JPH02236111A - System of correcting phase modulation degree in optical fiber gyro - Google Patents

Abstract

PURPOSE:To keep a constant phase modulation degree at all times by winding a phase modulation element with a correcting optical fiber, extracting one-fold, and two-fold frequency signal components related to a modulating wave and making an output of said element constant. CONSTITUTION:An optical fiber 14 for correction use is wound around a phase modulation element 7 which is a piezoelectric element. When a light is sent from a second light emitting element 11, a second photodetector 16 receives the interference light. A second synchronous wave detecting unit 18 obtains the size of a fundamental wave (p) or a two-fold higher harmonic (q) related to a modulating wave from an output of the photodetector 16. The synchronous wave detecting unit 18 sends such a control signal to a controlling unit 15 which controls oscillations of the phase modulation element that the fundamental wave (p) and two-fold higher harmonic (q) are kept constant in size. The controlling unit 15 adjusts the power to be applied to the phase modulation element 7, whereby the fundamental wave and two-fold higher harmonic are made constant. Accordingly, irrespective of the temperature change and the like, the phase modulation degree can be kept constant at all times.

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.
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 determined.

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

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 Yoshimasa who detects the phase difference Δθ.

In reality, the phase difference includes optical system offsets that are not based on rotation. This offset fluctuates significantly due to changes in temperature. Furthermore, in an optical fiber gyro with a basic configuration, the light receiving element output is (1 + cosΔθ)
appears in the form of 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 s1
It is obtained in the form of nΔθ.

Coherent light emitted from the light emitting element 1 is split into two beams by the 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.

Most of the optical fiber 5 is a sensor coil 6, but 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 modulation section 8 of the phase modulation element 7.
and sensor coil 8, and is emitted from the other end. 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 n be the refractive index of the optical fiber core. The time τ required for light to pass through the sensor coil 6 is given by nL τ = (2) 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.

The electric field intensities of the counterclockwise light and the clockwise light are Ei. If NER, then it becomes.

The counterclockwise light and clockwise light having such electric field strength are square-law detected by the light receiving element 9. Output S (Δθ.1
＞Hato becomes. These are expanded using Bessel functions. From the generating function expansion of the Bessel function, we get . 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, t) in the form of a sum, we get: If we set t=expiθ, oo 1iθ eXI) i Xsin θ= ΣJ. (
x) e (9)n=-oro. From the expansion of the real and imaginary parts of this equation, S(Δθ.
1) sln. Obtain the series expansion of the cos part S8vSc.

S(Δθ.1) = (S.cogΔθ+S, sinΔθ)EoQ(to)
Define it as follows.

By converting θ→θ+π/2, we can obtain the well-known property of Bessel function, J−n(x)=(−)” Jn(x)(1l
) (where n is a positive integer) and set φ ξ=2bsin 1(l2), then S0 ``Jo(ξ)+2 Σ(-)'''J2f1(ξ)
cos2nQt (13)ni1. Rewriting using these equations, the signal S(Δ
θ,1) is (DC component)+(2ω component)+EoQJo(ξ)cos Δθ n:1 n=0 (l5). This is an expansion due to harmonics of the modulation frequency Ω. By passing it through a filter, any harmonic component can be found. Among these, the first-order term is assumed to be the fundamental wave component P, and the second-order term is assumed to be the double harmonic component Q.

P (t)=2Eo”J t (ξ)cosΩ
t sin Δθ (IB)Q (t)”2EO2
J2(ξ) cos 2Ωt cos Δθ (1
7). In many cases, the fundamental wave P is detected and Δθ is determined. Maximize J1(ξ) so that the sensitivity of P is maximized. Therefore, 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. This is because otherwise, the value of Jl(ξ) would fluctuate.

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=2EO2J2(ξ)cosΔθ (l8). 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. J2 has ξ of 1.
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 ξ=! ．．
Keep it at 8.

(Engineering) Problems to be Solved by the Invention In the above method, there is an assumption that COSΔθ=1 in (18) representing the Q of the second harmonic. This is true when the rotational angular velocity is small.

In other words, it was assumed that the second harmonic level is constant both when stationary and when rotating. Only in this case, keeping J2 constant can be equivalent to keeping ξ constant.

However, if the rotation is fast, Δθ becomes thick.

If an accurate rotational speed is to be determined, an accurate value of coBΔθ must be used. However, Δθ is the value itself that this device attempts to find.

(E) Purpose It is an object of the present invention to provide a mechanism that can keep the degree of modulation constant in a phase modulation type optical fiber gyro.

(Force) Configuration The present invention newly uses an optical fiber for correction processing, a second light emitting element, and a light receiving element.

By wrapping an optical fiber for correction processing around the phase modulation element,
Light is passed through the optical fiber from the second light emitting element, and the light emitted from the optical fiber is caused to interfere with the light directly emitted from the second light emitting element, and is made to enter the second light receiving element. From this output, the magnitude of the fundamental wave or second harmonic of the modulated wave is determined.

This fundamental wave includes J+(ξ) and the second harmonic includes J2(ξ), but does not include COS Δθ, so the modulation degree ξ can be determined accurately. Then, by keeping the magnitude of the fundamental wave or the second harmonic constant, the modulation degree ξ is maintained at a constant value.

This will be explained with reference to drawings.

FIG. 1 shows the configuration of the present invention.

The light emitted from the first light emitting element 1 is split into two beams by a beam splitter 2, passed through coupling lenses 3 and 4, and made incident on both ends A1B of a single mode optical fiber 5.

The single mode optical fiber 5 has a sensor coil portion 6 and a phase modulation section 8 wound around a phase modulation element 7. The light entering from the A end propagates counterclockwise through the sensor coil 6, passes through the phase modulation section 8, and exits to the B end. The light entering from the B end first passes through the phase modulation section 8, propagates clockwise through the sensor coil 6, and exits from the A end.

The light combined by the beam splitter 2 enters the light receiving element 9. The intensity of the interference light appears in the output of the light receiving element 9.

This includes many harmonics.

The synchronous detection section 17 obtains the fundamental wave component P among these. For synchronous detection, a synchronous signal sinΩt from the phase modulation element excitation control section 15 is given to the synchronous detection section 17.

The above configuration is the basic form of a phase modulation type optical fiber gyro. The present invention has further been improved as follows.

The optical fiber 14 for correction processing is wound around the phase modulation element 7, which is a piezoelectric element or the like. There is no requirement that the length of the wrapped portion be equal to the length of the wrapped portion around the phase modulation of the main optical fiber 5. However, to simplify the explanation, it is assumed that both optical fibers have the same length of the portion wound around the phase modulation element 7.

A second light emitting element 11 and a second light receiving element 16 are newly provided. The light emitting element 11 must also produce interference light. However, the wavelength may be the same as or different from that of the first light emitting element 1.

The light generated from the second light emitting element 11 is split into two beams by a beam splitter 12.

One light beam passes through the optical coupling element 13 and passes through the light receiving element 16.
to go into. This is called the reference light.

The other light beam enters one end C of the optical fiber 14 for correction processing, passes through this optical fiber, and exits from the other end D. Since this optical fiber is wound around the phase modulation element 7, the optical path length changes according to expansion and contraction of the phase modulation element. This appears as a phase change in the emitted light. This light is called modulated light.

The modulated light is combined with the reference light by the optical coupling element 13 and enters the second light receiving element 16 . The intensity of the interference light is detected. The phase change of the modulated light can be detected as the intensity change of the interference light.

If the length of the portion 8 of the main optical fiber 5 wound around the phase modulation element 7 and the length of the portion of the correction processing optical fiber 14 wound around the phase modulation element 7 are equal, the correction processing optical fiber 1
4 and the phase modulation generated in the phase modulation section 8 of the main optical fiber are exactly the same.

In other words, the correction processing optical fiber 14 monitors the phase modulation of the main optical fiber.

The second synchronous detection section 18 determines the magnitude of only the fundamental wave p or the second harmonic q from the output of the light receiving element 16. For synchronous detection, a signal with a frequency of Ω or 2Ω is obtained from the phase modulation element excitation control section 15.

The fundamental wave and double harmonic of the main optical fiber detected by the first light receiving element 9 are referred to as P1Q, so to distinguish them from these, the fundamental wave and double harmonic of the output of the second light receiving element 16 are written in lowercase letters. Let it be expressed as p1q.

The synchronous detection section 18 sends a control signal to the phase modulation element excitation control section 15 so as to keep the magnitude of the fundamental wave or the second harmonic constant.

The phase modulation element excitation control section 15 adjusts the power applied to the phase modulation element 7 to maintain the magnitude of the fundamental wave or the second harmonic constant.

(G) Function The function regarding the main optical fiber 5 is the same as that of the conventional one. Light emitted from the first light emitting element 1 is split by a beam splitter 2. These rotate the sensor coil 6 clockwise,
The light propagates counterclockwise, is combined by the beam splitter 2, and enters the first light receiving element 9. The magnitude of the fundamental wave P in the interference light is determined by the first synchronous detection section 17. Since the optical fiber 5 has a phase modulation element 7, P is (l6)
From P (t)=2Eo”J+ (ξ)cosΩ
t sin Δθ (I9). When detected and passed through a low-pass filter, P=2Eo”Jt(ξ)sin
Δθ (20) This is the synchronous detection section 1
This is the output of 7.

Next, the functions of the parts unique to the present invention will be explained.

The angular frequency of the light from the second light emitting element 11 is ω. Then, the electric field strength at the reference light-receiving surface directly entering the light-receiving element 16 is E3sin(ωat +Φ) (2l). The electric field strength at the light receiving surface of the light receiving element 16 of the modulated light that has passed through the optical fiber 14 for correction processing is E4sln
(ωc t + bsln Ω t )
(22) can be written. E3 and E4 are amplitudes.

The degree of modulation is assumed to be b, which is the same as equations (4) and (5). Since the phase modulation section 8 of the correction processing optical fiber and the main optical fiber have the same length, the modulation degrees are the same.

However, this is only done for the convenience of explanation, and the lengths of the optical fibers do not have to be equal, and the degrees of modulation do not have to be equal.

Φ is the phase difference between the modulated light and the reference light. Since the two have different lengths, the phase difference is large, and its magnitude is not known in advance.

The phase difference can be increased or decreased by an integer multiple of 2π, so Φ is 0.
You can consider values between 2π and 2π.

Since the output S of the light-receiving element 16 is the sum of the above values and squared, s (t) = (DC component) + (2ω. term) + E
3 E 41cos+cos(bslnQt)+s
lnΦstn(bs+nQt)1 This is very similar to the equation already explained, so the derivation will not be described in detail.

The fundamental wave p is! ) (t) = 2 E a E 4 J t
(b) cos Ωt sin Φ (24). When this is synchronously detected and passed through a low-pass filter, the magnitude of the fundamental wave 1) = 2 E3 E4 J t(b) sin Φ
(25) is required. Synchronous detection section 1
8 seeks this. When the modulation degree b changes, the magnitude p of the fundamental wave changes. Therefore, a control signal is sent to the phase modulation element excitation control section 15 so as to keep p constant. Since the amplitudes E3 and E4 and the phase difference Φ are constants, the change in b and the change in p are equivalent.

Instead of the fundamental wave, a second harmonic can be used.

The second harmonic signal q is q(t)=2E3 E4 JQ(b) cos 2Ω
tcosΦ (2B). When synchronously detected and passed through a low-pass filter, the magnitude of the second harmonic is Q = 2 Ea Ea J 2 (b) cosΦ
(27) is required. This may be kept constant. In any case, the synchronous detection section 18
The phase modulation element excitation control section 15 can control the phase modulation element 7 according to the signal, and can maintain the degree of modulation constant.

Here, we have explained the fundamental wave and the second harmonic, but the
Double harmonic. You can use more than that.

There is one thing to note. This means that the variables in the Bessel function are different. The variable of the Bessel function of the signal entering the first light receiving element 9 is ξ. From equation (l2), φ ξ= 2 b sin
There is a relationship as (28). The variable of the Bessel function of the signal entering the second light receiving element 16 is b itself.

Therefore, even if the Bessel functions are the same, the values of the Bessel functions are different. The value of φ is determined by the length of the sensor coil, but the relationship between b and ξ cannot be uniquely determined in advance because φ is in between.

However, it is said that the phase modulation element excitation control unit is controlled so that the fundamental wave, second harmonic, etc. of the signal generated by interfering the modulated light that has passed through the optical fiber for correction processing and the reference light that does not pass through are constant. This poses no disadvantage to the present invention.

This is because there is no problem with this method even if φ is not determined in advance. On the contrary, the degree of freedom in control has increased.

This will be explained with reference to FIG.

This is a graph of a portion of a linear Bessel function. The fundamental wave of the output of the main optical fiber is shown above with ξ as a variable. The fundamental wave of the interference output between the optical fiber for correction processing and the reference light with b as a variable is shown below.

Assume that each of them is ξo1bo at the time of initial setting.

This is 2 points. The value and gradient at this point are arbitrary. There are only the constraint conditions of equations (12) and (28).

Assume that the modulation degree b fluctuates due to temperature fluctuations, aging, etc., and changes to the negative point. Along with this, ξ also changes and moves to point Nu. However, according to the present invention, the action of the phase modulation element excitation control section 15 causes the recovery movement from point N to point two to be performed immediately. Then ξ is also the initial setting value ξ. recover to.

(h) Effects In a phase modulation type optical fiber gyro, even if the modulation degree of the phase modulation element changes due to environmental temperature or changes over time, a constant phase modulation degree is always maintained at any sensor temperature. be able to.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the principle configuration of a phase modulation type optical fiber gyro according to the present invention. FIG. 2 is a diagram for explaining the deviation from the initial setting value and recovery of the two Bessel functions that appear in the main optical fiber and the optical fiber for correction processing. FIG. 3 is a diagram showing the principle configuration of a conventional phase modulation type optical fiber gyro. 1 ● ● 2 ● ● 3, 4 5 ● ● 6 ● ● 7 ● Fist 8 ● ● 9 ● ● 1 0 ● 1 l ● 1 2 ● ● Light emitting element ● Beam splitter fist ● Coupling lens ● Optical fiber ● Sensor Coil●Phase modulation element●Optical fiber phase modulation unit●Receiving optical element●Oscillator●Emitting element●Beam splitter●Optical coupling element●Optical fiber for correction processing●Phase modulation element excitation control unit●Receiving optical element Synchronous detection section ● Synchronous detection section ● Angular frequency of phase modulation ● Angular frequency of light of first light emitting element ● Angular frequency of light of second light emitting element ● Phase change of light passing through sensor coil Φ● ●● Phase difference between light passing through the optical fiber for correction processing and light not passing through it Δθ●● Phase difference Ω proportional to rotational angular velocity ● ω ● ω 0 φ ●

Claims (1)

[Claims] 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 light passing from the light emitting element through 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 receives the light that propagates through the optical fiber and combines the light emitted from both ends via the beam splitter; In a phase modulation type optical fiber gyro having at least a synchronous detection circuit for detecting a modulated frequency component, an optical fiber for correction processing is wound around the phase modulation element, and light from a second light emitting element that generates coherent light is emitted. Divide into two,
One is made to propagate through the optical fiber for correction processing and is made incident on the second light receiving element, and the other one is made to be made directly incident on the second light receiving element, and synchronous detection is performed from the output of the second light receiving element. The phase modulation element excitation control part extracts a signal component having a frequency of one or two times the phase modulation frequency, and the phase modulation element excitation control part drives the phase modulation element so that the output thereof is constant. Phase modulation depth correction method for optical fiber gyro.