JPH02236114A - Phase modulation optical fiber gyro - 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] (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 into a coil many times, and monochromatic light is introduced into both ends of the optical fiber, and the light is passed counterclockwise.
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 a phase difference of 80 degrees.

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 having a basic configuration, the light receiving element output appears in the form of (1+cosΔθ). 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 system, the optical fiber of 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 si
It is obtained in the form of nΔθ.

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

One is condensed by the coupling lens 4 to the A end of the optical fiber 5. incident on . 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 sensor coil 6, 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 10 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 6, and is emitted from the other end. These are combined by the beam splitter 2 and enter 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 6, 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 6, and let n be the refractive index of the optical fiber core. The time τ required for the light to pass through the sensor coil 6 is given by 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 passes through the sensor coil 6 and then enters the phase modulation element 7.

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 80, but due to phase modulation, the phase modulation part 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 as follows, Et ER means a number of components. Since the light receiving element 9 cannot detect such a fast signal, the signal is 0.

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, we get . Here, D. C. means a direct current component. ω
is the frequency of light, and 2ω is twice this frequency. t:
If we put expiθ, n=−oro. From the expansion of the real and imaginary parts of this equation, S(Δθ,
Obtain the series expansion of the s1n,cos part Ss1Sc in 1).

S (Δθ, 1) :(SacosΔθ+3ssinΔθ)E. ”(lO) Define it as follows.

Convert θ→θ+π/2, use the well-known property of the Bessel function, J−n(x)=Jn(X) (where n is a positive integer), and set ξ=2 b sin And, φ (I2) S0 = J o(ξ)+2 Σ(-) n=1 J 2n(ξ) cos2n(It (!3) S. ?2Σ(1)"J 2■,(ξ) n= 0 cos(2n+1)Ωt (l4).Rewriting using these equations, the signal S(Δ
θ, 1) is (DC component) + (2ω component) +Eo”Jo(ξ) cos Δθ n=1 n:O (l5). This is an expansion by harmonics of the modulation frequency Ω. By passing it through, arbitrary harmonic components can be obtained. Among these, the first-order term is taken as the fundamental wave component P, and the second-order term is taken as the double harmonic component Q.

p (t)=2Eo”Jt (ξ)cosΩ ts
ln Δθ (l6)Q (t)=2Eo”J2
(ξ) cos 2Ωt cos Δθ (17)
becomes. In many cases, the fundamental wave P is detected and Δθ is determined. Maximize J1(ξ) so that the sensitivity of P is maximized. Therefore, the modulation degree is set so that ξ=1.8. At this time, J. (ξ) is approximately 0.3.

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

(C) Prior Art Japanese Patent Application Laid-Open No. 81-147108 proposes a method of controlling the output of a light emitting element so that the DC component of the output of a light receiving element is constant.

Japanese Patent Application Laid-Open No. 1358111i proposes a method of dividing the fundamental wave component of the output of a light receiving element by a DC component to cancel out the influence of light intensity fluctuations.

(Engineering) Problems to be Solved by the Invention Until now, the amplitude has been expressed by Eo without distinguishing between counterclockwise and clockwise light. Hereinafter, for distinction, the amplitude of the clockwise light will be referred to as El, and the amplitude of the counterclockwise light will be referred to as E2.

In the optical fiber dial, there was a large variation in the amount of light incident on the optical fiber from the light emitting element.

Japanese Patent Application Laid-Open No. 2003-23001-T4';+toe proposes a control system that keeps the DC component constant. However, this seems to overlook the problem of reflected light and phase modulation degree fluctuations.

JP-A-13581G converts the fundamental wave component P into the DC component D.
This solves the problem of light intensity fluctuation. This is also reflected light,
The problem of phase modulation degree variation is overlooked.

Reflected light is light that is reflected by the surface of an optical component such as a lens or fiber and enters a light receiving element. This is light that does not pass through the sensor coil. It does not contribute to angular velocity measurement and becomes noise.

On the other hand, the light that passes through the sensor coil and enters the light receiving element is called signal light.

The above proposal seems to be based on the assumption that the amount of reflected light H does not exist. Even if there is, it is assumed that the reflected light amount H is assumed to vary in the same way as the signal light.

In reality, the amount H of reflected light exists at a level that cannot be ignored.

The amount of reflected light H varies in the same way as the signal light. do not. Rather, the amount of reflected light H hardly changes in many cases.

Furthermore, the degree of phase modulation may also vary.

Therefore, making the DC component constant or dividing the fundamental wave P by the DC component D does not necessarily cancel out the fluctuations in the amount of light.

Furthermore, dividing the fundamental wave by the DC component causes the following problem.

The DC component D of the interference light which is the output of the light receiving element is (E1”+E
2”)+Es E2 JO(ξ)cos Δθ+
It can be written as H(l8). H is the amount of reflected light.

The fundamental wave P is obtained by excluding the modulation frequency component of (l6) and rewriting Eo'' as El and E2 to obtain P=2E.E2 Jt (ξ) slnΔθ 09) In addition to the problem of reflected light, the following Assumptions are required.

Assumption 1: There is no change in the ratio of the light amounts of the clockwise light and the counterclockwise light of the fiber.

Assumption 2: There is no variation in phase modulation degree.

Assuming that Assumption 1 holds true, we can set E2/E1=K. The DC component D is (1 + K2)E *”+K E i”J
It can be written as o(ξ)cos Δθ(21). However, the amount of reflected light H is ignored here. When the fundamental wave component P is divided by the DC component D, it becomes.

-(1+K2)+KJo(ξ)cosΔθ A correct result is obtained by dividing the fundamental wave component by the DC component, and an output that does not depend on the amount of light from the light emitting element is obtained. K and J. Since (ξ) is known, Δθ can be found.

However, for such a relationship to hold, the above assumptions must hold. This is unrealistic.

Furthermore, obtaining Δθ from (22) requires quite complex calculations. It's not an easy formula to calculate.

Furthermore, it should not be forgotten that reflected light originally included in the DC component is ignored.

(o> +m) The phase modulation type optical fiber gyro of the present invention (1) determines the fundamental wave component P.

(2) Find the even-order harmonic component Q.

(3) Find even-order harmonic components other than Q.

(4) Even-order harmonic component Q is set to 0.

(5) Calculate P/T and find Δθ in the form of tanΔθ.

It has the following characteristics.

The amount of reflected light is included only in the DC component. The fundamental wave, second harmonic, fourth harmonic, etc. do not include the amount of reflected light. In order to avoid problems with the amount of reflected light, the present invention does not use a DC component.

Both the fundamental wave P and harmonics of arbitrary orders have the same coefficients with respect to the light emitting element light. Therefore, by dividing the fundamental wave P by a harmonic, the coefficient related to the amount of light can be canceled out.

When the fundamental wave is divided by odd harmonics, Δθ disappears. However, if we divide the fundamental wave by the even harmonic component T, we get
It takes the form of tanΔθ.

For this reason, the P/T provides an output that is not affected by the amount of reflected light or fluctuations in the output of the light emitting element. Output and Δ
The relationship between θ is stable and simple.

This invention requires two even harmonics Q1T. One harmonic Q is for making the phase modulation degree constant by setting Q=O. The other harmonic T provides a denominator for dividing the fundamental wave P.

Any order is fine as long as it is an even number, but here, Q
will be explained assuming that T is the second harmonic and T is the fourth harmonic.

A phase modulation type optical fiber gyro according to the present invention will be explained with reference to FIG.

The light emitting element 1 is a light source that generates coherent light. A semiconductor laser, a superluminescent diode, a gas laser, etc. can be used.

This light is split into two beams by a light branching element 2 such as a beam splitter.

The two light beams are condensed by lenses 3 and 4, and then connected to an optical fiber 5.
The light enters this from both ends A and B.

Optical fiber 5 is one single mode fiber.

This consists of a sensor coil 6 and a portion 8 wound around a phase modulation element 7.

The sensor coil 6 is in the form of a coil made by winding a fiber many times, and serves as a sensor portion that detects angular velocity.

The phase modulation element 7 is, for example, an optical fiber wound around the circumferential surface of a cylindrical piezoelectric element, so that a voltage can be applied between end face electrodes. When an alternating current voltage is applied to the modulation frequency Ω, the phase of light propagating through the optical fiber changes periodically at the frequency Ω.

Excitation AC power supply 10 applies a modulation voltage to phase modulation element 7 via phase modulation degree control section 12 .

The light incident from the A end of the optical fiber 5 is transmitted to the senna coil 6.
It propagates as light in a clockwise direction. The light incident from the B end propagates as counterclockwise light. These two lights are combined again at the optical branching element 2.

The intensity of the interference light is square-law detected by the light receiving element 9.

Since the phase modulation element 7 is asymmetrical with respect to the sensor coil 8, the light receiving element output includes the fundamental wave and all harmonics of the modulation frequency Ω.

As already explained, the n-th harmonic has an n-th Bessel function and the square of the amplitude of light as a coefficient.

Δθ enters the odd harmonics in the form of a sine function.

Δθ enters the even harmonics in the form of a cosine function.

The synchronous detection ml3 detects the fundamental wave component P of the output of the light receiving element.
Detect. The synchronization signal is obtained from the excitation AC power supply 10.

The second harmonic detection section 12 detects the second harmonic Q from the output of the light receiving element. A multiplier 20 doubles the signal from the excitation AC power supply 10 to produce a synchronization signal with a frequency of 2Ω.

The second harmonic detection section 12 controls the phase modulation degree b of the phase modulation degree control section 11 so that the second harmonic Q becomes zero.

Since ξ and b have a constant relationship, hereinafter, ξ will be simply referred to as the phase modulation degree.

The fourth harmonic detection section 15 detects the fourth harmonic T from the output of the light receiving element. The signal 20 from the multiplier 20 is further doubled by the multiplier 21 to produce a 4Ω synchronous signal, which is used for synchronous detection.

The divider 14 divides the fundamental wave P by the fourth harmonic T and outputs the result S.

(Force) Action The explanation of the action as a normal optical fiber gyro will be omitted.

The synchronous detection section 13 detects the fundamental wave component from the light receiving element output.

P”2EI E2 J t (ξ)sin Δ
θ (23) The fourth harmonic detection section 15 detects the fourth harmonic T from the light receiving element output.

T=2E+ E2 J4 (ξ) cos Δθ
(24) Since the divider 14 divides the fundamental wave P by the fourth harmonic T, the following equation is obtained, but this equation does not include the amplitude of light. Therefore, even if the amount of light from the light emitting element changes, this result is not affected. Furthermore, since the DC component is not used as the denominator, the problem of the amount of reflected light can be avoided.

However, the coefficient is a function of the phase modulation degree ξ. If this continues, there is a risk that the coefficients will change as the degree of phase modulation changes.

Therefore, in the present invention, the second harmonic Q is held constant. For example, hold it at O.

Q=-2E. EQ Ja (ξ) cos Δ
θ (2B) Setting Q=O means J2(ξ)=
This means setting it to 0. Bessel functions have many zeros. The quadratic Bessel function also has many zeros, but
It can be any zero point.

Since the value of ξ is fixed at a certain zero point, it is linear, 4
The following Bessel function Jt(ξ), J. The value of (ξ) becomes constant. This means that the coefficient of equation (25) remains constant. From this equation, the phase difference Δθ is uniquely determined.

Figure 3 shows a graph of the Bessel function. Quadratic J2(ξ
) becomes 0 at 5.1. It is called the first zero point. With this value, J, (ξ), J. (ξ) is −0 respectively
．． 35, 0.4. At this time, the coefficient of (28) is determined to be -0.85.

Of course, you can take any zero point. In a Bessel function, there is always one zero of another function between two adjacent zeros of a function of a certain degree. There is also a thumb rule that states that the sum of the squares of a function over all orders is always equal to 1. Therefore, near the zero point of the quadratic function, the value of the Bessel function of a similar order becomes large.

In this example as well, in the vicinity of the first zero point ξ=5.1, the first-order and fourth-order Bessel functions J. (ξ), J. (ξ) is large and its derivative is small. In other words, even if ξ changes, these values hardly change. Since the coefficient is difficult to change, the sensitivity does not change. Q=
Although the fluctuation of ξ is actively suppressed by setting it to 0, this has the double effect that even if it fluctuates, its influence is small.

In this way, the present invention can completely eliminate the effects of variations in the amount of light and variations in the degree of phase modulation of the light emitting element.

Another advantage is that the calculations are simple. (22), the final result S is given by tanΔθ. It is easy to calculate Δθ from tanΔθ. Furthermore, when Δθ is small, tanΔθzakiΔθ (27), so it is extremely simple. Δθ is obtained.

(G) Example Regarding the minimum configuration with the basic conditions of a phase modulation type optical fiber gyro, Ezk1el S. and Ardltty H. J
．． ：”FIBER OPTIC ROTATION S
ENSOR”, Sprlnger4erlag Ber
lln. I982 has a detailed explanation.

An example is shown in FIG. Description of parts common to FIG. 1 will be omitted.

The phase modulation degree control section 11 includes a PID controller 22 and a multiplier 23.

The second harmonic detection unit 12 detects the second harmonic Q, and the PI
The D controller 22 compares Q with O and determines the degree of phase modulation so that the difference becomes small. This is multiplier 23
is given as a signal to determine the multiplier.

The multiplier 23 multiplies the signal from the excitation AC power source 10 by a multiplier and applies the resultant signal to the phase modulation element. In this way, the degree of phase modulation is kept constant.

(H) Effect Even if the amount of coupled light changes due to temperature or changes over time, the present invention can eliminate this effect.

Since no DC component is used, there is no problem with the amount of reflected light.

The result is simple because the fundamental wave is divided by even harmonics. Calculations become easier.

Since the phase modulation degree is made constant by setting the even-numbered harmonics to 0, there is no fluctuation in the phase modulation degree due to changes in temperature or changes over time.

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 an optical fiber gyro according to an embodiment of the present invention. FIG. 3 is a configuration diagram of a conventional phase modulation type optical fiber gyro. Figure 4 is a graph of the Betzel function. 1 ● φ ● ● Departure. Optical element 2 ● ● ● ● Optical branching element 3, 4
●●●● Lens 5 1 ● ● Optical fiber
Bar 6●●●●Sensor coil 7●●●●Phase modulation element 8●●sha●Optical fiber portion wrapped around the phase modulation element 9●●●●Receiving light element 10●●●Exciting AC power supply 11●●● Phase modulation degree control section 12 ●●● Second harmonic detection section 13 ●●● Synchronous detection section 14 ●●● Divider 15 ●●● Quadruple harmonic detection section 2 0, 21- ● ● Multiplier 22 ●●●
PID controller 23 ● ● ● Multiplier

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. In a phase modulation type optical fiber gyro having at least a synchronous detection circuit for receiving and detecting a phase modulated frequency component, an even-order harmonic for receiving an output of the light-receiving element and detecting an even-order harmonic T of the phase modulation frequency. A wave detection unit, a synchronous detection unit that detects the fundamental wave component P of the light receiving element output, and a divider that divides the fundamental wave component P by an even-order harmonic component and outputs the result S. A phase modulation optical fiber gyro featuring: (2) The even-order harmonic T is other than second-order, and a double harmonic detection unit detects a harmonic with a frequency twice the phase modulation frequency, and sets the double harmonic component to a constant value. 2. The phase modulation type optical fiber gyro according to claim 1, further comprising a phase modulation degree control section for controlling the phase modulation degree. (3) A phase modulation type optical fiber gyro according to claim 1 or 2, wherein the phase modulation degree control section is constituted by a PID control system.