The implementation of a machine learning algorithm for measuring refractive index of liquid samples using Fresnel reflection at the tip of a fiber is proposed in order to overcome the measurement ambiguity between samples having refractive index values below and above the effective refractive index of the fiber fundamental mode. This is the first time that a machine learning algorithm is implemented in a fiber refractometer. The algorithm, used for pattern classification, is the Support Vector Machine (SVM). The sensing head is formed by two-cascaded cavities that generate an interference pattern that changes each time the fiber is immersed in a different sample. The changes in the interference pattern are classified by the proposed algorithm, which extends the sensing range and eliminates any ambiguity in the obtained RI values. The proposed system is also self-referenced, and therefore it is unaffected by any intensity change of the optical source. A theoretical model and experimental results are presented in detail to demonstrate the effectiveness of the proposed system.

A fiber sensor for directional bending monitoring is presented. The sensor is made of a single piece of a multimode elliptical-core fiber, and works in reflection regime. The interference spectrum generated by the proposed system has a non-uniform amplitude and non-periodic shape that makes the sensing signal adequate for a pattern recognition algorithm to be implemented. The elliptical-core fiber is fixed to a flexible rod and each bending position of the rod generates a specific optical spectrum, which is then identified by a pattern recognition algorithm. Using the algorithm identification, the trajectory of the flexible rod can be calculated. Experimental results demonstrating directional bending monitoring, based on pattern recognition and estimation of the flexible rod trajectory, are presented.

We propose an experimental setup to measure the refractive index (RI) of two independent liquid samples by analyzing a single optical signal. We spliced a section of a seven-core fiber (SCF) into each arm of a 1x2 fiber coupler and spliced additional single mode fiber (SMF) to use as fiber probe tips. As the spatial frequency of the characteristic sinusoidal response of an SCF is a function of the fiber length, we used two different lengths (3 and 10 cm) in our realization. A superluminescent diode (SLD) is used as the light source, and the reflected light by both fiber tips is acquired through an optical circulator. An optical spectrum analyzer (OSA) is used to analyze the spectral response of the combined signals from both fiber tips. We characterized this sensing system by immersing the fiber tips in liquid samples with a RI between 1.3175 and 1.4040. We show that the two independent RI measurements can be retrieved by performing a Fourier transform on the recorded optical spectrum. In particular, we demonstrate that with adequate calibration, the Fresnel coefficients can be calculated independently. The presented approach represents a straightforward and cost-effective fiber implementation for multipoint sensing.

We demonstrate a simple multi-point refractometer based on the coherent optical frequency-domain multiplexing technique. As local refractive index sensors, interferometers of different lengths formed between cleaved fiber tips and reference weak reflectors are employed. Referent reflectors were fabricated by splicing into lead SMF-28 fiber a very short section of hollow optical fiber using a standard fiber splicer and a cleaver with an optical microscope. Despite the very simple configuration and manufacturing of the sensor, the refractive index resolution of $6\times {10}^{-4}$ was demonstrated during proof-of-concept experiments.

We present an ultra-high sensitivity fiber loop mirror temperature sensor based on an asymmetric two-hole fiber (ATHF) filled with distilled water. The design of the ATHF allows for larger interaction between the optical mode and the region with large induced birefringence. This enhanced interaction provides large changes in the sensing signal when the temperature is varied. The sensing signal can easily be tracked by following the change in the frequency of the interference signal as a function of the temperature, thus allowing for absolute temperature measurements. Experimental results show that the ATHF birefringence is modified from 1.18 × 10-5 to 3.33 × 10-4 when the temperature changes from 40 to 80 °C; generating, in terms of frequency, a sensitivity of 6 × 10-3 nm-1/0C. In the case of very small temperature changes, where the wavelength shift is also small, simulations were performed using experimental parameters of the ATHF and an ultra-high sensitivity of 240 nm/°C was calculated. The sensitivity of the presented sensor can also be tuned by changing the liquid infiltrated into the fiber holes.

We propose and experimentally demonstrate a simple approach to realize a phase-sensitive correlation optical time-domain reflectometer (OTDR) suitable for detection and localization of dynamic perturbations along a single-mode optical fiber. It is based on the quantum phase fluctuations of a coherent light emitted by a telecom DFB diode laser. Truly random probe signals are generated by an interferometer with the optical path difference exceeding the coherence length of the laser light. Speckle-like OTDR traces were obtained by calculating cross-correlation functions between the probe light and the light intensity signals returned back from the sensing fiber. Perturbations are detected and localized by monitoring time variations of correlation amplitude along the fiber length. Results of proof-of-concept experimental testing are presented using an array of ultra-low-reflectivity fiber Bragg gratings as weak reflectors.