JPH063155A - Fiber optic gyro - Google Patents

Abstract

(57) [Abstract] [Purpose] In a phase modulation type optical fiber gyro, the output of the light receiving element is synchronously detected to stabilize the phase modulation degree ξ, and ξ becomes 5 when the double harmonic is controlled to 0. It is too big to be 2. Control ξ to a smaller value. [Structure] Two branching elements are provided in the optical paths at both ends of a fiber coil to extract phase-modulated light and non-phase-modulated light, interfere with each other, determine their intensities, and perform synchronous detection with a harmonic of phase modulation. The amplitude b of the phase modulation is made constant by setting the value of the synchronous detection to 0 or by setting the relationship between the outputs of the synchronous detection to be constant. It is possible to control ξ to a smaller value.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber gyro for measuring the rotational angular velocity of a moving body such as an automobile, an airplane or a ship. In particular, the present invention relates to an optical fiber gyro that can control a phase modulation degree of a phase modulator to be constant in an optical fiber gyro of a phase modulation system.

[0002]

2. Description of the Related Art An optical fiber gyro is used to obtain an angular velocity by utilizing the fact that the phase difference of light propagating counterclockwise and clockwise in a fiber coil is proportional to the angular velocity of the coil. The phase modulation method is a method in which a part of an optical fiber near one end of a fiber coil is expanded and contracted to modulate the phase of light propagating therein. When the fiber coil is rotating, the optical fiber gyro has a phase difference Δθ between the clockwise light and the counterclockwise light, which is proportional to the rotational angular velocity Ω _c.
Occurs, and this interferes with the light receiving element, and the intensity of the interference light changes, so that the angular velocity is obtained. In the case of the phase modulation method, the output of the light receiving element is synchronously detected using a carrier having an integral multiple of the modulation frequency. Then, the fundamental wave and harmonic components are obtained.

The odd-order (2m + 1) times higher harmonic wave (including the fundamental wave) after synchronous detection can be written as 2P ₀ J _{2m + 1} (ξ) sin Δθ (1). The even-order 2n times higher harmonic wave after synchronous detection can be written as 2P ₀ J _2n (ξ) cos Δθ (2). For example, the fundamental wave is 2P ₀ J ₁ (ξ) sin Δθ (3)

However, P ₀ is given as the square of the left-handed light and the right-handed light assuming that they have the same amplitude. In other words, it is the amount of light. J _{2m + 1} (ξ) is the (2m + 1) th order Bessel function, J _2n
(ξ) is a 2n-order Bessel function. Δθ is the phase difference between the clockwise light and the counterclockwise light, and this is the target to be obtained.
When the angular velocity of the rotating body is Ω ₀ and the phase difference between the clockwise light and the counterclockwise light is Δθ, there is a relationship of Δθ = 4πLaΩ ₀ / cλ (4). L is the total length of the fiber of the fiber coil. a is the radius of the fiber coil, c is the speed of light in vacuum, and λ is the wavelength in vacuum.

Ξ represents the magnitude of modulation, and ξ = 2bsin (LnΩ / 2c) (5). b is the amplitude of the phase modulation in the phase modulator, Ω is the phase modulation angular frequency, and n is the refractive index of the fiber. ξ is a term generated when the timings of receiving the phase modulation in the left-handed light and the right-handed light differ by Ln / 2c. In order to distinguish ξ from b, ξ is referred to as a phase modulation degree and b is referred to as a phase modulation amplitude.

Since the odd harmonics include Δθ in the form of sin Δθ, the phase difference Δθ can be obtained if the synchronous detection output is known. For example, the phase difference Δθ can be obtained from only the fundamental wave. Alternatively, Δθ can be obtained in the form of tan Δθ by dividing the fundamental wave component by the even harmonics. As for the phase modulation type optical fiber gyro, Japanese Patent Application No. 1-
57634-37, Japanese Patent Application Nos. 1-291628-31, 1
-295500, Japanese Patent Application Nos. 2-3809, 2-1005
Inventions such as 5, 2-225611 to 19 have been made.

[0007]

In order to be able to accurately obtain the phase difference with the phase modulation type optical fiber gyro, the modulation degree ξ of the phase modulator must be constant. This is because all the outputs of the coherent detection include Bessel functions, which include the degree of phase modulation as a parameter. If the degree of phase modulation changes, the value of the Bessel function also changes, so the scale factor changes.

The modulation degree ξ is proportional to the amplitude b of the phase modulation.
The phase modulator is, for example, one in which an electrode is attached to the end surface or inner and outer peripheral surface of a cylindrical piezoelectric element and an optical fiber is wound around the electrode. When an oscillating voltage with an angular frequency of Ω is applied to the electrodes, the piezoelectric element expands and contracts inward and outward, and the optical fiber expands and contracts, so that the phase of light passing therethrough changes. An electro-optical element can also be used as the phase modulator. For example, a waveguide is provided in a LiNbO ₃ single crystal, electrodes are provided on both sides of the waveguide, and a high frequency voltage is applied to the electrodes. Since the partial refractive index of this waveguide changes, the phase of the light passing therethrough changes.

In any case, the phase modulator is sensitive to temperature changes. In the case of a piezoelectric element, the amount of strain with respect to voltage changes due to temperature changes. In the case of electro-optic crystal, the refractive index changes with temperature. This changes the coefficient b of (5), so the phase modulation degree ξ changes. That is, ξ changes depending on the temperature, and the value of the Bessel function changes. Since the fundamental wave component and the harmonic wave component include ξ in the form of a Bessel function, the proportional constant between the value of these components and Δθ changes. This is called the scale factor, but if it changes, the accurate rotational angular velocity Ω ₀ cannot be obtained. Therefore, it is necessary to control ξ to be constant.

For example, control is performed so that even harmonics are constant. That is, the output of the light receiving element is synchronously detected by a carrier having an angular frequency of 2n times, and the phase modulation degree is controlled so that this output becomes zero. This means that ξ is fixed to the zero of the 2n-order Bessel function. The odd harmonics are 0 when the angular velocity of the object is 0 and cannot be used for controlling the degree of phase modulation. Therefore, this is set to 0 by using even harmonics.
Try to The lowest order is the second harmonic. If the phase modulation degree ξ is made constant by using this, ξ = 5.
It means fixing to 2. This means that the piezoelectric element or the like is driven with a large amplitude. Then, the following difficulties occur.

Since the voltage applied to the piezoelectric element increases, the induced noise induced in the electric circuit is large. If the noise is large, the electric circuit may malfunction. Further, the polarization plane is modulated or the amplitude is modulated due to the high voltage of the piezoelectric element. When ξ is large, not only the phase modulation but also noise, polarization plane, amplitude, etc. are affected. I want to reduce the degree of phase modulation and use it in the region where ξ is small. Since the minimum zero point of the Bessel function increases with the order n, if a smaller ξ is used, the control method of making the 2n-fold higher harmonic wave zero cannot be used.

It is desirable to be able to fix this with a smaller value of ξ. However, with the idea of making the value of the Bessel function 0, ξ cannot be made sufficiently small. The smallest zero of the Bessel function is the zero of the zero-order Bessel function, which is 2.4. But it is difficult to make the zero-order Bessel function zero. This is because the DC component is the sum of the constant and the zero-order Bessel function. After all, the conventional method of obtaining harmonics from the output of the light receiving element by synchronous detection and setting this to 0 to make ξ constant is too large and inappropriate. It is an object of the present invention to provide an optical fiber gyro capable of fixing the phase modulation degree to a smaller value. This is because if the phase modulation degree is small, it is possible to avoid the problems of rotation of the polarization plane and generation of noise.

[0013]

In the optical fiber gyroscope of the present invention, a fiber coupler is provided in the optical path at both ends of the fiber coil, and a part of the light that should pass through the fiber coil is branched as test light and interfered with this to cause interference. The light is synchronously detected at one or a plurality of frequencies that are integral multiples of the phase modulation frequency, and the phase modulation amplitude b is made constant by setting it to 0 or in a constant proportional relationship. Since the branched light does not include the phase difference Δθ of rotation, there is no limitation like the even harmonics as in the conventional case. It may be an odd harmonic. More freedom. Above all, ξ can be effectively reduced.

FIG. 3 shows a conceptual diagram of the present invention. Two couplers A and B are provided in the optical path that should be connected to both ends of the fiber coil.
And. Although the phase modulator is in the middle of the optical path leading to the fiber coil, the coupler is provided closer to the fiber coil than the phase modulator. The test lights branched from the coupler are combined by the coupler C, and the intensity of the interference light is detected by the light receiving element. This is synchronously detected with one or more harmonics of the phase modulation frequency. Then, the drive voltage of the phase modulator is controlled so that it becomes 0 or in a constant proportional relationship. This makes it possible to keep the phase modulation amplitude b constant.

[0015]

The test light from the coupler A is E _1, and the test light from the coupler B is E ₂ . These can be written as E ₁ = E ₁₀ sin (ωt + Φ) (6) E ₂ = E ₂₀ sin {(ωt + bsin Ωt)} (7) Where Ω is the angular frequency of phase modulation,
b is the phase modulation amplitude, Φ is the difference between the optical path lengths of the two test lights, and ω
Is the angular frequency of light. E ₁₀ and E ₂₀ are amplitudes. If the optical path lengths are perfectly equal, then Φ = 0, but generally not. Then the term Φ appears.

It should be noted that this expression does not include the phase difference Δθ due to rotation. This is because a branch optical path is formed in parallel with the fiber coil, and light passing through the fiber coil does not enter the branch optical path. That is, the signal b can be taken out regardless of whether the fiber coil is rotating or stationary. For this reason, the phase modulation amplitude b
Can be used to control When the test light is interfered and received by the light receiving element, the square of the sum of these appears as an output. Excluding the constant term, the product term of sin is

W = 2E ₁₀ E ₂₀ sin (ωt + Φ) sin {(ωt + bsin Ωt)} (8) = E ₁₀ E ₂₀ {cos (Φ−bsin Ωt) −cos (2ωt + Φ + bsin Ωt)} (9)

It is Since the second term is a term having a frequency twice the frequency of light, it cannot be detected by the light receiving element. Only the first term remains. This is even more

Cos (Φ−bsin Ωt) = cos Φcos (bsin Ωt) + sin Φsin (bsin Ωt) = cos Φ {J ₀ (b) + 2ΣJ _2n (b) cos 2nΩt} + sin Φ {2ΣJ _{2n + 1} (b) s in (2n + 1) t} (10)

It can be expanded by the Bessel function. It should be noted that in the conventional optical fiber gyro, the parameter was ξ in the expansion formula of the Bessel function. Here, not ξ, but b, which is the phase modulation amplitude itself, is a parameter. The coefficient of the even-order series is cos Φ. It should also be noted that this is not cos Δθ. Similarly, the coefficient of the odd-order series is sin Φ. This is not sin Δθ. Coefficient sin
Φ and cos Φ may depend on temperature, but not on rotation. That is, it is a constant.

This is the interference light intensity of the test light. This appears in the output of the light receiving element. This is synchronously detected with an arbitrary harmonic. It may be an odd order. When the odd-order 2m + 1-order harmonic U _{2m + 1} is calculated, U _{2m + 1} = E ₁₀ E ₂₀ sin ΦJ _{2m + 1} (b) (m = 0,1 ...) (11) Even-order harmonic When the wave V _2n is obtained, it becomes V _2n = E ₁₀ E ₂₀ cos ΦJ _2n (b) (n = 1, 2 ··) (12). Since an arbitrary order can be obtained by synchronous detection, the phase modulation amplitude b can be fixedly controlled to an appropriate value.

For example, the following case can be considered. The fundamental wave is obtained by synchronous detection and controlled to be zero. Since J ₁ (b) = 0, b = 3.8. FIG. 4 shows the value of the Bessel function, which has the parameter ξ, but the same applies to b. The second harmonic is obtained by synchronous detection and controlled so as to be zero. Since J ₂ (b) = 0, b = 5.2
Becomes The triple harmonic is obtained by synchronous detection and controlled to be zero. Since J ₃ (b) = 0, b = 6.3
Becomes The 10th harmonic is obtained by synchronous detection and controlled to be 0. Since J ₁ (ξ) = 0, b = 14.
It is 5.

The one described above is the minimum zero point among the zero points. However, the second zero point may be used instead of the minimum zero point. It may be the third zero point. It is known from the beginning how much value b is, since what value b is determined in advance by the drive voltage of the phase modulator. Therefore, it is easy to set b close to an arbitrary zero point.

The fourth harmonic is obtained by synchronous detection and fixed at the second zero point b = 11. The second harmonic is obtained by synchronous detection, and the third zero point b = 1
Fix to 1.7. Since there are innumerable zeros in the Bessel function, any of these may be set. However, when b becomes large, ξ also becomes large, and as described above, the plane of polarization of light may rotate or the noise generated by the drive circuit of the piezoelectric element may become large. Therefore, it is better that b is smaller.

Alternatively, two harmonics may be synchronously detected and compared to make them identical. This way
B can be fixed to a point other than the zero point. However, since the even expansion coefficient cosΦ and the odd expansion coefficient sin Φ are different, even two must be combined so that they are even if they are even and odd if they are odd. For example, zero order and 2
If the next is the same, b = 1.8 can be set. For example, if the third order and the fifth order are equal, then b = 5.2. For example, the fifth-order Bessel function is set to be half the third-order Bessel function. Then, in the minimum case, b = 4.4.
Becomes

What is important is that by fixing the value of b, ξ can be fixed to an extremely small value. If the rotational angular velocity is calculated using the fundamental wave, the sensitivity becomes maximum when ξ = 1.8. If the circuit for detecting the angular velocity is commonly used for the control of ξ as in the conventional case, it cannot be set to ξ = 5.2 or less. However, according to the present invention, it is easy to set ξ = 1.8 even if b is fixed to 10 or more.

The reason why this is the case will be explained. In the above-mentioned equation (5), the relation between the phase modulation amplitude b and the phase modulation degree ξ is determined as follows: ξ = 2b sin (LnΩ / 2c) (13). Consider the contents of the sin function. L = 100
If m, n = 1.4 and Ω = 2π × 60 kHz, then Ln
Ω / 2c = 0.088. In this case ξ = 0.18b
It becomes a relationship. This relationship is different for longer fiber lengths. The proportional coefficient becomes large. However, it can be seen that ξ is a value considerably smaller than b if the fiber length is in this range. For example, when ξ = 1.8 where the first-order Bessel function takes a maximum, b = 10.

It can be said that b takes a value close to 10 times ξ. The present invention controls b, but since it takes a relatively large value, it is easy to control as a zero of the Bessel function. In this example, b
There are three zeros in the vicinity of = 10: a sixth-order Bessel function (first zero), a third-order Bessel function (second zero), and a first-order Bessel function (third zero). Whichever one is used, it can be controlled in the vicinity of b = 10. If the above-mentioned proportional relationship exists, it means that if b = 10, then ξ = 1.8 can be fixed. Since the constant b, which is larger than ξ and proportional to this, is controlled in this way, the present invention can easily control the phase modulation degree ξ to a small value.

[0029]

【Example】

[Embodiment 1] FIG. 1 is a schematic view of an optical fiber gyro according to an embodiment of the present invention. In this, the optical path is composed entirely of optical fibers. The light emitting element 1 is a light source that emits monochromatic light. A laser diode and a super luminescent diode are used. A monochromatic light is emitted from the light emitting element 1 as a light source, and this emits an optical fiber 2, a first fiber coupler 3,
The light passes through the optical fiber 4 and the second fiber coupler 5 and enters both ends of a fiber coil 6 in which a single mode fiber is wound many times. This propagates inside the fiber coil 6 as left-handed light and right-handed light. A phase modulator 7 is provided at one end of the fiber coil 6 to modulate the phase of light as bsin Ωt. The phase modulator uses, for example, a piezoelectric element. When electrodes are attached to the inner and outer walls or end faces of a cylindrical or cylindrical piezoelectric element and an AC voltage is applied to this, the element expands and contracts in the radial direction due to the piezoelectric effect, so the optical fiber expands and contracts, and the phase of the light passing therethrough changes. Change. When the voltage amplitude is increased, the magnitude of the phase change also increases in proportion. Alternatively, an electro-optic crystal is used. This will be described later.

The right-handed light and the left-handed light merge at the fiber coupler 5 and pass through the optical fiber 4 and the fiber coupler 3 and enter the light receiving element 8. The light receiving element detects the intensity of the interference light of both and converts it into an electric signal. This is amplified by the preamplifier 9 and an appropriate harmonic or fundamental wave contained therein is synchronously detected by the synchronous detection circuit. Circuits after this are well known, and therefore not shown here.

In the present invention, the optical paths 2 at both ends of the fiber coil are used.
1 and 22 are provided with fiber couplers 23 and 24, respectively. It enters in parallel with the fiber coil 6. The light branched by the fiber couplers 23 and 24 does not enter the fiber coil 6. The branched light propagates through the branch fibers 25 and 26 and is combined by the fiber coupler 27. Here, the two branched lights join and further enter the light receiving element 28.
This light receiving element 28 is different from the main light receiving element 8 for detecting the rotational angular velocity.

The branched light does not include the phase difference Δθ related to rotation. This is natural because the light that has passed through the fiber coil 6 does not become branched light. However, since the fiber coupler 24 is located behind the phase modulator, the phase modulation signal is included in the light of the branch fiber 26. However, the light branched by the coupler 23 provided in the optical path 21 where the phase modulator 7 does not exist does not include the phase modulation signal. Therefore, when the two branched lights are combined, they interfere with each other and the phase modulation signal is taken out as a difference signal. The oscillator 11 generates a sine wave having an integral multiple of the frequency of phase modulation. The frequency is reduced by the frequency divider 12 to the phase modulation frequency Ω. This is given an appropriate amplitude through the amplitude controller 13 and applied to the phase modulator 7.

The intensity of the branched light is detected by the light receiving element 28, which is amplified by the preamplifier 29. This signal is synchronously detected by a carrier signal of an appropriate order, and a harmonic component of that order is obtained. This is U _{2m + 1} = E _{10 as} described above.
E ₂₀ sin ΦJ _{2m + 1} (b) or V _2n = E ₁₀
E ₂₀ cos ΦJ _2n (b). The comparator 31 compares this with a reference value. Then, by setting the value of the synchronous detection output to 0, the phase modulation amplitude b can be fixed to the zero point of the Bessel function. It is also possible to obtain two harmonic components and make them equal. In any case, b can be made constant. It is obtained by synchronously detecting the output of the light receiving element.

FIG. 2 shows a second embodiment. This uses electro-optic crystals. The optical waveguides 41 to 48 are provided on a single crystal having an electro-optical effect such as LiNbO ₃ . In this, a high refractive index portion is formed by doping impurity ions and the like according to a predetermined pattern by a lithography technique to form a waveguide. The light of the light emitting element enters the waveguide 41. The light emitted from the waveguide 42 is guided to the light receiving element. The waveguide 43 corresponds to the coupler 5 in FIG. Waveguide 44,
45 is connected to the fiber coil 6 of FIG. These are continuous waveguides with one branch. Apart from this, there are waveguides 46 and 47. This is a waveguide for extracting the branched light. These merge at the waveguide 48.
After being combined, this is guided to a newly provided light receiving element 29.

The waveguides 44 and 46 are evanescently coupled. This corresponds to the fiber coupler 23. The waveguides 45 and 47 likewise correspond to the fiber coupler 24. Electrodes 49 and 50 are formed at one location of the waveguide 45. By applying an alternating electric field here, the refractive index can be changed to be used as a phase modulator.

[0036]

In the optical fiber gyro of the phase modulation type, the modulation degree ξ of the phase modulator tends to change due to temperature fluctuation or the like. Conventionally, the output of the light receiving element for detecting the rotational angular velocity is synchronously detected, and the phase modulation degree ξ contained in this is made a constant. If this is the case, ξ will be fixed at a large value. If ξ is large, the plane of polarization of light rotates and noise occurs in the electric circuit. The present invention provides fiber couplers at both ends of a fiber coil to split light passing through a phase modulator and light not passing through a phase modulator to obtain a phase modulation degree ξ
Instead, only the amplitude b of the phase modulation is taken out. The value of b is fixed depending on whether this output is 0 or the two outputs are the same.

Although b is proportional to ξ, it is a large number of 5 to 10 times. Even if b is fixed as the zero point of the Bessel function, since b is large, a higher-order Bessel function can be used. For example, it is easy to fix ξ = 1.8 which gives the maximum of the first-order Bessel function as ξ. A high-order synchronous detection circuit is required, but this does not mean that a new multiplication circuit is required. Of course, an oscillator oscillates a high-frequency signal, divides it, and uses it for phase modulation. Since the original frequency is higher, the carrier of the desired harmonic can be obtained by using it directly or by dividing it slightly. According to the present invention, a highly accurate optical fiber gyro with a stable phase modulation degree and a stable scale factor can be constructed.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an optical fiber gyro according to an embodiment of the present invention.

FIG. 2 is a schematic perspective view of a main part of an optical fiber gyro according to a second embodiment of the present invention.

FIG. 3 is a schematic diagram showing the principle of the optical fiber gyro of the present invention.

FIG. 4 is a graph of a Bessel function.

[Explanation of symbols]

 1 light emitting element 2 optical fiber 3 fiber coupler 4 optical fiber 5 fiber coupler 6 fiber coil 7 phase modulator 8 light receiving element 9 preamplifier 11 oscillator 12 frequency divider 13 amplitude controller 23 fiber coupler 24 fiber coupler 27 fiber coupler 28 light receiving element

Claims (1)

[Claims] 1. A fiber optic gyro having a principle of propagating light counterclockwise in a fiber coil clockwise and obtaining a rotational angular velocity of the fiber coil from a phase difference between the two lights, which is a monochromatic light source. A light emitting element that generates light, a fiber coil in which a single-mode optical fiber is wound many times, a fiber coupler that connects both ends of the fiber coil to the light emitting element and the light receiving element, and a left and right rotation in the fiber coil. The light receiving element for detecting the intensity of the interference light by interfering the light propagated to the optical fiber, and the phase modulator for providing the sinusoidal phase modulation to the propagation light provided at one end of the fiber coil, It is incident on both ends of the coil and propagates as a right-handed light and a left-handed light in the fiber coil and combines them to detect the intensity of the interference light with a light receiving element, and the output of the light receiving element is phase-modulated. Synchronous detection is performed using the fundamental wave component of the same frequency or its n-fold harmonic component, and the rotational angular velocity is obtained by using the fact that the fundamental wave component or the n-fold harmonic component contains the Bessel function of the 1st or nth order as a constant. In a phase modulation type optical fiber gyro, two branch elements are provided in the optical paths at both ends of the fiber coil, and the light that has passed through the phase modulator and the light that does not pass through the phase modulator are taken out into different optical paths and interfered with each other. Photoelectric conversion is performed, and this is synchronously detected with one or two predetermined harmonics of phase modulation, and the synchronous detection output becomes 0, or the two synchronous detection outputs have a constant relationship. An optical fiber gyro characterized in that the amplitude of modulation is kept constant.