WO2005121697A1

WO2005121697A1 - Optical fiber strain sensor

Info

Publication number: WO2005121697A1
Application number: PCT/CA2005/000884
Authority: WO
Inventors: Edvard Cibula; Denis Donlagic
Original assignee: Fiso Technologies Inc.
Priority date: 2004-06-07
Filing date: 2005-06-07
Publication date: 2005-12-22
Also published as: SI21816A

Abstract

This invention presents manufacturing and tuning procedure of optical fiber sensor with diameter that equals to the diameter of optical fiber, e.g. of 125 µm or less. The sensor uses principle of Fabry-Perot interferometer with the optical resonator formed within the fiber in the form of an air cavity. The manufacturing procedure comprises etching of the fiber tip with the purpose of creating the cavity at the fiber tip (7), followed by the splicing of another fiber (8) to form and air cavity resonator (9). The calibration of the cavity length is achieved by controlled elongation of the segment of the optical fiber in the vicinity of the cavity by heating the vicinity of the cavity to the temperature that allows for plastic deformation of the fiber. The sensor is of small size, it has high sensitivity, low temperature dependence and it can be integrated in quasi distributed sensor networks.

Description

OPTICAL FIBER STRAIN SENSOR

FIELD OF THE INVENTION The current invention applies to manufacturing and calibration process of strain sensor based on optical fiber that can be used to measure static or dynamic strain and deformations of different materials and systems.

The technical problems that are solved by this invention relate to manufacturing of strain sensors with outer diameter that do not exceed the diameter of standard optical fiber, e.g. 125 μm and to a calibration procedure and setting of the sensor operating point. The sensor is exclusively designed from optical fibers (that base on pure or composite Si02+Ge02 glass) and can withstands broad temperature range (-270 C to 650 C). The sensor is based on air cavity Fabry-Perot that allows achieving high strain sensitivity while providing low temperature dependence. The use of standard optical fibers and commercially available manufacturing tools the invention allows for cost effective manufacturing of the sensor.

BACKGROUND OF THE INVENTION

Different fiber optic strain sensors are known in the art. The oldest presented solution is described in US patent 4,295,738. The special double core fiber is used and the optical power bounces between the two cores under applied strain. Solutions according to WO8901614, US patent 5,694,497 and US patent 6,003,340 base on bend loss phenomena in the fiber and the applied strain modulates the intensity at the sensor output. For those solutions relatively simple detection scheme can be applied, but with low sensitivity. The Brag grating fiber optic sensors, such as described in US patent 5,319,435, require more complex signal processing. In addition Bragg grating sensor usually exhibit high temperature dependence. The application of Fabry-Perot interferometer for strain measurements was presented in US patent 5,301,001. Two perpendicularly cleaved optical fibers were placed in thin capillary in a way to form the short air cavity between the fiber ends. This creates an optical resonator. The drawback of this approach is in use of adhesive and not well-defined point where fiber is adhered to the capillary as the adhesive randomly penetrates the capillary. The outer diameter of such sensor is also always larger than the fiber diameter. In the invention presented by US patent 6,056,435 hollow core optical fiber is used to create the spacer between two perpendicularly cleaved fibers. The sensor has the same diameter as optical fiber but it relies on manufacturing and splicing of the hollow core fiber. Other solutions of the optical fiber strain sensors rely on polarization effects in optical fibers (JP 62085805) and measurements of light pulse propagation time in the fiber that is exposed to the measured stress (US patents 5,649,035 and 4,928,004).

SUMMARY OF THE INVENTION

In this invention the strain sensor is created with splicing of two optical fibers where at least at one tip of the fiber a cavity is created before the splicing. The thin air gap that forms between the fibers by this procedure functions as an optical resonator with partially reflective mirrors. The applied strain causes the cavity length change and this can be detected by high resolution in two ways. In the first way, the sensor is illuminated by broadband source and the spectrum of input and reflected optical power is compared to determine the cavity length. In the second, a narrow line width laser is used to directly observe the reflectivity of the sensor at particular wavelength. The calibration and tuning of the sensor operating point is achieved by controlled elongation of the sensor (section of splice with the air cavity) at the temperature that allows for plastic deformation of the optical fibers. The calibration of the optical resonator length is necessary in case of application of signal processing system that relies on narrow line width laser source and in case when these types of sensor are used to build quasi distributed network that is compatible with signal processing that uses optical time domain reflectometer (OTDR) to determine stress applied at each individual sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and advantages of the invention will become apparent upon reading the detailed description and upon referring to the drawings in which :

Fig.1 illustrates a standard singlemode fiber with a spliced segment of a multimode optical fiber.

Fig.2 illustrates an etched tip of the optical fiber shown in Fig. 1. Fig.3 illustrates an optical fiber with the cavity, spliced to another section of single mode fiber.

Fig.4 illustrates an arrangement for performing calibration or fine-tuning of the sensor length with heating and sensor elongation.

While the invention will be described in conjunction with example embodiments, it will be understood that it is not intended to limit the scope of the invention to such embodiments. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included as defined by the appended claims. DESCRIPTION OF PREFERRED EMBODIMENTS

In the following description, similar features in the drawings have been given similar reference numerals and, in order to weight down the figures, some elements are not referred to in some figures if they were already identified in a precedent figure.

. Figure 1 shows a single mode optical fiber (SMF) 1 on which a section of multimode optical fiber (MMF) 2 is spliced and cleaved at desired distance 3 from the splice. This distance preferably ranges from 0 to 200 μm. The splicing is advantageously accomplished by the use of standard fusion splicer for optical fibers and the calving is performed by standard fiber cleaver. The outer diameter of fiber used in this example is preferably 125 μm. Preferably, the diameter of SMF is approximately 9 μm 5. MMF has step or graded index profile with the core diameter of approximately 60 μm 6.

The fiber tip prepared in accordance with previous paragraph is then dipped into HF acid that causes decomposition/etching of the MMF core. The etching is stopped at the interface of both fibers. The reflectivity at the fiber-air interfaces is then approximately 2%. This reflectivity is needed to achieve proper sensor operation. It is important to terminate the etching when the etching process reaches the interface between MMF and SMF, otherwise the SMF is damaged by HF that is reflected by drop for the cavity reflectivity. To achieve proper etching termination moment, the reflected optical power is advantageously monitored during the etching and when the reflectivity reaches the maximum the etching is terminated. After the etching the fiber tip is cleaned, for example in ultrasonic cleaner, in order to remove reminiscences of the HF and other impurities. Figure 2 shows etched cavity 7 at the tip of the fiber. Figure 3 is showing the spliced and etched fiber section after this section is spliced with another section of SMF 8. This creates an in-fiber cavity 9 at the position of the splice. In cases when longer air cavity is desired, it is possible to splice two etched fiber sections obtained by previously described procedure. The splicing procedure requires appropriate modification of fusion splicing parameters, e.g. modification of fusion temperature and fusion times. The fusion temperature is around 1800 C and the air that is trapped within the cavity expands if temperature increases after fibers come in contact. Under normal fusion conditions the cavity pressure rise and this leads to deformation of the cavity walls that results in decrease cavity reactivity or total destruction of the splice. In this invention the problem of splicing is solved in a way to perform the splicing in two steps. In the first step the fiber are preheated at higher temperature and when fibers come into the contact the temperature is slightly reduced until fusion is completed. This prevents deformation of the fibers during splicing.

Figure 4 demonstrates the calibration principle of the in-fiber air cavity. The section of the fiber with in fiber cavity is heated by resistive wire coil 10 and then pulled apart. The fiber is fixed 11 at one side of the heating coil, while it is attached 12 at the moving linear stage 13 at the other. The temperature that allows for the plastic (permanent) elongation of the optical fiber is approximately 850-1000 C for Si02 fiber. The calibration procedure requires on line monitoring of the cavity length, for example by application of broadband source and spectrum analyzer.

Figure 5 demonstrates typical reflectivity of the optical fiber strain sensors as a function of a strain. The sensor was calibrated in the vicinity of quadrature point and it was produced for the strain ranger of +- 5000 μm/m with approximately linear static characteristics.

Although preferred embodiments of the present invention have been described in detail herein and illustrated in the accompanying drawings, it is to be understood that the invention is not limited to these precise embodiments and that various changes and modifications may be effected therein without departing from the scope or spirit of the present invention.

Claims

WHAT IS CLAIMED IS:

1. An optical fiber strain sensor comprising: a two single mode fibers that are separated by segment of optical fiber with the cavity of step or concave shape that acts as an optical resonator

2. The sensor of claim 1 that is produced by: splicing of a multimode fiber segment (2) to the single mode fiber (1) that is followed by etching in HF acid

3. The sensor of claim 1 that is produced by: splicing of etched fiber with cleaved single mode fiber or another etched fiber (8).

4. An optical fiber strain sensor comprising: lead single mode fiber (1) spliced to spacer , followed by second single mode fiber (8) in a way to form an optical resonator between both single-mode fibers.

5. The sensor of claim 4 where spacer is produced by etching process

6. A calibration of the strain sensor cavity length obtained by heating of the short section of fiber surrounding the cavity with the step or concave shape (7) to the temperature that allows for plastic elongation of heated section and thereby allows fine tuning of the cavity length.

7. A calibration of the strain sensor cavity length obtained by heating of the short section of fiber surrounding the cavity created by between two optical fibers, separated by spacer to the temperature that allows for plastic elongation of heated section and thereby allows fine tuning of the cavity length.