JPH03158704A - Fiber optic ring interferometer - Google Patents

Abstract

PURPOSE:To suppress the disturbance to an object to be measured by utilizing two mutually independent waveguides that a polarized wave maintaining optical fiber has and detecting a phase shift of each mode independently by an interference phenomenon. CONSTITUTION:An X-polarized wave and a Y-polarized wave which are emitted by a laser 1 independently at the same time are guided to a polarization beam splitter(PBS) 5 through a 1/2-wavelength plate 2, a half-mirror 3, and a mirror 4. Then the PBS 5 separates the X-polarized wave and Y-polarized wave, which are made incident on the polarization maintaining optical fiber 10 in directions A and B. The X-polarized wave while propagated in the optical fiber 10 is shifted in phase by the application of an electric field at an electric field application part 10, but the electric field is not applied to the Y-polarized wave. Then the X-polarized wave and Y-polarized wave passed through the optical fiber 10 are multiplexed by the PBS 5 and aligned with the optical path of reference light by half-mirror 12, and the X-polarized wave and Y-polarized wave are separated by a PBS 13 and passed through objectives 14 and 15 to form concentric interference fringes of interference with the reference light on interference surfaces 16 and 17 respectively.

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Field of Industrial Application The present invention relates to an optical fiber ring interferometer that utilizes laser coherence.

(B) Conventional Technology Optical fiber ring interference needles are used in optical fiber functional sensors that use the fiber itself as a sensor to detect sharp changes in light propagation characteristics (phase, etc.) caused by external disturbances.

(c) 5! The problem that Ming is trying to solve The biggest problem with this optical fiber functional sensor is:
The problem lies in how to suppress disturbances other than the desired measurement target, and the current measures are not sufficient.

The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide an optical fiber ring interference needle that can eliminate the influence of disturbances other than the desired measurement target.

(d) Means for Solving the Problems The present invention provides means for exciting two orthogonal modes simultaneously and independently, means for separating reference light from the two modes, and two modes from which the reference light has been removed. a polarization-maintaining optical fiber through which the polarization-maintaining optical fiber is propagated, means for separating the two modes propagated through the polarization-maintaining optical fiber for each mode, and obtaining two interference fringes with each separated mode and the reference light. means and
This is an optical fiber ring interferometer characterized by comprising:

That is, two orthogonal modes (X polarization, Y polarization) are excited simultaneously and independently, and the two modes are propagated through the polarization-maintaining optical fiber. At the measurement site, the direction of application of the measured physical quantity is defined so as to be perpendicular to either the X polarization or the Y polarization. In such a case, phase changes induced by external disturbances other than the measurement target occur in common for both polarized waves, but phase changes induced by the directional measurement physical quantity that is the measurement target are subject to the above-mentioned regulation. Follow. Here, each polarized wave is separated, and the two phase changes observed due to the interference phenomenon between the reference light and each polarized wave are one obtained by superimposing a -like disturbance other than the measurement target and a change in the physical quantity of the measurement target. The other is based only on external disturbances other than the measurement target. Therefore, 2
The difference between the two phase changes is proportional to the change in the physical quantity to be measured.

(e) Operation Utilizing the two mutually independent waveguides of the polarization-maintaining optical fiber, the phase change of each mode can be detected independently by interference phenomenon, so it is possible to detect external disturbances other than changes in the physical quantity to be measured. The phase change caused by can be compensated for.

(to) Example When dealing with light wave propagation in an optically anisotropic medium, -
This will be explained using an index ellipsoid for a.

The phase velocity of a light wave traveling in an optically anisotropic medium differs depending on the polarization direction. Now, the principal axis of the refractive index (n) is x,
If y and z are the optical principal axes, then x"/nl"+ y'/n,' + z'/no' =
1-(1) can describe the behavior of light waves traveling in any direction.

The core of the optical 7-eyeper made of general silica glass is optically isotropic, and nw '' ny = no, but
In polarization-maintaining optical fibers, residual thermal stress is generated by the elliptical cladding, which has a coefficient of thermal expansion different from other parts, when it is drawn from a preform. This asymmetric residual thermal stress causes stress birefringence, which causes X-direction linear polarization (
A difference occurs in the refractive index within the core for X-polarized waves) and Y-direction linearly polarized waves (Y-polarized waves). The X and Y polarized waves excited in the X and Y directions, which are the main axis directions, are maintained and can be propagated without being coupled to each other.

Hereinafter, a case where the optical fiber ring interference needle of the present invention is applied to an electric field sensor will be described.

For use in electric field sensors, when an electric field is applied to an optical fiber,
This uses an electro-optic effect in which the refractive index of the core changes depending on the strength of the electric field. Here, for simplicity, only the first-order electro-optic effect, that is, the Bockels effect, will be considered.

Generally, when an electric field is applied, an index ellipsoid is deformed. If we rewrite equation (1) in general form, a+ 1X” ”
a*1y+ assz"◆2a*3yZ" 2a+
+ZX” 2a+tXy=1 (2).

The change in the coefficient in equation (2) is the applied electric field (E,).

Since they are proportional to E,, E,), they can be related by a tensor as shown in the following equation.

Δold! a eye γaI rafγ feeling ko°°(
3) Here, γ is the electro-optic coefficient, but when dealing with an amorphous optical fiber (silica glass), its electro-optic properties are isodirectional, and only the diagonal components of the tensor remain, and each The coefficients are equal (γ11=γ3.=γ, lmiγ). Therefore, equation (3) can be simplified, and if the electric field application direction is defined as the X direction, ΔaII γ E, ...(4) is obtained. From this, it can be seen that the deformation of the refractive index ellipsoid within the core when the electric field is applied in a specified direction maintains the main I#l shape after the electric field is applied, and that both the X and Y polarizations are linearly polarized during propagation. Moreover, it can be seen that the phase change caused by the refractive index change proportional to the applied electric field is only affected by the X polarized wave, and has no effect on the Y polarized wave even when an electric field is applied.

A specific configuration method of the present invention will be explained with reference to the drawings. FIG. 1 is a block diagram of an optical fiber ring interference needle according to an embodiment of the present invention.

Note that in FIG. 1, arrows indicate the traveling direction of light waves. Furthermore, the circle mark above the arrow indicates that this light wave is Y polarized wave (mode), and the horizontal line above the arrow indicates that this light wave is X polarized wave.

In order to make the incident light range of the X-polarized wave and Y-polarized wave simultaneously and independently excited by the laser (1) equal to the optical fiber described later] /2-wavelength plate: HP (2) makes the inclination of 45 degrees be held.

From the X-polarized wave and the Y-polarized wave with this inclination, the XW wave and Y-polarized wave are separated into reference beams by the HM (3), and the reference beams are removed via the mirror (4). , polarizing beam splitter: PBS (5).

P B S (5) is the guided X polarized wave and YB wave
Separate polarized waves and Y polarized waves. ) The two polarized waves are incident clockwise (direction B).In other words, the X polarized wave and the Y polarized wave are aligned with the principal axis by HP (6), (7) and objective lens = OB (8), (9). is,
The X polarization and Y polarization aligned with the principal axis are each PMS.
It is incident from both directions of M(1F+).

While the X-polarized wave is propagating through P M S M (lo), an electric field is applied at the electric field applying section (11) and the phase changes.

Note that no electric field is applied to the Y polarized wave.

The X-polarized wave and the Y-polarized wave that have passed through P M S M (10) are combined by P B S (5) and made to coincide with the optical path of the reference light at HM (12).

Each polarized wave matched with the optical path of the reference light is PBS (13
) is separated into X polarization and Y polarization, 0B(14), (
15), concentric interference fringes with the reference light are obtained for the X-polarized wave at the interference surface (16) and for the Y-polarized wave at the interference surface (17).

The two interference fringe movements observed here, that is, the phase changes, are caused by -like disturbances (noise) other than the measurement target.
This is a superimposition of the electric field change and the electric field change that is the object of measurement.

Since the electric field application direction is defined as the X-axis direction, phase changes due to refractive index changes proportional to the applied electric field are observed only at the interference surface (16), and disturbances (noise) are observed at the interference surface (17). Only phase changes due to are observed.

Therefore, the difference in the amount of phase change obtained between the two interference surfaces (16) and (17) is purely proportional to the electric field change that is the object of measurement.

In addition, the fiber link of the reference optical path: P ML (18
) is used to change the optical path length of the reference light in order to eliminate the optical path difference between the X polarization and the Y polarization and stabilize the interference fringes.

(G) Effects of the Invention The present invention is clear from the above description. In laser application measurement, by configuring a polarization-maintaining optical fiber to multiplex polarization, it is possible to It is possible to provide an interferometer that suppresses external disturbances.

[Brief explanation of the drawing]

FIG. 1 is a purchasing diagram of an optical fiber ring interferometer according to an embodiment of the present invention.

Claims (1)

[Claims] (1) means for exciting two orthogonal modes simultaneously and independently; means for separating reference light from the two modes;
a polarization-maintaining optical fiber in which the two modes from which the reference light has been removed are propagated; a means for separating the two modes propagated in the polarization-maintaining optical fiber into each mode; and a means for separating each separated mode and a reference beam. An optical fiber ring interferometer comprising: means for obtaining two interference fringes with light.