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In this work, we propose and experimentally demonstrate a new distributed short linear cavity fiber laser. At one of the cavity ends, fabricated by a commercial femtosecond fiber laser chirped pulse amplifier, an artificially controlled backscattering erbium doped fiber section has been connected. This distributed reflector acts also as a saturable absorber, leading to the generation of tunable and switchable single longitudinal-mode laser emissions. The distributed reflector consists of 9 micro-drilled sections of about 1cm each one and randomly spread throughout 2 meters of highly doped erbium fiber. The total length of the fiber laser is 9.5 m and the laser shows a single mode behavior at all the emitted wavelengths. Using this new kind of reflecting saturable absorber, single and multiple single-mode emissions can be obtained. The achieved laser presents a pump threshold as low as 45 mW and shows up to 8 different single-mode emission lines with an optical signal to noise ratio of 45dB.

In this work, a simple switchable dual-wavelength short-cavity fiber laser operating in a single-longitudinal-mode regime at room temperature and based on superposed fiber Bragg gratings is proposed and experimentally demonstrated. Only by introducing stress into one of the overwritten FBGs, single- or dual-wavelength laser oscillations can be attained. Either single- or multimode operation of the laser can be easily achieved. Single-longitudinal-mode emission has been verified in two different ways: the first corroboration has been conducted by the heterodyne detection of the output signal, and the second one has been carried out by using a high-resolution optical spectrum analyzer.

The ballistocardiogram (BCG) is a graphic representation of the movements of the body associated with cardiac activity. In this article, a 10-min BCG has been captured for ten different volunteers with a polymer optical fiber (POF) specklegram sensor. This transducer, which is composed of a charge-coupled device (CCD) camera, a laser emitting diode, and two meters of POF, allows capturing the BCG by analyzing how the induced speckle pattern changes over time. These changes are related to cardiac activity. Several processing methods have been compared to determine which method achieves the best performance: complex cepstrum, power of spectral density (PSD), Pam–Tompkins algorithm, wavelet, autocorrelation, Savitzky–Golay filter, mean absolute deviation, and Hilbert transform. Accuracy and resource consumption have been characterized and compared for these methods. Hilbert, PSD, and Savitzky–Golay exhibit both small errors and computational costs. This article describes a baseline for the main frequency determination of POF-based BCG signals in real-time. [ABSTRACT FROM AUTHOR]

This work presents an experimental analysis and comparison of the performance of quasi-randomly distributed reflectors inscribed into a single-mode fiber as a sensing mirror both in C- and L-band. Single-wavelength emission has been obtained in either band when using these artificially controlled backscattering fiber reflectors in a ring-cavity fiber laser. Single-longitudinal mode operation with an optical signal to noise ratio (OSNR) of 47 dB and an output power instability as low as 0.04 dB have been measured when employing a C-band optical amplifier. When replaced by an L-band optical amplifier, a single-longitudinal mode behavior has also been obtained, showing an OSNR of 44 dB and an output power instability of 0.09 dB. Regarding their performance as fiber-laser sensing systems, very similar temperature and strain sensitivities have been obtained in both bands, comparable to fiber Bragg grating sensors in the case of temperature and one order of magnitude higher in the case of strain variations. [ABSTRACT FROM AUTHOR]

In recent years, the Industry 4.0 paradigm has gained special relevance, focused on the integration of smart structures to increase the efficiency of processes. Photonics, with laser advanced manufacture and light-based sensors, can offer great possibilities in this field. Within advanced manufacturing, femtosecond lasers allow inscribing different optical structures. Waveguides are one of the most widely used structures in photonic sensors. This work proposes the design, simulation and manufacture of symmetric cladding waveguides in optical fibers using the slit beam shaping technique. Cladding waveguides characterized by their low losses (0.21 dB $/$ cm), quasi $-$ step-index refractive index profile, and circular cross-section are used to couple evanescent light from the core of a single-mode fiber to each of the four cores of a multicore fiber. [ABSTRACT FROM AUTHOR]

In this work, the development of plane-by-plane (Pl-b-Pl) fiber Bragg gratings (FBGs) by using a single pulse is reported. A slit oriented along the longitudinal axis of the fiber is employed for this. The purpose is to shape the beam so that the focal volume is spatially wider in the transverse direction of the fiber. In this way, it is possible to make 2-D modifications of the refractive index whose width and height depends on the slit width and the pulse energy, respectively. Through an analytical mathematical modelling, the relationship between the diameter of the Gaussian laser beam ($D_x$) and the slit width ($s=D_y$) is determined in order to obtain a circular cross-section. In the experimental results carried out, it can be observed that $D_x/D_y\text{{} = \text{}}3$ ratio cause positive and uniform refractive index changes, compared to the negative and inhomogeneous refractive index changes corresponding to point-by-point (PbP) FBGs. Likewise, Pl-b-Pl FBGs, presenting a broader modification of the core cross-section, show significantly lower losses in transmission (0.3 dB), as well as better reflectivity and FWHM. It is observed that these three parameters (losses, reflectivity and FWHM) have a monotonous tendency according to the slit width. Polarization-dependence is also evaluated. [ABSTRACT FROM AUTHOR]

In this paper, diffractive elements in one and two dimensions have been inscribed at the end-face of single and multicore optical fibers using femtosecond lasers. Different sensing applications have been also presented on the basis of their projected far-field patterns. The manufacturing of 2D structures was achieved using two approaches: line writing and its equivalent point-by-point technique in order to speed up the process. Specific properties of both the techniques have been exploited by inscribing 2-direction and circular gratings. When diffraction grating is inscribed at the end-face of a four core fiber, the 2D interference pattern is transferred into diffraction orders with the possibility of decoupling the interferometric contribution of selected core. Particularly, with a circular diffraction grating, the multicore far-field pattern exhibits two perpendicular and independent interference patterns, as only opposite cores are interfering with each other. These patterns exhibit period and, hence, sensitivity with phase perturbation, proportional to diffraction order angle. The 2D diffraction grating inscribed in the single mode fiber has been employed for bidimensional displacement insensitive sensing, exhibiting linear response, while aforementioned structures written in multicore fiber have been proposed for bending vector sensing, thereby showing higher sensitivity than that of its pristine counterpart. [ABSTRACT FROM AUTHOR]

Fiber Specklegram Sensors (FSSs) are highly sensitive to external perturbations, however, trying to locate perturbation's position remains as a barely addressed study. In this work, a system able to classify perturbations according to the place they have been caused along a multimode optical fiber has been designed. As proof of concept, a multimode optical fiber has been perturbated in different points, recording the videos of the perturbations in the speckle pattern, processing these videos, training with them a machine learning algorithm, and classifying further perturbations based on the spatial locations they were generated. The results show classifications up to 99% when the system has to categorize among three different locations lowering to 71% when the locations rise to ten. [ABSTRACT FROM AUTHOR]

A feasibility study of the employment of a fiber laser working as the laser source of a Brillouin distributed sensor is presented in this paper. The erbium fiber ring laser structure designed works on the single longitudinal mode regime. The main parameters of the ring cavity are analyzed in terms of the laser requirements and the performance of a Brillouin optical time domain analysis sensor using the fiber laser as light source. [ABSTRACT FROM PUBLISHER]

A system based on the use of artificial neural networks allowing discrimination of strain and temperature in a conventional Brillouin optical time domain analyzer setup is presented and demonstrated in this paper. This solution allows to perform an automatic discrimination of both parameters without compromising the complexity or cost of the interrogation unit. The classification results, achieved by considering a preprocessing stage with dimensionality reduction via principal component analysis and spatial filtering, improve those obtained in a previous feasibility study [ABSTRACT FROM AUTHOR]

In this paper, an in-fiber Mach–Zehnder interferometer inscribed by femtosecond laser for curvature sensing has been designed and manufactured. Its operating principle consists of a secondary waveguide inscription working as a sensing arm. This waveguide has been manufactured using asymmetric structures with an average refractive index change of \text1.1\times \text10^-\text2 in its guiding section measured by refracted near-field profilometry. The overall arm/cladding index difference is higher than its core/cladding counterpart, which is a suggested condition for device operation following preliminary simulations. The manufactured Mach–Zehnder interferometer exhibits a linear response to bending radius that is also dependent on an established bending axis. Sensitivity has been measured up to \text9.49\,\textnm/m^-1 for curvature ranges from 0 to 14 \textm^-1. Simulation results using the beam propagation method and conformal mapping transformation to convert bending action into a tilt refractive index agree with experimental results for the same index parameters. Preliminary temperature test shows a remarkable cross sensitivity of 0.0024(3)\,\textm^-1/^\circ C up to 180 $^\circ$C. [ABSTRACT FROM PUBLISHER]

A high-temperature distributed sensor system based on a Brillouin optical time-domain analyzer and a multimode gold-coated fiber is presented in this paper. Distributed measurements of temperatures up to 600 °C are demonstrated with a temperature accuracy of about 10 °C. The system shows a consistent response for repetitive measurements, even considering increasing or decreasing temperature changes. This is the first time to the best of our knowledge that a gold-coated fiber is used for high-temperature distributed measurements in a Brillouin-based system. The proposed solution via the gold-coated fiber allows a feasible deployment in field applications such as industrial scenarios. [ABSTRACT FROM PUBLISHER]

In this study, several multiplexing techniques have been combined to achieve a fiber specklegram multiplexed sensor. Using a single CCD camera as detector, two lasers of different wavelengths have been launched into two multimode polymer optical fibers and then combined through a 2 x 2 coupler before their interrogation. Both coupler exit fibers have been projected to different CCD positions and by analyzing each color of the video sequence, the four fiber channels can be independently obtained. In addition, the speckle sensitivity has been also studied analyzing different properties of speckle patterns such as their contrast or the speckle size. The achieved results will be very useful to the development of new fiber specklegram sensors with several sensing areas using only a CCD camera as detector, making possible low cost sensing devices. [ABSTRACT FROM PUBLISHER]

In this letter, different parameters of the spectral overlapping filtering technique have been analyzed for laser structures. Despite the promising preliminary experimental results obtained using this filtering technique, the optimization of the fiber Bragg grating (FBG) parameters to reduce the bandwidth of the achieved filter depending on the fiber laser requirements and configuration, should be performed. The simulated results have been confirmed during the experimental tests, where the same bandwidths and power evolutions have been measured using a ring laser configuration. This technique has been also tested for very narrow filtering purposes by achieving the single longitudinal mode operation with a ring cavity of L ≈ 8 m, using only two uniform FBGs. [ABSTRACT FROM PUBLISHER]

The recently proposed hierarchical temporal memory (HTM) paradigm of soft computing is applied to the detection and classification of foreign materials in a conveyor belt carrying tobacco leaves in a cigarette manufacturing industry. The HTM has been exposed to hyperspectral imaging data from 10 types of unwanted materials intermingled with tobacco leaves. The impact of the HTM architecture and the configuration of internal parameters on its classification performance have been explored. Classification results match or surpass those attained with other methods, such as Artificial Neural Networks (ANNs), with the advantage that HTM are able to handle raw spectral data and no preprocessing, spectral compression, or reflectance correction is required. It is also demonstrated that an optimized configuration of the HTM architecture and internal values can be derived from the statistical properties of the hyperspectral data, allowing the extension of the approach to other classification problems. [ABSTRACT FROM PUBLISHER]

Structural Health Monitoring (SHM) can be understood as the integration of sensing and intelligence to enable the structure loading and damage-provoking conditions to be recorded, analyzed, localized, and predicted in such a way that nondestructive testing becomes an integral part of them. In addition, SHM systems can include actuation devices to take proper reaction or correction actions. SHM sensing requirements are very well suited for the application of optical fiber sensors (OFS), in particular, to provide integrated, quasi-distributed or fully distributed technologies. In this tutorial, after a brief introduction of the basic SHM concepts, the main fiber optic techniques available for this application are reviewed, emphasizing the four most successful ones. Then, several examples of the use of OFS in real structures are also addressed, including those from the renewable energy, transportation, civil engineering and the oil and gas industry sectors. Finally, the most relevant current technical challenges and the key sector markets are identified. This paper provides a tutorial introduction, a comprehensive background on this subject and also a forecast of the future of OFS for SHM. In addition, some of the challenges to be faced in the near future are addressed. [ABSTRACT FROM PUBLISHER]