Susana O. Silva

Light requirements in microalgal photobioreactors: an overview of biophotonic aspects

In order to enhance microalgal growth in photobioreactors (PBRs), light requirement is one of the most important parameters to be addressed; light should indeed be provided at the appropriate intensity, duration, and wavelength. Excessive intensity may lead to photo-oxidation and -inhibition, whereas low light levels will become growth-limiting. The constraint of light saturation may be overcome via either of two approaches: increasing photosynthetic efficiency by genetic engineering, aimed at changing the chlorophyll antenna size; or increasing flux tolerance, via tailoring the photonic spectrum, coupled with its intensity and temporal characteristics. These approaches will allow an increased control over the illumination features, leading to maximization of microalgal biomass and metabolite productivity. This minireview briefly introduces the nature of light, and describes its harvesting and transformation by microalgae, as well as its metabolic effects under excessively low or high supply. Optimization of the photosynthetic efficiency is discussed under the two approaches referred to above; the selection of light sources, coupled with recent improvements in light handling by PBRs, are chronologically reviewed and critically compared.

Ultrahigh-sensitivity temperature fiber sensor based on multimode interference

The proposed sensing device relies on the self-imaging effect that occurs in a pure silica multimode fiber (coreless MMF) section of a single-mode-multimode-single-mode (SMS)-based fiber structure. The influence of the coreless-MMF diameter on the external refractive index (RI) variation permitted the sensing head with the lowest MMF diameter (i.e., 55 μm) to exhibit the maximum sensitivity (2800  nm/RIU). This approach also implied an ultrahigh sensitivity of this fiber device to temperature variations in the liquid RI of 1.43: a maximum sensitivity of -1880  pm/°C was indeed attained. Therefore, the results produced were over 100-fold those of the typical value of approximately 13  pm/°C achieved in air using a similar device. Numerical analysis of an evanescent wave absorption sensor was performed, in order to extend the range of liquids with a detectable RI to above 1.43. The suggested model is an SMS fiber device where a polymer coating, with an RI as low as 1.3, is deposited over the coreless MMF; numerical results are presented pertaining to several polymer thicknesses in terms of external RI variation.

Simultaneous measurement of multiparameters using a Sagnac interferometer with polarization maintaining side-hole fiber

A Sagnac interferometer with a section of a polarization maintaining side-hole fiber for multiparameter measurement is proposed. The sensor was experimentally demonstrated to be sensitive to torsion, temperature, and longitudinal strain, simultaneously. The birefringence in the investigated side-hole fiber is induced simultaneously by the elliptical shape of a germanium-doped core and by field overlap with the air holes surrounding the core. The latter effect is purely geometrical and causes high chromatic dispersion of the group birefringence in the long wavelength range, which results in a different period of spectral interference fringes. A different wavelength response is obtained for each interference fringe peak when the fiber is subjected to torsion, temperature, or longitudinal strain. A matrix equation for simultaneous measurement of the three parameters--torsion, temperature, and longitudinal strain--is also proposed.

All Fiber Mach–Zehnder Interferometer Based on Suspended Twin-Core Fiber

An all fiber Mach-Zehnder interferometer using suspended twin-core fiber is described. Due to the birefringence of the fiber cores, two interferometers are obtained when the fiber is illuminated by a polarized light. Applying curvature or temperature to the sensing head, different sensitivities are observed. In order to discriminate curvature from temperature in the suspended twin-core fiber Mach-Zehnder sensor, the matrix method is used.

Optical fiber refractometer based on a Fabry-Pérot interferometer

A fiber optic sensor for liquid refractive index measurement based on a Fabry-Perot interferometer is described. The interferometer is achieved between the reflection of a short fiber Bragg grating and the Fresnel reflection from the cleaved fiber end. This fiber end is then in contact with the liquid sample to provide refractive index measurements. The sensor is characterized by immersing the fiber tip in distilled water with different concentrations of ethylene glycol. A linear relation of the interferometer fringe visibility with refractive index variation is observed, and a resolution of ~10−3 is obtained. It is also shown that the sensor operation is independent of temperature effects, other than the one related to temperature-induced change of the liquid refractive index.

Strain sensitivity control of fiber Bragg grating structures with fused tapers

We report on the analysis and experimental validation of the strain sensitivity dependences of a fiber Bragg grating written in standard optical fiber when combined with fused tapers. By controlling the difference between the cross sections of the fused taper and the Bragg grating, the strain sensitivity of the Bragg wavelength can be changed by acting on the gauge length. The strain sensing characteristics of an interferometric structure formed by fabricating a fused taper in the middle of a fiber Bragg grating are also reported.

Optical fiber refractometry based on multimode interference

This paper presents an overview of optical fiber sensors based on multimode interference with a focus on refractometric applications. A specific configuration is presented to measure the refractive index of the surrounding liquid based on the Fresnel reflection in the fiber tip, combined with a simple interrogation technique that uses two fiber Bragg gratings as discrete optical sources, with the measurand information encoded in the relative intensity variation of the reflected signals. A resolution of 1.75×10−3 RIU is achieved.

Curvature and Temperature Discrimination Using Multimode Interference Fiber Optic Structures—A Proof of Concept

Singlemode-multimode-singlemode fiber structures (SMS) based on distinct sections of a pure silica multimode fiber (coreless-MMF) with diameters of 125 and 55 μm, were reported for the measurement of curvature and temperature. The sensing concept relies on the multimode interference that occurs in the coreless-MMF section and, in accordance with the length of the MMF section used, two fiber devices were developed: one based on a bandpass filter (self-image effect) and the other on a band-rejection filter. Maximum sensitivities of 64.7 nm·m and 13.08 pm/°C could be attained, for curvature and temperature, respectively, using the band-rejection filter with 55 μm-MMF diameter. A proof of concept was also explored for the simultaneous measurement of curvature and temperature by means of the matrix method.

Fiber Optic-Based Refractive Index Sensing at INESC Porto

A review of refractive index measurement based on different types of optical fiber sensor configurations and techniques is presented. It addresses the main developments in the area, with particular focus on results obtained at INESC Porto, Portugal. The optical fiber sensing structures studied include those based on Bragg and long period gratings, on micro-interferometers, on plasmonic effects in fibers and on multimode interference in a large spectrum of standard and microstructured optical fibers.

Strain-Temperature Discrimination Using Multimode Interference in Tapered Fiber

Tapering single-mode-multimode-single-mode structures to enhance sensitivity is proposed and experimentally demonstrated. 50-mm-long coreless multimode fiber sections are spliced between single-mode fibers (SMFs) and tapered. They are characterized in strain, and an increase in strain sensitivity is obtained with taper diameter reduction. Sensitivities as high as -23.69pm/με for the 15-μm taper are attained. Temperature sensitivities also depend on taper diameter. A combination of two different diameter tapered SMF MMF-SMF structures, with cross-sensitivity to strain and temperature, is proposed as a sensing system for the simultaneous measurement of strain and temperature with resolutions of ±5.6 με and ±1.6°C, respectively. A good condition number of 3.16 is achieved with this sensing structure.