SI21763A - Optical fibre pressure sensor with sio2 diaphragm - Google Patents

Abstract

The optical fibre pressure sensor solves the problem of implementing a pressure sensor, which is no wider that the cross section of a standard optical fibre, this is 125 micrometers. The key element of the sensor is the SIO2 diaphragm, which is welded on top of the optical fibre with a recess. The measuring of the convexity of the diaphragm or pressure is based on the phenomenon of light interference between the diaphragm and the end of the optical fibre. The procedure of its manufacture includes the preparation of the recess on top of the single-mode fibre, where a segment of any fibre can be welded to. This segment is then cut in the immediate vicinity of the seam and polished if required, so that the remaining welded part is only 1 - 10 micrometers long. This is followed by etching in a container, where the pressure is changing periodically. The continuous monitoring of the sensor response to the periodical change of pressure enables to stop the etching in the very moment, when the welded segment is etched in such a way that the required sensitivity of the sensor is obtained. This procedure is suitable for the manufacture of pressure sensors for ranges between 0 to 20 KPa and up to the range between 0 to 100 MPa.

Description

OPTICAL FIBER PRESSURE SENSOR WITH SiO ₂ MEMBRANE

The object of the invention is a pressure sensor and a method for manufacturing a pressure sensor located at the end of an optical fiber having a diameter smaller than or equal to the diameter of the optical fiber. The sensor is fully built and silicon dioxide. The sensor construction enables the realization of a highly sensitive sensor suitable for measuring lower pressures and higher industrial pressures. The complete SiO ₂ structure provides good temperature stability and resistance to other environmental influences.

A technical problem solved by the invention is the realization of a small diameter pressure sensor suitable for a range of applications requiring small sensor dimensions and / or electrical passivity. In addition, the sensor is built of SiO ₂ , which is crucial in the medical field. The invention also solves the problem of the high cost of manufacturing this type of sensor, due to its simple fabrication process and the ability to use standard fibers.

Some embodiments of optical fiber pressure gauges are known. The solution according to patent WO 02/23148 uses a housing made of silicon, to which is attached the end of the inlet optical fiber and a thin silicon membrane. The smoothly cut end of the optical fiber and the membrane represent two reflective surfaces that form the so-called. Fabry-Perot interferometer. This is where the optical wave is divided into two parts that interfere with each other. Their interference sum depends on the distance between the end of the fiber and the membrane, which is basically a measure of pressure. Other known solutions are those of EP 1 089 062, U.S. Pat. All of the above solutions are a common feature of the said housing, which is everywhere larger than the optical fiber. The only known solution where the entire sensor is equal to the diameter of the optical fiber is EP 0 997 721 and SI21242. According to EP 0 997 721, the manufacturing process is relatively complex and requires the use of special tools and materials. The SI21242 solution is based on the use of a polymeric membrane. The disadvantage of the polymer membrane system is the high sensitivity of the membrane to environmental influences, e.g. on water absorption, temperature sensitivity. The process of manufacturing a polymer membrane is also relatively demanding.

According to the invention, the problem is solved by placing the sensor at the end of the optical fiber. The invention will be described in the embodiment and in the drawings showing:

FIG. 1 Sensor layout and construction FIG. 2 The individual stages of sensor construction FIG. 3 Calibration and adjustment of membrane thickness FIG. 4 Typical sensor response for pressure range 0 to 40 KPa FIG. 5. Typical sensor response for pressure range 0 to 200 KPa

Sensor description

A key part of the sensor is the thin SiO ₂ membrane 1, which is separated by the short cylindrical spacer 2 from the tip of the single-mode optical fiber 3 (hereinafter EOV). The top of the EOV and the membrane play the role of two partially reflective surfaces that form a Fabry-Perot interferometer (hereinafter FPI), where, based on the interference of reflected light, a membrane pressure is detected by the appropriate processing system under the influence of the measured pressure.

Making a sensor

The sensor fabrication process is in two stages:

a) in the first phase, a miniature FPI is produced at the top of the fiber

b) in the second stage, the thickness of the membrane forming the FPI is adjusted

a) Production of FPI

A spacer 2. A spacer 2. This can be achieved by welding multipurpose optical fiber 4 (hereinafter MOV) at the top of the single-fiber 3, which transmits light from the light source to the sensor and back to the detector. The fiber is then cut at length 5 between typically 10 and 100pm from the joint. Welding is performed using a standard optical fiber welder. Both optical fibers used have an outside diameter of 125 pm or less. The optical fiber end thus prepared is immersed in 40% HF acid 6, where dissolution or resp. etching MOV. Because the fiber optic core is doped with GeO ₂ , it dissolves significantly faster than the coating. The measured etching rate of the MOV core with 20% by weight of GeO ₂ is about 7 pm per minute and the coating consisting of pure SiO ₂ is 0.5 pm. The etching runs all the way to the junction of the two optical fibers. It is important that the etching be interrupted at the moment when the joint of the two fibers is reached, otherwise the surface of the EOV at the joint is damaged and thus a sudden drop in reflectivity occurs. The resulting structure is neutralized and purified in an ultrasonic cleaner.

The resulting structure is then welded with an additional fiber segment 10 by a fiber optic welder. This may be standard or non-standard single-stranded or multi-stranded or non-core fiber. The welded fiber is then cut at length 12 between 5 and 100 pm away from the second joint. This is followed by polishing the top of the structure thus obtained, where the last welded fiber is reduced to a typical thickness of between 1 and 5 pm. The sensor structure obtained in this way takes the form of an FPI with a SiO ₂ membrane, which is relatively thick and in most cases does not allow the measurement of lower pressures.

b.) Adjusting membrane thickness:

In the previous step, the sensor structure obtained is connected via a single-stranded fiber 3 to an optical signal processing system 13. A buffer solution of HF and NH ₄ F 14 is prepared and inserted into the sensor structure. The buffered solution ensures that the germanium doped area now dissolves more slowly than the areas containing pure SiO ₂ . This prevents an unwanted hole in the membrane where the germanium doped nucleus is located. If core-free fiber is used, etching in HF acid can also be performed. The sensor structure, together with the etching solution, is enclosed in a container 15 in which the pressure in the area for which the sensor is desired is cyclically changed. Using the optical signal processing system, the sensor response is observed. The solution thus begins to dissolve the membrane, which is reflected in an increase in the sensitivity of the sensor. When the desired sensitivity is reached, the sensor is removed from the solution and neutralized. In this way, it is possible to achieve any sensitivity or working area of the sensor, while having good repeatability of the process.

Typical results

The sensor can be manufactured according to the described procedure for different pressure ranges. Examples of characteristics for fabricated sensors for the 0-40 KPa and 0-200 KPa ranges are shown in Figures 2 and 3. The figures show the characteristic reflectance of the sensor at a wavelength of 1550 nm as a function of pressure.

Claims (5)

PATENT APPLICATIONS Optical fiber pressure sensor, characterized in that it consists of a single inlet fiber (3) extending into a short spacer (2) with an internal diameter of between 20 and 200pm (7), an outer diameter of between 80 and 300μηη (8). and a length of 10 and 300 pm (9) and a SiO ₂ membrane welded to the end of the cylindrical element (2). A method for manufacturing an optical pressure sensor according to claim 1, characterized in that a weld with a short segment of any optical fiber (10) cut between 1 and 100 pm is made on a single-optical fiber on the top of which the recess is made. away from the weld, followed by etching in HF or buffered HF acid so that the weld segment is further reduced to a thickness that ensures the sensitivity of the sensor in the target pressure region. 3. A method for manufacturing an optical pressure sensor according to claim 1, characterized in that a weld with a short segment of any optical fiber (10) cut between 1 and 100 pm away is applied to a single optical fiber, at the top of which a recess is made. from the weld and then polished so that the welded segment is between 1 and 10 pm long, followed by etching in HF or buffered HF acid so that the welded segment is further reduced to a thickness that ensures the sensitivity of the sensor in the target pressure region. A method for manufacturing an optical pressure sensor according to claim 2, characterized in that the etching is carried out in a closed container in which the pressure cyclically changes while observing the sensor response such that the etching is interrupted when the desired sensitivity of the sensor is achieved. A method for producing an optical pressure gauge according to claim 3, characterized in that the etching is carried out in a closed container in which the pressure is cyclically changed and at the same time it observes the response of the sensor so that the etching is interrupted when the desired sensor sensitivity is achieved.