Abstract
In this paper, two novel hybrid multimode/single mode fiber Fabry-Perot (FP) cavities were compared. The cavities fabricated by chemical etching are presented as high temperature and strain sensors. In order to produce this FP cavity a single mode fiber was spliced to a graded index multimode fiber with 62.5 mu m core diameter. The multimode fiber was cut approximately 150 mu m away from the splice. Then the tip of the fiber containing the multimode fiber segment was dipped into a solution of 48% of HF during 8 minutes, creating a concavity due to the fact that the reaction between HF and the germanium doped fiber core is much faster than the reaction between HF and the pure silica cladding. By this method a concavity of approximately 100 mu m deep was created at the fiber tip. Two different FP cavities can be fabricated. The first cavity is obtained when a spliced with an identical tip concavity fiber (Sensor A) and the second is created when a tip concavity is spliced to a single mode fiber (Sensor B). The Fabry-Perot cavities were tested as a high temperature sensor in the range between room temperature and 800 degrees C and as strain sensors. A reversible shift of the interferometric peaks with temperature allowed to estimate a sensitivity of 0.75 +/- 0.03 pm/degrees C and 0.98 +/- 0.04 pm/degrees C for the sensor A and B respectively. For strain measurement sensor A demonstrated a sensitivity of 1.85 +/- 0.07 pm/mu epsilon and sensor B showed a sensitivity of 3.14 +/- 0.05 pm/mu epsilon. The sensors demonstrated the feasibility of low cost fiber optic sensors for high temperature and strain.

Abstract
In this work, an LPG-based Mach-Zehnder modal interferometer with a tapered fiber section between the two LPGs is studied as a sensing structure for measuring environmental refractive index. To interrogate this sensing device, coherence addressing and pseudo-heterodyne processing were used. A fiber taper was made between the two LPGs to improve and tailor the sensitivity of the sensor. Experimental results show that the sensitivity to external refractive index increases with the length of the taper and that the sensitivity enhancement is stronger for lower order cladding modes. © 2009 SPIE.

Abstract
A new interrogation method based on Fibre Bragg Gratings (FBG) for Surface Plamon Resonance (SPR) sensors in the region of refractive indices of aqueous solutions is described. Two FBGs are selected with their Bragg wavelengths at opposite sides of the plasmon resonance peak. The response of the system can be made independent of the fluctuations of the optical power source, and the linearity and the sensitivity of the sensor are improved. The use of the spectral selectivity of gratings for the interrogation of SPR sensors in different configurations is also promising in terms of multiplexing, temperature referencing or multiparameter detection. © 2009 SPIE.

Abstract
An optical fiber sensor for the measurement of oxygen in gaseous environments, which is based on the quenching of the fluorescence of a ruthenium complex, is presented. The sensing chemistry is immobilized in a sol-gel based solid matrix that is coated on a tapered optical fiber probe. Oxygen measurement is performed both by phase and fluorescence intensity spectroscopy. Experimental results show that the fluorescence intensity and the lifetime depend both on oxygen and temperature. A scheme for simultaneous determination of the temperature and the oxygen concentration is proposed. Temperature measurement is performed using the excitation radiation and an absorption long pass filter. Preliminary results are presented which show a temperature measurement independent of oxygen and of optical power level.

Abstract
A bulk interferometric configuration for electrical current remote sensing in high voltage environments based on the Faraday effect is described. The combination of Sagnac's reciprocal properties with a Mach-Zehnder processing interferometer in the same sensing head is used to implement serrodyne processing for current sensing. A theoretical analysis based on the Jones matrix is presented. Experimental results that validate the exploited concept are obtained, showing linearity up to 1800 A(rms) and waveform. reproduction at 50 Hz. The possibility of using the proposed interferometric concept to simultaneously fulfil the requirements associated with metering and relaying applications is also addressed.

Abstract
Optical tweezers based on metallic-coated tapered optical fibers with an aperture at the apex are fabricated and their sensing ability is tested. Preliminary results show robustness in differentiating between a trapped and no trapped state. © 2021 The Author(s).