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• Low-velocity impacts can cause microscopic and invisible damage to Carbon Fiber Reinforced Polymer (CFRP) structures, potentially compromising their integrity and leading to catastrophic failures. In this paper, an impact localization system for CFRP structures was developed by using Fiber Bragg Grating (FBG) sensors. The time difference of arrival (TDOA) between signals from different FBG sensors are collected to characterize the impact location, and an impact localization method of CFRP structure based on CNN-LSTM-Attention is proposed. The following conclusions can be made from this study: • A monitoring system of impact on CFRP structures was developed by using fiber Bragg grating (FBG) sensors, and impact signals detected by FBG sensors are demodulated by edge-filtering at high speed. • The Time Difference of Arrival (TDOA) between signals from different FBG sensors are collected to characterize the impact location, the occurrence time of the first trough in the low-frequency narrowband signal component is extracted as the arrival time of impact signal, which is found to be sensitive to the location of impact and thus can be used as features for impact localization. • An impact localization method of CFRP structure based on CNN-LSTM-Attention is proposed. The time differences in the arrival of impact signal detected by different FBG sensors are used as features, and attention mechanism is introduced into the CNN-LSTM model of impact localization to emphasizes the arrival time difference of impact signal detected by nearby FBG sensors. • The impact localization method was developed and validated on a CFRP plate by using FBG sensors. Comparative analysis of localization performance between different models confirms the effectiveness and satisfactory performance of the proposed method. Low-velocity impacts can cause microscopic and invisible damage to carbon fiber reinforced polymer (CFRP) structures, potentially compromising their integrity and leading to catastrophic failures. Therefore, obtaining precise information about the impact location is crucial for monitoring the health of CFRP structures. In this paper, an impact localization system for CFRP structures was developed by using fiber Bragg grating (FBG) sensors, and impact signals detected by FBG sensors are demodulated by edge-filtering at high speed. An impact localization method of CFRP structure based on CNN-LSTM-Attention is proposed. The time difference of arrival (TDOA) between signals from different FBG sensors are collected to characterize the impact location, and attention mechanism is introduced into the CNN-LSTM model to augment the significance of TDOA of impact signal detected by proximal FBG sensors. The model is trained using the training set, its parameters are optimized using the validation set and the localization performance of different models are compared by the test set. The proposed impact localization method based on CNN-LSTM-Attention model was verified on a CFRP plate with an experiment area of 400 mm*400 mm. Experimental results prove the effectiveness and satisfactory performance of the proposed method. [ABSTRACT FROM AUTHOR]

• In this paper, we propose a distorted pulse detection method based on odd function for pulse decomposition, to solve the problem of optical path mismatch in UWFBG array. • In theory, we build a mathematical model of the distortion pulse and derive the corresponding odd function. • Then, we analyze the peak trajectory obtained by the difference curve by simulation, and determine the optimal interference position of the distorted pulse. • Finally, we use the vibrating liquid column method to carry out underwater acoustic sensing experiments, under different conditions of optical path mismatch. • The results show that, when the distance between adjacent UWFBGs is 50 m and pulse width with 100 ns, the gain of harmonic SNR and average PSD is improved from 2.3 dB and 1.8 dB to 11.2 dB and 10.0 dB respectively, under different circumstances with length difference for 1 m, 1.5 m, and 2 m respectively. In this paper, we propose a distorted pulse detection method based on odd function for pulse decomposition, to solve the problem of optical path mismatch in UWFBG array. In theory, we build a mathematical model of the distortion pulse and derive the corresponding odd function. Then, we analyze the peak trajectory obtained by the difference curve by simulation, and determine the optimal interference position of the distorted pulse. Finally, we use the vibrating liquid column method to carry out underwater acoustic sensing experiments, under different conditions of optical path mismatch. The results show that, when the distance between adjacent UWFBGs is 50 m and pulse width with 100 ns, the gain of harmonic SNR and average PSD is improved from 2.3 dB and 1.8 dB to 11.2 dB and 10.0 dB respectively, under different circumstances with length difference for 1 m, 1.5 m, and 2 m respectively. So, the algorithm is more robust to length mismatch, and verifies the effectiveness of the algorithm, and has potential significance for reducing the difficulty of designing the UWFBG array interferometer. [ABSTRACT FROM AUTHOR]

• In view of the lack of online damage detection of CFRP driveshaft, this paper aims to establish a damage detection system for CFRP driveshaft and establishes a damage identification model for CFRP driveshaft based on strain and damage mechanism. • Considering the shortcomings of the existing research, this paper makes full use of the unique advantages of FBG sensors, combined with GWO-BP neural network, constructs a CFRP driveshaft damage identification system, and realizes the damage identification and localization of the CFRP driveshaft. • Combining theoretical methods, simulation calculations and experimental studies, torsion experiments were carried out on CFRP driveshafts, and damage identification was carried out by using the identification and localization system constructed in this paper to compare the actual damage and damage identification. Carbon Fiber Reinforced Plastic (CFRP) has the advantages of light weight, high strength, small coefficient of thermal expansion, good vibration damping performance, etc., the use of its production drive shaft can reduce weight, improve the intrinsic frequency, transmission efficiency and precision, and has been used in helicopters, heavy duty machine tools, wind turbines, ships and other high-end equipment. Once the damage occurs, it will cause adverse effects such as reduction of mechanical efficiency of the system, degradation of transmission system performance, and decrease of reliability and safety. The existing non-destructive testing technology for carbon fiber reinforced plastic has limitations such as inconvenient use, complex equipment, and no real-time online monitoring. To address this problem, this paper adopts Fiber Bragg Gratings (FBG) sensors to obtain real-time strain field data of CFRP drive shaft, and uses the linear and nonlinear mapping ability of Back Propagation Neural Network (BP neural network) to identify and locate the damage of CFRP drive shaft. At the same time, the GWO optimization algorithm is used to optimize the BP neural network to construct a CFRP drive shaft damage identification and localization system based on FBG sensing network and GWO-BP neural network. The experiments show that the CFRP drive shaft damage identification and localization system constructed in this paper can obtain the CFRP drive shaft strain field information in real time and accurately identify the damage and its location. [ABSTRACT FROM AUTHOR]

• High-Stability Single-Longitudinal-Mode Thulium-Doped Fiber Laser was proposed. • Using thulium holmium co doped fibers as saturable absorbers (SA). • The maximum wavelength drift and power fluctuation are 0.02 nm and 0.45 dB, respectively, and the linewidth of the laser is about 7 kHz. • Using a Quad Coupler Triple Ring Cavity (QC-TRC) filter based on passive four-coupler and a Uniform Fiber Bragg Grating (UFBG). This paper reports an experimental demonstration of a stable single-longitudinal-mode (SLM) thulium-doped fiber laser (TDFL) using a quad-coupler triple-ring cavity (QC-TRC) filter based on a passive quad coupler and a uniform fiber Bragg grating (UFBG). The TDFL adopts a ring cavity structure and utilizes a QC-TRC filter with four couplers to select a single longitudinal mode from a dense mode spectrum. The design and fabrication thinking on the QC-TRC filter are explained, and the selection principles for SLM are analyzed in detail. Experimental results show the filter features excellent performance, with the laser at 2048.40 nm exhibiting high stability with an optical signal-to-noise ratio (OSNR) of 69 dB. No significant wavelength drift is observed during the repeating measurement, the maximum wavelength drift and power fluctuation being 0.02 nm and 0.45 dB, respectively. The linewidth of the laser is approximately 7 kHz, measured using the β -separation linewidth method with an imbalanced Michelson interferometer linewidth measurement device. [ABSTRACT FROM AUTHOR]

• Revealed an error source of FBG straightness perception of scraper conveyor. • Proposed the 2D curve reconstruction accuracy compensation model. • Designed an experiment on the curvature error response characteristics. The straightness perception of scraper conveyor is a technology that helps to achieve intelligent operation of coal mine working faces. However, there is a problem of accuracy degradation caused by sensor torsion based on fiber Bragg grating (FBG) shape sensing technology. This paper proposes an accuracy compensation method based on torsion angle, and verifies the effectiveness of accuracy compensation through simulation and experimental research. Experimental tests have shown that when the torsion angle of the sensing substrate is 10° and 20°, the end accuracy of the reconstructed curve after accuracy compensation is increased by 2.77% and 1.93% compared to before compensation, respectively. This study indicates that compensation based on the torsion angle can significantly improve the accuracy of two-dimensional (2D) curve reconstruction, and it has broad engineering application prospects in fields such as straightness perception of scraper conveyors and monitoring of underwater pipelines. [ABSTRACT FROM AUTHOR]

We demonstrate a fiber optic accelerometer for low frequency vibration detection in this paper. A slider moving along a guide rail is suspended over an optical fiber into which a fiber Bragg grating (FBG) is inscribed. When acceleration signal acts on our sensing structure, the slider will vibrate vertically under the constraint of the optical fiber, inducing the change of FBG axial strain and we can therefore establish the relationship between vibrational acceleration and wavelength shift. Laboratory measurements validate the effectiveness of the derived theoretical results that the sensor owns a low natural frequency of 9.5 Hz and a high sensitivity up to 926.29 pm/g from 0.5 to 9 Hz. The sensor structure also shows a good ability to resist lateral interference that the cross-sensitivity is calculated as 0.3%. • Compact low-frequency FBG accelerometer using fiber itself as elastic element. • Low natural frequency of 9.5 Hz and high sensitivity up to 926.29 pm/g. • Cross sensitivity of 0.3% indicates its strong ability to resist lateral interference. [ABSTRACT FROM AUTHOR]

• This study used femtosecond laser processing technology to fabricate cascaded fiber Bragg gratings. • This study proposd an erbium-doped random fiber laser based on long-distance single mode fiber. • Random laser wavelength switching was achieved by changing the polarization state of the light in the system. This study proposed a wavelength switching system for the linear cavity erbium-doped random fiber laser (RFL) based on cascaded fiber Bragg gratings (FBGs). High reflection cascaded FBGs was fabricated by point-by-point inscription method based on femtosecond laser processing technology. The erbium-doped fiber (EDF) was used to provide effective gain for the laser system, the backward Rayleigh scattering (RS) in the long distance passive single-mode fiber (SMF) was used to provide random distributed feedback for the system, and the single-RS was changed into double-RS by the full reflection mirror to improve the feedback intensity. Combined with the frequency selection function of cascaded FBGs, the polarization state of the light in the system was changed by adjusting the polarization controller (PC) to achieve wavelength switching. The designed laser system achieved a low threshold power of 37 mW. When the pumping power was 57 mW, the output wavelength was 1549.6 nm and 1550.0 nm single-wavelength laser channel, respectively. The laser stability was observed within 50 min, and the maximum wavelength fluctuation was 0.01 nm and 0.09 nm with the maximum power fluctuation being 0.30 dB and 1.44 dB respectively. The dual-wavelength laser generation was obtained by adjusting the PC, the wavelength fluctuation was less than 0.07 nm and the power fluctuation was less than 1.01 dB. With the increase of input power, the fitting linear determination coefficient of output power was 0.979 and the correlation coefficient was 0.990. The wavelength switchable RFL proposed in this paper is uncomplicated in structure, switchable, and has excellent wavelength and power stability, which is of great significance in the fields of laser sensing, laser communication and diffuse spot imaging. [ABSTRACT FROM AUTHOR]

• This article presents an overview of FBGs sensors for high temperature measurements. • The fabrication and characteristics of the high temperature FBG sensing elements are described. • Packaging issues and applications of FBG temperature sensors are addressed. The demand for temperature sensors has been steadily rising as industry progresses. The miniature size of fiber Bragg grating (FBG) temperature sensors, their resistance to electromagnetic interference, multiplexing capability, remote detection ability, distributed measurement, among other advantages have made them crucial supplementations to the traditional electronic sensors. However, type I fiber grating is prone to failure at high temperatures due to thermal recession phenomenon, which limits their effectiveness in high temperature settings. This paper provides a comprehensive review of FBG sensors and their use at high temperatures. The article begins by giving a brief introduction to the principle of FBG thermal degradation, and then briefly describes the characteristics of various types of fiber gratings. The fabrication method of regenerated FBG and the encapsulation method are discussed, and fiber fabrication methods for improving thermal stability of fiber grating are summarized. [ABSTRACT FROM AUTHOR]

In this paper, we demonstrate a simple structure, ultra-narrow linewidth, extremely high stability and high optical signal to noise ratio single-longitudinal-mode (SLM) erbium-doped fiber laser (EDFL). Nested Fiber Ring (NFR), nested in the ring cavity of EDFL, is employed to serve as a high quality mode filter to eliminate the dense longitudinal mode of the EDFL. The mode-selection capability of the RIR is investigated and discussed theoretically and experimentally. Saturable absorber, which composes of a section of unpumped polarization maintaining erbium-doped fiber and a Fabry–Perot fiber Bragg grating (FP-FBG), is employed to produce a dynamic narrowband grating to ensure the SLM laser output. Saturable absorber, consisted of a section of unpumped polarization maintaining erbium-doped fiber, produces a dynamic narrowband grating to ensure the SLM laser output. We achieve a ultra-narrow linewidth SLM laser output of 550 Hz. The laser has an optical signal-to-noise ratio (OSNR) of more than 59 dB. In addition, the stability of the wavelength and power fluctuations below 0.03 nm and 0.02 dB, respectively. The single-longitudinal-mode fiber laser has a compact structure, ultra-narrow linewidth, and excellent performance, and is expected to be used in the fields of coherent communications, precision metrology, and so on. • We propose a new structure of ultra-narrow linewidth single-longitudinal-mode erbium-doped fiber laser based on Nested Fiber Ring structure and Saturable Absorber. • The new structure has the advantages of simple structure, easy preparation and high stability. • The output performance of the proposed fiber laser is excellent, the 3-dB linewidth is 550 Hz and the optical signal-to-noise ratio is 59 dB. • The filtering function of the Nested Fiber Ring was theoretically derived and its filtering effect was analyzed. [ABSTRACT FROM AUTHOR]

• Multiparameter sensing system using a single FBG with a polarization-assisted analysis. • Strain, torsion and temperature for the analysis of polarization and spectral components. • The relative errors are below 0.5% for each mechanical load condition. • A root mean squared error of 2.91 °C is obtained for the temperature analysis. This paper presents the development of a multiparameter sensing system using a single fiber Bragg grating (FBG) with a polarization-assisted analysis. In this case, the FBG sensor is subjected to axial strain, torsion angle and temperature for the analysis of polarization components and spectral features variations. In addition, the responses at two orthogonal polarization states are compared to evaluate the feasibility of developing a polarization-based analysis for multifeature regression. Results show the differences in the sensors responses considering the full spectrum and the polarization components, where non uniform strain components lead to differences in the polarization states, which are used on the simultaneous analysis of different mechanical loadings combinations. Furthermore, the simultaneous estimation of axial strain, torsion angle and temperature resulted in small relative errors when the whole range of the tests are considered. The relative errors are below 0.5 % for each mechanical load condition, whereas a root mean squared error of 2.91 °C is obtained for the temperature analysis. Thus, the proposed approach resulted in a suitable alternative for three parameters sensing using only one FBG in standard single mode silica optical fiber without any structural modification, which can lead to a new approach on spectrally efficient sensor systems for practical applications. [ABSTRACT FROM AUTHOR]

• Machine learning approach for automatic estimation of responses using different TFBGs. • Estimation based on the optical spectral characteristics of TFBGs as a function of RI. • Root mean squared errors as small as 0.008 RIU for the trained samples. • The approach can be used to enhance the reproducibility of large set of TFBG sensors. This paper presents the development of an approach for automatic estimation of the regression coefficients as a function of the refractive index (RI) in Tilted Fiber Bragg Gratings (TFBGs) based on their optical spectral characteristics. The methodology is based on machine learning approach, where different samples are analyzed in various test conditions to obtain a dataset for the training of a kNN regression algorithm. In this case, the spectral characteristics related to the wavelength spacing between adjacent cladding mode resonances, number of valleys and amplitude of the normalized spectra are correlated with the linear regression algorithm for RI estimation using the machine learning approach. The analysis are performed as a function of the wavelength shift and optical power, considering two regions on the transmitted spectrum and the envelope of the curve. Furthermore, the number of valleys and the area of the envelope curve are also analyzed as a function of the refractive index, which result in six analyzed features. The Principal Components Analysis (PCA) is used to reduce the number of features to two principal components. The results of the proposed algorithm are obtained with different trained and untrained samples, which indicates a root mean squared error (RMSE) of around 0.003 RIU for the trained samples and 0.008 RIU for the untrained ones. Therefore, the results indicate the usefulness of the proposed approach, which can be used to enhance the reproducibility of large set of TFBG sensors in practical applications. [ABSTRACT FROM AUTHOR]

• Over the years 2 µm mode-locked fiber laser has been widely developed as an important light source for various significant applications, such as ultrafast mid-infrared spectroscopy. Besides, as the solutions of CGLE governed by the composite balance among dispersion, nonlinearity, gain and loss in dissipative systems, the field of solitons has become subject to extensive study. • Worth to note that, the rise of time-stretched dispersive Fourier transform (TS-DFT) technique has significantly enhanced the understanding of soliton dynamics, including bound solitons, optical rogue waves and others. However, due to the lack of appropriate dispersive components at 2 µm band with low optical transmission loss for TS-DFT measurement, direct real-time observations of ultrafast soliton dynamical phenomena in the 2 µm mode-locked fiber lasers is still limited. • In the present paper, we utilize DFT method to investigate the ultrafast nonlinear soliton dynamics in a 2 µm Tm-doped mode-locked fiber laser. The real-time observation of the nonlinear dynamics of the soliton explosions and pulsations are successively investigated. Moreover, the real-time observation of the soliton explosions associated with soliton pulsations is demonstrated in 2 µm wavelengths for the first time. In addition, soliton rains with dual-wavelength operation are also observed. Our work will benefit the development of 2 µm ultrafast light source, as well as enhance a comprehensive understanding of the soliton nonlinear dynamics in mode-locked fiber lasers. Over the years, 2 μm mode-locked fiber laser has been widely developed as an important light source for various significant applications. Worth to note that, with the help of real-time spectroscopy technique, i.e., time-stretched dispersive Fourier transformation (TS-DFT), various complex nonlinear soliton dynamics have been revealed, such as soliton molecules, soliton explosions and soliton pulsations, enhancing our understanding of the ultrafast physics. However, due to the lack of appropriate dispersive components at 2 μm band with low optical transmission loss for TS-DFT measurement, the investigation of ultrafast soliton dynamical phenomena in the 2 μm mode-locked fiber lasers is still limited. In this work, we demonstrate the direct real-time observation of soliton explosion and pulsation phenomena at 2 μm band mode-locked fiber laser, for the first time as far as we know, by using specially designed linearly chirped fiber Bragg gratings as dispersive component for TS-DFT measurement. For explosions, we observe slight explosion and drastic explosion, respectively. Particularly, the soliton pulsating explosions are also characterized in real time where the explosions occur in the process of soliton pulsation. In addition, dual-wavelength soliton rains are also observed by carefully adjusting the cavity parameters. Our work provides supplements to the current investigation of 2 μm band ultrafast science, and might enrich our understanding of soliton dynamics. [ABSTRACT FROM AUTHOR]

• Aiming at the demand of pressure monitoring of flammable and explosive gases in industrial production, a diaphragm-type FBG high-pressure gas sensor is designed to realize long-term high accuracy pressure measurement in the range of 0–50 MPa, with good static characteristics under experimental conditions, which verifies the feasibility of the pressure sensing model. • The use of glass welding encapsulation instead of the traditional adhesive encapsulation to produce FBG pressure sensors improves the durability and long-term stability of FBG pressure sensors, and provides the necessary data support and reference for the subsequent use of glass welding encapsulation of FBG sensors. • The use of glass welding encapsulation helps to maintain the long-term high accuracy measurement of the FBG pressure sensor. Large quantities of flammable and explosive gases are often used in various production processes in the chemical and energy industries. these gases, if leaked or improperly operated, can cause serious production accidents. Therefore, in order to meet the requirement for miniaturized, highly accurate, long-term stable and large-range high pressure sensors for flammable and explosive gases in industrial production, a highly accurate diaphragm fiber Bragg grating (FBG) high pressure gas sensor is designed in this paper. In this work, the working principle of the sensor is first analyzed theoretically, the circular elastic metal diaphragm of the sensor will strain under the action of pressure load, and then the strain will be transmitted to FBG. The wavelength drift of FBG is measured to achieve the measurement of the pressure load. Then finite element simulation was carried out for the diaphragm strain of the sensor to verify the feasibility of the sensor design scheme. The traditional adhesive encapsulation is then replaced with a glass welded encapsulation to improve the durability and long-term stability of the sensor. Finally, calibration experiments was conducted on the sensor, and the experimental results were compatible with the theoretical simulation analysis. The results show that the sensor realizes long-term highly accurate measurement of pressure in a large range of 0–50 MPa, and has good static characteristics under experimental conditions. [ABSTRACT FROM AUTHOR]

• We designed and manufactured a 40-channel array waveguide grating (AWG) based on a silicon dioxide planar light wave circuit (PLC). • Using hybrid integration technology to couple AWG and photodetector arrays together. • The hybrid integrated demodulation chip achieves precise wavelength demodulation of C-band FBG sensors. • The designed AWG has a 3 dB bandwidth of 1.98 nm, insertion loss of approximately 3.5–4.1 dB, and adjacent channel crosstalk of approximately −7.84 dB. • The wavelength resolution of the demodulation system is 1 pm, and the demodulation accuracy is 2.11 pm. Fiber Bragg grating (FBG) sensors have been widely used in various fields. To develop a miniaturized, high-precision, and high-response FBG interrogation system, arrayed waveguide grating (AWG) has been widely used as a key device. In this paper, a 40-channel arrayed waveguide grating based on silica planar lightwave circuits (PLCs) is designed fabricated, coupled to a photodetector array using hybrid integration techniques. The hybrid-integrated demodulation chip enables precise demodulation of the FBG sensor wavelength in the C-band. The test results show that the AWG has a good transmission spectrum with a 3-dB bandwidth of 1.98 nm, insertion loss of about 3.5–4.1 dB, and adjacent channel crosstalk of about −7.84 dB. The established FBG interrogation system can realize continuous interrogation in the dynamic range of 1514.6–1578.6 nm, the wavelength resolution is 1 pm and the demodulation precision is 2.11 pm in the dynamic range of 1555.3–1556.1 nm. [ABSTRACT FROM AUTHOR]

• We build a system to sense the micro-deformation of connecting rods. • We design a connecting rod sensing network based on FBG. It not only detects the deformation of the connecting rod after compression but also detects the deformation of the connecting rod in the tensile state. • The work provides a new detection scheme to ensure the machining quality of connecting rods. As a key component of the ship power equipment, the deformation of the connecting rod directly affects its performance and life. Therefore, the deformation detection of the connecting rod plays an important role in ensuring the service life of the power equipment. In this paper, we propose a deformation perception and reconstruction method based on fiber Bragg grating (FBG) for the connecting rod of the marine diesel engine. According to the deformation characteristics under loads of different sizes and directions obtained from the finite element, a sensing distribution FBG-based is designed for the connecting rod. On this basis, we establish the FBG-based deformation sensing system and develop a three-dimensional deformation reconstruction model, which can realize the real-time monitoring of the deformation during the processing of the connecting rod. The experiments show that the deformation trends obtained by FBG and finite element are consistent. Therefore, the FBG-based deformation perception and reconstruction method is feasible. It provides a new and accurate detection scheme to ensure the processing quality of the connecting rod and a new idea for the deformation detection of other key parts of the ship power equipment. [ABSTRACT FROM AUTHOR]

• In linear erbium-doped fiber laser cavity, dual-wavelength lasing performances are compared with 3 dB FLM-FBG cavity and FBG-only cavity. • In the dual-wavelength EDFL with FBG-only cavity, the lasing performances are compared when the reflectivities of the output-mirror FBGs are the same or different, and the effect of the center wavelength spacing between the two pairs of FBGs on lasing performances is studied. There are few reports in the field of linear cavity dual-wavelength fiber lasers about the laser output characteristics when 3-dB fiber loop mirror (FLM) and Fiber Bragg grating (FBG) are used as full reflective mirrors, respectively. Additionally, there is scarce research on further tuning of the FBG parameters (reflectivity and center wavelength interval) to study dual-wavelength fiber lasers. In this paper, we compare the performances of dual-wavelength lasing outputs between the 3-dB FLM-FBG cavity and the FBG-only cavity in a linear-cavity erbium-doped fiber laser (EDFL) based on the polarization hole burning effect. The results show that the optical signal-to-noise ratios (OSNRs) of the dual-wavelength lasing outputs with the FBG-only cavity are approximately 4 ∼ 5 dB higher than those with the 3-dB FLM-FBG cavity. Furthermore, the lasing output performances based on the FBG-only cavity are compared when the reflectivities of the output-mirror FBGs are the same or different. It is observed that the dual-wavelength laser with the same output reflectivity leads to higher OSNR, slope efficiency, and stability. Moreover, by varying the center wavelength interval of pairs of FBGs in the FBG-only cavity to values of 4 nm, 8 nm, and 12 nm respectively. It is shown that wider intervals result in higher OSNRs but lower laser slope efficiency. Finally, we demonstrate a dual- wavelength linear cavity EDFL operating at room temperature with two lasing wavelengths of 1550 nm and 1562 nm. The laser results in OSNRs around 50.62 dB and 50.93 dB; center wavelength variations of less than 0.030 nm and 0.035 nm; output power fluctuations of less than 0.061 mW and 0.059 mW, respectively. [ABSTRACT FROM AUTHOR]

This paper presents the development, signal analysis and application of an FBG array embedded smart rubber carpet for 2D shape reconstruction. The proposed sensor system is based on the optical fiber integration between two rubber layers, where the optical fibers have 7 FBGs for the quasi-distributed force assessment. The sensor system analysis is performed at room temperature in difference set of tests, namely the stability tests (without any applied force), the force characterization tests (forces applied directly on the FBG region) and the shape reconstruction tests (different forces applied at different positions in the carpet). The stability tests indicated a maximum variation below 0.02 nm considering all 7 FBGs, whereas the force characterization tests indicated a mean sensitivity of around 80 N/nm. However, it is worth to mention that there is a variation on the sensitivities considering all FBGs, which is related to minor thickness differences in the rubber layer that directly effects the strain transmission from the rubber layer to the optical fiber beneath it. Thereafter, the shape reconstruction occurs in two steps, the first one is the cubic interpolation between the FBGs, considering the boundary conditions on the carpet following the beam theory. Then, the k-nearest neighbors regression algorithm is used on the force distribution assessment with a dataset divided into training and testing with a 70/30 proportion. The shape reconstruction results indicate the feasibility of the proposed approach, where root mean squared errors below 0.65 N are obtained in the range of applied force between 3 N and 35 N. Therefore, the proposed smart carpet as a feasible option for different 2D shape reconstruction applications ranging from biomechanics to structural health monitoring. • Force sensor array using Fiber Bragg Gratings embedded in rubber carpet. • 2D Shape reconstruction using machine learning algorithms. • Relative errors on the shape reconstruction below 3%. [ABSTRACT FROM AUTHOR]