JPH07128073A - Optical fiber gyro - Google Patents

Abstract

PURPOSE:To cancel intensity modulation gyro bias error caused by phase modulation. CONSTITUTION:A light from a light source 11 is split by an optical fiber coupler 14 into lefthand and righthand lights passing through an optical fiber loop 17. They are subjected to phase modulation by an optical phase modulator 18 based on a modulation signal from an oscillator 19 and the interference light thereof is converted through a light receiver 21 into an electric signal which is detected synchronously by a synchronous detector 22. Lights entering into the optical fiber loop 17 from the opposite ends thereof are subjected to polarizing dissociation by means of polarization maintaining fibers 15, 16. The optical phase modulator 18 comprises the polarization maintaining fibers 15, 16 wound around a tubular electrostrictive element and connected in series with a double refraction optical phase compensator 25. The modulator 18, the compensator 25 and the fiber 16 are connected while aligning the direction of polarization and the DC component of optical phase difference based on the difference of optical distance between two orthogonal optical polarization modes is set equal to integer times of pi radian.

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. 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. 2 with optical fiber coupler 14
The split lights are incident on the optical fiber loop 17 as right-handed light and left-handed light through the polarization maintaining optical fibers 15 and 16, respectively. 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 the simpler and more widely used type is that a polarization maintaining optical fiber is wound around a cylindrical electrostrictive oscillator, It expands and contracts the optical fiber. 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 θ The phase difference δφ is δφ due to the operation of the optical phase modulator 18. = 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 a first aspect of the present invention, there is provided a non-polarization phase modulation type optical fiber gyro, or an optical phase modulator, or a compound phase gyro connected to the optical phase gyro with their directions aligned with respect to the optical polarization axis. In the entire path length of the refractive polarization maintaining optical transmission line element, the DC component of the optical phase difference due to the optical distance difference between the two orthogonal optical polarization modes is set to an integral multiple of π radian.

According to the invention of claim 2, in the invention of claim 1, a birefringent optical phase compensator which is variable in direction and coincides with respect to the optical polarization axis is connected in series to the optical phase modulator. An integral multiple of π radian is set including the birefringent optical phase compensator. According to the invention of claim 3,
In the invention of claim 2, the output of the photodetector is synchronously detected by the second synchronous detector by the reference signal whose phase is different from the reference signal for the synchronous detector by approximately 90 °, and the birefringent optical phase compensator is obtained by the detected output. Negative feedback is controlled.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1A shows an embodiment of the invention of claim 2 and the same reference numerals are attached to the portions corresponding to those of FIG. In this embodiment, a birefringent optical phase compensator 25 is inserted in series between the polarization maintaining optical fiber 16 and the optical phase modulator 18. The birefringent phase compensator 25 is, for example, a mechanically movable bobbin around which a sufficiently long birefringent polarization-maintaining optical fiber is wound, and the optical fiber is expanded or contracted, or a babinet as shown in FIG. 1B. Compensators and Babinet-Soleil compensators are used. As the optical phase modulator 18, used is a cylindrical electrostrictive oscillator in which a polarization maintaining optical fiber is wound and the optical fiber is expanded and contracted. The optical fiber of the optical phase modulator 18 and the birefringent optical phase compensator 25 have the same direction with respect to the optical polarization axis. That is, when polarization maintaining optical fibers are used for both, their polarization maintaining directions are matched with each other and are connected to each other. Birefringent optical phase compensator 25
The polarization maintaining optical fiber 16 is also connected so that the directions of both optical polarization axes coincide with each other.

The total of the optical phase modulator 18, the birefringent optical phase compensator 25, and the polarization maintaining optical fiber 16 is 2
The DC component of the optical phase difference due to the difference in optical distance between the two orthogonal polarization modes is an integral multiple of π radian. That is, the control means 26 for the birefringent optical phase compensator 25 is adjusted to adjust the optical phase difference between the two orthogonal optical polarization modes in the phase compensator 25, and the direct current of the optical phase difference due to the optical distance difference is adjusted. Make the component an integer multiple of π radians.

As described above, the light intensity modulation based on the optical phase modulation is proportional to sin δφ _O (t) at the output of the synchronous detector 22, as shown in the equation (9). δφ _O
(T) is the DC component of the optical phase difference between orthogonal linear polarization components (orthogonal optical polarization mode) created by the birefringent optical transmission line element including the optical phase modulator 18, as shown in equation (7). And the absolute amount is the birefringence optical path difference of the transmission line element, that is, the retardation. According to the embodiment shown in FIG.
Since φ _O (t) is an integral multiple of π radian, s
in δφ _O (t) = 0, therefore, the output of the synchronous detector 22 does not show the optical intensity modulation effect based on the phase modulation of the optical phase modulator 18, and the bias error becomes zero.

By omitting the birefringent optical phase compensator 25, the optical phase modulator 18 and the polarization maintaining optical fiber 16 have δφ _O.
(T) may be an integral multiple of π radian.
This is the invention of claim 1, and in this case, in consideration of the beat length of the polarization maintaining optical fiber, it is necessary to adjust the length to be less than 1 mm. By using the point optical phase compensator 25, δφ _O (t) can be easily made an integral multiple of π radian.

FIG. 2A shows an embodiment of the invention of claim 3,
The same reference numerals are given to the portions corresponding to those in FIG. 1A. The synchronous detector 27 is provided, and in the synchronous detector 27, the light receiver 2
The output of 1 and the reference signal of the synchronous detector 22 have a phase of about 9
Synchronous detection is performed with reference signals that differ by 0 degrees. That is, the phase shifter 28 shifts the phase of the reference signal for the synchronous detector 22 by 90 degrees and supplies the reference signal to the synchronous detector 27. The detection output of the synchronous detector 27 is supplied to the driving unit 29, the birefringent optical phase compensator 25 is driven by the driving unit 29, and negative feedback control is performed so that the output of the synchronous detector 27 becomes zero.

The phase modulation frequency component based on the Sagnac phase difference in the output of the light receiver 21 can be expressed by the following equation from the equation (4). 2sinφ _s J ₁ (x) · sin (2πf _m t + ((π / 2) −πf _m τ)) (10) On the other hand, the phase modulation frequency component in the intensity modulation based on the phase modulation is expressed by the following formula from formula (8). Can be represented.

2 | E _x || E _y | sin2θ · sin δφ _O (x) J ₁ (δφ _m ) · sin (2πf _m t) (11) The length L of the optical fiber loop 17 and the modulation frequency f _m Π in equation (10) unless each is set to a particularly large value.
Since f _m τ is a much smaller value than π / 2,
For the gyro output expressed by equation (10),
The intensity modulation component represented by the equation is approximately 9 as shown in FIG. 2B.
There is a phase difference of 0 degree. Therefore, the bias error is removed by detecting the intensity modulation component from the synchronous detector 27 and controlling the birefringent optical phase compensator 25 through the drive unit 29 so that the intensity modulation component becomes zero.

In the above description, a single-mode optical fiber is used for the optical fiber loop 17 in a non-polarization type optical fiber gyro. Instead of the polarization maintaining optical fibers 15 and 16, other polarization dissociation means may be used.

[0026]

As described above, according to the present invention, in the optical phase modulation type non-polarization type optical fiber gyro, the optical intensity modulation component based on the optical polarization state changing at the modulation frequency due to the optical phase modulation is The DC component δφ _O (t) of the optical phase difference due to the optical distance difference between the two orthogonal optical polarization modes is π
Since it is an integral multiple of radians, the bias error is almost zero. According to the third aspect of the invention, the intensity modulation component is detected, whereby the phase difference between the two DC optical polarization modes is subjected to negative feedback control, and the bias error is made substantially zero.

[Brief description of drawings]

FIG. 1A is a block diagram showing an embodiment of the invention of claim 2, and B is a diagram showing a Babinet compensating plate.

2A is a block diagram showing an embodiment of the invention of claim 3, and FIG. 2B is a diagram showing an angular relationship between a modulation frequency component of a gyro output and a modulation frequency component of intensity modulation.

FIG. 3 is a block diagram showing a conventional optical phase modulation type non-polarization type optical fiber gyro.

Claims (3)

[Claims] 1. Light from a light source is converted into two by a branch coupling means.
The light is split into right and left lights at both ends of the optical fiber loop, and a polarization splitting means is inserted between at least one end of the optical fiber loop and the branch coupling means to obtain the right light. At least one of the left-handed light and the left-handed light is light having a sufficiently low degree of polarization, and an optical phase modulator that operates by expansion and contraction of the polarization maintaining optical fiber is provided between one end of the optical fiber loop and the branch coupling means. The right-handed light and the left-handed light are phase-modulated by being inserted, and the interfering light obtained by combining the right-handed light and the left-handed light propagating through the optical fiber loop by the branch coupling means is converted into an electric signal by a light receiver. An optical fiber gyro that is converted and whose electrical signal is synchronously detected by a synchronous detector using the modulation signal of the optical phase modulator as a reference signal to obtain a gyro output. The DC component of the optical phase difference due to the difference in optical distance between the two orthogonal optical polarization modes is an integral multiple of π radian in the total path length of the birefringent polarization maintaining optical transmission line elements connected in the same direction with respect to A fiber optic gyro characterized by being set to. 2. A birefringent optical phase compensator which is variably aligned in azimuth with respect to the optical polarization axis is connected in series to the optical phase modulator, and the birefringence is set to an integer multiple of π radian. 2. The optical fiber gyro according to claim 1, wherein the optical fiber gyro is set by including an optical phase compensator. 3. A second synchronous detector for synchronously detecting the output of the photodetector with a reference signal whose phase is approximately 90 ° different from that of the reference signal for the synchronous detector, and the output of the second synchronous detector is the above-mentioned. 3. The optical fiber gyro according to claim 2, which is negatively fed back as a control signal to the birefringent optical phase compensator.