JPH02236112A - Phase modulation optical fiber gyro - Google Patents

Abstract

PURPOSE:To detect true components of a direct current of a signal thereby to measure an angular velocity stably by removing components of reflected light because of a lens, a fiber and the like from the components of a direct current of a light receiving element. CONSTITUTION:A reflected light amount setting unit 14 is provided which sets the level of a reflected light reflected at an end face of an optical component such as a lens, an optical fiber or the like and not passing a sensor coil 6 among the components of a direct current of a photodetector 9. The amount H of the reflected light obtained by the setting unit 14 is subtracted from an output D of components of the direct current of a direct current component detecting unit 11 by a reflected light amount subtracting processor 15. Then, an output of a synchronous wave detecting unit 13 of the photodetector 9 is divided by an output R of the processor 15, the result of which is output from a dividing processor 16 as a correcting output. Accordingly, even when the amplitude of a signal light is varied, a correct phase difference can be obtained, thereby ensuring correct measurement of an angular velocity.

Description

[Detailed Description of the Invention] (A) Technical Field The present invention relates to a phase modulation type optical fiber gyro.

An optical fiber gyro is a device that measures the angular velocity of a moving body.

A single-mode optical fiber is wound into a coil many times, and monochromatic light is introduced into both ends of the optical fiber and is passed counterclockwise.
The light propagates clockwise, causing the light emitted from both ends to interfere. If the optical fiber coil is rotating around its axis,
A phase difference appears between the counterclockwise light and the clockwise light. Since this phase difference is proportional to the rotational angular velocity, if the phase difference is known, the rotational angular velocity can be calculated by 10.

When the phase difference is Δθ and the angular velocity is Ω, the following relationship exists.

C λ Here, L is the fiber length of the sensor coil, a is the diameter of the coil, C is the speed of light in vacuum, and λ is the wavelength of light in vacuum. This is called the Sagnac effect and is well known.

However, it is not easy to detect the phase difference Δθ.

In reality, the phase difference includes optical system offsets that are not based on rotation. This offset varies significantly with changes in temperature. Furthermore, in an optical fiber gyro having a basic configuration, the light receiving element output appears in the form of (1+cosΔθ). This has poor sensitivity when Δθ is small, and the direction of rotation cannot be determined.

To solve these difficulties, frequency modulation, phase modulation, and phase shift type optical fiber gyros are being considered.

The present invention relates to a phase modulation type optical fiber gyro.

(a) Phase modulation type optical fiber gyro The basic form of the phase modulation type optical fiber gyro will be explained with reference to FIG.

In this system, the optical fiber at one end of an optical fiber sensor coil is wound around a piezoelectric element to apply phase modulation. Taking the first-order term of the modulated wave, the phase difference is si
It is obtained in the form of nΔθ.

Coherent light emitted from a light emitting element 1 is split into two beams by a beam splitter 2.

One is focused by the coupling lens 4 and enters the A end of the optical fiber 5. This propagates counterclockwise in the sensor coil 6.

The other ray is narrowed down by the coupling lens 3,
The light enters the optical fiber 5 from the B end and propagates clockwise inside the sensor coil 6.

A portion of the optical fiber 5 is a sensor coil 8, and a portion near the B end is wound around a piezoelectric element or the like to form a phase modulation element 7.

Since the oscillator 10 applies an oscillating voltage to the piezoelectric element, the piezoelectric element expands and contracts. Since the phase modulation section 8 of the optical fiber is wound around the piezoelectric element, it expands and contracts together with the piezoelectric element, and the optical signal contains a modulation component.

The clockwise light and the counterclockwise light are transmitted through the phase Is portion 8 of the phase modulation element 7.
and the sensor coil 6, and is emitted from the other vents. These are combined by the beam splitter 2 and incident on the light receiving element 9. The light receiving element 9 performs square law detection of the interference light.

Since the phase modulation element 7 is provided at an asymmetrical position when viewed from the entire optical fiber 5, the counterclockwise light and the clockwise light receive phase modulation at different timings.

Let L be the length of the optical fiber of the sensor coil 6, and let n be the refractive index of the optical fiber core. The time τ required for light to pass through the sensor coil 6 is: n L τ = (2) given by C .

When the phase modulation element 7 is provided near the B end, the counterclockwise light first undergoes phase modulation and then enters the sensor coil 6. The clockwise light passes through the sensor coil 6 and then enters the phase modulation element 7.

Let the angular frequency of the modulation signal be Ω. Since the difference in time between receiving phase modulation at the phase modulation element 7 and entering the light receiving element 9 is τ, the phase difference φ between the modulation signals included in the interference light is φ = Ω τ (3) .

As mentioned above, due to the Sagnac effect, clockwise light and
The counterclockwise light has a phase difference of Δθ, but due to the phase modulation, the phase modulation part also has a phase difference of φ. Let b be the amplitude due to the action of the phase modulation element 7.

If the electric field strength of clockwise light and counterclockwise light is %ER%EL, Δθ ER =F, o sin {ωt+−+bsin(Ω
t+φ)}.

The counterclockwise light and clockwise light having such electric field strength are square-law detected by the light receiving element 9. The output of the light receiving element 9 is S(Δθ,
1> becomes. Here, D. C. means a direct current component. ω
is the frequency of light, and 2ω means a frequency component twice this frequency. Since the light receiving element 9 cannot detect such a fast signal, the signal is O.

Since the signal thus obtained includes phase modulation φ, the phase difference Δθ can be determined in relation to the amplitude of the modulation signal.

If we remove the DC component and rewrite S(Δθ, 1) in the form of a sum, we get: These are expanded using Bessel functions. From the generating function expansion of the Bessel function, S (Δθ, 1) ”(S.cos Δθ+Ss sln Δθ)Eo
” (to).

By converting θ→θ+π/2, we can obtain the well-known property of Bessel function, J−m(x)=(−)” Jn(x)( +
1) Using (where n is a positive integer) and setting φ ξ" 2 b slg (I2), we get. If we set j=expiθ, we get S."Jo(ξ)+2 Σ(-)" J2n(ξ)
cos2nQt (13)ni1. From the expansion of the real and imaginary parts of this equation, S(Δθ,
Obtain the series expansion of the sin and cos parts Ss1Sc of 1>.

becomes. Rewriting using these equations, the signal S(Δ
θ,1) becomes (DC component) + (2ω component) +Eo”Jo(ξ)cos Δθ ni1 n=0 (l5). This is an expansion by harmonics of the modulation frequency Ω. Passing through a filter An arbitrary harmonic component can be obtained by using the following formula.Among these, the first-order term is taken as the fundamental wave component P, and the second-order term is taken as the double harmonic component Q.

P (t)”2Eo2J t (ξ)cosΩ
t sin Δθ (1G)Q (t)”2EO
”J2(ξ) cos 2Ωt cos Δθ (
! 7). In many cases, by detecting the fundamental wave P, Δθ
seek. Maximize J1 (ξ) so that the sensitivity of P is maximized. For this reason, the modulation degree is set so that ξ = 1.8. At this time, J (ξ) is approximately 0.3. .

The above is the basic configuration of the phase modulation type optical fiber gyro.

(C) Prior art When detecting the fundamental wave P and finding Δθ, the modulation degree ξ must be constant. Otherwise J. This is because the value of (ξ) changes.

In order to keep the modulation degree constant, a method has been proposed in which, for example, the second harmonic Q is monitored and the value of J2(ξ) is determined.

Japanese Patent Application No. 59-244841 is like this.

A Ω signal and a 2Ω signal obtained by multiplying this signal are obtained from the drive circuit of the phase modulation element. The output of the light receiving element is synchronously detected using each signal. This is passed through a low-pass filter to obtain low frequency components. The second harmonic component Q is Q = 2 E O2J 2(ξ)cosΔθ (l
8). Since the modulation degree ξ must be constant, the phase modulation element is controlled so that Q is constant.

This is done so that ξ becomes 1.8. In J2, ξ is ivy.
8, it is about 0.3. If ξ increases beyond this, J2 increases, and if ξ decreases beyond this, J2 decreases. Therefore, keeping Q constant means that ξ=l.
Keep it at 8.

(Eng.) Problems to be Solved by the Invention If the amount of light from the light emitting element is constant, Δθ can be immediately determined from P (t) obtained by (1B).

However, in reality, the amplitude E of the light. varies, so even though the value of Δθ is the same, apparently different outputs are obtained due to variations in the amount of light.

Up until now, we have explained counterclockwise light and clockwise light for simplicity.
Both are the same E. was using. But these are actually not the same.

If it is necessary to distinguish, the amplitude of clockwise light is written as E1, and the amplitude of counterclockwise light is written as E2. What was written as Eo squared should be read as EI Ea.

Japanese Patent Laid-Open No. GG-13581fi described in the previous section proposes a control system in which the DC component of the signal is constant.

However, this does not discuss the issue of the amount of reflected light. Here, reflected light refers to a light component reflected by an end face of an optical component such as a lens or a fiber. Signal light is light that passes through the fiber coil and contributes to angular velocity measurement.

Signal light and reflected light enter the light receiving element.

Here, reflected light means light that does not pass through the sensor coil.

The aforementioned Japanese Patent Application Laid-Open No. 13581G was designed to have no reflected light.

Or even if there was, it is assumed that it was assumed to fluctuate in the same way as the signal light. That is why keeping the DC component constant is equivalent to keeping the amplitude of light constant.

However, in reality, a non-negligible amount of reflected light is incident on the light receiving element. The reflected light does not fluctuate in the same way as the signal light. Or it can be said that there is almost no change.

Therefore, keeping the DC component constant does not necessarily make the amplitude of the signal light constant.

(E) Configuration In the present invention, (1) A DC component D is determined from the output of the light receiving element, and a preset amount of reflected light H is subtracted from this to determine the DC component R of only the signal light.

R=DH (19) (2) From the output of the light receiving element, find the fundamental wave P or appropriate harmonics by synchronous detection. Divide this by R to reduce the amplitude and accurately find Δθ.

The phase difference Δθ is detected by the following procedure.

The present invention will be explained with reference to FIG.

Light from a light emitting element 1 that generates coherent light is split into two beams by a beam splitter 2. These are lens 3,
4 and enters both ends A1B of the optical fiber 5.

The optical fiber 5 is a single mode fiber and includes a senna coil 8 wound in a coil shape many times and a portion 8 wound around a phase modulation element 7.

The light incident from the B end propagates in the sensor coil 6 as light in a clockwise direction, and is emitted to the A end.

The light from the A end becomes counterclockwise light and is emitted to the B end. The counterclockwise light and the clockwise light are combined by the beam splitter 2 and enter the light receiving element 9.

The phase modulating element 7 located near one end of the optical fiber 5 is a piezoelectric element provided with an electrode, and is expanded and contracted by applying an alternating current voltage to change the optical path length of the optical fiber wound around the piezoelectric element.

An excitation AC power source 10 drives the phase modulation element 7 at a modulation frequency Ω.

The output of the light receiving element 9 includes various components such as a DC component, a fundamental wave, and harmonics. The reflected light is included only in the DC component.

The DC component detection unit 11 extracts a DC component from the light receiving element output. The light emitting element output control circuit 12 controls the drive current of the light emitting element 1 so as to keep the output of the DC component detection section 11 constant.

The DC component of the light-receiving element includes signal light and reflected light. However, since the amount of reflected light is constant,
If the DC component of the light-receiving element is made constant, the component that should become signal light becomes constant.

The synchronous detection unit 13 receives a synchronous signal from the excitation AC power supply 10 and synchronously detects a signal component of an appropriate order from the output of the light receiving element. This is preferably the fundamental wave P or odd harmonics. This is because sin includes Δθ.

The above configuration is the same as a normal phase modulation type optical fiber gyro.

In the present invention, further, Reflected light amount setting section 14 Reflected light amount subtraction processing unit 15 Division processing unit l6 will be established.

The reflected light amount setting section 14 sets the reflected light amount H in advance. The reflected light amount subtraction processing unit 15 subtracts the reflected light amount H from the DC component D of the DC component detection unit 11 to obtain the signal DC component R.

The division processing section 16 divides the output of the synchronous detection section 13 by the signal DC component R. This is the corrected output W. This does not include the amplitude of the light. From this result, the phase difference Δθ can be determined.

(Force) Effect: As with a normal optical fiber gyro, interference light output of clockwise light and counterclockwise light is obtained at the light receiving element. Assume that the synchronous detection section 13 detects, for example, a fundamental wave P.

If the amplitudes of the clockwise light and counterclockwise light are El and E2, the fundamental wave P obtained from the light receiving element output is P=2E. E2 J
．． (ξ)sln Δθ (2o).

The DC component D of the light receiving element output is - (El"+E2"
)+EI Ell JO(ξ)cosΔθ +H(
2l). H indicates the amount of reflected light. This light is reflected at the input end of the fiber, in addition to the clockwise and counterclockwise lights. By the way, in an actual optical system, E. is almost equal to E2, so (EI"+E2")=Et E2
(22) can be assumed.

As already mentioned, in order to maximize J1(ξ), ξ is often set to 1.8. For this value, J.
(ξ)&-=0.3.

At this time, the DC component D is DIR I E2 (1 +0.3 cos
Δθ)+H (23).

By the way, when the optical fiber gyro is not rotating,
Or, when there is almost no rotation, Δθ=O. Therefore, COSΔθ=1.

Therefore, when the optical fiber gyro is not rotating or hardly rotating, the DC component is D 1.3 EI E2 +H
(24).

The DC component D of the light receiving element output is the DC component R of the signal light,
This is the sum of the reflected light H and the reflected light H. In the reflected light amount setting section 14, set the correct
By setting the reflected light amount H and subtracting the reflected light amount H from D in the reflected light amount subtraction processing section 15, R can be obtained.

R″51.3 EI E2 becomes. This is important.

On the other hand, the fundamental wave output P of the synchronous detection section 13 is given by (20). The division processing unit 16 divides P by R.

1.3 @ 0.85 s1n Δθ By performing division, a result that does not depend on the amplitude of the light can be obtained. In (28) etc., ξ is set to a specific value, 1.8, but this value may be any value.

Generally, the coefficient is expressed as a Bessel function as follows.

The reason why it does not depend on the amplitude is because the amount of reflected light H is subtracted from the DC component. If the amount of reflected light is not subtracted, the ratio between the amplitude and H remains, and Δθ cannot be determined correctly.

(Force) Example Regarding the minimum configuration with the basic conditions of a phase modulation optical fiber gyro, see Ezeklel s. and Ardltty H.
J. ;”FIBER OPTIC ROTATION
SENSORS'', Sprlnger- Verlag
BerlIn. A detailed explanation can be found in 1982.

An embodiment will be explained with reference to FIG.

Components common to those in FIG. 1 are given the same numbers and their explanations will be omitted.

A light blocking element 20 such as an electronic shutter is placed between the fiber and one of the lenses into which either the counterclockwise light or the clockwise light enters. A light blocking element driving section 21 opens and closes this.

The light blocking element 20 is open when measuring angular velocity.

Close the light blocking element 20 only when measuring the amount of reflected light,
Block between the lens and the fiber. The signal light is transmitted to the light receiving element θ
It doesn't fit in. Only the reflected light enters the light receiving element 9. The DC component at this time is reflected light mH.

The operation of the driving section 2l of the light blocking element 20 such as an electronic shutter may be controlled by a simple signal such as a contact signal from a digital control section.

The reflected light amount H is detected by the DC component detection section 11, passes through the multiplexer 22 of the digital processing section 19, and is converted into a digital value by the A/D converter 23. This value is bus 2
6 is stored in a suitable storage element connected to 6.

The reflected light amount setting section 14 in FIG.
It includes a 01 light blocking element driving section 11, a DC component detection section 11, and a digital processing section 19.

Since the reflected light amount setting section 14 needs to determine the amount of reflected light and store it, it can also be configured with an analog circuit. In this case, a capacitor with a large capacity can be used as the memory element.

The output is voltage. The circuit is also simple.

However, since it is desirable to calculate the amount of reflected light when the optical fiber gyro is stationary, the reflected light amount storage element must correctly store the value of reflected light flH from one stationary state to the next stationary state. . If this time is long, a digital circuit is better.

In this example, the DC component detection section 11, the multiplexer 2
2. The A/D converter 23 is used to obtain and store the DC component during normal angular velocity measurement. These are shared by the reflected light amount setting section 14.

The CPU 25 calculates P/R. In this way, the phase difference Δθ is determined.

However, the reflected light amount setting section 14 may be configured in other ways.

The output of the light receiving element when the light is blocked by the light blocking element 20 may not be the amount of reflected light when signal light is present. In this case, multiply by an appropriate coefficient. If the light blocking element 20 is placed near the B end, the reflected light from the B end will not enter the light receiving element during blocking. This may need to be corrected.

Further, the amount of reflected light H can be determined by calculation.

The DC component D is due to the signal light (1 + cosΔ0)
Since it is the sum of Δθ and H, H can be estimated by changing Δθ and finding D as a function of Δθ. If H is a constant, this H can be used to further improve the approximation.

In this example, a half mirror 30, lenses 31 and 33, and a polarization constant fiber 32 are added between the light emitting element, the light receiving element, and the optical fiber 5. This is to match the planes of polarization of the clockwise light and the counterclockwise light. It is irrelevant to the main purpose of the invention.

(g) Effect Light reflected by lenses, fibers, and other optical systems is removed from the DC component of the light receiving element to obtain the true DC component of the signal light, so even if the amplitude of the signal light fluctuates, the correct A phase difference Δθ can be obtained.

Therefore, more accurate and stable angular velocity measurement is possible.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram of the present invention. FIG. 2 is a configuration diagram showing an embodiment of the present invention. FIG. 3 is a configuration diagram of a phase modulation type optical fiber gyro. 1 ● Fist 2 ● ● 3, 4 5 ● ● 6 ● ● 7 ● ● ● Light emitting element ● Half mirror ● ● Lens ● Optical phase ● Sensor coil ● Phase modulation element Ibar 8 φ ● 9 ● ● 1 0 ● 1 1 ● 1 2 ● 1 3 ● 14 fistl 5 ● 1 8 ● 1 9 ● 2 0 ● 2 1 ● 2 2 ● 241I 2 5 ● ● Fiber wrapped around phase modulation element ● Light receiving element ● Excitation AC power supply ● DC component detection section φ light emitting element output control circuit ● Synchronous detection section ● Reflected light amount setting section ● Reflected light amount subtraction processing section ● Division processing section ● Digital processing section ● Light blocking element ● Light blocking element drive section ● Multiplexer ●A/D converter ●Light blocking element drive unit interface ●CPU? Author Ken Okamoto
Patent Applicant Sumitomo Electric Industries Co., Ltd. Application Agent Patent Attorney Shigeru Kawase ■■■ Tsugu 1! l1'ji
lili'j+

Claims (4)

[Claims] (1) An optical fiber having a part constituting a sensor coil and a part provided with a phase modulation element, a light emitting element that generates coherent light, and light from the light emitting element or from the light emitting element via the optical fiber. a beam splitter that splits the light and supplies it to both ends of the optical fiber; a light receiving element that propagates through the optical fiber and combines the light emitted from both ends of the optical fiber and receives the light; and an output of the light receiving element. A phase modulation type optical fiber gyro including at least a synchronous detection circuit that receives the output of the light receiving element and detects the phase modulated frequency component, and a DC component detection section that receives the output of the light receiving element and detects the DC component, and a lens of the DC component. , a reflected light amount setting section that sets the level of the amount of reflected light that is reflected from the end face of an optical component such as a fiber and has not passed through the sensor coil, and a reflected light amount setting section that sets the level of the amount of reflected light that is reflected from the end face of an optical component such as a fiber, and from the output D of the DC component detection section. It is composed of a reflected light amount subtraction processing section that subtracts the reflected light amount H, and a division processing section that divides the output of the synchronous detector by the output R of the reflected light amount subtraction processing section and outputs the division result. Features: Phase modulation type optical fiber gyro. (2) The DC component detection section receives the output of the light receiving element and has a DC component detection function that detects the DC component and recognizes that the optical fiber gyro is stationary based on other sensor signals or the like. A DC component level holding unit receives the output signal of the optical fiber gyro, detects that the angular velocity is sufficiently small and is close to stationary, and holds the DC component level when the angular velocity is stationary or close to stationary. A phase modulation type optical fiber gyro according to claim 1. (3) The phase modulation type optical fiber according to claim 1 or 2, wherein the reflected light amount setting section is constituted by a part that supplies a constant voltage equivalent to the reflected light amount H. gyro. (4) The reflected light amount setting section includes, between the sensor coil and the beam splitter, a controllable light blocking element that passes or blocks light, a light blocking element driving section, and the output of the DC component detecting section when the light is blocked. A phase modulation type optical fiber gyro according to claim 1 or 2, characterized in that the phase modulation type optical fiber gyro is constituted by a processing section having a function of detecting the detected level and outputting the detected level while holding it.