Femtosecond laser auto-positioning direct writing of a multicore fiber Bragg grating array for shape sensing

Magnetic Soft Catheter Robot System for Minimally Invasive Treatments of Articular Cartilage Defects

Articular cartilage defects are among the most common orthopedic diseases, which seriously affect patients' health and daily activities, without prompt treatment. The repair biocarrier-based treatment has shown great promise. Total joint injection and open surgery are two main methods to deliver functional repair biocarriers into the knee joint. However, the exhibited drawbacks of these methods hinder their utility. The repair effect of total joint injection is unstable due to the low targeting rate of the repair biocarriers, whereas open surgery causes serious trauma to patients, thereby prolonging the postoperative healing time. In this study, we develop a magnetic soft catheter robot (MSCR) system to perform precise in situ repair of articular cartilage defects with minimal incision. The MSCR processes a size of millimeters, allowing it to enter the joint cavity through a tiny skin incision to reduce postoperative trauma. Meanwhile, a hybrid control strategy combining neural network and visual servo is applied to sequentially complete the coarse and fine positioning of the MSCR on the cartilage defect sites. After reaching the target, the photosensitive hydrogel is injected and anchored into the defect sites through the MSCR, ultimately completing the in situ cartilage repair. The in vitro and ex vivo experiments were conducted on a 3D printed human femur model and an isolated porcine femur, respectively, to demonstrate the potential of our system for the articular cartilage repair.

Length-extended 3D shape sensor using wavelength/space-division multiplexing grating arrays in a multicore fiber

Limited by the multiplexing number of fiber Bragg grating (FBG), further improvement in the length of 3D shape sensing based on FBG technology is challenging. In this Letter, a wavelength-division and space-division multiplexing multicore fiber grating method is proposed, which extends the sensing length. Employing the femtosecond-laser point-by-point technology, we inscribed WDM grating arrays in six outer cores of a seven-core fiber, respectively. Three cores were utilized as a segment for shape sensing, and two such segments were offset by a specific length and combined to form a shape sensor. Utilizing an FBG interrogator, the proposed shape sensor achieved 2D and 3D shape sensing at a length of 967 mm and effectively mitigated the effects of temperature variations. In experiments, maximum shape reconstruction errors per unit lengths are 1.89%, 2.72%, and 1.47% for 2D shape, 3D shape, and an arbitrary shape under variable temperature conditions, respectively. The proposed method holds promise for further extending the shape sensing length by utilizing multicore fibers or fiber clusters containing more cores.

Multi-Core Fiber Bragg Grating and Its Sensing Application

With the increase in the demand for large-capacity optical communication capacity, multi-core optical fiber (MCF) communication technology has developed, and both the types of MCFs and related devices have become increasingly mature. The application of MCFs in the field of sensing has also received more and more attention, among which MCF fiber Bragg grating (FBG) devices have received more and more attention and have been widely used in various fields. In this paper, the main writing methods of MCF FBGs and their sensing applications are reviewed. The future development of the MCF FBG is also prospected.

Highly birefringent side-hole fiber Bragg grating for high-temperature pressure sensing

We demonstrate a novel, to the best of our knowledge, high-temperature pressure sensor based on a highly birefringent fiber Bragg grating (Hi-Bi FBG) fabricated in a dual side-hole fiber (DSHF). The Hi-Bi FBG is generated by a femtosecond laser directly written sawtooth structure in the DSHF cladding along the fiber core through the slow axis (i.e., the direction perpendicular to the dual-hole axis). The sawtooth structure serves as an in-fiber stressor and also generates Bragg resonance due to its periodicity. The DSHF was etched by hydrofluoric acid to increase its pressure sensitivity, and the diameter of two air holes was enlarged from 38.2 to 49.6 µm. A Hi-Bi FBG with a birefringence of up to 1.8 × 10-3 was successfully created in the etched DSHF. Two distinct reflection peaks could be observed by using a commercial FBG interrogator. Moreover, pressure measurement from 0 to 3 MPa at a high temperature of 700°C was conducted by monitoring the birefringence-induced peak splits and achieved a high-pressure sensitivity of -21.2 pm/MPa. The discrimination of the temperature and pressure could be realized by simultaneously measuring the Bragg wavelength shifts and peak splits. Furthermore, a wavelength-division-multiplexed (WDM) Hi-Bi FBG array was also constructed in the DSHF and was used for quasi-distributed high-pressure sensing up to 3 MPa. As such, the proposed femtosecond laser-inscribed Hi-Bi FBG is a promising tool for high-temperature pressure sensing in harsh environments, such as aerospace vehicles, nuclear reactors, and petrochemical industries.

OFDR shape sensor based on a femtosecond-laser-inscribed weak fiber Bragg grating array in a multicore fiber

An optical frequency domain reflectometry (OFDR) shape sensor was demonstrated based on a femtosecond-laser-inscribed weak fiber Bragg grating (WFBG) array in a multicore fiber (MCF). A WFBG array consisting of 60 identical WFBGs was successfully inscribed in each core along a 60 cm long MCF using the femtosecond-laser point-by-point technology, where the length and space of each WFBG were 2 and 8 mm, respectively. The strain distribution of each core in two-dimensional (2D) and three-dimensional (3D) shape sensing was successfully demodulated using the traditional cross correlation algorithm, attributed to the accurate localization of each WFBG. The minimum reconstruction error per unit length of the 2D and 3D shape sensors has been improved to 1.08% and 1.07%, respectively, using the apparent curvature vector method based on the Bishop frame.

Roadmap on optical sensors

Optical sensors and sensing technologies are playing a more and more important role in our modern world. From micro-probes to large devices used in such diverse areas like medical diagnosis, defence, monitoring of industrial and environmental conditions, optics can be used in a variety of ways to achieve compact, low cost, stand-off sensing with extreme sensitivity and selectivity. Actually, the challenges to the design and functioning of an optical sensor for a particular application requires intimate knowledge of the optical, material, and environmental properties that can affect its performance. This roadmap on optical sensors addresses different technologies and application areas. It is constituted by twelve contributions authored by world-leading experts, providing insight into the current state-of-the-art and the challenges their respective fields face. Two articles address the area of optical fibre sensors, encompassing both conventional and specialty optical fibres. Several other articles are dedicated to laser-based sensors, micro- and nano-engineered sensors, whispering-gallery mode and plasmonic sensors. The use of optical sensors in chemical, biological and biomedical areas is discussed in some other papers. Different approaches required to satisfy applications at visible, infrared and THz spectral regions are also discussed.

Simultaneous Measurement of Microdisplacement and Temperature Based on Balloon-Shaped Structure

An optical fiber sensor for the simultaneous measurement of microdisplacement and temperature based on balloon-shaped single-mode fibers cascaded with a fiber Bragg grating with two core-offset joints is proposed. The interference between the core mode and cladding mode is caused by the stimulation of the cladding mode by the core-offset joints' structure. The cladding of the core has a distinct refractive index, which causes optical path differences and interference. The balloon-shaped structure realizes mode selection by bending. As the displacement increases, the radius of the balloon-shaped interferometer changes, resulting in a change in the interference fringes of the interferometer, while the Bragg wavelength of the fiber grating remains unchanged. Temperature changes will cause the interference fringes of the interferometer and the Bragg wavelength of the fiber grating to shift. The proposed optical fiber sensor allows for the simultaneous measurement of microdisplacement and temperature. The results of the experiment indicate that the sensitivity of the interferometer to microdisplacement is 0.306 nm/µm in the sensing range of 0 to 200 μm and that the temperature sensitivity is 0.165 nm/°C, respectively. The proposed curvature sensor has the advantages of a compact structure, extensive spectrum of dynamic measurement, high sensitivity, and simple preparation, and has a wide range of potential applications in the fields of structural safety monitoring, aviation industry, and resource exploration.

Supermode Bragg grating inscribed in a strongly coupled seven-core fiber and its responses to temperature and curvature

A Bragg grating is successfully inscribed in a piece of strongly coupled seven-core fiber (SCF). There are two separate Bragg resonance notches observed in the transmission spectrum, corresponding to backward coupling of HE₁₁-like and HE₁₂-like supermodes of the SCF. The mode coupling mechanism of the Bragg grating is theoretically investigated via modeling and analyzing modal properties of the SCF. The theoretical results agree well with the experimental results. Since the SCF is spliced between two standard single mode fibers with central alignments at both ends, the transmission spectrum of the device also contains a set of interference fringe due to modal interference between the supermodes. The device's responses to temperature and curvature are experimentally measured, respectively. The obtained temperature sensitivities and curvature sensitivities of the supermode Bragg grating notches are 9.55 pm/°C and 9.55 pm/°C, -1.8 pm/m^-1 and -112.3 pm/m^-1, respectively. The obtained temperature sensitivity and curvature sensitivity of one of the interference spectrum dips are 11.8 pm/°C and -3909.8 pm/m^-1, respectively. This device is potentially useful for simultaneous measurement of temperature and curvature.

Orthogonal single-mode helical Bragg gratings created in fiber cladding for vector bending measurement

We demonstrate a novel, to the best of our knowledge, two-dimensional vector bending sensor based on orthogonal helical Bragg gratings inscribed in the cladding of a conventional single-mode fiber (SMF). The helical cladding fiber Bragg gratings (HCFBGs) are created by using a femtosecond laser direct writing technology and a quarter-pitch graded index fiber (GIF) is used in front of the HCFBGs to diverge the core mode into fiber cladding. In contrast to the multimode resonance observed in conventional cladding Bragg gratings inscribed by using a femtosecond laser point-by-point (PbP) or line-by-line (LbL) technology, the proposed HCFBGs exhibit stable narrowband single-mode Bragg resonance. An HCFBG with a low peak reflectivity of -50.77 dB and a narrow bandwidth of 0.66 nm was successfully fabricated by using a lateral offset of 45 µm between the HCFBG and the fiber core axis. Moreover, two orthogonal HCFBGs were fabricated in the SMF cladding and used for vector bending sensing. Strong orientation dependence could be seen in omnidirectional bending measurement, exhibiting a maximum bending sensitivity of up to 50.0 pm/m^-1, which is comparable to that in a multicore FBG. In addition, both the orientation and amplitude of bending vector could be reconstructed by using the measured Bragg wavelength shifts in two orthogonal HCFBGs. As such, the proposed HCFBGs could be used in many applications, such as structural health monitoring, robotic arms, and medical instruments.

Ultrafast laser writing of arbitrary long low-loss waveguides in optical fibers

We propose an innovative femtosecond laser writing approach, based on a reel-to-reel configuration, allowing the fabrication of arbitrary long optical waveguides in coreless optical fibers directly through the coating. We report few meters long waveguides operating in the near-infrared (near-IR) with propagation losses as low as 0.055 ± 0.004 dB/cm at 700 nm. The refractive index distribution is shown to be homogeneous with a quasi-circular cross section, its contrast being controllable via the writing velocity. Our work paves the way for the direct fabrication of complex arrangements of cores in standard and exotic optical fibers.

Two-dimensional vector bending sensor based on a hole-assisted three-core fiber coupler

We demonstrate a two-dimensional vector bending sensor based on a hole-assisted three-core fiber (HATCF) coupler. The sensor is built by splicing a section of HATCF between two single-mode fibers (SMFs). The resonance couplings between the center core and the two suspended cores of the HATCF occur at different wavelengths. Two completely discrete resonance dips are observed. The bending response of the proposed sensor is investigated over a 360° range. The bending curvature and direction can be identified by interrogating the wavelengths of the two resonance dips, and a maximum curvature sensitivity of -50.62 nm/m^-1 is achieved at 0° direction. Moreover temperature sensitivity of the sensor is less than -34.9 pm/°C.

Vectorial distributed acoustic sensing based on a multicore fiber and phase-sensitive optical time-domain reflectometry

A vectorial distributed acoustic sensing (vDAS) system is proposed and demonstrated for distributed two-dimensional vector vibration measurements based on phase-sensitive optical time-domain reflectometry (ΦOTDR). An optical pulse compression (OPC) algorithm was used to achieve high spatial resolution and suppress fading noise, and a Rayleigh-enhanced seven-core fiber (eSCF) was used to magnify the differentials of Rayleigh backscattering (RBS) in various cores undergoing vibrations. A combination of OPC and eSCF allows the system to fully quantify perturbations with a spatial resolution of 1.1 m and a strain resolution of 1.1 pε/√Hz, achieving a maximum acceleration sensitivity of 1.04 mrad/g at 60 Hz and an orientation reconstruction error of 1.92°. The proposed vDAS system can achieve vectorial distributed vibration sensing without the need for specific fiber layouts or complex reconstruction algorithms, increasing its potential for applications in vertical seismic profiles or pipeline inspection.

High-Temperature-Resistant Fiber Laser Vector Accelerometer Based on a Self-Compensated Multicore Fiber Bragg Grating

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.

Direct Bragg Grating Inscription in Single Mode Step-Index TOPAS/ZEONEX Polymer Optical Fiber Using 520 nm Femtosecond Pulses

We experimentally report fiber Bragg gratings (FBGs) in a single mode step-index polymer optical fiber (POF) with a core made of TOPAS and cladding made of ZEONEX using 520 nm femtosecond pulses and a point-by-point (PbP) inscription method. With different pulse energies between 9.7 nJ and 11.2 nJ, 12 FBGs are distributed along the cores of two pieces of POFs with negative averaged effective index change up to ~6 × 10⁻⁴ in the TOPAS. For POF 1 with FBGs 1–6, the highest reflectivity 45.1% is obtained with a pulse energy of 10.6 nJ. After inscription, good grating stability is reported. Thanks to the post-annealing at 125 °C for 24 h, after cooling the grating reflectivity increases by ~10%. For POF 2 with FBGs 7–12, similar FBG data are obtained showing good reproducibility. Then, the FBGs are annealed at 125 °C for 78 h, and the average reflectivity of the FBGs during the annealing process increases by ~50% compared to that before the annealing, which could be potentially applied to humidity insensitive high temperature measurement.