We report a compact in-line fiber Mach-Zehnder (M Z) interferometer enabled by a femtosecond-laser two-photon polymerization (TPP) printed microfiber in a C-shaped fiber. The C-shaped segment is fusion-spliced between two standard single mode fibers, providing a mechanically robust platform, while the printed polymer microfiber bridges the two cores and supports multimode propagation to generate intermodal interference. The open cavity of the C-shaped fiber ensures strong evanescent-field interaction with the surrounding medium, resulting in high sensitivity. The device exhibits a high temperature sensitivity of 361 pm/°C and an air refractive-index sensitivity of 18300 nm/RIU. The sensor also demonstrates good stability and sub second response. Compared with conventional tapered fiber M-Z interferometers, the proposed M-Z interferometer represents a promising and scalable solution for high-sensitivity, multi parameter sensing, with potential applications in environmental monitoring, gas sensing, and biochemical detection.

This work presents a compact fiber Bragg grating (FBG)-based sensor that decouples curvature/strain and temperature effects using a configurable rectangular optical fiber design. The sensor structure integrates a side-hole rectangular fiber (SHRF) with dimension of 175 mu m & times; 358 mu m with a 97-mu m diameter air hole, along with embedded reduced-cladding fibers (RCFs). It features two FBGs, one inscribed in the SHRF, which is sensitive to both curvature/strain and temperature, and another within an RCF embedded in the air hole, which serves as a temperature reference. The design accommodates both reduced-cladding silica optical fiber (RC-SOF) and reduced-cladding polymer optical fiber (RC-POF), with the RC-POF exhibiting significantly enhanced temperature sensitivity (-43.64 pm/degrees C in the range 20-80 degrees C), four times greater than that of the SHRF (11.04 pm/degrees C), enabling precise thermal discrimination. Experimental results demonstrate a curvature sensitivity of -0.26 nm/m(-1), an axial strain sensitivity of 0.89 pm/mu & varepsilon;, and a bending-induced strain sensitivity of 1.09 pm/mu & varepsilon; for the SHRF. The embedded RCFs show negligible curvature/strain response, with the RC-SOF exhibiting 10.70 pm/degrees C sensitivity (50-180 degrees C). The SHRF's flat cladding enables clear orientation identification and enhances strain transfer/bonding. This all-fiber design provides thermal compensation without requiring complex packaging or separate reference elements.

An all-fiber Mach-Zehnder interferometer (MZI) is presented by embedding a two-photon-polymerized polymer microfiber inside a C-shaped fiber segment. The C-shaped fiber is fusion-spliced between two standard single-mode fibers to form a mechanically stable in-line configuration, while the directly written microfiber bridges the separated fiber cores and supports multiple guided modes. Interference among these modes generates a wavelength-dependent transmission spectrum that responds to thermal variations in the surrounding environment. Experimental characterization demonstrates a temperature sensitivity of 532 pm/°C. In contrast to MZIs relying on mechanically fragile tapered fibers, the proposed architecture combines compactness, structural robustness, and flexible microfabrication, offering a promising platform for high-sensitivity sensing of temperature and other environmental or biochemical parameters.

Tactile perception, particularly at the fingertips, is fundamental to human dexterity, enabling fine motor control and reliable manipulation of objects through the precise real-time modulation of normal and shear forces based on encountered frictional conditions. To bridge this capability gap in robotics, a novel tactile-sensitive artificial skin is designed to significantly enhance robot-object interaction and environmental recognition. The artificial skin, fabricated from a 2-mm thick silicone elastomer membrane embedded with polymer optical fiber Bragg grating (FBG) sensor array, achieves large measurement range (detecting forces up to 10 N normal and +/- 4 N shear) and high sensitivity for multiaxial forces. The skin's performance was evaluated through tests involving normal force loading of up to 10 N and shear force loading around +/- 4 N using a three-axis translation gantry. Additionally, the study examines slip-induced vibrations on various textured surfaces. A multi-input multi-output convolutional neural network (MIMO-CNN) was developed to simultaneously estimate force and recognize textures based on multichannel FBG inputs. The MIMO-CNN achieved an R-squared value of 0.96 for force estimation and classification accuracy of 94% for texture recognition across 20 fabric samples. These findings highlight the potential of tactile-sensitive artificial skin to enhance robotic perception and manipulation, paving the way for more advanced humanoid robotic systems.

In this paper, we propose a Vernier effect-based fiber-optic temperature sensor. The structure of the sensor is fabricated by cascading a single mode fiber (SMF) pigtail together with a C-shaped fiber segment and another SMF segment. In this original sensing architecture, the C-shaped fiber segment constitutes a Fabry-Perot interferometer (FPI) used as a sensing cavity while the SMF segment serves as a reference cavity. These two cavities have a slightly different optical path length to trigger the Vernier effect. Both theoretical calculations and experiments are carried out in the characterization of the sensor measuring temperature. The experimental sensitivity of the sensor is found to be -9.15 nm/degrees C for temperature measurement.

This work introduces a novel fiber Bragg grating (FBG) sensor design for discriminating strain and temperature effects using a side-hole rectangular fiber (SHRF) and embedded reduced-cladding fibers. An FBG in a single-mode fiber with a 75-mu m cladding diameter, embedded within the SHRF air-hole, allows independent temperature measurements, while the SHRF FBG senses both strain and temperature. By embedding a polymer fiber with a 76-mu m cladding diameter, the sensor achieved temperature sensitivity up to four times greater than that of the SHRF with SMF, enabling precise thermal discrimination. This compact, thermally-stable all-fiber sensor design effectively decouples strain and temperature without the need of complex packaging or a separate reference.

In this study, we propose a microfiber waveguide for temperature and air pressure measurement. To improve mechanical strength of the sensor, a C-shaped fiber is sandwiches between two single mode fibers (SMFs) by fusion splice. The microfiber waveguide is 3D printed between two SMFs to connect two fiber cores by two-photon polymerization technology. Due to multimode property of this printed waveguide, a Mach-Zehnder interferometer (MZI) is obtained. This sensor exhibits a high temperature sensitivity of 361 pm/°C at 25°C to 45°C and a high air pressure sensitivity of 55 pm/kPa from 300hpa to 1000hpa. The MZI sensor features significant advantages such as small size, high stability, and easy fabrication, without the need for complex post-processing, showing great potential and broad application prospects in many sensing applications.

Cochlear implantation surgery is currently the only effective treatment for restoring hearing in individuals with severe to profound sensorineural hearing loss. This procedure requires the precise insertion of a cochlear implant electrode array (EA) into cochlea, which has bending radii as small as 2 mm. Surgeons face two major challenges during this process: minimizing cochlear damage and achieving optimal EA positioning within the cochlear pathway. Although optical fiber sensors have been proposed to provide real-time feedback during insertion, the rigidity of traditional silica optical fibers makes them unsuitable for this application. This study examines the integration of three types of silica optical fibers-commercial 50-mu m single-mode fiber, customized 70-mu m eight-shaped fiber, and customized 80- mu m six- hole fiber-into straight EAs to evaluate their impact on insertion force and stiffness. The results indicate that EAs with integrated fibers measuring 50 mu m in diameter have comparable rigidity to those without fibers, up to an insertion depth of about 23 mm in the 25-mm long EAs. Additionally, an optical frequency domain reflectometry sensing system, utilizing 50-mu m single-mode fiber, is employed to facilitate precise real-time positioning of the EA during insertion. Results show that the proposed innovative sensing solutions for cochlear implantation have great potential to enhance surgical precision, improve outcomes, and reduce complications in cochlear implant procedures.

Accelerometers are crucial sensors that measure acceleration resulting from motion or vibration. Compared with their electromechanical counterparts, optical accelerometers are widely regarded as the most promising technology for high-requirement applications. However, compact integration of various optical and mechanical components to create a miniature optomechanical microsystem for acceleration sensing remains a challenge. In this study, we present a miniature optical fiber accelerometer based on a 3D microprinted ferrule-top Fabry-P & eacute;rot (FP) microinterferometer. In-situ 3D microprinting technology was developed to directly print a sub-millimeter-scale 3D proof mass/thin-film reflector-integrated FP microinterferometer on the inherently light-coupled end face of a fiber optic ferrule. Experimental results demonstrate that the optical fiber accelerometer has a flat response over a bandwidth of 2 to 3 kHz and its noise equivalent acceleration is 62.45 mu g/Hz under 1-g acceleration at 2 kHz. This ultracompact optical fiber interferometric accelerometer offers several distinct advantages, including immunity to electromagnetic interference, remote-sensing capability, and high customizability, making it highly promising for a variety of stringent acceleration-monitoring applications.

We present a compact, multidirectional force sensor based on a novel 8-shaped fiber inscribed with a fiber Bragg grating (FBG), designed to overcome the limitations of conventional force sensors in sensitivity, size, and directional discrimination. The 8-shaped fiber features two channels with a 43.3 mu m waist and an elliptical core (minor/major axes: 4.6/22.5 mu m). When subjected to normal forces along the minor and major axes, the sensitivities of 85.81 and 31.06 pm/N were achieved, respectively. Shear force evaluation on a 15 degrees inclined plane revealed sensitivities of 209.17 (minor axis) and 123.53 pm/N (major axis). This design eliminates the need for complex polymer embedding, offering a twofold improvement in both normal force sensitivity and shear force sensitivity compared to a sensor utilizing conventional single-mode fiber (SMF). This innovative sensor demonstrates precise detection of applied forces, enabling exceptional directional sensitivity, making it a valuable instrument for applications requiring accurate force measurement and monitoring in specific orientations.

long-period gratings (LPGs) exhibit superior flexibility and durability compared to conventional silica-based counterparts, allowing for reliable performance in dynamic environments and broadening fabrication options. This study presents the fabrication of LPGs in single-mode polymer optical fibers (POFs) made of ZEONEX, a cyclo-olefin polymer (COP), through mechanical compression. LPGs with grating length ranging from 20 to 40 mm were produced in 70-and 110-mu m cladding-diameter fibers via mechanical compression techniques. Key findings reveal that mechanically compressed LPGs require minimal force (2.2-3.4 N) to achieve significant resonance dips with attenuation up to 26.2 dB, along with strain-dependent spectral tun-ability of 1.64-1.74 pm/mu epsilon. Also, permanent mechanically produced LPGs were created by heating the POF during compression, exhibiting resonance strength of 1520 dB and insertion loss of 23 dB. Spectral tunability of 1.66-1.85 pm/mu epsilon and temperature sensitivity of 1.336-1.575 nm/degrees C were achieved. Furthermore, permanent LPGs in 110-mu m fibers demonstrate a surrounding refractive index (SRI) sensitivity of 236.6 nm/RIU at similar to 1.4441 threshold, while 70-mu m fiber LPGs display linear SRI response (47.3-49.6 nm/RIU) across a wide SRI range (1.34-1.45), highlighting their versatility. These results establish POFs as promising platforms for tunable and highly versatile LPG-based sensors, with mechanical compression offering a high-performance alternative to laser-based methods for diverse sensing applications.

We experimentally demonstrated in-line Mach-Zehnder interferometric sensors based on inter-mode interference (IMI) and inter-core-mode interference (ICMI) for refractive index (RI) measurements. The structure of the IMIbased sensor consisted of a C-shaped fiber spliced between two multi-mode fibers (MMF) with the opposite ends spliced to lead-in and lead-out single-mode optical fibers. The ICMI-based sensor has a similar structure as the IMI-based sensor but with an additional in-house fabricated three-core multi-core fiber (MCF) spliced between the MMF and C-shaped fiber. The performance of the two RI sensors was evaluated and compared using waterglycerol solutions with RI values ranging from 1.333 to 1.338. The RI sensitivity, as well as other characteristic properties such as free spectral range and extinction ratio in relation to three lengths of the C-shaped fiber of 2, 2.5 and 3 mm, were investigated in both structures. The open-cavity characteristic of the C-shaped fiber allows the solution to interact with the optical path of the sensors, hence improving their RI measurement sensitivities. Both types of sensors exhibit linear wavelength responses with variation in RI. The RI sensitivity of the IMI-based sensor exhibits minor variations when the length of the C-shaped fiber is altered, with a maximum sensitivity of -7999.76 nm/RIU. In contrast, the RI sensitivity of ICMI-based sensor exhibits variability when changing the Cshaped fiber length, potentially due to variations in the RI distribution among the cores within the MCF, with its maximum sensitivity being -6211.98 nm/RIU. The temperature sensitivity of all the tested sensors was around 0.7 nm/ degrees C, denoting a low temperature cross-sensitivity of -7.427 x 10-5 RIU/ degrees C. This work experimentally exhibits a comparison of two sensing effects in the same configuration. The long length (longer than 2 mm) of the sensing element facilitates the fabrication process, and the open-cavity present in the proposed sensors yields a high sensitivity, which is in the same order of magnitude as the IMI-based sensors and is one order of magnitude higher than the ICMI-based sensors, emphasizing their promising candidacy for label-free optical sensing of chemical and biological samples.

This study presents an innovative approach to pressure sensing by utilizing an 8-shaped birefringent optical fiber fabricated through a mechanical milling technique. The proposed sensor employs the Sagnac effect to monitor wavelength shifts in the interference pattern. Experimental results demonstrate that the sensor achieves a high sensitivity of 441 pm/kPa within the pressure range of 0 to 20 kPa, enabling accurate pressure measurements with a resolution of up to 2 Pa. The measurements exhibit good repeatability, and demonstrate a linearity of 99.65%. In addition, by integrating a fiber Bragg grating fabricated using a femtosecond laser into the 8-shaped fiber within the Sagnac loop, the temperature cross-sensitivity was determined to be 20 Pa/degrees C. The combination of the 8-shaped birefringent fiber and the Sagnac interferometric technique offers a promising solution for achieving precise and reliable pressure sensing in low-pressure environments.

An 8-shaped birefringent optical fiber fabricated by mechanical milling is proposed for low pressure sensing based on the Sagnac interferometric technique. The sensor provides a sensitivity of 441 pm/kPa with an accuracy up to 2 Pa.

提出基于特种微结构光纤的振动加速度传感器，将高性能的光纤振动传感器应用于铁路结构的健康监测，设计并研制了Sagnac型的振动传感器。通过对光纤结构的合理设计和优化，可以获得高双折射率特性，并能针对外界侧向压力的变化产生对应的相位改变量，进而建立起外界振动引起的动态压力与微结构光纤中传导模式之间的线性关系。基于此原理设计的振动传感器具有较高的加速度灵敏度，以及较宽的频率响应，灵敏度高于25pm/g，其实际谐振频率高于2500Hz，并与传感器机械结构封装有关。通过对铁路运营列车的实时监测，所提出的新型微结构光纤振动传感器可以准确获得低频范围内的大幅振动信息，并可较好的反映出高频扰动，有助于实时监测钢轨波纹形变。所提出的新型六孔微结构光纤振动传感器对于铁路、桥梁及机械等结构的振动实时监测提供了新思路和方法，具有很大的应用前景。

An optical accelerometer realized with a new side-hole fiber in a Sagnac interferometric scheme, was developed by utilizing the birefringent and stiffness properties of the optical fiber to achieve a high sensitivity, a broad frequency range, a wide acceleration range, and an excellent linearity. The proposed accelerometer exhibits a resonant frequency of 2300 Hz and an average sensitivity of 44.3 (±3.0) pm/g over a broad frequency range from 4 to 900 Hz. The linearity of the signal response exceeds 99.9% and its transverse sensitivity is less than 11% of the main axis. It exhibits a dynamic acceleration range of 62.5 dB which is related to the applied acceleration from 0.03 to 40 g, making the side-hole fiber a good candidate for acceleration/vibration sensing applications. Furthermore, the tradeoff relationship between the sensitivity and the resonant frequency was significantly improved when compared to that of fiber Bragg grating-based accelerometers.

We report, for the first time, the development of a rectangular fiber with two cores consisting of different refractive indices for the discrimination of temperature effects during strain or curvature measurements using fiber Bragg gratings written in a single segment of fiber. Two cores with a diameter of 8 μm and a rectangular cladding with dimensions of 120 μm × 255 μm permit convenient splicing to conventional single-mode fiber and support easy identification of the orientation of the fiber. When placed in the desired orientation, each core of the two-core rectangular fiber experiences a different spectral shift in the presence of applied strain or curvature. The sensitivities of the two cores are 0.87 and 1.08 pm/με for strain, 126 and -128 pm/m -1 for curvature, and 10.05 and 10.40 pm/ o C for temperature, respectively. In addition, the flat cladding structure of the fiber provides a large physical contact area between the sensing fiber and the substrate than a circular fiber and allows better strain coupling between the object and the fiber.

A simple, compact, and highly sensitive gas pressure sensor based on a Fabry–Perot interferometer (FPI) with a silicone rubber (SR) diaphragm is demonstrated. The SR diaphragm is fabricated on the tip of a silica tube using capillary action followed by spin coating. This process ensures uniformity of its inner surface along with reproducibility. A segment of single mode fiber (SMF) inserted into this tube forms the FPI which produces an interference pattern with good contrast. The sensor exhibits a high gas pressure sensitivity of −0.68 nm/kPa along with a low temperature cross-sensitivity of ≈ 1.1 kPa/°C.

We report a highly sensitive twist sensor based on a Sagnac interferometer constructed with a new type of optical fiber which contains an elliptical core and two large semicircular-holes, where the slow axis of the core orthogonal to the air-holes has a large sensitivity towards twist-induced birefringent changes. The novel fiber structure results in a highest twist sensitivity of 5.01 nm/° at a chosen dip over the range from 370°-400°. The resonance dips in the interference pattern respond with different rates in the wavelength shifts in the presence of physical parameters permitting to experimentally distinguish directional torsion, axial strain and temperature.

In this paper, we present a novel accelerometer based on the Sagnac interferometer configuration using a polarization-maintaining photonic crystal fiber (PM-PCF), which has a sensitivity of ~8 pm/G, and a resonant frequency exceeding 2.5 kHz. The proposed accelerometer is capable of functioning with a constant sensitivity in a large frequency range from 0 to 1 kHz which is much wider than many FBG-based accelerometers. Experimental results obtained from a field test in railway monitoring, demonstrate a broader frequency range for the proposed accelerometer compared to that of the FBG based accelerometer and is comparable to the conventional piezoelectric sensor. The abrupt change in the acceleration measured by the sensor aids in locating any defect or crack present on the railway track. To the best of our knowledge, this is the first demonstration of an accelerometer based on a fiber interferometer aimed for the railway industry. The proposed accelerometer operating at high accelerations (>40 G) and capable of functioning at a broad frequency range, shows significant potential in being used in applications which require detection of strong and fast vibrations, especially in structural health monitoring of trains and railway tracks in real time.