JPH02236113A - Phase modulation type optical fiber gyro - Google Patents

Abstract

PURPOSE:To make an output of a light emitting element and the phase modulation constant thereby to measure an angular velocity correctly by making components of a direct current of an output of a light receiving element and components of a two-fold higher harmonic when said light receiving element is static or nearly static closer to a predetermined value. CONSTITUTION:A direct current component detecting unit 10 detects components D of a direct current of an output of a light receiving element 9. The direct current components D are compared with a predetermined value D0 by a direct current component controlling unit 12. A controlling circuit 13 for controlling an output of a light emitting element controls the light emitting element 1 so that the direct current components D become closer to the predetermined value D0. On the other hand, a two-fold higher harmonic detecting unit 11 detects a two-fold higher harmonic Q among an output of the light receiving element 9. The two-fold higher harmonic Q is compared with a predetermined value Q0 by a controlling unit 14 which controls the two-fold higher harmonic. The power to be supplied to a modulating element 7 is controlled to reduce the difference between the Q and Q0 via a controlling unit 15 which controls oscillations of the phase modulating element. Accordingly, both an output of the light emitting element and the phase modulation are kept constant even when the temperature is changed or through the change with time, and a correct measurement is secured.

Description

[Detailed Description of the Invention] (A) Technical Field The present invention relates to a phase modulation type optical fiber gyro.

An optical fiber gyro is a device that measures the angular velocity of a moving body.

A single-mode optical fiber is wound around a coil many times, and monochromatic light is introduced into both ends of the optical fiber.
The light propagates clockwise, causing the light emitted from both ends to interfere. If the optical fiber coil is rotating around its axis,
A phase difference appears between the counterclockwise light and the clockwise light. Since this phase difference is proportional to the rotational angular velocity, if the phase difference is known, the rotational angular velocity can be determined.

When the phase difference is Δθ and the angular velocity is Ω, the following relationship exists.

Here, L is the fiber length of the sensor coil, a is the diameter of the coil, C is the speed of light in vacuum, and λ is the wavelength of light in vacuum. This is called the Sagnac effect and is well known.

However, it is not easy to detect the phase difference Δθ.

In reality, the phase difference includes optical system offsets that are not based on rotation. This offset varies significantly with changes in temperature. Furthermore, in an optical fiber gyro with a basic configuration, the light receiving element output is (1 + cosΔθ)
appears in the form of With this, the sensitivity is poor when 80 is small, and the direction of rotation cannot be determined.

To solve these difficulties, frequency modulation, phase modulation, and phase shift type optical fiber gyros are being considered.

The present invention relates to a phase modulation type optical fiber gyro.

(a) Phase modulation type optical fiber gyro The basic form of the phase modulation type optical fiber gyro will be explained with reference to FIG.

In this case, the optical fiber at one end of the optical fiber sensor coil is wound around a piezoelectric element to apply phase modulation. Taking the first-order term of the modulated wave, the phase difference is s1
It is obtained in the form of nΔθ.

Coherent light emitted from the light emitting element 1 is split into two beams by the beam splitter 2.

One is focused by the coupling lens 4 and enters the A end of the optical fiber 5. This propagates counterclockwise in the sensor coil 6.

The other ray is narrowed down by the coupling lens 3,
The light enters the optical fiber 5 from the B end and propagates clockwise inside the sensor coil 6.

Most of the optical fiber 5 is a senna coil 8, but a portion near the B end is wound around a piezoelectric element or the like to form a phase modulation element 7.

Since the oscillator 18 applies an oscillating voltage to the piezoelectric element, the piezoelectric element expands and contracts. Since the phase modulation section 8 of the optical fiber is wound around the piezoelectric element, it expands and contracts together with the piezoelectric element, and the optical signal contains a modulation component.

The clockwise light and the counterclockwise light are transmitted through the phase modulation section 8 of the phase modulation element 7.
and sensor coil 8, and is emitted from the other end. These are combined by the beam splitter 2 and incident on the light receiving element 9. The light receiving element 9 performs square law detection of the interference light.

Since the phase modulation element 7 is provided at an asymmetrical position when viewed from the entire optical fiber 5, the counterclockwise light and the clockwise light receive phase modulation at different timings.

Let L be the length of the optical fiber of the sensor coil 8, and let n be the refractive index of the optical fiber core. The time τ required for light to pass through the sensor coil θ is given by nL τ = (2) C .

When the phase modulation element 7 is provided near the B end, the counterclockwise light first undergoes phase modulation and then enters the sensor coil 6. The clockwise light enters the phase modulation element 7 after passing through the sensor coil θ.

Let the angular frequency of the modulation signal be Ω. Since the difference in time between receiving phase modulation at the phase modulation element 7 and entering the light receiving element 9 is τ, the phase difference φ between the modulation signals included in the interference light is φ = Ω τ (3) .

As mentioned above, due to the Sagnac effect, clockwise light and
The counterclockwise light has a phase difference of Δθ, but due to the phase modulation, the phase modulation part also has a phase difference of φ. Let b be the amplitude due to the action of the phase modulation element 7.

If the electric field strength of the counterclockwise light and the clockwise light is EL%ER, then the following equation is obtained. Here, D. C. means a direct current component. ω
is the frequency of light, and 2ω means a frequency component twice this frequency. Since the light receiving element 9 cannot detect such a fast signal, the signal is O.

Since the signal thus obtained includes phase modulation φ, the phase difference Δθ can be determined in relation to the amplitude of the modulation signal.

If we remove the DC component and rewrite S(Δθ.1) in the form of a sum, we get:

The counterclockwise light and clockwise light having such electric field strength are square-law detected by the light receiving element 9. The output of the light receiving element 9 is S(Δθ,
1) becomes. These are expanded using Bessel functions. From the generating function expansion of the Bessel function, +D. C. + {2ω or more} (7)2
t n: one φ. If we set t=expiθ, then S. =2Σ(-)”.r ...<ξ)cos(2n
+1) Ωt (14). From the expansion of the real and imaginary parts of this equation, the series expansion of the sin and cos parts Ss1Sc of S(Δθ, 1) is obtained.

S(Δθ, 1) = (SocosΔθ+SsS1nΔθ)Eo2(10)
Define it as follows.

By converting θ→θ+π/2, we obtain the well-known property of Betzel function, J−n(x)=(−)” Ja (x)
Using (II) (where n is a positive integer) and setting φ ξ: 2 b sln (!2), we get the following. Rewriting using these equations, the signal S(Δ
θ, 1) is (DC component) + (2ω component) 10E. "J.(ξ)cosΔθ n=1 n=O (l5). This is an expansion by harmonics of the modulation frequency Ω. By passing it through a filter, any harmonic component can be found. These Among them, the first-order term is assumed to be a fundamental wave component P, and the second-order term is assumed to be a double harmonic component Q.

P (t) = 2Eo”Jt (ξ)cos
Ω t sln Δ θ (l6)Q (t
)=2Eo”J2(ξ) cos 2Ωt cos
Δθ (I7). In many cases, the fundamental wave P is detected and Δθ is determined. In order to maximize the sensitivity of P,
Maximize J, (ξ). Therefore, the modulation degree is set so that ξ=1.8. At this time J. (ξ》 is approximately 0.3
It is.

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

(c) Problems to be Solved by the Invention In conventional optical fiber gyros, the amount of light incident on the fiber from the light emitting element fluctuates greatly. In other words, the amplitude E. There are large fluctuations. Therefore, for the same angular velocity, apparently different outputs were obtained due to variations in the amount of light.

Japanese Unexamined Patent Publication No. 1983-1971 Ichimoku 710B proposes a control system in which the DC component of the signal is constant.

Japanese Patent Laid-Open No. 135818 proposes dividing the phase modulation frequency component of the signal by the DC component to cancel out the influence of light intensity fluctuations.

In addition, there is a phase modulation type optical fiber adjuster which keeps the second harmonic constant.

This is patent application No. 59-244641.

The second harmonic includes J2(ξ), and it was thought that keeping this constant is equivalent to keeping the phase modulation degree constant.

However, keeping the DC component constant is not equivalent to keeping the amount of light constant. The DC component includes Δθ in the form CO8Δθ.

The invention described above makes the premise that the DC component is proportional to the light intensity of the light emitting element using the approximation ΔθzakiO, and the DC component is kept constant.

However, there are times when Δθ has a size that cannot be ignored. When Δθ is large, making such an approximation becomes inaccurate.

There are the following problems in keeping the second harmonic constant in order to keep the phase modulation degree constant.

As shown in (17), the second harmonic Q has a term of COSΔθ as well as J2(ξ). An approximation of Δθ10 is made, and the second harmonic is kept constant. However, when Δθ is large, this approximation is inaccurate. After all, the degree of phase modulation is not constant.

(Engineering) Purpose It is an object of the present invention to provide a phase modulation type optical fiber gyro that can suppress variations in the amount of light from a light emitting element and keep the degree of phase modulation constant.

(E) Configuration The optical fiber gyro of the present invention: (1) Recognizes whether the optical fiber gyro is stationary or has a sufficiently small angular velocity by a signal from the gyro or another sensor.

(2) When the above conditions are met, the light emitting element output is controlled so that the DC component of the light receiving element output is constant.

(3) When the above conditions are met, the degree of phase modulation is controlled so that the second harmonic component of the light receiving element output is constant.

This will be explained using drawings.

FIG. 1 is a block diagram of a phase modulation type optical fiber gyro according to the present invention.

The light emitting element 1 is a light source that generates coherent light.

Semiconductor lasers, superluminescent diodes, gas lasers, etc. are optional.

Beam splitter 2 splits this light into two beams. The beam splitter here means a light branching element in a broad sense.

The two light beams are focused by lenses 3 and 4 and then enter both ends A and B of optical fiber 5.

Optical fiber 5 is a single mode optical fiber. This includes a sensor coil 6 in which an optical fiber is wound in a coil shape many times, and a portion 8 in which the optical fiber is wound around a phase modulation element 7. A sensor coil detects rotational angular velocity.

The phase modulation element 7 is, for example, a cylindrical piezoelectric element provided with electrodes on the outer and inner surfaces thereof, and an optical fiber wound around the outer periphery. This gives periodic phase changes to the light propagating in the optical fiber.

The phase modulation element excitation control unit 15 applies a modulation voltage with a frequency Ω between the electrodes. This voltage amplitude is proportional to the phase modulation degree.

The light incident from the A end propagates in the sensor coil 6 as clockwise light. The light from the B end becomes counterclockwise light. Both lights are combined again in the beam stabilizer 2 and enter the light receiving element 9.

The light receiving element 9 detects the intensity of the interference light and outputs it. The light receiving element output includes a DC component D1, a fundamental wave component P1, a 2nd harmonic Q1, and higher harmonics.

The DC component D is detected by the DC component detection unit IO. This signal is sent to the DC component control section 12.

Although the DC component D is used to stabilize the output of the light emitting element, it does not always control the light emitting element. It is used to control the light emitting element output only when the stop signal U is given from another sensor or the final output S of this gyro itself is O.

The DC component control section 12 instructs the light emitting element control circuit 13 to drive current W for the light emitting element. This is to control the DC component D of the light receiving element output to be constant when the stop signal U is given or the final output S is O.

However, the stop signal U does not necessarily have to be a stop signal. The closer it is to the stop, the better. The same goes for the final output. It is good if S is close to O.

The DC component control unit 12 stores the DC component D when stopped or close to stopping, and stores the DC component D at a predetermined value D. An instruction is given to the light emitting element output control circuit 13 to approach this value.

Of the output of the light receiving element 9, the fundamental wave component P is transmitted to the synchronous detection section 17.
Detected in The synchronization signal is sent to the phase modulation element excitation control section 15.
given from.

Of the output of the light receiving element 9, the second harmonic Q is detected by the second harmonic detection section l1. Phase modulation element excitation control section 1
The modulated signal of No. 5 is doubled by a multiplier 20 and used as a synchronization signal.

The modulation degree of the phase modulation element excitation control section 15 is controlled to keep the second harmonic constant. Here, the degree of modulation is b, but since it is proportional to ξ, ξ will also be referred to as the degree of modulation.

The second harmonic is not always kept constant.

Only when a stop signal U is given from another sensor or when the final output S of this gyro is 0 or close to 0, the second harmonic is controlled to a predetermined value Q0.

The second harmonic control section 14 performs such intermittent control.

It is known that the vehicle is stationary or nearly stationary from a stop signal from another sensor or from the fact that the output S of this gyro is approximately 0. The value of the second harmonic Q at that time is adopted, and the predetermined Q is determined. The modulation degree ξ of the phase modulation element excitation control section 15 is given so that this difference is smaller compared to .

The fundamental wave P appears as the output of the synchronous detection section 17, and this becomes the final output S. This can be done by removing the vibration term from equation (l6) and calculating S”P=2Eo”Jt (ξ)sln Δθ (
I8). By keeping the DC component constant, the light intensity E. ′ can be kept constant. By keeping the second harmonic constant, the phase modulation degree ξ can be kept constant. The coefficients in (!8) are thus held constant. Δθ is determined from the final output S in the form of sin Δθ.

Here, the phase modulation degree is made constant by detecting the second harmonic and making it constant. However, the phase modulation degree is not limited to the second harmonic, but may be made constant by making the third or fourth harmonic constant. This is possible because all harmonics contain a phase modulation depth.

(Force) Action The operation of the phase modulation type optical fiber gyro has already been explained, so a description of this part will be omitted.

Until now, the amplitudes of clockwise light and counterclockwise light have not been distinguished as E0. Here, the amplitude of the clockwise light is assumed to be EI, and the amplitude of the counterclockwise light is assumed to be E2. What was E0' is E
．． It should be read as E2.

Of the light receiving element output, the DC component D is: It can be written as

By the way, in an actual optical system, the amplitudes of the clockwise and counterclockwise lights are almost equal, Et"lE+a, so (Et"+Eg")!=FEI EQ
(21) can be assumed. Although any modulation degree ξ may be used, a special example will be considered here. To maximize the sensitivity of the fundamental wave, the first-order Bessel function J. It is sufficient to maximize (ξ), and it is possible to select ξ→1.8.

At this time, the zero-order Bessel function Jo(ξ) is approximately 0.3. The DC component D becomes D:”Es l,a (1+0.a cos 80) (22), but from now on, when the fiber gyro is stationary or almost stationary, Δθ diagonal O cos Δθ→ 1 So, When the optical fiber is stationary or almost stationary, D = 0.3EIE2, and if D is equal to the predetermined D0 and controlled, the amount of interference light EIEQ can be made constant. These are the functions of the DC component detection section 10, the DC component control section 12, the light emitting element output control circuit 13, etc.

Next, control of the second harmonic Q will be explained.

Q”-2Et E2 JQ(ξ) cosΔθ
(2B), of which the light flEt of the light emitting element is
Es has already become constant. When stationary or near stationary,
COSΔθ is 1. At this time, Q=-2Et E2 JQ (ξ)
(27). The excitation voltage of the phase modulation element excitation control section 15 is controlled so as to keep this value constant.

If the phase modulation degree ξ can be made constant, the final output S
That is, from (18), the fundamental wave P is S=P=2E. Eg
J. (ξ) sin Δθ (2B),
The coefficient before slnΔθ becomes an unchanging constant, and the accurate Δθ can be found from the output S. This is an effect that cannot be obtained by simply keeping the amount of light or modulation constant.

(g) Example Regarding the minimum configuration of a phase modulation type optical fiber gyro, see Ezek (el S. and▲rdltt31 11
．． J. ：”FIBER OPTIC ROT▲TION
SENSOR” Springer-Verla
g Berlin. A detailed explanation can be found in 1982.

An embodiment of the present invention will be explained with reference to FIG. Components that are the same as in FIG. 1 are given the same reference numerals. I will not repeat the explanation for these.

The DC component detection unit 10 receives the output of the light receiving element and outputs the DC component as a voltage signal. This is converted into a digital signal by the A/D converter 22.

When receiving a stationary signal from another sensor or receiving the final output S of this optical fiber giro itself, this
Or when it is close to 0, perform the following operation.

DC component D is a preset value D. A control signal is calculated and created to control the amount of light emitted from the light emitting element so that the amount of light emitted from the light emitting element is

This control signal is converted into an analog signal by the D/A converter 21 and input to the light emitting element output control circuit 13.

When the optical fiber gyro is rotating rapidly, it retains and outputs the control signal from the previous time when it was stationary.

Since the signal is treated as a digital signal by the digital processing section 1B, it is easy to keep the time of the signal and continue outputting it.

The light emitting element output control circuit 13 receives the signal and controls the amount of light emitted from the light emitting element.

Oscillator 18 produces a modulated signal of frequency Ω.

The output of the oscillator 18 is amplified by a multiplier 19 with an appropriate amplification factor. The amplified oscillator phase modulation signal is applied to the electrodes of the phase modulation element 7. Therefore, the phase modulation element 7
The modulation degree b, that is, ξ, is freely given by the multiplier l9.

The synchronous detection section 17 receives a synchronous signal having a frequency of Ω from the oscillator 18 and obtains a fundamental wave component P from the output of the light receiving element. The fundamental wave component P is converted by the A/D converter 24 into
The signal is input to the digital processing section 16.

The multiplier 20 multiplies the phase modulation signal of the oscillator l8, and
Create a synchronization signal with a frequency of Ω.

The second harmonic detection section 11 detects the second harmonic Q from the output of the light receiving element using the 2Ω synchronization signal from the multiplier 20 . This value is converted into a digital value by the A/D converter 25 and input to the digital processing section 16.

The digital processing unit 18 recognizes that the optical fiber gyro is stationary or nearly stationary based on its own final output S1 or a stop signal U from another sensor.

When receiving a stop signal from another sensor, the status signal from the other sensor is input to the digital processing unit l6 through the status signal detection interface 26. The digital processing unit 16 determines whether the barking signal is stopped or not. Then, a stop signal U is created.

Alternatively, another sensor outputs a stop signal U when stopped,
It may also be input to the digital processing section 16.

It is also possible to obtain the output S→O from this optical fiber gyro itself.

The digital processing unit 18 performs two operations when the optical fiber gyro is stationary or nearly stationary.

One is to compare the DC component D with a preset reference D0 and control the light emitting element output so as to reduce this difference. The control signal is output from the D/A converter 21 to the light emitting element output control section 13.

The other is the second harmonic component Q, which is a preset standard. In order to reduce this difference compared to
The purpose is to control the phase modulation degree ξ. This is provided to the multiplier 19 by the D/A converter 23.

When the optical fiber gyro is rotating rapidly, the values of the DC component and double harmonic component at the time of the previous standstill are kept and continue to be output to the light emitting element output control section 13 and the multiplication section 19.

The rotational angular velocity can be determined from the fundamental wave component P.

(H) Effect: When stationary or close to stationary, the DC component and second harmonic component of the light receiving element output are compared with predetermined values and brought close to these values. In this way, the light emitting element output and the degree of phase modulation can be made constant.

Even if the coupling light intensity or phase modulation degree changes due to temperature or changes over time, according to the present invention, this can be corrected correctly and always
Accurate and stable angular velocity measurement becomes possible.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram of a phase modulation type optical fiber gyro according to the present invention. FIG. 2 is a configuration diagram of a phase modulation type optical fiber gyro showing an embodiment of the present invention. FIG. 3 is a configuration diagram of a conventional phase modulation type optical fiber gyro. 1 Self ●● 2 ● 1 3, 4 ● 5 ● ● ● 6 ● ● ● 7 1 ● 8 φ ● ● 9 1 ● 1 0 Fist ● 1 1 ● ● 1 2 φ ● l 3 ● ● ● Light emitting element ●Beam splitter ●Lens fist Optical fiber ●Sensor coil ●Phase modulation element ●Optical fiber part wrapped around phase modulator ●Receiver. Optical element Φ DC component component detection section - 2nd harmonic detection section ● DC component control section φ Light emitting element output control circuit 14 ●● 2nd harmonic control section 15 @● Phase modulation element excitation control section 17 ●● ●Synchronous detection unit 1 8 ●Life ●Oscillator 19●●●Multiplication unit 20 ●-●Multiplier 21, 23 ●-D/A converter 22, 24, 25●●●A/D converter invention
person

Claims (3)

[Claims] (1) An optical fiber having a part constituting a sensor coil and a part provided with a phase modulation element, a light emitting element that generates coherent light, and light from the light emitting element or from the light emitting element via the optical fiber. a beam splitter that splits the light and supplies it to both ends of the optical fiber; a light receiving element that propagates through the optical fiber and combines the light emitted from both ends of the optical fiber and receives the light; and an output of the light receiving element. A phase modulation type optical fiber gyro including at least a synchronous detection circuit that receives the output of the light receiving element and detects the phase modulated frequency component, and a DC component detector that receives the output of the light receiving element and detects the DC component, and an appropriate phase modulation frequency component. It has a harmonic detection section that detects harmonics of multiples, and a synchronous detection section that detects the fundamental wave component of the light receiving element output. A phase modulation type optical fiber gyro characterized by controlling the output of a light emitting element and the degree of phase modulation so that the DC component and the harmonic component approach a predetermined value when the proximity is recognized. (2) The phase according to claim 1, characterized in that the DC component of the light-receiving element output when it is stationary or nearly stationary is kept, and the light-emitting element output is controlled so that its level is constant. Modulation type optical fiber gyro. (3) The phase modulation degree is controlled so that the double harmonic Q of the light-receiving element output when it is stationary or nearly stationary is maintained and its level is constant. The phase modulation type optical fiber gyro according to item 2.