JPH0545162B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to an optical fiber iron. More specifically, it relates to a highly sensitive phase modulation type optical fiber iron with less drift.

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" IE General of Quantum Electronics (Giallorenzi TG, Bucaro
JA et al “Optical Fiber Sensor Technology”,
IEEE J. of Quantum Electronics) QE−14, No.
4, pp618-662 (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"
It is explained in detail in Laser Research, 11 , No. 12.pp889-902 (1983).

(a) Principle of optical fiber coil The principle of optical fiber coil will now be explained with reference to FIG. 3.

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 14 many times in a coil shape, that is, inputting it to both ends of the sensor coil 16.
to propagate light. At that time, the sensor coil 16
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.

The electric field strength of the light propagated clockwise and the light propagated counterclockwise in the sensor coil 16 E _CW , 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 sannyac effect The counterclockwise light and the clockwise light having such a phase difference Δθ are combined by the beam splitter 12 and are incident on the light receiving element 18. The phase difference Δθ can be determined from the detected intensity of the light receiving element 18. This 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 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 sensitivity when Δθ is close to 0 by 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 near one end of a sensor coil of an optical fiber, and a phase difference Δθ is determined by measuring the magnitude of a modulation signal.

The phase modulation type optical fiber coil will be explained with reference to FIG.
2C to the beam splitter 12.
and are coupled to both ends of the optical fiber 14. The optical fiber 14 includes a portion wound to form a sensor coil 16, and a phase modulation of the optical fiber wound around a phase modulator 22 such as a piezo element driven by an oscillator 20 at an angular frequency ω _n . It is divided into a section 24. The light coupled from both ends of the optical fiber is
The light propagates clockwise and counterclockwise within the optical fiber sensor coil 16, exits from the opposite end, is combined by the beam splitter 12, and enters the light receiving element 18. The output of the light-receiving element 18 is amplified by a preamplifier 26, sent to a synchronous detector 28 via a low-pass filter 27, and synchronously detected at an angular frequency ω _n .

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, it is assumed that the phase modulator 22 is provided near the input end of the counterclockwise light, and that the modulation signal of the phase modulator 22 is a sine wave with an angular frequency ω _n as described above.
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, the phase difference φ of the modulation signal is φ=ω _n τ = nLω _n /c = 2π _n nL/ c...(5) However, ω _n =2π _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 18, 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) co
s(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 value is 0 because theoretically it cannot be detected by a 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／2cos(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)t ⁿ ...(11) be. 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 and write S (Δθ, t) = (S _C cos Δθ + S _S sin Δθ) E _r E _L …(13), then we can convert θ → θ + π/2. Then, J _-o (x)=(-) ⁿ J _o (x) …(14) However, using the property that n is a positive integer, we set ξ=2b sinφ/2 …(15) and the above S Writing _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⁼⁰ (−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 −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 harmonic 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. 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.

Problems to be Solved by the Invention As can be seen from the above explanation, when extracting the component of the fundamental wave ω _n from the output of the light receiving element including the component shown in equation (10) b, conventionally, the angular frequency is used as the reference frequency. 2E _r by synchronous detection using a square wave of ω _n
E _L sinΔθ is obtained.

If the duty of the rectangular wave of the reference frequency is not 1/2, the 2ω _n component is also included as a result. Therefore, if the output of the photodetector is synchronously detected using such a rectangular wave as a reference signal, the output of the photodetector will be
The 2ω _n component is synchronously detected with the 2ω _n component of the reference signal. As a result, - of the components shown in equation (10)b
2E _r E _L J ₂ (ξ)cosΔθ is output. This becomes noise and becomes a major cause of drift when E _r and E _L change due to fluctuations in the amount of light.

In order to solve such problems, it is conceivable to provide a high-frequency rejection filter, that is, a low-pass filter 27, in front of the synchronous detector 28 as shown in FIG. It is not possible to completely remove the 2ω _n component with a low-pass filter. On the other hand, a low-pass filter that can completely remove the 2ω _n component cannot transmit the fundamental wave component without attenuation.

Therefore, the present invention solves the above-mentioned problems, and
It is an object of the present invention to provide a phase modulation type optical fiber gyro with small drift that does not affect measurement accuracy even if the amplitude of propagating light changes.

More specifically, the present invention aims to provide a phase modulation type optical fiber iron in which the second harmonic component of the phase modulation frequency does not affect the noise as noise.

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 at a phase modulation frequency according to A control device is further provided that detects a frequency component twice the phase modulation frequency of the phase modulation frequency and controls modulation by the phase modulator so that the component becomes zero.

Effect As can be seen from equation (10)a above, the harmonic component nω _n of the fundamental wave ω _n of the phase modulation frequency is proportional to J _o (ξ). And that ξ is ξ=2bsin(φ/
2). Therefore, ξ is proportional to the amplitude or depth of the phase modulation. The depth of this phase modulation depends on the output of the oscillator associated with the phase modulator.

FIG. 4 is a graph showing the Betzel function,
The above ξ corresponds to x. As can be seen from FIG. 4, by controlling x, that is, ξ, for example,
5.2, 8.3, 11.8..., J ₂ (x), that is, J ₂ (ξ), can be made zero.

Therefore, by controlling the depth of phase modulation, the J ₂ (ξ) component can be made zero.

The above-mentioned phase modulation type optical fiber iron according to the present invention is based on the above-described principle, and the controller detects the 2ω _n component of the light receiving element output and performs phase modulation so that it becomes zero. controlling the device. Therefore, it greatly affects drift.
The 2ω _n component can be removed.

Here, the ω _n component, which is the signal to be detected, is proportional to J ₁ (ξ), as seen from equation (10) b above. As can be seen from Figure 4, this J ₁ (ξ) reaches its maximum value when J ₂ (ξ), that is, J ₂ (x) becomes zero (x = 5.2, 8.3, 11.8...). It is close to a large value. Therefore, J ₁ (ξ)/
J ₂ (ξ) is sufficiently large, and even if the 2ω _n component cannot be completely suppressed to zero, the influence of the 2ω _n component can be almost eliminated. However, when x=5.2,
Since J ₁ (ξ) is the largest compared to when x=8.3, 11.8, etc., it is preferable to control the amplitude of the modulation signal so that x=5.2.

Note that when J ₂ (ξ) is zero, J ₄ (ξ) is large, but since this is the fourth harmonic of the fundamental wave, it can be almost completely suppressed by inserting a low-pass filter before the synchronous detector. can be removed. This is because a low-pass filter that transmits the fundamental wave component without attenuating it can easily remove the fourth harmonic component, although it cannot easily remove the second harmonic component.

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

FIG. 1 is a diagram showing the configuration of one embodiment of a phase modulation type optical fiber coil embodying the present invention. For the minimum configuration with the basic requirements of an optical fiber server, see Izekiel. S. and Ardety H. J.A. “Fiber Optic Rotation Sensor” Spriniger-Verlag Berlin (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 emitting element 30 such as a semiconductor laser is provided, and is driven by a drive power source (not shown) to generate a coherent light beam. Note that as the light emitting element, a gas laser such as a He-Ne laser, a superluminescent diode, etc. can also be used.

The light beam generated by the light emitting device 30 is sent to a beam splitter such as a half mirror 32. The light beam transmitted through the half mirror 32 is coupled to one end of a mode filter tough fiber 34 composed of a single mode fiber. The light beam output from the other end of the mode filter tough fiber 34 is transmitted to the half mirror 3 via the polarizer 36.
It is sent to another beam splitter such as 8. The light beam divided into two by the half mirror 38 is coupled to both ends of the optical fiber 40.

The optical fiber 40 is wound into a coil shape many times to form an optical fiber sensor, and includes a sensor coil 42 and an element made of a piezo semiconductor element or an LNO element (LiNbO ₃ ) driven at an angular frequency ω _n . It consists of a portion 46 wound around a phase modulator 44 such as .

The light beams propagated in both directions through the sensor coil 42 are output from both ends of the optical fiber 40 and combined by the half mirror 38, and the combined light beam is coupled to the mode filter tough fiber 34 via the deflector 36. The light beam propagated through the mode filter tough fiber 34 enters the light receiving element 48 via the half mirror 32.

The electrical output of the light receiving element 48 is
0 and a synchronous detector 5 via a low-pass filter 52
Connected to input 4. Note that the cutoff angular frequency of the low-pass filter 52 is ω _n . The synchronous detector 54 is supplied with a sine wave signal having an angular frequency ω _n from an oscillator 56 as a reference signal. On the other hand, the above-described phase modulator 44 is driven by an oscillator 56 at an angular frequency ω _n .

Therefore, the synchronous detector 54 synchronously detects the output of the light receiving element 48 at the angular frequency ω _n and outputs a voltage signal containing a component of the angular frequency ω _n .

Further, the output of the oscillator 56 is transmitted to a frequency multiplier 58.
and is converted into a sine wave signal with twice the angular frequency 2ω _n . The signal with the angular frequency 2ω _n is the second
is supplied to the synchronous detector 60 as a reference signal.
This synchronous detector 60 receives the output of the preamplifier 50 and performs synchronous detection at an angular frequency of 2ω _n . and,
Its output is fed to one input of comparator 62, the other input being grounded. Thus, comparator 62 compares the output of synchronous detector 60 with a ground or zero level and provides a voltage signal representing the difference to oscillator 56. The oscillator 56 controls the amplitude of the oscillation signal so that the difference signal is zero.

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

The light beam from the light emitting element 30 is sent to a half mirror 32 and split by the half mirror 32, and the transmitted light beam therein is coupled to one end of a mode filter tough fiber 34 to form a single mode laser beam. Only the light is outputted from the other end of the mode filter tough fibers, linearly polarized by the polarizer 36, and sent to the half mirror 38. The light beam divided into two by the half mirror 38 is coupled to both ends of the optical fiber 40.

The light beam input to the optical fiber 40 has a phase difference in the part of the sensor coil 42 undergoing rotation, and also has a phase difference in the part 46 wound around the phase modulator 44 driven by alternating current with an angular frequency ω _n . Phase modulated.

The right-handed light beam and the left-handed 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 half mirror 38, and are passed through the polarizer 36. and is coupled to the other end of the mode filter tough fiber 34. The light beam coupled to the mode filter tough fiber 34 enters the light receiving element 48 through the half mirror 32.

In this way, the electrical output of the light receiving element 48 is amplified by the preamplifier 50, and is input via the low-pass filter 52 to the synchronous detector 54, which receives the driving angular frequency ω _n of the phase modulator 44 as a reference frequency from the oscillator 56. and synchronous detection. Therefore,
The synchronous detector 54 outputs an electrical signal indicating an amount Δθ proportional to the rotational angular velocity.

Furthermore, the light receiving element 4 amplified by the preamplifier 50
The output of 8 is input to another synchronous detector 60 and is synchronously detected at the angular frequency of 2ω _n from the frequency multiplier 58 . Therefore, this synchronous detector 60 is
Outputs a voltage signal proportional to the amplitude of the 2ω _n component.
The voltage signal is then compared with a zero level in the comparator 62, and the difference signal is supplied to the oscillator 56. The oscillator 56 adjusts the amplitude of the AC drive signal of angular frequency ω _n supplied to the phase modulator 44 so that the input difference signal becomes zero.

As a result, 2ω _n included in the output of the light receiving element 48
components are maintained at substantially zero. That is, since the synchronous detector 54 receives an input signal without a 2ω _n component and performs synchronous detection, it can output a voltage signal exhibiting Δθ with less noise due to the 2ω _n component, that is, less drift fluctuation.

Effects of the Invention As is clear from the embodiments of the present invention described above, the phase modulation type optical fiber coil according to the present invention eliminates the influence of the second harmonic component of the phase modulation frequency, has less drift, and has high sensitivity. be.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the configuration of one embodiment of the phase modulation type optical fiber iron according to the present invention, FIG. 2 is a basic configuration diagram illustrating the principle of the phase modulation type optical fiber iron, and FIG.
The figure is a basic configuration diagram explaining the principle of the optical fiber iron, and FIG. 4 is a graph showing the Betzel function. Main reference number, 10...Light emitting element, 12, 12A
...Beam splitter, 12B...Mode filter tough fiber, 12C...Polarizer, 14...Optical fiber, 1
6... Sensor coil, 18... Light receiving element, 22... Phase modulator, 28... Synchronous detector, 30... Light emitting element, 3
2, 38...half mirror, 34...mode filter tough fiber, 36...polarizer, 40...optical fiber, 4
2... Sensor coil, 44... Phase modulator, 48... Light receiving element, 50... Preamplifier, 52... Low pass filter, 54, 60... Synchronous detector, 56... Oscillator,
58...Frequency multiplier.

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. , and a synchronous detector that synchronously detects the output of the light receiving element at the phase modulation frequency of the phase modulator, and a phase detector that measures the rotational angular velocity from the phase difference between the two directions of light generated when the sensor coil rotates. The modulation type optical fiber gyro further includes a control device that detects a frequency component twice the phase modulation frequency in the output of the light receiving element and controls modulation by the phase modulator so that the component becomes zero. A phase modulation type optical fiber gyro characterized by the following. 2. The phase modulator is attached with an oscillator with a predetermined angular frequency, and the control device includes a frequency multiplier that receives the output of the oscillator and outputs a signal with twice the angular frequency, and the light receiving element. a second detector that receives the output of the multiplier and performs synchronous detection with a signal having an angular frequency twice that of the multiplier; and a second detector that receives the output of the second detector and compares it with a reference value to control the output to the oscillator. 2. A phase modulation type optical fiber gyroscope according to claim 1, further comprising a comparator for adjusting the angle. 3. The output of the light receiving element is input to the synchronous detector via a high-frequency rejection filter that removes frequency components higher than the phase modulation frequency. The phase modulation type optical fiber gyro described in 2.