ES2343607A1 - Optical fiber optic sensor for the detection of liquid and/or measurement of liquid level. (Machine-translation by Google Translate, not legally binding) - Google Patents

Abstract

Self-referenced fiber optic sensor for liquid detection and/or liquid level measurement. Self-referenced plastic fiber optic sensor for detecting liquid and/or liquid level measurement, comprising an emitter system configured to generate an optical signal, and a power meter receiver system comprising at least two simultaneous inputs, comprising a coupler comprising two sections of optical fiber that are arranged in opposition to a critical radius of curvature rc, having eliminated the protective cover and partially eliminated the core in a coupling area between said two sections of optical fiber, which are separated by a liquid with a refractive index n, so that a coupling constant k of the sensor depends on said refractive index n, which makes it possible to detect the presence of said liquid, as well as its level. (Machine-translation by Google Translate, not legally binding)

Description

Self-referenced fiber optic sensor for liquid detection and / or liquid level measurement.

Object of the invention

The present invention relates to a sensor self-referenced optical fiber for the detection of liquid and / or liquid level measurement, said liquid being confined in a deposit, which has application in the device industry sensors

The sensor device that the invention proposes it comprises a coupler, by which self-reference is achieved,  based on the measurement or detection of power losses by curvature and polishing of part of a core of an optical fiber, for measuring the intensity of the optical power in a receiver. The coupling constant varies according to the type of liquid in which it is the sensor is immersed, allowing to detect the presence of liquid, as well as the type of liquid in which it is immersed the sensor.

The coupler comprises an input guide and two exit guides so that the amount of light coupled to each output guide varies when the coupling area is submerged in a liquid, said variation being such that the difference in power between both guides is level sensitive and not sensitive to variations in the power emitted by the emitter or to curvatures in the guide that connects to the reservoir that contains the liquid, getting an intensity sensor with self-reference, which allows its use for measurement of non-corrosive liquid level in deposits in general, such as fuel tanks of vehicles with low weight requirements, such as ultralight.

Background of the invention

In recent years they have developed multitude of optical sensors to measure both physical parameters as chemists because they have innumerable advantages over electrical sensors or mechanical sensors, such as immunity to electromagnetic interference in addition to easy handling, which allows them to be located in places that are both inaccessible and hostile

The use of sensors is also known fiber optic for level measurement in critical environments, such as fuel tanks, showing the suitable characteristics of the fiber optic for this type of measures by its inert and intrinsically safe character, between others.

Among the sensors currently used, it find the so-called intensity sensors in which the magnitude to be measured, in this case the level of a deposit, produce variations in the optical power that reaches the detector or receiver.

Within this type of sensors there are several subclassifications, according to the type of measurement as continuous or discrete, or according to the type of sensor as intrusive, is say, in contact with the liquid, or not intrusive.

In all these cases, the use of optical fiber is common, whose main parts have been represented in Figure 1, which is constituted by at least one filament that usually comprises a central core, in English called " core ", and a cover, in English called " cladding ", as concentric cylinders of different material, which allow the guidance of light by total reflection as a result of the difference in refractive indexes of both materials. The previous set is usually protected with an additional coating, called " coating " in English.

The central core of each filament can be of plastic or glass and has a high refractive index, said central core being surrounded by a cover, of a core-like material, which has an index of refraction slightly lower than the core.

Thus, when the light reaches a boundary surface with a lower refractive index, it is reflected almost in its whole. This reflection is greater the greater the difference of refractive indexes of both materials and the higher the angle of incidence in the interface, having in such cases a phenomenon of total internal reflection that are governed by the laws of geometric optics, and more specifically by the law of reflection, or principle of total internal reflection, and the law of Snell

The operation of the optical fiber is based on transmit a beam of light through the fiber core, so that said beam does not cross the nucleus, but is reflected and followed propagating through the nucleus, which is achieved whenever the index core refraction exceeds the refractive index of the cover or lining, as long as the angle of incidence exceed a limit angle.

In the field of optical sensing devices, plastic optical fibers, referred to in English as " Plastic Optical Fibers (POFs " ) , have been experiencing great growth and growth compared to optical glass fibers, because they have advantages such as greater ease of handling, greater flexibility, greater robustness and a noticeably lower cost. The majority of this type of sensors is based on the intensity of optical power at reception.

Currently, with the aim of increasing the Sensitivity of plastic optical fiber sensors against a certain parameter curves are made in the fiber, which produces losses due to curvature in the optical power of reception, which have been extensively studied and investigated by various authors In addition, different types of radii of curvature have been studied for the evaluation of said losses of power.

If besides introducing curvatures in the fiber the cover is removed and the fiber optic core is polished, eliminating a part of it, the losses due to curvatures increase, making the sensor more sensitive with respect to parameter a to size.

The inconvenience of this type of sensors are that they have poor robustness, and therefore reliability, since the sensitive parameter, which is the coupling constant of the sensor does not remain invariant before power fluctuations to the input or unwanted power losses in the link, which leads to false estimates of the type of liquid surrounding the sensor.

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Description of the invention

The present invention relates to a sensor fiber optic, preferably plastic, self-referenced for liquid detection and / or liquid level measurement, in the latter case once the sensor has been submerged in said liquid.

The sensor comprises a configured coupler to vary or modify a coupling constant K based on a refractive index of the liquid in which the sensor, thus obtaining a variation of said coupling constant K according to the type of liquid in question.

Against previous proposed schemes, the use of a coupler with sensor curvature losses of intensity for detection of level or presence of liquids that the The proposed invention has the advantage of being self-referenced. This technique gives the sensor greater robustness since the sensitive parameter to be measured, this is the coupling constant K of the sensor remains invariant before power fluctuations to the unwanted input or loss of power in the link, avoiding obtaining false estimates on the type of liquid that surrounds the sensor, presenting dependence only according to the type of liquid present in the outside environment.

Thus, different liquids with different indexes of refraction can be discriminated with the sensor of the invention.

In addition the measurement of the power intensity optics are subject to unforeseen variations due to degradation of components, different environmental conditions or fluctuations in the optical sources, for which the sensor of the invention it has a correction by means of self-reference, which endows of greater robustness to the link between the transmitter, the sensor and the receiver, due to the use of said means of self-reference

In the sensor of the invention the constant of coupling K presents variations of up to 4% with respect to its value nominal, which allows to discriminate the type of liquid that surrounds the sensor in each case as a result of said variation.

When generating a curvature over a given section of a multimode fiber, for example plastic, the modes of higher order circulating through it are refracted, due to that the angle of incidence increases at the interface or intercalates between the core and the lining or cover, producing a increase in power losses at reception.

These curvature losses occur for a critical radius of curvature R_ {C}, whose value can be estimated according to the following equation:

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one

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where l is the wavelength of operation, n_ {co} is the core refractive index, also referenced as n_ {1}, and n_ {cl} is the refractive index of fiber optic cover, also referenced as n_ {2}.

From the previous expression it follows that said critical radius of curvature R_ {C} is directly proportional to the operating wavelength l, and inversely proportional to the relative difference between core refractive indices n_ {co} and cover n_ {cl}.

Losses produced in a multimode fiber by curvature they will be less than less value of the radius of curvature Critical R_ {C}. Transient losses at (dB), at multimode fiber input for a radius of curvature R Applied to the fiber can be expressed by:

2

not being the radius value of curvature R too close to the radius of critical curvature R_ {C}. The study of curvature losses in multimode fibers can be based on the study of ray tracing inside the fiber in conditions of curvature.

If the core refractive index n_ {co} is approximately equal to the refractive index of the roof n_ {cl}, then the coefficient of Fresnel T transmission for refracted rays in a fiber Optics is independent of the polarization of light. The coefficient of Fresnel T transmission indicates losses due to radiation of the rays and has the following expression:

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where q is the angle of incidence of a certain ray from the normal to the fiber core and q_ {C} the critical angle defined as q_ {C} = sin ^ {1} (n_ {cl} / n_ {co}), such that if the angle of incidence that had approximately the same value as the angle critical q_ {C}, said ray will be refracted from the core losing power.

From the previous expression it follows that how much the greater the value of the critical angle q_ {C}, the greater the value of the Fresnel T transmission coefficient obtained. In that case, the refracted power P γ out of the fiber optics, that is the value of radiation losses, is given by the following expression:

4

where P_ {i} is the optical power at the entrance of the curvature created in the fiber.

Thus, in an arrangement in which the coupler comprises a port or input fiber p_ {1}, a port or fiber direct output p_ {2} and a coupled output port or fiber p_ {4}, the value of the coupling constant K, as a percentage, It is determined by the following expression:

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Thus, the losses of radiated optical power in a branch of the coupler depend on the refractive index of the liquid surrounding the sensor, which is detected by the variation of coupling constant K.

As the light propagates through the fiber, the optical power in the coupler is attenuated at the output due both to the curvature produced and by the polishing performed on the cover and part of the core of said fiber, so that you have a greater amount of optical power that is transferred and guided in The fiber cover. If the sensor is inserted into a liquid, the losses will vary because the refractive index of the external environment changes according to the type of liquid existing in the environment of the coupler modifying the power radiated to the medium exterior and power again guided by the fiber core optics through the Fresnel T transmission coefficient, which it implies a variation of the power coupled by the port of coupled output P4.

Thus, when there are differences in the index of refraction of the external environment, that is when they have different liquids, make the transmission coefficient of Fresnel T is modified and, therefore, the amount of light coupled by the output port coupled P4, whereby produces a variation of the coupling constant K of the sensor.

For example, if the outside medium is oil, which It has an index of refraction n_ {oil} equal to 1.46, its index of refraction becomes comparable with that of the fiber cover n_ {cl} with a value equal to 1,417 and all modes guided on the fiber cover are refracted outward, increasing the Fresnel T transmission coefficient and losing power through the coupled output port P4 with respect to the power at the direct output port p_ {2}, causing a increase in coupling constant K, ie the coupler is made more directive

According to the refractive index of the medium exterior decreases, for example n_ {water} is equal to 1.33 and n_ {air} is equal to 1, the rays guided by the cover of the fiber are increasingly reflected back towards the core, and by therefore the Fresnel T transmission coefficient decreases, increasing the power received at the coupled output port P_ {4} with respect to the power received at the output port direct p_ {2}, so the coupler becomes less directive and the coupling constant K decreases.

This variation of the coupling constant K of coupler is measured and evaluated producing the detection of liquid in which the sensor is immersed.

Description of the drawings

To complement the description that is being performing and in order to help a better understanding of the characteristics of the invention, according to a preferred example of practical realization of it, is accompanied as an integral part of said description, a set of drawings where with character Illustrative and not limiting, the following has been represented:

Figure 1.- Shows two schematic views, according to a cross section and a longitudinal section, of an optical fiber, in which its main ones can be appreciated constituent parts.

Figure 2.- Shows a representation schematic of the sensor coupling area, where they can appreciate the input port p_ {1}, the output port direct p_ {2} and the output port coupled p_ {4}.

Figure 3.- Shows a representation schematic of the sensor of the invention, in which the coupling area It is immersed in a liquid.

Figure 4.- Shows a longitudinal section. of the coupling and sensing zone of the sensor of the invention, where has schematically represented the propagation of light by reflection in the nucleus of the optical fiber.

Preferred Embodiment of the Invention

In view of the figures outlined you can be observed as in one of the possible embodiments of the invention  refers to a self-referenced plastic fiber optic sensor for liquid detection and / or liquid level measurement that it comprises a coupler configured to vary a constant of coupling K as a function of a refractive index n of the liquid in the that the sensor is immersed, thus obtaining a variation of said coupling constant K according to the type of liquid from which try.

The sensor is set so that the constant of coupling K remain invariant before power fluctuations at sensor input or unwanted power losses, presenting dependence only according to the type of liquid present in the outside environment.

The coupling constant K shows variations of up to 4% with respect to its nominal value, which allows discriminate the type of liquid surrounding the sensor in each case as consequence of said variation.

As can be seen in figures 2 and 4, the coupler has a critical radius of curvature R_ {C} which is directly proportional to the operating wavelength l, e inversely proportional to the relative difference between refractive indexes of core n_ {1} and cover n_ {2}. The coupler comprises an input port p_ {1}, a port of direct output p_ {2} and a coupled output port p_ {4}, of so that the value of the coupling constant K, in percent, It is determined by the following expression:

6

Thus, the losses of radiated optical power in a branch of the coupler depend on the refractive index of the liquid surrounding the sensor, which is detected by the variation of coupling constant K.

According to an embodiment example Preferred of the invention, the sensor comprises optical fiber of plastic methacrylate (PMMA) index jump, whose core it has a diameter of 980 µm and the cover has a diameter of 10 µm. The core refractive index n_ {co} is equal to 1,492 and the cover refractive index n_ {cl} is equal to 1,417.

According to the preferred embodiment, It has a 2x2 coupler comprising two fiber optic sections on part of which the protective cover has been removed and a part of its core has been polished by mechanical procedures

The nuclei of both sections have joined yes by means of a special glue obtaining the coupler and its corresponding sensing zone that has approximately 3 cm of length.

The sensor of the invention comprises a source of light, which may consist of a wavelength laser diode 650 nm and a power meter with two simultaneous inputs.

The sensor is excited with an optical signal from the light source, which is typically an LED or a semiconductor laser.

The device comprises an emitting system, configured to generate the optical signal, and a receiving system, configured to convert the optical signal back to the domain electronic. Additionally in the receiver can be incorporated systems for the graphic visualization of the liquid level in a display or an LED.

The advantage of the sensor of the invention is that in the case of unwanted power losses in any link section emitter-sensor-receiver these would be transparent to the extent, since the hypothetical decrease in power would affect both the direct output port equally P_ {2} as to the coupled output port P_ {4}, the relationship between the two, that is to say the constant of coupling K, invariant. For this same reason it is not necessary to perform modulation in the source to eliminate the effects of ambient light, which It allows to reduce the cost of the sensor.

The coupling constant K of the sensor is invariant to possible losses introduced in the link, with variations of the order of ΔK = \0.005, between different loss configurations and presents stability for a plurality of measurements in each configuration, with a variation of order of ΔK = pm0.002.

Curvature losses have been simulated on the link, with a radius of curvature equal to 10 mm. Loss Total power obtained was 4.2 dB, remaining identical in each case the coupling constant.

It has also been experienced to obtain the coupling constant K depending on the type of liquid on which the sensor is immersed for a certain number of samples and a variable radius of curvature. The results show that the coupling constant K of the sensor coupler It varies depending on the type of material in the outside environment. For any value of the radius of curvature an increase of ΔK = 1%, in the case of outside air or water, and a ΔK increase = 3.5% in the case of outside air or oil. This difference can be perfectly discriminated in the receiver through a control electronics.

It can also be seen that as the radius of curvature, the coupler becomes more directive, being able to establish discrimination of the type of liquid in the medium external depending on the value of the coupling constant K obtained.

In view of this description and game of figures, the expert in the field may understand that the Embodiments of the invention that have been described may be combined in multiple ways within the object of the invention. The invention has been described according to some embodiments. Preferred, but for the person skilled in the art it will be clear that multiple variations can be introduced in said preferred embodiments without exceeding the purpose of the claimed invention.

Claims (7)

1. Self-referenced plastic fiber optic sensor for the detection of liquid and / or liquid level measurement, comprising a transmitter system configured to generate an optical signal, and a power meter receiver system comprising at least two simultaneous inputs, characterized in that it comprises a coupler comprising two fiber optic sections that are arranged in opposition to a critical radius of curvature R C, the protective cover being removed and the core partially removed in a coupling area between said two fiber sections optics, which are separated by means with a refractive index n, such that a coupling constant K of the sensor depends on said refractive index n. 2. Self-referenced plastic fiber optic sensor for the detection of liquid and / or liquid level measurement, according to claim 1, characterized in that the coupling constant K remains invariant before power fluctuations at the sensor input. 3. Self-referenced plastic fiber optic sensor for liquid detection and / or liquid level measurement, according to any of claims 1 and 2, characterized in that the coupler comprises an input port p1 and an output port direct p2, which are in a first fiber optic segment, comprising a coupled output port p4, which is in the second fiber optic segment, said coupled output port p4 being located in correspondence with the direct output port p_ {2}. 4. Self-referenced plastic fiber optic sensor for liquid detection and / or liquid level measurement, according to any one of the preceding claims, characterized in that the two fiber optic sections are separated by a liquid in which the sensor, having a variation of said coupling constant K according to the type of liquid in question. 5. Self-referenced plastic fiber optic sensor for the detection of liquid and / or liquid level measurement, according to any of the preceding claims, characterized in that the fiber optic sections comprise a core having a core refractive index n_ { 1} slightly higher than a refractive index of a cover n2 of said optical fibers. 6. Self-referenced plastic fiber optic sensor for the detection of liquid and / or liquid level measurement, according to any of the preceding claims, characterized in that the critical radius of curvature R C is directly proportional to a wavelength of operation 1, and inversely proportional to the relative difference between the refractive index of the core n_ {1} and the refractive index of the cover n_ {2}. 7. Self-referenced plastic fiber optic sensor for liquid detection and / or liquid level measurement, according to any of the preceding claims, characterized in that the coupling area is approximately 3 cm in length.