Addressing the urgent need for high-temperature vector vibration monitoring in extreme environments such as aerospace and oil exploration, this paper proposes and develops a novel accelerometer based on femtosecond laser-engraved eccentric fiber Bragg gratings (FBGs) and nickel-coated reflectors. This sensor employs a highly localized FBG with a 10 mm length and 1μm eccentricity fabricated within a single-mode fiber. It utilizes asymmetric refractive index modulation to achieve direction-sensitive cladding mode coupling. Simultaneously, a 30-nm-thick nickel film is sputter-deposited onto the fiber end-face to form a single-ended reflection structure. This design eliminates complex processes like fiber taper drawing and eccentric fusion splicing, overcoming the stability limitations of existing vector sensors at elevated temperatures. Experimental results demonstrate spectral stability across the 25 ℃–1020 ℃ temperature range and effective vibration measurement at 800 ℃. At the room temperature and 800 ℃, the acceleration sensitivities are 0.169 V/g (R²=0.993) and 0.0743 V/g (R²=0.989), respectively, with a common frequency response range of 15 Hz–25 Hz. The maximum angular response sensitivities are 2.4 V/g and 1.2 V/g, respectively, fully validating its reliable vector detection capability across a wide temperature range. This study provides a compact, mechanically robust, and high-temperature-resistant solution for vibration monitoring in extreme environments.

To further improve sensor sensitivity, a strain and temperature sensor based on the harmonic Vernier effect with cascaded Sagnac interferometers (SIs) is proposed. Through a combination of simulation and experimentation, it is shown that the basic Vernier effect can be realized when the lengths of the polarization-maintaining fiber (PMF) in two SIs are slightly different. Furthermore, the first-order harmonic Vernier effect can be achieved when the lengths of two PMFs are approximately integer multiples. This sensor, leveraging the harmonic Vernier effect, demonstrates higher sensitivity. Compared to a single SI, the strain sensitivity based on the basic Vernier effect is improved to 61.93 pm/με with a magnification factor of 7.6, and the temperature sensitivity is improved to 14.29 nm/℃ with a magnification factor of 9.4. For the first-order harmonic Vernier effect, the strain sensitivity increases to 146.35 pm/με with a magnification factor of 18, and the temperature sensitivity increases to 24.92 nm/℃ with a magnification factor of 16.5. Additionally, the sensor based on the harmonic Vernier effect exhibits good stability in strain and temperature measurement. Unlike the basic Vernier effect, the harmonic Vernier effect does not require strict control of the reference and sensing interferometer lengths, further increasing sensitivity. Due to its simple structure and low cost, the proposed sensor shows significant potential for applications in high-precision measurement engineering and medical treatment.

A new type of human immunoglobulin G (IgG) sensors based on the surface plasmon resonance (SPR) in the low refractive index (RI) plastic optical fiber (POF) and an antibody immobilization method is presented. A 50-nm-thick gold film was formed on the polished D-shaped fiber surface by magnetron sputtering. The RI response of the POF sensor is 30049.61 nm/RIU, which is 26.5 times higher than that of single mode fiber (SMF) SPR sensors. The proposed SPR biosensor can be developed by simple and rapid modification of the gold film with 11-mercapto undecanoic acid (MUA). Upon immobilization of the goat anti-human IgG antibody, the resonance wavelength shifts by 11.2 nm. The sensor can be used to specifically detect and quantify the human IgG at concentrations down to 245.4 ng/mL with the sensitivity of 1.3277 nm per µg/mL, which offers an enhancement of 12.5-fold compared to that of the conventional SMF based SPR sensors. The proposed device may find the potential applications in the case of use at the point of care.

Ultrasensitive nanomechanical instruments, e.g. atomic force microscopy (AFM), can be used to perform delicate biomechanical measurements and reveal the complex mechanical environment of biological processes. However, these instruments are limited because of their size and complex feedback system. In this study, we demonstrate a miniature fiber optical nanomechanical probe (FONP) that can be used to detect the mechanical properties of single cells and in vivo tissue measurements. A FONP that can operate in air and in liquids was developed by programming a microcantilever probe on the end face of a single-mode fiber using femtosecond laser two-photon polymerization nanolithography. To realize stiffness matching of the FONP and sample, a strategy of customizing the microcantilever’s spring constant according to the sample was proposed based on structure-correlated mechanics. As a proof-of concept, three FONPs with spring constants varying from 0.421 N m⁻¹ to 52.6 N m⁻¹ by more than two orders of magnitude were prepared. The highest microforce sensitivity was 54.5 nm μN⁻¹ and the detection limit was 2.1 nN. The Young’s modulus of heterogeneous soft materials, such as polydimethylsiloxane, muscle tissue of living mice, onion cells, and MCF-7 cells, were successfully measured, which validating the broad applicability of this method. Our strategy provides a universal protocol for directly programming fiber-optic AFMs. Moreover, this method has no special requirements for the size and shape of living biological samples, which is infeasible when using commercial AFMs. FONP has made substantial progress in realizing basic biological discoveries, which may create new biomedical applications that cannot be realized by current AFMs.

Fiber Bragg grating (FBG) is the most widely used optical fiber sensor due to its compact size, high sensitivity, and easiness for multiplexing. Conventional FBGs fabricated by using an ultraviolet (UV) laser phase-mask method require the sensitization of the optical fiber and could not be used at high temperatures. Recently, the fabrication of FBGs by using a femtosecond laser has attracted extensive interests due to its excellent flexibility in creating FBGs array or special FBGs with complex spectra. The femtosecond laser could also be used for inscribing various FBGs on almost all fiber types, even fibers without any photosensitivity. Such femtosecond-laser-induced FBGs exhibit excellent thermal stability, which is suitable for sensing in harsh environment. In this review, we present the historical developments and recent advances in the fabrication technologies and sensing applications of femtosecond-laser-inscribed FBGs. Firstly, the mechanism of femtosecond-laser-induced material modification is introduced. And then, three different fabrication technologies, i.e., femtosecond laser phase mask technology, femtosecond laser holographic interferometry, and femtosecond laser direct writing technology, are discussed. Finally, the advances in high-temperature sensing applications and vector bending sensing applications of various femtosecond-laser-inscribed FBGs are summarized. Such femtosecond-laser-inscribed FBGs are promising in many industrial areas, such as aerospace vehicles, nuclear plants, oil and gas explorations, and advanced robotics in harsh environments.

A high-sensitivity all-fiber temperature sensor based on a Sagnac interferometer is demonstrated by splicing a section of polarization maintaining fiber (PMF) between two sections of standard single mode fibers (SMFs). In this sensor, the SMF-PMF-SMF structure in the Sagnac loop is bent into a circle to enhance the sensitivity. The length and curvature of the PMF in the loop are investigated and can be optimized to further increase the temperature sensitivity of the sensor. Results show that the radius of the circle has an important effect upon temperature sensitivity due to the bend-induced birefringence variation of the PMF. The SMF-PMF-SMF structure bent into a circle with a radius of 30 mm exhibits a high-sensitivity temperature of 1.73 nm/°C. The sensor is provided with the advantages of easy fabrication, low-insertion loss, and high sensitivity, which may find potential applications in the field of high precision temperature measurement.

Two promising post-treatment techniques, i.e. applying tensile strain and rising temperature, are demonstrated to enhance the mode-coupling efficiency of the CO₂-laser-induced long period fiber gratings (LPFGs) with periodic grooves. Such two post-treatment techniques can be used to enhance the resonant attenuation of the grating to achieve a LPFG-based filter with an extremely large attenuation and to tailor the transmission spectrum of the CO₂-laser-induced LPFG after grating fabrication.

Photonic crystal fibers are usually divided into two different types of fibers: solid-core photonic crystal fibers (PCFs) and air-core photonic bandgaps fibers (PBFs). We presented the fabrication methods and applications of long period fiber gratings (LPFGs) written in these two types of photonic crystal fibers by use of a CO₂ laser. A stain sensor with a high sensitivity was demonstrated by use of an LPFG written in solid-core PCFs. An in-fiber polarizer based on an LPFG was fabricated by use of a focused CO₂ laser beam to notch periodically on a PCF. A novel LPFG was written in an air-core PBF by use of a CO₂ laser to collapse periodically air holes in the fiber cladding.