JPH07306049A - Optical fiber gyro - Google Patents

Abstract

PURPOSE:To provide a fiber-optic gyro which eliminates polarization due to a double refraction at an extra coil-like part of an optical fiber generated when the gyro is assembled, thereby detecting an angular velocity highly accurately. CONSTITUTION:The fiber-optic gyro is provided with a light source 1, a first and a second photocouplers 2 and 4, a polarizer 3, and a sensing coil 6. A single-mode optical fiber 5 connecting the photocoupler 2 with the polarizer 3 is wound like a coil.l at arm extra part 8. A polarization-preserving optical fiber 9 is connected between t.he extra part, 8 and an extra part of a polarization-preserving optical fiber constituting the polarizer 3. At this time, a polarization axis of the optical fiber 9 is twisted 45 to a polarization axis of the polarization-preserving optical fiber constructing the 16 polarizer 3.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber gyro using a single mode fiber and is used as a gyro compass for automobiles, ships, aircrafts and the like.

[0002]

2. Description of the Related Art An optical fiber gyro is one of the gyro compasses used for detecting the rotation angle of a moving object such as an automobile, a ship or an aircraft. There are various types of optical gyros, and as one of them, there is a standard optical fiber gyro of the phase modulation type which has been often used conventionally. There is an optical system of this optical fiber gyro as shown in FIG. This is the light source A and the first and second optical fiber B
D, a polarizer C, a sensing coil E, a depolarizer (depolarizer) H, a phase modulator F, and a light receiver G, which are connected by a single mode optical fiber. By the way, the depolarizer H of FIG. 1 is a fiber type depolarizer (LYOT type fiber depolarizer) in which two polarization-maintaining fibers having a length of 1: 2 and twisted relatively 45 degrees and fused. Suitable.

In this optical system, the laser light emitted from the light source A passes from the first optical coupler B to the polarizer C, and is then passed through the second optical coupler D by the clockwise laser a, the counterclockwise laser light a and the counterclockwise laser light a. The laser light b is divided into the laser light b, of which the clockwise laser light a
Is depolarized by the depolarizer H and propagates in the sensing coil E together with the other counterclockwise laser beam b. The two lasers a and b are combined by the second optical coupler D and interfere with each other, and the interfered laser light reaches the light receiver G from the polarizer C through the first optical coupler B.

In this case, the sensing coil E has an angular velocity of Ω.
When rotated by, a phase difference 2φ is generated between the clockwise laser light a and the counterclockwise laser light b propagating in the sensing coil E due to the Sagnac effect.
The angular velocity can be known by detecting this phase difference.

One of the methods for measuring the phase difference 2φ is the phase modulation method described below. A sine wave signal is applied to the phase modulator F, and phase modulation represented by the following equation is applied to the laser light propagating in the fiber. ψ (t) = ψ _m Sin (ω _mt ) (1) where ψ _m = degree of modulation ω _m = modulation angular frequency t = time

At this time, when a signal of the same frequency as the modulation signal frequency is synchronously detected from the electric signal output of the light receiver G, a signal i represented by the following equation is obtained. i = S · J ₁ (η) sin (2φ) (2) where S = proportional constant J ₁ (η) is a first-order Bessel function η = 2ψsin (ω _m ΔT / 2) (3) Here, ΔT is the difference in time required for the clockwise laser light a and the counterclockwise laser light b to travel from the phase modulator F to the exit of the fiber coil E, respectively.

The proportional constant S is determined by the optical characteristics of the optical path from the light source A to the light receiver G and the photoelectric conversion efficiency of the light receiver G. The factors that determine the optical characteristics are the optical components of the optical system, that is, the insertion loss of the optical couplers B and D, the polarizer C, the sensing coil E, and the polarization characteristics.

Since the phase difference 2φ due to the Sagnac effect caused by the rotation of the fiber coil E shown in FIG. 2 is very small, for example, a gyro for inertial navigation needs to detect a minute angular velocity of 10 ^-2 to 10 ^-3 degrees / hour. Is. The phase difference 2φ generated at this angle is as small as -10 ^-6 rad.
In order to detect such a minute phase difference by the principle of the equation (2), the proportional constant S must be kept constant.

[0009]

By the way, when assembling an optical fiber gyro, the optical fiber connecting between the respective components in FIG. 2 is cut to a slightly longer length (with an extra length portion provided). The extra length of this optical fiber must be stored together as small as possible. Normally, as shown in FIG. 2, the extra length portions J and K are wound and stored in a small-diameter coil shape.
However, in this case, while the laser light emitted from the light source A reaches the polarizer C through the coupler B, birefringence is generated in the extra length portions J and K wound in a coil shape, and the laser light passing therethrough is converted into laser light. The polarization component is added. The polarization component due to this birefringence easily fluctuates due to ambient temperature changes, vibrations, etc., so that the intensity of light emitted through the polarizer C becomes unstable. As a result, there has been a problem that the proportional constant S, which must be kept constant, changes, and the angular velocity detection accuracy of the gyro becomes unstable.

An object of the present invention is to provide an optical fiber gyro capable of detecting the angular velocity with high accuracy by eliminating the polarization due to the birefringence of the coil-shaped extra length portion of the optical fiber generated at the time of assembly.

[0011]

In the optical fiber gyroscope of the present invention, as shown in FIG.
Through the optical coupler 2, the polarizer 3, and the second optical coupler 4 of the right-handed laser light a and the left-handed laser light b, and the single-mode optical fiber 5 is coiled into the sensing coil 6 at both ends separately. In the optical fiber gyro in which the angular velocity of rotation is detected from the phase difference between the laser beams a and b generated by the incident light and the rotation of the coil 6, the optical coupler 2 and the polarizer 3 are connected. A polarization-maintaining optical fiber 9 is provided between the extra length 8 of the single mode optical fiber 5 that is connected to each other in a coil shape and the extra length of the polarization-maintaining optical fiber that constitutes the polarizer 3. The polarization-maintaining optical fiber 9 is connected by twisting the polarization axis of the polarization-maintaining optical fiber 9 by 45 degrees with respect to the polarization axis of the polarization-maintaining optical fiber constituting the polarizer 3.

[0012]

In the optical fiber gyroscope of the present invention, the extra length portion 8 of the single mode optical fiber 5 for connecting the first optical coupler 2 and the polarizer 3 that is wound in the coil shape and the polarizer 3 are provided. The polarization maintaining optical fiber 9 is connected between the polarization maintaining optical fiber 9 and the extra length of the polarization maintaining optical fiber, and the polarization axis of the polarization maintaining optical fiber 9 constitutes the polarizer 3. Since it is connected by twisting 45 degrees with respect to the polarization axis,
The polarization maintaining optical fiber 9 and the extra length portion of the polarization maintaining optical fiber forming the polarizer 3 constitute a depolarizer. For this reason, even if the laser light is birefringently polarized by the extra length portion 8 wound in a small diameter, the polarization component is canceled by the depolarization element, and the laser light incident on the polarizer 3 becomes the polarization component. There is no unpolarized state. Therefore, even if the temperature of the external environment of the optical system of the optical fiber gyro is changed or the optical system, especially the optical fiber is vibrated, the intensity of the light emitted from the polarizer 3 is kept constant and the angular velocity of the gyro is kept constant. The detection accuracy is stable.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the optical fiber gyro of the present invention will be described in detail with reference to FIG. 1 in FIG. 1 is a light source that outputs a laser beam having a wavelength of 0.85 μm, for example, SL
D, 5 are single mode optical fibers for wavelength of 0.85 μm, 2 are first optical couplers, 4 are second optical couplers, 3 is a winding type fiber polarizer in which a polarization maintaining optical fiber is wound many times, 6 Is a sensing coil made by winding a single mode optical fiber 5 a number of times, and 10 is a depolarization element (LYOT type fiber depolarizer) in which two polarization maintaining fibers have a length of 1: 2 and are twisted by 45 degrees and fused. ) 1
Reference numeral 1 is a phase modulator, 12 is a light receiving element, and a single mode optical fiber 5 is provided between the respective parts, like the sensing coil 6.
Connected by.

The phase modulator 11 is for applying phase modulation with a time delay to both the lasers a and b in order to improve the sensitivity to minute rotations generated in the optical fiber gyro, and is usually a piezoelectric element. 0.85 μm in the cylinder
The single mode optical fiber 5 between the two is wound. A sinusoidal signal is added to the phase modulator 11 to apply the phase modulation represented by the formula (1) to the light wave propagating in the optical fiber 5.

This optical fiber gyro has a light source 1 when assembled.
And a first optical coupler 2 and a surplus portion 7 generated in the single mode optical fiber 5 connecting the optical coupler 2 and the polarizer 3,
8 is wound into a subtotal coil shape as shown in FIG. 1 (pigtail).

The above is the same structure as the conventional optical fiber gyro. In the present invention, a polarization maintaining optical fiber 9 is further connected between the extra length portion 8 and the extra length portion of the polarization maintaining optical fiber forming the polarizer 3, and both polarization maintaining optical fibers are connected. Twist 45 degrees relatively and fusion splice,
A depolarizer (LYOT type fiber depolarizer) is configured. As a result, even if the laser light emitted from the light source 1 is birefringent in the coil-shaped extra length portions 7 and 8 and is polarized, it is eliminated by this depolarizer, and the light emitted through the polarizer 3 is emitted. The strength was always constant.

When the zero-point drift at -20 ° C to 70 ° C was measured using the optical fiber gyro of this embodiment, the fluctuation range was about 1/10 of that of the conventional one.
Was improved.

[0018]

According to the optical fiber gyro of the present invention,
Since the polarization maintaining optical fiber 9 is connected between the extra length portion 8 and the extra length portion of the polarization maintaining optical fiber forming the polarizer 3, the depolarizer is formed. Even if the single mode optical fiber 5 up to 3 has coil-shaped extra lengths 7 and 8, the intensity of light passing through the polarizer 3 is stable, and the detection accuracy by the optical fiber gyro is stable.
Further, since the depolarizer has a simple structure, the size of the optical system does not increase, and the cost does not increase.

[Brief description of drawings]

FIG. 1 is an explanatory view showing an embodiment of an optical fiber gyro of the present invention.

FIG. 2 is an explanatory diagram showing an example of a conventional optical fiber gyro.

[Explanation of symbols]

 1 Light Source 2 First Optical Coupler 3 Polarizer 4 Second Optical Coupler 5 Single Mode Optical Fiber 6 Sensing Coil 8 Extra Length 9 Polarization Preserving Optical Fiber

─────────────────────────────────────────────────── ───Continued from the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical indication G02B 6/17

Claims (1)

[Claims] 1. A laser light from a light source (1) passes through a first optical coupler (2), a polarizer (3) and a second optical coupler (4), and a clockwise laser light (a) and a counterclockwise laser light (a). b) and the two laser beams generated by rotating the sensing coil (6), which is split into a single mode optical fiber (5) and is separately incident on both ends of the sensing coil (6). A coil of a single mode optical fiber (5) connecting the optical coupler (2) and a polarizer (3) in an optical fiber gyro in which the angular velocity of rotation is detected from the phase difference between (a, b). A polarization-maintaining optical fiber (9) is connected between the extra length (8) wound in a circular shape and the extra length of the polarization-maintaining fiber constituting the polarizer (3) to maintain the polarization. The polarization axis of the optical fiber (9) constitutes the polarization maintaining the polarizer (3). An optical fiber gyro, wherein the optical fiber gyro is characterized by being twisted by 45 degrees with respect to the polarization axis of the optical fiber.