JP2004309466A - Optical fiber gyroscope - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical fiber gyroscope capable of correctly turning an incident light from an optical integrated circuit, having the function of an optical polarizer and a lightguide path into a single-mode optical fiber into a non-polarized state. <P>SOLUTION: A light source 10 is successively connected with an end of a polarized plane preserving optical fiber 54 via a single-mode optical fiber 51, and a coupler 52 of the other end of the optical fiber 54 is connected with the lightguide paths 15 of an optical integrated circuit 14 having the branching light paths 15, the characteristic axes of which have the polarization functions, while being made to coincide with the electric field directions of the TE modes. Each one end of the polarized plane preserving optical fibers 55 and 57 is connected with the other two ends of the lightguide paths 15, respectively, while the inherent axes of the optical fibers 55, 57 coincide with the direction of the electric field of the TE modes; the other ends of the optical fibers 55, 57 are connected with the polarized plane preserving optical fibers 56, 58, while the characteristic axes each of which is shifted by 45°; and the other ends of the optical fibers 56, and 58 are connected with both the ends of an optical fiber coil 20, respectively. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber gyro having a substrate type optical integrated circuit having a function of a polarizer and a branched optical waveguide, and detecting an angular velocity applied to an optical fiber coil around the axis of the coil.

  In an optical fiber gyro, a light beam enters each end of an optical fiber coil, propagates through the optical fiber coil, and is emitted from both ends of the coil. The phase difference between these two emitted lights is zero in the state where the angular velocity around the axis is not applied to the optical fiber coil, but in the state where the angular velocity is applied, according to the applied angular velocity, The phase difference between the two emitted lights changes. Therefore, by detecting this phase difference, the angular velocity applied to the optical fiber coil is detected. If the optical fiber coil has birefringence, the light beam propagating therethrough has a different light propagation speed due to two orthogonal linearly polarized components. Accordingly, the angular velocity is detected by causing the same linearly polarized beams among the two linearly polarized lights in the outgoing light propagating through the optical fiber coil to interfere with each other. A polarizer is used to obtain an interference light beam between the one linearly polarized output beam. Further, the optical fiber coil is constituted by the polarization-maintaining optical fiber so that the linearly polarized light selected by the polarizer propagates through the optical fiber coil.

  However, polarization-maintaining optical fibers are considerably more expensive than single-mode optical fibers. From this point, even in a single mode optical fiber coil in which a single mode optical fiber is used as an optical fiber coil, the bending state and the like are slightly smaller, but the birefringence is generated, and the light propagating therethrough Changes easily due to some influence. Therefore, using a depolarizer, one linearly polarized light and the other linearly polarized light are of equal amplitude that do not interfere with each other, that is, light is incident on both ends of the optical fiber coil in a non-polarized state. .

  A splitter for splitting the light from the light source and entering both ends of the optical fiber coil, and a splitter for causing the emitted light beams from both ends of the optical fiber coil to interfere with each other, and a polarizer for making predetermined linearly polarized light, It has been proposed to make a substrate-type optical integrated circuit having an optical waveguide function. When an optical fiber gyro is configured using this substrate-type optical integrated circuit and a single-mode optical fiber, light incident on the optical waveguide of the substrate-type optical integrated circuit from the light source is transmitted in the TE mode, which is the propagation mode of the optical waveguide. The light propagates, is split into two, and enters both ends of the optical fiber coil. However, light in the TM mode, which is the extinction mode of the optical waveguide, leaks out of the optical waveguide and is reflected at the bottom of the substrate of the substrate-type optical integrated circuit. Then, not only the linearly polarized light based on the TE mode light but also the linearly polarized light orthogonal thereto is incident on the optical fiber coil based on the leaked stray light of the TM mode, and the extinction ratio as a polarizer is deteriorated. You. In order to solve this problem, US Pat. No. 5,475,772 (issued on December 12, 1995) forms a spatial filter on the bottom surface of a substrate of an optical integrated circuit. However, not only the leaked light in the TM mode enters the optical fiber coil as the TM mode light, but the leaked light is converted into the TE mode, is incident on the optical fiber coil, and propagates through the optical waveguide. It has been found that the extinction ratio as a polarizer is deteriorated as a result of adversely affecting the mode light.

Non-Patent Document 1 describes an optical fiber gyro that solves these problems when using a substrate-type optical integrated circuit. The optical fiber gyro will be described below with reference to FIG.
The light beam emitted from the light source 10 enters the substrate type optical integrated circuit 14 through the first optical fiber 11, the optical fiber coupler 12, and the second optical fiber 13. The first optical fiber 11, the optical fiber coupler 12, and the second optical fiber 13 are all composed of polarization-maintaining optical fibers. In the optical fiber coupler 12, the side surfaces of the intermediate portions of the two polarization maintaining optical fibers are fused to each other, and both core portions thereof are brought close to each other. The optical integrated circuit 14 is formed on a lithium niobate (LiNbO ₃ ) optical crystal substrate, a Y-branch optical waveguide 15 produced by a proton exchange method, and the two branched optical waveguide portions. Optical modulators 16 and 17 are provided.

  The light incident on the Y-branch optical waveguide 15 is split into two lights, a first light and a second light, and the first light is incident on one end of an optical fiber coil 20 through a third optical fiber 18. 20 in a clockwise direction (hereinafter, referred to as CW direction). The branched second light is incident on the other end of the optical fiber coil 20 through the fourth optical fiber 21 and propagates in the optical fiber coil 20 in a counterclockwise direction (hereinafter, referred to as a CCW direction). The third and fourth optical fibers 18 and 21 are composed of polarization-maintaining optical fibers, and each of these polarization-maintaining optical fibers 18 and 21 has its own axis (in general, the direction of the electric field of linearly polarized light having a higher propagation speed). Is connected to the optical waveguide 15 of the optical integrated circuit 14 at a position shifted by 45 degrees from the direction of the electric field of the propagation TE mode, and functions as a depolarizer together with the optical waveguide 15. The optical fiber coil 20 is composed of a single mode optical fiber.

When the optical fiber coil 20 is rotated around its axis, a phase difference occurs between light propagating in the CW direction and light propagating in the CCW direction in the optical fiber coil 20, and these lights are Y-branched. Interference light is generated by being incident on and coupled to the optical waveguide 15, the interference light is branched by the optical fiber coupler 12 and incident on the light receiver 25, and an electric signal corresponding to the intensity of the interference light is output from the light receiver 25. Is output. The output signal of the light receiver 25 is supplied to a detection circuit 26.
The light modulator 17 is used to increase the detection sensitivity. A phase modulation signal (for example, a sine wave signal) is applied from the modulation signal generator 27 to the optical modulator 17, and the light propagating in one of the branched optical waveguides is phase-modulated. Further, a signal synchronized with the phase modulation signal is supplied from the modulation signal generator 27 to the detection circuit 26, and the electric signal output from the light receiver 25 is synchronously detected by this signal.

  A detection output corresponding to the input angular velocity from the detection circuit 26 is supplied to a feedback signal generation circuit 28. The feedback signal generation circuit 28 generates a feedback signal corresponding to the magnitude of the detection output, which is the input, and supplies the feedback signal to the optical modulator 16 so that the detection output of the detection circuit 26 is controlled to be zero. An output signal of the optical fiber gyro is obtained from the feedback signal generated from the feedback signal generation circuit 28. In the configuration shown in FIG. 1, the entire optical path from the light source 10 to the optical integrated circuit 14 via the optical fiber coupler 12 is composed of a polarization-maintaining optical fiber. A polarization-maintaining optical fiber uses its birefringence to prevent the polarization plane from fluctuating during the propagation of light. A difference occurs in the propagation speed. The optical path length between the light source 10 and the optical integrated circuit 14 needs to be, for example, about 1 m. Therefore, in the case of a light source such as a super luminescence diode (SLD) used in a normal optical fiber gyro, the emitted light from the light source 10 is a polarization-maintaining optical fiber with respect to the coherence (coherence) of the emitted light. Is transmitted over about 10 cm, the wavefront distance between the linearly polarized lights based on the difference between the propagation velocities of the linearly polarized lights sufficiently exceeds coherence, and the coherence between the linearly polarized lights disappears. Accordingly, one linearly polarized light from the polarization-maintaining optical fiber 13 propagates through the optical waveguide as the TE mode, which is the propagation mode of the optical waveguide of the optical integrated circuit 14, but the other linearly polarized light is coupled into the optical waveguide as the TM mode. Then, the TM mode light leaks from the optical waveguide and is recombined into the optical waveguide or the polarization-maintaining optical fibers 18 and 21 irregularly as the TM mode as stray light as described above or as the TE mode after being mode-converted as described above. Also, since there is no coherence between the linearly polarized light propagating through the optical waveguide and the stray light as described above, the recombination does not affect the angular velocity detection at all. Therefore, it is not necessary to form a spatial filter on the bottom surface of the optical integrated circuit 14 for suppressing the influence of stray light.

In the optical fiber gyro shown in FIG. 1, the length L of the polarization-maintaining optical fibers 18 and 21 is set to such an extent that coherence between two mutually orthogonal linearly polarized light beams propagating therethrough becomes insufficient. . The length L is set so as to satisfy the condition of the following equation.
Lλ / B> Lc (1)
Here, B indicates the beat length (typical value of 2 mm), λ indicates the wavelength of light (typical value of 0.83 μm), Lc indicates the coherence length of light (typical value of optical fiber gyro: 50 μm), and these typical values will be described above. Substituting into the equation, L> 0.12m. In the above equation, L / B represents a phase difference due to a difference in propagation speed between orthogonal linearly polarized lights, and Lλ / B represents a distance between wave fronts of orthogonal linearly polarized lights.

The polarization-maintaining optical fibers 18 and 21 are connected to the optical waveguides with their intrinsic axes inclined by 45 degrees with respect to the electric field direction of the optical waveguide of the optical integrated circuit 14 in the TE mode. Accordingly, a depolarizer is formed by the optical waveguide and the polarization-maintaining optical fibers 18 and 21, and light from the optical integrated circuit 14 is incident on both ends of the single-mode optical fiber coil 20 in a non-polarized state.
With such a configuration, even if the optical fiber coil 20 is formed of a single mode optical fiber, the angular velocity can be detected with high accuracy. The problems associated with the use of the single-mode optical fiber coil and the problem of using the substrate type optical integrated circuit which also functions as a polarizer and a splitter appear as variations in the bias error of the optical fiber gyro. In other words, while the angular velocity is not applied, the detection output of the optical fiber gyro should be zero, but the detection output occurs as a bias, the bias error fluctuates, and the detection accuracy deteriorates.

Although the above document does not describe the length of both polarization-maintaining single-mode fibers 18 and 21, Non-Patent Document 2 describes the length. That is, each length L of both polarization-maintaining single-mode fibers 18 and 21 is set to 1: 2, thereby acting as a LYOT-type depolarizer. Even if the phase of the two linearly polarized lights is the worst, based on the birefringence of the above, the linearly polarized lights have such a length that gives a phase difference to such an extent that they do not interfere with each other.
To connect the polarization-maintaining optical fiber to the end face of the optical waveguide 15 of the substrate-type optical integrated circuit 14, first, a carrier made of the same material as the substrate of the substrate-type integrated circuit 14 is attached to the end of the polarization-maintaining optical fiber and bonded. The carrier was fixed and then the end face of the carrier was bonded and fixed to the end face of the substrate type optical integrated circuit 14. That is, for example, as shown in FIG. 2, the carrier 31 has a fiber holding groove 32 formed on one side surface of a quadrangular prism. One end of the polarization-maintaining optical fiber 13 is inserted into the fiber holding groove 32, and With the end faces of the two stress applying portions 13a parallel to the center of the optical fiber 13 with the center of the carrier 31 interposed between the two stress applying sections 13a, the two end faces of the two stresses are visually recognized with a microscope. The optical fiber 13 and the carrier 31 are fixed to each other with the adhesive 34 such that the arrangement direction 33 of the applying section 13a is parallel to one side surface (reference surface) 31a of the carrier 31. In this case, by aligning the hairline of the microscope with the reference surface 31a of the carrier 31, the arrangement direction 33 and the reference surface 31a can be relatively easily and accurately made parallel to each other.

  Next, the optical integrated circuit 14 is mounted on the connector so that the bottom surface of the substrate is aligned with the reference surface thereof, and the connector has a reference surface 31b corresponding to the side surface 31b perpendicular to the reference surface 31a of the carrier 31. Then, the carrier 31 is attached, and then the connector is operated to control the movement of the carrier 31 in the vertical direction and the horizontal direction with respect to the reference plane, and as shown in FIGS. 3A and 3B, the optical waveguide 15 and the optical fiber 13 are connected to each other. Are located on a straight line, and the end faces of the carrier 31 and the optical fiber 13 are brought into contact with the end faces of the optical integrated circuit 14 to adhere and fix them to each other. In this way, the intrinsic axis of the polarization-maintaining single-mode fiber 13 matches the direction of the electric field in the TE mode of the optical waveguide 15. Normally, the direction (fast axis) perpendicular to the arrangement direction 33 of the two stress applying portions 13a is made to coincide with the electric field direction of the TE mode. 3A and 3B show the state.

When connecting the polarization-maintaining optical fibers 18 and 21 to the optical waveguide 15, a carrier 31 is first mounted on the polarization-maintaining optical fiber 18, and at this time, as shown by broken lines in FIG. The optical fiber 18 and the carrier 31 are bonded together while visually observing with a microscope such that the arrangement direction of the 18a is 45 degrees with respect to the reference plane 31a. The subsequent fixing of the carrier 31 to the optical integrated circuit 14 is performed in the same manner as described with reference to FIG. The connection between the polarization-maintaining optical fiber 21 and the optical waveguide 15 is performed in the same manner.
However, when the arrangement direction of the stress applying portions is visually recognized at 45 degrees with respect to the reference plane 31a, the alignment accuracy of the angle is generally poor because the arrangement direction of the stress applying sections is parallel to the reference plane 31a. For example, the deviation of the angle alignment when making it parallel to the reference plane 31a is several degrees / hour due to the bias error of the optical fiber gyro, but the deviation of the angle alignment when making it 45 degrees with respect to the reference plane 31a is a bias error. It becomes several tens degrees / hour. That is, if the individual axes of the polarization-maintaining single-mode fibers 18 and 21 are not correctly 45 degrees with respect to the direction of the electric field in the TE mode of the optical waveguide 15, the light incident on the single-mode optical fiber coil 20 will be accurately non-polarized. The state does not occur, and the accuracy of the detected angular velocity decreases.

An open-loop optical fiber gyro using a substrate-type optical integrated circuit is disclosed in, for example, Japanese Patent Application Laid-Open No. H08-029184 (issued on February 2, 1996). Even for such an open-loop optical fiber gyro, as shown in FIG. 1, all of the optical path on the light source side of the optical integrated circuit is composed of a polarization-maintaining optical fiber, and is disposed between the optical integrated circuit and the optical fiber coil. By inserting a polarization-maintaining single-mode optical fiber, the problems that occur in the substrate-type optical integrated circuit can be similarly solved. However, there is a similar problem of the connection angle between the optical integrated circuit and the polarization-maintaining single-mode fiber.
Proceedings of SPIE, Vol.2292, P.166-176 Proceedings of SPIE, Vol. 2070, P. 152-163

  SUMMARY OF THE INVENTION It is an object of the present invention to provide an optical fiber gyro capable of properly setting light incident on a single mode optical fiber coil from an optical integrated circuit having a function of a polarizer and an optical waveguide to a non-polarized state.

  According to the present invention, light from a light source is made incident on a substrate type optical integrated circuit having a function of a polarizer and a branched optical waveguide through an optical fiber and an optical fiber coupler, and the light branched in the optical integrated circuit is converted into a single light. The clockwise light and the counterclockwise light are incident on both ends of the mode optical fiber coil as clockwise light and counterclockwise light, respectively, and the clockwise light and counterclockwise light propagated through the optical fiber coil are combined and interfere in the optical integrated circuit. In the optical fiber gyro, the light is introduced from the optical fiber coupler into the light receiver to convert the intensity into an electric signal, and the rotational angular velocity about the axis applied to the optical fiber coil is detected from the electric signal. First and second polarization planes in which the direction of the electric field of TM mode and the axis of the individual coincide with the two input / output end faces of the branch optical waveguide of the optical integrated circuit. One end of each of the preserved optical fibers is connected, and the other end of each of the first and second polarization-maintaining optical fibers is connected to one end of each of the third and fourth polarization-maintaining optical fibers with their individual axes shifted by 45 degrees. The other ends of the third and fourth polarization-maintaining optical fibers are connected to both ends of the optical fiber coil, and the third polarization-maintaining optical fiber meets the condition of inequality (1) described in the background section of the invention. And the length of the fourth polarization-maintaining single-mode fiber is 2 L or more.

  According to the above configuration, the 45-degree axis alignment for acting as a depolarizer can be performed by connecting the optical fibers, and this connection is performed by optically observing the stress applying portion of the polarization-maintaining optical fiber from the side. Image analysis is performed while the optical fiber is fusion spliced so as to be at an arbitrary relative angle by a commercially available polarization-maintaining optical fiber fusion splicer. Therefore, the light that has been correctly made unpolarized can be incident on both ends of the single mode optical fiber coil, and an optical fiber gyro with high detection accuracy can be obtained.

The configuration of an embodiment in which the present invention is applied to a closed-loop optical fiber gyro will be described with reference to FIG. In FIG. 4, elements and portions corresponding to those in FIG. 1 are denoted by the same reference numerals, and description thereof will be omitted unless necessary.
The light emitted from the light source 10 is incident on the substrate type optical integrated circuit 14 through the first optical fiber 51, the optical fiber coupler 52, the second optical fiber 53, and the third optical fiber 54. In this embodiment, the first and second optical fibers 51 and 53, and the optical fiber coupler 52 are composed of a single mode optical fiber, the third optical fiber 54 is composed of a polarization maintaining fiber having a length L, One of the individual axes of the third optical fiber 54 is made coincident with the electric field direction Ex of the TE mode of the optical waveguide 15 of the optical integrated circuit 14 (this state is similar to that of the fourth optical fiber 55 and the optical waveguide 15). Will be described with reference to FIG. 5). Here, one unit of the length of the polarization-maintaining single-mode fiber that gives a sufficient group delay time difference between two orthogonal linearly polarized lights so that coherence does not occur in the light from the light source 10 is L.

The substrate-type optical integrated circuit 14 includes a polarizer function and a branch optical waveguide, and is manufactured by a proton exchange method on a lithium niobate (LiNbO ₃ ) optical crystal substrate, for example, as in the above-described prior art. In this example, two optical modulators 16 and 17 are similarly provided. The use of a short polarization maintaining optical fiber for connecting the optical integrated circuit 14 to the optical integrated circuit 14 using a single mode optical fiber as the main light source 10 side optical path has been disclosed by the present inventors in US Patent Application No. 10 / 700,700. 312 proposed.
The light that has entered the Y-branch optical waveguide 15 is split into two light beams, a first light beam and a second light beam. In this embodiment, the first light beam passes through a fourth optical fiber 55 and further passes through a fifth optical fiber 56. The light enters one end of the optical fiber coil 20 and propagates in the optical fiber coil 20 in a clockwise direction. The branched second light is incident on the other end of the optical fiber coil 20 through the sixth optical fiber 57 and further through the seventh optical fiber 58, and propagates through the optical fiber coil 20 in the counterclockwise direction. The light wave propagating in the CW direction and the light wave propagating in the CCW direction in the optical fiber coil 20 are coupled by a Y-branch optical waveguide 15, further branched by an optical fiber coupler 52, input to the light receiver 25, and receive the intensity of the interference light. Is output as an electric signal indicating

  The fourth, fifth, sixth, and seventh optical fibers 55, 56, 57, and 58 are each composed of a polarization-maintaining optical fiber. The fusion points between these optical fibers are shown thick. As shown in FIG. 5, each of the fourth and sixth optical fibers 55 and 57 has its own axis, in this example, the intrinsic axis of the fast phase axis, that is, the arrangement direction 33 of the two stress applying portions 55a and 57a. The perpendicular direction is set to the same direction as the electric field direction Ex of the TE mode of each optical waveguide 15 of the optical integrated circuit 14, and as shown in FIG. 6, each of the fifth and seventh optical fibers 56 and 58 The axis (in this example, the direction perpendicular to each of the arrangement directions 35 of the two stress applying portions 56a and 58a) is shifted by 45 degrees. In the figure, the optical fibers 55 and 56 are slightly shifted for distinction.

Further, in this embodiment, the lengths L2, L3, L4 and L5 of the fourth,..., Seventh optical fibers 55, 56, 57 and 58 are 2L or more, 8L or more, 4L or more and 16L or more, respectively. , The difference between the two lengths is set to 1 L or more.
According to this configuration, since the length of the third optical fiber 54 is equal to or longer than L, the orthogonal linearly polarized lights entering the optical integrated circuit 14 from the third optical fiber 54 are in a state where they do not interfere with each other. . Therefore, even if stray light generated in the above-described substrate in the optical integrated circuit 14 is recombined during propagation in the optical waveguide 15 or when entering the fourth and sixth optical fibers 55 and 57, the recombined light causes The effect is not similar to that shown in FIG.

  The connection between the optical waveguide 15 and the fourth and sixth optical fibers 55 and 57 is such that the electric field direction of the TE mode of the optical waveguide 15 and the individual axis of each polarization-maintaining optical fiber are parallel to each other. The connection is angle-aligned with high precision, and the light incident on the fourth and sixth optical fibers 55 and 57 is substantially only the TE mode light propagating in the fast phase axis of the polarization maintaining optical fiber in this example. is there. The connection between the fourth and sixth optical fibers 55 and 57 and the fifth and seventh optical fibers 56 and 58 is such that the individual axes are shifted by 45 degrees with high accuracy. Accordingly, only the fast-phase linearly polarized light is substantially equally divided into the fast-phase axis and the slow-axis from the fourth and sixth optical fibers 55 and 57 to the fifth and seventh optical fibers 56 and 58. Incident.

  Since the lengths L3 and L5 of the fifth and seventh optical fibers 56 and 58 are each equal to or greater than L and are set to 1: 2, the fifth and seventh optical fibers 56 and 58 are separated from the optical fiber coil. The light incident on each of the optical fibers 20 is correctly in a non-polarized state, acts as a LYOT type depolarizer by the fifth and seventh optical fibers 56 and 58, and is incident on the optical integrated circuit 14 from the optical fiber coil 20. Each light is in the correct unpolarized state. Therefore, these lights are not affected by the recombination of stray light generated in the optical integrated circuit 14. Here, the birefringence of the single mode optical fiber of the optical fiber coil 20 is smaller by about two to three orders of magnitude than the birefringence of the polarization-maintaining optical fiber, and is randomly generated at various places and difficult to measure. As a whole, they have found that the birefringence can be neglected by mutually canceling each other.

  From the above description, the lengths L2 and L4 of the fourth and sixth optical fibers 55 and 57 are determined by the respective connections between the fourth and sixth optical fibers 55 and 57 and the optical waveguide 15, and by the fifth and seventh lengths. The connection may be made as short as possible within a range in which the work of connection with the optical fibers 56 and 58 is easy. In this case, L2 = L4 may be set. Thus, according to this optical fiber gyro, the parallel alignment can be performed relatively easily with relatively high accuracy, and the 45-degree alignment can be performed with high accuracy by using a commercially available connection device. And the accuracy of the detected angular velocity improves accordingly. In this example, the polarization-maintaining single-mode fiber used in the optical path from the optical integrated circuit 14 to the light source 10 can be set to, for example, more than 10 cm, compared with the case shown in FIG. And can be configured at low cost.

  When light enters the third optical fiber 54 from the optical integrated circuit 14, TM mode stray light is incident on the third optical fiber 54, and TE mode including stray light which is mode-converted and recombined is slightly changed to the third optical fiber. There is a possibility that it will be incident as 54 slow axis transmission light. However, in this embodiment, the difference between the sum L2 + L3 of the lengths of the fourth and fifth optical fibers 55 and 56 and the sum L4 + L5 of the lengths of the sixth and seventh optical fibers 57 and 58 is the third optical fiber. Since the length is longer than the length L of the third optical fiber 54, the phase difference between the wave fronts of the fast-phase axis propagation light and the slow-axis propagation light formed by the propagation of each polarization-maintaining optical fiber is different from that of the third optical fiber 54. Even if it becomes smaller due to propagation, there is no possibility that coherence occurs between the fast axis light and the slow axis light emitted from the third optical fiber 54 toward the light source 10, that is, orthogonal linearly polarized light.

Making the direction of the TE mode electric field of the optical waveguide 15 parallel to the fast axis of each of the fourth and sixth optical fibers 55 and 57 can be performed with relatively high precision as described above. However, since this axis alignment is performed by visual recognition, its accuracy is limited. For this reason, the slow axis transmission light of the fourth and sixth optical fibers 55 and 57 is slightly incident from the optical waveguide 15. Therefore, it is preferable to eliminate the influence of these. From this point, it is preferable that the difference between the length L of the third optical fiber 54 and each of the lengths L2 and L4 of the fourth and sixth optical fibers 55 and 57 is L or more.
Further, the fourth and fifth optical fibers 55 and 56 and the sixth and seventh optical fibers 57 and 58 constitute a LYOT type depolarizer, respectively, and are combined with the fourth and fifth optical fibers 55 and 56. The sixth and seventh optical fibers 57 and 58 may constitute a LYOT type depolarizer. In this regard, in this embodiment, L2 = 2L or more, L3 = 4L or more, L4 = 8L or more, L5 = 16L or more, and the difference between any two of these values is L or more. By doing so, an optical fiber gyro having extremely high angular velocity detection accuracy can be configured.

As can be understood from the above description, if the axes of the connections between the third, fourth and sixth optical fibers 54, 55 and 57 and the optical waveguide 15 are correctly aligned, the third optical fiber The group delay difference between the linearly polarized light that undergoes the TE mode coupling with the optical waveguide 15 and the linearly polarized light that undergoes the TM mode coupling at 54 is, for example, when the optical waveguide 15 and the fourth optical fiber 55 are coupled with each other, Even when coupled to the fourth optical fiber 55, the group delay difference between the two linearly polarized lights in the fourth optical fiber 55 is further increased. Therefore, in all cases, it is assumed that L1, L3, and L5 are each equal to or greater than L,
| (L1 + L2) −L3 | > L, | (L1 + L4) −L5 | > L (2)
And to avoid the effect of random birefringence on the propagation of the optical fiber coil 20,
| (L1 + L2) -L3 |-| (L1 + L4) -L5 || > L (3)
And it is sufficient. | Ab | represents the absolute value of the difference between a and b. Alternatively, while satisfying the condition of the expression (2), | L3-L5 | > L, or L3: L5
May be 1: 2 or 2: 1. When one of the two conditions of the equation (2) is equal, it can be said that the other condition may be set to 2L or more. If such a condition is satisfied, L2 = L4 may be satisfied, and L1 among L1 to L5 does not need to be minimized. | L1-L3 | > L, | L1-L5 | > L and || L1-L3 |-| L1-L5 || > L.

  Next, a case will be described in which the influence due to a slight displacement of the axis alignment of each connection between the third, fourth and sixth optical fibers 54, 55 and 57 and the optical waveguide 15 is suppressed. In this case, even if the fast axis polarization component of the third optical fiber 54 is, for example, the slow axis polarization component of the fourth optical fiber 55, and the slow axis polarization component, which has become stray light, is the fast phase axis polarization, only a small amount. Yes, but will propagate. Accordingly, there occurs a component in which the group delay difference between the fast-axis polarization component and the slow-axis polarization component provided by the third optical fiber 54, that is, the phase difference is reduced by the propagation of the fourth optical fiber 55. Therefore, L2 and L4 must also be equal to or larger than L and satisfy the condition of the following equation (4).

| L1-L2 | > L, | L1-L4 | > L (4)
Both linearly polarized lights from the fourth optical fiber 55 are equally distributed on the fast axis and the slow axis of the fifth optical fiber 56, respectively. For this reason, the propagation of the fifth optical fiber 56 generates a component in which the reduced group delay difference (phase difference) is further reduced. Therefore, it is necessary to satisfy the condition of the following equation (5).
|| L1-L2 | -L3 | > L, || L1-L4 | -L5 | > L (5)
Further, in order to avoid the influence of random birefringence on the propagation of the optical fiber coil 20, it is necessary to satisfy the following expression (6).

| L1-L2 | -L3 |-|| L1-L4 | -L5 || > L (6)
That is, if one of the conditions in the equation (5) is satisfied with the equal, the other condition is set to 2L or more. In the shortest case, the relationship may be 1: 2 or 2: 1. It is sufficient that these conditions are satisfied, and L1 does not have to be the minimum value among L1 to L5.
The most important point in the present invention is that the connection between the polarization-maintaining optical fiber and the optical waveguide on the side of the optical fiber coil 20 that is directly connected to the optical integrated circuit 14 does not shift the axis, and the connection that shifts the 45-degree axis does not maintain the polarization plane. The point is that the connection is made between optical fibers. Therefore, all the optical paths of the optical integrated circuit 14 on the light source 10 side may be constituted by polarization-maintaining optical fibers as in the case of FIG. That is, a configuration shown in FIG. 7 with the same reference numerals assigned to portions corresponding to FIG. 1 and FIG. 4 may be adopted.

In this case, the length of the polarization-maintaining optical fiber on the light source 10 side of the optical integrated circuit 14 is as long as, for example, about 1 m or more as described above, and therefore, the group delay difference (phase difference) of the orthogonal polarization generated in this polarization-maintaining optical fiber. ) Is significantly larger than the group delay difference (phase difference) of orthogonally polarized light generated on the optical fiber coil 20 side of the optical integrated circuit 14. Therefore, when the influence of the misalignment of the optical waveguide 15 and the third, fourth and sixth optical fibers 54, 55 and 57 is neglected, the following condition may be satisfied.
L3 > L, L5 > L, | L3-L5 | > L (7)
When it is desired to suppress the influence of the misalignment, the following condition may be satisfied.

| L2-L3 | > L, | L4-L5 | > L
|| L2-L3 |-| L4-L5 || > L (8)
In the above description, the conditions for the length of each polarization-maintaining single-mode fiber are as described above. However, from the viewpoint of economy, it is preferable that each condition is satisfied and the length is as short as possible.
In the above embodiment, the case where the present invention is applied to a closed loop type optical fiber gyro has been described. However, the present invention can be similarly applied to an open loop type optical fiber gyro, and the same operation and effect can be obtained. Needless to say. In the above embodiment, an optical integrated circuit having a Y-branch type optical waveguide manufactured by a proton exchange method was used for an optical crystal substrate of lithium niobate (LiNbO ₃ ). ₃ ). Alternatively, for example, light having a structure in which a certain polarizer is incorporated as a local device in a part of a titanium diffusion type lithium niobate optical waveguide capable of transmitting light in both polarization modes, that is, TE mode and TM mode. An integrated circuit may be used. In short, the optical integrated circuit only needs to have a function of a polarizer and a branch optical waveguide. Each polarization-maintaining optical fiber is not limited to one having two stress applying portions, and other various types can also be used.

In FIG. 4, the optical fibers 55 and 57 are omitted, and the optical fibers 56 and 58 are directly connected to the optical waveguide 15 with their individual axes shifted by 45 degrees with respect to the electric field direction of the TE mode. May satisfy the following expression.
L1 > L, L3 > L, L5 > L, | L1-L3 | > L, | L1-L5 | > L,
|| L1-L3 |-| L1-L5 || > L

FIG. 1 is a block diagram showing an example of a configuration of a closed-loop optical fiber gyro according to the related art. FIG. 4 is an enlarged view showing an end face side of the polarization-maintaining optical fiber in a state where a unique axis of the polarization-maintaining optical fiber is aligned in parallel with a reference plane of a carrier. 3A is a plan view showing a state where the polarization-maintaining optical fiber is held on a carrier and connected to an optical integrated circuit board, and FIG. 3B is a sectional view taken along line 3B-3B in FIG. 3A. FIG. 1 is a block diagram showing an embodiment of the present invention. The figure which shows the relationship between the electric field direction Ex of the TE mode of the optical waveguide 15 of the optical integrated circuit 14, and the axis of the polarization-maintaining optical fiber 55 (or 57). The figure which shows the relationship of the individual axis of the polarization-maintaining optical fiber 55 (or 57) and 56 (or 58). FIG. 9 is a block diagram showing another embodiment of the present invention.

Claims (6)

Light from a light source passes through an optical fiber and an optical fiber coupler in sequence, and is incident on a substrate type optical integrated circuit having a function of a polarizer and a branch optical waveguide, and the light branched in the optical integrated circuit is converted into a single mode optical fiber. The clockwise light and the counterclockwise light are respectively incident on both ends of the coil as clockwise light and counterclockwise light, and the clockwise light and the counterclockwise light propagated through the optical fiber coil are combined and interfere in the optical integrated circuit. In the optical fiber gyro, which is introduced from the fiber coupler into the light receiver and converts the light intensity into an electric signal, and from the electric signal, the optical fiber coil is detected, and an angular velocity applied around the axis is detected.
A first polarization-maintaining optical fiber connected between the optical fiber coupler and the optical waveguide of the optical integrated circuit, the direction of the electric field of the TE mode of the optical waveguide being aligned with a unique axis, and having a length L1;
The other two end faces of the optical waveguide of the optical integrated circuit are connected to one end of each of the optical waveguides in such a manner that the electric field direction of the TE mode of the optical waveguide and the individual axis are combined, and have lengths of L2 and L4, respectively. Second and third polarization-maintaining optical fibers;
The other ends of the second and third polarization-maintaining single-mode fibers are connected to each other with their respective axes shifted by 45 degrees from each other, and the other ends are respectively connected to both ends of the optical fiber coil. And fourth and fifth polarization-maintaining single-mode fibers having lengths L3 and L5, respectively.
All the optical fibers between the light source and the optical integrated circuit except the first polarization-maintaining optical fiber are constituted by single-mode optical fibers,
Assuming that the length that causes the group delay time difference between the orthogonal polarizations of the respective polarization-maintaining optical fibers to exceed the coherence length of the light from the light source is L,
L1 > L, L3 > L, L5 > L
| (L1 + L2) -L3 | > L, | (L1 + L4) -L5 | > L
|| (L1 + L2) -L3 |-| (L1 + L4) -L5 || > L
An optical fiber gyro characterized by satisfying the following conditions. The optical fiber gyro according to claim 1,
| L1-L3 | > L, | L1-L5 | > L
And ||| L1-L3 |-| L1-L5 || > L
An optical fiber gyro characterized by satisfying the following conditions. The optical fiber gyro according to claim 1,
L2 > L, L4 > L
|| L1-L2 | -L3 | > L, || L1-L4 | -L5 | > L
And ||| L1-L2 | -L3 |-|| L1-L4 | -L5 || > L
An optical fiber gyro characterized by satisfying the following conditions. The optical fiber gyro according to claim 3,
L2 > 2L, L3 > 4L, L4 > 8L, L5 > 16L
An optical fiber gyro characterized by the following. The light from the light source sequentially passes through the polarization-maintaining optical fiber and the polarization-maintaining optical fiber coupler, enters the substrate-type optical integrated circuit having the function of the polarizer and the branching optical waveguide, and is branched in the optical integrated circuit. The clocked light and the counterclockwise light propagating through the optical fiber coil are coupled and interfere with each other in the optical integrated circuit. An optical fiber that introduces the interference light from the optical fiber coupler into a light receiver, converts the light intensity into an electric signal, and detects an angular velocity applied from the electric signal to the optical fiber coil around its axis. In the gyro,
The other two end faces and one end of the optical waveguide of the optical integrated circuit are connected so that the direction of the electric field in the TE mode of the optical waveguide is aligned with the individual axis, and the second ends of the lengths L2 and L4, respectively. And a third polarization-maintaining optical fiber;
The other ends of the second and third polarization-maintaining single-mode fibers are connected to each other with their respective axes shifted by 45 degrees from each other, and the other ends are respectively connected to both ends of the optical fiber coil. And fourth and fifth polarization-maintaining single-mode fibers having lengths L3 and L5, respectively.
The group delay time difference between the orthogonal polarizations of the respective polarization-maintaining optical fibers, L is a length that causes the coherence length of the light of the light source to exceed L,
L3 > L, L5 > L, | L3-L5 | > L
An optical fiber gyro characterized by satisfying the following conditions. The optical fiber gyro according to claim 5,
| L2-L3 | > L, | L4-L5 | > L
|| L2-L3 |-| L4-L5 || > L
An optical fiber gyro characterized by satisfying the following conditions.

Images