M. Hernaez

Optical fiber refractometers based on lossy mode resonances supported by TiO 2 coatings

We obtain lossy mode resonances by the coupling of light from a multimode optical waveguide to a TiO(2)/PSS coating deposited with the layer-by-layer method. The resonances can be generated in a wide wavelength range from the ultraviolet to the infrared region of the optical spectrum. The transmission spectrum is monitored as a function of the number of bilayers deposited, and the experimental results agree with the theoretical predictions. Moreover, each of the resonances owns a particular sensitivity to the external refractive index. This permits us to use the sensor as a refractometer with multiple-wavelength monitorization.

Resonances in coated long period fiber gratings and cladding removed multimode optical fibers: a comparative study

Two optical fiber devices have been coated in parallel: a long period fiber grating (LPFG) and a cladding-removed multimode optical fiber (CRMOF). The progressive coating of the LPFG by means of the layer-by-layer electrostatic-self-assembly, permits to observe a resonance wavelength shift of the attenuation bands in the transmission spectrum. The cause of this wavelength shift is the reorganization of the cladding mode effective indices. The cause of this modal reorganization can be understood with the results observed in the CRMOF coated in parallel. A lossy-mode-resonance (LMR) is generated in the same wavelength range of the LPFG attenuation bands analyzed. Moreover, the thickness range where the wavelength shift of the LPFG attenuation bands occurs coincides exactly with the thickness range where the LMR can be visualized in the transmission spectrum. These phenomena are analyzed theoretically and corroborated experimentally. The advantages and disadvantages of both optical fiber devices are explained.

Optical fiber refractometers based on indium tin oxide coatings fabricated by sputtering.

This Letter presents the fabrication of optical fiber refractometers based on indium tin oxide (ITO) coatings deposited by sputtering with response in the visible region. ITO thin films have been sputtered by means of a rotating mechanism that enables the fabrication of smooth and homogeneous coatings onto the optical fiber core. The ITO coating acts as a resonance supporting layer. This permits us to couple light from the waveguide to the ITO-coating/external medium region at specific wavelength ranges. The device is sensitive to external medium refractive index, which allows its utilization as a refractometer. The sensitivity is dependent on the coating thickness, ranging from 523.21 to 1221 nm/refractive index unit in the explored sensors. The sensor development process is time effective compared to other techniques such as dip coating or layer-by-layer self-assembly, which is interesting in terms of mass production.

Lossy-mode resonance-based refractometers by means of indium oxide coatings fabricated onto optical fibers

In this work, lossy mode resonance (LMR) based optical fiber refractometers are fabricated by using a transparent coating (indium oxide), as the LMR supporting layer. The utilization of indium oxide coatings permits the fabrication of highly sensitive optical fiber refractometers and enables the tunability of the LMR by adjusting the fabrication parameters, such as the coating thickness. The detection technique is based on the wavelength shift of the LMR. In this work it has been studied the influence of the external refractive index, achieving a maximum sensitivity of 2.24e-4 refractive index units per nanometer. Moreover, by adequated parameterization, more than one LMR can be observed in the wavelength range analyzed in the experiments, which leads to a more accurate measurement of the refractive index.

LMR-based optical fiber refractometers based on transparent conducting and semiconducting oxide coatings: a comparative study

In this work, it is described the fabrication and characterization of optical fiber refractometers based on lossy-mode resonances (LMR) originated by deposition of different thin-film coatings around the optical fiber core. Two devices with different coating materials are compared: one coated with conducting tin doped indium oxide (ITO) coatings and the other one coated with semiconducting indium oxide. The response of these devices is characterized and compared as a function of the external refractive index. The sensitivity obtained for indium oxide based refractometers resulted 39% higher than that of ITO based ones when the resonance is located in the same spectral region. This behaviour is attributed to the spectral characteristics of indium oxide, which allow an earlier generation of the resonance thanks to its higher refractive index as well as permitting the accomplishment of LMR conditions in a wider spectral range. Moreover, these devices are an adequate platform for the development of a wide variety of sensors by the addition of the suitable layer onto the transparent oxide coating.

Optical Fiber Sensors Based on Lossy Mode Resonances

In the last decades, optical fiber sensors have played an important role in niche applications because of their advantages over electronic sensors. First of all, optical fiber makes possible the multiplexing of a large amount of sensor data over long distances. This feature allows placing the sensing devices at kilometers from the electronic systems used to process the information. In addition, optical fiber is made of dielectric materials. Consequently, optical fiber sensors are not affected by electromagnetic fields, what makes them suitable to be used in situations under high electromagnetic fields or radiation doses [1]. Furthermore, this technology can be also used in medical applications due to its biocompatibility and has acquired a great importance in the development of biomedical instrumentation. Other interesting advantages of optical fiber sensors are their small size or their wide temperature working range [2-5].

Optical fiber sensors based on indium tin oxide surface plasmon resonance supporting coatings.

In this work, surface plasmon resonance (SPR) based optical fiber sensors are proposed by using a transparent conductive coating, Indium Tin Oxide (ITO) in our case, as the SPR supporting layer. The utilization of these ITO coatings instead of using a thin film of gold or silver shifts the plasmon resonance band to the infra-red region and allows the tunability of the SPR wavelength by adjusting the film fabrication parameters. The spectral response of these novel devices is characterized, achieving a sensitivity of 0.00025 refractive index units per nanometer, which opens the door for a wide range of applications.

Optical fiber humidity sensor based on surface plasmon resonance in the infra-red region

Here, the fabrication and characterization of a novel optical fiber humidity sensor based on surface plasmon resonance (SPR) in the infra-red region is presented. Firstly, an indium tin oxide (ITO) coating is deposited onto a 200 μm core diameter optical fiber causing the shift of the SPR wavelength to the infra-red region. Then, the LbL method is used to deposit a polymeric coating onto the ITO layer. The variations in the external humidity originated changes in the thickness and refractive index of the polymeric coating and hence in the resonance.

High-sensitive lossy mode resonance-based optical fiber refractometers by means of sputtered indium oxide thin-films

Three different optical fiber refractometers based on lossy mode resonances (LMRs) have been fabricated by means of the deposition of indium oxide thin-films. The sensitivity of the devices as well as the full-width at half maximum (FWHM) has been characterized as a function of the surrounding medium refractive index (from 1.332 to 1.471. Obtained results revealed that thinner coating possess higher sensitivities. However, the FWHM is better for thicker coatings. As a general rule, the thicker the In2O3 coating the lower the sensitivity, but the better the FWHM. Thus, a compromise is required depending on the necessities of the application.

A comparative study between SMS interferometers and lossy mode resonace optical fiber devices for sensing applications

Optical fiber sensors are of great interest due to their intrinsic advantages over electronic sensors. In this work, the sensing characteristics of two different and novel optical fiber devices are compared, after simultaneously depositing a thin-film using the layer-by-layer assembly deposition process. The first one is an SMS structure, formed by splicing two single-mode fiber pigtails on both sides of a coreless multimode fiber segment. This structure induces an interferometric phenomenon that generates several attenuation and transmission bands along the spectrum. These bands are sensitive to variations in the surrounding refractive index, although this sensitivity has been enhanced by a TiO2/PSS thin-film. The other device is a 40 mm uncladded segment of a 200 μm-core multimode optical fiber. When coated by a TiO2/PSS thinfilm, part of the light transmitted into the uncladded core is coupled into the thin-film, generating a lossy mode resonance (LMR). The absorption peaks due to these phenomena red-shift as long as the thin-film thickness increases or the external RI becomes higher. The performance of these devices as refractometers and relative humidity sensors are tested. Results show that the LMR-based sensor is more sensitive in both situations, in spite of its lower sensitivity. Particularly, it presents a 7-fold sensitivity enhancement when measuring surrounding medium refractive index changes and a 10-fold sensitivity enhancement when measuring environmental relative humidity. To our knowledge, this is the first time that a comparative study between SMS and LMR sensors is performed.