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Abstract An acidic gas is an important basic chemical raw material used for synthesizing fertilizers, insecticides, explosives, dyes, and salts. Alternatively, inorganic acidic gases that leak into the air have harmful effects on the human health, infrastructure, and cultural relics. Therefore, the demand for inorganic acidic gas sensors for air quality monitoring and management has continuously increased, enabling the development of various sensing technologies. Among them, fiber-optic sensors are promising for acidic gas detection because of their excellent in-situ measurement, resistance to corrosion, anti-electromagnetic interference, long service life, and smart structure. In particular, fiber-optic sensors have proven to be very useful for the in-situ detection and distributed monitoring of multiple gas parameters. However, the sensitivity, selectivity, repeatability, and limits of detection of these sensors can be improved to achieve acceptable performance levels for practical applications. In this review, we introduce fiber-optic sensors based on structured optical fibers and fiber gratings for detecting H2S, SO2, NO2, CO2, and N2O. The structures of the sensing regions, gas-sensitive materials, and measurement principles of these sensors are presented. The sensitivity, selectivity, limit of detection, and response time of the sensors are summarized. Finally, the future of fiber-optic sensors for the detection of inorganic acidic gases is discussed.

An acidic gas is an important basic chemical raw material used for synthesizing fertilizers, insecticides, explosives, dyes, and salts. Alternatively, inorganic acidic gases that leak into the air have harmful effects on the human health, infrastructure, and cultural relics. Therefore, the demand for inorganic acidic gas sensors for air quality monitoring and management has continuously increased, enabling the development of various sensing technologies. Among them, fiber-optic sensors are promising for acidic gas detection because of their excellent in-situ measurement, resistance to corrosion, anti-electromagnetic interference, long service life, and smart structure. In particular, fiber-optic sensors have proven to be very useful for the in-situ detection and distributed monitoring of multiple gas parameters. However, the sensitivity, selectivity, repeatability, and limits of detection of these sensors can be improved to achieve acceptable performance levels for practical applications. In this review, we introduce fiber-optic sensors based on structured optical fibers and fiber gratings for detecting H2S, SO2, NO2, CO2, and N2O. The structures of the sensing regions, gas-sensitive materials, and measurement principles of these sensors are presented. The sensitivity, selectivity, limit of detection, and response time of the sensors are summarized. Finally, the future of fiber-optic sensors for the detection of inorganic acidic gases is discussed. [ABSTRACT FROM AUTHOR]

[Display omitted] An all-fiber-optic system for rapid detection of antibiotic concentration, based on an optical enzyme biosensor with microfiber interferometer (MFI) and fiber gratings (FBGs) power variation, is proposed and experimentally validated. During the experiment, β-lactamase(β-LS) is fixed on the polyaniline (PANI)-coated optical fiber by cross-linking through glutaraldehyde (GA) covalent bonding. β-LS can hydrolyze β-lactam antibiotics to generate acidic by-products that transform polyaniline from the form of the emerald base to emerald salt, which results in the surface refractive index (RI) variation of MFI, to convert MFI wavelength and FBGs power macroscopic change for feedbackingly detecting the concentration of β-lactam antibiotics. The detection of amoxicillin (AMX) in deionized water at concentrations in the range of 0.01–100 nM resulted in a wavelength change sensitivity of 0.6 nm/nM, and FBGs power difference change sensitivity of 1.3 dB/nM, with a detection limit LOD = 0.04 nM in real food and urine samples. The sensing system by the same calibration technique can detect antibiotic concentrations in different substances (tap water, milk and artificial urine). This developed all-fiber-optic system can be used as a rapid solution for the measurement of β-lactam antibiotic residues in food and the environment. [ABSTRACT FROM AUTHOR]

In this work, the tuning property and sensitivity enhancement of the long period fiber grating (LPFG) coated with the higher refractive index film are demonstrated theoretically by integrating a monolayer graphene. The general variation rule of the mode characteristics and the resonance with the chemical potential of the graphene are explored. The polarization-independent transmission and sensing characteristics are obtained even though the optical property of the graphene is polarization-dependent. The results reveal that the sensing characteristics are significantly dependent on the tunable state of the graphene, which can be used to greatly optimize the sensing performance. By this approach, an ultrahigh sensitivity up to 28 337.5 nm/RIU is achieved, which is 2.57 times higher than that of the conventional LPFG without graphene. The sensitivity can be further optimized by integrating few-layer graphene. This tunable property at a wide range makes the graphene integrated LPFG devices without destroying the fiber integrity ideal for wide applications, such as biochemical sensing and optical modulation. [ABSTRACT FROM AUTHOR]

Pd-based alloy coating on optical fiber is widely employed for hydrogen concentration sensing. However, it is difficult to distinguish the Pd-based alloy expansion from hydrogen adsorption or thermal effect. In this article, a novel fiber-optic hydrogen sensor based on $\pi $ -shifted fiber Bragg grating (FBG) with locally coated Pd/Ta alloy layer, which enables an in situ temperature compensation, is proposed and experimentally demonstrated. The hydrogen sensitivity difference between the extremely narrow peak of $\pi $ -shifted FBG and the sidelobe wavelength is employed to realize the hydrogen concentration detection with temperature compensation. Theoretical simulation and experiment show that the hydrogen sensitivity of the structure can be enhanced by optimizing the partially coating length. When the alloy length is 2 mm for a 10-mm grating, the wavelength shifts of central and sidelobe 19.2 pm and 6.0 pm are achieved, respectively, at 1% hydrogen concentration. Maximum hydrogen sensitivity after temperature compensation of 13.2 pm is obtained. The proposed local-compensated method based on partially coating on $\pi $ -shifted FBG is particularly potential for oxygen-free environments with high requirement on hydrogen accuracy. [ABSTRACT FROM AUTHOR]

The realization of multiple demodulation sche- mes of fiber Bragg grating (FBG) sensor is important for its engineering application. Here, we proposed a new edge filter that is formed by cascading dispersion compensation fiber (DCF) and no-core fiber (NCF) structure between two single-mode fibers (SMFs) for demodulation of quasi-distributed FBG sensor. The edge filter can be suitable for FBG with different center wavelengths by changing its structure parameters to achieve a wide range of FBG demodulation. In order to verify the performance of the edge filter, a demodulation system for quasi-distributed FBG sensor is successfully designed by introducing an optical time-domain reflectometer (OTDR), the wavelength shift of FBG caused by the change of temperature and strain is converted into power change through the edge filter, so as to realize temperature and strain demodulation. In the experiment, two FBG sensors separated by 500 m were set at the end of a 3000-m optical fiber for temperature and strain monitoring. The results show that the temperature sensitivity is 0.23 dB/°C from 50 °C to 100 °C, and the strain sensitivity is 0.01658 dB/ $\mu \epsilon $ from 0 to $669.6\,\mu \epsilon $. The edge filter for FBG demodulation has the advantages of high sensitivity, adjustable structural parameters, low cost, and independent multipoint/multiparameter demodulation. [ABSTRACT FROM AUTHOR]

An in-fiber Fabry–Perot interferometer (FPI) is proposed and designed for performance improvement of a fiber Bragg grating (FBG) accelerometer. It consists of two FBGs which are inscribed using the femtosecond laser point-by-point technology. The spectral characteristics of the FBG Fabry–Perot (FBG-FP) were studied. Edge filter interrogation configuration was employed in the vibration measurement system. In contrast to the conventional FBG accelerometer used in wavelength interrogation or edge filter interrogation configurations, the scheme has a higher sensitivity and lower detection threshold which are achieved improving the optical properties of the accelerometer. We fabricated a high-frequency accelerometer and experiments were carried out to verify its performance. The sensitivity and detection threshold were 27.3 mV/g and 1.5 mg, respectively. This study presents a feasibility analysis of the short cavity FBG-FP, as opposed to a single FBG, for the vibration accelerometer. [ABSTRACT FROM AUTHOR]

A vector flow velocity sensor with a stainless steel equal-strength cantilever beam as the elastic element and dual fiber-optic gratings as the sensitive element has been proposed and experimentally demonstrated. And it uses differential measurement to eliminate the effect of temperature. To improve the sensitivity and stress transfer efficiency, we focus on the structural design of the cantilever beam through theoretical derivation and simulation. The cantilever beam structure combines two lug bosses on both sides, a rectangular beam, and an enlarged thrust surface. Three sensors A–C with cantilever beams that are 0.3, 0.4, and 0.4 mm thick with double size, are selected for experiments. The displacement sensitivities of the three sensors at static stress tests are 1.4405, 1.6455, and 0.6662 pm/ $\mu \text{m}$ , respectively. For practical measurements of underwater velocity, the quadratic term coefficients of the flow velocity sensitivity fitting curves as 210.2, 48.9, and 354.6, the linear term coefficients as −6.67, +5.05, and −19.76, and the ${R}^{{2}}$ as 0.9963, 0.9887, and 0.9981, have been calculated respectively. The error between the forward and reverse displacement sensitivities of sensor C is 0.404%, and its average error between the forward and reverse flow velocity measurements is 4.98%. Temperature-controlled experiments are further conducted and verify that the sensors can resist the effect of temperature. Experiments show that the sensor with large size and small thickness of cantilever beam has better velocity sensitivity. And experiment results prove that the sensors have application prospects in 3-D ocean current measurements. [ABSTRACT FROM AUTHOR]

Bolt joints are widely used in large-span truss and bridge structures. The integrity of bolt joint is paramount to the safety of the structure. For distributed multiple bolt joints in large-span structures, a multiplexed angle sensor based on fiber Bragg grating (FBG) for bolt loosening monitoring is proposed. Two FBGs characterized by different wavelength values are symmetrically arranged inside the specially designed sensor structure for the measurement of the bolt loosening angle in terms of their wavelength shift difference. The detail structure design of the FBG angle sensor with temperature self-compensation, automatic reset, and simple reuse is presented. The theoretical and numerical analyses are adopted to derive the measurement principle and optimize the structure of the sensor. Experimental results show that the measuring sensitivity of the FBG angle sensor prototype is about 109.6 pm/° with excellent linearity in the measuring range from 0° to 20°, and the maximum resolution of the bolt loosening angle obtained is 0.38°. The proposed sensor has stable static characteristics and can be expediently reused and developed into a quasi-distributed sensing network, which offers a new approach to real-time quantitative bolt loosening monitoring for distributed bolt joints. [ABSTRACT FROM AUTHOR]

This study proposes a novel monitoring method that combines photogrammetry and fiber Bragg grating (FBG) sensors, which provide overall shape measurement and high-precision strain sensing, respectively. A system was built and tested on a cantilever structure and a gantry crane model; it used two industrial cameras and six or 11 FBG sensors pasted along the beam. Formulas are derived and finite-element simulations of beam strain are presented, along with theoretical derivations of the relationship between image-point displacement (IPD) and 3-D spatial displacement. A series of monitoring experiments were conducted, combining FBG sensing and photogrammetry to monitor the beam under various loads. The strain and deflection curves of the beam based on FBG sensors and photogrammetry were obtained and compared. The bendings measured by photogrammetry were consistent with the strains measured by FBG sensors. To describe the relationship between the beam’s displacements at the marked points monitored by photometry and local strains, an H matrix is proposed. The proposed system allows real-time and long-term monitoring of critical structures, thereby increasing their safety. [ABSTRACT FROM AUTHOR]

A fiber Bragg grating (FBG) sensor includes three main parts, an FBG, a sensor substrate, and a packaging material. The most commonly used packaging material is epoxy resin adhesive, which is prone to aging and failure, which further affects the performance and life of a sensor. This research provides an innovative approach that enables all-silicon sensor fabrication, where the substrate and packaging material are prepared by silicon glass, which is consistent with silicon optical fiber. In the packaging process, there are three main bottleneck problems, which are considered in this study: 1) highly elastic processing of glass used in the sensor elastic element; 2) thermal fixation between the silicon packaging material and substrate; and 3) selecting appropriate FBG parameters and fixing methods. In the proposed approach, an elastic glass cantilever beam and an FBG are packaged with molten glass powder. The proposed approach is verified by tests, and the test results show that the packaged FBG has good package quality and sensing performance. The theoretical analysis of the wavelength drift phenomenon during the packaging is presented. Finally, the introduced packaging method is applied to the fabrication of an FBG tilt sensor. The sensor shell, cantilever beam, temperature compensation beam, mass block, and other components are designed and made of silicon glass. The sensing principle and performance tests of the tilt sensor have been described in detail. The tilt sensor shows good linearity and repeatability. The presented packaging technology shows good applicability and has promising applications in the fabrication of high-performance FBG sensors. [ABSTRACT FROM AUTHOR]

Real-time monitoring of the dynamic vibrations of beam-type structures indicates important implications in aerospace engineering applications, in which the additional sensing information about vibration parameters, including displacements, rotational angles, angular rates, as well as accelerations, facilitates the improvement of the system’s safety. This article proposed a virtual sensing method with the usage of fiber Bragg grating (FBG) strain sensors to directly estimate the vibration parameters from the measured strains. The algebraic relationships of displacement–strain and rotation–strain on the sensor’s installing points as well as the boundary points are obtained based on the Birkhoff interpolation method, and meanwhile, the explicit expressions on angular rates and accelerations are derived. Furthermore, a tracking differentiator is used to recover the strain signals and their first- and second-order derivatives from the noisy measurements in order to real-time estimate the angular rates and accelerations of the vibrating beam. Differing from the classical modal-based method, complex modal decomposition is not required in the proposed method. The comparison of numerical simulation results reveals that the proposed method has advantages over the modal-based method in accuracy and efficiency. Furthermore, the laboratory experiments indicate that the proposed method is effective for real-time estimating vibration parameters from the noisy measured strains. [ABSTRACT FROM AUTHOR]

Flexible bevel-tipped needles are often used for needle insertion in minimally invasive surgical techniques due to their ability to be steered in cluttered environments. Shape-sensing enables physicians to determine the location of needles intraoperatively without requiring radiation of the patient, enabling accurate needle placement. In this article, we validate a theoretical method for flexible needle shape-sensing that allows for complex curvatures, extending upon a previous sensor-based model. This model combines curvature measurements from fiber-Bragg grating (FBG) sensors and the mechanics of an inextensible elastic rod to determine and predict the 3-D needle shape during insertion. We evaluate the model’s shape sensing capabilities in C- and S-shaped insertions in single-layer isotropic tissue, and C-shaped insertions in two-layer isotropic tissue. Experiments on a four-active area, FBG-sensorized needle were performed in varying tissue stiffnesses and insertion scenarios under stereo vision to provide the 3-D ground truth needle shape. The results validate a viable 3-D needle shape-sensing model accounting for complex curvatures in flexible needles with mean needle shape-sensing root-mean-square errors (RMSEs) of $0.160 \boldsymbol {\pm } 0.055$ mm over 650 needle insertions. [ABSTRACT FROM AUTHOR]

This article studies the influence of fiber Bragg grating (FBG) head length on insulated gate bipolar transistor (IGBT) direct on-chip thermal sensing performance of FBG sensors. To this end, the surface of a commercial IGBT chip is thermally simulated and experimentally characterized. Uniform FBG sensors with three head sizes are then tested in two promising thermal sensing locations. The study has found that the large thermal gradients in this application create an additional constraint when using longer head lengths. A distortion in the reflected spectrum of the 5-mm FBG sensor is used to illustrate the underlying physical effect, which causes this limitation for IGBT junction temperature measurement. This additionally affects the length of head sizes providing accurate temperature readings of the IGBT surface hotspots, and significantly, this limit is location-dependent in a given IGBT geometry. [ABSTRACT FROM AUTHOR]

Fiber Bragg gratings (FBGs) are point optical fiber sensors that allow the monitoring of a diversity of environmental parameters, e.g., temperature or strain. Several research groups have studied radiation effects on the grating response, as they are implemented in harsh environments: high energy physics, space, and nuclear facilities. We report here the advances made to date in studies regarding the vulnerability and hardening of this sensor under radiation. First, we introduce its principle of operation. Second, the different grating inscription techniques are briefly illustrated as well as the differences among the various types. Then, we focus on the radiation effects induced on different FBGs. Radiation induces a shift in their Bragg wavelengths, which is a property serving to measure environmental parameters. This radiation-induced Bragg wavelength shift (RI-BWS) leads to a measurement error, whose amplitude and kinetics depend on many parameters: inscription conditions, fiber type, pre- or post-treatments, and irradiation conditions (nature, dose, dose rate, and temperature). Indeed, the radiation hardness of an FBG is not directly related to that of the fiber where it has been photo-inscribed by a laser. We review the influence of all these parameters and discuss how it is possible to manufacture FBGs with limited RI-BWS, opening the way to their implementation in radiation-rich environments. [ABSTRACT FROM AUTHOR]

This article proposes a novel hinge-type fiber Bragg grating (FBG) accelerometer for measuring medium-frequency vertical vibrations that employ a single-notch circular flexure hinge as the elastomer of the accelerometer. The proposed elastomer allows for the simultaneous realization of high sensitivity, a compact structure, and a relatively wide working frequency range, all of which are critical for accelerometer performance. The working principle and detailed theoretical analysis of the FBG accelerometer are illustrated and conducted. Moreover, the sensitivity of the accelerometer is optimized further through a genetic algorithm in order to strike a balance between miniaturization and resonance frequency. Finite element analysis and experimental tests were carried out to validate the performance of the accelerometer. The results indicate that the proposed FBG accelerometer has a resonant frequency of 800 Hz and a working frequency range of 50–450 Hz, with a sensitivity of approximately 65 pm/g at an exciting frequency of 100 Hz between the fiber central wavelength and vibration acceleration. The results also demonstrate that the proposed accelerometer exhibits a good linear relationship with the frequency of vibration accelerations, as well as good anti-interference properties with the horizontal vibrations. It has the potential to provide a novel and simple solution for monitoring medium-frequency vibration signals in complex environments characterized by high levels of electromagnetic interference (EMI), such as in aeronautical hydraulic systems. [ABSTRACT FROM AUTHOR]

In this article, a compact fiber-optic current sensor (FOCS) based on an in-fiber Fabry–Pérot interferometer (FPI) and a magnetostrictive transducer is presented. The in-fiber FPI sensor employed in the proposed FOCS was fabricated with a 273- $\mu \text{m}$ Section of a capillary fiber spliced between two pieces of a standard single-mode fiber (SMF). The in-fiber FPI sensor and a commercial fiber Bragg grating (FBG), which was used as a reference sensor for sensitivity comparison, were bonded to a small magnetostrictive actuator (Terfenol-D) and assembled in the proposed FOCS for electric current monitoring up to $800 {A}_{\text {rms}}$ using a commercial FBG interrogator. The in-fiber FPI sensor presented a sensitivity 84.4% higher than that of the FBG sensor. An alternative interrogation system based on an edge-filter technique with temperature compensation was also developed and employed for the characterization of the proposed FOCS under the same range of electric current. Using the edge-filter interrogation system developed, the in-fiber FPI sensor presented a sensitivity 169.8% higher than that shown by the FBG sensor. The results show that the proposed FOCS using an in-fiber FPI sensor presented high sensitivity to electric current sensing and, therefore, is a potential candidate for current monitoring in complement to conventional current transformers (CTs) as well as to conventional optical CTs (OCTs). [ABSTRACT FROM AUTHOR]

Optical fiber in-line Mach-Zehnder interferometer for strain and refractive index sensing is proposed based on a pair of multi-column diffraction gratings. In the device operation, part of the light traveling in the fiber core diffracted to the cladding region through the first set of multi-column diffraction gratings, and directed back into the fiber core by the second set of multi-column diffraction gratings after being reflected at the cladding-air interface. The rest of the light remains traveling along the core, and the two parts of light recombine and produce interference. Thus, the multi-column diffraction gratings pair essentially forms an optical fiber in-line Mach-Zehnder interferometer. The transmission spectra of different free spectral ranges can be obtained by controlling the separation of two set of multi-column diffraction gratings. The sensitivities achieved for strain and refractive index are ~−1.43 pm / $\mu \varepsilon $ and 363.41 nm/RIU (refractive index unit), respectively. As the device has a low temperature sensitivity 8.03 pm/°C, the temperature cross sensitivity can be greatly reduced. Such a device is small in size, robust in structure, and convenient in operation, and has potential in photonic applications. [ABSTRACT FROM AUTHOR]

Pressure sensors are used in various industrial applications assisting in preventing unintended disasters. This paper presents the design and fabrication of a novel Seesaw device incorporating a diaphragm and Fiber Bragg Grating (FBG) sensor to measure the pressure of liquids. The designed sensor has been tested in a static water column. The proposed design enables the user to easily make and modify the diaphragm based on the required pressure range without interfering with the FBG sensor. The developed pressure sensor produces improved accuracy and sensitivity to applied liquid pressure in both low and high-pressure ranges without requiring sophisticated sensor construction. A finite element analysis has been performed on the diaphragm and on the entire structure at 10 bar pressure. The deformation of the diaphragm is comparable to theoretical deformation levels. A copper diaphragm with a thickness of 0.25 mm is used in the experiments. All experiments are performed in the elastic region of the diaphragm. The sensor's sensitivity as 19.244 nm/MPa with the linearity of $99.64 \%$ is obtained based on the experiments. Also, the proposed sensor's performance is compared with recently reported pressure sensors. [ABSTRACT FROM AUTHOR]

Battery storage has an important role to play in integrating large-scale renewable power generations and in transport decarbonization. Real-time monitoring of battery temperature profiles is indispensable for battery safety management. Due to the advantages of small size, resistance to corrosion, immunity to electromagnetic interference, and multiplexing, fiber Bragg-grating (FBG) sensing has received substantial interest in recent years for battery temperature measurement. However, traditional temperature calibration for FBG sensors often requires a high-standard reference and causes the sensors to fail to be consistent during the calibration or recalibration processes. To tackle the challenges, an ensemble data-driven calibration method is developed in this article for FBG sensors. The calibration model consists of a linear part and a nonlinear part. First, the fuzzy ${C}$ -means (FCM) algorithm is used to extract the linear relationship between the measured FBG wavelength shift and temperature variation. Then, the empirical mode decomposition (EMD) technique is used to classify the intrinsic mode functions (IMFs) and the remainder of the unmodeled nonlinear information. The unmodeled nonlinear information is further compensated using battery state of charge (SOC) and cycle number information. The experimental results confirm that the proposed temperature calibration method achieves desirable accuracy and reliability, with both the mean absolute error (MAE) and root mean square error (RMSE) being around 0.2 °C, respectively. Compared with the traditional temperature calibration method, the proposed approach can be used online in real-life applications. [ABSTRACT FROM AUTHOR]

Regenerated fiber Bragg grating (RFBG) sensors for thermal aging at high temperatures from 600 °C to 1000 °C are produced and analyzed in this work. Seed FBGs are regenerated at 850 °C–900 °C and thermal characterized at the increasing steps of 50 °C–100 °C. Signal decay is measured from the reflected spectra until imminent erasure. The objective is to analyze the RFBG sensors lifetime above the regeneration temperature for long periods after annealing cycles with increasing thermal levels. Lifetime results from RFBG were reported to be a few hours above their regeneration temperature. Our results are significantly better. The sensors resisted for the time intervals of one to eight days up to 100 °C above the regeneration temperature. In addition, some sensors remained stable in terms of the spectral amplitude for more than four days in that thermal condition. [ABSTRACT FROM AUTHOR]

In recent years, tactile perception has attracted more and more attention as one of the important sensing technologies. Fiber Bragg grating (FBG) can be used as an advanced tactile sensing element based on the change of wavelength reflection spectrum under the tiny tactile force, which also has the characteristics of its small size and the fact that it is easy to be encapsulated in the industrial manipulator. This article proposes an object classification scheme for the FBG tactile sensing system based on the Gramian angular field (GAF) algorithm and convolutional neural network (CNN). Three identical FBGs are pasted on the surface of a flexible three-claw manipulator to obtain a three-channel tactile sensing signal, which is demodulated by the structure of wavelength-swept optical coherence tomography. In principle, any number of channels is applicable. The FBG tactile sensing signal belongs to 1-D small volume data, which transmits fast and occupies a small bandwidth. GAF maintains the correlations of time stamp during the process of encoding 1-D time series into 2-D images. CNN extracts deep features of data without a manual sign. Four typical CNN models are compared, which shows the feasibility of the proposed scheme. Finally, Resnet18 is chosen as the classifier of six types of object, and the accuracy of classification can reach 99.75% and the classification response time is only about 1.1 ms, which is suitable for application in any scenes, especially in smart industry with precious bandwidth, high accuracy, and low delay requirements. [ABSTRACT FROM AUTHOR]

We demonstrated an all-fiber chirped-pulse amplifier with a stretcher stage based on a triple-cladding fiber and a final amplification stage based on a newly-developed highly Yb-doped large-mode area fiber with a Ge-doped pedestal. The stretcher fiber was designed to match a diffraction grating compressor in the third-order dispersion and allowed the stretching of 6 ps 1026 nm chirped pulses up to ~500 ps. These pulses were amplified in the developed Yb-doped fiber and then compressed using a transmission grating compressor down to 670 fs duration with 3.5 W average power and $61.8~\mu \text{J}$ pulse energy, corresponding to a record-high peak power of 92 MW. [ABSTRACT FROM AUTHOR]

In order to meet the needs of accelerometers in vibration monitoring, a fiber Bragg grating (FBG) accelerometer based on a miniature slide rail is proposed. The accelerometer consists of two FBGs, a miniature slide rail, two spring plungers, and a mass block. Since the motion of the two fiber gratings is in opposite directions, the induced displacement of the center wavelength can wipe one another out. So, the effect of temperature can be reduced and the sensitivity of the fiber grating can be improved. Through theoretical analysis and experimental verification, the resonant frequency, sensitivity, and lateral immunity of the sensor are studied. The experimental results show that the effect of temperature on the sensor structure is reduced to 0.3 pm/°C in the range of 22.5 °C–90 °C. The resonant frequency of the accelerometer is 200 Hz, and the FBG accelerometer has a broad frequency range from 0.5 to 150 Hz, with a corresponding sensitivity range from 307.3 to 319.2 pm/g, and the lateral immunity is less than 0.5%. The sensor is simple in structure and easy to manufacture. Therefore, it has great prospects for application in structural health monitoring and other fields. [ABSTRACT FROM AUTHOR]

This work reports the design, fabrication, and characterization of a miniaturized metallic package for optical fiber Bragg grating (FBG) sensors. The aim of the package is to enhance the temperature sensitivity of the grating and eliminate the cross-sensitivities to mechanical effects, i.e., strain or impulsive forces. The package is simple in its design and developed using off-the shelf components: the FBG is first encapsulated into a steel tube, which is in turn placed inside another larger steel tube, where the fiber is kept loose to prevent that any mechanical effect is transferred to the grating. The package has a length of 25 mm and a diameter of about 2 mm, hence it is very compact. Concerning the performance, the packaged FBG has a linear temperature response in the range 5–50 °C with a sensitivity of 28.9 pm/°C, that is about three times that of a standard FBG. Whereas, concerning the longitudinal force sensitivity, it is decreased by two orders of magnitude in comparison to an unpackaged FBG. The temporal evolution of the device is also evaluated when subjected to a temperature step of about 20 °C, demonstrating a fast response time of 5 s. Finally, the effect of an impulsive force is evaluated by means of a falling mass attached to the fiber, demonstrating a negligible response and a strong resistance of the packaged FBG in comparison to unprotected one, which broke after the same test. [ABSTRACT FROM AUTHOR]

We propose and experimentally demonstrate a slit beam shaping method for femtosecond laser Point-by-Point (PbP) inscription of highly localized Fiber Bragg Gratings (FBGs). The influence of slit width on the shape and area of Refractive Index Modulation Region (RIMR) induced by a single femtosecond laser pulse was investigated. The shape of RIMR can be changed from a spot to a line with an enlarged RIMR, and hence enhances the coupling strength of core mode and cladding modes. The RIMRs were precisely assembled along the fiber core, producing highly localized FBGs with a spectral comb of pronounced Cladding Mode Resonances (CMRs) intensity of more than 30 dB, a wide wavelength span of 240 nm and a low insertion loss of 0.3 dB. Note that the total processing time for fabricating such a highly localized FBG only requires ∼3.7 s. Subsequently, by including tilted angle on the slit, highly localized tilted FBGs were fabricated and these FBGs show adjustable envelope on the CMRs. Moreover, we investigated the surrounding Refractive Index (RI) response and thermal characteristics of the fabricated highly localized FBGs, which exhibit a sensitivity of 510.87 nm/RIU in a wide RI measurement range and excellent high temperature resistance at 1000°C. Therefore, such highly localized FBGs could potentially be used for multi-parameter sensing in many extreme environments. [ABSTRACT FROM AUTHOR]

High performance grating coupler array for coupling of a silicon photonic chip to a multi-core fiber mounted perfectly vertical to the chip surface is proposed and demonstrated. Experimental results show a low coupling loss of −0.8 dB with 1-dB bandwidth of 47 nm, a low back reflection of <−15.8 dB in about 40 nm wavelength range, a channel non-uniformity of 0.9 dB, and crosstalk of <−40 dB. These improved results attribute to the design of embedding the diffractive grating in a one-port cavity formed by partially and perfectly reflecting Bragg gratings on the silicon chip and a metal mirror on the chip surface. [ABSTRACT FROM AUTHOR]

Steerable needles are a promising technology to provide safe deployment of tools through complex anatomy in minimally invasive surgery, including tumor-related diagnoses and therapies. For the 3-D localization of these instruments in soft tissue, fiber Bragg gratings (FBGs) based reconstruction methods have gained in popularity because of the inherent advantages of optical fibers in a clinical setting, such as flexibility, immunity to electromagnetic interference, nontoxicity, and the absence of line-of-sight issues. However, methods proposed thus far focus on shape reconstruction of the steerable needle itself, where accuracy is susceptible to errors in interpolation and curve fitting methods used to estimate the curvature vectors along the needle. In this study, we propose reconstructing the shape of the path created by the steerable needle tip based on the follow-the-leader nature of many of its variants. By assuming that the path made by the tip is equivalent to the shape of the needle, this novel approach paves the way for shape reconstruction through a single set of FBGs at the needle tip, which provides curvature information about every section of the path during navigation. We propose a Kalman-filter-based sensor fusion method to update the curvature information about the sections as they are continually estimated during the insertion process. The proposed method is validated through simulation, in vitro and ex vivo experiments employing a programmable bevel-tip steerable needle (PBN). The results show clinically acceptable accuracy, with 2.87-mm mean PBN tip position error, and a standard deviation of 1.63 mm for a 120-mm 3-D insertion. [ABSTRACT FROM AUTHOR]

Minimally invasive surgical procedures have become the preferable option, as the recovery period and the risk of infections are significantly lower than traditional surgeries. However, the main challenge in using flexible tools for minimal surgical interventions is the lack of precise feedback on their shape and tip position inside the patient's body. Shape sensors based on fiber Bragg gratings (FBGs) can provide accurate shape information depending on their design. One of the most common configurations in FBG-based shape sensors is to attach three single-mode optical fibers with arrays of FBGs in a triangular fashion around a substrate. Usually, the selected substrates dominate the bending stiffness of the sensor probe, as they have a larger diameter and show less flexibility compared to the optical fibers. Although sensors with this configuration can accurately estimate the shape, they cannot be implemented in flexible endoscopes where large deflections are expected. This paper investigates the shape sensor's performance when using a superelastic substrate with a small diameter instead of a substrate with dominating bending stiffness. A generalized model is also designed for characterizing this type of flexible FBG-based shape sensor. Moreover, we evaluated the sensor in single and multi-bend deformations using two shape reconstruction methods. [ABSTRACT FROM AUTHOR]

This letter presents a method of measuring the strain during rapid maxillary expansion (RME) using fiber Bragg grating (FBG) sensors. The activation of the central jackscrew of the Hyrax expander provides the forces required to expand the maxilla that are transmitted to the teeth by a metallic wire. The strain resulted from the opening forces was measured by a FBG sensor bonded to the posterior arm of the appliance. A proof of concept is shown using a 3D printed maxillary model of the biomechanical structure of a human maxilla, in which the appliance was mounted. The sensor was able to measure static and dynamic events, such as the strain accumulation and viscoelastic strain patterns. An increase in the strain is observed as the number of activations increases. A fast decrease in the strain between activations was also observed. In spite of using a plastic model with viscoelastic properties, this decrease may be related to limitations of the central jackscrew. The instrumentation of the Hyrax expanders using FBG sensors assists in clinical studies on the biomechanics and the efficiency of RME. Different expanders can be compared using the present sensor, and tests on their components, especially the central jackscrew, contributes to improvements or new procedures on such treatments. [ABSTRACT FROM AUTHOR]

In this article, a novel long-period fiber grating (LPFG) sensor based on a core-cladding misalignment structure is described. The LPFG consists of multiple short-length singlemode fiber (SMF) segments joined with core-cladding misalignment splicing. Due to the refractive index (RI) difference between the core and the cladding, periodic RI modulation along fiber axis is achieved. The resonance wavelength responses of the core-cladding misalignment structure based LPFG to RI, strain and temperature are investigated experimentally, and a maximum RI sensitivity of 992.8 nm/RIU, a strain sensitivity of −7.4 pm/μϵ, and a temperature sensitivity of −31 pm/°C are obtained, respectively. Such a core-cladding misalignment structure provides a low-cost fabrication route for LPFGs while offering high sensitivities, which has potential applications for accurate measurement and sensing in industrial and harsh environments. [ABSTRACT FROM AUTHOR]

A fast FBG sensor interrogator on chip based on SOI ring resonator (RR) filters is presented. The measurement of the Bragg wavelength shift is based on the detection of multiple signals at the output ports of the RR filter, providing fast dynamic strain measurement capabilities, fundamentally limited by the photodetector bandwidth and signal processing. The ring resonator resonance wavelength is tuned and fixed by an integrated heater and the ring acts as a passband filter for the reflected FBG spectrum. After a theoretical analysis of the interrogation method, which allows the optimization of the RR characteristics depending on the specific application and requirements, we present experimental results on a single RR based interrogation scheme applying high frequency dynamic strain up to 40 kHz. We also theoretically evaluate the performance of multi-ring resonators. The proposed scheme provides low cost, high scalability to mass-volume market applications as well as an extremely fast interrogation method. [ABSTRACT FROM AUTHOR]

A low frequency-noise ring random fiber laser (RFL) based on random Bragg grating array (RBGA) feedback is proposed. A dual-cavity fiber Bragg grating Fabry-Perot (FBG-FP) filter, which matches the wavelength of RBGA, is specially designed. Due to the narrower bandwidth, flatter response and steeper rolloff, the dual-cavity FBG-FP filter can significantly suppress the competition-induced frequency jitter and undesired residual random lasing modes, and contribute to establish stable lasing and further reduce frequency noise of RFL. The frequency noise of the proposed RFL is ∼3 Hz/√Hz above 1 kHz, and the frequency jitter is ∼3.11 × 10−11 over 10 s. The 3-dB linewidth is ∼490 Hz with a ∼40 dB of optical signal-to-noise ratio. [ABSTRACT FROM AUTHOR]

A miniaturepolymer Bragg grating (PBG) sensor is fabricated and employed for ultrasonic imaging of seismic physical models (SPMs). The sensing Bragg grating is inscribed into an ultraviolet (UV) glue polymer waveguide with a femtosecond laser. The uniform polymer waveguide is fabricated by sealing the UV glue into a capillary fibre through capillary effect. By using line-by-line inscription technique, the laser beam is scanned transversely to periodically modulate the refractive index of the polymer waveguide, leading to the formation of the PBGs with various grating lengths. The sensor response to ultrasonic waves are investigated experimentally. When compared to single-mode fiber Bragg grating and phase-shifted fiber Bragg grating, the PBG with the same reflectivity presents a higher response amplitude due to its lower Young’s modulus. Besides, the sensor has a good spectral stability when transferred from air to water due to the waterproof coating on the sensor end. Finally, the sensor is used to scan a large-scale 3D SPM and the structural features, such as fault, fluctuation, and depositional termination, can be distinctly reconstructed. The proposed PBG sensor provides a new technique with easy fabrication, high sensitivity, and good stability for high-fidelity ultrasonic imaging of seismicphysical models. [ABSTRACT FROM AUTHOR]

Retinal microsurgery, which requires surgeons to perform precise operations on delicate tissue in a tiny eye, is considered one of the most technically demanding operations. Lack of force perception is one of the major problems faced during retinal microsurgery. In order to solve the problem of lack of force sensing, many 3 degree of freedom (DOF) force sensing instruments have been developed. However, they all adopt complex structures, and the accuracy of axial force still has room for improvement. In this paper, we presented a 3-DOF force sensor based on fiber Bragg grating (FBG) sensors for retinal microsurgery. An improved FBG configuration which simplifies the structure of the force sensor and the corresponding temperature compensation method was designed. Two optical fibers with single FBG sensor and an optical fiber with dual FBG sensors are longitudinally attached along the grooves of a stainless steel tube. The calibration method used in the force sensor is a combination of linear calibration method and the least squares support vector regression (LSSVR), which can effectively reduce the influence of linear and nonlinear errors. The results of calibration experiment demonstrate that the hybrid calibration method has better accuracy than using linear or nonlinear method alone. Verification experiment and temperature compensation experiment show that the 3-DOF force sensor proposed has the ability to provide accurate and stable estimation of 3 dimensional forces in a temperature changing environment. [ABSTRACT FROM AUTHOR]

A 10 kW fiber amplifier seeded by the random fiber laser (RFL) with 1018 nm tandem-pumping scheme is reported in this letter. The amplified RFL centered at 1085 nm is seeded by the RFL with a full-open cavity. An array of chirped and tilted fiber Bragg gratings placed before the main amplifier prevent the stimulated Raman scattering with a suppression ratio of 55.8 dB. Control experiments prove that the fiber amplifier seeded by the RFL can suppress the spectral broadening during the amplification process effectively. We demonstrate experimentally that this phenomenon is due to the better temporal stability of the RFL compared with that of the fixed-cavity fiber laser. This fiber amplifier seeded by the RFL is the highest output power record for the amplified RFL to the best of our knowledge, and the spectral evolution suggests the potential for further power scaling-up. [ABSTRACT FROM AUTHOR]

Novel fiber Bragg grating (FBG) relative humidity (RH) sensors based on polyimide (PI) doped with graphene quantum dots (GQDs) are proposed and experimentally demonstrated. The effects of GQDs doping on the sensor performance were investigated. The sensitivity of the humidity sensor could be increased by 96.36% when the doping mass fraction of GQDs was optimized at 0.2%. The FBG humidity sensor based on PI-doped GQDs exhibited good linearity at different operating temperatures in the RH range of 10–90%. Moreover, the excellent linearity of the temperature sensitivity facilitates the temperature compensation of these sensors. GQDs-doped PI is an excellent moisture-sensitive material that significantly improves the sensitivity of FBG humidity sensors, making it a promising candidate for humidity sensing in various applications. [ABSTRACT FROM AUTHOR]

Photothermal actuation that leverages optical fiber and plasmonic materials would not only add a practical function but also spur many important applications for the fiber-optic sensors, for example, the self-temperature governing and fiber-optic photothermal medicine probe. In this paper, we propose and demonstrate a microfiber Bragg grating device that integrates the functionality of self photothermal modulation mediated by the plasmonic gold nanostar (AuNSt). As an excellent plasmnoic nanomaterial, AuNSt stands out due to its tailorable surface plasmon resonance(SPR) spectrum and pronouced “lightning rod” effect. After exploring the optimized AuNSt colodial solution for immobilization on the fiber surface, the AuNSt-functionalized microfiber Bragg grating presented a high pump-heat slope ${\sim }$ 0.06 $^{\circ }$ C/mw and the maximum temperature elevation of 18.5 $^{\circ }$ C in the liquid environment. Furthermore, the enhanced electromagnetic field of AuNSt effectively improves the sensitivity of the microfiber Bragg grating. The functional grating probe also performs good stability and fast on-off switch of the photoheating. The demonstration heralds the potential of the plasmonic microfiber grating for steering the interfacial temperature through the photothermal effect, which will be beneficial for the bio-chemical sensing and medical therapeutics. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive review of optical fiber sensors (OFSs), including FBG, distributed optical fiber sensor and Fabry-Perot interferometer, and their applications within harsh environments, which include extremely high temperatures (from 275 °C to 1750 °C) and low temperatures (from −271.15 °C to −40 °C, namely cryogenic conditions: from 2 K to 233.15 K), and high levels of ionizing radiation (with a maximum gamma dose up to 2 GGy, and a maximum neutron fluence of approximately $5 \times 10^{19}$ n/cm2). After a brief introduction of the principles of OFSs and mechanisms of interrogation, this paper focuses on the existing works for the above three operating environments. Attention have been paid to material selection for fabricating fibers, effects of doping with rare earth elements, femtosecond laser engraving, pre-processing and post-processing (i.e., annealing) that are employed to overcome issues faced byOFSs in extreme temperatures and radiation environments. Application examples and practical test cases are also presented. Through these examples, the limitations in the current state-of-the-art are acknowledged and the key problems are identified. Potential solutions to some of these problems are also elucidated. A feature of this paper is the amalgamation of many research methodologies and outcomes in three seemingly distinct environmental conditions in one place so that different solution techniques can be integrated to advance OFS technologies, especially for extreme environment applications. [ABSTRACT FROM AUTHOR]

Temporal multiplexed fiber Bragg grating (FBG) sensing system for detecting physical parameters with a high spatial resolution is desirable for applications such as structure health monitoring, oil and gas exploration, geophysical research, etc. However, due to the trade-off between bandwidth and sensitivity of the linear photodetection, the wavelength resolution of dense temporally multiplexed FBG sensing system is severely affected under a spatial resolution of centimeters. In this paper, we propose and demonstrate a dense temporal multiplexed FBG sensing scheme with single-photon detection at 1.5 μm. A proof-of-principle demonstration is implemented by using three FBG sensors spliced along a piece of fiber. The spatial resolution can reach 5.0 cm which is determined by a time jitter of superconducting nanowire single-photon detector and a pulse width of laser. In our experiment, by using the dual-wavelength differential detection method, a wavelength resolution of 0.5 pm over a range of 550 pm is obtained with a corresponding temperature resolution of 0.05 °C, which could be further improved to 0.01 °C by increasing the repetition rate of laser pulse. [ABSTRACT FROM AUTHOR]

This paper presents a new method of demodulating the spectrum of fiber Bragg grating (FBG) based sensors by employing deep convolutional neural networks (DCNN). As a proof of demonstration, FBG-based temperature sensor was utilized to conduct temperature measurement and over 1700 samples of the spectral raw data were recorded to train and validate the DCNN model. Using such method, the temperature information can be directly extracted from the experimentally obtained FBG spectra without any peak tracking algorithms. Since it makes full use of the information containing the full spectrum rather than only the central wavelength, it overcomes the limit of traditional fitting method and could improve the measurement accuracy of FBG effectively, which can reach 99.95% and its mean square error (MSE) is just 0.1080 °C, an order of magnitude less than that achieved by the traditional maximum peak method. The proposed method could reduce the need of high-performance hardware of equipment, whose accuracy can still maintain a high level when the sampling rate is reduced. Additionally, the universality of the method was experimentally demonstrated through the accurate demodulation of tilted FBG spectrum, and the relevant measurand can be retrieved directly from the entire spectrum instead of detecting the change of particular peaks. The proposed approach provides a cost-effective solution for the FBG based sensing system, and is promising for establishing sensing networks to implement smart monitoring. [ABSTRACT FROM AUTHOR]

This article describes the development of a fiber Bragg grating (FBG) sensor encapsulated in carbon-fiber reinforced polymer to detect faults in rotating electrical machine bearings. The proposed sensor was based on the temperature and strain sensitivity of the FBG sensor and the robustness of carbon fiber as an encapsulation. The sensor was composed of three FBGs, one of which was packaged in a glass tube. The behavior characteristics of the developed sensor were evaluated in the calibration process. The experimental setup and measurement system were discussed and analyzed under dynamic conditions. The sensor was implemented in an induction motor operating at a controlled full load. The tests were performed with new and faulty bearings. The results, for the faulty bearing frequencies of 147.5, 296.6, and 439.0 Hz, were identified. These frequency components were generated by defects in the inner raceway. Comparing the amplitude spectrum between the signal of the new and faulty bearing, a variation of approximately 96% was found. The FBG reinforced sensor can identify bearing integrity in real-time and be a preventive maintenance tool for bearing monitoring. [ABSTRACT FROM AUTHOR]

In this article, a newly developed reduced Graphene Oxide (rGO) – Titanium Dioxide (TiO2) coated on a Fiber Bragg grating (FBG) based sensor is presented for the detection of ethanol in Petrochemicals i.e., Petrol and Diesel are reported. An experimental procedure has been carried out to find the adulteration level of ethanol in petrochemicals. Initially, the FBGs were etched using 40% concentrated hydrofluoric acid to interact with the outer medium and then coated with an optically active thin film to enhance the sensing abilities. Furthermore, the addition of rGO layer served as a large surface area due to which a significant further shift in the Bragg wavelength was observed. The sensitivity calculation of the proposed sensor is found to be better than the etched FBG (eFBG) and TiO2-coated eFBG sensor. The final sensitivities achieved are 5.91 nm/RIU and 6.32 nm/RIU with the increment of 9% and 13% in petrol and diesel, respectively. Also, the proposed sensor is temperature insensitive due to the double coated layer of thin film and the Limit of Detection (LOD) of the sensor is found to be ${2.19}\times {10} ^{\text {-3}}$ RIU for petrol and ${2.05}\times {10} ^{\text {-3}}$ RIU for diesel, respectively. The characterizations of the sensor are also done using XRD, FESEM, Raman Spectroscopy, UV-VIS-NIR Spectroscopy, and 3D Optical profiler. This work will significantly improve the petrochemical industry’s ability to detect adulteration using eFBG-based fiber optic sensors. [ABSTRACT FROM AUTHOR]

Numerous studies have been carried out aiming to improve wheelchair users’ quality of life. Based on the muscle effort evaluation during their daily activities, wheelchair users can adopt different postures to reduce their effort. However, most of the current solutions for muscle effort assessment are affected by uncontrolled factors. Here, a solution immune to these factors is proposed. The system, based on six fiber Bragg gratings embedded in epoxy resin, was distributed on both arms of six wheelchair users’ volunteers. The arms’ muscle effort was estimated through the fiber Bragg grating’s wavelength shift, which was related with the epoxy resin deformation during some of the wheelchair users’ daily movements, such as horizontal plane locomotion (using different wheelchair hand movement patterns), ramp up and down, and dips. The slightest hand clearance, in relation to the rim (pattern A), implies smaller sensor deformations and, therefore, a lower effort. Comparing to pattern A, volunteer 5 increased 17% its effort in pattern B and 27% in pattern D, in the left bicep sensor. Also, ramp displacements require higher muscle effort, in relation to the horizontal plane. Of all the exercises performed, dips involve the most intense arms’ muscle effort (volunteer 5 had an 82% deformation increase, comparing to pattern A, in the left bicep sensor). The developed system revealed promising results, providing deeper knowledge about the muscle effort during daily movements. Based on this information they can adopt different postures, resulting in minor muscle fatigue, and consequently an improvement of their quality of life. [ABSTRACT FROM AUTHOR]

A broadband fiber-optic acoustic sensing system is proposed and demonstrated, which is configured by a fiber Bragg grating (FBG) and an erbium-doped fiber (EDF) ring laser. As the acoustic waves can shift the FBG reflective spectrum, the wavelength of the laser output shifts accordingly and this shift is subsequently converted into a corresponding phase shift and demodulated by an unbalanced fiber-optic Mach-Zehnder interferometer (MZI). The sensitivity of the presented sensing system is also investigated, which is a function of the optical path difference of the MZI and the linewidth of the FBG. Due to the adaptability and high sensitivity of the system, it can detect signals with frequencies from tens of hertz to hundreds of kilohertz, which shows the potential for acoustic emission detection in real applications. [ABSTRACT FROM AUTHOR]

A method for simultaneous measurement of static strain and dynamic vibration with high spatial resolution based on millimeter sized ultra-short fiber Bragg grating (FBG) is proposed and experimented in this paper. Dual-parameter can be measured on the single unit of cascaded FBGs, which interrogate with optical coherence tomography method by a designed wavelength swept light source. In experiments, the sensing units were precisely located with mm degrees by inverse FFT of coherent signals and the phase difference calculation. And also the strain and vibration could be simultaneously measured in time and frequency domain: demodulating strain with low frequency component’s phase shift of coherent signals by extracting envelope and interpolation; demodulating vibration by finding peak positions of sidebands in FFT spectrum. Spatial resolution of positioning can reach 0.77 mm. Sensitivity of strain measurement reaches 0.811 $\boldsymbol{\times} 10^{-3} rad/\mu \varepsilon $ under a wavelength sweep rate of 0.2 nm/ms, and forces of several milli newtons can be perceived. The measurement accuracy of kHz vibration frequency is over 98%. Moreover, by comparing simultaneous and respective measurement of dual-parameter, it can be seen that there is almost no crosstalk affectting the results. The results shows great application potential in the field of mechanical sensing, especially in bionic tactile sensing for its simultaneous dynamic and static measurement with accurate positioning performance. [ABSTRACT FROM AUTHOR]

We developed a passive silica based large mode area fiber with the nanostructured core with a diameter of 30 μm devoted to fiber Bragg grating inscription for application in an all-fiber laser cavity. The fiber is perfectly matched to the commercial active fiber with NA = 0.06 and core/cladding diameters 30/250 μm. Measured modal characteristics and bending loss of the fiber confirm single-mode operation for bending diameter of 8 cm with bending losses as low as 0.15 dB/m. Although the nanostructured fiber core is low germanium doped, we fabricated high reflectivity (98.5%) UV induced fiber Bragg grating (FBG) and verified its performance in fiber laser setup. The laser cavity formed with highly reflective FBG inscribed in the fiber and Fresnel reflection on the free end of active fiber results in lasing efficiency of 65.5%. The flexibility of the nanostructuring approach gives prospects for the development of free-form large mode area fibers for passive fiber components and matched with any type of active fibers. [ABSTRACT FROM AUTHOR]

Highly localized fiber Bragg gratings (HLFBGs) with strong cladding mode have been successfully inscribed in hydrogen loaded single mode fiber by low intensity femtosecond laser and phase mask. The HLFBGs possess strong cladding mode resonances which are deep than 20 dB and wide than 100 nm in the transmission spectrum. Experimental results reveal that localized refractive index (RI) modulation in fiber core leads to the emergence of strong cladding mode and higher localization leads to stronger cladding mode resonances. The HLFBGs can withstand high temperature up to 800 °C, and the strength of cladding mode still remains about 8 dB after thermal annealing. With surrounding RI varying from 1.3330 to 1.4583, the cut-off wavelength of cladding mode shows a good linear shift with sensitivity of 527.43 nm/RIU. The HLFBGs are expected for using in chemical and biological detections. [ABSTRACT FROM AUTHOR]

The main factors limiting the power scaling of high-peak-power high-brightness quasi-continuous wave (QCW) fiber laser oscillator and a possible solution are analyzed in this paper. The impacts of fiber length and grating parameters on the output power are studied by rate equations. Under the optimized parameters, a high-brightness QCW fiber laser oscillator with a peak power of 7.3 kW is presented experimentally by using a 24 meter long spindle-shaped Yb-doped fiber with a constant core cladding ratio, and that is the best performance of peak power and brightness in near-single-mode QCW fiber lasers at present to the best of our knowledge. The corresponding pulse energy is 0.57 J and the beam quality M2 factor is about 1.43 under the condition of repetition frequency of 1 kHz and pulse width of 100 μs. The details are analyzed, and also the limiting factors of further power scaling, strategies of suppressing nonlinear effects and the principle of optical device parameter optimization. [ABSTRACT FROM AUTHOR]