WO2011027016A1

WO2011027016A1 - Coated fibre optic sensors based on near cutoff lossy mode resonance

Info

Publication number: WO2011027016A1
Application number: PCT/ES2010/070574
Authority: WO
Inventors: Ignacio DEL VILLAR FERNÁNDEZ; Carlos RUIZ ZAMARREÑO; Miguel HERNÁEZ SÁENZ DE ZÁITIGUI; Francisco Javier ARREGUI SAN MARTÍN; Ignacio Raúl MATÍAS MAESTRO
Original assignee: Universidad Pública de Navarra
Priority date: 2009-09-07
Filing date: 2010-08-31
Publication date: 2011-03-10
Also published as: ES2363285A1; ES2363285B2

Abstract

The invention relates to a coated fibre optic sensor device based on near cutoff lossy mode resonance (LMR) that uses a thin film of absorbent material placed on the core of the fibre optic. The device is advantageous in that it eliminates the optical prism from the Kretschmann configuration in favour of a portable, small fibre optic design and allows both measurements to be taken remotely and multiplexing. The invention is also advantageous in that it does not require the use of polarised light in TM mode, which is a requirement of surface plasmon resonance sensors. It is possible to adjust the sensitivity of the device as a function of the width of the thin film and to generate multiple resonances in the electromagnetic spectrum, thereby allowing the use thereof as a fibre optic.

Description

COATED OPTICAL FIBER SENSORS BASED ON RESONANCE ORIGINATED BY MODES WITH LOSSES NEAR TO THE CONDITION OF

CUT D E S C R I P C I Ó N

FIELD OF THE INVENTION

The following invention relates to fiber optic sensors coated with a thin film of absorbent material. The detection technique is based on the generation of one or several resonances originated by modes with losses close to the cut-off condition (the lossy mode resonance - MRL).

BACKGROUND OF THE INVENTION

During the last decades, work has been carried out on the study of waveguides covered with thin films of absorbent material of less than the wavelength of light; that is, with a non-null imaginary part of its optical permittivity (Batchman and Me Wright, IEEE J. Quantum Electron., 18: 782-788, 1982; Yang and Sambles, J. Mod. Opt., 44: 1 155-1 163, 1997; RC Jorgenson and SS Yee, Sens. &

Actuators B 12: 213, 1993). Depending on the properties of the coatings that form the thin films, three different cases are distinguished. The first case occurs when the real part of the permittivity of the absorbent material is negative and its absolute value is greater than the absolute value of its imaginary part and greater than the real part of the permittivity of the dielectric surrounding the thin film. In this case, there is a coupling between the light that propagates through the waveguide and a surface plasmon called surface plasmon resonance (SPR). The second case occurs when the real part of the permittivity of the material is positive and its absolute value greater than that of its imaginary part and also greater than the real part of the permittivity of the dielectric that surrounds the thin film. In this case there is a coupling between the light that propagates through the waveguide and what some authors consider as long-range guided modes while others call them as loss modes. These modes give rise to a resonance consisting of an attenuation band. which originates in the spectrum in transmission by light coupling to a mode with the electrical component perpendicular to the direction of propagation of light (electrical transverse mode or TE), to a mode with the magnetic component perpendicular to the direction of propagation of the light (magnetic transverse mode or TM) or the combination between them, both with losses and close to the cutting condition. Therefore, resonance is called resonance caused by loss modes (in English lossy mode resonace - MRLs) close to the cut-off condition. The cutting condition marks the point from which a mode becomes guided in the coating and is fundamentally controlled by two variables: the wavelength of the light that is propagated by the waveguide and the width of the coating. If, for example, the wavelength remains fixed, there is a width from which a mode is guided in the coating. Therefore, for widths close to this value it can be said that the modes are close to the cutting condition and there is a transfer of energy between the waveguide and the coating that causes the appearance of resonance.

The third case occurs when the real part of the material that forms the thin coating approaches zero while its imaginary part is elevated. This case is known as long-range exciton polariton.

To date, the phenomenon that has given rise to the greatest number of applications and that has focused the interest of the scientific community are SPR-based devices (J. Homola, SS. Yee, G. Gauglitz. Sens. Actuators B, 54, 3-15, 1999). Traditionally, the Kretschmann configuration (Kretschmann and Raether, Z. Naturforsch. Teil A 23: 2315-2136, 1968) has been used for the measurement of SPR. In this configuration, a thin layer of a highly reflective metal (for example, gold or silver) is deposited on the base of a prism (US Pat. No. 7015471; US Pat. No. 6738141). The metal surface comes into contact with the sample and, using a monochromatic polarized light in TM mode that crosses the prism and the measurement of the intensity of reflected light as a function of the angle of incidence, the SPR reflection spectrum is calculated of the sample. The angle with minimum reflection intensity is the resonance angle at which the maximum coupling between the incident light and the surface plasmon waves originates. This angle, together with the resonance spectrum and the intensity at the angle at which the minimum reflection intensity is obtained, It can be used to characterize or determine the sample in contact with the metal surface.

Various types of SPR-based sensors have been described in the literature, sensitive to changes in both refractive index and sample thickness. These systems, together with the appropriate sensitive chemical coatings have led to the development of a wide variety of SPR-based chemical sensors (such as C. Nylander, B. Liedberg and T. Lindt, Sens. & Actuators, 4: 299, 1983 use angular scanning; K. Matsubara, S. Kawata and S, Minami, Appl. Opt, 27: 1 160, 1988 use a multiple linear detector; LM Zhang and D. Uttamchandani, Electron. Lett., 24: 1469, 1988 make a wavelength scan; Liedberg et al., Sens. & Actuators, 4: 299, 1983; Daniels et al., Sens. & Actuators, 15: 1 1, 1988; Jorgenson et al., LEEE / Engineering Medicine and Biology Society Proceedings, 12: 440, 1990 manufacture sensors for immunological assays).

Although the Kretschmann configuration for chemical sensors based on SPR offers considerable sensitivity, it has drawbacks derived from its geometric configuration that have greatly restricted its application. As a clear example of limitation, it is worth mentioning the need to include a relatively large, expensive and inappropriate prism for remote applications. In addition, such devices require precise instruments or complicated processing of the data obtained; This makes them non-portable devices and difficult to implement in industrial environments. Likewise, they mostly use a monochromatic light source due to configuration limitations (such as the presence of the prism), and they need the incident light to sweep across a wide range of incident angles because they use, mostly , as a measurement parameter the angle at which the maximum attenuation of the reflection intensity occurs.

Based on the needs outlined above, new types of SPR-based sensors have been developed on fiber optics that provide reasonable sensitivity by improving the limitations of the systems with the Kretschmann configuration set forth above, such as the need to use a prism or use of a monochromatic light source and enabling remote sensing applications. These new devices use a broad spectrum incident light as a source of excitation of the sensor device, which is composed of an optical fiber with a part of the exposed core and a thin layer of a highly reflective metal (commonly gold or silver) deposited on the core, which promotes the effect of superficial plasmons. In this way, by measuring the intensity of the light for each wavelength at the exit of the fiber after crossing the sensitive area we can determine or characterize the sample in contact (US Pat. No. 5327225;

Esp. Pat. No. 21 14175). As before in the Kretschmann configuration, this system also allows the use of layers or coatings of particles adhered to the metal layer and sensitive to different chemical substances or compounds depending on the application in both the transmission configuration (A. Ten , MV Andrés and JL Cruz, Sens. & Actuators B, 73:95, 2001) as in reflection (R. Slavik, J. Homola, Z. Manikova & J. Ctyroky, Sens. & Actuators B, 51: 31 1, 1998). On the other hand, the applications that are derived from the appearance of several resonances caused by surface plasmons are interesting, which allows the generation of filters in multiple wavelengths suitable for optical communications in addition to allowing several reference points to be available for use in applications with sensors (Monzón-Hernández et al. App Opt, 43: 1216-1220, 2004). However, it is worth highlighting the problems involved in the manufacture of a device based on the generation of multiple SPRs suitable for this phenomenon to occur due to the difficulty of substrate design. In other words, the phenomenon of multiple resonance is due to the modification of the geometry of the fiber optic device. In addition, SPR sensors require polarized light for resonance to occur, which greatly complicates measuring devices. OBJECT OF THE INVENTION

The object of the present invention is to present a new type of fiber optic devices that overcome the limitations of SPR sensors based on optical fiber, where resonance is only produced in the event that the polarization of the incident light is TM and where Multiple resonances can only be achieved by modifying the geometry of the substrate on which the coating is deposited. For this, the invention proposes a fiber optic sensor based on the phenomenon of resonance by modes with losses close to the cutting condition, which comprises: - an optical fiber with a waveguide core and at least one film of absorbent material located in a sensitive area in direct contact with at least a part of the fiber waveguide core

- a source of broad-spectrum electromagnetic radiation whose output is applicable to one of the ends of the optical waveguide core so that the radiation propagates through the fiber and out of the optical fiber; Y

- a detector device for measuring the radiation that exits through the fiber

The film is formed by an absorbent material in which the real part of its permittivity is positive and its absolute value is greater than the absolute value of its imaginary part and greater than the real part of the permittivity of the dielectric surrounding the thin film and capable of producing at least one mode close to the cutting condition.

The choice of a thin film with adequate absorption in the range of the electromagnetic spectrum in which it is going to work is essential. An additional advantage of the device of the invention is that the range of deposited materials capable of producing resonance goes from being practically limited to the use of metals (especially gold and silver) in the case of SPR, to the use of metals, semiconductors and any other material with absorbent properties such as polymers.

Some suitable materials are polymers deposited with the monolayer electrostatic self-assembling technique or with the Langmuir Blodgett technique which, due to the fact that they are constituted by multiple layers of molecular size linked together, acquire a roughness that translates into a part value Imagination of non-zero permittivity in the aforementioned optical range. Another group of materials are transparent conductive metal oxides, which after an adequate parameterization (thickness, tempered, doped, etc.), also meet the above conditions for polymers in the ultraviolet, visible and, depending on the parameterization, in the infrared The deposition process of the thin film must take into account that the deposition is carried out on a substrate that is the fiber itself, being necessary the adequate adaptation of the deposition process used to said substrate. The sensor may be based on direct transmission, where the radiation source is applicable to the input end of the fiber optic core so that the radiation is propagated through the fiber by total internal reflection from the input end to the end. of exit, or based on reflection, where the propagation mechanism is also by total internal reflection, but when the light reaches the other end, a specular layer in contact with the end of the waveguide core, causes the light to reflect The original end. In both configurations (transmission and reflection) at least one additional layer of particles sensitive specifically to the species to be detected can be incorporated into the absorbent material film. The sensitive area may be in the center of the guide or at the end in reflection.

The preferred compound for the thin film is (without the invention limiting this) a transparent conductive metal oxide of an element chosen from the elements zinc, indium, tin, iridium, cadmium, yttrium, scandium and nickel, or alloys, doped or binary, ternary or quaternary combinations of the oxides of the above elements among themselves, with other elements such as fluorine, copper, gallium, magnesium, calcium, strontium or aluminum or combinations of the latter among them. Also suitable are polymers obtained from the elements poly (vinyl pyrrolidone), poly (vinyl alcohol), polyacrylamide, polyacrylic acid, polystyrene sulfate, polyaniline sulfate, poly (thiophene-3-acetic acid), polyaniline, polypyrrole, poly (3-hexyl thiophene) ), poly (3,4-ethylenedioxythiophene) and poly (dimethyl ammonium dichloride). The film can be of a thickness adapted to generate multiple resonances.

In addition, it may be advantageous for the sensor to incorporate another optical fiber capable of generating an output reference signal. The source of electromagnetic radiation can consist of an LED, an array of LEDS, a semiconductor laser or a halogen lamp. The light detection system will be adapted to detect the wavelengths produced by the chosen source.

The detector device preferably comprises a spectrometer. In the mode of reflection, in addition, the specular layer comprises a material of high reflectivity, preferably gold, silver, chromium, aluminum or platinum.

DESCRIPTION OF THE DRAWINGS For the best understanding of what is described herein, some drawings are attached in which, by way of example only, a preferred embodiment of the invention is presented.

Figure 1 a - Schematic representation of the operation of the transmission configuration when a broad-spectrum white light source is used as the emitting device.

Figure 1 b - Schematic representation of the operation of the configuration in reflection when a broad-spectrum white light source is used as the emitting device.

Figure 2 - Detailed representation of the sensitive part of the resonance-based fiber optic device caused by modes with losses close to the cut condition in the online transmission based configuration shown in Figure 1 a.

Figure 3a - Detailed representation of the sensitive part of the resonance-based fiber optic device caused by modes with losses close to the cut condition in the reflection-based configuration shown in Figure 1 b.

Figure 3b - Detailed representation of a second embodiment of the reflection-based configuration, where the sensitive part of the resonance-based fiber optic device originated by modes with losses close to the cutting condition is at the end of the fiber.

Figure 4a - Representation of the cross-sectional and longitudinal section to the direction of light propagation by the sensor.

Figure 4b - Representation of the cross-sectional and longitudinal section to the direction of light propagation by the sensor, in which an additional sensitive layer is included.

Figure 5 - Spectral responses in sensor absorption for different refractive indices on the thin film and for a small width thereof.

Figure 6 - Wavelength variation of the resonance peak for different refractive indices on the thin film and for a small width thereof. Figure 7 - Spectral responses in sensor transmission for two different refractive indices on the thin film and for a large width thereof.

Figure 8 - Simulation of the spectral responses in sensor transmission for two different refractive indices on the thin film and for a large width thereof.

DETAILED DESCRIPTION OF THE INVENTION The fiber optic sensor device coated with absorbent material (the real part of its permittivity is positive and with absolute value greater than that of its imaginary part and also greater than the real part of the permittivity of the dielectric surrounding the thin film) is suitable for the generation of one or several MRLs originated by modes with losses close to the cutting condition. Said device combines the advantages of the elimination of the optical prism of the Kretschmann configuration mentioned above in favor of a design in fiber optic, portable, of small size and with the possibility of performing remote measurements and multiplexing, together with the advantage of using a material absorbent that, in conditions where the width of the thin film of absorbent material allows for a TE mode with losses close to the cutting condition, a TM mode with losses close to the cutting condition, or the combination of both, will be generated an MRL in the spectrum in transmission or in reflection whose displacement in the ultraviolet, visible or infrared range will allow measurements of chemical, biomedical or environmental parameters. On the other hand, the sensor of the invention can be used in the same applications as SPR sensors. The device is highly sensitive to changes in the surrounding environment, so simply by adding, on the absorbent material, a layer sensitive to the parameter to be detected, chemical, environmental, biochemical sensors, etc. can be developed. Without said sensitive layer, sensor devices can also be developed based on the variation of the index of the external medium (refractometers), based on the variation of the properties of the film, or even optical filters. In addition, the fact that the MRL is produced for angles that approximate the propagation of light by the fiber is very suitable, because after traveling a distance from the end of the fiber through which light is introduced, it tends to adopt these angles for the most part.

For the manufacture of these devices a thin film of absorbent material is deposited, which makes it possible to adjust its manufacturing parameters so that the MRL is in the appropriate range of electromagnetic spectrum wavelengths. The combinations are very varied. If a single MRL is generated, it can be placed in one of the infrared windows, leaving other visible wavelengths free, for example to perform complementary measures such as absorption or fluorescence. The width of the film will determine the sensitivity of this unique MRL. It should be noted that as the thickness increases, the resonance sensitivity is also modified, which decreases with increasing thickness. In other words, the benefit of generating multiple resonances by increasing the thickness occurs to the detriment of a decrease in sensitivity. Another possibility is to increase the thickness of the thin film of absorbent material to generate several MRLs distributed over the ultraviolet, visible and infrared range. Each of them will have a different sensitivity, which will allow multiple measures to improve errors caused by interference and noise.

The manufacture of these devices as well as their operation is not obvious either, as this will require the adaptation of the different existing deposition techniques so that they are appropriate for the proper deposition of the coatings regardless of the shape of the substrate used, fiber optics instead of the glass prism, on its different surfaces and depending on the configuration of the device.

As regards the MRL itself, it generates an attenuation band in the electromagnetic spectrum. The central wavelength of the MRL will experience variations depending on the parameters that are detected, reaching very large variations of up to 470 nm, as seen in Fig. 6.

In summary, it is possible to improve the sensitivity of the sensors, the tuning of the MRL in the ultraviolet, the visible or the infrared is allowed through the manufacturing parameters of the coating of absorbent material, the need to introduce polarized light is eliminated, it is enhanced The effect of the MRL thanks to the confinement that the optical fiber exerts on the light beam, and what is still more novel and advantageous, can be controlled with the width of the film The sensitivity of the MRL and the appearance of several MRLs in the electromagnetic spectrum are thin, as shown in Fig. 7. As the width increases, more modes are guided in the absorbent material. Therefore, at each wavelength where it is fulfilled that the mode is located near the cut-off condition, the MRL will occur. In the case of Fig. 7, a width of about 440 nm is available, which causes the appearance of 4 MRLs in the spectrum. Each of them will have a different behavior, which implies different data for the monitoring of parameters.

This device uses, to couple light to the optical fiber (3), a "broad spectrum" light source (1) with multiple wavelengths, where "broad spectrum" means a minimum of two wavelengths although a wide enough range to cover the resonance spectrum of the sample, such as a white light source or black body radiation. The range of angles will be defined by the numerical aperture of the optical fiber.

The optical fibers that can be used for the present invention include all commercial ones that allow the transmission of light by internal total reflection. Such fibers will generally be characterized by three parameters: fiber core material, numerical fiber aperture and fiber optic core diameter. The choice of one type or another of optical fiber will vary the position of the resonance peak (the wavelength at which the MRL takes place).

The embodiment of this device is presented in two preferred configurations as shown in FIG. 1 a and 1 b. These configurations are based on optical detection systems based on reflection and transmission respectively. In the reflection-based system (FIGS. 3 ^a and 3b) an additional element (12) located at one end of the optical fiber (1 1) is necessary, so that it reflects in reverse the light that propagates to through the fiber, which can consist of a layer of a highly reflective metal, such as gold, silver or chrome, adhered to the end of the fiber and thick enough to provide adequate reflection. It also requires a coupler (FIG. 1b, ref. 4).

The use of the present invention for the detection of the sample is carried out, by placing the sample (13) on the thin film of absorbent material (9) deposited on the optical fiber. This movie is deposited in different places depending on the configurations used (5) in FIGS 2, 3a and 3b. The process of deposition of the thin film (9) is carried out by removing the cover (8) adhered to the core of the optical fiber (6) and the subsequent deposition of the material itself in the exposed area of the core (7). Then, the thin film (9) is exposed to the sample (13) as shown in FIG. 4a thus allowing to determine the refractive index of the sample through the MRL (s) generated by combining the power coupling to the TE and TM modes close to the cut in the thin film. A variant of this consists in the deposition of an additional sensitive layer (14) that will act as a mediator between the sample (13) and the thin film (9) as shown in FIG. 4b

The removal of the fiber cover is carried out through the use of known techniques, such as the use of appropriate chemical agents or tools. Once the fiber optic core is exposed, the thin film is adhered by using known techniques. The characteristics of the absorbent material are that the imaginary part of its permittivity is non-zero. In addition, its real part will be positive and greater in absolute value than the permittivity of the surrounding dielectric (fiber and external medium) and its imaginary part. Tin doped indium oxide (ITO) meets this condition in ultraviolet, visible and infrared (200-1500nm). In addition, we can use other transparent conductive metal oxides that allow us to place ourselves in other areas of the spectrum. Depending on the thickness of the film, one (FIG. 5) or several MRLs (FIG. 7) will appear.

In turn, a single optical fiber can contain one or more thin films of the same or different types, with the same or with different geometries and located along or at the end of it. In addition, any core portion exposed in the optical fiber can be used to place the thin film thereon, although one of the preferred embodiments consists in removing the entire circumference of the shell surrounding the core and depositing the thin film symmetrically and with uniform thickness over the exposed core area.

A detection system (2) suitable for the present invention will consist of any device capable of detecting the intensity of all or a part of the wavelengths that exit through the optical fiber. As an example of Detector device can be used a spectrometer capable of measuring the intensity of light as a function of wavelength.

In the configurations of the present invention described, the optical power injected by the emitting device (1) at one end of the optical fiber (3) travels through it through the sensitive area and reaching the detector device (2) directly in the case of the transmission configuration, FIG. 1 a, or once reflected by the specular layer (12) in the case of the configuration in reflection, FIG. 1 B. This optical power that reaches the detector device is a function of the refractive index of the external medium in contact with the thin film (9), by which the mode close to the cutting condition that is guided by the fiber (6) is guided. ) to be guided in the thin film (9).

In this way, by measuring the spectrum at the fiber exit we can determine the refractive index of the sample in contact with the sensitive area of the fiber optic sensor.

The sensor device can also incorporate a dynamic self-calibration signal by bifurcation of the optical fiber that comes from the light source so that we have a reference signal of the light that passes through the optical fiber without being affected by the sensitive area .

In general, the sensor device can be used in multiple applications: refractometers, optical filters, and in the chemical or biochemical field, for the detection of species that are present in liquid or gas solutions. For applications in biosensors or selective detection, the thin film of absorbent material can be coated with one or more additional layers that include immobilized compounds, specifically sensitive to the species to be detected (for example enzymes and coenzymes, antigens and antibodies, etc.). Moreover, most biological reactions occur in the ultraviolet range, so the possibility of obtaining resonances in this range will allow sensors to adapt to these applications. The refractive index and the thickness of the additional layer must also be suitable for the application. This layer also provides protection against external physical and chemical agents that can damage or affect the behavior of the sensor.

DESCRIPTION OF A PREFERRED EMBODIMENT This embodiment is based on an optical transmission system in line transmission like the one shown in FIG. 1 a.

The light source used (1) corresponds to a DH-2000-H halogen light lamp (Avantes Inc.), the optical fiber used corresponds to a silica optical fiber with polymeric cover and buffer of diameters 200/225/500 μηι for the core, cover and buffer respectively, and numerical aperture 0.39 (Thorlabs Inc.). The buffer was removed by using the appropriate tools while the cover was removed by chemical procedures for several fibers with lengths of 1 cm, 2cm, 4 cm and 7cm. Once the fiber core was exposed, the dip-coating technique was used, which will allow us a homogeneous deposition of an 85 nm film of transparent conductive metal oxide (ITO on the optical fiber), resulting in the sensitive area that is represented in FIG. 2. This process was carried out using a solution of indium (In) and tin (Sn) ions in relation to 90:10 to which a surfactant element was added to improve adhesion and finally subjecting the sensor to a curing process or Annealing at 500 ⁵ C to improve the characteristics and homogeneity of the transparent conductive film. Finally, the ends of the sensor are attached to two fiber optic hoses.

The fiber optic output was connected to a NIR-512 spectrometer (Oceanoptics Inc.) with a detection range between 850 nm - 1700 nm and a spectral resolution of less than 5 nm using an SMA connection and connected in turn to a computer for the acquisition of the spectra.

Five different solutions of glycerin diluted in deionized water in different concentrations were used with refractive indices 1,378, 1,400, 1,424, 1,436 and 1,446 respectively, which correspond to glycerin concentrations in water of 45 %, 60%, 75%, 85% and 100%. An increase in the resonance wavelength is observed with the refractive index as it appears in FIG. 5 and said resonance is in the range of minimum attenuation of the optical fiber around the first (850nm), second (1310 nm) and third (1550nm) communications windows. In FIG. 6, the correspondence between the wavelength of the resonance peak and the refractive index for each of the solutions is presented, where a spectral variation of 470 nm is observed for the range of indices analyzed. This variation assumes a sensitivity of 1.74x10 ^"4 units of refractive index per nanometer. If you want to cover a range of indexes of higher refraction, the option is to use a wider width of ITO. In the case of 1 15 nm, indices between 1.321 (water) to 1.46 (glycerin) can be monitored. This has a dynamic range of 0.14 units of refractive index, which could be increased in case of measuring solutions of another substance.

In addition, in FIG. 7 shows the effect of depositing a film of absorbent material of width 440 nm. The increase in width causes a greater number of modes to be guided in the film, which results in a greater number of wavelengths at which the condition of proximity to the cut is fulfilled in a film-guided manner. An MRL is generated in each of these wavelengths. In the case shown in FIG. 7 shows up to 4 MRLs that for an external water medium have their central wavelength at wavelengths 310 nm, 471 nm, 726 nm and 1257 nm. To obtain these results, in addition to the NIR-512 spectrometer (Oceanoptics Inc.), another HR4000 spectrometer (Oceanoptics Inc.) is used, measuring between 200 and 1200 nm. As can be seen from the data obtained with water and with 75% glycerin solution in water, it is clear that each of the peaks is sensitive to the external environment and with a different sensitivity. Another application is the development of optical filters at different wavelengths. The above data are contrasted with the results of a theoretical analysis based on electromagnetic wave theory presented in FIG. 8.

In short, with these new devices based on MRLs on optical fiber, a qualitative leap is achieved with respect to those based on SPR. We have seen the possibility of working in a spectrum of wavelengths as wide as the one we have just seen (ultraviolet, visible and infrared), being able to obtain multiple MRLs independently of the substrate, with the implications for the detection of multiple parameters That this implies. This is related to the width of the thin film deposited on the fiber, such that the number of MRLs increases with the thickness. On the other hand the width also allows to control the sensitivity of the devices. In addition, the range of materials with which these types of sensors can be developed is very wide.

Here we have shown the application of the use of transparent conductive metal oxides, specifically ITO, although there are a large number of materials that can be used, such as a rough polymeric material deposited with the monolayer electrostatic self-assembling technique or the Langmuir Blodgett.

Although the device has been oriented to the field of sensors, and more specifically to chemical or biochemical applications, it can also be used as an optical sensor to detect the variation of any physical or chemical parameter that affects the optical properties of the external medium under control and you can even leave the field of sensors to be used as an optical filter of various wavelengths in optical communications.

Claims

one . Coated fiber optic sensor based on the phenomenon of resonance by modes with losses close to the cutting condition, comprising:

- an optical fiber (3) with a waveguide core (6) and at least one film of absorbent material (9) located in a sensitive area in direct contact with at least a part of the fiber waveguide core,

- a broad-spectrum electromagnetic radiation source (1) whose output is applicable to one of the ends of the optical waveguide core so that the radiation propagates through the fiber and out of the optical fiber and

- a detecting device (2) for measuring the radiation exiting through the fiber, characterized in that the film (9) in direct contact with at least a part of the fiber waveguide core is formed by an absorbent material in which the real part of its permittivity is positive and its absolute value is greater than the absolute value of its imaginary part and greater than the real part of the permittivity of the fiber (3) and capable of producing at least a mode close to The cutting condition.

2. Sensor according to claim 1 characterized in that the radiation source is applicable to the input end of the fiber optic core so that the radiation is propagated through the fiber by total internal reflection from the input end to the end of exit.

3. Sensor according to claim 1 characterized in that it comprises a reflecting end (1 1) defined by a specular layer (12) in contact with the end of the waveguide core.

4. Sensor according to claim 3 characterized in that the thin film of absorbent material is located at the reflective end of the optical fiber.

5. Sensor according to claims 3 or 4, characterized in that the specular layer comprises a material with high reflectivity.

6. Sensor according to claim 5, characterized in that the specular layer comprises gold, silver, chrome, aluminum or platinum.

7. Sensor according to any of the preceding claims characterized in that at least one additional layer (14) of particles sensitive specifically to the species to be detected is deposited on the thin film of absorbent material.

A sensor according to any of the preceding claims characterized in that the thin film comprises a transparent conductive metal oxide of an element chosen from the elements zinc, indium, tin, iridium, cadmium, yttrium, scandium and nickel, or alloys, doped or binary combinations , ternary or quaternary of the oxides of the previous elements among themselves, with other elements such as fluorine, copper, gallium, magnesium, calcium, strontium or aluminum or combinations of the latter among them.

9. Sensor according to any one of claims 1-7, characterized in that the thin film comprises a polymeric material deposited by the electrostatic self-assembling techniques monolayer or the Langmuir Blodgett.

10. Sensor according to claim 9 characterized in that the thin film comprises one of the compounds poly (vinyl pyrrolidone), poly (vinyl alcohol), polyacrylamide, polyacrylic acid, polystyrene sulfate, polyaniline sulfate, poly (thiophene-3-acetic acid), polyaniline, polypyrrole, poly (3-hexyl thiophene), poly (3,4-ethylenedioxythiophene) and poly (dimethyl ammonium dichloride) or combinations thereof.

eleven . Sensor according to any of the preceding claims characterized in that it incorporates another optical fiber capable of generating an output reference signal.

12. Fiber optic sensor according to any of the preceding claims characterized in that the source of electromagnetic radiation is one of the group consisting of one or more LEDs, a semiconductor laser or a halogen lamp and where the light detection system is adapted to detect the wavelengths produced by the chosen source.

13. Fiber optic sensor according to any of the preceding claims, characterized in that the detector device (2) comprises a spectrometer.

14. Fiber optic sensor according to any of the preceding claims characterized in that the film is of a thickness adapted to generate multiple resonances.

15. Use of a sensor according to any of claims 1-14 as a filter in optical communications.