JP2751599B2 - Hikaribaiyairo - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial applications]

The present invention relates to an optical fiber gyro for measuring a rotational angular velocity of a moving body such as an automobile, an airplane, and a ship. In particular, the present invention relates to a phase modulation type optical fiber gyro in which the entire optical path is constituted by optical fibers.

[Prior art]

The optical fiber gyro obtains the angular velocity by utilizing that the phase difference of light propagating left and right in the fiber coil is proportional to the angular velocity of the coil. The phase modulation method modulates the phase of light propagating in a part of an optical fiber near one end of a fiber coil by expanding and contracting the optical fiber. The intensity of the interference light is detected by the light receiving element, and the modulation frequency and its higher harmonic signal are included in the form of an expansion formula using the Bessel function as a coefficient.
Therefore, if a carrier signal having a modulation wave frequency or an integer multiple thereof is created and the output of the light receiving element is synchronously detected by this, a fundamental wave component or an arbitrary harmonic component can be obtained. Odd-order harmonics (including the fundamental) can be written as 2E ₁ E ₂ J _{2m + 1} (ξ) sinΔθ (1). Where E ₁ and E ₂ are the left-handed and right-handed light amplitudes, J _{2m + 1} (ξ) is the (2m + 1) -order Bessel function, Δθ
Is the phase difference between left-handed light and right-handed light. ξ represents the magnitude of the modulation, It is. b is the phase modulation amplitude, Ω is the phase modulation angular frequency,
L is the fiber tone of the fiber coil, n is the fiber refractive index, and c is the speed of light in vacuum. Even order harmonics can be written as 2E ₁ E ₂ J _2n (ξ) cosΔθ (3). If the amplitude of light and the magnitude of modulation ξ are stable, the phase difference Δθ can be obtained from only the fundamental wave. In order to keep the modulation magnitude 一定 constant, the phase modulator driving circuit may be controlled so that the appropriate even-order harmonic becomes zero. Then, ξ is fixed to the zero of the 2n-order Bessel function such that J _2n (ξ ₀ ) = 0. If the amplitude of light fluctuates, the fundamental wave may be divided by the fourth harmonic to find the phase difference of tan Δθ. Regarding the optical fiber gyro of the phase modulation method, Japanese Patent Application Nos. 1-57634-37, 1-2291628-31 and 1-295550.
0, inventions such as Japanese Patent Application Nos. 2-3809 and 2-10055 have been made. Since the optical fiber gyro causes left-handed light and right-handed light to interfere with each other, the polarization planes must be the same.
If the polarization planes are different, the interference light has a value proportional to the cosine of the included angle of the polarization plane. If the polarization planes are orthogonal, the light does not interfere. Therefore, the planes of polarization of left-handed light and right-handed light must be aligned. In the case of a single-mode optical fiber, the light on the two degenerated polarization planes propagates with the same phase constant, so that the polarization planes rotate. Therefore, it is conceivable to construct a large part of the optical path by using a polarization-maintaining optical fiber instead of a single-mode optical fiber, and to make the light linearly polarized through a polarizer before the light is split into two. The plane of polarization is preserved for two orthogonal axes. Since the rotation of the polarization plane does not occur, the polarization planes of the left-handed light and the right-handed light can be aligned to interfere with each other. However, the polarization-maintaining optical fiber is more expensive than a simple single-mode optical fiber, and is therefore an extremely expensive optical fiber gyro. Again, most of the fiber coils and optical paths are to be made with simple single mode optical fibers. However, single mode optical fibers have several problems. The single mode means that only one mode is established for the phase constant, and there are actually two modes whose polarization planes are orthogonal to each other. Modes with different polarization planes are ideally independent, but rotation of the polarization plane can occur because the phase constants are macroscopically the same. Modes having different polarization planes have different microscopic phase constant fluctuations, so that even if they propagate for the same distance, they do not have the same effective optical path length. Therefore, if propagation of two modes having different polarization planes is allowed, left-handed and right-handed lights having different optical path lengths interfere with each other, and the interference light includes an offset. Here, the offset refers to a deviation of Δθ from 0 when the phase difference Δθ is not 0 although the angular velocity Ω _{c of} the coil is 0. This is natural if the effective optical path lengths are different. Left-handed right-handed light must have exactly the same experience. For this purpose, it is effective to fix the plane of polarization in one direction by passing through a polarizer before splitting into left-handed light and right-handed light. In this case, since only one mode of the polarization plane passes through the single mode fiber, the optical path length becomes the same. Up to this point, the operation is the same as that of the polarization-maintaining optical fiber described above. However, in a single-mode optical fiber, the rotation of the plane of polarization can occur, so this is not sufficient. After being linearly polarized through the polarizer, it propagates through the fiber coil,
Again it passes through the polarizer in the opposite direction. At this time, the plane of polarization does not always coincide with the main axis of the polarizer. Assuming that the angle between the main axis and φ is φ, the amount of light passing through the polarizer decreases in proportion to cos φ. This angle is not always the same for left-handed and right-handed light, and varies with temperature. Therefore, when using a single mode optical fiber,
In addition to the polarizer, a depolarizer is required. This converts arbitrary linearly polarized light into non-polarized light. For example, K. Bhm et al .: “Low-Drift Fiber Gyro Using a Sup
erluminescent Diode ", ELECTRONICS LETTERS, vol.17, N
o.10, p352 (1981), has proposed such an optical fiber gyro. FIG. 2 shows the structure. Light emitted from the light emitting element 1 is incident on one end of an optical fiber 25 via a lens 21, a beam splitter 22, a polarizer 23, and a lens 24. This condenses light and enters a small fiber core, but is linearly polarized because of the presence of the polarizer 23. That is, only one mode of polarization plane is passed. This fiber 25 is connected to another fiber 27 by a coupler 26. Here, the light is separated into left-handed light and right-handed light. The right-handed light exits the space once from the fiber 25 and enters the lens 28, the depolarizer 29, and the lens 30.
, And again enters the fiber 3 and propagates the fiber coil 4 clockwise. Thereafter, the light passes through the phase modulator 5. The counterclockwise light propagates from the fiber 27 through the phase modulator 5 to the fiber coil 4 counterclockwise. After this depolarizer 29
Pass through. The depolarizer 29 converts the linearly polarized light into non-polarized light, and has a function opposite to that of the polarizer. This is Lyot depolar
It is called zer, which is made by bonding two birefringent crystals so that the optical principal axis is twisted by 45 °. Its thickness is 1:
Has become 2. In addition, the thickness of the difficult crystal is such that the optical path difference between extraordinary light and ordinary light is longer than the coherent length of light. In order to use a thin depolarizer, a light emitting element having a short coherence length is required. FIG. 2 uses a single mode optical fiber, and solves the problem of output fluctuation due to polarization plane rotation by using a polarizer and a depolarizer. Bhm et al. Also proposed an optical fiber gyro shown in FIG. The light emitted from the light emitting element 1 is squeezed through the cylindrical lens 33, the lenses 34 and 35, and
It is incident on one end of 36. The fiber 26 is connected to a fiber 32 connected to a light receiving element 36 by a coupler 37.
The light emitted from the fiber 36 enters another fiber 41 through a lens 38, a polarizer 39, and a lens 40. This light is split by the coupler 42 into left-handed light and right-handed light. The clockwise light exits from the fiber end 43 into free space, passes through the lens 45, the depolarizer 46, and the lens 47, and enters the core of the optical fiber 3. Then, the light passes through the fiber coil 4 clockwise, passes through the phase modulator 5, and returns from the coupler 42 to the polarizer 39. The counterclockwise light passes through the phase modulator 5 from the coupler 42, turns the fiber coil 4 counterclockwise, and passes through the depolarizer 46 in the opposite direction. Both the device shown in FIG. 2 and the device shown in FIG. 3 fix the polarization plane to one with a polarizer, and then split the light into two lights and make them unpolarized through a depolarizer. The left-handed light and the right-handed light pass through the phase modulator 5 and undergo phase modulation at different times. The output of the light receiving element is amplified by a lock-in amplifier (not shown) in synchronization with the modulation signal. The processing as the phase modulation method is the same as the conventional one. As it passes through the depolarizer, it becomes unpolarized, which passes through the polarizer again. Each light component passes through only the cosine of the angle formed by the main axis of the polarizer. Therefore, even if the rotation of the polarization plane occurs, it is possible to avoid the problem of the fluctuation and decrease of the light amount due to passing through the polarizer.

[Problems to be solved by the invention]

The apparatus shown in FIGS. 2 and 3 is an apparatus assembled in a laboratory and is not a practical machine. It is strongly desired that a polarizer or a depolarizer be made into a fiber in order to make it practical and lightweight. The polarizer and depolarizer use parc optics in this experiment and are much larger than the fiber. To pass light as a plane wave through these components, lenses must be placed in front and behind. This results in a bulky device. Polarizers and depolarizers can be made of optical fibers. This is well known. These components become practical only after they are converted to optical fibers. Also, it is equally undesirable to use a beam splitter consisting of a prism, which should be replaced by a fiber optic coupler. Then, the structure as shown in FIG. 3 is obtained, but it is not so if it is sufficient to replace the polarizer and the depolarizer, which are the Parc optical components, with an optical fiber. The light emitted from the light emitting element 1 is linearly polarized light, which may rotate in the plane of polarization in the optical fiber reaching the polarizer. Although the relay fiber in this portion is short, it is difficult to match the polarization direction of the light emitted from the light emitting element 1 with the polarization direction of the fiber polarizer. If they do not match, the amount of light passing through will be small.
In the case of using a polarizer made of a Parc optical crystal as shown in FIGS. 2 and 3, the polarizer can be adjusted by rotating the polarizer so that the amount of light reaching the light receiving element is maximized. However, in the case of a fiber-type polarizer, the light is transmitted only after fusion bonding with the single mode optical fiber, so that the polarization direction of the polarizer cannot be adjusted. Even if the polarization direction can be matched by rotating the light emitting element, the plane of polarization may be rotated due to a temperature change or stress due to the single mode optical fiber. Then, the amount of light passing through the polarizer also decreases, and the scale factor varies. The problem of polarization plane rotation in the optical fiber nearer to the fiber coil than the polarizer could be solved by inserting a depolarizer near the fiber coil. However, the problem of polarization plane rotation in the optical fiber closer to the light emitting element than the polarizer has not been solved yet.

[Means for Solving the Problems]

In the optical fiber gyro of the present invention, a main part of an optical path including a polarizer and a depolarizer is formed into an optical fiber.
Then, another depolarizer is added to the single mode optical fiber from the light emitting element to the polarizer. The depolarizer may be provided between the light emitting element and the first coupler, or may be provided between the first coupler and the fiber polarizer. As the depolarizer, a two-polarization preserving optical fiber in which the main optical axis is twisted by about 45 ° and connected in the axial direction is used. The length ratio shall be 1: 1 or more. The length of the shorter polarization maintaining fiber is set so that the optical path difference due to birefringence is longer than the coherent length of the light emitting element. Such depolarizers are known.

[Operation]

In the apparatus of the present invention, a depolarizer is newly inserted in the middle of the single mode optical fiber connecting the light emitting element and the fiber type polarizer, so that the light passing therethrough changes from linearly polarized light to non-polarized light. Since the light becomes non-polarized light and then enters the fiber-type polarizer and is changed to linearly-polarized light, the amount of light appearing at the output of the fiber-type polarizer is constant. Even if the rotation of the polarization plane occurs, it is unpolarized because it occurs for light having all the polarization planes. Since the amount of light appearing at the output of the fiber-type polarizer is constant, it is not necessary to adjust the axes of the fiber, the light-emitting element, and the fiber-type polarizer. Also, the output light quantity does not fluctuate due to a stress applied to the fiber or a change in temperature. Since the amount of light passing through the fiber polarizer is constant, the scale factor does not change. Highly accurate angular velocity measurement can be performed.

【Example】

FIG. 1 shows an embodiment of the present invention. In this, the optical path is entirely constituted by optical fibers. However, although a single mode optical fiber is mainly used, a polarization maintaining optical fiber is partially used. The optical fiber gyro includes a light emitting element 1, depolarizers 2, 3, a fiber coil 4, a phase modulator 5, a light receiving element 6, fiber couplers 7, 8, a fiber polarizer 9, and the like. Interconnected. The light emitting element 1 is a light source that emits monochromatic light. Laser diodes and pearlescent diodes are used.
However, the coherent length must be short. The depolarizers 2 and 3 are elements for converting linearly polarized light into non-polarized light. Introducing the second depolarizer is shown in FIG.
It also appears in the figure and is well known. However, in the present invention, a first depolarizer is added. This is new. The depolarizer is also made of optical fiber. As shown in FIG. 4, two polarization-maintaining optical fibers are connected in the axial direction such that the main optical axis forms 45 °. Then, the optical path difference of the light having two polarization planes is set to be equal to or longer than the coherent length of the light emitting element. The length of two optical fibers is 2: 1
Therefore, the shorter optical fiber must satisfy the above condition. Polarization plane x-direction, the refractive index of the optical n _x a y-direction, n _y, the length of the fiber l, when the coherence length of the light emitting element and _{h, (n x-ny)} l> h (4) That's what it means. Although this is known, a fiberized depolarizer is not widely used and will be described here. It is assumed that light having a polarization plane that forms an angle 入射 with respect to the principal axis of the polarization plane of the first polarization-maintaining optical fiber is incident. This is divided into light whose main axis of polarization is in the x direction and light in the y direction. The phase velocities of these lights are different, and the substantial refractive indexes are different. The lights on the respective polarization planes passing through the optical fiber are separated by a distance that can no longer interfere with each other. This is the condition (4). The component in the x direction is represented by cosψ, and the component in the y direction is represented by sinψ. This light enters the second polarization-maintaining optical fiber.
It is assumed that this is twisted by Θ with respect to the first polarization maintaining fiber. X of the second polarization-maintaining optical fiber
The components come from cos {cos} and -sin {sin}. They no longer interfere because the optical path length is greater than the coherent length. Therefore, the intensity P _X of light having a polarization plane in the x direction can be written as P _x = (cosscosΘ) ² + (sinψsinΘ) ² (5). Since (4) has occurred, the cross term that should appear on the right side of equation (5) disappears. Similarly, the y component comes from cos {sin} and sin {cos}. Since they do not interfere with each other, the intensity P _y of the light having the polarization plane in the y direction is P _y = (cosψsinΘ) ² + (sinψcosΘ) ² (6). Non-polarized light means that light having polarization planes in all directions of the xy plane exists equally. This is P _x
= Is that P _y. Regardless of the direction ψ of the plane of polarization of the incident light, P _x = P _y is _satisfied by Θ = π / 4 (45
Only at the time of ゜). For this reason, the depolarizer connects two birefringent objects such that the principal axes of polarization differ by 45 °. A polarization-maintaining optical fiber can also be used because it has birefringence. The reason why the thickness is set to 1: 2 is that the optical path difference caused by the first polarization-maintaining optical fiber is longer than the coherent length, and then approaches again by passing through the second polarization-maintaining optical fiber. It is to prevent. If the length is 1: 1, the optical path difference between the aforementioned cos {sin} and sin {cos} becomes zero, causing interference. The fiber coil 4 is formed by winding a single mode optical fiber many times. The phase modulator 5 is formed by winding an optical fiber near one end of a fiber coil around a cylindrical piezoelectric vibrator. When an AC excitation voltage is applied to the piezoelectric vibrator, it expands and contracts in the radial direction, so that the optical fiber expands and contracts, and the phase of light propagating in the optical fiber changes. The light receiving element 6 is a pin photodiode or the like, and makes the light propagating counterclockwise and counterclockwise through the fiber coil interfere to detect the intensity of the interference light. The fiber couplers 7 and 8 are obtained by peeling the coatings of the two optical fibers, approaching each other, fusing them, and stretching them. Since the distance between the cores is small, evanescent coupling occurs. It is made by adjusting so that when light enters from one end, half the optical power comes out to the other two ends. It is a small branching element that is not bulky like a beam splitter. The fiber type polarizer 9 is obtained by winding a polarization maintaining optical fiber around a cylinder. One of the two modes having orthogonal polarization planes is a radiation mode and is attenuated, so that only the light of the remaining one mode can pass therethrough. Therefore, it works equivalent to a polarizer. Next, an optical path formed by a fiber will be described. The first fiber optical path 11 communicates between the light emitting element 1 and the first fiber coupler 7. It is a single mode optical fiber.
There is a focusing optical system 10 between the light emitting element 1 and the fiber end.
The second fiber optical path 12 communicates between the second fiber coupler 7 and the second fiber coupler 8. Third fiber optical path 13
The fourth fiber optical path is connected to one end of the second fiber coupler 8 at both ends of the fiber coil 4. The fifth fiber optical path 15 connects the first fiber coupler 7 and the light emitting element 6. The sixth fiber optical path 16 is a remainder of the fiber connected to the fiber coupler 8 and has a free end. The seventh fiber optical path 17 is connected to the first fiber coupler 7.
And the free end of the fiber leading to If the branching element is a fiber coupler, a surplus portion ending at the free end is inevitably generated. A fiber polarizer is provided in the second fiber optical path 12 between the first and second fiber couplers 7 and 8. The phase modulator 5 and the second depolarizer 3 are provided in any of the third and fourth fiber optical paths 13 and 14. In this example, both are allocated to different optical paths, but they may be in the same optical path. The first depolarizer 2 is on the first fiber optical path 11. Although this is the case in this example, a first depolarizer 2 may be provided in the second fiber optical path 12 between the first fiber coupler 7 and the fiber polarizer 9 (as shown by the dashed line). It suffices if it is before the fiber polarizer 9. The light emitted from the light emitting element 1 is linearly polarized light, but becomes unpolarized by the first depolarizer 2. This reaches the fiber type polarizer 9 and becomes linearly polarized light in a certain direction. Once the light becomes non-polarized, the amplitude of the light when it becomes linearly polarized by the polarizer 9 is constant regardless of the orientation of the light emitting element. This is an important point. There is no need to align the axes of the light emitting element and the fiber polarizer. Also, even if the plane of polarization rotates in the optical fiber due to external force or temperature, there is no effect because it is non-polarized.

【The invention's effect】

A depolarizer is provided between the light-emitting element and the fiber polarizer, whereby the linearly polarized light is once depolarized and then passed through the fiber polarizer, so that the amount of light passing through the polarizer is constant. An optical fiber gyro having a stable scale factor in which all optical paths including a polarizer, a depolarizer, and the like are configured by optical fibers can be provided.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an optical fiber gyro according to an embodiment of the present invention. FIG. 2 is a configuration diagram of an optical fiber gyro according to a conventional example. FIG. 3 is a configuration diagram of an optical fiber gyro according to another conventional example. FIG. 4 is a schematic diagram of a depolarizer using a polarization-maintaining optical fiber. DESCRIPTION OF SYMBOLS 1 ... Light emitting element 2 ... 1st depolarizer 3 ... 2nd depolarizer 4 ... Fiber coil 5 ... Phase modulator 6 ... Light receiving element 7 ... 1st fiber coupler 8 ... 2nd fiber coupler 9 ... Fiber type polarizer 10 Focusing optical system 11 First fiber optical path 12 Second fiber optical path 13 Third fiber optical path 14 Fourth fiber optical path 15 Fifth fiber optical path

Continuation of the front page (56) References JP-A-63-106519 (JP, A) JP-A-63-95312 (JP, A) JP-A-4-106417 (JP, A) JP-A-4-106420 (JP) JP-A-3-233315 (JP, A) JP-A-3-152416 (JP, A) JP-A-58-500458 (JP, A) U.S. Pat. No. 4,529,313 (US, A) Polarization and depolarization in the fiber -Optic gyroscope, R.C. Ulrich, (Fiber-Optical Rotation Sensors and Related Technologies, 52-77, 1982), Selected Papers on Fiber Optics Gyroscopes P. 239-264.

Claims (2)

(57) [Claims] An optical fiber gyro based on the principle that light is propagated left and right in a fiber coil to determine the rotational angular velocity of the fiber coil from the phase difference between the two lights, which emits monochromatic light. An element, a fiber coil in which a single mode optical fiber is wound many times, a light receiving element for detecting the intensity of the interference light by interfering light propagating left and right in the fiber coil, and a first light emitting element connected to the light emitting element.
A first fiber coupler coupling the fiber optical path and a fifth fiber optical path connected to the light receiving element to a second fiber optical path and a seventh fiber optical path having a free end; a third fiber optical path connected to both ends of a fiber coil; A second fiber coupler that couples the fiber optical path to the second fiber optical path and a sixth fiber optical path having a free end; and a polarization-maintaining optical fiber provided in the middle of the second fiber optical path, which is formed by winding a polarization-maintaining optical fiber into a cylindrical shape. A fiber polarizer that attenuates one of the two modes of the wavefront and passes the other mode to obtain linearly polarized light, and the phase of light propagating in the optical fiber provided near one end of the fiber coil A phase modulator that modulates light, and an optical path difference of light having an orthogonal polarization plane provided in the middle of a first fiber optical path connecting the light emitting element and the first fiber coupler. A first depolarizer for connecting the polarization preserving fiber 2 having a length equal to or longer than the optical fiber length in the axial direction so that the optical principal axis forms 45 degrees and randomizing the polarization plane of the light, and the both ends of the fiber coil and the second fiber coupler. The polarization preserving fiber 2 which is provided in the middle of the third fiber optical path or the fourth fiber optical path and has an orthogonal polarization plane and whose optical path difference is equal to or longer than the coherent length of the light source is axially oriented so that the optical principal axis is at 45 degrees. And a second depolarizer for connecting and randomizing the polarization plane of the light, wherein the optical path is entirely constituted by an optical fiber, and the light emitted from the light emitting element is depolarized by the first depolarizer, and then a fiber polarizer is provided. An optical fiber gyro, characterized in that it passes through the optical fiber gyro. 2. An optical fiber gyro which is based on the principle that light is propagated left and right in a fiber coil and the rotational angular velocity of the fiber coil is determined from the phase difference between the two lights. An element, a fiber coil in which a single mode optical fiber is wound many times, a light receiving element for detecting the intensity of the interference light by interfering light propagating left and right in the fiber coil, and a first light emitting element connected to the light emitting element.
A first fiber coupler coupling the fiber optical path and a fifth fiber optical path connected to the light receiving element to a second fiber optical path and a seventh fiber optical path having a free end; a third fiber optical path connected to both ends of a fiber coil; A second fiber coupler that couples the fiber optical path to the second fiber optical path and a sixth fiber optical path having a free end; and a polarization-maintaining optical fiber provided in the middle of the second fiber optical path, which is formed by winding a polarization-maintaining optical fiber into a cylindrical shape. A fiber polarizer that attenuates one of the two modes of the wavefront and passes the other mode to obtain linearly polarized light, and the phase of light propagating in the optical fiber provided near one end of the fiber coil And a phase modulator that modulates the light, and an optical path difference between the light having the orthogonal polarization plane provided between the first fiber coupler and the fiber polarizer in the second fiber optical path. A first depolarizer for connecting two polarization-maintaining fibers having a coherence length or more in the axial direction so that the optical principal axis forms 45 degrees and randomizing the polarization plane of light, and both ends of a fiber coil and a second fiber coupler; The optical principal axis is connected to two polarization plane preserving fibers that are provided in the middle of the third fiber optical path or the fourth fiber optical path and that have orthogonal polarization planes and whose optical path difference is equal to or longer than the coherent length of the light source.
A second depolarizer that is connected in the axial direction so as to form an angle of 45 degrees and randomizes the plane of polarization of light, wherein the entire optical path is formed of an optical fiber;
An optical fiber gyro characterized in that the light is depolarized by a depolarizer and then passed through a fiber polarizer.