JPH07128076A - Optical fiber gyro - Google Patents

Abstract

PURPOSE:To cancel bias error by removing the intensity modulation component caused by optical phase modulation. CONSTITUTION:An optical phase modulator comprises first and second polarization maintaining optical fibers 18b1, 18b2 wound around a tubular electrostrictive oscillator 18a while being connected therewith. The first and second polarization maintaining optical fibers 18b1, 18b2 have identical characteristics and length and the end faces thereof are jointed such that the direction of polarization intersects perpendicularly each other.

Description

Detailed Description of the Invention

[0001]

This invention relates to an optical fiber loop,
The present invention relates to an optical fiber gyro that detects an angular velocity applied around its axis from the phase difference between right-handed and left-handed light propagating in the optical fiber loop.

[0002]

2. Description of the Related Art A conventional optical fiber gyro will be described with reference to FIG. 2A. The light emitted from the semiconductor laser light source 11 is incident on the optical fiber coupler 12 as an optical branching means, and further through the polarizer 13 to the optical fiber coupler 14 as an optical branching / coupling means. The lights divided into two by the optical fiber coupler 14 are polarization maintaining optical fibers 15 and 16 respectively.
Through the optical fiber loop 17 as right-handed light and left-handed light. The polarization axes of the polarization-maintaining optical fibers 15 and 16 are 45 degrees with respect to the respective polarization directions of these two lights.
And the polarization maintaining optical fibers 15 and 1
The length of 6 is made sufficiently long, and the optical fiber loop 1
The polarization maintaining optical fibers 15 and 16 respectively act as polarization dissociation means for reducing the degree of polarization so that the x-direction polarization component and the y-direction polarization component of the light incident on 7 will not interfere with each other.

An optical phase modulator 18 is inserted in series between one end of the optical fiber coupler 14 and the polarization maintaining optical fiber 16, and the optical phase modulator 18 is supplied with a modulation signal from an oscillator 19.
Is driven, and the left-handed and right-handed lights propagating through the optical fiber loop 17 are phase-modulated at the frequency f _m . The left-handed and right-handed lights propagating through the optical fiber loop 17 are combined by the optical fiber coupler 14, and the interference light reaches the light receiver 21 through the polarizer 13 and further through the optical fiber coupler 12, and is converted into an electric signal. This electric signal is the output of the oscillator 19, is synchronously detected in the synchronous detection circuit 22, and the detected output is output to the output terminal 23 as a gyro output.

When the angular velocity around the axis of the optical fiber coil 17 is applied, a phase difference occurs between the two-direction light due to the Sannyak effect. This phase difference φ _S is expressed by the following equation. φ _S = 4πPLΩ / (cλ) (1) where R and L are the loop radius and the total length of the optical fiber loop 17, respectively, c is the optical velocity, λ is the optical wavelength, and Ω is the input angular velocity.

[0005] change in the interference light intensity has no sensitivity phi _S = 0 near proportional to cos [phi _S to determined phi _S (1). Therefore, the optical phase modulator 18 is installed at one end of the optical fiber loop 17, and the optical phase modulation of φ _m sin (2πf _m t) (2) is applied to the light transmitted through the optical phase modulator 18. Here, φ _m is the optical phase modulation amplitude, f _m is the optical phase modulation frequency, and t is the time. If the light propagation time of the optical fiber loop 17 is written as τ, the phase difference Δφ (t) of the two-way light that determines the intensity of the interference light can be expressed by the following equation.

Δφ (t) = φ _S + φ _m sin (2πf _m (t−τ)) −φ _m sin (2πf _m t) (3) Thus, the phase difference Δφ (t) oscillates and the interference light The cos Δφ (t) that gives the intensity change can be expressed by the following equation. _{cosΔφ (t) = cosφ S Σε} n (-1) n cos2n (2πf m (t- (τ / 2)) J 2n (x) -sinφ S · 2Σ (-1) n cos (2n + 1) (2πf m ( t- (τ / 2)) J _{2n + 1} (x) (4) ε = 1 (n = 0); ε = 2 (n ≧ 1), ε is from n = 0 to infinity x = 2φ _m sin πf _m τ where J _i is the i-th order Bessel function of the first kind (4)
From the equation, it can be seen that sinφ _S can be detected as an odd multiple frequency component of the modulation signal from the intensity vibration of the interference light. Thus, it is the phase modulation type optical fiber gyro that obtains the sensitivity at φ _S ≈0, but normally, the fundamental wave component (n = 0) of the output of the light receiver 21 is synchronously detected as described above. .

Optical fibers are transmitted with special exceptions.
The light mode is usually not single, but so-called single
Even in optical fiber called mode optical fiber
HE ₁₁ ^x, HE₁₁ ^yTwo orthogonal linear polarization modes
Is degenerate. This degeneracy is easily solved by the action of stress.
However, polarization mode dispersion occurs. Moreover, these dispersed models
Mode coupling often occurs due to stress disturbance.
It Optical fiber is usually used to eliminate these error factors.
Optical branching means connected to the loop 17, that is, an optical fiber cable
In front of the plastic 14, a linear polarizer 1 is used as a mode filter.
Insert 3. However, the performance of the polarizer 13, that is, the extinction
Since the ratio has a finite value in a real device, the optical
In order to obtain sufficient resolution as a gyro,
One of two methods is adopted. That is, the first one
The method uses a polarization maintaining optical fiber for the optical fiber loop 17.
To prevent mode coupling between polarization modes,
The second method is polarization dissociation means, and in FIG.
Correlation of vibration of light between polarization modes by aver 15 and 16
Is a way to eliminate. Optical fiber with the above mode dispersion
The gyro error is equivalent to the phase difference and can be expressed by the following equation.

Δφ <εγ (| E _y | / | E _x |) (| α _xy | + | α _yx |) / | α _xx | (5) where ε is the extinction ratio of the polarizer 13 and γ is the polarization Correlations between wave mode propagating lights, E _x and E _y are amplitudes of polarization main axis and multi-axis component of incident light, and α _xx and α _xy are amplitudes between polarization modes in the optical fiber loop 17. It is assumed that the transfer coefficient and ε are sufficiently small. Here, the first method is (| α _xy | + | α
_yx |) / | α _xx | is 0, and the second method is to set γ to 0. The former is called a polarization system and the latter is called a non-polarization optical fiber gyro.

The polarization dissociation is relatively easy to realize as described above, particularly when linearly polarized light is targeted. When linearly polarized light is incident on a birefringent medium such as a polarization-maintaining optical fiber at an angle of 45 ° with respect to the birefringence axis, the correlation between the two components decreases as the propagation distance increases, and Dissociates. In this case, the birefringent transmission line should be long enough to have a sufficient optical distance difference between the polarization modes as compared with the coherence length of light.

The non-polarization type optical fiber gyro in the phase modulation system among the optical fiber gyros and the polarization dissociation means have been described above. As the optical phase modulator 18, there is a type that uses the electro-optic effect in an optical crystal, but a simpler and more widely used type is a cylindrical electrostrictive oscillator 18a that maintains polarization as shown in FIG. 2B. The optical fiber 18b is wound and the optical fiber is expanded and contracted. The linear polarizer 13 is typically formed by winding a polarization-maintaining optical fiber having a large bending loss in the sub-axis polarization mode and winding the fiber with a sufficiently small diameter.

[0011]

As described above, in the phase modulation type optical fiber gyro, the Sagnac phase difference due to the rotational angular velocity is detected as the modulation synchronous AC component of the light quantity of the light reaching the light receiver 21. is there. If there is a case where a modulation-synchronized AC component is generated in the amount of light reaching the light receiver due to other reasons, this is a bias error of the gyro because it is not separated from the Sagnac phase difference. In particular, not by forming interference light as a combination of double-sided light, but when the fluctuation of oscillation due to phase modulation synchronization has already occurred in the amount of transmitted light at the stage of single-sided light, the gyro error caused by the change in light intensity can be modulated. Called bias. Although there is no single cause for causing such light intensity modulation, what is important in the non-polarization type phase modulation type optical fiber gyro having the above-mentioned configuration is the modulation synchronous optical polarization induced by the optical phase modulator 18. It is a state change. This will be described below.

The extinction ratio of the optical fiber polarizer 13 as an actual device is typically about 20 dB, and the sub-axis polarization mode does not extinct as ideal. The optical fiber coupler 14 as an actual device has some crosstalk, that is, coupling between orthogonal polarization modes even if it is polarization maintaining. In the actual optical fiber gyro manufacturing process, it is impossible to completely eliminate the azimuth error of polarization axis alignment in the polarization maintaining optical fiber by splicing the optical fibers. Under these circumstances, for example, for transmission of counterclockwise light in FIG.
3. The light passing through the optical phase modulator 18 via the optical fiber coupler 14 has a polarization sub-axis excitation having a non-zero amplitude, and the optical phase modulation of a modulation amplitude different from that of the main axis is effective for this component. . In this case, the relative phase difference between the light on the main axis and the light on the sub axis oscillates in an alternating current in synchronization with the driving of the phase modulator 18.

On the other hand, in the non-polarization type optical fiber regyro, it is assumed that the light propagating through the optical fiber loop 17 is completely polarization-dissociated, but the extinction of the polarizer 13 is insufficient, and the optical fiber coupler 14 is used. Crosstalk, a 45 ° connection angle error between polarization maintaining optical fibers 15 and 16 for polarization dissociation, a connection angle error between birefringence axes at other polarization maintaining optical fiber connection points, and light caused by the spectrum of the light source 11. Due to practical constraints such as non-monotonic coherence attenuation characteristics, it is inevitable that the light propagating through the optical fiber loop 17 has a non-zero degree of polarization and a slight partial polarization component.
Such a minute partial polarization component is the above-mentioned optical phase modulator 18
The polarization state may be changed in synchronism with the operation described above, and light having a polarization state change synchronized with this phase modulation is used for the polarizer 1 having an angle difference with respect to the birefringence axis direction.
When light is received via 3, the light reaching the light receiver 21 undergoes intensity modulation in synchronization with phase modulation.

To describe the above situation very generally, if the orthogonal linearly polarized wave component having correlation and defining the component involved in such polarization state fluctuation is written as E _x and E _y , the phase difference between them is expressed as δφ. The intensity Iθ of the partially polarized light transmitted through the polarizer having the azimuth θ can be expressed by the following equation. I θ = I _x + I _y + 2√I _x I _y cos δφ (6) I _x = | E _x | ² cos ² θ, I _y = | E _y | ² sin ² θ The phase difference δφ is the operation of the optical phase modulator 18. Therefore, when it changes to δφ = δφ _O (t) + δφ _m sin (2πf _m t) (7), Iθ becomes the equation (8).

Iθ = I _x + I _y + 2√I _x I _y [cos δφ _O (t) Σε _n cos2n (2πf _m t) · J _2n (δφ _m ) −sin δφ _O (t) · 2Σsin (2n + 1) (2πf _m t) · J _{2n + 1} (δφ _m )] ε = 1 (n = 0); ε = 2 (n ≧ 1) (8) Σ is from n = 0 to infinity, that is, Iθ is a phase modulation It has a component synchronized with the fundamental wave, and its intensity Iθ ′ is Iθ ′ = 2 | E _x || E _y | sin2θ · sin δφ _O (t) J ₁ (δφ _m ) (9).

[0016]

According to the present invention, in a non-polarization phase modulation type optical fiber gyro, an optical phase modulator has a polarization maintaining optical fiber wound around an electrostrictive oscillator, and the polarization maintaining optical fiber is maintained. The optical fiber is configured by connecting first and second polarization-maintaining optical fibers having the same length in directions orthogonal to each other with respect to the optical polarization axis. In the invention of claim 2, the connection point of the first and second polarization maintaining optical fibers is separated from the electrostrictive oscillator.

[0017]

The embodiment of the present invention has the same overall structure as that shown in FIG. 2A, but in the present invention, the optical phase modulator 18 has a cylindrical electrostrictive oscillator as shown in FIG. 1A. 1
Polarization maintaining fiber 18b is first being wound in 8a, second polarization maintaining optical fiber 18b _1, consists 18b _2, these first, second polarization maintaining optical fiber 18b _1, 1
8b ₂ has the same length of the portion wound around the electrostrictive oscillator 18a, and is connected in the directions orthogonal to each other with respect to the optical polarization axis. That is, for example, in FIG.
The second polarization-maintaining optical fibers 18b ₁ and 18b ₂ have the same characteristics in that two cores 25 and 26 are provided symmetrically with respect to the axial center of the circular clad 27 in cross section, and the cores 25 and 26 are The end faces of the first and second polarization-maintaining optical fibers 18b ₁ and 18b ₂ are butt-joined so that the winding directions 28 are orthogonal to each other.

The first and second polarization maintaining optical fibers 18b
₁, 18b₂The connecting point 29 of is electrostrictive as shown in FIG. 1C.
The electrostrictive vibrator 18a is driven away from the vibrator 18a.
Even if the radius is expanded or contracted, the connection point 29 is mechanical.
You should not be affected and damaged. In this configuration
According to this, the polarization maintaining optical fiber 18b of the optical phase modulator 18
₁, 18b ₂Two orthogonal polarization modes at
DC component δφ of optical phase difference due to optical distance difference_O(T)₁When
δφ_O(T)₂And have the same magnitude and opposite polarities
It Therefore, δφ in equation (9) above_O(T) is zero
Therefore, the strength based on the phase modulation during the output of the synchronous detector 22 becomes
The degree modulation component becomes zero, and the bias error is eliminated.

When the optical fiber coupler 14 is composed of a polarization maintaining fiber and is connected to the polarization maintaining optical fiber of the optical phase modulator 18, desired stable operation can be obtained regardless of the optical polarization. . In the above description, one of the polarization maintaining optical fibers 15 and 16 may be omitted, and the other polarization dissociation means may be used. As the optical fiber loop 17, an inexpensive single mode optical fiber is preferable. The first and second polarization-maintaining optical fibers 18b ₁ and 18b ₂ are not limited to one, and one or both of them may be plural, but the sum of their lengths may be equal to each other.

[0020]

As described above, according to the present invention, the polarization maintaining optical fibers of the optical phase modulator 18 are configured as the first and second optical fibers, and have the same length and the same characteristics. Since the connection is made so that the polarization directions are orthogonal to each other, the phase difference caused by the phase modulation between the two orthogonal polarization modes causes the first and second polarization-maintaining optical fibers 1.
8b _1, at 18b ₂ the same size, are canceled each other become opposite polarity, intensity modulation component becomes zero, the bias error is also zero gyro output.

[Brief description of drawings]

FIG. 1A is a perspective view showing an embodiment of an optical phase modulator which is a main part of the present invention, and B is a view showing a connection state of the first and second polarization maintaining optical fibers 18b ₁ and 18b ₂ . C is a perspective view showing another example of the optical phase modulator.

2A is a block diagram showing a conventional phase modulation type non-polarization type optical fiber gyro, and FIG. 2B is a perspective view showing an optical phase modulator 18 thereof.

Claims (2)

[Claims] 1. Light from a light source is split into two by a branch coupling means and is made incident on both ends of an optical fiber loop as clockwise light and counterclockwise light, and at least one end of the optical fiber loop and the branch coupling means. In the meanwhile, a polarization dissociation means is inserted, and at least one of the right-handed light and the left-handed light is light having a sufficiently low degree of polarization, and between one end of the optical fiber loop and the branch coupling means, An optical phase modulator in which a polarization maintaining optical fiber is wound around an electrostrictive oscillator is inserted in series, the right-handed light and the left-handed light are phase-modulated, and the right-handed light propagated through the optical fiber loop and In the optical fiber gyro, the counterclockwise light is converted into an electric signal by the light receiver in the form of interference light combined by the branch coupling means, and the electric signal is synchronously detected by the modulation signal of the optical phase modulator to obtain a gyro output, Optical phase above The polarization maintaining fiber modulator, first the same length which are connected with the orientation orthogonal to each other with respect to the optical polarization axis, the optical fiber gyro characterized by made of the second polarization maintaining optical fiber. 2. The optical fiber gyro according to claim 1, wherein a connection point of the first and second polarization maintaining optical fibers is separated from the electrostrictive oscillator.