WO2016137348A1

WO2016137348A1 - A fibre optic gas sensor and manufacturing method of fibre optic gas sensor

Info

Publication number: WO2016137348A1
Application number: PCT/PL2016/050003
Authority: WO
Inventors: Marek NAPIERAŁA; Tomasz NASIŁOWSKI; Michał SZYMAŃSKI; Michał MURAWSKI; Tomasz STAŃCZYK; Karol WYSOKIŃSKI; Małgorzata BROCZKOWSKA; Agnieszka KOŁAKOWSKA
Original assignee: Inphotech Sp. Z O. O.
Priority date: 2015-02-28
Filing date: 2016-02-26
Publication date: 2016-09-01
Also published as: PL226779B1; WO2016137348A4; PL411431A1

Abstract

A fibre optic gas sensor and its manufacturing method are described. The fibre optic gas sensor consists of at least one optical fibre coated with a silica gel layer which changes its colour with a change in the concentration of gases, such as C0₂, N0₂, S0₂, NH₃and H₂0, wherein the silica gel layer is applied on a stripped optical fibre surface and contains at least one indicator substance, such as phenol red or thymol blue, sensitive to a change of pH in the optical fibre working environment within a range from 0 to 14 and wherein the indicator substance is placed on a surface of treated optical fibre prepared in the process of tapering, etching, bending, or cutting. The manufacturing method of the fiber optic gas sensor consists of stripping an optical fibre, preferably a single-mode optical fiber, treating a visible section of the optical fibre and coating it with a chemically active substance, wherein the surface of the optical fibre is tapered, etched or cut and silica gel is the chemically active substance placed on the surface of treated section of optical fibre in an immersion process.

Description

A fibre optic gas sensor and manufacturing method of fibre optic gas sensor This invention relates to the fibre optic gas sensor used to assess the content of C0₂, N0₂, S0₂, NH₃, H₂0 and other gases in the air which is based on the phenomenon of colour change of the pH indicators and to the manufacturing method of fibre optic gas sensor.

In technology, there is a demand for an effective method to measure the concentration of gases i.a. while conducting measurements in potentially explosive environments. Gas measurements take usually place in potentially explosive environments and therefore in many cases requirements concerning the sensors are very rigid. In particular, sensors must not generate sparks. Thus, optical fibres being passive elements are the perfect candidates for manufacturing sensing components.

Technology knows various ways to use optical fibres to measure gas concentrations.

Patent US5714121 presents optical carbon dioxide sensor and its manufacturing method. The sensor is designed for detection of C0₂ in the liquid sample and consists of a cap/capsule slid over the end of optical fibre with fluorescent indicator particles which react with C0₂. The capsule is made of silicone.

Patent US5408999 relates to the fibre optic sensor to measure parameters of liquids. The solution disclosed is intended for analysis of pH and concentration of blood gases (including oxygen and C0₂). The invention works on the basis of the reactive coating or layer applied on the surface of optical fibre. Change in propagation of the light which results from the reaction of reactive layer with environment allows to assess pH/ concentration.

Patent US4842783 presents manufacturing method of fibre optic chemical sensor which consists in applying a photocrystalline gel, preferably a polymeric one, on the waveguide part which increases its volume while crosslinking. Gel is saturated with a dye which changes propagation of light and may be used to assess pH and concentration of oxygen, C0₂ or other gases.

Patent US5280548 relates also to fibre optic sensor to analyse C0₂, which may be used to assess oxygen concentration or pH.

DEI 9900019 - invention relates to the fibre optic sensor to assess pH, C0₂ and other physicochemical parameters. Within detection area, the optical fibre is covered with a membrane in which there is a substance (indicator) changing its absorption properties with changes in environment.

According to the invention, the sensor consists of at least one optical fibre coated with silica gel layer which changes its colour with the change in the concentration of gases, such as C0₂, N0₂, S0₂, NH₃ and H₂0. Silica gel layer is applied on the jacket-free optical fibre surface and contains at least one substance sensitive to changes of pH in the optical fibre working environment; preferably the substance should involve methyl orange, thymol blue or phenolphthalein or other substance sensitive to the change of pH in the range of 0 to 14.

Preferably, the silica gel layer is applied on the surface of optical fibre tapered and/or etched and/or bent or on the optical fibre's face. Whereas an optical fibre is a single-mode, multi-mode or undoped fibre (made of pure silica). Other manufacturing method involves also microstructural fibres. Whereas the fibres with silica gel have low attenuation.

Depending on the manufacturing method, silica gels in which there is at least one indicator are made in two ways: with the use of inorganic silicates, e.g. sodium silicate or with the use of organic silanes.

According to the invention, the manufacturing method of the gas sensor consists in the fact that optical fibre, preferably a single-mode one, is stripped of a part of the coating, visible section of optical fibre is tapered, etched or cut and afterwards treated surface of the optical fibre is placed preferably by putting treated optical fibre section in the silica gel solution.

It is preferable if stripped optical fibre is tapered so thatthe fibre is heated and stretched with the use of dedicated fusion splicer. It is preferable if thetaper is made with the use of GPX-3400 fusion splicer, maintaining the parameters of taper performed on typical single-mode SMF-28 optical fibre in the following form:

- taper length: at least 1 mm,

- taper diameter: preferable not smaller than 5 μπι,

- length of transition zones: at least 1 mm.

In a preferable manufacturing method, silica gel solution is obtained in such a way that diluted sodium silicate solution is added drop by drop to the acidic aqueous solution until obtaining assumed pH in the range from 1 to 9. Afterwards, after a certain amount of time of at least 10 sec, solution prepared in such a way gels which occurs faster if higher pH has been reached. Then gel is washed and dried at room conditions. Alternatively, this reaction may be conducted in the opposite way, i.e. a silicate may be added drop by drop to the acid, but the reaction occurs rapidly and obtained silica may not be suitable for sensors.

Taper area is coated with silica gel containing 10-60% w/w of ethanol solution consisting of 0.1% w/w of thymol blue and 20-70% w/w of triethoxymethylsilane with addition of 0-20% w/w of tetraethoxysilane and 5-30% w/w of water along with 1-10% w/w of aqueous solution of tetramethylammonium hydroxide, which form a homogeneous mixture. Silica gel is placed on the optical fibre surface in an immersion process, after which one part of taper is located above the other and after 1 minute this process should preferably be repeated at least 3 times and then taper is dried for at least 24 hours.

Other manufacturing method involves organic silanes in amount of at least 20%, which contains in particular methyl or octyl group, resulting in silica gels with different parameters and no need of washing which in turn simplifies the manufacturing process. The use of ethoxysilanes with additional alkyl groups allows to obtain different level of porosity and permeability for various gases. Then, silanes (preferably ethoxysilanes) are mixed with a solvent (e.g. ethyl alcohol), water and catalytic agent. Water and catalytic agent are necessary to conduct hydrolysis, whereas a solvent facilitates mixing of the ingredients which may be difficult.

In another manufacturing method, a part of single-mode optical fibre is stripped and afterwards for at least 10 minutes it is treated with 40% hydrofluoric acid solution, maintaining the parameters of etching as:

- diameter of etched optical fibre preferably should amount to at least: 5 μπι,

- etching length is equal to at least: 0.5 cm.

Afterwards, the etched fragment is immobilised, preferably ona platform, so that it will not move during coating and sensor work and thenit is coated with silica gel.

Silica gel which covers the etched taper surface is in a form of ethanol solution containing 0.1% phenol red and thymol blue in 10-60% w/w with addition of 20-70% w/w of triethoxymetylsilane, with addition of 0-20% w/w of tetraethoxysilane and 5-30% w/w of water along with 1-10% of aqueous solution of tetramethylammonium hydroxide, which form a homogeneous mixture in which etched fragment of optical fibre is immersed. After immersion, the optical fibre is pulled out from it so that one end of the etched part is above the other one and after 1 minute this activity is repeated at least 3 times and then the optical fibre is dried for at least 24 hours.

In other manufacturing method, undoped optical fibre is stripped at length of approx. 12 cm, which in turn is coated with silica gel layer. Silica gel which covers stripped optical fibre surface is in a form of ethanol solution containing 0.1% phenol red in 10-60% w/w, 20-70% w/w of triethoxymethylsilane, 0-20% w/w of tetraethoxysilane and 5-30% w/w of water with addition of 1-10% of aqueous solution of tetramethylammonium hydroxide, which form a homogeneous mixture.

After a bath, optical fibre is pulled out from it so that one end is above the other one and after 1 minute this activity is repeated at least 3 times and then the optical fibre is dried for at least 24 hours. Coated section of the optical fibre is bent in such a way that a bending radius amounts to approx. 2 cm and then is immobilised.

Substances being pH indicators are used to create sensory layer, i.e. such which change their colour with change in pH in an appropriate range. These include i.a. the following: brilliant green, erythrosine B, crystal violet, malachite green, thymol blue, methyl yellow, dinitrophenol, bromophenol blue, anthocyanidins, anthoxanthins, anthocyanins, Congo red, methyl orange, bromocresol green, methyl red, methyl violet, resazurin, nitrazine, aurine, litmus, chinolidine red, nitrophenol, chlorophenol red, bromocresol purple, bromocresol violet, bromothymol blue, phenol red, toluidine red, naphtolophtaleine, cresol red, cresolophtaleine, orange yellow, phenolphthalein, thymolphthalein, alizarin yellow, indigo carmine, tropeoline, polyphenols and other similar compounds.

The invention operates based on the phenomenon consisting in the fact that changes in gas concentration may be assessed when light of a wavelength, at which the various forms of the active layer (in the presence of test gas and not) have different absorption, is introduced to the optical fibre.

While interacting in particular with indicated sensitive substances, each gas listed changes its colour, whereas C0₂, N0₂, S0₂ are acidic and thus decrease pH of solutions and solids, while alkaline NH₃ increases pH. Presence of water is necessary to perform hydrolysis which also alters pH; therefore, increase in humidity enhances the change of signal caused by the presence of assayed acidic or alkaline gas. This may be illustrated by hydrolysis of C0₂ presented below: C0₂ + 2H₂0 <→ HCO^" ₃ + H₃0⁺ (1)

HCO^" ₃ + H₂0 <→ C0₃ ^" + H₃0⁺. (2)

To carry out the assessment of gases tested, light should be introduced to the material containing any indicative substance and afterwards it should be taken away and changes in transmission should be measured. Taking into consideration the fact that diffusion of gases to the inside of material is usually a slow process, it is preferable to use it in form of thin layer. In such a case, gas will require very little time to change layer's colour at its entire cross-section.

pH indicators in pure form usually do not exhibit change of colour in presence of previously listed gases. This may be a result of i.a. intermolecular interactions between adjacent molecules of an indicator which may alter their electron properties and thus also the absorption spectrum. The second cause may include diffusion limitations, because gases may not be able to pass through relatively thickly packed structures of indicative substances. Therefore, it is necessary to dissolve the indicator in the porous material. Silica gels in a form of e.g. organically modified silica are an ideal candidate for this type of application. They are made of organic silicate derivatives, in which dyes being pH indicators may be dissolved. As a result of hydrolysis, silicates transform from liquid state to a solid state and when solvents are evaporated, they become very porous. Porous silica may be obtained if e.g. tetraethoxysilane, triethoxymetylsilane, triethoxyoctylosilane, silicates or other similar compounds with silicon are used as reagents. These dyes may also be dissolved in a number of polymeric substances, such as ethyl cellulose, however they do not exhibit so rapid reaction time.

According to the invention, the light is introduced to silica gel layer containing pH indicators by means of optical fibres with the use of one of the following methods:

- coating the taper made of a single-mode, multi-mode or undoped optical fibre with the active layer, whereas microstmctural optical fibre may also be used.

- coating the etched section of a single-mode, multi-mode or undoped optical fibre with the active layer, whereas microstmctural optical fibre may also be used.

- coating the section of a multi-mode or undoped optical fibre with the active layer, whereas microstmctural optical fibre may also be used.

- coating optical fibre face with the active layer.

In the first three cases, the assessment may take place by evaluating fibre transmission or by examining changes in intensity of light reflected from the fibre's face opposite to the one, through which the light is introduced which requires the need to use fibre optic coupler. In the last case, the assessment may be performed by receiving the light from the optical fibre with faceadjacent to another optical fibre, which may be done if they are accurately set against each other. Alternatively, this assessment may also be conducted by means of reflection, measuring the intensity of light reflected from the fibre face coated with the active layer.

According to the invention, three properties are typical for the sensor:

1) sensing component of the sensor contains optical fibre coated with a layer made of porous silica or organically modified silica, hereinafter sensor layer (described below),

2) in the layer of silica or organic silica (sensor layer), there is at least one pH indicator, such as: phenol red, methyl red, bromothymol blue, thymol blue or other similar substances, preferably substances which change their colour with pH in the range from 0 to 14,

3) sensor layer is placed on a tapered, etched or bent, undoped or multi-mode optical fibre or on the optical fibre face, whereas microstructural optical fibre may also preferably be used.

An optical fibre being at the same time a sensing component of the sensor must primarily be able to provide the signal to the sensor layer containing a dye and afterwards to receive it.

Depending on the configuration selected, various measuring systems may be constructed.

1) With the use of fibre optic taper

Taper made of the single-mode optical fibre coated with silica gel layer with addition of phenol red is immobilised so that it cannot move during the assessment. Afterwards, light with wavelength of approx. 530 nm is introduced to one end of the optical fibre. The amount of light transmitted by the optical fibre is measured with the use of detector adapted to operate at the above-mentioned wavelength. The rise of carbon dioxide concentration increases the transmission. Sensor calibration is preferably performed in a chamber with the use of a reference gas sensor.

2) With the use of etched optical fibres

A single-mode optical fibre with etched section coated with silica gel layer with addition of thymol blue is immobilised so that it cannot move during an assessment. Afterwards, light with wavelength of approx. 635 nm is introduced to one end of the optical fibre. The amount of light transmitted by the optical fibre is measured with the use of detector adapted to operate at the above-mentioned wavelength. The rise of carbon dioxide concentration increases the transmission. Sensor calibration is preferably performed in a chamber with the use of a reference gas sensor.

3) With the bent optical fibre

Undoped bent optical fibre coated with silica gel layer with addition of cresol red is immobilised so that it cannot move during assessment. Afterwards, light with wavelength of approx. 530 nm is introduced to one end of the optical fibre. The amount of light transmitted by the optical fibre is measured with the use of detector adapted to operate at the above- mentioned wavelength. The rise of carbon dioxide concentration increases the transmission. Sensor calibration is preferably performed in a chamber with the use of reference gas sensor.

4) with the use of coated optical fibre face

A single-mode optical fibre coated with silica gel layer with addition of thymol blue is set opposite to an undoped fibre and immobilised so that they cannot move during an assessment. Afterwards, light with wavelength of approx. 635 nm is introduced to an uncoated end of a single-mode optical fibre. The amount of light transmitted at the free end of an undoped optical fibre is measured with the use of detector adapted to operate at the above-mentioned wavelength. The rise of carbon dioxide concentration increases the transmission. Sensor calibration is preferably performed in a chamber with the use of a reference gas sensor.

In the above configurations, a light-emitting diode, gas laser, solid- state laser, etc. are the preferable source of the signal. Signal detection is preferably carried out with a detector adjusted to a wavelength of the light source. It should be a semiconductor detector or a spectrum analyser.

According to the invention, the sensor may be used to carry out the assessment away from the light source; therefore, there are no electrical signals at the assessment site (optical fibre as a passive component which does not generate sparks). There is a possibility of placing several sensors in one fibre and this expands measuring capabilities of the systems.

According to the invention, the device will allow to assess the following ambient properties:

- C0₂ concentration range: 0.01%-100%,

- humidity: 0-100%,

- concentration range of N0₂, S0₂, NH₃: up to approx. 1%

Example 1

According to the invention, the sensor consists of an optical fibre coated with silica gel layer which changes its colour with the change of the concentration of gases, such as C0₂, N0₂, S0₂, NH₃ and H₂0. Silica gel layer is applied on the stripped optical fibre surface and contains a substance sensitive to changes of pH in the optical fibre working environment; the substance is thymol blue sensitive to the change in pH in the range of 8.0 to 9.6.

Silica gel layer is placed on the surface of tapered optical fibre. Whereas an optical fibre is a single-mode fibre and has low attenuation.

Fibre optic gas sensor is manufactured in such a way that single- mode optical fibre is stripped and tapered with the use of a dedicated fusion splicer by heating and extending it. The taper is made with the use of GPX- 3400 fusion splicer. The parameters of the taper made of a typical single- mode SMF-28 optical fibre are as follows:

- taper length: 10 mm,

- taper diameter: 13 μπι,

- length of transition zones: 5 mm.

Taper area is coated with a silica gel containing 23% w/w of ethanol solution consisting of 0.1% w/w of thymol blue and 52% w/w of triethoxymetylsilane with addition of 7% w/w of tetraethoxysilane and 15% w/w of water along with 3% w/w of aqueous solution of tetramethylammonium hydroxide, which form a homogeneous mixture. Silica gel is placed on the optical fibre surface in an immersion process, after which one part of the taper is located above the other and after 1 minute this process should preferably be repeated at least 3 times and then the taper is dried for at least 24 hours.

After being calibrated, sensor manufactured in such a way may be used to detect C0₂, N0₂, S0₂ and H₂0. Concentration of the selected gas may be measured with the use of reference sensors.

Example 2

305 According to the invention, the sensor consists of an optical fibre coated with a silica gel layer which changes its colour with the change of the concentration of gases, such as C0₂, N0₂, S0₂, NH₃ and H₂0. Silica gel layer is applied on the stripped optical fibre surface which was etched with hydrofluoric acid and contains a substance sensitive to changes of pH

310 in the optical fibre working environment; the substance was phenol red sensitive to the change of pH in the range of 6.8 to 8.2.

Silica gel layer is placed on the surface of tapered optical fibre after it was etched, whereas an optical fibre is a single-mode fibre and has low attenuation.

315 Fibre optic gas sensor is manufactured in such a way that single- mode optical fibre is stripped and its section is treated with 40% hydrofluoric acid solution for 57 minutes, maintaining the following etching parameters:

- diameter of etched optical fibre: 20 μπι,

320 - etching length: 2 cm.

Afterwards, the etched fragment is immobilised on the platform so that it will not move while coating and sensor work and then it is coated with silica gel.

Silica gel coating at the etched surface of the taper is produced of 325 0.1% w/w of ethanol solution consisting of 24% w/w of phenol red which is added to 55% w/w of triethoxymethylsilane with addition of 3% w/w of tetraethoxysilane and 15% w/w of water in turn with addition of 3% w/w of aqueous solution of tetramethylammonium hydroxide, which form a homogeneous mixture, in which etched part of the optical fibre is immersed. After immersion, optical fibre is pulled out from the solution so that one end of the etched part is above the other one and after 1 minute this activity is repeated for 3 times and then the optical fibre is dried for 24 hours.

After being calibrated, a sensor manufactured in such a way may be used to detect C0₂, N0₂, S0₂ and H₂0. Concentration of the selected gas may be measured with the use of reference sensors.

Example 3

According to the invention, the sensor consists of an optical fibre coated with silica gel layer which changes its colour with the change of the concentration of gases, such as C0₂, N0₂, S0₂, NH₃ and H₂0. Silica gel layer is applied on the stripped optical fibre surface which contains a substance sensitive to changes of pH in the optical fibre working environment; the substance was phenol red sensitive to change of pH in the range of 6.8 to 8.2.

Silica gel layer is placed on the surface of optical fibre, whereas an optical fibre is a single-mode fibre and has low attenuation.

Fibre optic gas sensor is manufactured in such a way that a single- mode fibre is stripped and its section is coated with silica gel layer, made of ethanol solution containing 0.1% of phenol red in 20% w/w, 61% w/w of triethoxymethylsilane as well as 3% w/w of tetraethoxysilane and 14% w/w of water in turn with addition of 2% w/w of aqueous solution of tetramethylammonium hydroxide, which are mixed until a homogeneous mixture is formed.

After immersion, optical fibre is pulled out from the solution so that one end is above the other one and after 1 minute this activity is repeated at least 3 times and then the optical fibre is dried for at least 24 hours. Coated section of the optical fibre is bent in such a way that a bending radius amounts to approx. 2 cm and is immobilised.

Example 4

According to the invention, the sensor consists of an optical fibre coated with silica gel layer which changes its colour with the change of the concentration of gases, such as C0₂, N0₂, S0₂, NH₃ and H₂0. Silica gel layer is applied on the optical fibre face and it contains a substance sensitive to changes of pH of the optical fibre working environment; the substance was thymol blue sensitive to the change of pH in the range of 8.0 to 9.6.

Fibre optic gas sensor is manufactured in such a way that a single- mode optical fibre is stripped and cut, whereas in this section it is coated

Claims

Patent claims

1. A fibre optic gas sensor consisting of at least one optical fibre coated with silica gel layer which changes its colour with the change of the concentration of gases such as C0₂, N0₂, S0₂, NH₃ and H₂0 is characterised in that silica gel layer is applied on stripped optical fibre surface and contains at least one substance sensitive to change of pH in the optical fibre working environment within a range from 0 to 14 and this substance sensitive to change in pH is placed on the surface of treated optical fibre - prepared in the process of: tapering and/or etching and/or bending and/or cutting.

2. According to claim 1, sensor is characterised in that an optical fibre is a single-mode, multi-mode or undoped fibre (made of pure silica), or a microstructural fibre.

3. According to claim 2, sensor is characterised in that silica gel in which there is at least one indicator contains inorganic silicates.

4. According to claim 2, sensor is characterised in that silica gel in which there is at least one indicator contains organic silanes.

5. According to the invention, the manufacturing method of the gas sensor consists in the fact that optical fibre, preferably a single- mode, is stripped, visible section of optical fibre is treated and coated with chemically active substance and it is characterised in that the surface of the optical fibre is tapered, etched or cut, whereas silica gel is a chemically active substance placed on the surface of treated section of optical fibre in immersion process.

6. According to claim 5, the method is characterised in that optical fibre coated with silica gel is stripped, heated and extended, maintaining taper parameters:

a. taper length: at least 1 mm,

b. taper diameter: not smaller than 5 μπι,

c. length of transition zones: at least 1 mm,

and it is coated with at least one silica gel layer.

7. According to claim 6, the method can be characterised in that the taper is made with the use of GPX-3400 fusion splicer and the optical fibre is a SMF-28 single-mode optical fibre.

8. According to claim 5, the method is characterised in that optical fibre coated with silica gel is stripped and afterwards etched with 40% hydrofluoric acid solution for at least 10 minutes, so that: a. diameter of etched optical fibre amounts to at least: 5 μπι, b. etching length is equal to at least: 0.5 cm.

and etched section is immobilised, preferably on the platform, for 24 hours and coated with at least one silica gel layer.

9. According to claim 5, 6, 7 or 8, the method is characterised in that at the section of approx. 12 cm, the undoped optical fibre coated by silica gel is stripped and coated with at least one silica gel layer.

10. According to claim 5, 6, 7, 8 or 9, the method is characterised in that a section of optical fibre coated by silica gel is stripped, cut and its face is coated with at least one silica gel layer.

11. According to claim 5, 6, 7, 8, 9 or 10, the method is characterised in that a silica gel solution is obtained in such a way that diluted sodium silicate solution is added drop by drop to the acidic aqueous solution until obtaining assumed pH in the range from 1 to 9 and after 10 seconds the solution prepared in such a way becomes a gel. The gel is washed and dried at room conditions.

12. According to claim 5, 6, 7, 8, 9, 10 or 11, the method is characterised in that sodium silicate is added drop by drop to the acid.

13. According to claim 5, 6, 7, 8, 9, 10, 11 or 12, the method is characterised in that the silica gel contains organic silanes, in particular the one which contain methyl or octyl group.

14. According to claim 13, the method is characterised in that the silica gel contains silanes selected from ethoxysilanes with additional alkyl groups, a solvent (in particular ethyl alcohol), water and catalytic agent which are successively mixed in the following proportions: 59%, 23%, 15%, 3%.

15. According to claim 6, 7, 8, 9, 10, 11, 12, 13 or 14, the method is characterised in that the silica gel is in a form of ethanol solution containing 0.1% phenol red in 10-60% w/w with addition of 20- 70% w/w of triethoxymethylsilane, to which 0-20% w/w of tetraethoxysilane and 5-30% w/w of water is added, to which in turn 1-10% of aqueous solution of tetramethylammonium hydroxide is added; these are mixed to create a homogenous mixture in which etched fragment of optical fibre is immersed.

16. According to claim 15, the method is characterised in that the silica gel used for coating the etched surface of the taper is in form of ethanol solution consisting of 0.1% phenol red in 24% w/w with addition of 55% w/w of triethoxymethylsilane, with addition of 3% w/w of tetraethoxysilane and 15% w/w of water, in turn with addition of 3% w/w of aqueous solution of tetramethylammonium hydroxide, which form a homogeneous mixture, in which etched section of optical fibre is immersed after which is pulled out so that one end of etched part is above the other one and after 1 minute this activity is repeated at least 3 times and optical fibre is dried for at least 24 hours.

17. According to claim 15, the method is characterised in that the silica gel used for coating the stripped optical fibre surface is in form of ethanol solution containing 0.1% phenol red in 20% w/w, 61% w/w of triethoxymetylsilane, 3% w/w of tetraethoxysilane and 14% w/w of water with addition of 2% w/w of aqueous solution of tetramethylammonium hydroxide, which form a homogeneous mixture.

18. According to claim 6, 7, 8, 9, 10, 11, 12, 13 or 14, the method is characterised in that the silica gel which is used for coating the stripped optical fibre surface is in a form of solution containing 0.1% phenol red in 10-60% w/w, 20-70% w/w of triethoxymethylsilane, 0-20% w/w of tetraethoxysilane and 5- 30% w/w of water with addition of 1-10% w/w of aqueous solution of tetramethylammonium hydroxide, which form a homogeneous mixture.

19. According to claim 18, the method is characterised in that the silica gel which is used for coating the stripped optical fibre surface is in a form of solution containing 0.1% thymol blue in 23% w/w, 59% w/w of triethoxymethylsilane, 15% w/w of water and 3% w/w of aqueous solution of tetramethylammonium hydroxide, which are mixed to form a homogeneous mixture.

20. According to any claim from 5 to 19, method is characterised in that after a bath (immersion) optical fibre is pulled out from it so that one end is above the other one and after 1 minute this activity is repeated at least 3 times and optical fibre is dried for at least 24 hours, whereas coated optical fibre section is bent in such a way that a bending radius amounts to approx. 2 cm and is immobilised.