JPH02266216A - Optical fiber gyroscope - Google Patents

Abstract

PURPOSE:To enhance reliability and to improve detection efficiency by using a double refraction fiber as a polarizing filter instead of using a polarizer. CONSTITUTION:The double refraction fiber is used as the polarizing filter 3 and it can propagate linear polarization modes independent of each other. Since two polarization modes which are simultaneously made incident on the filter 3 have different propagating speeds, the time when they exit from the fiber is different. It is desirable to make the length of the filter 3 longer than the coherent length of light from a light source 1. Since there is no coherent relation between two polarized light beams by thus doing, two linearly polarized light beams which are incoherent and which are independent and orthogonal each other are made incident on a fiber coil 7 through a coupler 4. Ordinarily, two couplers are combined to be used in order to correct the difference of the optical path length of clockwise light 5 and counterclockwise light 6 in an optical fiber gyroscope.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an optical fiber gyro used in a gyro compass for automobiles and aircraft.

(Prior Art) An optical fiber gyro is one of the gyro compasses used for detecting the rotation angle of a moving object such as a car, a ship, or an aircraft. The optical system of a standard optical fiber gyro using the phase modulation method that has been used in the past is as shown in FIG.

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

In this optical system, light emitted from a light source A passes through a first optical coupler B, an optical polarizer C, and is split into a clockwise light a and a counterclockwise light beam by a second optical coupler D, and then sent to a fiber coil E. propagated inside. After propagation, they are combined and interfered at the first optical coupler D, and the interfered light waves are transferred from the optical polarizer C to the first optical coupler B.
The light reaches the photoreceiver G through the . From receiver G will be described later (
2) An electrical signal is output according to the principle of 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 a 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 to apply phase modulation expressed by the following equation to the light wave propagating in the fiber.

ψ(t)=ψs stn (C+Jm t)...
・(1) However, ψ = modulation degree ω, = modulation angular frequency t = waiting time When a signal with the same frequency as this modulation signal frequency is synchronously detected from the electrical signal output of the optical device G, a signal l expressed by a linear equation is obtained. is obtained.

i = S ・, J An) sin (2φ)・ ・
(2) However, S = proportionality constant J, (r)) is the first-order Bessel function η = 2ψsjn (ω, ΔT/2) --- (3) where ΔT is the difference between the clockwise light and the counterclockwise light. The difference in time required 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 to the light receiver G and the photoelectric signal conversion efficiency of the light receiver G.

In addition, the factors that determine the optical characteristics of the optical path are the insertion loss and polarization characteristics of the optical components that make up the optical system, such as the optical coupler B, the optical leaker C, and the optical fiber coil E. be.

In order to detect the shift 2φ caused by the rotation of the fiber coil E in the system shown in FIG. 3 by measuring the electric signal i based on the principle of equation (2), the proportionality constant S and the magnitude of the modulation parameter η must be kept stable during measurement.

External factors that cause S and η to fluctuate include temperature changes, vibrations,
Examples include power fluctuations.

Moreover, the phase difference 2ψ due to the Sagnac effect caused by the rotation of the fiber coil E in the third cabinet is very small, for example, in a gyro for inertial navigation, it is 10-2~]0-3 degrees/
It is necessary to detect minute angular velocities called hours. The phase difference 2φ caused by this angular velocity is very small at ~10-'rad, so in order to detect such a minute phase difference using the principle of equation (2) above, the proportionality constant S must be kept constant. If that's the case.

In order to reduce this variation in S, conventional methods have been to use a polarization-maintaining fiber for the fiber coil E, or to install a depolarizer I between the second optical coupler D and the fiber coil E as shown in Figure 4. A method of inserting it has been proposed.

(Problems with the prior art) The conventional fiber-optic gyro has the following problems.6 ■ In the optical fiber gyro shown in Fig. 3, the clockwise light a and the counterclockwise light propagating through the fiber coil are separated by the light leaker C. Although the polarization direction of both light waves of the optical beam is specified to be the same, the opening polarizer C (n) only specifies the polarization state of the human output light of the fiber coil E. The polarization direction of the light wave propagating through may vary due to disturbances.

When the polarization direction of the light wave changes, the proportionality constant S in the above equation (2)
It appears as a fluctuation in

■Since the optical polarizer C is used, the polarized wave component (
only one of the x and y polarization components),
The amount of light was small and the sensitivity was not good.

(2) In the method of using a polarization-maintaining fiber for the fiber coil E, since the optical leaker C is also used, a polarization-maintaining type must be used for the first optical coupler B. Therefore, the principal axes of the light opening coupler B, the second optical coupler D, the light leaker C, and the fiber coil E must be made to coincide with each other, making it troublesome to connect the optical components.

■As shown in Figure 4, optical gyros that use devolizer r (and polarization-maintaining fiber) have a large number of optical components, resulting in high costs, and the optical components must be connected with their main axes aligned with each other. Therefore, connection was troublesome.

(Objective of the Invention) The object of the present invention is to provide a highly sensitive optical fiber gyro with a small number of optical circuit components, easy interconnection of optical circuit components, and little variation in detection signals due to polarization fluctuations. It is about realization.

(Means for solving the problem) The optical fiber gyro of the present invention has a light source 1 as shown in FIG.
The light from is incident on the optical coupler 4 without passing through the optical polarizer,
In the coupler 4, the clockwise light 5 and the counterclockwise light 6 are split into two, and are separately incident on both ends of the fiber coil 7, and the clockwise light 5 and the counterclockwise light 6 are generated when the coil 7 is rotated. This is an optical fiber gyro in which the rotational angular velocity of the same rotation is detected from the phase difference between the two, and the fiber coil 7 is a polarization maintaining fiber.
The length of is set longer than the coherent length.

(Function) In the optical fiber gyro of the present invention, the light Q=t emitted from the light source 1 is transmitted to the optical coupler 2. It passes through the optical coupler 4 and reaches the fiber coil 7. In this case, since no optical polarizer is used before the optical coupler 4 in the present invention, the polarization state of the light incident on the fiber coil 7 is determined by the optical configuration from the light source 1 to the optical coupler 4. The polarization state is generally a partially polarized state and consists of a polarized component and a non-polarized component. Of the light incident on the fiber coil 7, the polarized components (X polarized waves and Y polarized waves that are mutually coherent) are propagated through the fiber coil 7 into separate X polarized waves and 5 polarized waves. In the invention, the length of the fiber coil 7 is set longer than the coherent length, so the polarized light components, X polarized wave and Y polarized wave, have an incoherent relationship and do not interfere with each other, and also have non-polarized light wave components. is decomposed into an X-polarized wave and an X-polarized wave and propagated. This applies to both clockwise and counterclockwise light.

The clockwise light and counterclockwise light propagated through the fiber coil 7 as described above are combined by the second coupler 3b. In this case, as mentioned above, the X-polarized waves and the Y-waves have an incoherent relationship and do not interfere with each other, so the X-polarized waves and the X-polarized waves of the clockwise and counterclockwise lights interfere with each other. .

The intensity of this interference light is the sum of the light intensities of the X polarization component and the X polarization component. This interference light is detected as an electric signal by the photoreceiver G, and its output shows a dependence as shown in equation (2) above on the phase change 2φ due to the Sagnac effect.

(Embodiment) FIGS. 1 and 2 show an embodiment of the optical fiber gyro of the present invention. The optical fiber gyro shown in these figures does not use an optical polarizer and uses a polarization-maintaining fiber for the fiber coil 7.

In Figures 1 and 2, [ is a light source, 2.4 is an optical coupler,
7 is a fiber coil, 8 is a modulation signal generator of a phase modulator, and 9 is a synchronous detection device.

In the embodiment shown in FIG. 2, a spalluminescence diode with a low degree of polarization was used as the light source 1, and a polarization-maintaining fiber with a crosstalk of less than 30 dB was used as the fiber coil 7. For the optical couplers 2 and 4, single-mode fibers with a branching ratio of 50:50 were used. In this example, since a light source with a low degree of polarization was used, the precision of alignment of the principal axes of the connection between the optical coupler 4 and the fiber coil 7 was moderate, and there was no need for exact alignment.

Note that an optical fiber gyro can be implemented with just one coupler, but usually two couplers 2.4 are combined as shown in the figure to correct the optical path length difference between the clockwise light and the counterclockwise light. is used.

(Effects of the Invention) The optical fiber gyro of the present invention has the following effects.

■Since a polarization maintaining fiber is used in the fiber coil 7, the x of the light wave propagated through the fiber coil 7 is
, y circularly polarized components are preserved, and there is no problem of fluctuations in the -wave direction causing fluctuations in the proportionality constant S, as in the case of using conventional optical polarizers.

(2) Since no optical polarizer is used, the polarization component of the light transmitted to the fiber coil 7 is not regulated, and therefore the polarization-maintaining coupler used in the past becomes unnecessary, and the optical coupler 4
Since the alignment of the optical axes between the fiber coil 7 and the fiber coil 7 becomes loose, the connection between the two becomes easier.

■Since the length of the fiber coil 7 is set longer than the coherent length, the polarized light components, X-polarized wave and Y-polarized wave, have an incoherent relationship and do not interfere with each other. Since the X-polarized waves and the y-polarized waves of the circulating light interfere with each other, the light intensity of the interference light is the sum of the X-polarized wave component and the y-polarized wave component. For this reason, the detected light intensity is stronger and the sensitivity is higher than in conventional optical fiber gyros in which only the X-polarized wave or the y-polarized wave component is propagated due to the presence of the optical polarizer.

(2) Since neither a light polarizer nor a devolizer is required, the number of optical parts is reduced, which is economical, and the mutual connection of optical parts becomes easy.

(2) Since no optical polarizer is used, there is no optical loss caused by the optical polarizer.

[Brief explanation of drawings]

1 and 2 are embodiments of the optical fiber gyro of the present invention, and FIGS. 3 and 4 are different explanatory diagrams of the conventional optical fiber gyro. l is the light source 2.4 is the optical coupler 5 is the clockwise light 6 is the counterclockwise light 7 is the fiber coil

Claims (1)

[Claims] Light from a light source 1 is incident on an optical coupler 4 without passing through an optical polarizer, and the coupler 4 splits the light into a clockwise light 5 and a counterclockwise light 6, which are separately incident on both ends of a fiber coil 7.
This is an optical fiber gyro in which the rotational angular velocity of the same rotation is detected from the phase difference between the clockwise light 5 and the counterclockwise light 6 caused by the rotation of the fiber coil 7. An optical fiber gyro characterized in that a holding fiber is used and the length of the coil 7 is set longer than the coherent length.