Pressure and temperature are two important parameters in the well and reservoir down-hole monitoring. Accurate, continuous, and real-time data of pressure and temperature helps in oil and reservoir management.¹ Sensors with dual-parameter sensing capability and survivability in the down-hole harsh environment are highly necessary in such applications. Fiber-optic sensors, with advantages such as small size, low loss, high sensitivity, resistance to harsh environments, and multiplexing capability, are good candidates for pressure and temperature down-hole monitoring.

We report a reflection-based phase-shifted long period fiber grating (PS-LPFG) and demonstrate its capability for simultaneous measurement of temperature and external reflective index (RI). The sensor device comprises a grating directly written by CO2 laser and silver-coated end face. A π-shifted LPFG is presented with two attenuation bands through its reflection spectrum. These two bands have different sensitivity towards temperature and external RI that can be used for simultaneous measurement of the two variables. The experimental results show that this probe-type PS-LPFG performs well in terms of linearity and sensitivity.

Optical fiber-based sensors have gained wide application for strain monitoring due to their compact size, immunity from electromagnetic interference, multiplexing capabilities etc. thus offering an alternative to traditional electrical sensors and conventional pneumatic-based sensors.¹ 2. ^{– 3} Different types of optical fiber instruments like fiber Bragg grating sensors, extrinsic Fabry–Perot interferometric (EFPI) sensors, intrinsic Fabry–Perot interferometric (IFPI) sensors, long-period fiber grating sensors, and related hybrid combination sensors have been used for monitoring strain, stress, temperature, and pressure.³ 4. 5. 6. ^{– 7} Temperature sensitivity and temperature maximums are important practical limitations for many of these sensors. For instance, the temperature sensitivity of Bragg grating and IFPI sensors are well known.^8,9

We report a two-axis fiber inclinometer fabricated using an ultraweak terahertz-range fiber Bragg grating. Three sensing grating structures were inscribed along a single-mode optical fiber using a femtosecond laser, bound together into a sensing array using thermoformed plastic, and fixed to a two-axis rotational stage. Inclination tests were performed in which the fiber was deflected from 0 to 1.7 deg. These tests were repeated at eight azimuthal angles in increments of 45 deg (from 0 to 315 deg). The standard deviation of the largest inclination angle error was 0.048 deg and the stability of the inclination angle was 0.030 deg.

A type of extrinsic Fabry–Perot interferometer (EFPI) fiber optic sensor, i.e., the microcavity strain sensor, is demonstrated for embedded, high-temperature applications. The sensor is fabricated using a femtosecond (fs) laser. The fs-laser-based fabrication makes the sensor thermally stable to sustain operating temperatures as high as 800°C. The sensor has low sensitivity toward the temperature as compared to its response toward the applied strain. The performance of the EFPI sensor is tested in an embedded application. The host material is carbon fiber/bismaleimide (BMI) composite laminate that offer thermally stable characteristics at high ambient temperatures. The sensor exhibits highly linear response toward the temperature and strain. Analytical work done with embedded optical-fiber sensors using the out-of-autoclave BMI laminate was limited until now. The work presented in this paper offers an insight into the strain and temperature interactions of the embedded sensors with the BMI composites.