JPH01175099A - Optical fiber sensor unit - Google Patents

Abstract

PURPOSE:To prevent measurement from being unstabilized due to oscillation of air in an optical path by connecting a light transmitting/receiving part to an optical sensor head part through one constant polarizing optical fiber. CONSTITUTION:A laser beam radiated from a laser light source 10 is rotated by 45 deg. in the clockwise direction at its polarizing face by a Faraday rotor 14 and inputted to a constant polarizing optical fiber 20 through a condenser lens 8. At that time, straight polarized light for generating reference light and measured light is transmitted through the fiber 20 in one mode. In the returning route, the reference light and the measured light are transmitted by two transmission modes whose polarizing faces intersect with each other at right angles. Thereby, the influence of the fiber 20 due to a temperature change or the like, e.g. a phase variation, can be canceled. Thus, the rotational angle or the like of a substance 42 to be measured can be highly accurately measured.

Description

DETAILED DESCRIPTION OF THE INVENTION TECHNICAL FIELD The present invention relates to an optical fiber sensor device that uses an optical fiber to transmit light between an optical transmitter/receiver and an optical sensor head.

Prior Art An optical transmitter/receiver having a light source for generating reference light and measurement light, and a light receiver that receives the reference light and measurement light for measuring physical quantities such as length, angle, and pressure; and this optical transmitter/receiver. Optical measurement system that measures physical quantities such as length, angle, and pressure with high precision, and includes an optical sensor head that changes transmission parameters of at least one of the reference light and measurement light sent from the sensor in relation to external conditions. device is being considered.

For example, in the device shown in FIG.
Wavelength-stabilized He that is feedback-controlled by
The laser beam output from the -Ne laser 70 and containing linearly polarized components whose polarization planes are orthogonal to each other and has two types of frequencies f and f2 is transmitted to the reference beam splitter 71.
The light reflected by this reference beam splitter 71 is split by a beam splitter 66, one of which is detected by a feedback optical sensor 67, and the other is detected by a reference optical sensor 72 via a mirror 68. The light transmitted through the reference beam splitter 71 is split by the polarizing beam splitter 73 into a reference light having a frequency of 12 and a measurement light having a frequency of f. Then, the reference light is reflected by the fixed mirror 74 and the measurement light is reflected by the movable mirror 75, after which they are combined by the polarizing beam splitter 73 and detected by the first optical sensor 76. The beat frequency of the composite light is detected by the first optical sensor 76, but since the measurement light includes a frequency change Δf due to Doppler shift due to the movement of the movable mirror 75, the actual frequency f
t (f+ ±Δr,) is detected. On the other hand, the reference optical sensor 72 detects light having a beat frequency fz-f. The first sensor 76 and the reference light sensor 72
The frequency difference Δf detected by causes the pulse converter in the output device 77 to count one pulse for every 90° movement of the movable mirror 75, so the moving distance can be accurately determined based on the counted value of this pulse. It is. That is,
In this device, a polarized beam beam splitter 73, a fixed mirror 74,
and the portion excluding the movable mirror 75 constitutes an optical transmitting/receiving section,
A polarizing beam splitter 73, which corresponds to the optical sensor head, and a fixed mirror 7 are located at a predetermined distance away from the optical transmitter/receiver.
4. A movable mirror 75 is provided.

In addition, in the device shown in FIG. 10, a frequency-stabilized He
The -Ne laser 80 outputs linearly polarized light with a polarization plane of 45 degrees, and the linearly polarized light is converted into linearly polarized light (P waves and S waves) with mutually orthogonal polarization planes by the polarization beam splitter 81. be divided. One linearly polarized light is reflected by an interference corner cube 83 into which a λ/8 plate 82 is inserted, and the other is reflected by a moving corner cube 84. Since one linearly polarized light passes through the λ/8 plate 82 twice, the phase of the S-polarized light component and the P-polarized light component of the reflected light is shifted by 90''.

The reflected light is combined by the polarizing beam splitter 81 and then split into a P-polarized component and an S-polarized component by the polarized beam splitter 85, which are independently detected by a pair of optical sensors 86 and 87. The sin signal and c detected by the pair of optical sensors 86 and 87
The os signal is synthesized and divided by electrical processing in the fringe division circuit 90, and interference fringes are counted by waveform processing with a resolution of, for example, about λ/16, so that the moving amount or moving speed of the moving corner cube 84 can be determined with high accuracy. Measured at

In this device as well, the optical transmitter/receiver 88, the interferometer 89 and the movable corner cube 84 constituting the optical sensor head section
are arranged at positions separated by a predetermined distance.

Problems to be Solved by the Invention However, in conventional optical measurement devices, light is transmitted through space between the optical transmitter/receiver and the optical sensor head. It is not possible to block the space between It had the disadvantage that it was large and expensive.

Means for Solving the Problems The present invention has been made against the background of the above circumstances.
The purpose of this is to connect the optical transmitting/receiving section and the optical sensor head section with a single optical fiber, and to provide the optical sensor head section with reference light and measurement light based on the optical transmitting/receiving section. It is an object of the present invention to provide a small and inexpensive optical fiber sensor device that detects changes in transmission parameters with high accuracy.

In order to achieve such an object, the gist of the present invention is to provide (a single polarization-controlled optical fiber for transmission that transmits light in two transmission modes in which al polarization planes are orthogonal to each other, and fb) a reference light and A light for generating measurement light is outputted to the polarization-controlled optical fiber, and the light is transmitted in one of the two transmission modes within the polarization-controlled optical fiber, and the two transmission modes are transmitted through the polarization-controlled optical fiber. (C1) an optical transmitting/receiving unit that receives the reference light and measurement light returned in the transmission mode and detects a relative change in the transmission parameter of the measurement light based on the reference light and measurement light; an optical sensor head that separates the light transmitted by the light into a pair of reference light and measurement light, combines the lights and returns them to the polarization-constant optical fiber, and changes at least a transmission parameter of the measurement light in relation to external conditions; and +d) is provided in the optical sensor head, and the reference light and the measurement light are linearly polarized lights whose phases are shifted by 90' and whose polarization planes are orthogonal to each other, so that the reference light and the measurement light have the constant polarization. and phase polarization plane changing means for causing the wave to be transmitted in the two transmission modes in the optical fiber.

Operation and Effect of the Invention In this way, in the polarization-controlled optical fiber, the light for generating the reference light and the measurement light is transmitted in one of the two transmission modes on the outward path, and the light for generating the reference light and the measurement light is transmitted in one of the two transmission modes on the return path. The light and the measurement light are converted into linearly polarized light whose phases are shifted by 90 degrees from each other and whose polarization planes are orthogonal to each other by the phase polarization plane polarization means, and are transmitted in two different transmission modes. Therefore, the phase change due to the constant polarization optical fiber for transmission is are canceled out. Therefore, in the optical transmitter/receiver section, a change in the transmission parameter given to the measurement light is detected in the optical sensor head based on the reference light and measurement light, which are returned through the constant polarization optical fiber and whose phases are shifted by 90 degrees from each other. Ru. Therefore, according to the present invention, the transmission parameter given to the measurement light is detected with maximum detection sensitivity based on the reference light and the measurement light that have a phase difference of 90° from each other, and
In order to form a 90° phase difference between the reference light and the measurement light, there is no need to apply mechanical external force such as providing an electrostrictive element to the optical sensor head, which increases the size of the optical sensor head. This has the effect that the device can be made smaller and less expensive.

In addition, in the present invention, since it is only necessary to connect the optical transmitting/receiving section and the optical sensor head section with one polarization-controlled optical fiber, there is no problem even if the space between them is blocked, and the optical path Since the stability of measurement is not impaired due to air fluctuations in the air, the device is small and inexpensive.

EXAMPLE Hereinafter, an example of the present invention will be described in detail based on the drawings.

FIG. 1 shows an embodiment applied to the optical homodyne system.

In the figure, a laser light source 1 such as a HeNe laser
A monochromatic laser beam emitted from 0 has a radius of 45 mm around the optical axis.
After passing through the polarizing beam splitter 12, the Faraday rotator 14, and the polarizing beam splitter 16, which are arranged to rotate by degrees, the light is made incident on the end face of the polarization-constant optical fiber 20 by the condenser lens 18. Said polarizing beam splitter 12
, the Faraday rotator 14, and the polarizing beam splitter 16 also function as an optical isolator,
The reflected light is prevented from entering the laser light source 10. The polarizing beam 1 and the splitter 16 also have a function of splitting the reflected light according to the plane of polarization, and the light reflected by the polarizing beam splitter 16 is made incident on the first photosensor 24 by the condensing lens 22. Further, the reflected light that has passed through the polarizing beam splitter 16 is transmitted to the Faraday rotator 1.
4, the plane of polarization is rotated by 45 degrees around the optical axis, so that it is reflected by the polarizing beam splitter 12 and is made incident on the second optical sensor 28 by the condensing lens 26. In this embodiment, the above-described optical elements and optical sensors constitute an optical transmitter/receiver 100, and one end of a polarization-controlled optical fiber 20 is fixed to a machine frame (not shown) of the optical transmitter/receiver 100.

The polarization constant optical fiber 20 has a core 30 and a core 30.
It is composed of a pair of stress applying parts 32 sandwiching the stress applying part 32 and a cladding 34 covering them, and for example, the HE mode and the HE II mode which transmit light with polarization planes orthogonal to each other.
Polarized light is transmitted while preserving the plane of polarization in two transmission modes: y mode. As mentioned above, the light incident on one end of the polarization-controlled optical fiber 20 is linearly polarized light that has passed through the polarization beam splitter 16, so that within the polarization-controlled optical fiber 20 it is transmitted in one of the two transmission modes. transmitted.

The other end of the constant polarization optical fiber 20 is fixed to a machine frame (not shown) of the optical sensor head 102, and the signal is transmitted in one of the two modes and from the other end of the constant polarization optical fiber 20. The emitted linearly polarized light passes through the condensing lens 36
The light is converted into parallel light through a non-polarizing beam splitter 38 and separated into reference light and measurement light. The measurement light that has passed through this non-polarized beam splitter 38 is
λ that rotates the plane of polarization of linearly polarized light by 45 degrees by passing through the
The light passes through the /4 plate 40 and is projected toward one of a pair of first corner cube prism 44 and second corner cube prism 46 mounted on the object to be measured 42 .

The reference light reflected by the non-polarizing beam splitter 38 has a wavelength of λ, which converts linearly polarized light into circularly polarized light by transmitting it twice back and forth.
After passing through the /8 plate 48, the direction is changed by the mirror 50, so that the measurement light that has passed through the non-polarizing beam splitter 38 and the measurement light are parallel to each other, and the first corner cube prism is mounted on the object to be measured 42. 44 and the second corner cube prism 46 .

Note that the first corner cube prism 44 and the second
The reflecting surface of the corner cube prism 46 is coated with metal to prevent the polarization characteristics from being changed by reflection.

In this embodiment, the non-polarizing beam splitter 38, mirror 50, etc. correspond to the optical sensor head section 1.2. Further, the λ/4 plate 40 and the λ/8 plate 48 are provided in the optical sensor head 102 to convert the reference light and measurement light into linearly polarized light whose phases are 90 degrees and whose polarization planes are orthogonal to each other. , corresponds to a phase polarization plane changing means that allows the reference light and the measurement light to be transmitted on the return path in the polarization constant optical fiber 20 in two transmission modes.

The operation of this embodiment will be explained below.

As shown in FIG. 2, the laser light emitted from the laser light source 10 is linearly polarized light, and is passed through a polarization beam splinter 12. In FIG. 2, a short arrow perpendicular to the direction of travel of the light indicates the plane of polarization of the linearly polarized light. After the plane of polarization of this linearly polarized light is rotated 45 degrees clockwise by the Faraday rotator 14, the polarization beam splinter 1
6 and is condensed by a condenser lens 18 and made incident on the core 30 of the polarization-constant optical fiber 20 . In this polarization-controlled optical fiber 20, two types of HE,
," mode and HE, ,' mode.

As shown in detail in FIG. 3, the laser light emitted from the constant polarization optical fiber 20 is made parallel by a condenser lens 36, and then split into two beams by a non-polarizing beam splitter 38. This non-polarized beam splinter 38 needs to have the property of not only distributing linearly polarized incident light evenly in terms of energy, but also maintaining the branched light as linearly polarized light, but commercially available products usually have this property. is met. The beam transmitted through the non-polarized beam splinter 38 is the measurement light 1e in this embodiment, and is converted into elliptically polarized light by being transmitted through a λ/4 plate 40 whose crystal axis is tilted by 22.5 degrees with respect to the polarization direction. After that, the light is radiated toward the first corner cube prism 44. In addition, the non-polarized beam splinter 38
The beam reflected by is the reference beam 1c in this embodiment, and after being converted into elliptically polarized light by being transmitted through the λ/8 plate 48, it is reflected by the mirror 50 and directed toward the second corner cube prism 46. is emitted.

As shown in FIG. 4, the first corner cube prism 44
The measurement light reflected by the λ/4 plate 40 is converted into linearly polarized light tilted by 45° with respect to the polarization plane of the outgoing path.

Further, the reference object reflected by the second corner cube prism 46 is transmitted through the λ/8 plate 48 again, thereby being converted into circularly polarized light. The axis of the λ/4 plate 40 is inclined by 22.5 degrees with respect to the polarization plane of the outgoing measurement light 1e, and the axis of the λ/8 plate 48 is inclined with respect to the polarization plane of the outgoing reference light IC. This is because they are arranged at an angle of 45°. The measurement light and the reference light, which have been made into linearly polarized light and circularly polarized light as described above, are combined by the non-polarized beam splinter 38 to become interference light, but the planes of polarization of this interference light are orthogonal as shown in the figure. When the linearly polarized lights 1a and 1b are separated, they become interference lights whose phases are shifted by 906 points from each other. Such interference light is transmitted within the transmission polarization optical fiber 20 in the two types of HE, `` mode and HE, `` mode.

As shown in FIG. 5, the transmission light emitted from the polarization-constant optical fiber 20 is made into parallel light by a condenser lens 18, and then separated by a polarization beam splitter 16. That is,
The linearly polarized light 1b included in the reflected light transmitted by the constant polarization optical fiber 20 is reflected by the polarization beam splinter 16, and is therefore converted into an electrical signal by the first optical sensor 24.

Furthermore, since the linearly polarized light 1a included in the reflected light transmitted by the constant polarization optical fiber 20 passes through the polarizing beam splitter 16, it is converted into an electrical signal by the second optical sensor 28 via the Faraday rotator 14 and the polarizing beam splitter 12. be done. As mentioned above, since the phases of the linearly polarized light 1a and the linearly polarized light 1b are 90' out of phase, the above electric signals are treated as a sine signal and a cosine signal, respectively, and a measurement method having a function of detecting the phase difference between these signals is used. The pulse counter in the circuit 60 counts, for example, in 90° increments. Therefore, for example, if the object to be measured 42 rotates within a plane that includes the optical axis of the incident light or reflected light of the first corner cube prism 44 and the second corner cube prism 46, the object to be measured 42 is rotated by the above count value. The rotation angle of the measurement object 42 is detected with high accuracy.

As shown in FIG. 6, when the object to be measured 42 having two corner cube prisms 44 and 46 is placed on the surface plate 62, the surface plate is 62 straightness can be detected with high accuracy.

Further, as shown in FIG. 7, if one of the second corner cube prisms 46 is fixed to the optical sensor head section 102 and the other first corner cube prism 44 is fixed on the moving table 64, the measurement circuit Based on the count value at 60, the optical sensor head section 102 of the moving table 64
The amount of displacement can be detected with high accuracy.

Similarly to the above, if the other first corner cube prism 44 is configured to be slidable and connected to a movable body of a contact type length measuring device such as a caliper or a micrometer, the measurement circuit 60 The length can be measured with high accuracy using the count value based on the detected phase difference. An example is shown in FIG.

As described above, according to the above embodiment, the linearly polarized light for generating the reference light and the measurement light is transmitted through the polarization-controlled optical fiber 20 in one transmission mode on the outgoing route, and the reference light and the measuring light are transmitted on the returning route. Since the measurement light is transmitted in two transmission modes in which the planes of polarization are orthogonal to each other, the effects (changes in transmission parameters) from the constant polarization optical fiber 20 on external stimuli such as temperature fluctuations, distortion, and vibrations, such as phase fluctuations, are canceled out. It will be done. Therefore, the first corner cube prism 44 and the second corner cube prism 46
Only the relative displacement of the object 42 is expressed as a phase difference, and the rotation angle of the object to be measured 42, the straightness of the surface plate 62, the moving distance of the moving table 64, etc. can be measured with high accuracy. In the above-mentioned embodiment, a He-Ne laser is used as the laser light source 10, and a pulse counter is used to
By counting in increments, measurements with a resolution of 0.1 μm are obtained.

Furthermore, in this embodiment, one polarization-controlled optical fiber 20 is connected between the optical transmitting/receiving section 100 and the optical sensor head section 102.
Since the space between them only needs to be connected by a It is.

Although one embodiment of the present invention has been described above, the present invention is also applicable to other aspects.

For example, the optical sensor head section 10 in the above-mentioned embodiment
2 is of a type in which measurement light and reference light are emitted toward a first corner cube prism 44 and a second corner cube prism 46 fixed to the object to be measured 42 or a housing (not shown) of the optical sensor head section 102. However,
It may be a constant polarization optical fiber for use in a sensor with a reflective film provided on one end surface. In this case, during the process in which the measurement light and reference light similar to the above-mentioned embodiment are reciprocated within the polarization-controlled optical fiber for the sensor, sound pressure, vibration, etc. are applied to the polarization-controlled optical fiber for the sensor. Transmission parameters such as phase are changed in response to an external stimulus, and this change is detected in the optical transceiver unit lOO.

Further, in the above embodiment, the λ/4 plate 40 and the λ/8 plate 48 constituted the phase polarization plane changing means, but the means may be constituted by other parts that change the phase of light.

The above-mentioned embodiment is merely one embodiment of the present invention, and various modifications may be made to the present invention without departing from the spirit thereof.

[Brief explanation of the drawing]

FIG. 1 is a diagram illustrating the configuration of an embodiment of the present invention. 2, 3, 4, and 5 are diagrams illustrating the operation of the embodiment of FIG. 1. Figures 6, 7, and 8 are diagrams explaining other measurement objects of the present invention, in which Figure 6 is for measuring straightness, Figure 7 is for measuring displacement, and Figure 8 is for measuring length. It shows the measurement of FIG. 9 and FIG. 10 are diagrams showing the configuration of a conventional optical fiber sensor device, respectively. 20: Constant polarization optical fiber 100: Optical transmitter/receiver section 102: Optical sensor head section

Claims (1)

[Claims] (1) A single polarization-controlled optical fiber for transmission that transmits light in two transmission modes whose polarization planes are orthogonal to each other, and outputting light for generating reference light and measurement light to the polarization-controlled optical fiber; The light is transmitted in one of the two transmission modes within the polarization constant optical fiber, and the reference light and measurement light returned in the two transmission modes through the polarization constant optical fiber are received. an optical transmitter/receiver that detects a change in a transmission parameter of the measurement light based on the reference light and measurement light; an optical sensor head unit that returns the measurement light to the constant polarization optical fiber and changes at least a transmission parameter of the measurement light in relation to external conditions; a phase polarization plane changing means for transmitting the reference light and the measurement light in the two transmission modes in the polarization constant optical fiber by making the reference light and the measurement light linearly polarized light with a 90° shift and orthogonal polarization planes; An optical fiber sensor device comprising: