Fiber Optic Gyroscope

Abstract

To provide an optical fiber gyroscope that can stabilize an amount of light passing through a single mode optical fiber coil.SOLUTION: On each end of both ends of a single mode optical fiber 16 and respective ends of either end of a single mode optical fiber coil 15, total six pieces of polarization holding optical fibers 21, 22, 23, 24, 25 and 26 are arranged that have the same beat length. An angle is 45 degrees that is formed with a polarization principal axis of the polarization holding optical fiber 21, and a polarization plane of a linear polarization from a light source 11. Respective optical lengths of the total six pieces of polarization holding optical fibers 21, 22, 23, 24, 25 and 26 are greater than a coherent length of the linear polarization from the light source 11. A sum of optical lengths of the six pieces of polarization holding optical fibers 21, 22, 23, 24, 25 and 26 in consideration for polarization rotation in a process of passing through single mode optical fibers 15 and 16 is greater than the coherent length of the linear polarization from the light source 11.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to fiber optic gyroscopes, and more particularly to depolarizing (i.e., artificially creating a depolarized state) of light used in fiber optic gyroscopes. [Background technology]

[0002] As a prior art for depolarizing light used in an optical fiber gyroscope, for example, Patent Document 1 is known. The optical system of an optical fiber gyroscope 900 disclosed in Patent Document 1 has the following basic components: a) a light source 11; b) an optical element 13 (for example, an optical crystal of lithium niobate (LiNbO3)) in which a Y-shaped optical waveguide 13a is formed; c) a single-mode optical fiber coil 15; d) a single-mode optical fiber 16; and e) a second polarization-maintaining optical fiber 22; and f) a third polarization-maintaining optical fiber 23; and g) a fourth polarization-maintaining optical fiber 24; h) a fifth polarization-maintaining optical fiber 25; i) Sixth polarization-maintaining optical fiber 26 For ease of explanation, the first ordinal number is designated as "second."

[0003] These connections are as follows: 1) One end of the single-mode optical fiber 16 is connected to the light source 11; 2) One end of the second polarization-maintaining optical fiber 22 is connected to the other end of the single-mode optical fiber 16 (in the drawing, for ease of distinction, the line width of the polarization-maintaining optical fiber is shown thicker than the line width of the single-mode optical fiber), 3) The first end of the Y-shaped optical waveguide 13a is connected to the other end of the second polarization-maintaining optical fiber 22; 4) One end of the third polarization-maintaining optical fiber 23 is connected to the second end of the Y-shaped optical waveguide 13a, 5) One end of the fourth polarization-maintaining optical fiber 24 is connected to the other end of the third polarization-maintaining optical fiber 23 (in the figure, the boundary between the third polarization-maintaining optical fiber 23 and the fourth polarization-maintaining optical fiber 24 is indicated by a dashed line), 6) One end of the single-mode optical fiber coil 15 is connected to the other end of the fourth polarization-maintaining optical fiber 24; 7) One end of the fifth polarization-maintaining optical fiber 25 is connected to the other end of the single-mode optical fiber coil 15; 8) One end of the sixth polarization-maintaining optical fiber 26 is connected to the other end of the fifth polarization-maintaining optical fiber 25 (in the figure, the boundary between the fifth polarization-maintaining optical fiber 25 and the sixth polarization-maintaining optical fiber 26 is indicated by a dashed line), 9) The third end of the Y-shaped optical waveguide 13a is connected to the other end of the sixth polarization-maintaining optical fiber .

[0004] Furthermore, with respect to the principal axis of the polarization-maintaining optical fiber, A) The angle between the polarization main axis of the third polarization-maintaining optical fiber 23 and the polarization main axis of the fourth polarization-maintaining optical fiber 24 is 45 degrees, B) The angle between the polarization main axis of the fifth polarization-maintaining optical fiber 25 and the polarization main axis of the sixth polarization-maintaining optical fiber 26 is 45 degrees, C) The polarization main axis of the second polarization-maintaining optical fiber 22 coincides with the electric field direction of the TE mode (Transverse Electric mode) of the Y-shaped optical waveguide 13a, D) The polarization main axis of the third polarization-maintaining optical fiber 23 coincides with the electric field direction of the TE mode of the Y-shaped optical waveguide 13a, E) The polarization main axis of the sixth polarization-maintaining optical fiber 26 coincides with the electric field direction of the TE mode of the Y-shaped optical waveguide 13a.

[0005] Furthermore, the second polarization-maintaining optical fiber 22, the third polarization-maintaining optical fiber 23, the fourth polarization-maintaining optical fiber 24, the fifth polarization-maintaining optical fiber 25, and the sixth polarization-maintaining optical fiber 26 each have the same beat length. b The wavelength of the light from the light source 11 is λ, and the coherence length of the light from the light source 11 is L c between two orthogonal linearly polarized lights, Lc Let L be the length of the polarization-maintaining optical fiber that gives a longer group delay than the first polarization-maintaining optical fiber. Regarding the length L2 of the second polarization-maintaining optical fiber 22, the length L3 of the third polarization-maintaining optical fiber 23, the length L4 of the fourth polarization-maintaining optical fiber 24, the length L5 of the fifth polarization-maintaining optical fiber 25, and the length L6 of the sixth polarization-maintaining optical fiber 26, L2≧L, L4≧L, L5≧L, |L2+L3-L4|≧L, |L2+L6-L5|≧L, |L2+L3-L4|-|L2+L6-L5|≧L, For these technical features, please refer to claim 1 and FIG. 4 of Patent Document 1.

[0006] The optical element 13 actually includes a phase modulator for phase-modulating each of the CW (clockwise) light propagating clockwise through the single-mode optical fiber coil 15 and the CCW (counter-clockwise) light propagating counterclockwise through the single-mode optical fiber coil 15. However, since the phase modulator is not directly related to the present invention and its configuration and function are well known, illustrations and descriptions thereof are omitted. Furthermore, the optical fiber gyroscope 900 actually includes an optical coupler attached to the single-mode optical fiber 16 for extracting interference light formed by optically coupling the CW light and the CCW light, an optoelectronic converter for optoelectronic conversion of the light from the optical coupler, and a signal processing circuit for performing signal processing such as angular velocity detection based on the electrical signal from the optoelectronic converter. However, since these components are not directly related to the present invention and their configurations and functions are well known, illustrations and descriptions thereof are omitted.

[0007] According to Patent Document 1, most of the optical system can be configured with inexpensive single-mode optical fibers, and unpolarized light can propagate through the single-mode optical fiber coil. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2004-309466 Summary of the Invention [Problem to be solved by the invention]

[0009] When a depolarizer is configured using a polarization-maintaining optical fiber, it is preferable to use a light source that has high optical coupling efficiency with the single-mode optical fiber 16, high power density, a broadband optical spectrum, and emits light with low temporal coherence in the optical fiber gyroscope 900. For example, a superluminescent diode (SLD) is used as such a light source 11. Light from an SLD has polarization properties. Regarding the light from the light source 11, if the polarization extinction ratio of the two orthogonal linearly polarized lights is at most approximately 0 to 3 dB, even if polarization rotation occurs during the process of the two orthogonal linearly polarized lights passing through the single-mode optical fiber 16, there is no significant difference in the light intensity of the two linearly polarized lights entering the optical element 13. Therefore, even if the optical element 13, which functions as a polarizer, passes one of the linearly polarized lights, there is no significant fluctuation in the amount of light.

[0010] However, in recent years, with improvements in the performance of semiconductor light-emitting elements, the polarization extinction ratio of the two orthogonal linearly polarized lights that are light from the light source 11 has increased to, for example, about 10 to 18 dB. If polarization rotation occurs in the light from the light source 11 having such polarization characteristics as it passes through the single-mode optical fiber 16, the optical element 13 acting as a polarizer may mainly pass linearly polarized light with extremely low light intensity, resulting in a large fluctuation in the amount of light. In other words, the amount of light passing through the single-mode optical fiber coil 15 is greatly reduced, resulting in a deterioration in the signal-to-noise ratio and degradation of the performance of the optical fiber gyroscope 900.

[0011] A simple solution to this technical problem would be to connect the light source 11 to the first end of the Y-shaped optical waveguide 13a with a polarization-maintaining optical fiber. However, from the viewpoints of connection with the optical coupler and cost, it is preferable to avoid such a solution.

[0012] In view of the above technical problems, there is provided a fiber optic gyroscope that can stabilize the amount of light passing through a single-mode optical fiber coil. [Means for solving the problem]

[0013] The technical matters described herein are not intended to explicitly or implicitly limit the invention described in the claims, and are not provided to limit the invention described in the claims to anyone other than those who will benefit from the present invention (e.g., the applicant and the right holder), but are described simply to facilitate understanding of the gist of the present invention. The outline of the present invention from other perspectives can be understood, for example, from the claims at the time of filing of this patent application. The fiber optic gyroscope disclosed in this specification includes one more polarization-maintaining optical fiber in addition to the five polarization-maintaining optical fibers described above. Each of the six polarization-maintaining optical fibers has the same beat length. The light source and one end of the single-mode optical fiber are connected to each other with an additional polarization-maintaining optical fiber. The angle between the main polarization axis of the added polarization-maintaining optical fiber and the polarization plane of the linearly polarized light from the light source is 45 degrees. The optical length of each of the six polarization-maintaining optical fibers is greater than the coherence length of the linearly polarized light from the light source. c Let the physical length of the i-th polarization-maintaining optical fiber be L i and the birefringence of the polarization-maintaining optical fiber is n, L i ×n>L c is. The total optical length of the six polarization-maintaining optical fibers, taking into account the polarization rotation that occurs during transmission through the single-mode optical fiber, is greater than the coherence length of the linearly polarized light from the light source. For details, see the embodiments described below. [Effects of the Invention]

[0014] The fiber optic gyroscope disclosed in this specification can stabilize the amount of light passing through the single-mode optical fiber coil. [Brief explanation of the drawings]

[0015] [Figure 1] 1 shows the optical configuration of a prior art fiber optic gyroscope. [Figure 2] 1 shows an optical configuration of an embodiment of a fiber optic gyroscope. DETAILED DESCRIPTION OF THE INVENTION

[0016] The optical configuration of an optical fiber gyroscope 100 according to an embodiment will be described with reference to the drawings. The optical fiber gyroscope 100 actually includes a phase modulator for phase-modulating each of CW light and CCW light, an optical coupler attached to a single-mode optical fiber for extracting interference light formed by optically coupling the CW light and CCW light, a photoelectric converter for photoelectrically converting the light from the optical coupler, and a signal processing circuit for performing signal processing such as angular velocity detection based on the electrical signal from the photoelectric converter. However, since these components are not directly related to the present invention and their configurations and functions are well known, illustrations and descriptions thereof will be omitted.

[0017] The optical system of the fiber optic gyroscope 100 has the following basic components: a) a light source 11 that emits linearly polarized light; b) a single-mode optical fiber 16 having one end and another end; c) a first polarization-maintaining optical fiber 21 having one end and another end; d) a second polarization-maintaining optical fiber 22 having one end and an opposite end; e) a third polarization-maintaining optical fiber 23 having one end and the other end; f) a fourth polarization-maintaining optical fiber 24 having one end and another end; g) a fifth polarization-maintaining optical fiber 25 having one end and the other end; h) a sixth polarization-maintaining optical fiber 26 having one end and another end; i) a single-mode optical fiber coil 15 having one end and another end; j) an optical element 13 having a Y-shaped optical waveguide 13a formed thereon, the Y-shaped optical waveguide 13a having a first end, a second end, and a third end; The optical element 13 is, for example, a lithium niobate optical crystal, and the Y-shaped optical waveguide 13a is, for example, a proton-exchanged LiNbO3 optical waveguide. The proton-exchanged LiNbO3 optical waveguide has a large polarization extinction ratio and functions as an excellent polarizer.

[0018] These connections are as follows: 1) One end of the first polarization-maintaining optical fiber 21 is connected to the light source 11 (in the drawing, for ease of distinction, the line width of the polarization-maintaining optical fiber is shown to be thicker than the line width of the single-mode optical fiber), 2) One end of the single-mode optical fiber 16 is connected to the other end of the first polarization-maintaining optical fiber 21; 3) One end of the second polarization-maintaining optical fiber 22 is connected to the other end of the single-mode optical fiber 16; 4) The first end of the Y-shaped optical waveguide 13a is connected to the other end of the second polarization-maintaining optical fiber 22; 5) One end of the third polarization-maintaining optical fiber 23 is connected to the second end of the Y-shaped optical waveguide 13a, 6) One end of the fourth polarization-maintaining optical fiber 24 is connected to the other end of the third polarization-maintaining optical fiber 23 (in the figure, the boundary between the third polarization-maintaining optical fiber 23 and the fourth polarization-maintaining optical fiber 24 is indicated by a dashed line), 7) One end of the single-mode optical fiber coil 15 is connected to the other end of the fourth polarization-maintaining optical fiber 24; 8) One end of the fifth polarization-maintaining optical fiber 25 is connected to the other end of the single-mode optical fiber coil 15; 9) One end of the sixth polarization-maintaining optical fiber 26 is connected to the other end of the fifth polarization-maintaining optical fiber 25 (in the figure, the boundary between the fifth polarization-maintaining optical fiber 25 and the sixth polarization-maintaining optical fiber 26 is indicated by a dashed line), 10) The third end of the Y-shaped optical waveguide 13a is connected to the other end of the sixth polarization-maintaining optical fiber .

[0019] Of course, the axial core of the first polarization-maintaining optical fiber 21, the axial core of the second polarization-maintaining optical fiber 22, and the axial core of the single-mode optical fiber 16 are all aligned with one another, and the axial core of the third polarization-maintaining optical fiber 23, the axial core of the fourth polarization-maintaining optical fiber 24, the axial core of the fifth polarization-maintaining optical fiber 25, the axial core of the sixth polarization-maintaining optical fiber 26, and the axial core of the single-mode optical fiber coil 15 are all aligned with one another.

[0020] Furthermore, with respect to the principal axis of the polarization-maintaining optical fiber, A) The angle between the main polarization axis of the first polarization-maintaining optical fiber 21 and the polarization plane of the linearly polarized light from the light source 11 is 45 degrees, B) The angle between the polarization main axis of the third polarization-maintaining optical fiber 23 and the polarization main axis of the fourth polarization-maintaining optical fiber 24 is 45 degrees, C) The angle between the polarization axis 25 of the fifth polarization-maintaining optical fiber and the polarization axis of the sixth polarization-maintaining optical fiber 26 is 45 degrees; D) The polarization main axis of the second polarization-maintaining optical fiber 22 coincides with the electric field direction of the TE mode of the Y-shaped optical waveguide 13a, E) The polarization main axis of the third polarization-maintaining optical fiber 23 coincides with the electric field direction of the TE mode of the Y-shaped optical waveguide 13a, F) The polarization main axis of the sixth polarization-maintaining optical fiber 26 coincides with the electric field direction of the TE mode of the Y-shaped optical waveguide 13a.

[0021] Under the above conditions A), B), and C), the incident light is divided into two orthogonal polarization modes with equal intensity, i.e., the polarization mode of the fast axis and the polarization mode of the slow axis of the polarization-maintaining optical fiber. Under the above conditions D), E), and F), the polarization mode of, for example, the fast axis of the polarization-maintaining optical fiber and the TE mode of the Y-shaped optical waveguide 13a are matched.

[0022] Furthermore, the optical lengths of the first polarization-maintaining optical fiber 21, the second polarization-maintaining optical fiber 22, the third polarization-maintaining optical fiber 23, the fourth polarization-maintaining optical fiber 24, the fifth polarization-maintaining optical fiber 25, and the sixth polarization-maintaining optical fiber 26 are each greater than the coherence length of the linearly polarized light from the light source 11. In other words, the first polarization-maintaining optical fiber 21, the second polarization-maintaining optical fiber 22, the third polarization-maintaining optical fiber 23, the fourth polarization-maintaining optical fiber 24, the fifth polarization-maintaining optical fiber 25, and the sixth polarization-maintaining optical fiber 26 each have the same birefringence index n, and the coherence length of the linearly polarized light from the light source 11 is greater than L. c and the physical length of the first polarization-maintaining optical fiber 21 is L1, the physical length of the second polarization-maintaining optical fiber 22 is L2, the physical length of the third polarization-maintaining optical fiber 23 is L3, the physical length of the fourth polarization-maintaining optical fiber 24 is L4, the physical length of the fifth polarization-maintaining optical fiber 25 is L5, and the physical length of the sixth polarization-maintaining optical fiber 26 is L6, then α)L1×n>L c , β)L2×n>L c , γ)L3×n>L c , δ)L4×n>L c , ε)L5×n>L c , ζ)L6×n>L c These are the conditions for two orthogonal polarization modes passing through a polarization-maintaining optical fiber to have a group delay difference between the two orthogonal polarization modes, and as a result, the two orthogonal polarization modes cannot interfere with each other. In short, each of the first polarization-maintaining optical fiber 21, the second polarization-maintaining optical fiber 22, the third polarization-maintaining optical fiber 23, the fourth polarization-maintaining optical fiber 24, the fifth polarization-maintaining optical fiber 25, and the sixth polarization-maintaining optical fiber 26 has a group delay difference between the two orthogonal polarization modes. c It is sufficient that the physical length is longer than / n.

[0023] Generally, the beat length of a polarization-maintaining optical fiber is defined as L b where n=λ / L, where λ is the wavelength of the linearly polarized light from the light source 11. b Therefore, the above conditions are as follows: α)L1×λ / L b >L c , β)L2×λ / L b >L c , γ)L3×λ / L b >L c , δ)L4×λ / L b >L c , ε)L5×λ / L b >L c , ζ)L6×λ / L b >L c It can be rewritten as follows.

[0024] Due to conditions A) and α), the linearly polarized light from the light source 11 is distributed with equal intensity to the fast axis polarization mode and the slow axis polarization mode of the first polarization-maintaining optical fiber 21, and the correlation between the fast axis polarization mode and the slow axis polarization mode is lost during the propagation process, resulting in a depolarized state of the light by the first polarization-maintaining optical fiber 21. In other words, the first polarization-maintaining optical fiber 21 depolarizes the linearly polarized light from the light source 11 (specifically, the first polarization-maintaining optical fiber 21 converts the linearly polarized light from the light source 11 into two orthogonal polarization modes that are equal in intensity and uncorrelated with each other).

[0025] The second polarization-maintaining optical fiber 22 preserves the unpolarized state obtained by the first polarization-maintaining optical fiber 21 due to condition β), but the fast axis polarization mode of the second polarization-maintaining optical fiber 22 travels in the Y-shaped optical waveguide 13a in the TE waveguide mode due to condition D).

[0026] The Y-shaped optical waveguide 13a splits the light into equal intensity beams.

[0027] One of the lights in the TE waveguide mode from the Y-shaped optical waveguide 13a (i.e., CW light) enters the fast axis of the third polarization-maintaining optical fiber 23 due to condition E), but due to condition γ), the third polarization-maintaining optical fiber 23 converts the light in the TE waveguide mode from the Y-shaped optical waveguide 13a into two orthogonal polarization modes that are uncorrelated with each other.

[0028] The fourth polarization-maintaining optical fiber 24 depolarizes the light obtained by the third polarization-maintaining optical fiber 23 under conditions B) and δ) (specifically, the fourth polarization-maintaining optical fiber 24 converts the light into two orthogonal polarization modes that are equal in intensity and uncorrelated with each other).

[0029] The fifth polarization-maintaining optical fiber 25 preserves the unpolarized state of the light from the fourth polarization-maintaining optical fiber 24 that has propagated through the single-mode optical fiber coil 15 due to the condition ε).

[0030] The sixth polarization-maintaining optical fiber 26 preserves the unpolarized state of the light from the fifth polarization-maintaining optical fiber 25 due to conditions C) and ζ), but due to condition F), the polarization mode of the fast axis of the sixth polarization-maintaining optical fiber 26 travels in the Y-shaped optical waveguide 13a in the TE waveguide mode.

[0031] The other light in the TE waveguide mode from the Y-shaped optical waveguide 13a (i.e., CCW light) enters the fast axis of the sixth polarization-maintaining optical fiber 26 due to condition F), but due to condition ζ), the sixth polarization-maintaining optical fiber 26 converts the light in the TE waveguide mode from the Y-shaped optical waveguide 13a into two orthogonal polarization modes that are uncorrelated with each other.

[0032] The fifth polarization-maintaining optical fiber 25 depolarizes the light obtained by the sixth polarization-maintaining optical fiber 26 due to conditions C) and ε) (specifically, the fifth polarization-maintaining optical fiber 25 converts the light into two orthogonal polarization modes that are equal in intensity and uncorrelated with each other).

[0033] The fourth polarization-maintaining optical fiber 24 preserves the unpolarized state of the light from the fifth polarization-maintaining optical fiber 25 that has propagated through the single-mode optical fiber coil 15 under condition δ).

[0034] The third polarization-maintaining optical fiber 23 preserves the unpolarized state of the light from the fourth polarization-maintaining optical fiber 24 due to conditions B) and γ), but due to condition E), the polarization mode of the fast axis of the third polarization-maintaining optical fiber 23 travels in the Y-shaped optical waveguide 13a in a TE waveguide mode.

[0035] Generally, single-mode optical fibers can transmit light of any polarization state. However, the polarization state of light inside a single-mode optical fiber can easily change due to environmental disturbances. That is, polarization rotation of two orthogonal polarization modes can occur during transmission through the single-mode optical fiber. Furthermore, although optical element 13 is an excellent polarizer and does not cause polarization rotation, it is not a perfect polarizer with an infinite polarization extinction ratio. Therefore, optical element 13 does not completely select only one of the two orthogonal polarization modes. Furthermore, as mentioned above, each polarization-maintaining optical fiber generates a non-polarized state of light, but this is an artificially created non-polarized state, unlike the non-polarized state of natural light. Therefore, if polarization rotation of two orthogonal polarization modes occurs during transmission through a single-mode optical fiber, the correlation between the two orthogonal polarization modes may be restored after the orthogonal polarization modes pass through multiple polarization-maintaining optical fibers. This is because when the fast-axis polarization mode and the slow-axis polarization mode, which have been given a group delay difference depending on the optical length of the upstream polarization-maintaining optical fiber by the upstream polarization-maintaining optical fiber, respectively enter the slow-axis and fast-axis of the downstream polarization-maintaining optical fiber through a 90-degree polarization rotation by the single-mode optical fiber, the group delay difference obtained depending on the optical length of the upstream polarization-maintaining optical fiber is reduced depending on the optical length of the downstream polarization-maintaining optical fiber. In other words, even if all of the conditions α), β), γ), δ), ε), and ζ) are satisfied, there is a possibility that the correlation between the two orthogonal polarization modes will be restored for each of the CW light and CCW light entering the optical element 13. Therefore, a condition to eliminate this possibility is required for the CW light and CCW light entering the optical element 13.

[0036] This condition is as follows: "The total optical length of the six polarization-maintaining optical fibers, taking into account the polarization rotation during the process of passing through the single-mode optical fiber 16 and the single-mode optical fiber coil 15, is greater than the coherence length of the linearly polarized light from the light source." Specifically, I) In the case where a 90-degree polarization rotation occurs during the process of passing through the single-mode optical fiber 16 and a 90-degree polarization rotation occurs during the process of passing through the single-mode optical fiber coil 15, |L1-(L2+L3+L4)-(L5+L6)|×n>L c If the condition is satisfied, the possibility of the correlation between the two orthogonal polarization modes being restored can be eliminated. II) In the case where 90-degree polarization rotation occurs during the process of passing through the single-mode optical fiber 16, and 90-degree polarization rotation does not occur during the process of passing through the single-mode optical fiber coil 15, |L1-(L2+L3+L4)+(L5+L6)|×n>L c If the condition is satisfied, the possibility of the correlation between the two orthogonal polarization modes being restored can be eliminated. III) In the case where 90-degree polarization rotation does not occur during the process of passing through the single-mode optical fiber 16, and 90-degree polarization rotation occurs during the process of passing through the single-mode optical fiber coil 15, |L1+(L2+L3+L4)-(L5+L6)|×n>L c If the condition (2) is satisfied, the possibility of the correlation between the two orthogonal polarization modes being restored can be eliminated.

[0037] The conditions I), II), and III) are as follows: |L1-(L2+L3+L4)-(L5+L6)|×λ / L b >L c |L1-(L2+L3+L4)+(L5+L6)|×λ / L b >L c |L1+(L2+L3+L4)-(L5+L6)|×λ / L b >L c can be rewritten as

[0038] Therefore, in the optical fiber gyroscope 100, even if the polarization extinction ratio of the two orthogonal linearly polarized lights from the light source 11 is large, the CW light and CCW light interfere with each other at the branch point of the Y-shaped optical waveguide 13a with equal intensity and without any fluctuation in light quantity.

[0039] From the viewpoint of cost, it is preferable that the sum of lengths L1, L2, L3, L4, L5 and L6 be the smallest among the combinations of lengths L1, L2, L3, L4, L5 and L6 that satisfy the above-mentioned conditions α), β), γ), δ), ε), ζ), I), II) and III).

[0040] <Addendum> While the present invention has been described with reference to exemplary embodiments, those skilled in the art will recognize that various changes can be made and equivalents can be substituted for elements thereof without departing from the scope of the invention. Furthermore, many modifications can be made to adapt a particular system, device, or component thereof to the teachings of the present invention without departing from the essential scope of the invention. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed for carrying out this invention, but rather that the invention will include all embodiments falling within the scope of the appended claims.

[0041] Furthermore, the use of terms such as "first" and "second" does not denote order or importance, and terms such as "first" and "second" are used to distinguish elements. The terms used herein are for the purpose of describing embodiments and are not intended to limit the present invention in any way. The term "comprises" and its conjugations, when used in this specification and / or the appended claims, reveal the presence of the mentioned features, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, steps, operations, elements, components, and / or groups thereof. The term "and / or" includes any and all combinations of one or more of the associated listed elements, if any. In the claims and the specification, unless otherwise specified, the use of words such as "connected," "coupled," "joined," "connected," or their equivalents, and all forms thereof, does not necessarily negate the presence of one or more intermediate elements between two elements that are, for example, "connected" or "coupled" to each other or "coupled" to each other. In the claims and the specification, the term "optional," if any, should be understood as a term that represents the same meaning as the universal symbol ∀, unless otherwise specified. For example, the phrase "for any X" has the same meaning as "for all X" or "for each X."

[0042] Unless otherwise specified, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Furthermore, terms such as those defined in commonly used dictionaries should be interpreted to have a meaning consistent with their meaning in the context of the relevant art and this disclosure, and should not be interpreted ideally or excessively formally unless explicitly defined.

[0043] It will be understood that in describing the present invention, many techniques and steps are disclosed. Each of these has distinct advantages, and each can be used in combination with one or more, or in some cases all, of the other disclosed techniques. Therefore, to avoid cluttering, this specification will refrain from describing every possible combination of individual techniques or steps. Nevertheless, the specification and claims should be read with the understanding that such combinations are fully within the scope of the present invention and claims.

[0044] The corresponding structure, material, acts, and equivalents of all functional elements combined with means or steps in the following claims are intended to include the structure, material, or acts, if any, that perform the function in combination with other elements.

[0045] Although the present invention has been described above with reference to exemplary embodiments, it is not limited to these embodiments. Various modifications and variations are possible without departing from the spirit of the present invention. The selected and described embodiments are intended to illustrate the principles of the present invention and its practical application. The present invention may be used in various embodiments with various modifications and variations, which are determined according to the expected use. All such modifications and variations are intended to be included within the scope of the present invention, as defined by the appended claims, and are intended to be accorded the same protection when interpreted in accordance with the breadth that is fairly, legally, and equitably to be given. [Explanation of symbols]

[0046] 11 Light source 13 Optical Elements 13a Y-shaped optical waveguide 15 Single-mode optical fiber coil 16 Single-mode optical fiber 21 First polarization-maintaining optical fiber 22 Second polarization-maintaining optical fiber 23 Third polarization-maintaining optical fiber 24 Fourth polarization-maintaining optical fiber 25 No. 5 polarization-maintaining optical fiber 26 No. 6 Polarization-Maintaining Optical Fiber 100 Fiber Optic Gyroscope 900 Fiber Optic Gyroscope

Claims

1. 1. A fiber optic gyroscope, comprising: a light source that emits linearly polarized light; a single-mode optical fiber having one end and another end; a first polarization-maintaining optical fiber having one end and an opposite end; a second polarization-maintaining optical fiber having one end and an opposite end; a third polarization-maintaining optical fiber having one end and an opposite end; a fourth polarization-maintaining optical fiber having one end and an opposite end; a fifth polarization-maintaining optical fiber having one end and another end; a sixth polarization-maintaining optical fiber having one end and an opposite end; a single-mode optical fiber coil having one end and another end; an optical element having a Y-shaped optical waveguide formed thereon, the Y-shaped optical waveguide having a first end, a second end, and a third end; Including, the one end of the first polarization-maintaining optical fiber is connected to the light source; the one end of the single-mode optical fiber is connected to the other end of the first polarization-maintaining optical fiber, the one end of the second polarization-maintaining optical fiber is connected to the other end of the single-mode optical fiber, the first end of the Y-shaped optical waveguide is connected to the other end of the second polarization-maintaining optical fiber, the one end of the third polarization-maintaining optical fiber is connected to the second end of the Y-shaped optical waveguide, the one end of the fourth polarization-maintaining optical fiber is connected to the other end of the third polarization-maintaining optical fiber, the one end of the single-mode optical fiber coil is connected to the other end of the fourth polarization-maintaining optical fiber; the one end of the fifth polarization-maintaining optical fiber is connected to the other end of the single-mode optical fiber coil; the one end of the sixth polarization-maintaining optical fiber is connected to the other end of the fifth polarization-maintaining optical fiber, the third end of the Y-shaped optical waveguide is connected to the other end of the sixth polarization-maintaining optical fiber, the angle between the polarization main axis of the first polarization-maintaining optical fiber and the polarization plane of the linearly polarized light is 45 degrees; the angle between the polarization main axis of the third polarization-maintaining optical fiber and the polarization main axis of the fourth polarization-maintaining optical fiber is 45 degrees; the angle between the polarization main axis of the fifth polarization-maintaining optical fiber and the polarization main axis of the sixth polarization-maintaining optical fiber is 45 degrees; the polarization main axis of the second polarization-maintaining optical fiber coincides with the electric field direction of the TE mode of the Y-shaped optical waveguide; the polarization main axis of the third polarization-maintaining optical fiber coincides with the electric field direction of the TE mode of the Y-shaped optical waveguide; a polarization main axis of the sixth polarization-maintaining optical fiber coincides with the electric field direction of the TE mode of the Y-shaped optical waveguide; the first polarization-maintaining optical fiber, the second polarization-maintaining optical fiber, the third polarization-maintaining optical fiber, the fourth polarization-maintaining optical fiber, the fifth polarization-maintaining optical fiber, and the sixth polarization-maintaining optical fiber each have the same beat length; The beat length is L b The wavelength of the linearly polarized light is λ, and the coherence length of the linearly polarized light is L c and the length L of the first polarization-maintaining optical fiber 1 and the length L of the second polarization-maintaining optical fiber 2 and the length L of the third polarization-maintaining optical fiber 3 and the length L of the fourth polarization-maintaining optical fiber 4 and the length L of the fifth polarization-maintaining optical fiber 5 and the length L of the sixth polarization-maintaining optical fiber 6 The minimum value of L s As, L s ×λ / L b >L c ,and, |L 1 -(L 2 +L 3 +L 4 )-(L 5 +L 6 )|×λ / L b >L c and |L 1 -(L 2 +L 3 +L 4 )+(L 5 +L 6 )|×λ / L b >L c and |L 1 +(L 2 +7 3 +7 4 )-(L 5 +7 6 )|×λ / L b >7 c is true Fiber optic gyroscope.

2. 2. The fiber optic gyroscope according to claim 1, L s ×λ / L b >L c ,and, |L 1 -(L 2 +L 3 +L 4 )-(L 5 +L 6 )|×λ / L b >L c and |L 1 -(L 2 +L 3 +L 4 )+(L 5 +L 6 )|×λ / L b >L c and |L 1 +(L 2 +7 3 +7 4 )-(L 5 +7 6 )|×λ / L b >7 c The length L 1 and the length L 2 and the length L 3 and the length L 4 and the length L 5 and the length L 6 Among the combinations of the length L 1 and the length L 2 and the length L 3 and the length L 4 and the length L 5 and the length L 6 The sum of is the smallest 1. A fiber optic gyroscope comprising:

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