JPS61277013A - Optical fiber sensor - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION FIELD OF INDUSTRIAL APPLICATION The present invention relates to fiber optic sensors, and more particularly to fiber optic sensors that allow measurements in different directions, dimensions or types. .

BACKGROUND OF THE INVENTION Currently, various measuring devices using optical fibers have been developed, and attempts are being made to use them in a wide range of fields.

For example, various measuring devices have already been developed, such as an optical fiber gyro that measures ζ rotational speed and an optical hydrophone that measures temperature and pressure. When an optical fiber is used as a sensor in this way, the light and the optical fiber through which the light propagates are less affected by magnetic fields and electric fields, so it can be used in any environment without shielding problems. Furthermore, by using optical fibers, it is possible to reduce the size and completely eliminate movable parts. Therefore, it is thought that the measurement of physical quantities using optical fibers will become even more widely used.

Next, an optical fiber gyro will be explained as a typical example of such an optical fiber sensor.

(a) Optical Fiber Gyro [Research] Currently, gyros are used as angular velocity sensors for navigation and attitude control of aircraft, flying objects, automobiles, robots, etc. By using this gyro, you can obtain not only angular velocity but also data such as orientation by integrating it.

Among such gyros, fiber optic gyros have no moving parts and can be miniaturized. Furthermore, they have excellent minimum detectable angular velocity (sensitivity), drift, image side range (dynamic range), and scale-of-actor. It has attracted attention and development in recent years because it has superior stability compared to conventional gyros.

Examples of such fiber optic gyros are, for example, Gear Engine, G-1, and Bukaro-Jyro. knee.

Other "Optical fiber sensor technology" IEE Journal of Quantum Electronics (Giallor)
Enzi T, GlBucaro J, A, et a.
l ”0pticalFiber 5ensor
Technology”, tBBB J, o
f Quantum Blectronics) QB
-18, No, 4. pp626-662 (1982
) and Tarasho and I.P. Gilles, "Optical Fiber Gyroscope" Journal of Physics Electronics Science Instruments (Culshaw)
nd 1. P, G11es “Fiber 0pti
c Gyroscopes”J, Phys, B: Sci.
Instrum, ) 16 PP5-15. (
1983), Tsubo Yo, Otsuka "Optical Fiber Gyroscope" Laser Research, 11. No, 12. pp889-
902 (1983) and others.

Here, the principle of the optical fiber gyro will be explained with reference to FIG.

The light from the light emitting element 10 is split by the beam splitter 12, and the single mode optical fiber 1 is coiled many times.
Optical fiber loop wound with 8 or sensor coil 2
0 manually, turn the sensor coil 20 clockwise (CW).
) and counterclockwise (CCW). At this time, when the sensor coil 2o 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 strengths E, , E, of the light propagated clockwise and the light propagated counterclockwise in the sensor coil 20 are expressed as follows.

However, El, E2 = amplitude of counterclockwise light and clockwise light ω: angular frequency of light t: time Δθ: phase difference due to Sagnac effect. The beams are combined by the beam splitter 12 and incident on the light receiving element 26. From the detection intensity of the light receiving element 26, the phase difference Δ′θ
can be known. The phase difference Δθ can be expressed as follows.

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 Su÷Yak effect.

There are various methods of detecting the phase difference Δθ, and various methods have been proposed.

Most simply, when the sum of the counterclockwise light and the clockwise light is square-law detected by a light receiving element, the output I becomes Icx: (4+cos(Δθ)) (2).

This has the disadvantage that since Δθ exists in the COS, sensitivity is poor when Δθ is close to 0.

Therefore, the phase of either the counterclockwise or clockwise light is changed to 90°.
An optical mechanism has been proposed in which the beam is shifted and square detection is performed. In this case, the output I has the form l cx (1+5in(80>) −・−−(3), so Δθ is O
Sensitivity is good when close to .

However, in order to separate either one of the lights, three new beam splitters 5 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 O using a static optical detection mechanism as described above has the above-mentioned difficulties.

(3) Phase modulation type optical fiber gyros Therefore, many optical fiber gyros 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.

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

The phase modulation optical fiber gyro will be explained with reference to FIG. 3. The coherent light from the light emitting element 10 is transmitted to the beam splitter 12
The optical fiber 18 is divided into two parts and coupled to both ends of an optical fiber 18 via coupling lenses 14 and 16. 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. . 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.

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 input end of counterclockwise light. When the length of the sensor coil of the optical fiber is L1, the refractive index of the fiber core is n1, and the speed of light is C, the time τ required for light to pass through the sensor coil is τ=nL/C·reference Φ(4).

Assume that the modulation signal is a sine wave of angular frequency, as described above. When the light emitted from the light emitting element at the same time is divided into clockwise light and counterclockwise light, and each undergoes phase modulation, the phase difference φ of the modulation signal is φ=ωyoτ =nLωm/c =2πfonL/c ・・・(5) However, ω1=2
πf.

becomes.

Due to the Sagnac effect, clockwise light and counterclockwise light are ±Δθ
Although it has a phase difference of /2, 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, , E, of clockwise light and counterclockwise light are as follows.

Clockwise light and counterclockwise light having the above electric field strength are
They are combined by the beam splitter 12 and square-law detected by the light receiving element 26, so the output of the light receiving element S (80, 1)
is proportional to the sum of Er and E squared.

S(Δθ, B=(E, +EL)” ---(s)
Calculating this, +D, C1+ (2ω or more) ・ ・ ・ (9) However, D, C, mean DC components.

(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, we omit D, C, and expand S(Δθ, 1) into a series using the Bessel function.

...αQ From the generating function expansion of the Bessel function, it can be expressed as n=-■ n=-flash. 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 the six-port equation. Divide S(Δθ, 1) into these parts, S(Δθ, 1) = (SccosΔθ+3.sinΔθ)E1E2...
(13), after converting θ−θ1π/2, J
−, (x) = (−)”J,, (x) ・・・(
14) However, using the property that n is a positive integer and setting φ ξ = 2b sin −φ・φ (15) and writing the above Sc and Ss, Sc”Jo(ξ) ・ ・ ・ (16)・ ・ ・(17) Then, expressing S(Δθ, 1) again is as follows.

S (Δθ, 1) = 'A (E + '' 1 E 2') + (components greater than or equal to 2ω) + E I E 2 J o (ξ) cos
Δθ...αO" = DC component + 2EIE2Jl (ξ) cos 0mt-5inΔθ2
EIE2. L2(ξ)cos 2ω, t'cosΔθ+
High-order component...α1° This is the sum of the series of the fundamental wave of the modulation signal ω and the high-frequency signal.

By using an appropriate filter, it is possible to extract the fundamental wave ω or a harmonic signal of any order.

No matter which signal is adopted, the magnitude of cos Δθ or sin Δθ can be known.

In that case, the amplitude b1 of the modulation by the phase modulation element, the modulation angular frequency ω1, and the sensor coil transit time τ should be set so that the value of the Bessel function J, , (ξ) of that order becomes large.

The highest sensitivity can be expected from the first-order term (
n=o), that is, the second term on the right side of the αQ″″ equation.

This is the fundamental wave component. This fundamental wave component is P(Δθ
, 1), then P(Δθ, 1) = 2E, E2J, (ξ) cos ω@t-5inΔθ
...(18). In this way, the output proportional to sin Δθ is changed, and Δθ can be found by finding the amplitude of the fundamental wave component.

Problems to be Solved by the Invention Among the moving objects that are equipped with optical fiber gyros and move as described above, many move in three-dimensional space, such as aircraft, rockets, and even missiles. In order to measure the angular velocity of such a moving body, three optical fiber gyros were conventionally installed. Furthermore, when measuring angular velocity and pressure, it is necessary to install, for example, an optical fiber gyro and an optical hydrophone, respectively.

However, for example, installing an angular velocity measuring device consisting of three fiber optic gyros poses a problem in that the entire device is seriously damaged.

Further, it is necessary to adjust the light source and the light receiving element so that each sensor operates under the same conditions, but this adjustment is extremely troublesome. This is because if the same conditions cannot be ensured, the scales of each measurement item cannot be maintained.
This is because the reliability of measurements cannot be maintained. For example, if the scales in each three-dimensional direction do not match, the movement direction and speed in the movement direction obtained by combining them will become unreliable, making it difficult to achieve accurate navigation and attitude control. Therefore, it is a big problem that the number of necessary adjustments increases.

In addition, for each optical fiber sensor, the light source and light receiving element must operate stably to ensure measurement reliability.
Therefore, stable driving is required. Therefore, the devices attached to the light source and the light receiving element are expensive and difficult to adjust. Therefore, installing multiple optical fiber sensors requires a great deal of expense.

As described above, in the past, optical fiber sensors were installed independently for different directions, dimensions, and types of measurements, which resulted in the problem of increasing costs and increasing the size of the entire device.

SUMMARY OF THE INVENTION Therefore, the present invention aims to solve the above-mentioned problems and provide an optical fiber sensor that can measure items of different directions, dimensions, or types, and that is relatively simple, compact, and inexpensive with fewer adjustment points. It is.

Means for Solving the Problems The present invention has conducted various studies for the above-mentioned purpose, and has focused on the fact that in many optical fiber sensors, propagating light is phase-modulated or frequency-modulated. That is, if the modulation frequencies are different, even if the light propagated through a plurality of sensor optical fibers is received by the same light receiving element, it is possible to separate the lights based on the modulation frequencies.

The present invention has been made based on this knowledge.

That is, according to the present invention, in an optical fiber sensor that measures a physical quantity by modulating propagated light propagating through a sensor optical fiber and detecting a modulated component of the propagated light, a plurality of sensor optical fibers are used. It modulates the light propagating through each sensor optical fiber at different frequencies, and detects the light propagating through each sensor optical fiber with the same light-receiving element, and extracts each modulated frequency component from the output of the light-receiving element. The physical quantities detected by the optical fibers for each sensor are measured separately.

According to the above-described optical fiber sensor, the components of light transmitted through each sensor optical fiber can be separated and extracted based on the modulation frequency. Therefore, by performing measurements using the separated and extracted components, it is possible to measure the items assigned to each sensor optical fiber without being affected by the light propagated through other sensor optical fibers. .

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

FIG. 1 is a diagram showing the configuration of an embodiment of a phase modulation optical fiber gyro according to the present invention. This phase modulation type optical fiber gyro has a minimum configuration that meets the basic requirements of an optical fiber gyro. Regarding the minimum configuration, see Izekiel S. and Aditi H. J.

"Optical fiber rotation sensor" Springer-Verlag Berlin (Bzekil S, and Arditt)
yHoJ.

”Fiber Optic Rotation 5en
sors”, Springer-VerlagBerl
There is a detailed explanation in 1982.

In the illustrated phase modulation optical fiber gyro, a light source such as a light emitting element 30 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 30 is sent to beam splitters 32, 34, and 36 such as half mirrors arranged in series. The beam splitter 32 is for splitting the light to the light receiving element 38, and the beam splitters 34 and 36 are for splitting the light from the light source 30 into three parts.
It brings together two lights into one.

The light beam split by the beam splitter 34 is sent to a beam splitter 40 such as a half mirror, and the light beam split into two by the beam splitter 40 is coupled to both ends of a first optical fiber 42. .

Similarly, the two light beams split by the beam splitter 36 are sent to beam splitters 44 and 46, such as half mirrors, respectively. The light beams each split into two by the beam splitters 40 are
Coupled to opposite ends of second and third optical fibers 48 and 50, respectively.

These three optical fibers 42, 48 and 50 are each wound into a coil many times to form an optical fiber sensor and are connected to sensor coils 42A, 48A and 50A and piezoelectric vibrating elements 52, 54 and 56, respectively. It consists of wrapped portions 42B, 48B and 50B. The sensor coils 42A, 48A and 50A are
They are arranged on each plane of orthogonal three-dimensional coordinates: the XY plane, the YZ plane, and the ZX plane.

Furthermore, the piezoelectric vibrating elements 52, 54 and 56 are connected to AC excitation power supplies 58, 60 and 62 for phase modulation, respectively, and are driven by AC with angular frequencies ω, 1, ω, 2, and ωp3, respectively. ing.

The light beams propagated clockwise and counterclockwise through the three optical fibers 42.48 and 50, respectively, pass through the optical fiber 42.4.
8 and 50, are combined by beam splitters 40, 44 and 46, further combined by beam splitters 34 and 36, and incident on light receiving element 38 via beam splitter 32.

The electrical output of the light receiving element is transmitted through three synchronous detectors 64.6
6 and 68 inputs. These synchronous detectors 64, 66 and 68 are composed of the piezoelectric vibrating elements 52, 5,
4 and 56, an AC excitation power source 58 for phase modulation.
60 and 62 with angular frequencies ω28 and ω, respectively.
It is adapted to receive alternating currents of 2□ and ωp3.

The outputs of the synchronous detectors 64, 66 and 68 are outputted via low-pass filters 70, 72 and 74, respectively. Thus, the synchronous detectors 64, 66 and 6
8 extracts components of angular frequencies ω,1, ωP2 and ωp3, and low-pass filters 70, 72 and 74 extract components of angular frequencies ω□, ω,2 and ω1 . The signal component corresponding to sinΔθ is extracted and output from among the components.

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

The light beam from the light emitting element 30 driven by the power source is
After passing through the beam splitter 32, the beam splitter 3
4.36, it is branched into three beams, and furthermore, the beam splitter 4
0.44.46, respectively, and are branched into two and coupled to both ends of optical fibers 42, 48, and 50.

The light beams input into these optical fibers 42, 48, 50 are connected to sensor coils 42A, 48A5 which are undergoing rotation.
A phase difference is created at the 0A portion, and the AC excitation power source 58.
It is phase modulated in the portions 42B, 48B and 50B wound around piezoelectric vibrating elements 52, 54 and 56, respectively, driven by alternating currents of angular frequencies ω□, ωp2 and ω,3 from 60.62.

The clockwise light beam and the counterclockwise light beam, which have a phase difference and are phase modulated in the optical fiber 42, 48, 50, are outputted from both ends of the optical fiber 42, 48, 50, and then sent to the beam splitter. 40.44
．． 46, and the beam splitter 34.
36 and enters the light receiving element 38 via the beam splitter 32.

Here, the output of the light receiving element 38 is expressed as follows.

■ (Δθ1, Δθ2, Δθ3) = DC component + harmonic component However, b, , φ. is a constant Δθ regarding the phase modulation of the nth optical fiber coil. is the phase difference due to the Sagnac effect experienced by the n-th optical fiber coil E n l N E n 2 is the amplitude of the left and right light of the n-th optical fiber coil The electrical output of such a light receiving element is the AC excitation power supply 58.
60.62, the signals are input to synchronous detectors 64, 66 and 68 which receive the drive angular frequencies ωPisω2° and AC of ω,3 of the piezoelectric vibrating elements 52, 54 and 56 as reference angular frequencies.

Since the synchronous detector 64 receives the angular frequency ωp1 as a reference angular frequency, it outputs an AC signal with a component of the angular frequency ω, 1, and thus the low-pass filter 70
The phase difference Δ generated on the right side of the sensor coil 42Δ on the surface
A voltage signal indicating θ1 is output.

That is, the angular velocity of rotation about the Z-axis can be measured.

Furthermore, since the synchronous detector 66 receives the angular frequency ω,2 as a reference angular frequency, it outputs an AC signal of the 2□ component of the angular frequency, and thus the low-pass filter 72 detects the sensor located on the YZ plane. A voltage signal indicating the phase difference Δθ2 generated in the coil 48A is output. That is, the angular velocity of rotation about the X-axis can be measured.

Since the synchronous detector 66 receives the angular frequency 3 as a reference angular frequency, it outputs an AC signal of the component of the angular frequency ωp3, and thus the low-pass filter 74
outputs a voltage signal indicating the phase difference Δθ generated in the sensor coil 50A. That is, the angular velocity of rotation about the Y-axis can be measured.

Therefore, from the outputs of the three low-pass filters, X1Y, Z
The angular velocities in three directions can be obtained, and by combining them, the moving direction and the velocity in that moving direction can be determined.

The phase modulation type optical fiber gyro described above has three sensor coils, but if necessary for measurement,
Four or more sensor coils may be included. In any case, the phase modulation frequency may be different for each sensor coil, and the electrical output of the light receiving element may be synchronously detected at each phase modulation frequency to separate and extract components of the corresponding modulation frequency.

Further, in the above embodiment, a common light source is used, but a light source may be provided independently for each sensor optical fiber.

In the above embodiment, the present invention is applied to an optical fiber gyro for measuring angular velocity. However, the present invention is also applicable to other optical fiber sensors, such as to optical hydrophones having multiple sensor optical fibers. Furthermore,
The sensor coil can also be used for multiple different types of measurements, such as angular velocity measurement, temperature measurement, and pressure measurement.

In the above embodiments, the light source and the light receiving element are common to a plurality of sensor optical fibers. Therefore, since there is only one light source and one light-receiving element, which must be driven stably to ensure measurement reliability, the burden of troublesome adjustment is light. In addition, because stable operation is required, there is only one light source and one light-receiving element, which must be expensive, so optical fiber sensors that can measure multiple items are cheaper than conventional ones. can be manufactured. Moreover, the entire device does not suffer major damage.

Further, since the light source and the light receiving element are common to the sensor optical fiber, each sensor can be operated under the same conditions by simply adjusting them. Therefore, when measuring speed in three directions as in the above example, the scales in each three-dimensional direction match, and the reliability and accuracy of the moving direction and the speed in that moving direction obtained by combining them is extremely high and accurate. It is possible to use the robot for navigation and posture control. Also,
This eliminates the troublesome adjustment work required to operate the light source and light receiving element attached to each sensor optical fiber under the same conditions, making it extremely practical. As is clear from the above description, in the optical fiber sensor according to the present invention, the light receiving element is common to all sensor optical fibers. Therefore, items with different directions, dimensions, or types can be measured with a relatively simple, compact, and inexpensive device.

Furthermore, by using a common light source, both the light source and photodetector, which affect measurement accuracy the most among optical fiber sensors, are expensive, and are difficult to adjust for stable driving, can be shared by all sensor optical fibers. Therefore, the entire device can be downsized and highly reliable.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the optical system configuration of a phase modulation optical fiber gyro according to the present invention, FIG. 2 is a basic configuration diagram explaining the principle of the optical fiber gyro, and FIG. 1 is a basic configuration diagram illustrating the principle of a fiber optic gyro. [Main reference numbers] 10...Light emitting element 12...Beam splitter 14
．． 16...Coupling lens 18...Optical fiber" fiber 20.
- Sensor coil 22... Phase modulation element 26... Light receiving element 30... Light source 32.34.36.40
．． 44.46... Beam splitter 38... Light receiving element

Claims (4)

[Claims] (1) An optical fiber sensor that measures a physical quantity by modulating propagated light propagating through a sensor optical fiber and detecting a modulated component of the propagated light, which has a plurality of sensor optical fibers, and each sensor The light propagating through the sensor optical fibers is phase-modulated at different frequencies, and the light propagating through each sensor optical fiber is detected by the same light-receiving element, and each modulated frequency component is separated from the output of the light-receiving element. An optical fiber sensor that measures a physical quantity detected by a sensor optical fiber. (2) The optical fiber sensor according to claim (1), wherein light from the same light source is coupled to each sensor optical fiber. (3) Each of the sensor optical fibers is an optical fiber coil, and detects the angular velocity that the optical fiber coil receives. Optical fiber sensor described in . (4) The optical fiber sensor according to claim (3), wherein there are three optical fiber coils arranged on each plane of orthogonal three-dimensional coordinates.