JPS5967414A - Optical fiber gyroscope - Google Patents

Abstract

PURPOSE:To make it possible to form a device of an integrated circuit composed of a simple optical system having few optical components, by constructing optical paths of lights falling on and coming out of an optical fiber loop so that they do not overlap one upon another. CONSTITUTION:A light being emitted from a semiconductor laser 102, passing through an optical wave-guide 105 and a Y-branch wave-guide 107 on a Z-cut niobic acid lithium single crystal plate 101, being transmitted through a directional coupling type wave- guide 109 on the same plate and falling on an optical fiber loop 103 in a TE mode turns round along the loop 103 clockwise, falls on a directional coupling type wave- guide 110 thereafter in a TM mode and proceeds to a Y branch 108. Meanwhile, a light proceeding from the Y branch 107, passing through the wave-guide 110 and falling on the loop 103 in the TE mode turns round along the loop counterclockwise and falls on a wave-guide 109 in the TM mode, and it proceeds to the Y branch 108. The light procedding from the Y branch 108 passes through a wave-guide 106 and is detected by an optical detector 104. The rotation of the loop 103 is detected from a change in the quantity of light reaching the optical detector 104. Thus, the device is formed of an integrated circuit composed of a simple optical system having few optical components.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a gyroscope, and more particularly to an optical fiber gyroscope constructed from an optical fiber, a simple integrated optical circuit, and a trap.

A gyroscope (gyro), which detects the rotational movement of a moving object, is an essential and important component of navigation equipment used in various aircraft, ships, and the like.

The gyro system currently in general use is a mechanical type that utilizes the fact that when a shaft is rotated at high speed, the axis of rotation maintains a constant direction in inertial space. However, this type of gyro has an extremely short lifespan because it uses high-speed mechanical rotation, requires high-precision machining technology, and is expensive.

In recent years, as optical fibers have become less optically structured than mechanically, it has become possible to obtain sufficiently high transmittance even when the fibers are made long. Therefore, light is input from both sides of the input and output ends of a circularly wound optical fiber, and the difference in position of the output light is measured by interference, and the rotational angular velocity being received by the optical 7 eyeper is detected. Development of fiber gyro is being promoted. The basic optical system of the optical fiber laser gyro is as shown in FIG. 1, where 1 is a laser, 2 is a long single mode optical fiber, 3 is a photodetector, and 4 is a beam splitter. The oscillation light of the laser 1 is split into two by a beam splitter 4, and each of the two split lights is inputted from both ends of a single mode optical fiber 20. The light propagating clockwise through the loop of the chapter-mode optical fiber 2 is again interfered by the beam splitter 4, and the intensity of the interference light is measured by the photodetector 3. This light? When the system rotates at an angular velocity of Ω in the plane of a single mode optical fiber loop, a phase difference of Δ0−4πL⇠/(cλ) Ω (1) occurs between the light waves in both left and right directions. where Ll is the length of the single mode optical fiber and the radius of the loop J), and CIλ is the speed and wavelength of the light wave in vacuum. Sensitivity can be increased by using long optical fibers. The interference light intensity P is p ccl −) cc) sli
It is given by (2). By detecting this interference light intensity, the phase difference Δg and the rotational angular velocity Ω obtained from this can be measured.

When the rotational angular velocity experienced by the optical fiber loop is extremely small, that is, when the phase difference between the left and right optical waves is extremely small, (2) As can be seen from Nika et al., the interference light intensity P is a cosine function of △g. Therefore, the change is small and the sensitivity is extremely low.

One of the methods usually considered to solve this problem is to apply a phase difference bias of 90° to the phase difference between the left and right light waves shown in equation (1), and to apply the interference light intensity in equation (2). The goal is to maximize the sensitivity of To achieve this with an optical system, you can either insert a non-reciprocal phase shifter with different amounts of phase change, as shown in Figure 1, or, if a reciprocal phase shifter is used, use multiple beam splitters or reflectors in the optical path. A method is being considered in which the phase shift is applied only to light traveling isometrically in one direction using a complex configuration. Both methods are susceptible to disturbances such as a decrease in the amount of detected light due to light beam splitting, changes in ambient temperature, and vibrations, making it difficult to detect rotational speed with high sensitivity. There is also a method of heterogain detection by inserting an acousto-optic element instead of a reciprocal phase shifter, but this also has the ability to split multiple light beams and detect insensitivity. My boyfriend thinks that the system is very complicated.
It is not a practical construction method that can withstand various disturbances in the surroundings. Also, the configuration of the optical system is complicated, but
It is also difficult to realize this with a waveguide optical integrated circuit that is less susceptible to surrounding disturbances.

SUMMARY OF THE INVENTION An object of the present invention is to provide an optical fiber gyro that has a simple optical system with fewer optical components and can be easily constructed using an optical integrated circuit.

According to the present invention, there is provided a laser, a first branching waveguide that splits the output light of the laser into two guided waves, and a first branching waveguide that transmits the first guided light split by the first branching waveguide. A directionally coupled optical waveguide, a second directionally coupled optical waveguide that transmits the second guided light, and a first guided light that has passed through the first directionally coupled optical waveguide are incident from the first end surface. and the second guided light transmitted through the second directionally coupled optical waveguide is made incident from the second end surface, and the light is arranged such that the polarization of the incident light and the polarization of the output light are orthogonal to each other. a fiber loop, a guided light that is emitted from a first end face of the optical fiber loop and that is emitted from a waveguide different from a waveguide into which the first guided light of the first directionally coupled optical waveguide is incident; The second end face force of the optical fiber loop is emitted from the second end face of the optical fiber loop, and the second
a second branching waveguide that interferes with guided light emitted from a waveguide different from the waveguide into which the second waveguide light enters the directional metal-type optical waveguide; and an output of the second branched waveguide. An optical fiber gyro with high sensitivity and precision can be obtained by a photodetector for detecting incident light and a means for providing a phase difference or a frequency difference between two guided light beams entering or exiting the optical fiber loop.

The details of the present invention will be further explained based on embodiments and with reference to the drawings. FIG. 2 is a diagram showing the principle of groove formation in one embodiment of the present invention.
1 is a 7-of-(1+) lithium single crystal plate, 102 is a semiconductor laser, 103 is an optical fiber loop, 104 is a photodetector, and 1o5 to 110 are channel-type optical waveguides formed on the surface of a lithium niobate crystal. , 105.10
6 is a single channel waveguide, 107 and 108 are Y branch waveguides, 109 and 110 are directionally coupled optical waveguides, 112
is a metal film that gives a phase shift to the guided light. The oscillation light of the semiconductor laser 102 is guided as a Tl13 mode in the waveguide 105, and is split into two branch waveguides with approximately equal intensity at the Y branch 107. One of the two branch waveguides is connected to a directionally coupled waveguide 109, and the other is connected to another directionally coupled optical waveguide 110. In the directional coupling waveguides 109 and 110, the TI3 mode is set to traverse the same waveguide as the incident waveguide, and the 1M mode is set to move the optical path to an adjacent waveguide provided close to each other./ρ2 Ru. The light is branched at the Y-branch waveguide, enters one end of the directional coupling groove, and circulates around the loop clockwise. on the other hand,
From the Y-branch waveguide 107 to the directionally coupled waveguide 11
The TE mode which is guided to 0 and has a force of 1 is the directionally coupled waveguide 1.
10, enters the other end of the optical fiber loop 103, and goes around the loop counterclockwise. The optical fiber loop 1103 is installed in the same manner as described in the description of the first and second embodiments.

That is, the optical fiber loop 103 is composed of an optical fiber such as a polarization-maintaining fiber in which the relationship between the polarization of the input light and the polarization of the output light is fixed, and the polarization of the input light and the polarization of the output light are fixed. are arranged so that they are orthogonal.

The light that passes through the directionally coupled waveguide 109 from the Y branch 107 and enters the optical fiber loop 103 in TE mode goes around the loop clockwise, and then passes through another directionally coupled waveguide connected to the other end. 110 as a 1M mode. The 1M mode incident on the directional coupling waveguide 110 transfers energy to the adjacent waveguide, and the Y-branch waveguide 10
Head to 8.

On the other hand, Y branch 107 nine directionally coupled waveguides 110
The light that passes through the optical fiber loop 103 and enters the optical fiber loop 103 in TE mode goes around the loop counterclockwise, and then enters the directionally coupled waveguide 109 connected to the other end as an 'rM mode. The 1M mode incident on the directional coupling waveguide 109 transfers to a channel adjacent to the channel into which it entered, and heads toward the Y-branch waveguide 108. The straight waveguide that connects the directionally coupled waveguide 109 and the Y-branch waveguide 108 and the straight waveguide that connects the directionally coupled waveguide 110 and the Y-branch waveguide 107 intersect at 111, but the intersection angle is If the angle is set to about 4° or more, the amount of light traveling through each straight waveguide will leak into the other waveguide, and the light traveling from the two branch waveguides will be the same. If the phases are in phase, they merge and proceed to a single waveguide 106, and if they are in opposite phases, they are emitted as synchrotron radiation into the crystal substrate, and the light traveling through the waveguide 106 has an interference function in which it disappears. However, the light propagates through two branch waveguides and reaches the waveguide 106 depending on the phase relationship of the light, and the amount of light detected by the photodetector 104 changes. The phase shifter 112 is a metal film placed on the waveguide.

When a material with optical properties different from air, such as a dielectric or a metal film, is placed on the waveguide, the phase velocity of the guided light propagating through the waveguide will change from that when the waveguide surface is air. do. By adjusting the length of the metal film in the light transmission direction, an appropriate phase difference can be provided between the light waves propagating through the two branch waveguides of the Y branch waveguide 107.

When rotation is applied to the optical fiber loop 103, a phase difference occurs between the light waves traveling in the clockwise and counterclockwise directions, and depending on the phase difference, the optical cover that exits the Y-branch waveguide and reaches the photodetector is The change in rotational angular velocity is detected.

As explained above, the optical paths of the light emitted from and incident on the optical fiber loop are separated spatially, so it is possible to apply a phase bias to only the light waves traveling in one direction, making it extremely easy to use optical fibers. system can be constructed.

Furthermore, since there are almost no light waves returning to the laser 1, the oscillation of the laser does not become unstable, and the rotational angular velocity can be detected with extremely low noise.

Differences in transmission characteristics depending on the mode of a directionally coupled optical waveguide occur as follows.

Even if the waveguide channel part and the surrounding medium are optically isotropic, the equivalent refraction skewer of the TE mode (the value obtained by dividing the imaginary carrier number of the guided human wave by the wave number in air) is T M
It is larger than that of the mode, and the mode confinement within the waveguide is strong. Therefore, when a directionally coupled groove waveguide is formed, the complete coupling length, that is, the propagation distance required for all the energy to transfer from one waveguide to the other nearby waveguide, controls the refractive index of the channel part, and the coupling part By setting the length, when the TE mode is input, more than 90% of its energy will go straight through the same channel as the input waveguide, and only 10% or less will be coupled to the adjacent waveguide, allowing the TM mode to enter. Then, it is possible to easily create a state in which 90% or more of the waveguide goes straight to the adjacent waveguide and 10% or less of the waveguide goes straight. 1
If the leakage light of 0% or less results in a loss of 5 degrees in the measurement, this is removed as follows. In FIG. The TB mode light absorbed in the waveguide 109 slightly transfers its energy to the adjacent waveguide, but at the end 11 of the waveguide.
4, the light can be scattered and radiated outside the crystal substrate 101. This is similarly done by the end portion 115 for the TN mode light that travels through the Y-branch waveguide 107 to another directionally coupled groove waveguide 110. Furthermore, most of the TM wave that enters the directionally coupled groove waveguide 109 from the optical fiber loop 103 transfers its energy to the adjacent waveguide at the coupling portion and proceeds to the Y-branch waveguide 108, but a small amount of energy does not propagate straight. , through the Y-branch waveguide 107, and may re-inject into the semiconductor laser 102, making the oscillation characteristics of the laser unstable and causing noise. The wave path] is absorbed by the metal film 112 provided on the way to 07. In addition, from the optical fiber loop 103 to the directional coupling groove waveguide L1
3, the directionally coupled groove waveguide 10
By providing a metal film on the waveguide from 3 to the Y-branch waveguide IQ7, BH6q can be changed.

The phase shift function can be performed without any problem by providing the two metal films 112 and 113 with different lengths in the light traveling direction.

In the description of this embodiment, a case has been described in which the phase shifter is provided on the propagation path of the Ill I) mode from the semiconductor laser force) to the optical fiber loop.
A dielectric film or the like may be loaded on the TM mode propagation path from 09.110 to the Y branch waveguide lo8. In addition, in Tip 5 above, it was described that the phase shifter should provide a fixed phase shift, but since the substrate crystal has an electro-optical effect, polarization can be achieved by applying an electric field on the crystal. A variable phase shift can be provided. It is known that if the applied electric field waveform is a sawtooth waveform, a wavelength shifter can be obtained. This also allows heterodyne detection.

Furthermore, although a case has been described in which a four-Y branch waveguide is used for the function of splitting the semiconductor laser light and the function of interfering with the light transmitted through the optical fiber, it is of course possible to use a directional coupling groove waveguide instead of the Y-branch waveguide. Two photodetectors may be provided in each of the two output side waveguides of the directionally coupled groove waveguide to differentially detect the interference light. The optical fiber used in the optical fiber loop is a polarization-maintaining fiber, but the optical fiber installed in the optical fiber gyro device is fixed and does not move due to vibration etc., and the length is still several hundred. Since it is relatively short, from m to 'lft7jl km or less, a normal single mode fiber can be used instead of a polarization maintaining fiber. This is because, as long as the fiber is short and there is no spatial positional movement, the relationship between the input polarization of light that passes through a single mode fiber and the polarization at the time of output is fixed, and the polarization at the time of output can be controlled. b et al. That is, when linearly polarized light is incident on the -carrier, it does not emerge in an unpolarized state, but generally becomes elliptically polarized light.

As is well known, by forming one or more small loops in a single mode fiber with a diameter of several tens of mm, and rotating the loop around the loop diameter, the loop can be adjusted as desired. Can you generate linearly polarized light that oscillates in the direction of

In addition, although the case where an electro-optic crystal is used for the substrate has been described in the explanation of this embodiment, paraelectric materials such as glass, light sources, photodetectors, and even optical circuits can be integrated on the same substrate. It is also possible to use a body material or the like.

As described above, in the optical fiber gyro of the present invention, the optical paths entering and exiting the optical fiber loop do not shift or overlap, so it is easy to insert the phase shift means into the optical path. In addition, there is little loss of light and the level of light received by the photodetector is high. Therefore, it is possible to detect the rotational angular velocity with high sensitivity and accuracy.

Furthermore, since the optical configuration is simple and integrated, the optical components do not shift due to changes in ambient temperature and are stable.

[Brief explanation of the drawing]

Figure 1 is a diagram explaining the basic principle of an optical fiber gyro, in which 1 is a laser, 2 is an optical fiber loop, 3 is a photodetector,
4 is a half mirror. FIG. 2 is a diagram showing the basic structure of one embodiment of the present invention, in which 101 is a substrate, 102 is a laser, 103 is an optical fiber loop, 1
04 is a photodetector, 107.108 is a Y branch optical waveguide, l
Q9.110 is a directional coupling type optical waveguide, and 112 is a phase shifter.

Claims (1)

[Claims] A laser, a first branching waveguide that splits the output light of the laser into two four-wave lights, and a first directionally coupled optical waveguide that transmits the first guided light split by the first branching waveguide. , a second directionally coupled optical waveguide that transmits the second waveguide light, and a first directionally coupled optical waveguide that transmits the first directionally coupled optical waveguide.
Sakaki 1-wave light is made to enter from the first end surface, and the second waveguide light that has passed through the second directionally coupled optical waveguide is made to be made to enter from the second end surface, and the polarization of the incident light and the output light are different. The optical fiber loop is arranged such that the polarized light is perpendicular to the first end face force λ of the optical fiber loop, and the twelfth guided light of the first directionally coupled optical waveguide enters the optical fiber loop. The guided light emitted from a waveguide different from the waveguide and the second end face of the optical fiber loop are also emitted, and the waveguide into which the second guided light of the second directionally coupled optical waveguide is incident. A second branching waveguide that interferes with guided light emitted from different waveguides, a photodetector that detects the light emitted from the second branching waveguide, and two guideways that enter or exit the light 7 eye pulses. An optical fiber gyro comprising means for imparting a phase difference and a frequency between waves and light.