JPS622121A - Optical fiber gyroscope - Google Patents

Abstract

PURPOSE:To achieve a higher accuracy, by directly monitoring a propagation light with a shifted frequency and that without it to measure the frequency shift corresponding to the angular velocity of rotation from the frequency of an interfering light. CONSTITUTION:A laser light from a light source 11 turns to two pieces of propagation light via a polarizer 19 and a Y branch 172 and converge at the branch 172 being propagated reversely through an optical fiber loop 13 to be inputted into a photo detector 14. In this case, the first propagation light is modulated in the phase with a phase modulator 24 at a fixed frequency and the shifting of the frequency is done with a frequency modulator 23 which is controlled by an error signal to be outputted from the detector 14 based on the results to match the phase between both the first and second propagation lights. This frequency shift of an interference frequency proportional to the angular velocity of rotation is received with a photo detector 30 in a monitor system which directly detects the first propagation light modulated with the modulator 23 and the second propagation light not and measured directly with a frequency measuring device 31 for measuring the frequency shift when the phase coincides between both the propagation lights. This can always maintain a high measuring accuracy free from the effect from aging or the like of the modulator 23 or the like.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical fiber gyroscope that optically measures rotational angular velocity and the like.

[Conventional technology]

2. Description of the Related Art Gyroscopes are used in ships, aircraft, etc. for purposes such as direction measurement during navigation. Conventionally, devices that utilize the inertia of a high-speed rotating body have been used exclusively for this purpose, but many complex mechanisms are required, such as a device to maintain the rotation of the rotating body and a device to detect the direction of the rotational axis of the rotating body. There was a limit to miniaturization and high precision.

On the other hand, with the practical use of laser oscillators, a gyro called a ring laser was developed as part of laser application technology.

As shown in FIG. 4, the laser beams 11 and 12 output from the laser oscillator 1 are propagated in both directions, counterclockwise and clockwise, using mirrors 2 to 24.
, and guides both to the same optical path 3 by means of mirrors 25-27. When this optical system rotates at an angular velocity ω in the direction of an arrow 5 about a point 4, for example, a difference occurs in the optical path length between light traveling in the same direction and light traveling in the opposite direction. As a result, both laser beams 11 and 1
Since a difference occurs in the resonance frequencies of □, these interference lights are detected by the photodetector 6 provided on the optical path 3.

Since the frequency of this interference light is proportional to the rotational angular velocity ω of the optical system, the frequency can be counted by the frequency measuring device 7 and the rotational angular velocity of the optical system can be measured.

Ring lasers that operate on this principle are easy to handle because they have no driving parts, and have already been put into practical use as an alternative to conventional gyroscopes. However,
While this laser oscillator generally has a lifespan of about one hour, some semiconductor lasers, which have been used in various applications such as optical communications in the last nine years, have a lifespan of over one million hours. Therefore, an optical fiber gyroscope constructed from this semiconductor radar and an optical fiber has been developed, and its practical use is progressing.

This propagates clockwise and counterclockwise light in a loop formed by optical fibers.

This optical fiber gyroscope also has a ring type, which is called a passive ring type. In this method, a ring resonator is formed using an optical fiber, and the rotational angular velocity is determined by detecting the difference in resonance frequency between clockwise light and counterclockwise light propagating within the ring resonator.

In addition, there are non-ring types.

In this method, light is propagated from both ends of an optical fiber wound in a loop toward the other end, and the rotational angular velocity is determined from the phase difference between the propagating lights in both directions caused by the rotation. In this non-ring type, various methods such as a phase modulation method, a frequency modulation method, a phase bias method, and a hetero gain method have been developed in order to detect the phase difference of propagating light.

Figure 5 is a block diagram showing an example of such an optical fiber gyroscope that uses a frequency modulation method.
This was introduced on page 120 of WBB4 lecture material of S (Optical Fiber Conference).

This device includes a light source 11 made of a semiconductor laser or the like, a light introduction section 12 that guides laser light outputted from the light source 11 to a predetermined path, and a light source 11 that is connected to the light introduction section 12 at both ends and is wound into a loop. A fiber loop 13 and a photodetector 14 that detects the laser beam output from the light introduction section are provided. Further, the laser beam outputted from the light source 11 is inserted into the light introduction section 12 by the arrow 16. ．． Y branch 17. which distributes in two directions shown at 162. After passing through the optical fiber loop 13, this arrow 16. ．． A Y branch 172 is provided for guiding the light propagated in the opposite direction to the Y branch 171 and collected at the Y branch 171 to the photodetector 14 through the waveguide 18 . These Y branches 517, . 172 is, for example, the third
As shown in Figure a, it is formed by heat-sealing single-mode optical fibers 10 into a figure-7 shape. These two Y branches 17. ．． 1
A polarizer 19 is inserted between 7□.

Here, the light propagating in the direction of arrow 16 in the figure will be referred to as the first propagating light, and the light propagating in the direction of arrow 16□ will be referred to as the second propagating light. In the waveguide 182 for guiding this first propagation light to the optical fiber loop 13, there are two modulation elements 21.
22 is inserted. Each of the modulation elements modulates the radiation by applying an electric field to a piezoelectric crystal through which Rade light passes, for example, using a so-called electro-optic effect. A frequency modulator 23 that causes a frequency shift in the first propagating light is connected to one of the modulation elements 21, and a so-called sawtooth wave is output from the frequency modulator 23. Also,
A phase modulator 24 that shifts the phase of the first propagation light is connected to the other modulation element 22, and this phase modulator 24
outputs a sine wave with a constant frequency. For example, when the wavelength of the propagating light is 0.85 micrometers (μm), the frequency modulator 23 has a frequency of 0 to 100 kilohertz CK.
Hz), and the phase modulator 24 can change the amount of phase shift to 0, for example.
It is changed at a constant period within the range of ~2π.

Further, the frequency modulator 23 is connected to receive a detection signal 141 photoelectrically converted by the photodetector 14, and a monitoring device that monitors the frequency of the sawtooth wave output from the frequency modulator 23 over time. 25 are provided.

Such a fiber optic gyroscope operates as follows.

First, the laser light outputted from the light source 11 passes through the waveguide 18.
3, is distributed to two waveguides 18□, 184 via a polarizer 19 and a Y branch 17□, propagates clockwise and counterclockwise through the optical fiber loop 13, and returns to a Y branch 17. The light is collected at the polarizer 19, the Y branch 17□, and the waveguide 181, and then input to the photodetector 14. When this optical fiber gyroscope rotates in the direction of arrow 5 about point 4 at the center of optical fiber loop 13, it is positioned between the clockwise first propagating light and the counterclockwise second propagating light. A phase difference occurs, and predetermined interference light is output from the waveguide 181 in addition to the first propagation light and the second propagation light. The photodetector 14 photoelectrically converts this light and generates a detection signal 14. and this is the frequency modulator 23
send to. Frequency modulator 23 receives this detection signal 14. Based on this, a necessary frequency shift is applied to the first propagating light so that the phases of the first propagating light and the second propagating light match.

When the rotational angular velocity of the optical fiber gyroscope changes, the amount of frequency shift to be added to the first propagation light changes accordingly. The monitoring device 25 determines the temporal change in the rotational angular velocity of the optical fiber gyroscope from the change in the oscillation frequency of the frequency modulator 23, and records this or outputs it to another device.

Here, the phase modulator 24 is provided to facilitate detection of a state in which the phases of the first propagating light and the second propagating light match accurately.

That is, the phase modulator 24 applies phase modulation to the first propagating light at a constant frequency. When the phases of the first propagating light and the second propagating light completely match, when phase modulation is applied, an error signal having a frequency twice the modulation frequency is output from the photodetector 14. However, if the phase of the first propagating light is shifted to either side with respect to that of the second propagating light, an error signal having the same frequency as the phase modulation frequency is output from the photodetector 14. If the phase is shifted in the opposite direction, a similar signal but an error signal with a reversed phase is output. Therefore, the frequency shift amount of the first propagation light is always tracked so that the photodetector 14 does not output an error signal having the same frequency as the phase modulation frequency.

In this way, the temporal change in rotational angular velocity can be determined with extremely high accuracy.

[Problem that the invention seeks to solve]

Now, in such an optical fiber gyroscope, the amount of frequency deviation of propagating light is proportional to the frequency of the sawtooth wave output from the frequency modulator. However, the proportionality coefficient depends on the amplitude of the sawtooth wave and the characteristics of input voltage versus refractive index change of the modulation element.

Therefore, if there is a change in these over time, for example, a detection error in the rotational angular velocity will occur.

The present invention has been made to solve the above problems, and an object of the present invention is to provide an optical fiber gyroscope that can detect rotational angular velocity with higher accuracy.

[Means for solving problems]

The optical fiber gyroscope of the present invention includes a light source, an optical fiber loop, and distributes the output light of the light source into a first propagating light and a second propagating light, and transmits both to the optical fiber loop, respectively. a light introduction part that propagates in the optical fiber loop; a photodetector that detects the first propagated light and the second propagated light that have been propagated through the optical fiber loop after guiding them to the same waveguide;
a frequency modulator that causes a frequency shift in the first propagating light or the second propagating light, and the first propagating light and the first propagating light are matched in phase by the photodetector. , and calculate the rotational angular velocity from the amount of frequency deviation at that time. Then, the propagating light before the frequency deviation occurs and the propagating light with the frequency deviation are guided to the same waveguide. The present invention is characterized in that it is provided with a monitor line and a frequency measuring device that is connected to the monitor line and measures the amount of frequency deviation from the frequency of the interference light outputted from the monitor line.

Here, a light guide section for propagating the first propagation light and a light guide section for propagating the second propagation light are provided in the light introducing section, and the frequency modulator is provided for each of the light guide sections, respectively. Furthermore, a monitor line may be provided that guides the propagating light input to each frequency modulator and the propagating light output therefrom to the same waveguide, and the frequency measuring device may be connected to each monitor line.

[Effect]

In such an optical fiber gyroscope, when it rotates and a phase difference occurs between the first propagating light and the second propagating light, a frequency modulator is used to change the first propagating light. Alternatively, a frequency shift is caused in the second propagation light to cancel out the phase difference between the two. The point where the phases match is detected using a photodetector. When the phases match in this manner, the propagating light before the frequency shift and the propagating light after the frequency shift are taken out from the monitor line, and the frequency of the interference light is measured. The frequency of this interference light corresponds to the net frequency shift amount of the propagating light. Thereby, the rotational angular velocity can be detected with high accuracy without being influenced by the characteristics of the frequency modulator, the characteristics of the modulation element, etc.

〔Example〕

(First Embodiment) FIG. 1 is a block diagram showing an embodiment of the optical fiber gyroscope of the present invention. Here, the same parts as in FIG. 5 are denoted by the same reference numerals, and redundant explanation thereof will be omitted.

The optical fiber gyroscope of this embodiment differs from that of FIG. 5 in the following points. First, the frequency modulator 23
There is no monitoring device installed to read the oscillation frequency. Furthermore, the intermediate portion between the two modulation elements 21 and 22, and the waveguide 18. Y branch 17. which branches the propagating light in the directions of arrows 16□ and 162, respectively. ．． 17. is provided.

This branched propagation light is guided to one waveguide 185 at the Y branch 175 and is configured to be input to the photodetector 30. In the present invention, these three Y branches 1
73-17. The portion consisting of the waveguide and the waveguide 18° will be referred to as the monitor line 29. This photodetector 30 also uses a photoelectric conversion element similar to the photodetector 14, and its detection output signal 30. is input to the frequency measuring device 31,
The monitoring device 32 is configured to read this measured value.

Such a fiber optic gyroscope operates as follows.

The laser light is emitted from the light source 11 and passes through the waveguide 183,
The Y branch 171 passes through the polarizer 19 and the Y branch 172 and is distributed into first propagating light and second propagating light. The first propagating light is
Arrow 16. The light passes through the waveguide 182, the modulation element 21, and the Y branch 173, and is again split into two parts, one of which propagates toward the optical fiber loop 13 and the other toward the photodetector 30. On the other hand, the second propagating light passes through the waveguide 18 in the direction of the arrow 16□. From Y branch 17. , where it is split into two parts, one propagating toward the optical fiber loop 13 and the other propagating toward the photodetector 30 .

The first propagating light that propagated clockwise in the optical fiber loop 13 reaches the Y branch 172 via the waveguide 184, and the second propagating light that propagated counterclockwise in the optical fiber loop 13 passes through the waveguide 182. A Y-branch 172 is reached. Here, the first propagating light and the second propagating light merge into the waveguide 18. is input to the photodetector 14.

Here, the frequency modulator 23 has the following functions for the first propagating light:
A frequency shift is applied so that the phases of the first propagating light and the second propagating light match. Then, the phase modulator 24 phase-modulates the first propagating light at a constant frequency, thereby outputting an error signal 14. from the photodetector 14. Based on this, the frequency modulator 23 performs tracking so that the phases of the first propagating light and the second propagating light match accurately. This operation is also similar to that previously explained using FIG. 5. At this time,
The first propagating light that has been subjected to a frequency shift and the second propagating light that reaches here directly via the Y branch 172 and the Y branch point 174 are introduced into the monitor line 29, and the interference light of both is input to the detector 30. Arrow 16 at Y branch 17□. ．．
Since the first propagating light and the second propagating light distributed in 162 directions have exactly the same phase, the photodetector 30 receives the first propagating light before inputting to the frequency modulator 23 through the monitor line 29. The result is the same as when the first propagating light and the first propagating light, which is to be output after causing a frequency shift, are input.

That is, the frequency of the interference light corresponds to the net frequency shift amount of the first propagation light.

Therefore, when the phases of the first propagating light and the second propagating light propagating through the optical fiber loop 13 completely match, the frequency measuring device 31 counts the frequency of the interference light input to the photodetector 30, and A monitoring device 32 reads the frequency and determines the rotational angular velocity. In this way, the rotational angular velocity can be detected in the same manner as described with reference to FIG. In addition, this optical fiber gyroscope measures the amount of frequency shift by directly extracting the first propagating light before the frequency shift occurs and the first propagating light after the frequency shift occurs, so it has a high Accurate measurement is possible.

(Second Embodiment) FIG. 2 is a block diagram showing another embodiment of the optical fiber gyroscope of the present invention.

In this embodiment, the rotational angular velocity can be detected even when the entire gyro rotates to the left or right as shown by the arrow 5 about the axis 4. This is suitable for use in, for example, an automatic flight recorder for aircraft.

The difference between the embodiment shown in this figure and the embodiment shown in FIG. 1 is that the frequency modulator 23. ．． 23□ and phase modulators 24, . 24□
and 1 for the first propagating light and the second propagating light, respectively.
Furthermore, a monitor line 29. ．． 29°, photodetector 301, 302 and frequency measuring device 31□,
31□ is also being expanded accordingly.

Further, a bidirectional coupler 27 is used to introduce the first propagation light and the second propagation light into the optical fiber loop 13. For example, as shown in Figure 3,
A known leading wave element is used, which is formed on a substrate 35 and has a waveguide 36 formed in an X-shape.

The fiber optic gyroscope of this embodiment operates as follows.

First, when this rotates in one direction about the axis 4, only the frequency modulator and the phase modulator prepared for modulating one of the propagating lights operate depending on the direction of rotation.

For example, here, the waveguide 182 is connected to the modulation element 21.122□
It is assumed that a frequency shift is applied to the first propagating light that propagates in the order of . At this time, the first propagating light is transmitted through the modulation element 21,
When passing through the frequency modulator 23. When it passes through the modulation element 221, it is modulated by the phase modulator 241.
It is modulated by

This first propagation light is transmitted to the modulation element 21. Y branches 17s and 17-s are provided immediately before and after the input to the monitor line 2, and together with the Y branch 177, the same monitor line 2 as described above is provided.
9. It consists of This monitor line 29. The photodetector 30° photoelectrically converts the interference light output from the frequency measuring device 31. Measure this by. The rotational angular velocity is detected in the same manner as explained below. Furthermore, when the optical fiber gyroscope rotates in the opposite direction, a frequency shift is applied to the second propagating light that propagates through the waveguide 181 in the order of modulation elements 21□ and 22□. This frequency deviation amount is measured at a monitor line of 29 degrees. This monitor line 29□ has Y branches 178.17, . 17. .

However, since the following operation description is similar and will be redundant, it will be omitted. In this way, the monitoring device 32
By reading the measurement data of the two frequency measuring devices 31□ or 31□, the rotational direction and rotational angular velocity can be detected.

[Modified example]

The optical fiber gyroscope of the present invention is not limited to the above embodiments.

As the light source, various light emitters capable of extracting interference light from an optical fiber loop, such as a gas laser or a solid-state laser, can be used in addition to a semiconductor laser.

In addition to the light introducing section that guides the first propagating light and the second propagating light to the optical fiber loop, in addition to the one shown in FIG. ), or one using a crystal 37 having a structure to branch light, a mirror 38, etc. (see d and e in the figure).

Further, as the optical fiber loop, various types of fibers that can propagate light with low mode dispersion and good coherence, such as a single mode fiber or a polarization maintaining fiber, can be used. The configurations of the frequency modulator, photodetector, and frequency measuring device may also be replaced with various known devices.

Furthermore, the light introduction section may have an integral structure in which the entire light introduction section including the monitor line is formed by combining a predetermined waveguide 36 on one substrate 35 as shown in FIG. 3C. . In this way, this part can be made smaller and its reliability can also be improved. Furthermore, the configuration, branching position, branching direction, etc. of the optical path in this light introducing section do not necessarily have to be as in the above embodiment. That is,
The frequency modulator may be mounted anywhere as long as the first propagation light or the second propagation light can be individually modulated.

〔Effect of the invention〕

The optical fiber gyroscope of the present invention described above applies a frequency shift to one of the first propagating light and the second propagating light that are propagated in opposite directions in the optical fiber loop. When the phases match, the propagating light before and after the frequency shift is directly taken out and the frequency of the interference light is measured, so the amount of frequency shift can be detected very accurately. Moreover, this allows detection of rotational angular velocity with higher accuracy.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing an embodiment of the optical fiber gyroscope of the present invention, Fig. 2 is a block diagram showing another embodiment of the optical fiber gyroscope of the invention, and Figs. 3 a to e are used for this. FIG. 4 is a conceptual diagram of a conventional ring laser gyroscope, and FIG. 5 is a block diagram of a conventional optical fiber gyroscope. 11...Light source, 12...Light introduction part, 1
3... Optical fiber loop, 14.30... Photodetector, 23... Frequency modulator, 24... Phase modulator, 29... ...Monitor line, 31...Frequency measuring device. Figure 3

Claims (1)

[Scope of Claims] 1. A light source, an optical fiber loop, and distributing the output light of the light source into a first propagating light and a second propagating light, and transmitting both to the optical fiber loop in mutually opposite directions. a photodetector that detects the first propagated light and the second propagated light that have propagated through the optical fiber loop after guiding them to the same waveguide; a frequency modulator that causes a frequency shift in the light or the second propagating light, and the photodetector detects a phase match between the first propagating light and the second propagating light; A monitor line that guides the propagating light before the frequency shift occurs and the propagating light after the frequency shift occurs to the same waveguide, and this monitor line 1. An optical fiber gyroscope, comprising: a frequency measuring device that is connected to a frequency measuring device that measures the amount of frequency deviation from the frequency of interference light that will be output from the optical fiber gyroscope. 2. A waveguide for propagating the first propagation light and a waveguide for propagating the second propagation light are provided in the light introducing section,
The frequency modulator is provided on each of the frequency modulators, and a monitor line is provided that guides the propagating light input to each frequency modulator and the propagating light to be outputted from the frequency modulator to the same waveguide, and the frequency measuring device is provided for each monitor line. 2. The optical fiber gyroscope according to claim 1, wherein: