JPH0376845B2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a device for optically measuring the amount of displacement of a measurement target by shining light onto the measurement target.
In particular, the present invention relates to an optical measuring device in which light is guided to an object to be measured using an optical fiber.

[Conventional technology]

This type of conventional technology is shown in FIG. 4, in which a laser beam b emitted from a laser oscillator 1 is divided into two beams b ₁ and b 2 by a beam splitter 2, and the beam b ₁ is divided into two beams b 1 and b ₂ . The light b2 is reflected by the reflective surface 3 and enters the photodetector 4, while the light _b2 transmitted through the beam splitter 2 passes through the lens 5, the optical fiber 6, and the lens 7, and is reflected by the surface of the object to be measured 8 and returns to the lens. 7, an optical fiber 6, a lens 5, and a beam splitter 2 to input the light into a photodetector 4. The light b ₁ and b ₂ interfere with each other in the photodetector 4 , and the brightness of the interference light changes depending on the amount of displacement of the object to be measured 8 . After converting it into an electrical signal, it is measured by a measuring circuit 9, thereby making it possible to know the amount of displacement of the object to be measured 8. The code 10 is the display.
A signal as shown in FIG. 5 is displayed, and from the number n of waves of this signal, the displacement δ of the object to be measured 8 (δ=
nλ) is now being measured.

Furthermore, as shown in Japanese Patent Application Laid-Open No. 151509/1983, a technique for optically measuring surface roughness in which a plurality of quarter-wave plates are provided in the middle of the optical path is also known.

[Problem to be solved by the invention]

In the two conventional techniques described above, only one of the two lights having different optical paths passes through the optical fiber. Therefore, the refractive index of the optical fiber changes due to changes in the temperature around the optical fiber, changes in atmospheric pressure, or vibrations applied to the optical fiber. There was a problem that this appeared as a measurement error.

Furthermore, in the conventional technique shown in FIG.
Although a signal as shown in Fig. 5 appears in response to the displacement of the object to be measured 8, it is not possible to determine even the direction of displacement of the object to be measured 8, so even if the amount of displacement is known, it is difficult to determine in which direction the displacement occurs. It was not possible to detect what had happened.

Conventional examples of this type include:
There are JP-A-59-72005 and JP-A-60-100002.

JP-A-59-72005 does not disclose the type of transmission optical fiber. If a multimode fiber is used, the light incident on the semi-transparent mirror will no longer be coherent light. Therefore, the reflected lights from the semi-transparent mirror and the reflecting mirror do not interfere with each other.

Therefore, if we use a single mode fiber, the reflected light from the semi-transparent mirror and the reflecting mirror will interfere with each other, but
It was not possible to determine the sign of the phase change occurring within the sensor optical fiber, and it was also impossible to measure the direction of displacement.

Furthermore, since the intensity of the interference light changes depending on the intensity of the reflected light from the semi-transparent mirror and the reflecting mirror, there is a problem in that the measurement accuracy decreases.

Furthermore, when a disturbance such as a temperature change or a force is applied to the transmission optical fiber, the polarization plane of the laser beam propagating through the transmission optical fiber changes. Therefore, it was not possible to distinguish whether the polarization plane of the light had changed in the sensor fiber due to the current to be detected, or whether the polarization plane had changed due to disturbance in the transmission fiber.

On the other hand, in JP-A No. 60-100002, the detection section provided at the tip of the transmission optical fiber is complicated, and the light receiver/display device and the detection section must be connected with another fiber. It was hot.

Furthermore, even if linearly polarized light is input, if a disturbance acts on the optical fiber while it is propagating through the optical fiber, it will be emitted as elliptically polarized light. That is,
The light emitted from the optical fiber is a combination of two linearly polarized lights with orthogonal polarization planes and different phase differences. Thereafter, the light is split into two by a polarizing beam splitter into reflected light and transmitted light, but in the reflected light, the two linearly polarized light components interfere and the intensity of the light changes. Similarly, the intensity of transmitted light also changes. Therefore, even if the reference light and measurement light are made to interfere, the intensity of the interference will also change due to disturbances, resulting in a decrease in measurement accuracy.

In this case as well, it was not possible to determine the sign of the measured quantity, and it was also impossible to measure the direction of displacement.

An object of the present invention is to provide an optical displacement measuring device that is not affected by changes in the refractive index of an optical fiber due to various disturbances and is capable of measuring not only the amount of displacement but also the direction of displacement of an object to be measured. The goal is to provide the following.

[Means for solving problems]

In order to achieve the above object, the present invention provides a laser oscillator that emits linearly polarized light, an electro-optic crystal that is driven by a voltage and modulates the phase of light emitted from the laser oscillator, and an electro-optic crystal that modulates the phase of light emitted from the electro-optic crystal. A polarization-preserving optical fiber that outputs light to the object to be measured while maintaining its polarization plane, and a polarization-preserving optical fiber that outputs the reflected light from the object to the electro-optic crystal while maintaining its polarization plane. It is located between the end of the optical fiber on the side of the object to be measured and the object to be measured, and separates the transmitted light and reflected light on the incident surface that serves as the measurement reference surface, and the transmitted light is polarized in the opposite direction without changing the polarization direction. A quarter-wave plate that changes the polarization direction of light by 90 degrees when it enters the fiber, and a polarizing beam splitter that splits the reflected light emitted from the electro-optic crystal side end of the polarization-preserving optical fiber into two parts according to the polarization plane. , detect the voltage applied to the electro-optic crystal when the brightness of each light split by the polarizing beam splitter is maximum or minimum, and determine the direction and amount of displacement of the object to be measured from this applied voltage and the difference between them. The present invention proposes an optical displacement measuring device equipped with a measuring section that calculates displacement.

[Effect]

Next, the operation of the present invention will be explained with reference to FIG. 1 showing the overall configuration of an embodiment of the present invention.

A measurement reference plane 22 is provided in front of the fiber end 16F, which is the output end of the polarization preserving optical fiber 16. Since the light reflected from the reference surface 22 also passes through the optical fiber 16 in the same way as the light reflected from the surface of the object to be measured 18, there is an optical path difference between the two lights I ₁ and I ₂ separated by the polarizing beam splitter 26. The presence of optical fiber 16 has no effect on this. As a result, the optical fiber 16
Measurement errors are no longer caused by changes in the refractive index of.

In addition, the measurement section detects the voltage applied to the electro-optic crystal when the brightness of each light split by the polarizing beam splitter is maximum or minimum, and uses this applied voltage and the difference between the two to detect the measurement target. By calculating the direction and amount of displacement, it is possible to accurately grasp not only the amount of displacement but also the direction of displacement.

〔Example〕

Next, embodiments of the present invention will be described.

FIG. 1 is an overall schematic diagram of an embodiment of the present invention,
In this figure, reference numeral 11 denotes a laser oscillator, and on the optical axis of the laser beam emitted by this laser oscillator 11, there is an electro-optical device driven by a high-frequency power source 13 for modulating the phase of the incident light. A crystal 12 is installed. A beam splitter 14 is arranged on the optical axis of the light that has passed through the electro-optic crystal 12 to reflect part of the light and to transmit part of the light. On the optical axis of the transmitted light B of the beam splitter 14, one end 16E of a polarization preserving optical fiber 16 is arranged, which guides the incident light with its polarization plane unchanged. The light is guided and emitted from the other end 16F of the fiber toward the object to be measured 18. Polarization preserving optical fiber 16
is the center core 16 as shown in FIG.
A, it is composed of a cladding 16B on its outer periphery, an elliptical jacket 16C on its outer periphery, and a support tube 16D on its outermost periphery, and the light focused on the core 16A is repeatedly totally reflected at the interface with the cladding 16B. It is adapted to proceed through the core 16A.

Lenses 20 (20A, 20B) are arranged near both ends 16E, 16F of the optical fiber 16 to condense light to the input end of the optical fiber 16 and to collimate the light emitted from the optical fiber 16. It's summery. In front of the lens 20B, the incident light B is separated into reflected light _B1 and transmitted light _B2 on the surface that becomes the measurement reference surface 22, and when the transmitted light _B2 enters from the opposite direction without changing the polarization direction, it becomes light. A 1/4 wavelength plate 24 is installed which has the effect of changing the plane of polarization by ₉₀ degrees from the point of incidence. The light B ₂ transmitted through the plate 24, reflected on the surface of the object to be measured 18, and transmitted through the quarter-wave plate passes through the optical fiber 16 and enters the beam splitter 14 again.

On the optical axis of the reflected light I separated by the beam splitter 14, two lights are separated due to the difference in the plane of polarization.
A polarizing beam splitter 26 is provided to separate the lights I ₁ and I ₂ , and the lights I ₁ and I ₂ separated by the polarizing beam splitter 26 are placed on their respective optical axes by photoelectric conversion. Photodetector 2 that converts into electrical signals
8, 29, local maximum (or minimum) detection circuits 30, 31 for detecting the maximum value (or minimum value) of the electrical signals detected by these photodetectors 28, 29; Electro-optic crystal 12 when indicating the detected maximum (or minimum) value
Sample and hold circuits 32 and 33 are sequentially provided to capture the voltage applied to the voltage and store the voltage value at that time, and the sample and hold circuits 32 and 33 are connected to a subtraction circuit 34, 33, the difference between the stored voltages V ₁ and V ₂ is calculated. That is, the photodetectors 28, 29, the maximum (or minimum) detection circuit 30,
31. The sample and hold circuits 32 and 33 and the subtraction circuit 34 calculate the difference between the maximum (or minimum) brightness values of the two lights I ₁ and I ₂ separated by the polarizing beam splitter 26 to determine the object to be measured 18 A measuring section 40 is configured to measure the amount of displacement.

Furthermore, displacement measurement of the object to be measured 18 according to this embodiment will be explained.

The electro-optic crystal 12 has an orthogonal structure 3 as shown in FIG.
It has axes X, Y, and Z, and has a length l _c in the direction of incidence of light I. Now, when linearly polarized light I _x whose polarization plane is parallel to the XZ plane travels in the Z direction, the refractive index of the crystal is n _x , and similarly, linearly polarized light I _y whose polarization plane is parallel to the YZ plane is When the refractive index of the crystal when traveling in the Z direction is n _y , n _x and n _y are expressed by the following formula.

n _x = n _x0 +K _x V...(1) n _y = n _y0 +K _y V...(2) However, V is the voltage applied to the electro-optic crystal 12,
K _x and K _y are constants determined depending on the type of electro-optic crystal, and n _x0 and n _y0 are refractive indices when the voltage applied to the crystal is zero.

If the difference between the refractive index n _x and n _y is Δn, then Δn = ny − n _x = _ny0 − n _x0 + (K _y − K _x ) V …… ₍ 3) The polarization plane of the laser beam is electro-optic. The electro-optic crystal is installed so that it is parallel to the X'Z plane that is inclined at 45 degrees to the X-axis of the crystal 12, and the laser beam is a light I _x whose polarization plane is parallel to the XZ plane of the electro-optic crystal. and light I _y with a plane of polarization parallel to the YZ plane. here
Letting φ _XC and φ _YC be the phases when I _X and I _Y exit the electro-optic crystal 12, respectively, the following equation is obtained.

_{φ _ _ _ _ _ _} wavelength.

The lights I _X and I _Y that have passed through the beam splitter 12 are focused on a polarization preserving optical fiber 16 (indicated by symbol B).
However, as shown in FIG. 3, the optical fiber 16 is arranged so that the X-axis of the electro-optic crystal 12 shown in FIG. The plane of polarization of the light _IX incident on 16 is parallel to the XZ plane in FIG. 3, and the plane of polarization of the light _IY is parallel to the YZ plane in FIG. At this time, the light
Phase changes φ _xF and φ _yF caused by I _X and I _Y traveling through the optical fiber 16 are expressed by the following equations. Here, φ _xF and φ _yF are changes in phase that the lights I _X and I _Y receive, respectively.

φ _xF = 2πn _xF / λl _F ... (6) φ _yF = 2πn _yF / λl _F ... (7) However, n _xF is _the refractive index of the optical fiber 16 with respect to the light _I The refractive index for _IX , _lF is the length of the optical fiber.

The light emitted from the optical fiber 16 passes through the lens 20B and enters the measurement reference plane 22 formed on the surface of the 1/4 wavelength plate 24. The arrangement relationship between the 1/4 wavelength plate 24 and the output end 16A of the optical fiber 16 is such that the X-Y axis of the optical fiber 16 and the crystal axis X-
It is arranged so that it is inclined at 45 degrees relative to the Y axis. In other words, the optical fiber 16 and the quarter-wave plate are arranged so that the polarization plane of the light _I
A wavelength plate 24 is arranged. A part of the light B exiting the lens 20B is reflected by the reference surface ₂₄ , as shown by the symbol B1 in FIG. 1, and the rest is transmitted through the quarter-wave plate 24, as shown by the symbol _B2 . and is reflected on the surface of the object to be measured 18. In other words, two types of _incident light _I There is. Considering the polarization planes of these four types of reflected light, (1) A part of _the light _I It is. This light I _(X

Claims (1)

[Scope of Claims] 1. A laser oscillator that emits linearly polarized light; an electro-optic crystal that is driven by a voltage and modulates the phase of the light emitted from the laser oscillator; and a polarization plane of the light incident from the electro-optic crystal. a polarization-preserving optical fiber that outputs the reflected light to the object to be measured while maintaining the polarization plane thereof, and outputs the reflected light to the electro-optic crystal side while maintaining the polarization plane of the reflected light returned from the object to be measured; The fiber is located between the end of the fiber to be measured and the object to be measured, and is separated into transmitted light and reflected light on the incident surface that serves as the measurement reference surface, and the transmitted light is transmitted from the opposite direction without changing the polarization direction. a 1/4 wavelength plate that changes the polarization direction of light by 90 degrees when it enters; and a quarter-wave plate that changes the polarization direction of the light by 90 degrees;
A polarizing beam splitter to be split and a voltage applied to the electro-optic crystal when the brightness of each light split by the polarizing beam splitter is maximum or minimum are detected, and from this applied voltage and the difference therebetween, What is claimed is: 1. An optical displacement measuring device comprising: a measuring section that calculates a displacement direction and a displacement amount of an object to be measured.