JPH0511845B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to an optical fiber gyro, and more specifically, to a highly accurate phase modulation type optical fiber gyro.

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 gyroscopes 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. Other ``Optical Fiber Sensor Technology'' Institute of Quantum Electronics (Giallorenzi TG, Bucaro JA
et al “Optical Fiber Sensor Technology”,
IEEE J. of Quantum Electronics) QE−18,
No. 4, pp626-662 (1982) and Kurashiyo and I. P. Culshaw and IPGiles “Fiber Optic” Journal of Physics Electronics Science Instruments
Gyroscopes”J.Phys.E:Sci Instrument) 16 pp5
-15, (1983) and Tsubokawa, Otsuka, “Optical Fiber Gyroscope” Laser Research, 11 , No.. 12, pp889
-902 (1983) and other publications.

(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 12
The optical fiber loop is made by winding the single mode optical fiber 18 many times in the form of a coil, and is input to both ends of the sensor coil 20.
to propagate light. At that time, the sensor coil 20
When 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 propagated clockwise and 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 clockwise light with such a 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 counterclockwise light and clockwise light is square-law detected using a light receiving element, the output I is: 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 using an 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 modulation element 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 coil will be explained with reference to FIG.
is connected to both ends of the The optical fiber 18 includes a portion wound to constitute a sensor coil 20 and a phase modulation portion 24 of the optical fiber wound around a phase modulation element 22 such as a piezo element driven at an angular frequency ω _n . It is divided into and,
The light coupled from both ends of the optical fiber propagates clockwise and counterclockwise within the sensor coil 20 of the optical fiber, exits from the opposite end, is combined by the beam splitter 12, and is sent to the light receiving element 26.
incident on .

When the phase modulation element is installed at an asymmetric position with respect to the sensor coil, the light emitted from the light emitting element at the same time passes through the sensor coil and the phase modulation element winding part in clockwise and counterclockwise directions, but the light is not modulated. Since the times are different, 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, if 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, then the time required for light to pass through the sensor coil is τ
is τ=nL/c (4).

Here, the phase modulation element 22 is provided near the incident end of the counterclockwise light, and the modulation signal of the phase modulation element 22 is
As mentioned above, it is assumed that it is a sine wave with an angular frequency ω _n . When the light that exits the light emitting element at the same time is split into clockwise light and counterclockwise light, and each undergoes phase modulation, the phase difference φ of the modulation signal is 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 modulation element. If the amplitude of the phase modulation element is b, the electric field strengths E _CW and E _CCW of the clockwise and counterclockwise lights are E _CW = E _r sin {ωt + Δθ/2 + bsin (ω _n t + φ)}...(6
) E _CCW = E _L sin {ωt−Δθ/2+bsin(ω _n t)} …(7).

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 E _CCW 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+φ/2)}+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 modulation element, 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 _L {cosΔθcos[2bsinφ/2c
os(ω _n t+φ/2)] −sinΔθsin[2bsinφ/2cos(ω _n t+φ/
2)]...(10) On the other hand, from the generating function expansion of the Betzel function, e ^x/2(t-1/t) = _∞ 〓 ^n=-∞ J _o (X)tn...(11). By setting t= e ⁱ 〓, it can be expressed as e ^ixsin 〓= _∞ 〓 ^n=-∞ J _o (X) e ⁱ 〓 …(12). From the expansion of the real and imaginary parts of equation (12), we can obtain the series expansion of the cosi and sn 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 …(1
3) After converting θ→θ+π/2, J _-o (X) = (-)nJ _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 _p (ξ) + 2 _∞ 〓 ⁿ⁼¹ (−) ⁿ J _2o (ξ) cos2nω _n t …(16 ) S _s =2 _∞ 〓 ⁿ⁼⁰ (−)nJ _2o+1 (ξ)cos(2n+1)ω _n t
…(17) becomes. 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 _p (ξ) cos Δθ + E _r E _L 2 _∞ 〓 ⁿ⁼¹ (-1) nJ _2o (ξ)cos2nω _n t・cosΔθ +E _r E _L 2 _∞ 〓 〓 ⁿ⁼⁰ (−1)nJ _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 −2E _r E _L J ₃ (ξ) cos3ω _n t・sinΔθ +2E _r E _L J ₄ (ξ) cos4ω _n t・cosΔθ + higher-order component...(10)b This is the series sum of the fundamental wave of the modulation signal ω _n and the harmonic signal.

By using an appropriate filter, it is possible to extract the fundamental wave ω _n or a high frequency signal of any order.
No matter which signal is adopted, the magnitude of cosΔθ or sinΔθ can be known.

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

The highest sensitivity can be expected from the first-order term (n=0) in equation (17), that is, the second term on the right side of equation (10)b.
This is the fundamental wave component. 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.

Problems to be Solved by the Invention In the conventional optical fiber gyro, when extracting the fundamental wave ω _n component from the signal shown in equation (10)b above, a rectangular wave with an angular frequency ω _n is used as a reference signal and synchronous detection is performed. Then, we obtain the DC output 2E _r E _L sin Δθ.

However, a rectangular wave with an angular frequency ω _n whose duty is not exactly 1/2 also contains a 2ω _n component that cannot be completely removed even with a low-pass filter, and as a result, the equation (10) b above is of the signal shown
The 2ω _n component is also synchronously detected and the DC component −2E _r E _L J ₂ (ξ)cosΔθ is output.
This caused noise, making it difficult to maintain high accuracy with the optical fiber gyroscope.

Therefore, the present invention aims to eliminate the above noise and provide a highly accurate phase modulation type optical fiber iron.

Means for Solving the Problems According to the present invention, as shown in FIG. A bidirectional fiber 18 is provided near one end of the optical fiber to couple the light propagated to both ends of the sensor coil and output the light propagated in both directions from both ends. a phase modulator 30 that performs phase modulation at a frequency ω _n ;
It is equipped with a light-receiving element 26 that receives the two-way light propagated through the optical fiber, and a synchronous detection device that receives the output of the light-receiving element and performs synchronous detection, and detects the two-way light generated when the sensor coil 20 rotates. In a phase modulation type optical fiber gyro that measures rotational angular velocity from a phase difference, the synchronous detection device receives the output of the light receiving element 26 and performs synchronous detection at an angular frequency 2ω _n that is twice the modulation angular frequency. A phase modulation method characterized by comprising a detector 40 and a second synchronous detector 42 that receives the output of the first synchronous detector 40 and performs synchronous detection at the same angular frequency ω _n as the modulation angular frequency. Optical fiber gyros are provided.

Function In the above-described phase modulation type optical fiber coil, light from the light emitting element propagates through the mode filter tough fiber via a beam splitter, and is further split into two beams by the beam splitter. (CW), counterclockwise light (CCW) propagates through the optical fiber.

One of the clockwise light and the counterclockwise light is first subjected to phase modulation by the phase modulation element, and the other light is subjected to phase modulation after a delay. At the same time, due to the rotation that the sensor coil receives, a phase difference Δθ is generated between the clockwise light and the counterclockwise light.

The clockwise light and counterclockwise light propagating through the optical fiber are
The light is emitted from the optical fiber, combined by the beam splitter, and propagated through the mode filter tough fiber.

The light beam propagated through the mode filter tough fiber enters the light receiving element via the beam splitter.

The light-receiving element generates an electric signal proportional to the light intensity, and upon receiving the output, the first synchronous detector extracts only a component of the angular frequency twice the phase modulation angular frequency. Furthermore, upon receiving the output of the first synchronous detector, the second synchronous detector extracts only the component having the same angular frequency as the phase modulation frequency.

The above signal processing can be analyzed as follows.

Assuming that the DC component is removed in advance from the output from the light receiving element, the first synchronous detector outputs an electric signal expressed by the following equation.

S ₁ =2E _r E _L J ₄ (ξ)cosΔθ・cos2ω _n t+2E _r E _L・
sinΔθ{J ₁ (ξ)−J ₃ (ξ)}・cosω _n t−2E _r E _L J ₂ (
ξ)
cos Δθ (19) Further, when the DC component is removed and the signal S ₁ is synchronously detected by a second synchronous detector, an electric signal expressed by the following equation is output.

S ₂ = 2E _r E _L J ₄ (ξ) cosΔθ・cosω _n t+2E _r E _L {J ₁
(ξ)−J ₃ (ξ)}・sinΔθ …(20) If only the DC component is extracted from the above output signal,
An output proportional to sinΔθ, that is, an angular velocity signal is obtained.

At this time, if an angular frequency component of 2ω _n is included in the rectangular wave of the reference angular frequency ω _n of the second synchronous detector, the output after the synchronous detection shown in equation (20) above will contain a DC current. The component 2ηE _r E _L J ₄ (ξ)cosΔθ will also be mixed in as noise. Here, η represents the ratio of the 2ω _n component in the reference frequency signal to the ω _n component in the reference frequency signal.

Note that, as mentioned above, maximizing J ₁ (ξ) improves the sensitivity, so it is set to ξ = 1.8. At this time, as can be seen from Figure 4, which shows the Betzel function as a function of x instead of ξ, J ₁ (1.8) − J ₃
(1.8) = 0.491, J ₄ (1.8) 0.023, and the signal component 2E _r
The above noise component 2ηE _r E _L J ₄ (ξ)cosΔθ has a sufficiently small value with respect to E _L {J ₁ (ξ)−J ₃ (ξ)}sinΔθ.
Also, since J ₂ (1.8) = 0.306, the above noise component is
This is sufficiently small compared to the noise component -2E _r E _L J ₂ (ξ) cos Δθ in the above-mentioned conventional fiber optic gyroscope.

Embodiments Hereinafter, embodiments of the optical fiber coil according to the invention will be described with reference to the accompanying drawings.

FIG. 5 is a diagram showing the configuration of an embodiment of a phase modulation type optical fiber coil embodying the present invention. For the minimum configuration with the basic requirements for optical fiber optics, please refer to Izekiel S. and A.D.H. Ezekil S. and Arditty HJ “Fiber Optic
Rotatior 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 a He--Ne laser, a semiconductor laser, a superluminescent diode, etc. can be used as the light source. The light beam generated by the light emitting element is transmitted through a beam splitter 1 like a half mirror.
2 and enters the mode filter tough fiber 16. The light beam propagated through the mode filter tough fiber 16 is split into two by the beam splitter 14 and coupled to both ends of the optical fiber 18 .

The optical fiber 18 consists of a sensor coil 20 wound into a coil shape many times and a portion coupled to a phase modulator 30 disposed near one end of the sensor coil 20 so as to constitute an optical fiber sensor. There is.

The phase modulator 30 is composed of, for example, a piezoelectric vibrating element, is connected to the AC excitation power source 32 for phase modulation via a frequency divider 34, and divides the rectangular wave of the angular frequency 2ω _n of the AC excitation power source 32 into 1/2. It is designed to be driven by an alternating current rectangular wave with an angular frequency ω _n .
In this case, the optical fiber 18 is wound around a phase modulator 30, for example.

The light beams propagated clockwise and counterclockwise through the optical fiber 18 are emitted from both ends of the optical fiber 18, are combined by the beam splitter 14, and enter the mode filter tough fiber 16. The light beam propagated through the mode filter tough fiber 16 enters the light receiving element 26 via the beam splitter 12.

The electrical output of the light receiving element 26 is connected to the input of the synchronous detector 40 via a DC blocking filter 50. The synchronous detector 40 synchronously detects the output from the light receiving element 26 using the reference frequency signal 2ω _n supplied from the AC excitation power source 32, and outputs a voltage signal having a component of the angular frequency 2ω _n .

The electrical output of the synchronous detector 40 is connected to the input of the synchronous detector 42 via a DC blocking filter 52. The reference frequency signal ω _n of the synchronous detector 42 is the angular frequency output from the AC excitation power source 32.
A frequency divider 34 that receives 2ω _n and outputs an angular frequency ω _n
Supplied from. The synchronous detector 42 synchronously detects the output from the synchronous detector 40 at an angular frequency ω _n ,
A voltage signal of the angular frequency component is outputted via the high-frequency rejection filter 54. That is, an angular velocity signal indicating the phase difference Δθ generated in the sensor coil 20 is obtained.

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 propagates through a mode filter fiber 16 via a beam splitter 12, is split into two by a beam splitter 14, and is coupled to both ends of an optical fiber 18.

The light beam input to the optical fiber 18 has a phase difference at the part of the sensor coil 20 undergoing rotation, and also has a phase difference at the part coupled to the phase modulator 30 driven by a rectangular alternating current with an angular frequency ω _n . is phase modulated at

The clockwise light beam and the counterclockwise light beam, which have a phase difference and are phase-modulated in the optical fiber 18, are outputted from both ends of the optical fiber 18, are combined by the beam splitter 14, and then enter the mode filter tough fiber 16. propagates and enters the light receiving element 26 via the beam splitter 12.

After the DC component is removed from the output of the light receiving element 26 by a filter 50, it is synchronously detected at an angular frequency of 2ω _n by a synchronous detector 40, and a voltage signal expressed by the above-mentioned equation (19) is output. After the DC component is removed from the output of the synchronous detector 40 by a filter 52, the angular frequency ω _n is determined by the synchronous detector 42.
The voltage signal expressed by the above equation (20) is output.

Only the DC component is extracted from the output signal of the synchronous detector 42 by the filter 54, and an output proportional to sin Δθ, that is, an angular velocity signal is obtained.

At this time, as described above, the noise component induced by the 2ω _n angular frequency component included in the rectangular wave from the frequency divider is included in the angular velocity signal.

However, by setting ξ=1.8 as described above, this noise component is sufficiently small relative to the angular velocity signal, and is also sufficiently small compared to the noise component in the conventional optical fiber gyro described above. can do.

Therefore, in the phase modulation type optical fiber gyro, noise induced during synchronous detection can be significantly reduced and high accuracy can be ensured.

Effects of the Invention As is clear from the above description, the phase modulation type optical fiber coil according to the present invention significantly reduces noise induced during synchronous detection and ensures high accuracy. Therefore, the optical fiber according to the invention
It can be used in a wide range of applications.

[Brief explanation of the drawing]

FIG. 1 is an optical system diagram illustrating the principle of the optical fiber coil according to the present invention, FIG. 2 is a basic configuration diagram illustrating the principle of the optical fiber coil, and FIG. 3 is a diagram of the phase modulation type optical fiber coil. FIG. 4 is a diagram showing the values of the Bessel function J _o ; FIG. 5 is a block diagram showing the configuration of one embodiment of the optical fiber coil according to the present invention. . Main reference number, 10... Light emitting element, 12, 14
... Beam splitter, 16 ... Mode filter tough fiber, 18 ... Optical fiber, 20 ... Sensor coil, 22 ... Phase modulation element, 26 ... Light receiving element, 30 ... Phase modulator, 321 ... AC excitation Power supply, 34... Frequency divider, 40, 42... Synchronous detector, 50, 52, 54... Filter.

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 phase-modulate the light propagating through the optical fiber at a given angular frequency; and a phase modulator that receives the light propagating in both directions through the optical fiber. A phase modulation type optical fiber coil comprising a light receiving element and a synchronous detection device that receives the output of the light receiving element and performs synchronous detection, and measures the rotational angular velocity from the phase difference between the two directions of light generated when the sensor coil rotates. The synchronous detection device includes a first synchronous detector that receives the output of the light receiving element and performs synchronous detection at an angular frequency twice the modulation angular frequency; A second device that performs synchronous detection at the same angular frequency as the modulation angular frequency.
A phase modulation type optical fiber gyro characterized by comprising a synchronous detector. 2. The output of the light receiving element is input to the first synchronous detector via a DC component removal filter, and the output of the first synchronous detector is input to the second synchronous detector via another DC component removal filter. 2. The phase modulation type optical fiber gyro according to claim 1, wherein a high-frequency rejection filter is connected to the input to the wave detector and to the output thereof to take out the DC component. 3. The phase difference φ and the modulation degree b of the two-way light by the phase modulator are set so that 2bsinφ/2 is approximately 1.8. Phase modulation optical fiber gyro.