JPH0469731B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to an optical fiber iron, and more specifically, to an optical fiber iron having a stable scale factor and a wide dynamic range.

BACKGROUND OF THE INVENTION Currently, gyros are used as angular velocity sensors for navigation and attitude control of aircraft, flying objects, automobiles, robots, and the like. By using this gyro, you can obtain not only angular velocity but also data such as direction 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. In addition, it is superior to conventional gyros in terms of minimum detectable angular velocity (sensitivity), drift, dynamic range, and scale factor stability. has been attracting attention and development in recent years.

Examples of such fiber optic tires include, for example, gear range gear. G. Bukalo J. , A. Other “Optical Fiber Sensor Technology”
EEEE Annual 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 Kurashio and I.
P. Gilles “Optical Fiber Gyroscope” Journal of Physics Electronics Science Instruments (Culshawand
IPGiles “Fiber Optic Gyroscops” J.Phys.E:
Sci Instrument) 16 pp5−15, (1983), Tsubokawa,
This is detailed in Otsuka's "Optical Fiber Gyroscope," Laser Research, 11 , No. 12, pp. 889-902 (1983). (a) Principle of optical fiber coil The principle of optical fiber coil will now be explained with reference to FIG. 6.

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.

The electric field strength E _CW of the light propagated clockwise and the light propagated counterclockwise in the sensor coil 20,
E _CCW 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 Sunyack 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 is of the form I∝{1+cos(Δθ)}...(2).

This has the disadvantage that since Δθ is included in cos, the sensitivity is poor when Δθ is close to 0, and the direction of rotation cannot be determined.

Therefore, an optical mechanism has been proposed in which the phase of either the counterclockwise or clockwise light is shifted by 90 degrees and square-law detection is performed. In this case, the output I has 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 gyro 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. The optical fiber 18
is divided into a portion wound to constitute a sensor coil 20 and a portion 24 wound around a phase modulation element 22 such as a piezo element driven at an angular frequency ω _n . The light coupled from both ends of the optical fiber is
It propagates clockwise and counterclockwise within the optical fiber sensor coil 20, exits from the opposite end, is combined by the beam splitter 12, and is sent to the light receiving element 2.
6.

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, assume that a phase modulator is provided near the incident 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 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 strengths 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 + 2hsin (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 introduced 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/2cos(
ω _n t+φ/2)} −sinΔθsn{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)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)
If we write E _r E _L ...(13), after converting θ→θ+π/2, J _-o (x)=(-) ⁿ J _o (x) ...(14) where n is positive. Using the property of being an integer, set ξ=2b sinφ/2 ...(15) and write the above S _c and S _s , then 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, once again, S(Δθ, t) is expressed as follows.

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⁼⁰ (−1) ⁿ J _2o+1 (ξ)cos(2n+1)ω _n t・sinΔθ
……(10)′ = 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)'' 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 modulation element, 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 the first-order term (n=0) in equation (17), that is, the second term on the right side of equation (20).This is the fundamental wave component.This fundamental wave component When is P(Δθ, t), P(Δθ, t) = 2E _r E _L J ₁ (ξ) cosω _n t・sinΔθ ...(18).Thus, an output proportional to sinΔθ is obtained, Δθ can be found by finding the amplitude of the fundamental wave component.

Furthermore, since sensitivity improves when J ₁ (ξ) is maximized, ξ is set to 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 As can be seen from equation (18) above, clockwise light,
The oscillation waves E _r and E _L of the counterclockwise light are included in the amplitude of the fundamental wave component P of the light receiving element output. Also, J ₁ (ξ)
There is also a coefficient.

In order to accurately determine Δθ from such an output, the values of E _r and E _L J ₁ (ξ) must be stable.

However, the values of E _r , E _L , and J ₁ (ξ) vary. In particular, the light amplitudes E _r and E _L tend to fluctuate.
E _r and E _L are fluctuations in the output of the light source, fluctuations in the amount of light passing through the polarizer inserted to remove the optical path difference,
It can easily change due to misalignment of the optical system, etc.

Furthermore, even if the direction of polarization when entering the fiber is determined, the state of polarization will fluctuate as it propagates through the fiber due to the effects of temperature, pressure, strain, etc.
The polarization of the emitted light is not constant.

As a result, the amplitudes of light E _r and E _L for the light receiving element are
will change.

In addition to this variation in scale factors,
Since Δθ is detected in the form of sin Δθ, linearity is poor in the high-speed range, and Δθ can only be detected in the range of π/2, making it difficult to achieve a wide dynamic range.

Therefore, the present invention improves linearity and provides a wide dynamic range without making the scale factor dependent on the above-mentioned E _r and E _L and without impairing the high performance characteristic of optical fiber coils. The present invention aims to provide an optical fiber gyroscope having the following characteristics.

Means for Solving the Problems According to the present invention, as shown in FIG. An optical fiber 1 whose light is split by a beam splitter 28, coupled to both ends, and which propagates through the sensor coil 20 in both directions and outputs the light from both ends.
8 and a light-receiving element 26 that receives the bidirectional light propagated through the optical fiber 18, and measures the rotational angular velocity from the phase difference between the bidirectional light generated when the sensor coil rotates. A phase modulator 30 provided near one end of the sensor coil and driven by a triangular wave 32 receives the output of the light receiving element so that the phase difference between clockwise and counterclockwise light within one cycle of the triangular wave is positive. An optical fiber gyroscope is provided which is equipped with a detector 34 that detects rotational angular velocity from the difference between a certain time and a negative time.

Effect In the above configuration, let us assume that the period of the modulated triangular wave is T and that the peak value of the modulated signal is φ _nax . When the fiber coil is stationary, clockwise light (hereinafter referred to as CW light) and counterclockwise light (hereinafter referred to as CW light) are emitted.
When the CCW light (abbreviated as CCW light) passes through this phase modulator, there is a difference in time τ (=nL/c) in which the light propagates through the fiber coil, as shown in FIG. 2a. As a result, there is a difference between CW light and CCW light.
As shown in FIG. 2b, a phase difference is generated that repeats with a period T. When the fiber coil is stationary, the time _T1 during which the CW light is ahead in phase with the CCW light is equal to the time _T2 during which the phase is delayed.
However, when the fiber coil rotates, a phase difference Δθ occurs, as shown in Figure 3.
The zero level shifts by Δθ, and a difference occurs between T ₁ and T ₂ .

At that time, T ₁ and T ₂ are respectively expressed as T ₁ =T/2+T/φ _nax Δθ, T ₂ =T/2−T/φ _nax Δθ, and the difference ΔθT between T ₁ and T ₂ is If measured, ΔT=2T/φ _nax Δθ (19), and an output proportional to Δθ, that is, the sannyac phase difference, can be obtained. In this case, the scale factor depends only on φ _nax and is not affected by changes in the amount of light, etc.

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

FIG. 4 is a diagram showing the configuration of one embodiment of a phase modulation type optical fiber coil embodying 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 ARDT H. J.A. "Optical fiber rotation sensor" Springer-Verlag Berlin (EzekilS.and Arditty HJ
“Fiber Optic Rotation Sensors”, Springer−
VerlagBerlin.) 1982 has a detailed explanation.

In the illustrated phase modulation type optical fiber iron, a light source such as a light emitting element 36 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 36 is sent to beam splitters 36A and 36B, which are like half mirrors arranged in series. The beam splitter 36A is for splitting light to the light receiving element 38, and the beam splitter 36B splits the light from the light source 36 into two and couples them to both ends of the optical fiber 40.

The optical fiber 40 is wound into a coil many times to form an optical fiber sensor, and includes a sensor coil 42 and a portion coupled to a phase modulator 44.

The phase modulator 44 is for phase modulating a triangular wave. However, phase modulation with a triangular wave is
This is difficult to achieve with a method that utilizes the resonance of a piezoelectric vibrator. For this reason, a method using an electro-optic crystal such as LiNbO ₃ is considered to be effective. From the viewpoint of modulation efficiency, it is desirable to use a waveguide type element. For example, as shown in FIG. 5, an optical waveguide 52 is provided on the surface of a LiNbO ₃ substrate 50, and electrodes 54 and 56 are embedded so as to sandwich the optical waveguide 52. In this case, an optical waveguide 52 is inserted into one end of the optical fiber 40 and between the electrodes 54 and 56,
Triangular wave modulation voltages with a period T supplied from the triangular wave generators 48A, 48B, . . . , 48N via the selection switch 46 are applied.

The light beams propagating clockwise and counterclockwise through the optical fiber 40 are output from both ends of the optical fiber 40 and are combined by the beam splitter 36B.
Through the beam splitter 36A, the light receiving element 38
incident on .

The electrical output of the light receiving element is connected to the input of a phase difference zero cross detector 58. The output of this phase difference zero cross detector 58 is input to a signal processor 60 and is also supplied to a selection switch 46 so that a triangular wave signal having an optimum period T is supplied to a phase modulator 44. be done.

The reason for adopting the above method is that the upper and lower limits of the dynamic range of this method are determined as follows.

The maximum detected angular velocity is determined by the maximum value of the time difference between T ₁ and T ₂ and must satisfy |T ₂ −T ₁ |<2τ.

Now, if the fiber length is 200m, τ=nL/c=1×10 ^-6 (sec) Δθ _nax =φ _nax /T×2×10 ^-6 ...(20) On the other hand, the minimum detectable angular velocity is T ₁ It depends on the accuracy of measuring the time difference between and T ₂ , and is generally |T ₂ −T ₁ |>20 nsec. Therefore, Δθmin= _φnax /T×20×10 ^-3 (21). If φ _nax and T are constant, the dynamic range is limited to two digits from equations (20) and (21).

The sanyac phase difference Δθ is Δθ=4πLa/cλΩ, so for example, assuming L=200 (m), a=25×10 ^-5 (m), and λ=0.83×10 ^-6 (m),
In order to detect with the dynamic range of 1×10 ^-4 (deg/sec) <Ω<300 (deg/sec), if Δθ=0.25Ω, then 2.5×10 ^-5 (deg)<Δθ<75( By setting φ _nax = 50 (deg) and varying T, the dynamic range expands, and the upper limit is 75 < 50/T x 2 x 10 ^-6 , or T < 1.4 x 10, from equation (20). ^-6 (sec), and the lower limit is 2.5×10 ^-5 >50/T×2×10 ^-9 , that is, T>0.04 (sec) from equation (21).

So, for example, if the frequency of the triangular wave is variable between 100Hz>f>1MHz, 1×4 ^-4
(deg/sec) to 300 (deg/sec) is possible. That is, as shown in FIG. 4, the dynamic range can be expanded by detecting the level of input angular velocity and switching the phase modulation frequency to a frequency corresponding to the detected level.

In reality, the output of the light receiving element is I(φ)=cosφ as a result of the interference between the CW light and the CCW light with respect to the phase difference φ between the two-way beams, so the zero cross of φ is Not easy to detect. For this purpose, a well-known method of CW light and
Higher accuracy is expected to be achieved by applying a sinusoidal phase bias at a constant frequency to the CCW light in the form of sinφ.

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

A light beam from a light emitting element 36 driven by a power source passes through a beam splitter 36A, is split into two by a beam splitter 36B, and is coupled to both ends of an optical fiber 40.

The light beam input to the optical fiber 40 has a phase difference at the portion of the sensor coil 42 undergoing rotation, and is also phase modulated at the portion coupled to the electro-optic effect phase modulator 44 .

The clockwise light beam and the counterclockwise light beam, which have a phase difference and are phase-modulated in the optical fiber 40, are output from both ends of the optical fiber 40, are combined by the beam splitter 36B, and are further combined by the beam splitter 36B. Light receiving element 3 via 36A
8.

The output of the light receiving element 38 is outputted to the signal processor 60 via the zero phase difference cross detector 58. The signal processor 60 determines the time T ₁ when the input signal is negative and the time T ₂ when it is positive, calculates the difference, and divides it by φ _nax according to equation (19) and outputs a signal representing Δθ. do.

Here, when the required modulation voltage of the electro-optic effect phase modulator 44 is calculated, it is as follows. The phase change due to the electro-optic effect in the electro-optic effect phase modulator 44 is expressed as follows.

φ=kΔnl Δn=1/2n ³ rE where k=2π/λ λ: wavelength n: refractive index r: electro-optic constant E: electric field (V _nax /t) V _nax : applied between electrodes 54 and 56 Maximum voltage t: Distance between electrodes 54 and 56 l: Length of electrodes 54 and 56 In addition, by appropriately selecting the polarization of light and the direction of applying the electric field, it is possible to utilize γ ₃₃ , which maximizes γ of LiNbO ₃ . . In LiNbO ₃ , n = 2.2, r ₃₃ = 30.8 × 10 ^-12 , φ = 360 / λ2.2 ³ × 30.8 × 10 ^-12 × lE (deg) λ = 0.83 × 10 ^-6 (m), l = 10×10 ^-3 (m), t=10×10 ^-6 (m), then φ=360/0.83×10 ^-6 ×2.2 ³ ×1/2 ×30.8×10 ^-12 ×10×10 ^-3 ×V/10×10 ^-6 =71V When V=0.7 (V), φ=50 (deg) is obtained.

In addition, phase modulators using triangular waves can use not only LiNbO ₃ but also phase modulators that use bulk electro-optic crystals such as LiTaO ₃ or optical waveguides using these as substrates. Any type of element may be used as long as it is capable of phase modulation using a triangular wave, without being limited to one that utilizes optical effects.

Effects of the Invention As is clear from the above explanation, the optical fiber coil according to the present invention has a scale factor that does not depend on the amplitudes E _r and E _L of the light propagating through the optical fiber, has excellent linearity, and has a wide dynamic range. has. Therefore, the optical fiber according to the present invention can be used in a wide range of applications.

[Brief explanation of drawings]

FIG. 1 is an optical system diagram illustrating the principle of the optical fiber coil according to the present invention, and FIG. FIG. 3 is a waveform diagram showing the phase difference of light in both directions, and FIG. 3 is a waveform diagram showing the relationship between the phase difference due to the triangular wave and the phase difference due to the sannyac effect when a phase difference due to the sannyac effect occurs due to rotation, FIG. 4 is an optical system configuration diagram of an optical fiber gyro according to the present invention, and FIG. 5 is a schematic perspective view illustrating an example of an electro-optic phase modulator used in the optical fiber gyro of FIG. 4. and
FIG. 6 is a basic configuration diagram illustrating the principle of an optical fiber tester, and FIG. 7 is a basic configuration diagram explaining the principle of a phase modulation type optical fiber tester. [Main reference numbers], 10... Light emitting element, 12...
... Beam splitter, 14, 16 ... Coupling lens, 18 ... Optical fiber, 20 ... Sensor coil, 22 ... Phase modulation element, 26 ... Light receiving element,
30... Phase modulator, 36... Light source, 36A, 3
6B... Beam splitter, 38... Light receiving element,
40...Optical fiber, 42...Sensor coil, 4
4... Electro-optic effect phase modulator, 46... Selection switch, 48A, 48B... 48N... Triangular wave generator, 50... LiNbO ₃ substrate, 52... Optical waveguide, 54, 46... Electrode, 58... ... Phase difference zero cross detector, 60 ... Signal processor.

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 coil is equipped with an optical fiber that outputs from both ends and a light receiving element that receives light in both directions propagated through the optical fiber, and measures the rotational angular velocity from the phase difference between the lights in both directions generated when the sensor coil rotates. A phase modulator driven by a triangular wave provided near one end of the sensor coil receives the output of the light receiving element to determine the phase difference between clockwise and counterclockwise light during one cycle of the triangular wave. An optical fiber gyro comprising: a detector for detecting rotational angular velocity from the difference between positive time and negative time. 2. The optical fiber gyro according to claim 1, wherein the period of the triangular wave is made variable according to the level of the input rotational angular velocity. 3. The optical fiber gyroscope according to claim 1 or 2, wherein the phase modulator is a phase modulator that utilizes an electro-optic effect. 4. The phase modulator according to claim 1 or 2, wherein the phase modulator is a phase modulator using a bulk of LiNbO ₃ , LiTaO ₃ or the like or an optical waveguide using these as a substrate. Hikari Faiba Jairo.