A novel ultra-high sensitivity and temperature-compensated in-line Fabry–Perot (FP) strain sensor based on tapered FBG-in-capillary structure is proposed and experimentally demonstrated. The proposed sensor can be easily fabricated by inserting a fiber taper with FBG into the capillary, making it possess the ultra-long active length and the ultra-short interference length to achieve strain sensitivity enhancement. In addition, the strain-free FBG in the capillary is a good candidate for temperature compensation. The strain sensitivity of this structure is as high as 1129.44 pm/µe, which is about two orders of magnitude higher than that of common FP strain sensors reported. The crosstalk of the temperature can be compensated by sensitivity coefficient matrix demodulation. Such structure has ultra-high sensitivity, good linearity, temperature-compensation, robust and easy-fabrication, which is expected to have potential applications in strain sensing.

Ultra-high sensitivity and temperature-compensated Fabry–Perot strain sensor based on tapered FBG

Reconfigurable Optical Magnetometer for Static and Dynamic Fields

In this paper, an all-single-mode fiber strain sensor based on a bent structure Mach-Zehnder interferometer (BS-MZI) is proposed. This sensor consists of two identical bent structures, one of which excites the cladding modes, and the other couples them back to the core. Interference occurs when the cladding and the core modes meet each other. Therefore, the bent structures function as an MZI. The bent structures are fabricated by releasing the pulse of the CO2 laser along the specific designed paths. The fiber shaping method based on CO2 laser polishing is flexible, which is demonstrated in detail. The unique geometrical shape of the bent structures brings high strain sensitivity to the sensor. As a consequence, the sensor with a length of 9.6 mm exhibits an ultrahigh strain sensitivity, which reaches 165 pm/μϵ. Experimental results show that the strain responses of the BS-MZIs are highly correlated with their interference lengths and the modulated lengths. Its temperature-strain cross-sensitivity is as low as 0.21 μϵ/°C. Moreover, the sensor exhibits high stability and repeatability in strain measurement, which makes it a promising sensor in strain measurement.

Ultrasensitive Strain Sensor Based on Mach-Zehnder Interferometer With Bent Structures

Recent progress in optical fiber sensors based on Fabry-Perot interferometers (FPIs) has achieved much attention. In this paper, we designed and fabricated an FPI sensor by building a bubble in an ultra-thin core optical fiber. The size of the bubble decreased gradually with further arc discharges, which was inverse to the bubble fabrication in standard single mode fibers and multimode fibers. The bubble microcavity in the ultra-thin core optical fiber demonstrated a strain sensitivity of 2.08 pm/μe in the range of 0–5000 μe and a gravity sensitivity of 2.908 nm/N in the range of 0–4.89 N with high linear responses. Meanwhile, the bubble microcavity sensor was insensitive to temperature and bending, which was benefit to avoid the cross influences. The FPI sensor based on a bubble microcavity in an ultra-thin core optical fiber possesses the properties of easy to fabricate, small size, robustness, and high sensitivity.

Bubble microcavity strain and gravity sensor with temperature and bending insensitivity using an ultra-thin core optical fiber

This study presents an extrinsic Fabry–Perot interferometric (EFPI) fiber-optic strain sensor with a very short cavity. The sensor consists of two vertically cut standard single-mode fibers (SMFs) and a glass capillary with a length of several centimeters. The two SMFs penetrate into the glass capillary and are fixed at its two ends with the use of ultraviolet (UV) curable adhesives. Based on the use of the lengthy glass capillary sensitive element, the strain sensitivity can be greatly enhanced. Experiments showed that the microcavity EPFI strain sensor with initial cavity lengths of 20 μm, 30 μm, and 40 μm, and a capillary length of 40 mm, can yield respective cavity length–strain sensitivities of 15.928 nm/μe, 25.281 nm/μe, and 40.178 nm/μe, while its linearity was very close to unity for strain measurements spanning a range in excess of 3500 μe. Furthermore, the strain–temperature cross-sensitivity was extremely low.

https://www.mdpi.com/1424-8220/19/19/4097/pdf

Sensitivity-Enhanced Extrinsic Fabry–Perot Interferometric Fiber-Optic Microcavity Strain Sensor

We present a laser machining method for fabricating an all-fiber pillar-in-bubble Fabry-Perot interferometer (FPI), which is used for strain sensors with high sensitivity. The micro-structure of the air-bubble is fully controllable, especially the cavity length and sidewall thickness. The measured sensitivity of this strain sensor is as high as 56.69 pm/μe, which is several times higher than that of most FPI strain sensors reported to date. This sensor also has a low-temperature sensitivity of 0.682 pm/°C, reducing the cross-sensitivity between tensile strain and temperature to 0.012 μe/°C. Furthermore, such a sensor has the benefits of flexible design, simple fabrication, and high reproducibility, making it attractive for practical applications.

High-sensitivity strain sensor with an in-fiber air-bubble Fabry-Perot interferometer

The Kibble-Zurek (KZ) mechanism is a universal framework that can in principle describe nonequilibrium phase transition phenomena in any system with the required symmetry properties. However, a conflicting phenomenon termed anti-KZ behavior has been reported in the study of ferroelectric phase transitions, in which slower driving results in more topological defects [S. M. Griffin et al., Phys. Rev. X 2, 041022 (2012)]. Although this finding is significant, related experimental work remains scarce. In this work, we experimentally demonstrate anti-KZ behavior under a noisy control field in three different quantum phase transition protocols using a single trapped ${}^{171}{\mathrm{Yb}}^{+}$ ion. The density of defects is studied as a function of the quench time and the noise intensity. We experimentally verify that the optimal quench time to minimize excitations scales as a universal power law of the noise intensity. Our work provides an experimental verification of anti-KZ behavior in a two-level system.

Experimental verification of anti–Kibble-Zurek behavior in a quantum system under a noisy control field

We demonstrate an active acoustic sensor based on a high-finesse fiber Fabry-Perot micro-cavity with a gain medium. The sensor is a compacted device lasing around 1535 nm by external optical pumping. The acoustic pressure acting on the sensor disturbs the emitted laser frequency, which is subsequently transformed to beat signals through a delay-arm interferometer, and directly detected by a photo-detector. In this configuration, the sensing device exhibits a high sensitivity of 2.6 V/Pa and a noise equivalent acoustic signal level of 230 μPa/Hz1/2 at a frequency of 4 kHz. Experimental results provide a wide frequency response from 100 Hz to 18 kHz. As the sensor works at communication wavelength and the output laser can be electrically tuned in the 10 nm range, a multi-sensor network can be easily constructed with the dense wavelength division multiplexing devices. Extra lasers or demodulators are unnecessary thus the proposed sensor is low cost and easy fabrication. The proposed sensor shows broad applications prospect in remote oil and gas leakage exploration, photo-acoustic spectrum detection, and sound source location.

An Acoustic Sensor Based on Active Fiber Fabry–Pérot Microcavities

We have demonstrated a mode matching method between two different fibers by a hybrid thermal expanded core technique, which can be applied to match the modes of fiber-based Fabry–Perot cavities. Experimentally, this method has achieved an expansion of the ultraviolet fiber core by 3.5 times while keeping fundamental mode propagation. With the experiment parameters, the fundamental mode coupling efficiency between the fiber and micro-cavity can reach 95% for a plano-concave cavity with a length of 400 μm. This method can not only have potential in quantum photonics research but also can be applied in classical optical fields.

Thermal expanded core ultraviolet fiber for optical cavity mode matching

A narrow-linewidth laser operating at the telecommunications band combined with both fast and wide-band tuning features will have promising applications. Here we demonstrate a single-mode (both transverse and longitudinal mode) continuous microlaser around 1535 nm based on a fiber Fabry–Perot microcavity, which achieves wide-band tuning without mode hopping to the 1.3 THz range and fast tuning rate to 60 kHz and yields a frequency scan rate of 1.6×1017  Hz/s. Moreover, the linewidth of the laser is measured as narrow as 3.1 MHz. As the microlaser combines all these features into one fiber component, it can serve as the seed laser for versatile applications in optical communication, sensing, frequency-modulated continuous-wave radar, and high-resolution imaging.

Xin-Xia Gao

Papers

High-sensitivity strain sensor with an in-fiber air-bubble Fabry-Perot interferometer

Experimental verification of anti–Kibble-Zurek behavior in a quantum system under a noisy control field

An Acoustic Sensor Based on Active Fiber Fabry–Pérot Microcavities

Thermal expanded core ultraviolet fiber for optical cavity mode matching

Fast and wide-band tuning single-mode microlaser based on fiber Fabry–Pérot microcavities