JPH0979827A - Optical fiber sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To constitute an optical fiber sensor whose manufacture is easy and inexpensive and which has high tensile sensitivity by using single mode type optical fiber formed of a core, a clad arranged around the core and a through hole arranged in the clad. SOLUTION: Retardation (phase difference) sensitivity of a polarization interference type optical fiber sensor to elongation strain largely depends on tensile stress induced birefringence Bt, and a parameter Bt (ΔL/L) (L is a length of fiber, and ΔL is elongation) exterts large influence on a phase difference. Then, when single mode type optical fiber F composed of a core 3, a clad 1 arranged around it and a through hole 2 arranged in the clad 1 is used as a light sensor, since the large through hole 2 exists near the core 3, when tensile stress and circumferential directional stress are applied, since asymmetric large stress is generated in the core 3, a parameter Bt (ΔL/L) value becomes large, and large phase difference sensitivity is obtained, a highly sensitive sensor can be formed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber sensor which performs various measurements by measuring strain applied in the longitudinal direction of an optical fiber.

[0002]

2. Description of the Related Art In the field of optical measurement, many polarization interference type optical fiber sensors using a birefringent single mode optical fiber have been proposed. In these sensors, the phase difference (retardation) of light waves between orthogonal eigenmodes propagating in an optical fiber
The measured amount is converted to and the sensing is performed. Birefringence A sensor that applies strain to a single-mode optical fiber and measures the birefringence that changes depending on the stress by retardation is one of the important ones.

Conventionally, as an optical fiber for this purpose, a panda type optical fiber in which a foreign substance is arranged in a clad with a core sandwiched therein and an elliptic clad single mode polarization maintaining fiber have been used.

[0004]

However, in such an optical fiber of the related art, the steps of disposing a foreign substance in the clad without a gap and the step of processing the clad into a non-circular shape are complicated, and the optical fiber is inexpensive. It could not be manufactured.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and an object thereof is to provide an optical fiber sensor which is easy to manufacture and has high sensitivity.

The present invention is an optical fiber sensor which measures the phase difference between orthogonal eigenmodes of a light wave propagating in an optical fiber and detects the stress applied in the longitudinal direction of the optical fiber. An optical fiber sensor characterized by being a mode type and comprising a core, a clad arranged around the core, and a through hole arranged in the clad.

When tensile stress is applied to a birefringent single-mode optical fiber having no circularly symmetric cross-sectional structure, asymmetric stress is applied to the core of the optical fiber in the cross section, and the birefringence depends on the tensile stress. Is known to change. For example, when tensile stress is applied to an optical fiber of length L and elongation ΔL occurs, its birefringence B
Is expressed by the following equation from the original birefringence Bo of the optical fiber and the tensile stress induced birefringence Bt. B = Bo + Bt

Further, the retardation change ΔΦ at this time
The sensitivity ΔΦ / ΔL for ΔL is ΔΦ / ΔL = (2π / λ) {Bo + Bt / (ΔL /
L)}. As described above, the retardation sensitivity of the polarization interference type optical fiber sensor with respect to extension strain largely depends on not only the fiber's original birefringence Bo but also the tensile stress induced birefringence Bt. In an actual optical fiber, B
o << Bt / (ΔL / L) and the parameter Bt
/ (ΔL / L) greatly affects the retardation. The original birefringence Bo of the optical fiber is already well known for each of various structures of the optical fiber. However, the parameter Bt / (ΔL / L) is not well known.

Under these circumstances, when the present inventors searched for an optical fiber having a large parameter Bt / (ΔL / L), they discovered the present optical fiber and invented an optical fiber sensor using this optical fiber. Table 1 shows examples of optical fibers useful in the present invention.

[0010]

[Table 1]

In Table 1, No is the type of optical fiber, L
b is the beat length, P is the c / a value in FIG. 3, Q is the s / a value in FIG. 3, and ΔΦt is the tensile stress-induced retardation.

In FIG. 3, 1 is a clad, 2 is a through hole symmetrically provided in the clad, 3 is a core, a is the radius of the optical fiber, S is the radius of the through hole, and c is the center of the core. The distance from the center of the through hole is shown.

The retardation change Φ with respect to elongation and the birefringence (beat length) Lb of the optical fiber were measured by the optical heterodyne method and the cutback method, respectively. The measurement wavelength is
The light source was a helium neon laser at 633 nm. These measurements are described by Y. Ohtsuka, T. Ando and M. Imai, Journal.
al of Lightwave Technology Magazine, Vol.LT-5, No.4, April19
87, pp602-607. In the measurement of the retardation change with respect to elongation, 1.5 cm of the optical fiber was fixed to the movable table with an adhesive, and the movable table was moved to measure.
The magnitude of elongation strain was measured by stretching 100 μm in 5 seconds to 3 minutes. In the cutback method, the optical fiber was cut back 0.5 to 5 mm and cut back 20 to 30 times for measurement.

The reason why the optical fiber of the present invention has a large parameter Bt / (ΔL / L) value is that there is a large through hole in the vicinity of the core of the optical fiber, and due to this structure, tensile stress and stress in the circumferential direction are caused. It is considered that this is because a large asymmetric stress is generated in the core when added.

Table 1 shows the parameter Bt / (ΔL /
Since it seems that the value of L) depends on the structure of the optical fiber, Bt / (ΔL
/ L) was calculated. As a result, when P is about 0.5 and Q is greater than about 0.4, the parameter Bt / (ΔL /
It was found that an extremely large value of L) of 7 × 10 ⁻³ can be expected. Note that Bt / (ΔL / L) = of a conventionally known panda optical fiber (Lb = 0.8 mm)
This is an extremely large value even compared with 1.41 × 10 ⁻⁴ .

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, the shape of the core and the shape of the clad are not limited. The core may have a perfect circle or an ellipse in cross section, and the clad may have a perfect circle or a polygon. For example,
An optical fiber with a circular clad cross section is convenient for coupling with an ordinary core circular optical fiber, and an optical fiber with a non-circular cross-sectional shape such as a rectangle allows the orthogonal eigenmode light propagating in the optical fiber. It is convenient because it can be easily identified.

The present invention does not limit the number of cores or the number of through holes. In the present invention, the core material of the optical fiber can be freely selected from those used for ordinary optical fibers. The material of the core is silica glass containing germanium oxide, phosphorus oxide, titanium oxide and the like, and the material of the clad is not only pure silica glass but also silica glass containing fluorine or boron oxide.

According to a second aspect of the invention, the sensor is constituted by an optical fiber having through holes arranged in the clad arranged on both sides of the core. Such an optical fiber is preferable because the core is stressed bilaterally. The optical fiber according to the present invention is manufactured by forming a through hole in the longitudinal direction in the cladding of the optical fiber preform of an ordinary single-mode optical fiber, then heating and melting and drawing.

Since the optical sensor of the present invention measures the stress applied in the longitudinal direction or the circumferential direction of the optical fiber by the phase difference between mutually orthogonal light waves propagating in the core, the object of sensing is In addition to the direct pulling force applied to the optical fiber, the temperature and pressure are indirectly measured by applying a mechanical movement generated in the longitudinal direction of the optical fiber due to temperature and pressure liquid level fluctuations. can do.

The strain applied to the optical fiber can be measured by using a differential interference displacement sensor, a double Mach-Zehnder strain sensor, or the like for elongation strain, and a differential type strain sensor for bending strain of the optical fiber. A polarization interference optical fiber coil displacement sensor, a coil displacement sensor in which two optical fibers are welded, or the like can be used.

[0021]

An embodiment of the present invention will be described below with reference to the drawings. In FIG. 1, BS is a beam splitter that splits the input light into two, H is a half-wave plate, L is a condenser lens, T is a flexible and airtight tube, and F is a winding on the outer circumference of the tube T as described later. The optical fibers shown in the above Table 1, A is an analyzer, DS and DR are optical heterodyne type photodetectors, and I is a phase meter for detecting a phase difference.

Next, the operation of this apparatus will be described. A light wave emitted from a semiconductor laser (not shown) is also converted into orthogonal linearly polarized light having different frequencies by an orthogonal polarization frequency shifter (not shown). This light wave is ω in the figure
1 and ω2. The light waves ω1 and ω2 having these two orthogonal frequencies as components are split into two by the beam splitter BS. These light waves ω1 transmitted through the beam splitter BS
By adjusting the half-wave plate A, ω2 is incident via the condenser lens L so as to coincide with the X axis and the Y axis of the birefringence of the optical fiber F. The incident light waves ω1 and ω2 propagate through the optical fiber F as described later, and then are emitted from the other end. Two-component light wave ω1 · ω emitted from the optical fiber F
After being multiplexed by the analyzer A, 2 is converted into a beat photocurrent having a carrier frequency of a difference frequency of two components by an optical heterodyne type photodetector DS.

The light wave reflected downward in the figure by the beam splitter BS is converted into a reference beat photocurrent by the photodetector DR. The obtained two beat photocurrents are input to the phase meter I. Here, the phase of the beat photocurrent obtained by the photodetector DS is the orthogonal 2 propagated through the optical fiber.
The phase difference between the components is shown. By detecting this phase difference with the phase meter I, the retardation is measured.

As the low coherence light wave emitted from the semiconductor laser, one having a center wavelength of 670 nm and a coherence length of 0.5 mm was used. When the flexible tube T receives water pressure from the surroundings, the flexible tube T is deformed into an elliptical shape as shown in the drawing from the left side to the right side of FIG. The optical fiber F for sensing is divided into two parts in the middle part, and they are spliced by rotating the birefringence axis by 90 degrees to form one fiber, and then tightly wound on the tube T as shown in the figure. , A sensor unit is configured.

When the sensor portion thus constructed is submerged in water and water pressure is applied to the tube T, the tube T is deformed as described above, and bending strain is applied to the optical fiber F. As a result, the two propagating through the optical fiber F are orthogonal to each other.
A phase difference is given to the propagation of the two light waves ω1 and ω2. By measuring the magnitude of this phase difference with the phase meter I, the pressure applied to the tube T can be measured.

[0026]

As described in detail above, the present invention is an optical fiber type which detects a strain applied in the longitudinal direction of an optical fiber by measuring the phase difference between orthogonal eigenmodes of a light wave propagating in the optical fiber. In the sensor, an optical fiber type sensor characterized in that the optical fiber is composed of a single mode type including a core, a clad arranged around the core, and a through hole arranged in the clad. Is. In such an optical fiber type sensor, an optical fiber sensor having a high tensile stress-induced birefringence, that is, an optical fiber sensitive to extension strain and bending strain is used to provide a highly sensitive optical fiber sensor. Further, the optical fiber used in the present invention can be manufactured by performing a process of forming a through hole in the longitudinal direction of the optical fiber preform of a normal single mode optical fiber, then heating and melting and drawing. This sensor can be provided at low cost.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an embodiment of the present invention.

FIG. 2 is an operation explanatory diagram of a sensor unit according to an embodiment of the present invention.

FIG. 3 is an end view of an optical fiber used in an embodiment of the present invention.

[Explanation of symbols]

 F is an optical fiber 1 is a clad 2 is a through hole 3 is a core

Claims (2)

[Claims] 1. An optical fiber sensor for detecting a strain applied in a longitudinal direction of an optical fiber by measuring a phase difference between orthogonal eigenmodes of a light wave propagating through the optical fiber, wherein the optical fiber is a single mode type. An optical fiber sensor comprising a core, a clad arranged around the core, and a through hole arranged in the clad. 2. The optical fiber sensor according to claim 1, wherein the through holes are arranged on both sides of the core.