A. M. R. Pinto

Photonic Crystal Fibers for Sensing Applications

Photonic crystal fibers are a kind of fiber optics that present a diversity of new and improved features beyond what conventional optical fibers can offer. Due to their unique geometric structure, photonic crystal fibers present special properties and capabilities that lead to an outstanding potential for sensing applications. A review of photonic crystal fiber sensors is presented. Two different groups of sensors are detailed separately: physical and biochemical sensors, based on the sensor measured parameter. Several sensors have been reported until the date, and more are expected to be developed due to the remarkable characteristics such fibers can offer.

High precision micro-displacement fiber sensor through a suspended-core Sagnac interferometer

A sensing system for micro-displacement measurement based in a suspended-core fiber Sagnac interferometer is presented. The suspended-core fiber characterization was made through the use of an optical backscatter reflectometer, screening its multimodal and birefringent behavior. Its sensitivity to displacement measurements is shown to be due only to birefringence, being that core-cladding mode coupling is negligible. High precision (~0.45 μm) was obtained using three different measurement instruments, showing an extremely high stability and high insensitivity to temperature, demonstrating that the sensing system has the ability for low cost applications.

Temperature Fiber Laser Sensor Based on a Hybrid Cavity and a Random Mirror

In the present work, a simple temperature fiber laser sensor configuration is proposed. The temperature fiber laser sensor is based in the combination of a Fabry-Perót hybrid cavity and a random mirror. The Fabry-Perót hybrid cavity is fabricated by splicing a single mode fiber with a small piece of suspended-core fiber. The random mirror is created by multiple Rayleigh scattering events running along the dispersion compensation fiber, as a direct consequence of Raman gain in this fiber. In the proposed configuration, the Fabry-Perót cavity presents simultaneously a double function: laser reflective mirror and temperature sensing cavity. The proposed temperature fiber laser sensor presents maximum output power of ~4 mW in a 15nm wavelength range while providing a temperature sensibility of ~6 pm/°C, in a 200°C temperature range.

Interrogation of a Suspended-Core Fabry–Perot Temperature Sensor Through a Dual Wavelength Raman Fiber Laser

The interrogation of a Fabry-Perot cavity through a dual wavelength Raman fiber laser is reported. The proposed sensing system is based on the use of a dual wavelength Raman fiber laser to generate two quadrature phase-shifted signals that allow the recovery of the temperature change sensed by the Fabry-Perot interferometric cavity. The dual wavelength Raman fiber laser is based on fiber Bragg gratings combined with a distributed mirror. The Fabry-Perot cavity is fabricated by splicing a short length of a suspended-core microstructured fiber to a single mode fiber. The use of this sensing system allows a passive and accurate interrogation of the temperature, while taking advantage of the Rayleigh scattering growth as a distributed mirror in the laser.

Suspended-core fiber Sagnac combined dual-random mirror Raman fiber laser

In the present work, a multiwavelength fiber laser based in the combination of a double-random mirror and a suspended-core Sagnac interferometer is presented. The double-random mirror acts by itself as a random laser, presenting a 30dB SNR, as result of multiple Rayleigh scattering events produced in the dispersion compensating fibers by the Raman amplification. The suspended-core fiber Sagnac interferometer provides the multi peak channeled spectrum, which can be tuned by changing the length of the fiber. The result of this combination is a stable multiwavelength peak laser with a minimum of ~25dB SNR, which is highly sensitive to polarization induced variations.

Micro-Displacement Sensor Based on a Hollow-Core Photonic Crystal Fiber

A sensing head based on a hollow-core photonic crystal fiber for in-reflection measurement of micro-displacements is presented. The sensing structure takes advantage of the multimodal behavior of a short segment of hollow-core photonic crystal fiber in-reflection, being spliced to a single mode fiber at its other end. A modal interferometer is obtained when the sensing head is close to a mirror, through which displacement is measured.

An in-reflection strain sensing head based on a Hi-Bi photonic crystal fiber.

A photonic crystal fiber-based sensing head is proposed for strain measurements. The sensor comprises a Hi-Bi PCF sensing head to measure interferometric signals in-reflection. An experimental background study of the sensing head is conducted through an optical backscatter reflectometer confirming the theoretical predictions, also included. A cost effective setup is proposed where a laser is used as illumination source, which allows accurate high precision strain measurements. Thus, a sensitivity of ∼7.96 dB/ms was achieved in a linear region of 1,200 με.

Experimental and Numerical Characterization of a Hybrid Fabry-Pérot Cavity for Temperature Sensing

A hybrid Fabry-Pérot cavity sensing head based on a four-bridge microstructured fiber is characterized for temperature sensing. The characterization of this cavity is performed numerically and experimentally in the L-band. The sensing head output signal presents a linear variation with temperature changes, showing a sensitivity of 12.5 pm/°C. Moreover, this Fabry-Pérot cavity exhibits good sensitivity to polarization changes and high stability over time.

Fiber Optic Sensing System for Temperature and Gas Monitoring in Coal Waste Pile Combustion Environments

It is presented an optical fiber sensing system projected to operate in the demanding conditions associated with coal waste piles in combustion. Distributed temperature measurement and spot gas sensing are requirements for such a system. A field prototype has been installed and is continuously gathering data, which will input a geological model of the coal waste piles in combustion aiming to understand their dynamics and evolution. Results are presented on distributed temperature and ammonia measurement, being noticed any significant methane emission in the short time period considered. Carbon dioxide is also a targeted gas for measurement, with validated results available soon. The assessment of this technology as an effective and reliable tool to address the problem of monitoring coal waste piles in combustion opens the possibility of its widespread application in view of the worldwide presence of coal related fires.

Fiber optic sensing system for monitoring of coal waste piles in combustion

The combustion of coal wastes resulting from mining is of particular environmental concern and therefore the importance of the proper management involving real-time assessment of their status and identification of probable evolution scenarios is recognized. Continuous monitoring of combustion temperature and emission levels of certain gases opens the possibility to plan corrective actions to minimize their negative impact in the surroundings. Optical fiber technology is well-suited to this purpose and in this work it is described the main attributes of a fiber optic sensing system projected to gather data on distributed temperature and gas emission in these harsh environments.