axis and the beam via the Y-polarization mode axis is, for example, as shown in Fig. 5, and the longer the optical delay distance, the less likely they are to interfere with each other. In other words, if the interference intensity is 1 when the optical delay distance is 0 (the position of peak P11), there are peaks P12, P13, P14, ... where the interference intensity gradually weakens as the optical delay distance moves away from 0 by a certain distance, and after a certain optical delay distance, the interference intensity becomes almost 0.
Here, the length that avoids interference between the beam via the X polarization mode axis and the beam via the Y polarization mode axis means, for example, a length having an optical delay distance where the interference intensity between peaks P11 and P12 is approximately 0. It may also be a length having an optical delay distance where the interference intensity between peaks other than between peaks P11 and P12 is approximately 0.

[6. Example of using a broad-band light source with poor coherence]
Among low-coherence light sources with a coherence length, there are light sources that do not have multiple oscillation spectra associated with the above-mentioned resonator length, but have a specific wavelength width and the intensity changes broadly with respect to the wavelength. For example, there are superluminescence diodes and LEDs (light-emitting diodes). Alternatively, as a solid-state laser, there is a titanium sapphire laser.

When using light sources of these types as the light sources 101, 201, and 301, the coherence length L _low of the light source is measured in advance.
Fig. 6 shows an example of a configuration for measuring the coherence length L _low of a light source. Note that Fig. 6 shows the principle of measuring the coherence length L _low , and omits configuration that is not necessary for explaining the basic principle of measurement.
Here, a light source 401 is the light source to be measured. The beam output by the light source 401 is incident on a polarizing beam splitter 402 and is split into one and the other polarized light components. One polarized light component is reflected by a fixed mirror 403 and returns to the polarizing beam splitter 402, and the other polarized light component is reflected by a movable mirror 404 and returns to the polarizing beam splitter 402.
The position of the movable mirror 404 moves in the X4 direction.
The two beams returning to the polarizing beam splitter 402 are then superimposed on each other and enter the light receiving element 405 .

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

7, when the optical path length difference (La-Lb) is 0, the interference intensity is a maximum value of 1, and as the optical path length difference becomes longer, the interference intensity becomes approximately 0. For example, the optical path length difference at which the interference intensity becomes 1/1000 of the maximum value 1 is defined as the coherence length L _low of the light source.

Then, the product of the coherence length L _low of the light source thus determined and the beat length determined from the difference in the propagation constants of the X- and Y-polarization modes of the polarization-maintaining fiber 150 is calculated, and the length of the polarization-maintaining fiber 150 is made longer than the product of this beat length divided by the wavelength of the light source 101. The displacement detection device 100 using a polarization-maintaining fiber 150 of 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, enabling stable displacement detection against stress on the polarization-maintaining fiber 150.
In general, the above-mentioned low-coherence light source has a wider wavelength width than a multimode semiconductor laser and a very short coherence length L _low , so that the optical fiber length L _fiber rarely becomes very long.
The formula [5] mathematically expresses the condition for determining the length L _fiber of the polarization-maintaining fiber 150 described above.

As described above, regardless of which light source is used, by determining the length of the polarization-maintaining fiber under the above-mentioned conditions, it is possible to prevent the beam via the X-polarization mode axis and the beam via the Y-polarization mode axis from interfering with each other. Therefore, the displacement detection device of this example enables stable displacement detection against stress such as bending applied to the optical fiber, and improves the detection accuracy of the displacement. In addition, although it is preferable that the optical axis of the polarized beam emitted from the light source is incident on the polarization-maintaining fiber so as to be aligned with the optical axis that maintains polarization in the cross section of the core, in this example, accuracy is maintained even if the optical axis of the end face of the polarization-maintaining fiber is not strictly aligned.
In the explanation so far, the conditions for the length of the polarization-maintaining fiber 150 provided in the displacement detection device 100 shown in FIG. 1 have been described, but the conditions for the lengths of the polarization-maintaining fibers 250 and 350 provided in the displacement detection devices 200 and 300 shown in FIGS. 2 and 3 can also be determined in a similar manner.

[7. Examples of combining multiple fibers]
The polarization-maintaining fibers 150, 250, and 350 provided in the displacement detection devices 100, 200, and 300 may be formed by connecting a plurality of polarization-maintaining fibers.
8, a beam from a light source 101 may be incident on three polarization-maintaining fibers 150a, 150b, and 150c that are fitted together and connected via a first lens 102. The beam that travels through the three polarization-maintaining fibers 150a, 150b, and 150c is incident on a displacement detector 110.
Here, fitting the polarization-maintaining fibers 150a, 150b, and 150c refers to a state in which the end faces of the polarization-maintaining fibers 150a, 150b, and 150c are connected without any gaps.
Such a mating can be achieved, for example, using a connector (not shown) attached to the polarization maintaining fiber.

The length of each of the polarization-maintaining fibers 150a, 150b, and 150c must be set in consideration of the coherence of the light source according to the type of light source as described above, but the following condition must also be met: That is, it is necessary to take measures against interference between the beam via the X-polarization mode axis and the beam via the Y-polarization mode axis due to leakage of the beam of the X-polarization mode axis and the beam of the Y-polarization mode axis that occurs at the mating portion of each of the polarization-maintaining fibers 150a, 150b, and 150c.
For example, if polarization-maintaining fibers of the same length are mated, there is a possibility that a beam passing through the X+Y polarization mode axis and a beam passing through the Y+X polarization mode axis will interfere with each other due to leakage at the mating portion.

When there are multiple fitting parts, the order of connection of the polarization-maintaining fibers must be such that the total length of the polarization-maintaining fibers of the last-stage polarization-maintaining fiber (e.g., polarization-maintaining fiber 150a in FIG. 8) and any combination of the polarization-maintaining fibers other than that (e.g., polarization-maintaining fibers 150b and 150c in FIG. 8) is not equal to the product of an even multiple of the resonator length multiplied by the resonator refractive index and the beat length calculated from the difference in the propagation constants of the X and Y polarization modes of the polarization-maintaining fiber, divided by the wavelength of the light source. By setting such a condition, it becomes possible to reduce the combination of interferences that have a large effect on the interference of the main component in the interference between the lights with different propagation phase differences output from the last-stage polarization-maintaining fiber caused by the fitting parts. In addition, the interference of each component other than the combination of interferences that has a large effect on the main interference is originally small enough to be ignored.
For example, in the case of a polarization-maintaining fiber that propagates light with the X-polarized mode as the main component, when the main component is mated with the X-polarized mode of the next stage of polarization-maintaining fiber, some components leak into the Y-polarized mode. Here, by having multiple mating parts, various leakage components occur, but the interference between the Y-polarized modes is sufficiently small that it can be ignored.

Furthermore, when using a light source with a wide bandwidth and poor coherence, the length of each polarization-maintaining fiber 150a, 150b, 150c must be 2 ^m ×L (m is an integer greater than or equal to 0, and L is the length of the shortest polarization-maintaining fiber), and there must be no combination of the same length of each polarization-maintaining fiber.
For example, the lengths of the polarization-maintaining fibers must be such that L1≠L2≠L3, such as L1=2×L3 and L2=4×L3.

8. Modifications
The displacement detection device to which the present invention is applied may be one that detects displacement with a configuration or principle different from that of the three displacement detection devices 100, 200, and 300 shown in Figures 1, 2, and 3. In other words, the present invention is applicable to other configurations as long as the displacement detection device has a configuration in which light from a light source is guided to a displacement detector via a polarization-maintaining fiber.

In the example of FIG. 8 using multiple polarization-maintaining fibers, an example is shown in which it is applied to the displacement detection device 100 of FIG. 1, but the same can be applied to the conditions for the length of each fiber when multiple polarization-maintaining fibers are used in the displacement detection devices 200 and 300 shown in FIG. 2 and FIG. 3. Furthermore, although FIG. 8 shows an example in which three polarization-maintaining fibers 150a, 150b, and 150c are fitted together, the same conditions as in the above example can be applied when two or four or more polarization-maintaining fibers are fitted together.

100... Displacement detection device, 101... Light source, 102... First lens, 110... Displacement detector, 111... Second lens, 112... First polarizing beam splitter, 113... First mirror, 114... Second mirror, 115... Diffraction grating, 116... Third lens, 117... Fourth lens, 118... First phase plate, 119... Third mirror, 130... Light receiving unit, 131... Beam splitter, 132... Second phase plate, 133... Second polarizing beam splitter, 134... Third polarizing beam splitter, 141... First light receiving element, 1 42... second light receiving element, 143... third light receiving element, 144... fourth light receiving element, 150, 150a, 150b, 150c... polarization maintaining fiber, 151... core, 152, 153... stress applying member, 154... cladding, 200... displacement detection device, 201... light source, 202... first lens, 210... displacement detector, 211... second lens, 212... first polarizing beam splitter, 213... first phase plate, 214... target mirror, 215... second phase plate, 216... reference mirror, 230... light receiving unit, 231...beam splitter, 232...third phase plate, 233...second polarizing beam splitter, 234...third polarizing beam splitter, 241...first light receiving element, 242...second light receiving element, 243...third light receiving element, 244...fourth light receiving element, 250...polarization maintaining fiber, 300...displacement detection device, 301...light source, 302...first lens, 310...displacement detector, 311...second lens, 312...first polarizing beam splitter, 313...first phase plate, 314...second phase plate, 315...target mirror , 316...reference mirror, 317...transmission type diffraction grating, 318, 319...mirror, 330...light receiving unit, 331...beam splitter, 332...third phase plate, 333...second polarizing beam splitter, 334...third polarizing beam splitter, 341...first light receiving element, 342...second light receiving element, 343...third light receiving element, 344...fourth light receiving element, 350...polarization maintaining fiber, 401...light source, 402...polarizing beam splitter, 403...fixed mirror, 404...movable mirror, 405...light receiving element

Claims (8)

a multimode laser light source having low coherence and having multiple interference intensity peaks caused by optical path length differences;
a lens for collecting light from the laser light source;
a beam splitter that splits the light collected by the lens and transmitted through the polarization-maintaining fiber;
a diffraction grating onto which the branched light is irradiated;
a light receiving element for receiving interference light produced by overlapping and interfering with the diffracted light from the diffraction grating;
A displacement detection device that detects a displacement amount of the diffraction grating from a change in intensity of the interference light,
The fiber length L of the polarization-maintaining fiber was set to a length at which the intensity of the interference light due to the light passing through two orthogonal propagation modes reaches a peak, and the interference intensity between (2mn _{eff LCAV} L _beat )/λ and (2(m+1)n _{eff LCAV} L _beat )/λ becomes approximately 0 (where m is a positive integer of 0 or more, n _eff is the effective refractive index of the resonator of the laser light source, _LCAV is the resonator length of the laser light source, L _beat is the beat length of the polarization-maintaining fiber, and λ is the wavelength of the laser light source).
Displacement detection device. 2. The displacement detection device according to claim 1, wherein a plurality of the polarization-maintaining fibers are connected to a detachable connector, and when a plurality of fitting portions are present, the total length of the polarization-maintaining fibers, including the last polarization-maintaining fiber in the connection order and all combinations of the polarization-maintaining fibers other than that, is set to a length at which the intensity of the interference light due to the light passing through two orthogonal propagation modes peaks, and the interference intensity between (2mn _{eff LCAV} L _beat )/λ and (2(m+1)n _{eff LCAV} L _beat )/λ is set to be approximately zero. a lens for focusing light from a laser light source;
a beam splitter that splits light transmitted through the polarization-maintaining fiber;
a target mirror that moves in conjunction with the movement of a measurement surface that reflects the branched light;
a light receiving element for receiving interference light obtained by overlapping and interfering with the light reflected from the target mirror;
A displacement detection device that detects a displacement amount of the target mirror from a change in intensity of the interference light,
The fiber length L of the polarization-maintaining fiber was set to a length at which the intensity of the interference light due to the light passing through two orthogonal propagation modes reaches a peak, and the interference intensity between (2mn _{eff LCAV} L _beat )/λ and (2(m+1)n _{eff LCAV} L _beat )/λ becomes approximately 0 (where m is a positive integer of 0 or more, n _eff is the effective refractive index of the resonator of the laser light source, LCAV is the resonator length of the laser light source, L _beat is the beat length of the polarization-maintaining fiber, and λ is the wavelength of the laser light source).
Displacement detection device. 4. The displacement detection device according to claim 3, wherein a plurality of the polarization-maintaining fibers are fitted with a detachable connector, and when a plurality of fitting portions are present, the order of connection of the polarization-maintaining fibers is set so that the total length of the polarization-maintaining fibers, including the last polarization-maintaining fiber and all combinations of the polarization-maintaining fibers other than that, is set to a length at which the intensity of the interference light due to light passing through two orthogonal propagation modes peaks, and the interference intensity between (2mn _{eff LCAV} L _beat )/λ and (2(m+1)n _{eff LCAV} L _beat )/λ is set to be approximately zero. a low-coherence light source having a coherence length that emits light;
a lens for collecting light from the light source;
a beam splitter that splits the light collected by the lens and transmitted through the polarization-maintaining fiber;
a diffraction grating onto which the branched light is irradiated;
a light receiving element for receiving interference light produced by overlapping and interfering with the diffracted light from the diffraction grating;
a displacement detection device for detecting an amount of displacement of the diffraction grating from a change in intensity of the interference light,
a length L of the polarization-maintaining fiber that is longer than the product of a coherence length L _low and a beat length L beat calculated from the difference between the propagation constants of two polarization modes, divided by the wavelength λ of the light source, and the intensity of interference light due to light passing through two orthogonal propagation modes of the polarization-maintaining fiber is approximately zero when the light emitted from the light source is incident on the polarization-maintaining fiber, the length L of the polarization-maintaining fiber being longer than the product of a coherence length L low and a beat length L _beat calculated from the difference between the propagation constants of two polarization modes, divided by the wavelength λ of the light source, and the intensity of interference light due to light passing through two orthogonal propagation modes of the polarization-maintaining fiber is approximately zero. 6. The displacement detection device according to claim 5, wherein a plurality of the polarization-maintaining fibers are connected to a detachable connector, the length of each polarization-maintaining fiber is 2 m × L (where m is an integer equal to or greater than 0, and L is the length of the shortest polarization-maintaining fiber), and no two combinations of the polarization-maintaining fibers have the same length. a lens for focusing light from a laser light source;
a beam splitter that splits the light collected by the lens and transmitted through the polarization-maintaining fiber;
a target mirror that moves in conjunction with the movement of a measurement surface that reflects the branched light;
a light receiving element for receiving interference light obtained by overlapping and interfering with the light reflected from the target mirror;
A displacement detection device that detects a displacement amount of the target mirror from a change in intensity of the interference light,
a length L of the polarization-maintaining fiber that is longer than the product of a coherence length L _low and a beat length L _beat calculated from the difference between the propagation constants of two polarization modes divided by the wavelength λ of the laser light source, and when the light emitted from the laser light source is incident on the polarization-maintaining fiber, the intensity of the interference light due to the light passing through the two orthogonal propagation modes of the polarization-maintaining fiber becomes approximately zero. 8. The displacement detection device according to claim 7, wherein a plurality of the polarization-maintaining fibers are connected to a detachable connector, the length of each polarization-maintaining fiber is 2 m × L (where m is an integer equal to or greater than 0, and L is the length of the shortest polarization-maintaining fiber), and no two combinations of the polarization-maintaining fibers have the same length.

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