FI77327C - FOERFARANDE FOER FRAMSTAELLNING AV EN TRYCKGIVARFIBER. - Google Patents

Description

77327

The present invention relates to a method for producing a pressure sensor fiber according to the preamble of claim 1.

The invention relates to a pressure-sensitive optical capillary fiber in the wall of which a single-mode light waveguide is made. The operation of the fiber is based on the stress caused by the external and internal pressure difference in the capillary wall, which affects the birefringence of the light-conducting single-mode core made inside the capillary fiber wall. Sensor fibers of this type have been known in the past in which two holes are left next to the core of the fiber during the manufacturing step.

The pressure sensor fiber can be used in awkward applications where conventional pressure sensors cannot be used, e.g. for underwater pressure wave measurements (hydrophone) and for strong electric or magnetic field pressure measurements. In addition, the capillary fiber may be coated or the hole may be filled with a material which, due to some other physical quantity, does not. . puts pressure inside and / or outside the capillary. Such substances include e.g. Magnetostrictive materials (sensitive to magnetic field) and materials with a different coefficient of thermal expansion other than quartz (sensitive to temperature).

; Two pressure sensor solutions of the same type implemented in fiber optics are known from the literature. One of these is a fiber-optic interferometer, as described e.g. in B. Culshaw, "Optical fiber sensing and signal process sing", Peter Peregrinus Ltd., London, UK, 1984, pp. 87-112. This solution has taken advantage of the dependence of the optical length of ordinary fiber on the external pressure of the fiber. The solution presented uses two different fibers, of which 2 77327 one acts as the sensor itself and the other as the reference. When light is connected to both fibers from a coherent light source by means of a beam splitter and the rays passing through both fibers are combined, an interference pattern is formed from which a change in the optical travel difference between the sensor fiber and the reference fiber can be inferred. Another solution that is even closer to the pressure sensor fiber described herein is in H.

M. Xie, Ph. Dabkiewicz, R. Ulrich and K. Okamoto, "Sidehole fiber for fiber-optic pressure sensing," Optics Letters, vol. 11, no. 5, May 1986, pages 333-335. Here, both modes of polarization proceed along the same fiber. The fiber is an ordinary single-mode fiber in which two holes are left on both sides of the single-mode light channel during manufacture. The holes are typically made by drilling. The difference between external and internal pressure causes stresses in the core and thus changes the birefringence of the core. This can be measured, for example, by coupling circularly polarized light to this special fiber, whereby equal amounts of light are applied to both polarization modes, i.

The disadvantage of interferometric solutions has been the effect of physical changes other than pressure on both the sensor fiber itself and the reference fiber. Indeed, the main problem has been how to make these other factors affect both fibers in exactly the same way. The solution described in the publication, in which both polarization modes run in the same fiber, is technically cumbersome to manufacture, and it is almost impossible to produce long fibers.

The object of the present invention is to obviate the disadvantages of the technique described above and to provide a completely new type of method for manufacturing a pressure sensor fiber.

The invention is based on making a tubular groove in the outer surface of the light-transmitting first tube, fitting a single-shaped fiber blank having a diameter substantially equal to the width of the groove, fitting a second tube on top of the first tube and the single-shaped fiber blank, fusing the first and second tubes together , the tube-single-mode fiber preform arrangement is drawn into a pressure sensor fiber, and the tube ends are closed if necessary.

Thus, the sensor is formed, for example, by a capillary fiber made of quartz glass, inside the wall of which a light-conducting uniform shape core is made. The difference between the internal and external pressure of the capillary causes a tension in the capillary wall which is mainly parallel to the circumference of the capillary. This stress acts on the core made inside the wall so that the polarization state of the light propagating in the core changes due to a change in birefringence.

More specifically, the method for producing a pressure sensor fiber according to the invention is characterized by what is set forth in the characterizing part of claim 1.

The invention provides considerable advantages.

Special sensor fibers such as H. M. Xie et. Oh. and a fiber according to the invention, provide a solution to the disadvantage of interferometric fiber sensors. In these solutions, both modes of polarization propagate down the same physical fiber. In this case, other external physical variables, such as temperature, affect both forms in almost the same way, so their effect is considerably smaller than in interferometric fiber sensors. The sensor design also becomes simpler than using an interferometric approach. H. M. Xie et.

ai. The capillary k according to the invention differs considerably from the solution presented by et al. and the production of the fiber is simpler. The hole of the fiber (capillary) is also considerably larger than that described in said reference, so that, for example, filling the capillary is easier.

4 77327

The invention will now be examined in more detail by means of an application example according to the accompanying drawings.

Figure 1 shows the different steps of the method for manufacturing a pressure sensor fiber according to the invention in perspective views.

Figure 2 shows a cross-sectional end view of a pressure sensor fiber made by the method according to the invention.

Figure 3 schematically shows a system for measuring pressure with a pressure sensor fiber produced by the method according to the invention.

Figure 4 shows a cross-sectional perspective view of the effect of forces on a pressure sensor fiber produced by the method according to the invention.

The sensor fiber according to the invention can be manufactured, for example, as shown in Figure 1. A groove 3 is ground on the surface of the quartz tube 1, for example with a diamond saw, the size of which corresponds to the diameter of a suitable single-shape fiber blank 2, which may be e.g. 0.6 mm. In this case, the outer tube 1 has an outer diameter of 14 mm and a wall thickness 1. The diameter 15 of the core 15 of the monofilament fiber blank 2 is then about 0.3 mm and the refractive index difference between the core 15 and the shell part is about 4 * 10-3 The monofilament fiber blank 2 can then be fused into the groove 3 in the tube 1 by means of a small hand torch The larger quartz tube 4 is then placed on the smaller tube 1, and the larger tube 4 is further collapsed by heating onto the smaller tube 1. The collapse can be performed, for example, by rotating the tube assembly in a lathe and heating the larger tube 4 with a gas flame. The tube obtained in this way, in the wall of which there is a blank 2 of the single-mode fiber, can be drawn into a capillary, the wall 5 of which has a single-mode light waveguide 6.

The smaller 1 and larger 4 quartz tubes can also be connected to each other by pulling them together in connection with the heat treatment 5 77327, whereby the heat treatment melts the uppermost tube 4 onto the outer surface of the smaller, inner tube 1.

A schematic view of the obtained capillary is shown in Fig. 2. The figure shows the capillary hole 7, the wall 5 and the single-mode light guide 6.

If necessary, several optical waveguides 6 can be placed in the sensor fiber.

Other grades of quartz glass can be used as the material for pipes 1 and 4.

Figure 3 shows how the sensor fiber in question can be utilized. The light is switched from a coherent light source, such as a HeNe laser 8, via a 1/4-wave plate 9 by means of a lens 10. to the single-mode light channel of the sensor fiber 11. The function of the I / 4 waveguide 9 is to ensure that approximately the same amount of light power is applied to both modes of polarization. The output of the sensor fiber 1 has a lens 12 which aligns the light coming from the fiber 11, and a polarizer 13 so as to transmit a linearly polarized light at a 45 ° angle to the polarization shape of the sensor fiber 11. . in relation to work. In this case, the phase difference between these shapes is seen as changes in intensity in the light transmitted by the polarizer 13. Thus, in the sensor fiber 11, the phase difference (= birefringence) created for the two different polarization modes changes into an intensity difference that can be measured by the light sensor 14.

The birefringence of the sensor fiber 11, on the other hand, depends on the pressure difference inside and outside the capillary tube. This can be explained, for example, as follows: Figure 4 shows a half of a pipe whose outer wall is subjected to a hydrostatic pressure p (not shown). This causes the pipe to shrink while the pipe retains its shape. If the thickness t of the pipe wall 5 is small compared to the pipe diameter 2r, the following can be stated: due to the circular cross-section of the pipe 6 77327 no bending moment is generated in this wall 5 and the tensile stress is evenly distributed over the wall thickness, i.

-2rlp = σ 211t => σ = -pr / t (1) here r = pipe radius 1 = pipe length p = hydrostatic pressure σ = circumferential stress

The inhomogeneous stress acting on the wall of the capillary tube, on the other hand, causes a change in birefringence, which can be measured by utilizing a .. single-mode light channel made in the wall of the tube.

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Claims (6)

7 77327 A method of manufacturing a pressure sensor fiber (11), characterized in that - a tubular, substantially flat plate groove (3) is made in the outer surface of the light-transmitting first tube (1), - a single-shaped fiber blank (2) with a diameter substantially fixed is fitted in the groove (3). the width of the groove (3) is equal to, - a second tube (4) of at least approximately the same shape as the first tube is arranged on the first tube (1) and the monofilament fiber blank (6), - the tube-single-mode fiber preform arrangement (1, 6, 4) is drawn, for example by heat treatment, into a pressure sensor fiber (11), and - the ends of the tube (5) are closed if necessary. Method according to Claim 1, characterized in that the groove (3) is made with a diamond saw. Method according to Claim 2 or 3, characterized in that the second tube (4) is arranged on top of the first tube (1) by rotating a combination of the first tube (1), the monofilament blank (6) and the second tube (4) in a lathe and heating the second tube (4) to collapse this onto the first tube (1). β 77327 Method according to Claims 1 to 3, characterized in that the first combination of tube (1) and single-mode fiber blank (4) is drawn together with the second tube (4) during the heat treatment, the heat treatment fusing the second tube (4) together with the first tube (4). 1) with. Method according to one of the preceding claims, characterized in that glass is used as the material for the tubes (1 and 4). Method according to one of the preceding claims, characterized in that quartz glass is used as the material for the tubes (1 and 4). 77327 9