JPH02266217A - Optical fiber gyroscope - Google Patents

Abstract

PURPOSE:To keep the polarized component of light wave propagated in a fiber coil and to prevent fluctuation in a polarization direction from occurring by using a polarization maintaining fiber for the fiber coil. CONSTITUTION:Light emitted from a light source 1 passes through an optical coupler 4 and reaches the fiber coil 7. In this case, the polarized state of the light made incident in the coil 7 is decided in accordance with optical constitution which means from the light source 1 to the coupler 4. The polarized components out of the light made incident in the coil 7 is separately propagated in the coil 7 as (x) polarized wave and (y) polarized wave. Since the length of the coil 7 is set longer than coherent length, the (x) polarized wave and the (y) polarized wave are made incoherent and do not interfere with each other. Non-polarized component of the light wave is separated to the (x) and (y) polarized waves to be propagated. Then, clockwise light and counterclockwise light propagated in the coil 7 are synthesized by a 2nd coupler, and the (x) polarized wave and the (y) polarized wave which are incoherent do not interfere with each other, so that the (x) polarized waves of the light mutually interfere one another and the (y) polarized waves of the light mutually interfere one another. Therefore, the polarized component is kept.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to an optical fiber gyro used in a gyro compass for automobiles, aircraft, etc. (Prior art) Rotation angle of moving objects such as automobiles, ships, aircraft, etc. An optical fiber gyro is one of the gyro compasses used for detection. An optical system of a standard optical fiber gyro using a phase modulation method that has been commonly used in the past is as shown in FIG.

This is composed of a light source A, first and second optical couplers B, D, a polarizer C, an optical fiber coil E, a phase modulator F, and a photoelectric converter (light receiver) G. They are connected by optical fibers.

In this optical system, light emitted from a light source A passes through a first optical coupler B, a polarizer C, and a second optical coupler D, where it is split into a clockwise light a and a counterclockwise light beam, and then enters a fiber coil E. propagate. After propagation, they are combined and interfered by the second optical coupler D, and the interfering light waves travel from the polarizer C to the first optical coupler B and reach the light receiver 2'AG. From receiver G, it will be described later (2
) An electrical signal is output based on the principle of the equation.

In this case, when the optical fiber coil E rotates at an angular velocity Ω, a phase difference of 2φ occurs between the clockwise light and the counterclockwise light propagating in the optical fiber coil E due to the Sagnac effect. By detecting this phase difference, the angular velocity can be determined.

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 expressed by the following equation is applied to the light wave propagating in the fiber.

+p(t)=ψs sin (ωll t)...
・(1) However, when ψ, = modulation degree ω, = modulation angular frequency is synchronously detected from the electrical signal output of the two-hour photoreceiver G, a signal with the same frequency as this modulation signal frequency is obtained, and a signal i expressed by the following equation is obtained. is obtained.

1=s-J+fyy) sin (2φ)=12) However, S=proportionality constant J, (η) is the first-order Bessel function η=2ψsin ((IJI@△T/2)−・−(
3) Here, 8T is the difference in time required for the clockwise light and the counterclockwise light to travel from the phase modulator F to the exit of the fiber coil E, respectively.

The proportionality constant S is determined by the characteristics of the optical path from the light source A to the light receiver G and the photoelectric signal conversion efficiency of the light receiver G. The factors that determine the optical characteristics are the insertion loss and polarization characteristics of the optical components constituting the optical system, such as the optical couplers B and D, the polarizer C1, and the optical fiber coil E.

The phase difference 2φ due to the Sagnac effect caused by the rotation of the fiber coil E in FIG.
It is necessary to detect minute angular velocities called hours. The phase difference 2φ that occurs at this angle is very small at ~1O-1lrad. In order to detect such a minute phase difference using the principle of equation (2) above, the proportionality constant S must be kept constant. In addition, it is necessary to devise ways to satisfy the first-order condition.

■The optical paths followed by light in both left and right directions are equal.

(2) The left and right light propagates in the same propagation mode.

(2) Left and right light propagates in the same polarization state.

If these conditions are not met, 5 in the above equation (2)
in(2φ) becomes sin (2φ+φ,), resulting in a Sagnac shift. It looks like a bias has been added. Moreover, in this case, although it is sufficient that φ is kept constant without depending on time, it usually changes due to changes in the external environment, resulting in zero point drift.

Conventionally, in order to solve these problems, single mode fibers have been used for the polarizer C and the fiber coil E in FIG.

(Problems with conventional technology) The conventional optical system shown in FIG. 4 had the following problems.

In the case of a polarizer C using a (+) crystal, a collimating lens system, an optical connector, etc. are required for coupling with a fiber, so it is difficult to miniaturize, and the number of parts is large, resulting in lack of reliability.

(2) As the polarizer C, a fiber type is sometimes used, which is a birefringent fiber with one polarization plane configured to have a high loss and made into a small diameter coil to make it easier to connect to the fiber. Child C cannot be called very small and its reliability is not sufficient.

(3) Linearly polarized light polarized by polarizer C enters the fiber, and when the polarization direction of the linearly polarized light propagates through the fiber, due to some factor, the polarization of the polarizer changes as shown in Figure 2. When tilted by θ from the direction, if E is the magnitude of the electric field in which both the left and right lights are combined, the optical power that can pass through the polarizer C is lEl” cos2 (θ).
Only the (θ) factor changes.

(4) When the light from the light source A is linearly polarized by the polarizer C, a considerable loss of optical power occurs.

For example, in principle, to obtain linearly polarized light from unpolarized light, the optical power is halved.

(5) In order to avoid loss of optical power, polarizer C may not be used, but in this case it is necessary to use a light source that can generate linearly polarized light, and in that case, the direction of polarization must be changed. A considerable amount of work is required to match, which is troublesome, and requires a highly accurate light source, resulting in high costs.

(Objective of the Invention) An object of the present invention is to realize an optical fiber gyro with high reliability and good detection efficiency without using a polarizer.

(Means for Solving the Problems) In the optical fiber gyro according to claim 1 of the present invention, as shown in FIG. The light is split into a clockwise light 5 and a counterclockwise light 6 and separately enters both ends of the fiber coil 7, and the same rotation occurs due to the phase difference between the two lights 5 and 6 caused by the rotation of the coil 7. This optical fiber gyro is adapted to detect the rotational angular velocity of the polarizing filter 3, and is characterized in that a birefringent fiber is used as the polarizing filter 3.

The optical fiber gyro according to a second aspect of the present invention is characterized in that the length of the birefringent fiber is longer than the coherent length of light from the light source.

(Function) In the optical fiber gyro according to the present invention, the light emitted from the light source 1 is passed through the optical coupler 2, the polarizing filter 3. It reaches the fiber coil 7 through the optical coupler 4. At this time, it is assumed that the electric fields of the light emitted from the optical coupler 4, that is, the light incident on the fiber coil 7, are Ex and Ey, respectively, and these are orthogonal to each other and have an incoherent relationship. For example, after Ex is split by the optical coupler 4 and becomes left and right light and propagated through the fiber coil 7, the light is passed through the optical coupler 4 and the polarizing filter 3.
, is received by the light receiver 9 through the optical coupler 2.

In this case, since the polarizing filter 3 is capable of propagating two independent light waves vibrating in two mutually orthogonal directions, for example, the X and y directions, the light emitted from the fiber coil 7 is directed in the direction of the main axis The light is divided into a component in the y direction and a component in the principal axis y direction, propagates within the polarizing filter 3, and propagates toward the optical coupler 2.

In this case, the Ex and Ex lights pass through the polarizing filter 3 with components in the main axis X and y directions of the polarizing filter 3, but even though the electric field directions are the same, the Ex and Ex lights are incoherent as described above. There is no mutual interference due to the close relationship.

This effect is the same even when the Ex light is split by the optical coupler 4 to become left and right light and propagates through the fiber coil 7.

(Embodiment) FIG. 1 shows an embodiment of the optical fiber gyro of the present invention.

In the figure, 1 is a light source, 2 and 4 are optical couplers, 7 is a fiber coil, and 8 is a phase modulator, all of which are the same as those in the conventional optical fiber gyro shown in FIG.

In FIG. 1, reference numeral 5 indicates clockwise light, and reference numeral 6 indicates counterclockwise light, in which light from a light source 1 is split through an optical coupler 2, a polarizing filter 3, and an optical coupler 4.

The phase modulator 8 is used to apply time-delayed phase modulation to the two lights 5.6 in order to improve the sensitivity to minute rotations occurring in the optical fiber gyro. Wrapped with optical fiber.

3 in FIG. 1 is a polarizing filter, which is a feature of the present invention. In the present invention, a birefringent fiber is used for this polarizing filter 3. This birefringent fiber allows mutually independent linear polarization modes to propagate.

The two polarization modes incident on the same polarization filter 3 at the same time have different propagation velocities, so when exiting the fiber there is a
1 is different.

It is preferable that the length of the polarizing filter 3 is longer than the coherent length of the light from the light source. If this is done, there will be no coherent relationship between the two polarized lights. Orthogonal linearly polarized light is incident on the fiber coil 7 via the coupler 4.

Although an optical fiber gyro can be used with just one optical coupler, two couplers are usually combined as shown in Fig. 1 in order to correct the optical path length difference between the clockwise light 5 and the counterclockwise light 6. has been used.

(Effect of the invention) The optical fiber gyro of the present invention can be used without using a polarizer.
Since a birefringent fiber is used as the polarizing filter 3, the following effects can be obtained.

■ The various problems associated with using a polarizer are eliminated, resulting in an optical fiber gyro that is highly reliable and has good detection efficiency.

(2) Since a polarizer only allows light waves whose electric field oscillates in a certain direction (for example, the X direction) to pass through, the polarizer
As shown in the figure, a light wave whose electric field oscillates in a direction tilted by θ from the X direction is E in the X direction.

Since it has a component of cos θ, the intensity of the light wave passing through the polarizer is E. "cos" θ.

On the other hand, a birefringent fiber can propagate two independent light waves vibrating in two directions orthogonal to the U direction, for example, the X and X directions, so in the optical fiber gyro of the present invention, the fiber coil 7 If the polarization direction of the linearly polarized light that propagated is tilted by θ as shown in Fig. 3, and the combined electric field of the left and right lights at that time is Eo in Fig. 3, then as shown in Fig. 3 A light wave whose electric field oscillates in a direction tilted by θ from the X direction is E in the X direction. cos e, has a component of Eosin θ in the X direction. Therefore, the intensity of the light wave passing through the birefringent fiber is.

Ea"cos"θ+E n2sin "θ:Eo2. Therefore, the transmittance of the birefringent fiber is better than the transmittance of the polarizer, and the optical fiber gyro of the present invention using the birefringent fiber has a high transmittance. This results in high detection efficiency.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing an embodiment of the optical fiber gyro of the present invention, FIG. 2 is an explanatory diagram of the intensity of light waves passing through a polarizer,
Figure 3 is an illustration of the intensity of light waves passing through a birefringent fiber.
FIG. 4 is an explanatory diagram of a conventional optical fiber gyro. 1 is a light source 2 and 4 is an optical coupler 3 is a polarizing filter 5 is a counterclockwise light 6 is a clockwise light 7 is a fiber coil 8 is a phase modulator 9 is a light receiver

Claims (2)

[Claims] (1) Light from one light source 1 passes through an optical coupler 2, a polarizing filter 3, and an optical coupler 4, and is split into clockwise light 5 and counterclockwise light 6, which are separately input to both ends of the fiber coil 7, and the same light is transmitted. The optical fiber gyro is such that the rotational angular velocity of the same rotation is detected from the phase difference between the two lights 5 and 6 caused by the rotation of the coil 7, and a birefringent fiber is used as the polarizing filter 3. An optical fiber gyro that is characterized by (2) An optical fiber gyro, wherein the length of the birefringent fiber is longer than the coherent length of light from the light source.