Respiration rate is an essential vital sign that requires monitoring under various conditions, including in strong electromagnetic environments such as in magnetic resonance imaging systems. To provide an electromagnetically-immune breath-sensing system, we propose an all-fiber-optic wearable breath sensor based on a fiber-tip microcantilever. The microcantilever was fabricated on a fiber-tip by two-photon polymerization microfabrication based on femtosecond laser, so that a micro Fabry–Pérot (FP) interferometer was formed between the microcantilever and the end-face of the fiber. The cavity length of the micro FP interferometer was reduced as a result of the bending of the microcantilever induced by breath airflow. The signal of breath rate was rebuilt by detecting power variations of the FP interferometer reflected light and applying dynamic thresholds. The breath sensor achieved a high sensitivity of 0.8 nm/(m/s) by detecting the reflection spectrum upon applied flow velocities from 0.53 to 5.31 m/s. This sensor was also shown to have excellent thermal stability as its cross-sensitivity of airflow with respect to the temperature response was only 0.095 (m/s)/°C. When mounted inside a wearable surgical mask, the sensor demonstrated the capability to detect various breath patterns, including normal, fast, random, and deep breaths. We anticipate the proposed wearable breath sensor could be a useful and reliable tool for respiration rate monitoring.

A simple vibration sensor is proposed and demonstrated based on an optical fiber Fabry-Perot interferometer (FPI) with an in-fiber collimator. The device was fabricated by splicing a quarter-pitch graded index fiber (GIF) with a section of a hollow-core fiber (HCF) interposed between single mode fibers (SMFs). The static displacement sensitivity of the FPI with an in-fiber collimator was 5.17 × 10⁻⁴ μm⁻¹, whereas the maximum static displacement sensitivity of the device without collimator was 1.73 × 10⁻⁴ μm⁻¹. Moreover, the vibration sensitivity of the FPI with the collimator was 60.22 mV/g at 100 Hz, which was significantly higher than the sensitivity of the FPI without collimator (11.09 mV/g at 100 Hz). The proposed FPI with an in-fiber collimator also exhibited a vibration sensitivity nearly one order of magnitude higher than the device without the collimator at frequencies ranging from 40 to 200 Hz. This low-cost FPI sensor is highly-sensitive, robust and easy to fabricate. It could potentially be used for vibration monitoring in remote and harsh environments.

A distributed acoustic sensor (DAS) with a one- centimeter spatial resolution is proposed and demonstrated using pulse-compression phase-sensitive optical time-domain reflectometry (Φ-OTDR). A positive- and negative- swept pulse was used to expand the frequency modulation range of a heterodyne DAS system to achieve a high spatial resolution. A Rayleigh-enhanced single-mode fiber (eSMF) was used to magnify the backscattering signal and suppress the fading noise. The combination of pulse compression and eSMF allows the system to fully quantify perturbations with a high spatial resolution of 1.2 cm and a strain resolution of 1.46 nϵ/√Hz. In addition, the detection and positioning of multiple acoustic emission (AE) sources at frequencies of 120 ∼ 130 kHz is also demonstrated. Hence, the high-spatial-resolution DAS system proposed in this work could potentially be used for structural health monitoring for large equipment, such as spacecrafts, hypersonic vehicles, and deep-sea submersibles.

Raman-optical time domain reflectometry (R-OTDR) -based distributed temperature sensing (DTS) with high temperature resolution is of great significance in extremely harsh environment of deep wellbore. In this work, we experimentally demonstrate a Fast-non local means (Fast-NLM) algorithm for performance improvement without modifying the hardware architecture of a conventional R-OTDR system. In the proposed method, the first-in-first-out (FIFO) data transmission protocol is used to reconstruct the two-dimensional (2D) temperature image in real time. An integral image is employed to simplify the calculation of Euclidean distance between similarities in the 2D space-time domain. The overall response time by using the method decreases from 27.9 s to 1.7 s, marking a 16-fold enhancement over traditional NLM denoising methodologies. An average temperature resolution is below 0.01 °C at the end of fiber. The evolution of the measured temperature at the hot spot position does not produce delay or distortion. Moreover, two segments fiber at 300 m and 9.68 km are rigorously tested with the temperature range from 50 °C to 300 °C and time span of 4 hours, compared with conventional wavelet denoising algorithm, the root means square error decrease from 1.02 °C to 0.42 °C. In all, the Fast-NLM denoising based R-OTDR illustrates great improvement in temperature resolution while maintaining fast response time. This achievement holds immense potential for diverse applications, including dynamic monitoring in the realms of energy development, oil and gas exploration, and allied fields.

We demonstrate a silica diaphragm-based fiber tip Fabry–Perot interferometer (FPI) for high-pressure (40 MPa) sensing. By using a fiber tip polishing technique, the thickness of the silica diaphragm could be precisely controlled and the pressure sensitivity of the fabricated FPI sensor was enhanced significantly by reducing the diaphragm thickness; however, the relationship between the pressure sensitivity and diaphragm thickness is not linear. A high sensitivity of −1.436 nm/MPa and a linearity of 0.99124 in hydraulic pressure range of 0 to 40 MPa were demonstrated for a sensor with a diaphragm thickness of 4.63 μm. The achieved sensitivity was about one order of magnitude higher than the previous results reported on similar fiber tip FPI sensors in the same pressure measurement range. Sensors with a thinner silica diaphragm (i.e., 4.01 and 2.09 μm) rendered further increased hydraulic pressure sensitivity, but yield a significant nonlinear response. Two geometric models and a finite element method (FEM) were carried out to explain the nonlinear response. The simulation results indicated the formation of cambered internal silica surface during the arc discharge process in the fiber tip FPI sensor fabrication.

We propose and demonstrate a novel high-temperature-resistant vector accelerometer, consisting of a ring cavity laser and sensing probe (i.e., fiber Bragg gratings (FBGs)) inscribed in a seven-core fiber (SCF) by using the femtosecond laser direct writing technique. A ring cavity laser serves as a light source. Three FBGs in the outer cores of SCF, which are not aligned in a straight line, are employed to test the vibration. These three FBGs have 120° angular separation in the SCF, and hence, vibration orientation and acceleration can be measured simultaneously. Moreover, the FBG in the central core was used as a reflector in the ring cavity laser, benefiting to resist external interference factors, such as temperature and strain fluctuation. Such a proposed accelerometer exhibits a working frequency bandwidth ranging from 4 to 68 Hz, a maximum sensitivity of 54.2 mV/g, and the best azimuthal angle accuracy of 0.21° over a range of 0–360°. Furthermore, we investigated the effect of strain and temperature on the performance of this sensor. The signal-to-noise ratio (SNR) only exhibits a fluctuation of ~1 dB in the range (0, 2289 με) and (50 °C, 1050 °C). Hence, such a vector accelerometer can operate in harsh environments, such as in aerospace and a nuclear reactor.