Arthur M. B. Braga

A Magnetostrictive Composite-Fiber Bragg Grating Sensor

This paper presents a light and compact optical fiber Bragg Grating sensor for DC and AC magnetic field measurements. The fiber is coated by a thick layer of a magnetostrictive composite consisting of particles of Terfenol-D dispersed in a polymeric matrix. Among the different compositions for the coating that were tested, the best magnetostrictive response was obtained using an epoxy resin as binder and a 30% volume fraction of Terfenol-D particles with sizes ranging from 212 to 300 μm. The effect of a compressive preload in the sensor was also investigated. The achieved resolution was 0.4 mT without a preload or 0.3 mT with a compressive pre-stress of 8.6 MPa. The sensor was tested at magnetic fields of up to 750 mT under static conditions. Dynamic measurements were conducted with a magnetic unbalanced four-pole rotor.

Dengue immunoassay with an LSPR fiber optic sensor.

Dengue fever is a viral disease that affects millions of people worldwide. Specific tests for dengue are not usually performed due to high costs, complicated procedures and, in some cases, long time to yield a result. For widespread use of specific tests to be possible, fast, reliable and fairly simple methods are needed. In this paper, we present a new dengue diagnostic method for the acute phase of the infection. The method proposed uses an all-optical fiber sensor based on Localized Surface Plasmon Resonance (LSPR) and specular reflection from gold nanoparticles (AuNPs). Dengue anti-NS1 antibody was immobilized on AuNPs deposited on the endface of a standard multimode fiber (62.5 µm/125 µm). The sensor is able to detect NS1 antigen at different concentrations, with limit of quantification estimated to be 0.074 μg/ml = 1.54 nM. These results indicate that the sensor could potentially be used for dengue diagnosis in the acute phase of the infection.

Hydrostatic Pressure Sensing with High Birefringence Photonic Crystal Fibers

The effect of hydrostatic pressure on the waveguiding properties of high birefringence photonic crystal fibers (HiBi PCF) is evaluated both numerically and experimentally. A fiber design presenting form birefringence induced by two enlarged holes in the innermost ring defining the fiber core is investigated. Numerical results show that modal sensitivity to the applied pressure depends on the diameters of the holes, and can be tailored by independently varying the sizes of the large or small holes. Numerical and experimental results are compared showing excellent agreement. A hydrostatic pressure sensor is proposed and demonstrated using an in-fiber modal interferometer where the two orthogonally polarized modes of a HiBi PCF generate fringes over the optical spectrum of a broad band source. From the analysis of experimental results, it is concluded that, in principle, an operating limit of 92 MPa in pressure could be achieved with 0.0003% of full scale resolution.

Design of a highly-birefringent microstructured photonic crystal fiber for pressure monitoring

We present the design of an air hole microstructured photonic crystal fiber for pressure sensing applications. The air-hole photonic crystal lattices were designed to produce a large intrinsic birefringence of 1.16 x 10(-3). The impact of the surrounding air holes for pressure sensing to the propagation mode profiles and indices were studied and improved, which ensures single mode propagation in the fiber core defined by the photonic crystal lattice. An air hole matrix and a practical chemical etching process during the fiber perform preparation stage is proposed to produce an optical fiber with a birefringence-pressure coefficient of 43.89 x 10 (-6)MPa(-1) or a fiber Bragg grating pressure responsivity of 44.15 pm/MPa, which is a 17 times improvement over previous photonic crystal fiber designs.

Magnetic Field Measurements Based on Terfenol Coated Photonic Crystal Fibers

A magnetic field sensor based on the integration of a high birefringence photonic crystal fiber and a composite material made of Terfenol particles and an epoxy resin is proposed. An in-fiber modal interferometer is assembled by evenly exciting both eigenemodes of the HiBi fiber. Changes in the cavity length as well as the effective refractive index are induced by exposing the sensor head to magnetic fields. The magnetic field sensor has a sensitivity of 0.006 (nm/mT) over a range from 0 to 300 mT with a resolution about ±1 mT. A fiber Bragg grating magnetic field sensor is also fabricated and employed to characterize the response of Terfenol composite to the magnetic field.

In-fiber Fabry-Perot interferometer for strain and magnetic field sensing.

In this paper we discuss the results obtained with an in-fiber Fabry-Perot interferometer (FPI) used in strain and magnetic field (or force) sensing. The intrinsic FPI was constructed by splicing a small section of a capillary optical fiber between two pieces of standard telecommunication fiber. The sensor was built by attaching the FPI to a magnetostrictive alloy in one configuration and also by attaching the FPI to a small magnet in another. Our sensors were found to be over 4 times more sensitive to magnetic fields and around 10 times less sensitive to temperature when compared to sensors constructed with Fiber Bragg Grating (FBG).

Gold nanoparticles on the surface of soda-lime glass: morphological, linear and nonlinear optical characterization

Materials presenting high optical nonlinearity, such as materials containing metal nanoparticles (NPs), can be used in various applications in photonics. This motivated the research presented in this paper, where morphological, linear and nonlinear optical characteristics of gold NPs on the surface of bulk soda-lime glass substrates were investigated as a function of nanoparticle height. The NPs were obtained by annealing gold (Au) thin films previously deposited on the substrates. Pixel intensity histogram fitting on Atomic Force Microscopy (AFM) images was performed to obtain the thickness of the deposited film. Image analysis was employed to obtain the statistical distribution of the average height of the NPs. In addition, absorbance spectra of the samples before and after annealing were measured. Finally, the nonlinear refractive index (n2) and the nonlinear absorption index (α2) at 800 nm were obtained before and after annealing by using the thermally managed eclipse Z-scan (TM-EZ) technique with a Ti:Sapphire laser (150 fs pulses). Results show that both n2 and α2 at this wavelength change signs after the annealing and that the samples presented a high nonlinear refractive index.

Optical fibre Bragg gratings for high temperature sensing

The development of new techniques for writing and tailoring the properties of Bragg gratings has generated a suite of distinct grating types that are optimised for performance within different temperature windows. These cover gratings produced by recipes such as hypersensitisation, thermal processing and single and multiphoton writing. In this paper, we review four types of high temperature gratings that offer comprehensive coverage of temperature space for most applications of interest. Up to 1200°C novel processing methods allow standard silica-based optical fibres to be used. However, beyond these temperatures, optical fibres made from other materials, such as sapphire, need consideration.

Simultaneous measurement of temperature and pressure using single fiber Bragg grating and fixed filter demodulation technique

Fiber Bragg grating (FBG) sensors offer many advantages for monitoring strain and temperature, other physical parameters can be measured through the use of mechanical transducers. However, FBGs are sensitive to strain and temperature and, in many cases, it is difficult to discrminate both measurements. To overcome this problem several techniques have been proposed, most of them employing more than one grating. This work demonstrates the possibility to discriminate temperature and pressure measurements using only one FBG sensor and a low cost demodulation technique based on two fixed filters. A pressure transducer has been used to transfer a lateral force to the fiber, proportional to the applied pressure, generating birefringence in the Bragg grating. The system allowed to measure pressure in the range of 0 to 400 psi with uncertainty of 4 psi and, simultaneously, measure temperature in a range of 22°C with uncertainty of 0.1°C.

Internal specular reflection from nanoparticle layers on the end face of optical fibers

Metal nanoparticles deposited on a dielectric substrate exhibit an absorption peak due to localized surface plasmon resonance (LSPR). Changes in the refractive index of the surrounding medium cause the LSPR band to shift in wavelength. In this paper, gold nanoparticles were placed on the end face of an optical fiber spliced to a reflection setup. As expected in reflection geometries, the back-reflected signal exhibits a dip at wavelengths in the LSPR band when the fiber tip is surrounded by ambient air (n = 1). However, as the refractive index of the surrounding medium is increased, there is a gradual inversion from a dip (n = 1.0) to a peak (n = 2.0). To explain these results, a model based on absorption and interference between the specular reflections from the fiber/nanoparticle and nanoparticle/ambient interfaces is proposed, showing that our reflection configuration can be used in sensing applications.