JPH0658227B2 - Light fiber gyro - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Field of Application> The present invention relates to a gyro composed of an optical fiber,
It is suitable for use in attitude control of flying objects and robots.

<Prior Art> A configuration of a conventional optical fiber gyro using a birefringent fiber is shown in FIG. In FIG. 8, the light emitted from the laser 1 is collimated by the lens 2 and converted into linearly polarized light by the bulk polarizer 4 in the direction of the principal axis of the birefringent fiber 7.
Is incident on the fiber. The light incident on the fiber is
The 3 dB optical coupler 6 bisects one-to-one into a fiber coil 7
Is divided into light propagating clockwise and light propagating counterclockwise. The light propagating clockwise and counterclockwise in the fiber coil is
After being combined again by the 3 dB optical coupler 6 and passing through the lens 5 and the polarizer 4, it is reflected by the half mirror 3 and passes through the lens 10 before entering the photodetector 9. The photodetector 9 produces a photocurrent proportional to the intensity of interference light between the light propagating clockwise in the optical fiber and the light propagating counterclockwise. In the optical fiber gyro, the optical path length of light propagating in the optical fiber in both left and right directions varies depending on the rotation of the optical fiber. Such a phenomenon is known as the Sagnac effect and is known as EJPost “Sagn
ac Effect "Rev.of Modern Phys.vol.39-2, P.475-493 (19
67). A phase difference occurs between the two-direction light corresponding to the optical path difference between the two-direction light.

Here, λ is the wavelength of the light source in vacuum, C is the speed of light in vacuum, R is the radius of the optical fiber coil, L is the length of the optical fiber coil, and Ω is the rotational angular velocity. Therefore, the photocurrent output I from the photodetector becomes I∝1 + cos (2) due to this phase difference, as shown in FIG. Therefore, the rotational angular velocity Ω can be detected by reading the change in the photocurrent output. However,
In the case of the formula (2), the output I becomes a substantially constant value in the case of a minute rotation (<< 1) and the rotation cannot be detected.
As shown in the figure, wind a fiber around the PZT oscillator 8
An AC voltage is applied to the ZT oscillator to apply phase modulation to the light propagating in the fiber. Now, assuming that the angular frequency of the AC voltage applied to the PZT is ω _m and the phase modulation degree is _m , the output current I (t) of the photodetector is I (t) ∝1 + cos [+ _m cos (ω _m t) ] = 1 + cos cos [ _m cos ( _m t)]-sinsin [ _m cos (ω
_m t)] = 1 + cos {J _o ( _m ) + 2J ₂ ( _m ) cos (2ω _m t) +…} + sin {2J ₁ ( _m ) sin (ω _m t) +…} (3) Become. However, J _o , J ₁ , and J ₂ are Bessel functions. The output I ₁ ∝sin · 2J ₁ ( _m ) (4) can be obtained by synchronously detecting the component of ω _m in the angular frequency component of I (t). In the case of minute rotation << 1, formula (4) becomes I ₁ ∝ · 2J ₁ ( _m ) (5), and an output proportional to the rotational angular velocity is obtained.

Here, there is conventionally known an optical fiber gyro in which the half mirror 3 and the polarizer 4 shown in FIG. 8 are replaced with a 3 dB coupler and a polarizer. However, when a single mode optical fiber (optical fiber that does not intentionally cause birefringence) is used as the fiber coil 7, the polarization state of light changes due to vibration or temperature change, and the performance of the gyro is degraded. Therefore, the coherent light emitted from the light source 1 is depolarized (polarization elimination or coherence elimination) so that coherent noise does not occur even when disturbance such as vibration or temperature change is applied. There is a need. Therefore, it is necessary to interpose a depolarizer between the light source 1 and the 3 dB coupler 3, and conventionally, a pair of birefringent optical fibers 11 and 12 as shown in FIG. A depolarizer made by connecting Y with a shift of 45 degrees was used. In FIG. 10, 13 is a core and 14 is a core 13.
Is a stress-applying layer that applies stress to the waveguide to cause birefringence in the waveguide.

<Problems to be Solved by the Invention> In order to insert a depolarizer composed of a birefringent optical fiber into an optical fiber gyro composed of a single mode optical fiber as shown in FIG. Optical fibers of different types must be fusion spliced. Therefore, there is a problem in that the mechanical strength of the connection portion is reduced, and when the protection member is provided in the connection portion, the optical fiber gyro becomes large in size.

The present invention has been made in view of the above conventional circumstances, and a depolarizer is also configured by a single-mode optical fiber to eliminate a fusion splicing portion of an optical fiber, and to provide an optical fiber gyro with a small size and high mechanical strength. The purpose is to provide.

<Means for Solving Problems> The optical fiber gyroscope of the present invention is a light source, a depolarizer for eliminating coherence of light from the light source, and a light from the depolarizer is split into one to one. And a second coupler for guiding the light emitted from both ends of the optical fiber coil to the detector. , The light propagating in the same direction as the rotating direction and the light propagating in the opposite direction to the rotating direction are incident on the optical fiber coil placed in the rotating system, and the effective optical path length difference of these lights is proportional to the rotational angular velocity. In the optical fiber gyro that detects the rotational angular velocity by utilizing the above, the first coupler, the second coupler, and the optical fiber coil are formed of a single mode optical fiber, and the depolarizer is a single mode light. First coil made of fiber and second And the center of the first coil and the center of the second coil are set to be inclined by 45 degrees with respect to each other, and the deviation of the wave packet due to the first coil is larger than the coherence length of the light source light and the wave packet due to the second coil. The deviation of the wave packet due to the first coil is 2
The number of turns of the first coil and the second coil, the diameter, and the diameter of the optical fiber are set so as to be more than double.

<Operation> When the optical fiber gyro is configured with a single mode optical fiber, the depolarizer is also configured with a single mode optical fiber,
The optical fiber gyro has a small size and high mechanical strength without a fusion splicing part.

<Embodiment> FIG. 1 is a block diagram showing an optical fiber gyro according to an embodiment of the present invention. In the figure, 16 is a laser as a light source,
Reference numerals 17 and 18 are first and second coils forming the depolarizer 19, 20 and 21 are 3 dB couplers, 2 respectively.
Reference numeral 2 is an optical fiber coil for rotation detection, 23 is a PZT oscillator, and 24 is a photodetector. The depolarizer 19, the 3 dB couplers 20, 21, the optical fiber coil 22, the PZT oscillator 2
3 is disposed in a continuous system path 25 composed of a single mode optical fiber.

Both the 3 dB couplers 20 and 21 are made by arranging two single-mode optical fibers in parallel and heating and extending a part thereof, and split the light at an intensity ratio of 1: 1.

Further, the optical fiber coil 22 placed in the rotating system (not shown) to be detected is also composed of a single mode optical fiber, and the PZT oscillator 23 is also composed of a single mode optical fiber wound. There is.

When the optical fiber coil 22 is composed of a single-mode optical fiber, the coherent light emitted from the laser 16 is depolarized to prevent the gyro performance from deteriorating as described above. 3dB combiner 20
A depolarizer 19 is interposed between the and. The first coil 17 and the second coil 18 that compose the depolarizer 19 are both composed of a single-mode optical fiber, and have a predetermined number of turns, a diameter, and an optical fiber diameter, respectively. The light from the laser 16 is depolarized by the two coils 18, which realizes stable rotation angular velocity detection by the optical fiber gyro.

Therefore, the light emitted from the laser 16 is transmitted to the depolarizer 1
After being depolarized by 9, the 3 dB coupler 21 divides the light into two equal parts one by one and enters the optical fiber coil 22 from both ends thereof, and propagates in the optical fiber coil in the same direction as the rotation direction and in the opposite direction. The propagating light is a 3 dB coupler 20.
Are combined again and enter the photodetector 24, and the rotational angular velocity of the system in which the optical fiber coil 22 is placed is detected.

The setting conditions and actions of the first coil 17 and the second coil 18 will be described below. As shown in FIG. 2, the first coil 17 and the second coil 18 are set such that their centers (corresponding to the main axes Y ₁ and Y ₂ ) are inclined by 45 degrees. The diameters 2R ₁ and 2R _{2 of} the coils 17 and 18, the numbers of turns N ₁ and N ₂ and the outer diameters 2b ₁ and 2b ₂ of the optical fiber are such that the deviation of the wave packet due to the first coil 17 causes the interference of the light of the laser 16. Greater than distance and second
The deviation of the wave packet due to the coil 18 is set to be at least twice the deviation of the wave packet due to the first coil 17, and 2b ₁ = 2
If b ₂ = 2b It is set to the condition.

The conditions of such coils 17 and 18 are set for the following reasons. As shown in Figs. 3 (A) and (B), the outer diameter 2b
When the optical fiber 27 of E is bent with a bending radius R, E
Is the Young's modulus, and the stress σ in each axial direction in the optical fiber
_x , σ _y and σ _z are given by the following equations.

Further, the propagation constant difference Δβ _BEND between the linear polarization modes polarized in the X-axis and Y-axis (center axis of the coil) directions is Given in. Where k (= 2π / λ: λ is the wavelength) is the wave number, n is the refractive index of the core, P ₁₂ and P ₁₁ are Pockels coefficients,
ν is the Poisson's ratio, typically n = 1.45, P ₁₁ = 0.
121, P ₁₂ = 0.270, E = 7830 (kg / mm ² ), and ν = 0.186. Since the core diameter of a single-mode optical fiber is sufficiently smaller than the outer diameter, the stress felt by the light inside the core is
X | << b Becomes Then, by substituting the above numerical values into the equation (9), the birefringence _BBEND generated by bending is Is expressed as Further, the polarization mode dispersion τ _P (delay time difference between X and Y polarization modes) is calculated by using B _BEND , Is expressed as From this, the deviation Δl ₁ of the wave packet of both polarization modes due to the first coil 17 is Is. If the deviation Δl ₁ of the wave packet due to the first coil 17 is larger than the coherence length l _c of the light of the laser 16, the coherence of the laser light can be eliminated. Therefore, when the wave number k is the above value, It suffices to set R ₁ and N ₁ so as to satisfy the following condition. Further, here, in the main axis direction of the first coil 17 (X ₁ , Y ₁
When linearly polarized light polarized in the axial direction is incident, the light passes through the first coil 17 as it is as linearly polarized light, and coherence cannot be eliminated. Therefore, in order to eliminate the coherence of light in all incident states, as shown in FIG. 10, the inclination of 45 degrees with respect to the first coil 17 and the deviation Δl ₁ of the wave packet due to the first coil 17 Wavelet deviation more than twice Δl ₂
It is necessary to provide the second coil 18 having That is, the deviation Δl ₂ of the wave packet due to the second coil 18 is Must. Therefore, from equations (13) and (15), The condition is that R ₁ and N ₁ of the first coil 17 and the second coil 18
Must be satisfied between R ₂ and N ₂ . The formula
When the outer diameter of the optical fiber of the first coil and the outer diameter of the optical fiber of the second coil are different from each other, (16) includes the relationship between these outer diameters, but the mechanical strength is improved as in the present invention. In consideration of the above, it is preferable that the outer diameters of these optical fibers are made equal. In addition, the first coil 17 and the second coil 1
It is preferable that the relative inclination with respect to 8 is 45 degrees,
This tilt angle can be changed within a range where there is no practical problem.
In addition, the wave packet that has passed through the first coil 17 is
In order not to overlap by passing through,
The condition of 2Δl ₁ ≦ Δl ₂ is required, but the magnification to be set is appropriately set within the range in which the above function is achieved.

Fig. 4 shows the relative refractive index difference Δ = 0.55%, core diameter 2a = 5.0μ.
m, the outer diameter of the fiber is 2b = 125 μm, and the coil of various diameter R is made with the single mode optical fiber, and the birefringence B _BEND and polarization mode dispersion τ _P of the coil are made to enter the light of the wavelength λ = 1.3 μm. Is the result of measurement. In the figure, the solid line is equation (9) and
It is a theoretical value given by (12), and it can be seen that the measurement results plotted in the figure are in good agreement with the theoretical value. Then, R ₁ = R ₂ = 2.3 mm, N ₁ = 21 (times), N ₂ = 4
If it is set to 2 (times), from equation (10) and FIG. 4, | B _BEND | = 1 × 10 ⁻⁴ (17), so the deviation of the wave packet by the first coil 17 and the second coil 18 is Δl ₁ = 30.3 (μm) (18) Δl ₂ = 60.6 (μm) (19)

A in FIG. 5 shows a polarization state when the central axes of the first coil 17 and the second coil 18 (main axes Y ₁ and Y _{2 in} FIG. ₂ ) are inclined by 45 degrees with respect to each other. The measurement was performed using a semiconductor laser with a coherence length l _c = 25 μm as a light source (wavelength 1.3 μm).
m, spectrum width Δλ = 70 mm), this laser light was made incident on the depolarizer 19 shown in FIG. 2, and the emitted light was measured through a polarizer. On the other hand, B in FIG. 5 is obtained by measuring the laser light directly through the polarizer, and it can be seen that the intensity changes depending on the polarization direction. That is, it can be seen that the light passing through the depolarizer 19 has a constant light intensity regardless of the direction of the polarizer, and the polarization (coherence) is almost completely eliminated.

FIG. 6 is a zero point fluctuation of the gyro output when the central axis of the rotation detecting optical fiber coil 22 of the optical fiber gyro shown in FIG. 1 is oriented in the east-west direction (condition where the rotation due to the rotation of the earth is zero). Indicates. According to this, the output fluctuation due to noise is within 0.1 degree in the measurement for 30 minutes, and it can be understood from this that a high-performance optical fiber gyro with a minimum detection sensitivity of about 0.1 degree / hr can be realized. The optical fiber gyro shown in FIG. 1 has a high mechanical strength because it uses a continuous single-mode optical fiber and there is no fusion splicing point between the optical fibers, and is excellent in downsizing and mass productivity. .

If a single mode optical fiber with a large relative refractive index difference is used as the optical fiber forming the depolarizer or the optical fiber coil for rotation detection, the bending loss will not increase even if the coil diameter is reduced. Suitable for miniaturization. FIG. 7 shows Δ = 0.9%, core diameter 2a = 4.0 μm, Δ = 1.9%, core diameter 2a = 2.3 μm and Δ = 2.9%, core diameter 2a = 2.3 μm.
The bending loss characteristic of the single mode optical fiber of
The results are obtained by spectroscopic measurement with 3 mmφ. Where λ
_c is the cutoff wavelength for single mode, and λ is the measurement wavelength. Normally, the relationship between the laser wavelength λ and the cutoff wavelength λ _c is about 1.05λ / λ _c 1.2 (20), so the relative refractive index difference Δ of the single-mode optical fiber is
It can be understood that the polarization canceller and the rotation detecting optical fiber coil can be made smaller in diameter without increasing the bending loss if is less than 1%.

<Effects of the Invention> According to the present invention, an optical fiber gyro can be configured by a single-mode optical fiber without forming an optical fiber connection portion, and is small in size, resistant to mechanical disturbance, and stable in the long term. It is possible to obtain an optical fiber gyro having.

[Brief description of drawings]

FIG. 1 is an overall configuration diagram of an optical fiber gyroscope according to an embodiment of the present invention, FIG. 2 is a configuration diagram of its depolarizer, and FIG.
(A) and (B) are explanatory diagrams of the bending stress of the optical fiber, respectively.
FIG. 5 is a graph showing the bending radius dependence of the bending birefringence and the polarization mode dispersion, FIG. 5 is a graph explaining the action of the depolarizer, FIG. 6 is a graph showing the zero point variation of the optical fiber gyro, FIG. 7 is a graph showing bending loss characteristics of a single mode optical fiber, FIG. 8 is an overall configuration diagram of a conventional optical fiber gyro, FIG. 9 is a graph showing output of a photodetector, and FIG. It is a block diagram shown in the state which separated the wave canceller. In the drawing, 16 is a laser, 17 is a first coil, 18 is a second coil, 19 is a depolarizer, 20 is a 3 dB coupler, 21 is a 3 dB coupler, 22 is an optical fiber coil, and 24 is a photodetector. is there.

Claims (1)

[Claims] 1. A light source, a depolarizer for eliminating coherence of light from the light source, a first coupler for splitting light from the depolarizer in a one-to-one relationship, and a first coupler for halving the light. It is equipped with an optical fiber coil in which the generated light is incident from both ends and a second coupler that guides the light emitted from both ends of the optical fiber coil to a detector, and is rotated by the optical fiber coil placed in a rotating system. An optical fiber that receives light propagating in the same direction as that of light and light propagating in the opposite direction of the rotation direction and detects the rotation angular velocity by utilizing the fact that the effective optical path length difference between these lights is proportional to the rotation angular velocity. In the gyro, the first coupler, the second coupler, and the optical fiber coil are composed of a single-mode optical fiber, and the depolarizer is composed of a first coil and a second coil composed of a single-mode optical fiber. The center of the first coil and the second carp And the center of each is set to be inclined by 45 degrees with respect to each other, and the deviation of the wave packet due to the first coil is larger than the coherence length of the light source light and the deviation of the wave packet due to the second coil is at least twice the deviation of the wave packet due to the first coil. First to be
An optical fiber gyro, wherein the number of turns of the coil and the second coil, the diameter, and the optical fiber diameter are set.