JPS63138208A - Optical fiber gyro by phase modulation system - Google Patents

Abstract

PURPOSE:To secure the high accuracy by eliminating the variation of a scale factor, by bringing a light beam inputted and propagated from both ends of an optical fiber which has been wound round in a shape of a coil, to a phase modulation, bringing the output of a light receiving element to a synchronous detection by two frequencies, and calculating an angular velocity from the ratio of a voltage signal output. CONSTITUTION:Light beams of each different wavelength from light emitting elements 10, 11 are synthesized by a BS 14 through beam splitters (BS) 12, 12', moder filter fibers 16, 16', and polarizers 15, 15', and furthermore, made to branch and inputted to both ends of an optical fiber 18. Its light beams are propagated by the right turn and the left turn in the optical fiber 18 having a sensor coil 20, and a phase modulator 30 modulated by an angular frequency omegam, and emitted from both ends, and made incident on light receiving elements 26, 27 through the BSs 12, 12' after passing through the reverse optical path. Its output signal is brought to a synchronous detection by frequencies of omegam and 2omegam by synchronous detectors, 40, 42, and by calculating an output ratio by a divider, an angular velocity signal is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to an optical fiber gyro, and more specifically, to a highly accurate phase modulation type optical fiber gyro with small scale factor fluctuations. .

BACKGROUND OF THE INVENTION Gyroscopes are currently used in various fields, particularly as angular velocity sensors for navigation and attitude control of moving objects such as aircraft, flying objects, and automobiles. By using this gyro, you can obtain not only angular velocity but also data such as orientation by integrating it.

Among such gyros, optical fiber gyros 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. There is no angular velocity (sensitivity), trift,
It has attracted attention and development in recent years because it is superior to conventional gyros in terms of measurable range (dynamic range) and scale factor stability.

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

Other "Optical fiber sensor technology" IEE Journal on Quantum Electronics (Giallor)
enzi T, G, Bucaro J, A, et al.
al ”0pticalFiber 5ensor T
technology”, IEEE J, of Q
uantturnε1 electronics) QB-18
, Na4. pp. 626-662 (1982) and Culshaw and I., P., and Gilles, "Optical Fiber Gyroscope," Journal on Physics Electronics Science Instruments.
1. P, G11es “Fiber Optic Gyr
oscopes”J, Phys, E:Sci Ins
trum, ) 16 pp5-15. (19
83), human urine "optical fiber gyroscope"
Laser research, 11. No, 12. pp889-90
2 (1983) and others.

(a) Principle of the optical fiber gyro The principle of the optical fiber gyro will now 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 and clockwise (CW) to the sensor coil 20.
) 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 cw and E c c w of the light propagated clockwise and the light propagated counterclockwise in the sensor coil 20 are expressed as follows.

However, El, E2: Amplitude of the counterclockwise light and clockwise light ω: Previous angular frequency t: Time Δθ: Phase difference due to the Sagnac effect. The lights are combined by the splitter 12 and enter 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.

However, L: fiber length of sensor coil a: Radius of sensor coil C: Speed of light in vacuum λ: Destination wavelength Ω: rotational angular velocity This is called the Sagnac effect.

There are various methods of detecting the phase difference Δθ, and various methods 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 1'' (1+cos(Δθ)) -----(2)
It will look like this.

This has the disadvantage that since Δθ exists in CO3, the 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 l is I'' (1+5in(Δθ
))... (3), so the sensitivity is good when Δθ is close to 0.

However, in order to separate either 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.

There are the above-mentioned difficulties in changing the sensitivity when Δθ is close to 0 using an optical detection mechanism as described above.

(b) Phase Modulation Type Optical Fiber Gyro 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 type 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
The optical fiber 18 is divided into two parts and coupled to both ends of the optical fiber 18. 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. It is divided into The light coupled from both ends of the optical fiber propagates clockwise and counterclockwise within the sensor coil 20 of the optical fiber, and is emitted from the opposite end, and is emitted from the beam splitter 1.
2 and enters 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, if the length of the optical fiber sensor coil is L1, the refractive index of the fiber core is n, and the speed of light is C, then the time τ required for light to pass through the sensor coil is τ=nL/c ・
...(4).

Here, it is assumed that the phase modulation element 22 is provided near the input end of the counterclockwise light, and that the modulation signal of the phase modulation element 22 is a sine wave with an angular frequency ω1, as described in ``1''.

When the light emitted from the light emitting element at the same time is split into clockwise light and counterclockwise light and undergoes phase modulation, the phase difference φ of the modulation signal is as follows: φ=ω, τ = nLωlI/c = 2πf, nL/c ...(5) However, ω, = 2
πf.

becomes.

Due to the Sagnac effect, clockwise light and counterclockwise light are ±Δθ
Although it has a phase difference of /2, it is further phase modulated by a phase modulation element. If the amplitude of the phase modulation element is b, the electric field strengths E eV and E ccw of the clockwise light and counterclockwise light are as follows.

Clockwise light and counterclockwise light having the above electric field strength are
Since the beams are combined by the beam brick 12 and square-law detected by the light receiving element 26, the output of the light receiving element S(Δθ, 1)
is proportional to the square of the sum of Ecw and Eccw.

S(Δθ, t) = (EC, +Ecc,) 2・
-(s) Calculating this, +D, C, + (2ω or more) ... (9)
However, D and C mean direct current 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, D and C are omitted and S(Δθ, 1) is expanded into a series using a Bessel function. First, equation (9) is expressed as follows.

S(Δθ, 1) ... αQ On the other hand, from the generating function expansion of the Bessel function, it can be expressed as n=-Senn=-■. 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 0-unit equation. Divide S(Δθ, 1) into these parts, S(Δθ, 1) Q=(SccosΔθ+3SsinΔθ)EIE2...
(13), after converting θ→θ+π/2, J
−h(x) = (−)” J , (x) −・
-(14) However, using the property that n is a positive integer, we set φ ξ-2bsln-...(15) and write the above Sc and SS as 5C-Jo(ξ)...(16 ) ...(17) becomes. Therefore, once again, S(Δθ, 1) is expressed as follows.

S(Δθ, 1) 1′//2(E+′+Ea”)+(components over 2ω)
+E+E2Jo(ξ)cosΔθ =DC component+2EIE2Jl(ξ)cosa+, t-5inΔθ2
EIE2J2(ξ)cos 2ω, t-cosΔθ−2
E, E2J3(ξ) cos 3 ω, t-5inΔθ+
2EIE2J4(ξ)cos4ω, t・cosΔθ+higher-order component...αlbThis is
This is the sum of the series of the fundamental wave of the modulation signal ω1 and the harmonic 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 Bessel function J of that order. The amplitude b of 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 (ξ) becomes large.

The highest sensitivity can be expected from the first-order term (
n=0), that is, the second term on the right side of the arJb equation.

This is the fundamental wave component. This fundamental wave component is P(Δθ
, 1), then θ P (Δθ, 1) = 2 EI E2 J + (ξ) cos ωI, 1t
−sinΔθ・・・(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.

Note that the sensitivity improves when J, (ξ) is maximized, so
Set to ξ-1,8. At this time, DC component Jo(ξ)
is almost 0.

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

Problems to be Solved by the Invention As mentioned above, in the conventional phase modulation type optical fiber/Ijiiro, the outputs 2E, E2J, (ξ)cosω of the light receiving element
, t-5inΔθ is synchronously detected using a rectangular wave with an angular frequency ω as a reference signal to obtain an angular velocity output 2EIE2JI.
(ξ) Obtain sinΔθ. However, the angular velocity output 2
EIE2JI (ξ) sinΔθ is EI in addition to Δθ
It also depends on scale factors such as E2, J, (ξ), etc.

Especially for E and E2, even if the light output of the light emitting element is stabilized,
It also fluctuates due to fluctuations in the coupling efficiency of light to the optical fiber, polarization fluctuations of light propagating in the optical fiber, etc. When the scale factor fluctuates, the output fluctuates even in response to human power at the same angular velocity, resulting in an error in the detection of the input angular velocity.

Therefore, the present invention aims to provide a highly accurate phase modulation type optical fiber gyro by eliminating the above-mentioned scale factor fluctuation.

According to the present invention, the means for solving the problem, as shown in FIG.
An optical fiber that includes a light-emitting element and a sensor coil portion 20 wound in a coil shape many times, and from 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. 18, a phase modulator 30 that is provided near one end of the optical fiber and modulates the phase of the light propagating in the optical fiber at a given angular frequency ω1, and a light receiving element that receives the bidirectional light propagated in the optical fiber. , a phase modulation type optical fiber gyro that is equipped with 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 20 rotates. , the light emitting elements include a first light emitting element 10 that emits light of different wavelengths, and a second light emitting element 10 that emits light of different wavelengths.
a light-emitting element 11, and the light-receiving element is a first light-emitting element that receives propagating light having the emission wavelength of the first light-emitting element 100.
The synchronous detection device includes a light receiving element 26 and a second light receiving element 27 that receives propagating light having the emission wavelength of the second light emitting element, and the synchronous detection device synchronously detects the output of the first light receiving element 26. a first synchronous detector 40 that synchronously detects the output of the second light receiving element 27; a second synchronous detector 42 that synchronously detects the output of the second light receiving element 27;
A phase modulation method characterized by comprising an analog divider 49 that receives the output signal of the first synchronous detector and the output signal of the second synchronous detector and outputs a ratio of the two output signals. A fiber optic gyro is provided.

]” The wavelength of the first light emitting element lO is set to λ3, and the wavelength of the second light emitting element 11 is set to λ3.
Assuming that the wavelength of The signal obtained by synchronous detection in is given by the following equation.

P,: Output P2 of the first light emitting element: Output η of the second light emitting element: Loss of the optical system Here, the wavelength λ2 of the second light emitting element is set to twice the wavelength of the first light emitting element, i.e. If we set λ2=2λ1, Δ
The relationship θ5=2Δθ2 is obtained.

Therefore, by taking the ratio of the signal component shown in equation (19) and the signal component shown in equation (20), the following relational expression can be obtained.

P2ηJ2(ξ)cosΔθ2 P2J2(ξ)
The signal component ratio expressed by cosΔθ2 (21) has the optical system loss η eliminated and is not affected by loss fluctuations in the optical system. Thus, the scale factor is the output p, ,p of the two light emitting elements and the phase modulation parameter Jl(ξ
), J2 (ξ) only.

All of these scale factors are extremely easy to control when compared to loss fluctuations in the optical system. For example, regarding the output of the light emitting element, as shown in FIG. 4, a part of the light from the light emitting element IO is received by the light receiving element 28, and the drive current of the light emitting element IO is fed back so that the output level does not fluctuate. Easy to control. In addition, phase modulation usually involves winding an optical fiber around a piezoelectric vibrator and modulating the phase of propagating light by mechanical strain of the piezoelectric vibrator.

Therefore, regarding the phase modulation parameters, as shown in FIG.
It is also easy to provide feedback control so that the degree of phase modulation is constant by monitoring the above-mentioned element in reverse.

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

FIG. 6 is a diagram showing the configuration of one embodiment of a phase modulation type optical fiber gyro in which the present invention is implemented using discrete components. For a minimal configuration with basic requirements for a fiber optic gyro, see Izekiel Niss and Arpt.H.

J., “Optical Fiber Rotation Sensor” Splinigaveerlag Berlin (Ezekil S, and A
rdityHoJ, “Fiber 0ptic
Rotatior 5ensors", Sprin
A detailed explanation can be found in Ger-Verlag Berlin, ) 1982.

In the illustrated phase modulation type optical fiber gyro, a first light emitting element 10 and a second light emitting element 11 are provided,
It is driven by a power source (not shown) to generate light beams with different wavelengths. On the right, He-N is used as a light emitting element.
E-lasers, semiconductor lasers, superluminescent diodes, etc. can be used. In this case, for example, an AlGaAs-based semiconductor laser is used as the first light-emitting element and a 1nGaAs P-based semiconductor laser is used as the second light-emitting element, so that the emission wavelength of the second light-emitting element 11 is different from that of the first light-emitting element 100. It can also be set to twice the emission wavelength. The light beams generated by the two light emitting elements 10 and 11 pass through beam splitters 12 and 12, such as half mirrors, respectively.
The light beams propagated through the mode filter fibers 16 and 16', respectively, are combined by the beam splitter 14 via polarizers 15 and 15°, and further split into two. It is branched 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,
It is connected to an AC excitation power supply (not shown) for phase modulation, and is driven by a rectangular AC wave having an angular frequency ω1. 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, combined by the beam splitter 14, and further split into two, which enter the mode filter fibers 16 and 16°. do. The light beams propagated through the mode filter fibers 16 and 16'' respectively enter the light receiving elements 26 and 27 via the beam splitters 12 and 12'.The first light receiving element 26 and the second light receiving element 27 are It is set to receive only the propagating light having the emission wavelength of the first light emitting element 10 and the second light emitting element 11. Therefore,
For example, the first light-receiving element 26 may be a Si-based photodiode, and the second light-receiving element 27 may be a Ge-based or InG-based photodiode.
An aAs-based or InGaAn-based photodiode can be used.

The electrical outputs of the light receiving elements 26 and 27 are connected to the inputs of synchronous detectors 40 and 42, respectively, via DC blocking filters. The synchronous detectors 40 and 42 synchronously detect the outputs from the light receiving elements 26 and 27 using frequency signals ω1 and 2ω, and obtain angular frequencies ω and 2ω.

The voltage signals of the respective components are output.

The analog divider 49 calculates the ratio of the electrical output of the synchronous detector 40 and the electrical output of the synchronous detector 42, and calculates the ratio (
As shown in equation 21), an angular velocity signal indicating the phase difference Δθ generated in the sensor coil 20 is obtained.

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

Light beams from the light emitting elements 10 and 11 driven by a power source propagate through mode filter fibers 16 and 16' via beam splitters 12 and 12°, respectively, and are combined by a beam splitter 14 and further split into two. and is coupled to both ends of the 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 an angular frequency ω. The phase is modulated in a portion coupled to a phase modulator 30 driven by a square wave alternating current.

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, combined by the beam splitter 14, and further split into two. The light beams propagate through the mode filter fibers 16 and 16' and enter the light receiving elements 26 and 27 via the beam splitters 12 and 12'', respectively.

The outputs of the light receiving elements 26 and 27 are sent to synchronous detectors 40 and 4.
2, synchronous detection is performed at angular frequencies ω and 2ω, respectively, and voltage signals expressed by the above-mentioned equations (19) and (20) are output, respectively. The ratio of the above two voltage signal outputs is calculated by the analog divider 49, and the above (21) is calculated.
An output, that is, an angular velocity signal, proportional to sin Δθ expressed by the formula is obtained. At this time, as described above, errors caused by loss fluctuations in the optical system are eliminated from the angular velocity signal.

Therefore, in a phase modulation type optical fiber gyro,
It is possible to significantly reduce fluctuations in scale factor due to loss fluctuations in the optical system and ensure high accuracy.

FIG. 7 is a schematic diagram of the configuration of a second embodiment in which the phase modulation optical fiber gyro of FIG. 1 is made entirely of fibers. The illustrated optical fiber gyro differs in structure from the optical fiber gyro shown in FIG. 1 only in that fiber couplers 45 to 48 are used as branching/merging elements and a fiber type polarizer 17 is used as a polarizer.

In addition, it is also possible to configure the optical system on a single planar waveguide to create a so-called integrated optical structure.

Effects of the Invention As is clear from the above description, the phase modulation type optical fiber gyro according to the present invention 1. significantly reduces the fluctuation of the scale factor, which has been a problem in the past, and ensures high accuracy. Therefore, the phase modulation optical fiber gyro according to the present invention can be used over a wide range of applications.

[Brief explanation of the drawing]

FIG. 1 is an optical system diagram illustrating the principle of the phase modulation type optical fiber gyro of the present invention, FIG. 2 is a basic configuration diagram illustrating the principle of the optical fiber gyro, and FIG. 4 is a diagram showing a basic configuration diagram explaining the principle of a modulation type optical fiber gyro; FIG. 4 is a diagram showing a method for monitoring the light emission output of a light emitting element; FIG. 5 is a diagram showing a method for monitoring the degree of phase modulation. 6 is a schematic diagram of the configuration of the first embodiment of the phase modulation optical fiber gyro of the present invention using discrete components, and FIG. FIG. 2 is a schematic configuration diagram of a second example of a fiber gyro. (Main reference numbers) 10.11...Light emitting element, 12.14...Beam splitter, 15...Polarizer, 17...Fiber type polarizer, 18...Optical fiber, 20...Sensor coil, 26. 27.28... Light receiving element, 30... Phase modulator 40, 42... Synchronous detector, 45... Monitor electrode patent applicant Kozo Iizuka, Director of the Agency of Industrial Science and Technology Figure 2 18... Optical fiber 20°... Sensor coil Fig. 4 Fig. 5 10.11... Light emitting element 26.27.28... Light receiving element 40.42... Periodic detector 45... Monitor electrode

Claims (3)

[Claims] (1) Includes a light emitting element and a sensor coil portion wound in a coil shape many times, and light from the light emitting element is branched and coupled to both ends, and light propagated in both directions through the sensor coil is output from both ends. an optical fiber, 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 propagated through the optical fiber, and an output of the light-receiving element. A phase modulation type optical fiber gyro is equipped with a synchronous detection device that receives and performs synchronous detection, and measures rotational angular velocity from the phase difference between the two directions of light generated when the sensor coil rotates, wherein the light emitting elements are different from each other. The first one that emits light of the wavelength
and a second light emitting element, the light receiving element being the first light emitting element.
The synchronous detection device includes a first light receiving element that receives propagating light having an emission wavelength of the light emitting element, and a second light receiving element that receives propagating light having the emission wavelength of the second light emitting element. , a first synchronously detecting the output of the first light receiving element;
a synchronous detector, a second synchronous detector that synchronously detects the output of the second light receiving element, and a synchronous detector that receives an output signal of the first synchronous detector and an output signal of the second synchronous detector. A phase modulation optical fiber gyro comprising: an analog divider that outputs a ratio of the two output signals. (2) The emission wavelength of the second light emitting element is twice that of the first light emitting element, and the first synchronous detector detects the output of the first light receiving element synchronously at the phase modulation frequency. Claim 1, characterized in that the ratio between the output signal and the output signal of a second synchronous detector which synchronously detects the output of the second light-receiving element at a frequency twice the same wave number as the phase modulation is used as the angular velocity output. 1
The phase modulation type optical fiber gyro described in . (3) The first light-emitting element is an AlGaAs-based semiconductor laser, the second light-emitting element is an InGaAsP-based semiconductor laser, the first light-receiving element is a Si-based photodiode, and the second light-receiving element is a Si-based photodiode. The element is Ge-based, InGaAs
3. The phase modulation optical fiber gyro according to claim 1, wherein the optical fiber gyro is selected from InGaAn-based photodiodes and InGaAn-based photodiodes.