We propose and demonstrate a Fabry-Perot (F-P) optical fiber tip sensor for high-resolution refractive-index measurement fabricated by using 157-nm laser micromachining, for the first time to our knowledge. The sensor head consists of a short air F-P cavity near the tip of a single-mode fiber and the fiber tip. The external refractive index is determined according to the maximum fringe contrast of the interference fringes in the reflective spectrum of the sensor. Such a sensor can provide temperature-independent measurement of practically any refractive index larger than that of air and offers a refractive-index resolution of ~4×10^-5 in its linear operating range. The experimental data agree well with the theoretical results.

This paper reviews a wide variety of fiber-optic microstructure (FOM) sensors, such as fiber Bragg grating (FBG) sensors, long-period fiber grating (LPFG) sensors, Fabry-Perot interferometer (FPI) sensors, Mach-Zehnder interferometer (MZI) sensors, Michelson interferometer (MI) sensors, and Sagnac interferometer (SI) sensors. Each FOM sensor has been introduced in the terms of structure types, fabrication methods, and their sensing applications. In addition, the sensing characteristics of different structures under the same type of FOM sensor are compared, and the sensing characteristics of the all FOM sensors, including advantages, disadvantages, and main sensing parameters, are summarized. We also discuss the future development of FOM sensors.

A miniature in-line fiber-optic Fabry–Perot etalon is fabricated on a photonic crystal fiber (PCF) by using $157\mathrm{nm}$ laser micromachining for the first time to our knowledge. Experimental results show that such a PCF-based etalon has an excellent fringe visibility of up to $\sim 26\mathrm{dB}$ due to the mirror-finish quality of the two cavity surfaces inside the PCF. This etalon can be used as an ideal sensor for precise strain measurement under high temperature of up to $800°\mathrm{C}$. It can also offer some other outstanding advantages, such as fast and easy fabrication, high reproducibility, capacity of mass production, low cost, low temperature–strain cross-sensitivity, and high signal-to-noise ratio.

Fiber-optic distributed acoustic sensing (DAS) technology has many outstanding advantages, such as long sensing range, high detection efficiency, wide frequency response, low operation cost, long working lifetime, et al. It can be anticipated that DAS has great potential to become a next generation technology of long-distance vibration and acoustic wave detection in the future. However, despite the rapid development of DAS technology, the sensitivity of optical cable is still a main limitation in terms of the applications of DAS technology. In this paper, we propose and demonstrate a sensitivity-enhanced optical cable with smaller diameter, lighter weight, and greater flexibility than conventional cables. This novel cable is experimentally tested both in lab and field. The results show that the acoustic sensitivity of the cable has been significantly improved by ∼10 times, comparing with a rigid cable that has much higher reflection and attenuation of acoustic waves. Hence, such a sensitivity-enhanced cable could be one of the best candidates for DAS applications as a general sensing cable. In addition, we found that the experimental results are in agreement with our theoretical analysis, providing a guidance for design of sensitivity-enhanced cables. Finally, this cable has been successfully demonstrated in the field test of borehole vertical seismic profiling (VSP), and good performances have been achieved. The perfect combination of sensitivity-enhanced cables and high-performance DAS systems would provide a powerful tool for many DAS applications.

We propose two types of temperature-compensated all-fiber sensors based on a novel laser machined Fabry-Perot interferometer (FPI) whose sensitivity is adjusted by controlling the FPI gauge length and fiber material. The sensors are demonstrated to achieve precise multiparameter measurements under high temperatures. One sensor is an enhanced sensitivity FPI cascaded with a femtosecond laser (FSL) inscribed fiber Bragg grating (FBG). We propose a modified calculation formula considering the temperature dependence of strain/pressure sensitivities to ensure measurement accuracies. The experiment shows that the cascaded sensor exhibits excellent sensing performance with the maximum strain and pressure sensitivities of $\rm 41.6 pm/\mu \varepsilon and -222 pm/MPa$ , respectively, and in a stable environment, its strain-temperature and pressure-temperature crosstalk are $\rm 0.139 \mu \varepsilon/^{\circ} C$ and 0.027 MPa/°C, respectively. The other sensor is the FPI itself with a length-matched gutter. It achieves an ultralow average thermal drift of ∼0.1 pm/°C over 16-400°C. Experimental results show that it exhibits low temperature-pressure and temperature-strain cross sensitivities of $\rm \sim -0.0017 MPa/^{\circ}C and \sim 0.015 \mu \varepsilon/^{\circ} C$ , respectively.

A novel tunable fiber ring laser configuration with a combination of bidirectional Raman amplification and dual erbium-doped fiber (EDF) amplification is proposed for realizing high optical signal-to-noise ratio (SNR), long-distance, quasi-distributed fiber Bragg grating (FBG) sensing systems with large capacities and low cost. The hybrid Raman–EDF amplification configuration arranged in the ring laser can enhance the optical SNR of FBG sensor signals significantly owing to the good combination of the high gain of the erbium-doped fiber amplifier (EDFA) and the low noise of the Raman amplification. Such a sensing system can support a large number of FBG sensors because of the use of a tunable fiber Fabry–Perot filter located within the ring laser and spatial division multiplexing for expansion of sensor channels. Experimental results show that an excellent optical SNR of $\sim 60\mathrm{dB}$ has been achieved for a $50\mathrm{km}$ transmission distance with a low Raman pump power of $\sim 170\mathrm{mW}$ at a wavelength of $1455\mathrm{nm}$ and a low EDFA pump power of $\sim 40\mathrm{mW}$ at a wavelength of $980\mathrm{nm}$, which is the highest optical SNR achieved so far for a $50\mathrm{km}$ long FBG sensor system, to our knowledge.