A methodology to design a multipoint fiber refractometer based on the Coherent Optical Frequency Domain Reflectometry (C-OFDR) technique is presented; in this refractometer, the nonlinearity effect generated by the scanning in optical frequency is reduced by a proper design of each sensing head; eliminating the need for the auxiliary interferometer commonly used for reducing this effect. Experimental demonstration on the nonlinearity effect in the sensing signal is presented. This effect is reduced by cutting the length of the processed signal in a cycle of scanning; to demonstrate this behavior, 100%, 50%, and 25 % of the sensing signal from a scanning cycle are processed. However, it is not only about reducing the length of the sensing signal, but proper sensing head design also allows the reduced signal to be form by an integer number of cycles from each interference signal generated at the sensing heads, which in turns permit concatenation of the reduced signal without losing information. These three considerations, reduction of the signal in a scanning cycle, an integer number of cycles, and concatenation, generate stable and strong sensing signals. In this sense, the design of the sensing heads allows reducing the nonlinearity effect and possibility of a multipoint refractometer as well. Theoretical description and experimental results to demonstrate the effectiveness of the proposed method are presented. Design, construction, characterization and simultaneous implementation of three sensing heads are demonstrated.

<!-- *** Custom HTML *** --> A multipoint fiber optic refractometer that uses an algorithm to compensate for the nonlinearity generated by an optical frequency sweeping laser is presented. This compensation allows for designing the multipoint sensing system. Otherwise, only single-point sensing is possible. Also, the nonlinearity compensation, applied to all sensing points simultaneously, permits an improvement in the resolution of each sensing point after the compensation is performed. The mathematical model and experimental results of the proposed multipoint refractive index sensor are presented.

We propose to use the nonlinearity in wavelength sweeping of a distributed feedback (DFB) diode laser to generate a reliable synthetic database implemented in a pattern recognition-based fiber sensor. Firstly, the experimentally extracted wavelength-sweeping nonlinearity permits obtaining the time-varying tuning rate along the sweeping period. Secondly, this tuning rate is used to simulate a two-point interferometric sensing system under a complete set of temperature variations to generate a reliable synthetic database used to train a pattern recognition algorithm. Thirdly, the trained algorithm is successfully implemented for correctly identifying the experimental sensing signals of a dual-point temperature sensor. In fiber sensor systems employing machine learning algorithms, a huge amount of experimental data is required to ensure accurate pattern classification, which becomes a challenging task. The methodology for generating a reliable synthetic database provides time and resource savings in the lab, without compromising the accuracy of the results. A standard DFB diode laser, wavelength tuned over a few tens of pico-meters, is used as an optical source, and a PIN photodetector is used as an optical detector. A description of the wavelength-nonlinearity extraction approach, a mathematical model of the interferometric fiber sensor, and experimental results confirming the effectiveness of the proposed sensing system are reported. Also, classification results using a database generated without including the nonlinearity effect are presented to highlight the importance of considering this nonlinearity.

Implementation of the Hilbert Transform (HT) for temperature monitoring using an interferometric fiber sensor is presented. The method’s effectiveness is confirmed through modeling, simulation, and experimental results. This approach simplifies the process of monitoring the phase of an interferometric signal from a fiber sensor. Using a Fabry-Perot interferometer as the sensing element, it is demonstrated that the HT method enables accurate temperature measurement. The results show a high sensitivity and precision, with an error margin of less than 0.1°C, demonstrating the method’s reliability and potential for various applications.