JPH0470562B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a phase modulation type optical fiber iron, and more specifically, to a signal detection circuit for the phase modulation type optical fiber iron.

BACKGROUND ART Currently, gyros are used in various fields, particularly as angular velocity sensors for navigation and attitude control of moving objects such as aircraft, flying objects, and automobiles. If you use this gyroscope,
Not only angular velocity, but also data such as orientation can be obtained by integrating it.

Among these types of gyroscopes, optical fiber gyroscopes can be used in any environment without shielding problems because the light and the optical fiber through which the light propagates are not easily affected by magnetic fields or electric fields, and they have no moving parts. Furthermore, it is superior to conventional gyros in terms of minimum detectable angular velocity (sensitivity), drift, measurable range (dynamic range), and scale factor stability. Therefore, it has been attracting attention and being developed in recent years.

Examples of such fiber optic tires include, for example, gear range gear. G. , Bukalo J. A. Others, ``Optical Fiber Sensor Technology,'' International Institute of Quantum Electronics (Giallorenzi TG, Bucaro J.
A.et al “Optical Fiber Sensor Technology”,
IEEE J. of Quantum Electronics) QE−18, No.
4, pp626-662 (1982) and Kurashio and I.
P. Gilles “Optical Fiber Gyroscope” Journal of Physics Electronics Science Instruments (Culshaw
and IPGiles “Fiber Optic Gyroscopes”J.
Phys.E: Sci Instrum.) 16 pp5−15, (1983)
Ya, Tsubokawa, Otsuka "Optical fiber gyroscope"
Laser Research, 11 , No. 12, pp. 889-902 (1983), etc.

(a) Principle of the optical fiber coil The principle of the optical fiber coil will now be explained with reference to FIG. 2.

The light from the light emitting element 10 is transmitted to the beam splitter 1
The optical fiber loop is made by winding the single mode optical fiber 18 many times in a coil shape, that is, the input is input to both ends of the sensor coil 20. Propagate light. At that time, if the sensor coil 20 is rotating at an angular velocity Ω,
A phase difference Δθ occurs between the clockwise light and the counterclockwise light, and the angular velocity Ω is detected by measuring Δθ.

Electric field strengths E _cw and E _ccw of the light that propagated clockwise and the light that propagated counterclockwise in the sensor coil 20
is expressed as follows.

E _cw = E _r sin (ωt + Δθ/2) E _ccw = E _l sin (ωt − Δθ/2) However, E _r , E _l : Amplitude of counterclockwise light and clockwise light ω : Angular frequency of light t : Time Δθ: Phase difference due to the sannyac effect The counterclockwise light and the clockwise light with the phase difference Δθ are combined by the beam splitter 12 and are incident on the light receiving element 26. The phase difference Δθ can be determined from the detection intensity of the light receiving element 26.
The phase difference Δθ can be expressed as follows.

Δθ＝4πLa／cλΩ ……(1) however, L: Fiber length of sensor coil a: radius of sensor coil c: speed of light in vacuum λ: wavelength of light Ω: rotational angular velocity This is called the sanyatsuk effect.

There are various methods of detecting the phase difference Δθ.
Various things have been proposed.

Most simply, when the sum of the counterclockwise light and the clockwise light is square-law detected using a light receiving element, the output I becomes I∝{1+cos(Δθ)}...(2).

Since Δθ is included in cos, this has the disadvantage of poor sensitivity when Δθ is close to 0.

Therefore, an optical mechanism has been proposed in which the phase of either counterclockwise or clockwise light is shifted by 90 degrees and square law detection is performed. In this case, the output I takes the form I∝{1+sin(Δθ)} (3), so the sensitivity is good when Δθ is close to 0.

However, in order to separate one of the lights, three new beam splitters are required to separate the optical paths. Furthermore, the lengths of the separated optical paths must always be made equal.

Improving the sensitivity when Δθ is close to 0 by using a static optical detection mechanism as described above has the above-mentioned difficulties.

(b) Phase modulation type optical fiber irons Therefore, many optical fiber irons that attempt to detect Δθ using a dynamic mechanism have been proposed. For example, a phase modulation method, a frequency modulation method, etc. are used. Among them, the phase modulation optical fiber iron is the most superior in terms of minimum detectable angular velocity.

The phase modulation type optical fiber iron is a method in which a phase modulator is provided at one end of an optical fiber sensor coil, and the phase difference Δθ is determined by measuring the magnitude of the modulation signal.

The phase modulation type optical fiber iron will be explained with reference to FIG.

The coherent light from the light emitting element 10 is split into two by a beam splitter 12 and coupled to both ends of an optical fiber 18. The optical fiber 18
is divided into a part wound to form the sensor coil 20 and a phase modulation part of an optical fiber wound around a phase modulator 24 such as a piezo element driven at an angular frequency ω _n . .
Then, the light coupled from both ends of the optical fiber is transmitted to the sensor coil 20 of the optical fiber.
The light beams propagate clockwise and counterclockwise within the center, exit from the opposite end, are combined by the beam splitter 12, and enter the light receiving element 26.

If the phase modulator is placed asymmetrically with respect to the sensor coil, the light emitted from the light emitting element at the same time will
The light passes through the sensor coil and the phase modulator winding section in clockwise and counterclockwise directions, but the modulation times are different, so when the output is square-law detected by the light receiving element, a modulated signal appears in the output. Since Δθ is included in the amplitude of the modulation signal, Δθ can be determined by knowing the magnitude of the modulation signal.

For example, assume that a phase modulator is provided near the input end of counterclockwise light. When the length of the optical fiber sensor coil is L, the refractive index of the fiber core is n, and the speed of light is c, the time τ required for light to pass through the sensor coil is τ=nL/c (4).

As mentioned above, the modulation signal has an angular frequency ω _n
Suppose that it is a sine wave. The phase difference φ of the modulation signal when the light emitted from the light emitting element at the same time is divided into clockwise light and counterclockwise light, each undergoing phase modulation.
φ=ω _n τ =nLω _n /c =2πf _n nL/c (5) However, ω _n =2πf _n .

Due to the sannyac effect, the clockwise light and the counterclockwise light have a phase difference of ±Δθ/2, but the phase is further modulated by the phase modulator. If the amplitude of the phase modulator is b, the electric field strengths E _cw and E _ccw of the clockwise light and counterclockwise light are E _cw = E _r sin {ωt + Δθ/2 + bsin (ω _n t + φ/2)}
...(6) E _ccw = E _l sin{ωt+Δθ/2+bsin(ω _n t−φ/2)
}...(7) becomes.

The clockwise light and counterclockwise light having the electric field strength as described above are combined by the beam splitter 12 and square-law detected by the light receiving element 26, so the output S(Δθ, t) of the light receiving element is E _cw and _Eccw squared.

S (Δθ, t) = {E _cw + E _ccw } ² ...(8) Calculating this, S (Δθ, t) = E _r E _l cos {Δθ + 2bsin (φ/2) cos
ω _n t}+D.C.+{2ω or more}...(9) However, DC means a direct current component.

{2ω or more} means a term with a frequency twice the angular frequency of light. Note that this is 0 because it is not applied to the detector.

becomes. Thus, because of the phase difference φ provided by the phase modulator, Δθ can be obtained in relation to the amplitude of the modulation signal.

Therefore, DC is omitted and S(Δθ, t) is expanded into a series using the Betzel function. First, equation (9) is expressed as follows.

S(Δθ,t)=E _r E〓{cosΔθcos〔2bsin(φ/2co
sω _n t〕−sinΔθsin〔2bsin(φ/2cosω _n t〕}…
…(10) On the other hand, from the generating function rotation of the Betzel function, e ^x/2(t-1/t) = _∞ 〓 ^n=-∞ J _o (x)t ⁿ …(11). By setting t=e ⁱ 〓, it can be expressed as e ^ixsin 〓= _∞ 〓 ^n=-∞ J _o (x) e ⁿⁱ 〓 ...(12). From the expansion of the real and imaginary parts of equation (12), we can obtain the series expansion of the cos and sin parts of equation (10). Divide S (Δθ, t) into these parts, S (Δθ, t) = (S _c cos Δθ + S _s sin Δθ) E _r E _l ... (
13) After converting θ → θ + π/2, J _-o (x) = (-) ⁿ J _o (x) ... (14) However, using the property that n is a positive integer, , ξ=2bsinφ/2 ...(15) and writing the above S _c and S _s , S _c = J ₀ (ξ) + 2 _∞ 〓 ⁿ⁼¹ (-) ⁿ J _2o (ξ) cos2nω _n t ...(16) S _S =2 _∞ 〓 ⁿ⁼⁰ (-) ⁿ J _2o+1 (ξ)cos(2n+1)ω _n t ...(17) Therefore, S(Δθ, t) is expressed as follows again.

S (Δθ, t) = 1/2 (E _r ² E _l ² ) + (component over 2ωt) + E _r E _l J ₀ (ξ) cosΔθ + E _r E _l 2 _∞ 〓 ⁿ⁼¹ (-1) ⁿ J _2o (ξ)cos2nω _n t・cosΔθ +E _r E _l 2 _∞ 〓 〓 ⁿ⁼¹ (−1) ⁿ J _2o+1 (ξ)cos(2n+1)ω _n t・sinΔθ
...(10)a = DC component +2E _r E _l J ₁ (ξ)cosω _n t・sinΔθ−2E _r E _l J ₂
(ξ) cos2ω _n t·cosΔθ+higher-order component...(10)b This is the sum of the series of the fundamental wave of the modulation signal ω _n and the high-frequency signal.

By using an appropriate filter, it is possible to extract the fundamental wave ω _n or a harmonic signal of any order. No matter which signal is adopted, cosΔθ or sinΔθ
You can know the size of

In that case, the Betzell function J _o (ξ) of that order
The amplitude b of modulation by the phase modulator, the modulation angular frequency ω _n , and the sensor coil transit time τ should be set so that the value of ω n is large.

The highest sensitivity can be expected from equation 1 in (17).
This is the next term (n=0), that is, the second term on the right side of equation (10)b. This is the fundamental wave component. If this fundamental wave component is P(Δθ, t), then P(Δθ, t)=2E _r E _l J ₁ (ξ)cosω _n t・sinΔθ...
(18). In this way, an output proportional to sin Δθ is obtained, and Δθ can be found by finding the amplitude of the fundamental wave component.

Note that since sensitivity improves when J ₁ (ξ) is maximized, it is set to ξ = 1.8. At this time, the DC component J ₀ (ξ) is approximately 0.

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

As the output of the light-receiving element, a component of phase modulation angular frequency ω _n and its harmonic components are obtained as expressed by equation (10) a. The even-numbered harmonic components of ω _n have amplitudes proportional to cosΔθ, and are maximum when the gyro is not rotating, that is, when Δθ=0. On the other hand, the amplitudes of ω _n and its odd harmonic components are proportional to sin Δθ, and are 0 when Δθ=0. Therefore, in the region where Δθ is small, ω _n
By extracting the amplitude of the component or its odd harmonic components, an output proportional to the rotational angular velocity can be obtained. Normally, the ω _n component is extracted, and the amplitude of this ω _n component, including the sign representing the rotational direction of the gyro, is detected by synchronous detection.

FIG. 4 is a schematic diagram showing the configuration of a signal detection circuit of a conventional phase modulation type optical fiber coil.

In the illustrated circuit, the light-receiving signal output from the light-receiving element 26 is amplified by the amplifier 32 and input to the synchronous detector 34 . On the other hand, the output of the drive circuit 22 that drives the phase modulator at a given phase modulation frequency is connected to the input of the phase adjustment circuit 30, and the output of the phase adjustment circuit 30 is supplied as a reference signal to the synchronous detector 34. be done. Phase adjustment circuit 30, amplifier 32 and synchronous detector 3
4 constitutes the signal detection circuit 28.

The signal detection circuit configured as described above operates as follows.

The light-receiving signal output from the light-receiving element 26 is amplified by the amplifier 26 and then input to the synchronous detector 34 . The frequency signal that excites the phase modulator output by the phase modulator wave number drive circuit 22 is as follows:
The signal is input to the phase adjustment circuit 30, and the phase shift with respect to the above-mentioned light reception signal is corrected. The signal whose phase shift has been corrected is sent to a synchronous detector 34 as a reference signal.
is input. The synchronous detector 34 synchronously detects the received light signal at the angular frequency of the reference signal.

In this way, conventionally, a reference signal for synchronous detection has been obtained from a signal that excites a phase modulator. At this time, since there are uncertain phase changes such as phase changes during electromechanical conversion in the phase modulator,
The phase of the ω _n component expressed by equation (10) a is generally not determined. During synchronous detection, if the phases of the received light signal and the reference signal are shifted by ψ, the output will be the value obtained by multiplying the right side of equation (18) by cosψ. In order to obtain the maximum output, it was necessary to provide a phase adjustment circuit 30 for correcting the phase shift.

Problems to be Solved by the Invention As mentioned above, the signal detection circuit of the conventional phase modulation type optical fiber coil is equipped with a phase adjustment circuit, but the phase adjustment accuracy of the phase adjustment circuit itself is
There was a problem that the gyro's angular velocity detection accuracy was affected. Further, normally, the phase adjustment by the phase adjustment circuit is performed only once and is fixed after adjustment. However, due to the temperature characteristics of the phase modulator, variations occur in the phase change during electromechanical conversion during use. As a result, there was a problem that the angular velocity detection accuracy of the gyro was affected. Furthermore, the phase adjustment work has to be performed for each individual phase modulation type optical fiber coil, specifically for each phase modulator, which is very complicated.

Therefore, the present invention provides a signal detection circuit for a phase modulation type optical fiber coil that can effectively compensate for fluctuations in phase changes during electromechanical conversion due to temperature characteristics, etc., and does not require adjustment for each phase modulator. This is what we intend to provide.

Means for Solving the Problems According to the present invention, a light emitting element;
An optical fiber including a sensor coil portion wound in a coil shape many times, into which light from the light emitting element is branched and coupled to both ends, and the light propagated in both directions through the sensor coil is output from both ends; a phase modulator that is provided near one end of the optical fiber and modulates the phase of light propagating through the optical fiber; a light receiving element that receives the bidirectional light that has propagated through the optical fiber; and a synchronous detector that performs synchronous detection using a reference signal of a phase modulation frequency, and the rotational angular velocity is measured from the phase difference between the two directions of light generated when the sensor coil rotates. As shown in the figure, a reference signal extraction circuit 35 is further provided, which receives the output of the light receiving element 26, extracts a frequency component equal to the phase modulation frequency, and outputs it to the synchronous detector 34 as the reference signal.

Operation The signal detection circuit configured as described above operates as follows.

Letting the phase modulation frequency of the phase modulator be ω _n ,
The reference signal extraction circuit 35 detects the frequency ω _n component or its harmonic component from the light reception signal output from the light receiving element 26 , and in the case of a harmonic component, further divides the frequency to obtain the frequency ω _n component. It is extracted and output to the synchronous detector 34 as a reference signal. Therefore, the reference signal is a signal that has undergone fluctuations such as phase changes during electromechanical conversion due to temperature characteristics, etc., and is in phase with the frequency ω _n of the light reception signal input to the synchronous detector. Therefore, if the received light signal is synchronously detected with the reference signal described above, it will not be subject to fluctuations such as phase changes during electromechanical conversion due to temperature characteristics, etc.
Maximum output can be obtained.

The output signal of the light receiving element is expressed as in equation (10)a. Usually, the modulation degree is set around ξ=1.8 so that the coefficient J ₁ (ξ) of the ω _n component is maximized. At this time, J ₁ (ξ)≒0.58. On the other hand, the amplitude of the 2ω _n component is proportional to J ₂ (ξ), and J ₂ (ξ)≈0.31. Therefore, 0.58sinΔθ<0.31cosΔθ, i.e. 0°<Δ
θ
In the region <28°, the 2ω _n component of the output of the light receiving element is larger than the ω _n component.

In general, phase modulation type optical fiber coils are
It is used to detect rotational angular velocity limited to a region where sinΔθ≒Δθ can be approximated. When Δθ=28°, sinΔθ
Approximating Δθ causes an error of about 4%, so
Normally, it is not applied to detection where Δθ exceeds 28°.

Further, even when performing detection in which Δθ exceeds 28°, it is easy to make the 2ω _n component larger than the ω _n component using a filter.

Therefore, the reference signal extraction circuit extracts the signal of the 2ω _n component, which has the largest component, and sets the threshold value to
2ω Divide by a frequency divider lower than the amplitude of the _n component to obtain ω _n
Preferably, the composition is configured to obtain the components. This ω _n component is used as a reference signal during periodic detection.

Embodiments Hereinafter, embodiments of a signal detection circuit for a phase modulation type optical fiber gyro according to the present invention will be described with reference to the accompanying drawings.

FIG. 5 is a diagram showing the configuration of one embodiment of a phase modulation type optical fiber coil using the signal detection circuit of the present invention. This phase modulation type optical fiber coil has a minimum configuration that meets the basic requirements of an optical fiber coil. Regarding the minimum configuration, please refer to Izekiel S. and ADT H. J.A. "Optical fiber rotation sensor" Springer-Verlag Berlin (Ezekil S.and
Arditty HJ “Fiber Optic Rotation Sensors”
Springer-Verlag Berlin.) 1982 has a detailed explanation.

In the illustrated phase modulation type optical fiber iron, a light source such as a light emitting element 10 is provided, and is driven by a power source (not shown) to generate a light beam. Note that as a light source, a He--Ne laser, a semiconductor laser, a superluminescent diode, etc. can be used. The light beam generated by the light emitting element 10 is sent to beam splitters 12 and 14, which are like half mirrors arranged in series. The beam splitter 12 is for splitting light to the light receiving element 26, and the beam splitter 14
splits the light from the light source 10 into two and couples them to both ends of the optical fiber 18.

The optical fiber 18 is wound into a coil a number of times to form an optical fiber sensor, and includes a sensor coil 20 and a portion coupled to a phase modulator 24.

The phase modulator 24 is composed of, for example, a piezoelectric vibrating element, is connected to the phase modulator drive circuit 22, and is configured to be driven at an angular frequency ω _n . In this case, the optical fiber 18 is wound around, for example, a piezoelectric vibrating element.

The light beams propagated clockwise and counterclockwise through the optical fiber 18 are output from both ends of the optical fiber 18, are combined by the beam splitter 14, and are incident on the light receiving element 26 via the beam splitter 12.

The electrical output of the light receiving element 26 is input to an amplifier 32. The output of the optical signal amplifier 32 is passed through a bandpass filter 36 that passes the component of angular frequency ω _n .
It is connected to the input of the synchronous detector 34 via.

The output of the light receiving element 26 is further input to the 1/2 frequency divider 42 via a bandpass filter 37 that passes the component of angular frequency 2ω _n . The output of the 1/2 frequency divider is inputted to the synchronous detector 34 as a reference signal via the phase correction circuit 40. The synchronous detector synchronously detects the received light signal using the reference signal and outputs the result.

The phase modulation type optical fiber coil configured as described above operates as follows.

A light beam from a light emitting device 10 driven by a power source passes through a beam splitter 12 and is split into two by a beam splitter 14 and then split into two optical fibers 18.
is connected to both ends of the

The light beam input to the optical fiber 18 has a phase difference in the part of the sensor coil 20 undergoing rotation, and also in the phase modulator 24 driven by the angular frequency ω _n from the phase modulator drive circuit 22. Phase modulated.

The clockwise light beam and the counterclockwise light beam, which have a phase difference and are phase modulated in the optical fiber 18, are output from both ends of the optical fiber 18, are combined by the beam splitter 14, and are further combined by the beam splitter 14. 12 to the light receiving element 26
incident on .

The light receiving signal outputted by the light receiving element 26 is transmitted to the amplifier 3.
2, and the 2ω _n component in the received light signal is extracted by a bandpass filter 37 and then passed through a frequency divider 42.
The reference signal having an angular frequency of ω _n is output to the synchronous detector 34 via the phase correction circuit 40 . On the other hand, the ω _n component is extracted from the received light signal by the bandpass filter 36 and input to the synchronous detector 34, where it is periodically detected using the reference signal and a signal indicating the rotation angle is output.

Note that since there is a phase change in the electric circuits such as the internal circuits and attached circuits of the filter 37 and frequency divider 42, the ω _n component in the optical signal and the reference signal ω _n
The phase with the component does not necessarily match. Therefore, a phase correction circuit 40 is provided to equalize the phases.
However, since this phase correction circuit 40 corrects phase changes caused only by the electric circuit, it corrects a predetermined phase amount. Therefore, the phase correction circuit 40 does not need to perform adjustment for each individual phase modulation type optical fiber coil, specifically, for each phase modulator.

Effects of the Invention As is clear from the above explanation, in the signal detection circuit of the phase modulation type optical fiber coil according to the present invention, since the reference signal for synchronous detection is extracted from the received light signal, the temperature characteristics of the phase modulator are Even if variations occur in the phase change or the like during electromechanical conversion during use due to factors such as this, it is possible to effectively compensate for it and obtain the maximum output.

The reference signal extraction circuit that extracts the reference signal for synchronous detection from the received light signal eliminates the need for a phase adjustment circuit that requires adjustment for each individual phase modulation type optical fiber coil. Therefore, since extremely complicated phase adjustment is not required, installation and adjustment of the phase modulation type optical fiber gyro can be simplified.

Although the signal detection circuit of the phase modulation type optical fiber iron according to the present invention uses a phase adjustment circuit that compensates for phase changes in the electric circuit, it can be set independently as an electric circuit.
Therefore, phase correction can be performed with high accuracy, and the accuracy of phase correction does not depend on the characteristics of the phase modulator, so that the measurement performance of the optical fiber gyro is improved.

Therefore, the signal detection circuit for the phase modulation type optical fiber gyro according to the present invention can be utilized in a wide range of applications.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of the configuration of a signal detection circuit of a phase modulation type optical fiber coil according to the present invention.
FIG. 3 is a basic configuration diagram illustrating the principle of an optical fiber gyro, and FIG. 3 is a basic configuration diagram illustrating the principle of a phase modulation type optical fiber gyro.
FIG. 4 is a schematic configuration diagram of a signal detection circuit of a conventional phase modulation type optical fiber gyro, and FIG. 5 is a configuration schematic diagram of an embodiment of a phase modulation type optical fiber gyro using the signal detection circuit of the present invention. It is. [Main reference numbers], 10... Light emitting element, 12...
...Beam splitter, 18...Optical fiber, 20
... Sensor coil, 22 ... Phase modulator drive circuit, 24 ... Phase modulator, 26 ... Light receiving element, 2
8... Signal detection circuit, 30... Phase adjustment circuit, 3
2... Optical signal amplifier, 34... Synchronous detector, 3
6, 37...filter, 40...phase correction circuit,
42... Frequency divider.

Claims (1)

[Scope of Claims] 1. A device comprising a light emitting element and a sensor coil portion wound in a coil shape many times, and in which light from the light emitting element is branched and coupled to both ends, and light propagated in both directions through the sensor coil is split. an optical fiber that outputs from both ends; a phase modulator that is provided near one end of the optical fiber to modulate the phase of light propagating through the optical fiber; and a light receiving element that receives light that has propagated in both directions through the optical fiber. , a synchronous detector that synchronously detects the output of the light receiving element with a reference signal of the phase modulation frequency of the phase modulator, and detects the rotational angular velocity from the phase difference between the two directions of light generated when the sensor coil rotates. The phase modulation type optical fiber iron to be measured further comprises a reference signal extraction circuit that receives the output of the light receiving element, extracts a frequency component equal to the phase modulation frequency, and outputs it as the reference signal to the synchronous detector. A phase modulation type optical fiber gyro. 2. The reference signal extraction circuit includes a filter that receives the output of the light receiving element and extracts a predetermined harmonic component of the phase modulation frequency, and a filter that receives the output of the filter and divides the frequency to a frequency equal to the phase modulation frequency. 2. A phase modulation type optical fiber iron according to claim 1, further comprising a frequency divider. 3. The reference signal extraction circuit further includes a phase correction circuit that compensates for phase changes in electric circuits that constitute the filter and the frequency divider. Phase modulation optical fiber gyro. 4. Claim 2 or 4, wherein the filter is a filter that extracts a harmonic component twice the phase modulation frequency, and the frequency divider is a 1/2 frequency divider. The phase modulation optical fiber gyro according to item 3.