JP2571870B2 - 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, a carrier signal having a modulation frequency or a frequency that is an integral multiple of the modulation frequency is generated, and a fundamental wave component or any harmonic component can be obtained by synchronously detecting the output of the light receiving element. 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 length 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 ( _{ｏ o} ) = 0. If the amplitude of the light fluctuates, the fundamental wave may be divided by the fourth harmonic to obtain the phase difference in the form 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, since the polarization maintaining optical fiber is more expensive than a simple single mode optical fiber, it becomes 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 light 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 with the main axis is ψ, the amount of light passing through the polarizer decreases in proportion to cosψ. 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 S
uperluminescent Diode ", ELECTRONICS LETTERS, vol.17,
No. 10, p352 (1981), such an optical fiber gyro is proposed. 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 makes it incident on a small fiber core. However, the light is linearly polarized due to the presence of the polarizer 23. That is, only the mode of one polarization plane is passed. This fiber 25 is a coupler
26 couples with another fiber 27. Here, the light is separated into left-handed light and right-handed light. The clockwise light once exits the space from the fiber 25, passes through the lens 28, the depolarizer 29, and the lens 30, enters the fiber 3 again, and propagates clockwise through the fiber coil 4. 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 an “izer” and 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 each 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 36 is connected to the fiber 32 connected to the 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 bulk 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 when they are converted to optical fibers. It is also undesirable to use a beam splitter consisting of prisms,
This 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 bulk optical components, with an optical fiber. Although the light emitted from the light emitting element 1 is linearly polarized light, there is a possibility that the light is rotated in the plane of polarization in the optical fiber up to 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.
When a bulk optical crystal polarizer is used as shown in FIGS. 2 and 3, the polarizer can be rotated and adjusted 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, a substantial depolarizer for making the light emitted from the light emitting element unpolarized is added. This depolarizer utilizes the fact that the light emitted from the light emitting element is linearly polarized light, and arranges a birefringent crystal so that the optical principal axis is inclined by about 45 ° with respect to the linearly polarized light. The birefringent crystal is provided between the light emitting element and the single mode optical fiber on which the light is incident. The light emitting element and the birefringent crystal are combined to substantially form a depolarizer. However, the following conditions are imposed on the length of the birefringent crystal. That is, the time delay between the orthogonal polarizations due to the birefringence of the birefringent crystal must be longer than the coherence time of the light emitting element. Only in this way does the interference between the orthogonal polarizations disappear and the polarization becomes non-polarized. A birefringent crystal has a difference in refractive index between lights having orthogonal polarizations.

[Operation]

In the device of the present invention, a new birefringent crystal is placed immediately after the light emitting element so that the optical principal axis is at 45 ° to the direct polarization of the light emitting element, so that the light passing through the crystal substantially acts as a depolarizer. From linearly polarized light to unpolarized 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, a birefringent crystal 2, a depolarizer 3, a fiber coil 4, a phase modulator 5, a light receiving element 6, fiber couplers 7, 8, a fiber polarizer 9, a condensing optical system 10, and the like. including. These components are interconnected by an optical fiber. The light emitting element 1 is a light source that emits monochromatic light. Laser diodes and super luminescent diodes are used. However, the coherent length must be short. The depolarizer 3 is an element for converting linearly polarized light to non-polarized light. The insertion of the depolarizer 3 also appears in FIGS. 2 and 3 and is well known. However, in the present invention, another substantially depolarizer made of a birefringent crystal whose main optical axis is inclined by 45 ° with respect to the polarization of the light emitting element is added. This is new. The depolarizer 3 is also made of an optical fiber. In this case, as shown in FIG.
These are connected in the axial direction so as to form ゜. And 2
The optical path difference of light having two polarization planes is equal to or longer than the coherent length of the light emitting element. The length of the two optical fibers is 2:
Is one. In the present invention, the birefringent crystal 2 is inserted immediately after the light emitting element to form a substantial depolarizer. The length of this l, the refractive index of the optical n _x, n _y, if the coherence length of the light emitting element is is h, should be _{(n x -n y) l>} h (4). Otherwise, it will not be unpolarized. Since the polarization of the birefringent crystal and the light-emitting element forms 45 °, the amplitude of the light in the x direction is cos 45 °, and the amplitude of the light in the y direction is sin 45 °, and the intensity is the mean square of this. Each time. Thereafter, the amplitude of light having an arbitrary polarization plane becomes equal. 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. Evanescent coupling occurs because the distance between the cores is small. 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.
A condensing optical system 10 is provided between the tip of the single mode optical fiber and the light emitting element.
It is sandwiched between pieces. The second fiber optical path 12 is connected to the first fiber coupler 7 and the second fiber
It communicates with the fiber coupler 8. The third fiber optical path 13 and the fourth fiber optical path are both 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 receiving element 6. The sixth fiber optical path 16 is a remainder of the fiber connected to the second fiber coupler 8 and has a free end. The seventh fiber optical path 17 is a remainder of the fiber connected to the first fiber coupler 7 and has a free end. If the branching element is a fiber coupler, a surplus portion ending at the free end is inevitably generated. A fiber polarizer 9 is provided in the middle of the second fiber optical path 12 between the first and second fiber couplers 7 and 8.
The phase modulator 5 and the 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 light emitted from the light emitting element 1 is linearly polarized light, but becomes non-polarized light by passing through the birefringent crystal 2 whose optical main axis forms 45 ° with respect to the polarized light. This reaches the fiber type polarizer 9 and becomes linearly polarized light in a certain direction. Since the light is once 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 substantial depolarizer made of a birefringent crystal is inserted between the light-emitting element and the fiber-type 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 ... Birefringent crystal 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 ... Condensing 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

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; a fiber polarizer provided in the middle of the second fiber optical path; A phase modulator that modulates the phase of light propagating in the optical fiber, and a first optical path between the light emitting element and the first fiber coupler, the optical axis of which is approximately 45 with respect to the linear polarization of the light emitting element.設 け Provided in the middle of the third or fourth fiber optical path connecting the inclined birefringent crystal and both ends of the fiber coil and the second fiber coupler. And a depolarizer, the light emitted from the light emitting element is depolarized by a birefringent crystal and then passed through a fiber type polarizer to become linearly polarized light. After being polarized, the fiber coil is propagated in a certain rotation direction and the other light is propagated in the opposite direction to the fiber coil to be depolarized by a depolarizer. An optical fiber gyro characterized in that the polarization planes are aligned and made incident on a light receiving element to interfere with light of the same polarization to detect the intensity of the interference light. 2. The light according to claim 1, wherein the time delay between the orthogonal polarizations caused by the birefringence of the birefringent crystal is longer than the coherence time of the light emitting element. Fiber gyro.