David Barrera

University of Alcalá | UAH · Department of Electronics

This paper introduces a novel technique to simultaneously measure temperature and strain using a single 5 mm femtosecond laser-inscribed superstructure fiber Bragg grating (SFBG). This SFBG enables improved spectral capabilities, resulting in side-resonances within the optical spectrum. The characteristics of the device depend on three key factors:...

We have methodically implemented and measured fiber tapers of different waist lengths and diameters in a seven-core optical fiber to describe and analyze the inter-core crosstalk between all the cores of the fiber. We describe the crosstalk between the central and external cores but also between outer cores, where the distance to the rest of the co...

We have methodically implemented and measured fiber tapers of different waist lengths and diameters in a seven-core optical fiber to describe and analyze the inter-core crosstalk between all the cores of the fiber. We describe the crosstalk between the central and external cores but also between outer cores, where the distance to the rest of the co...

An elastomeric optical fiber core was fabricated by drawing a 1160-D thermoplastic polyurethane preform. The cut-back technique was used to measure fiber attenuation for a light spectrum ranging from 550 nm to 875 nm, resulting in an average value of less than 0.25 dB/cm, which is suitable for use in short-distance applications. The potential of th...

This paper analyses the waist diameter effect over the sensitivity of a sensor based on a multicore fiber taper. We report the maximum sensitivities of 56.94 pm/°C and -5.2 pm/µɛ to measure temperature and strain variation, respectively.

This paper introduces a novel technique to simultaneously measure temperature and strain using a single 5 mm femtosecond laser-inscribed superstructure FBG. Analyzing resonance distances, it allows distinguishing and complements the contributions of strain and temperature, improving measurements in sensing applications.

Forward Brillouin scattering interactions support the sensing and analysis of media outside the cladding boundaries of standard fibers, where light cannot reach. Quantitative point-sensing based on this principle has yet to be reported. In this work, we report a forward Brillouin scattering point-sensor in a commercially available, off-the-shelf mu...

We monitored the curing temperature and strain of multidirectional carbon-reinforced polymer test pieces cured by oven or microwave radiation heating method through embedded fibre optical sensors based on fibre Bragg gratings.

Forward Brillouin point sensing is demonstrated in a multi-core fiber. Acoustic waves are stimulated by light in one core and monitored using a grating in another. Measurements distinguish between ethanol and water outside the cladding.

This paper presents a technique to acquire measurements of curvature, twist and pose for two multi-core fibers; one with straight cores and the other with helical cores. Both the fibers have multiple fiber Bragg grating (FBG) sensors inscribed in the cores and the fibers are placed in known configurations in order to compare their measurement accur...

Optical fiber sensors are a promising solution for monitoring composites cured by microwave radiation. We embedded several fiber Bragg gratings into a multidirectional fiber reinforced composite test piece for monitoring its curing process using a microwave heating method.

Optical fibers containing multiple cores are being developed towards capacity enhancement in space-division multiplexed optical communication networks. In many cases, the fibers are designed for negligible direct coupling of optical power among the cores. The cores remain, however, embedded in a single, mechanically-unified cladding. Elastic (or ac...

In the last decades, fiber Bragg gratings (FBGs) have become increasingly attractive to medical applications due to their unique properties such as small size, biocompatibility, immunity to electromagnetic interferences, high sensitivity and multiplexing capability. FBGs have been employed in the development of surgical tools, assistive devices, we...

Multiple regenerated fiber Bragg gratings were inscribed in all the cores of a seven-core fiber in order to develop a multi-parameter sensor designed to operate at high temperatures. We experimentally showed the possibility of measuring strains of up to 1200 με, curvatures from 0° to 360° and temperatures up to 1000 °C, with a maximum curvature mag...

This work describes the use of FBGs inscribed in optical fiber with electrodes for voltage sensing. The results show a quadratic voltage dependence. The device can be explored for a multipoint, single-ended voltage sensing device.

Optical fibers containing multiple cores are being developed towards capacity enhancement in space-division multi-plexed optical communication networks. In many cases, the fibers are designed for negligible direct coupling of optical power among the cores. The cores remain, however, embedded in a single, mechanically-unified cladding. Elastic (or a...

We have designed and implemented a fiber optic shape sensor for high-energy ionizing environments based on multicore optical fibers. We inscribed two fiber Bragg gratings arrays in a seven-core optical fiber. One of the arrays has been inscribed in a hydrogen-loaded fiber and the other one in an unloaded fiber in order to have two samples with very...

We propose and experimentally demonstrate distributed microwave photonics signal processing over a few-mode fiber link by implementing 4-sample true time delay line operation. The inscription of a set of long period gratings at specific locations along the few-mode fiber allows the excitation of the higher-order modes while adjusting the individual...

Multicore optical fibers are of great interest in the optical sensing field. Their core diversity and spatial distribution enable the development of sensing mechanisms that are not possible in single-core fibers. In this paper we study the use of the inter-core crosstalk phenomena for the implementation of a surrounding refractive index (SRI) senso...

This article presents a technique to reconstruct the shape of a flexible instrument in three dimensional Euclidean space based on data from Fiber Bragg Gratings (FBG) that are inscribed in multi-core fibers. Its main contributions are the application of several multi-core fibers with FBGs as shape sensor for medical instruments and a thorough prese...

We report, for the first time to our knowledge, distributed radiofrequency signal processing built upon true time delay operation on a step-index few-mode fiber. Two 3-sample configurations with different time delay properties are implemented over the same 60-meter 4-LP-mode fiber link. The inscription of a long period grating at a specific fiber p...

An interrogation technique of cascaded FBGs sensors based on a Microwave Photonics filtering technique under coherent regime is presented. The sensing information of a 5m fiber coil containing 500 weak FBGs with a similar central wavelength is retrieved. The FBGs are 9mm long and the spatial separation between consecutive FBGs is 1mm. The principle...

We have produced a device for the selective light coupling among the seven cores of a multicore optical fiber. We have used a tilted fiber Bragg grating for increasing 40dB the crosstalk between the cores.

We study the use of multicore optical fibers for external refractive index sensing taking advantage of the spatial distribution of the cores by partially coating the surface of the optical fiber with a thin film SnO2 layer.

We selectively inscribe 2º and 7º Tilted Fiber Bragg Gratings (TFBGs) in a seven-core fiber. We use the crosstalk between cores induced by the TFBGs for temperature and refractive index sensing. We have demonstrated that the higher the tilted angle the more sensitive sensor. Nearly 20 dB of optical power variation has been retrieved in a refractive...

Point analysis of forward stimulated Brillouin scattering is demonstrated for the first time, using a Bragg grating in a multi-core fiber. Measurements distinguish between air and water outside a cm-scale fiber section.

We review the introduction of the space dimension into fiber-based technologies to implement compact and versatile signal processing solutions for microwave and millimeter wave signals. Built upon multicore fiber links and devices, this approach allows the realization of fiber-distributed signal processing in the context of fiber-wireless communica...

Multicore optical fibers are especially attractive for the fabrication of curvature and shape sensors due to the spatial distribution of the different cores. Fiber Bragg gratings have been used in the past for the implementation of these sensors, however, despite their inherent properties, they have a very limited sensitivity. In this paper, we stu...

This paper is focused on the characterization of a fiber Bragg grating (FBG) sensor interrogation system based on a fiber ring laser with a semiconductor optical amplifier as the gain medium, and an in-loop electro-optical modulator. This system operates as a switchable active (pulsed) mode-locked laser. The operation principle of the system is exp...

A simple, spectral-drift-insensitive interrogation scheme for a multicore fiber Bragg grating (FBG)-based directional curvature sensor is proposed. The basic principle is to transform the wavelength shift of FBGs into the reflected power variation, which is accomplished by utilizing a broadband source with a sinusoidal spectrum. The closed-form exp...

We propose and experimentally demonstrate an approach to perform high-resolution and temperature-insensitive interrogation of a fiber Bragg grating (FBG)-based multicore fiber (MCF) curvature sensor using a microwave photonics filtering technique. By combining two reflected sample signals from the two FBGs inscribed in MCF and a dispersion device,...

The Karhunen-Loeve transform (KLT) is used to retrieve the wavelength information of several fiber Bragg gratings (FBGs) that are acting as a multiplexed sensor. The modulated light of a broadband source is launched to the FBG cascade in order to capture the electrical frequency response of the system. Thanks to a dispersive media, the wavelengths...

We envision the application of optical Space-division Multiplexing (SDM) to the next generation fiber-wireless communications as a firm candidate to increase the end user capacity and provide adaptive radiofrequency-photonic interfaces. This approach relies on the concept of fiber-distributed signal processing, where the SDM fiber provides not only...

We have inscribed a tilted fiber Bragg grating (TFBG) in selected cores of a multicore optical fiber. The presence of the TFBG permits to couple light from the incident-guided mode to the cladding modes and to the neighbor cores, and this interaction can be used for optical sensing. We measured different magnitudes: strain, curvature magnitude and...

The use of Spatial Division Multiplexing for Microwave Photonics signal processing is proposed and experimentally demonstrated, for the first time to our knowledge, based on the selective inscription of Bragg gratings in homogeneous multicore fibers. The fabricated devices behave as sampled true time delay elements for radiofrequency signals offeri...

We present and experimentally demonstrate the implementation of true time delay lines for microwave photonics signal processing based on the selective inscription of Fiber Bragg gratings along the individual cores of a multicores fiber.

We implement “fiber-distributed signal processing” in the context of fiber-wireless communications by using different multicore fibers, providing both radio access distribution and microwave photonics signal processing in the same fiber medium.

In this work, we propose and experimentally demonstrate an efficient and scalable photonic superstructure employed for UWB pulses modulation purposes. This design is based on the combination of a phase-modulation to intensity modulation (PM-IM) stage with a customized pulse shaper. Pulse shaping is achieved by means of a photonic structure composed...

We present novel multi-cavity fibre-based devices built upon the fabrication of cavities along each one of the cores of a homogeneous 7-core fibre. Since the multi-cavity structure is confined in a single optical fiber, this approach translates into optical true time delay lines that offer at the same time 2D operation by exploiting the optical wav...

New optical fiber based spectroscopic tools open the possibility to develop more robust and efficient characterization experiments. Spectral filtering and light reflection have been used to produce compact and versatile fiber based optical cavities and sensors. Moreover, these technologies would be also suitable to study N-photon correlations, wher...

Single indium arsenide quantum dot emission spectra have been recorded using a four-core, crosstalk-free, multicore fiber placed at the collection arm of a confocal microscope. We developed two different measurement setups depending on the relative configuration of the excitation and collection spots. In the single-matched mode, the emission from t...

The use of multicore optical fibres (MCF) in optical sensing applications has gained increasing interest over the past years due to the benefits directly brought from their inherent spatial diversity. This property allows measuring either multiple physical magnitudes at the same time or the same magnitude with slight differences in order to compens...

The Karhunen-Loeve transform is applied to the coarsely sampled impulse response generated by an FBG cascade in order to calculate the temperature change suffered by the FBGs. Thanks to a dispersive media, the wavelength change performed by the temperature change produces a delay shift in the sample generated by an FBG, delay shift which is recorde...

Incoherent Optical Fourier-Domain Reflectometry incorporating a dispersive delay line is used for the interrogation of an array of three identical fiber Bragg gratings with a Bragg wavelength of 1552.81 nm, reflectivity of -19.3 dB and 10-cm separation. The dispersive delay line induces different delays in the wavelengths reflected by each grating,...

A humidity and temperature optical fiber sensor based on a long-period grating (LPG), which can provide simultaneous response to both magnitudes, is proposed and demonstrated via experiments. Previously, the LPG was fully coated with humidity sensitive nanostructured polymeric thin films by the Layer-by-Layer (LbL) nano assembly technique. Hence th...

We have designed a temperature and strain monitoring system for a magnetic actuator based on the giant magnetostrictive material Terfenol-D (Tb0.3 Dy0.7Fe1.92) with Fiber Bragg grating (FBG) sensors. Magneto-elastic properties of Terfenol-D depend on magnetization, stress pre-history, and temperature. In order to simultaneously monitor these effect...

An optical fiber sensor composed of six standard FBGs in cascade is interrogated by use of a technique based on wavelength to delay mapping. A microwave-modulated optical signal enters the FBG cascade and, after being sliced and reflected, experiences dispersion in standard single-mode fiber. The Bragg wavelengths of the FBGs are mapped into the de...

In this work, a scheme that relies on phase-to-intensity (PM-IM) conversion for high order UWB pulses generation is proposed and demonstrated by using a customized optical filter. The obtained triplet pulse reveals high efficiency and proper fitting for FCC standards.

We present a proof-of-concept experiment aimed at increasing the sensitivity of Fiber-Bragg-gratings temperature sensors by making use of a weak-value-amplification scheme. The technique requires only linear optics elements for its implementation and appears as a promising method for increasing the sensitivity than state-of the-art sensors can curr...

We have implemented a multipoint two-dimensional curvature optical fiber sensor based on a nontwisted homogeneous four-core fiber. A theoretical approach to model the mechanical behavior of these fibers under curvature conditions has been developed. Two shape sensors composed of an array of FBGs inscribed in the four-core fiber have been implemente...

A technique is proposed to interrogate a distributed photonic sensor composed of seven standard fiber Bragg gratings in cascade. A microwave-modulated optical signal is launched into an FBG cascade and subsequently undergoes dispersion in standard single-mode fiber, so that the different Bragg wavelengths are mapped into the radio-frequency (RF) de...

We have designed a magneto-optical sensor based on fiber Bragg grating (FBG) and Metglas 2605CO (Fe67Co18B14Si1) material with magnetostriction coefficient of 35ppm. The FBG sensor is glued between two sheets of Metglas. FBG-based detectors are sensitive to both temperature changes (approximately 10pm/°C) and mechanical stress (approx. 1.15pm). Als...

Structural Health Monitoring (SHM) is presently having a great development. Strains and temperatures are usually the key parameters to be monitored due to their relevance when explaining structural behavior. Several types of sensors are used in SHM, but fiber optic sensors are especially interesting due to their advantages with respect to conventio...

A fiber optic liquid-level sensor based on a long period grating (LPG) is proposed and experimentally validated. The principle of operation is based on a technique used to analyze microwave photonics filters. A 4-cm-long LPG cascaded with a high-reflectivity fiber Bragg grating is employed to achieve a continuous liquid-level sensor. The measuremen...

A system to interrogate photonic sensors based on a very weak fiber Bragg grating cascade fiber is presented and experimentally validated and dedicated to detecting the presence and location of a spot event. The distributed sensor proposed consists of a 5-m-long fiber, containing 500 9-mm-long Bragg gratings with a grating separation of 10.21 mm. T...

A technique to enhance the response and performance of Brillouin distributed fiber sensors is proposed and experimentally validated. The method consists in creating a multi-frequency pump pulse interacting with a matching multi-frequency continuous-wave probe. To avoid nonlinear cross-interaction between spectral lines, the method requires that the...

A photonic temperature sensor based on a 10 cmlong fiber Bragg grating (FBG) is presented and experimentally validated. The system is based on the measurement of the central frequency distribution of the grating based on time-frequency domain analysis. A short optical pulse is coupled into the FBG and the back-reflected pulse is scanned by means of...

A system to interrogate photonic sensors based on long weak fiber Bragg gratings (FBGs) is illustrated and experimentally demonstrated. The FBG sensor is able to detect and measure the precise location of several spot events. The principle of operation is based on a technique used to analyze microwave photonics filters. The long weak FBGs are used...

A photonic sensor based on a 10-cm-long fiber Bragg grating (FBG) is presented and experimentally validated that is dedicated to detect the presence and the position of a temperature gradient. The system is based on the measurement of the central frequency distribution of the grating based on time-frequency domain analysis. A short optical pulse, h...

Recent events such as the fires on St. Gotthard (2001), the Twin Towers in New York (2001) and the Windsor Building in Madrid (2005) show the importance and the necessity of a monitoring strategy for fire vulnerable structures. Through this strategy, the assessment of fire damaged structures would be possible and if necessary, their demolition oper...

A microwave photonic technique is presented to interrogate a 5 m-long fiber, containing 9 mm-long weak FBGs having 10 mm grating separation, used as a distributed sensor. Hot-spots as small as 20 cm have been measured with an accuracy of 0.5 cm

A novel sensor based on a 10 cm-long fiber Bragg grating and a simple scheme is proposed, dedicated to detect the position and distribution of a temperature gradient with a 1 mm spatial resolution.

A multi-point curvature sensor composed by an array of fifteen wavelength multiplexed FBGs has been inscribed in a multicore optical fiber and is demonstrated in order to measure non-uniform curvatures with a resolution of 0.5 10-3 m-1

Space-division multiplexing (SDM) through Multicore fibers (MCFs) has been intensively researched last years as a solution to the upcoming transmission capacity bottleneck in the framework of Digital Communications. We propose the novel extension of this concept to the areas of analog photonics and radio-over-fiber systems by implementing novel dev...

A technique to enhance the response of Brillouin distributed sensors is proposed and experimentally validated. The method consists in creating a multi-frequency pump pulse interacting with a multi-frequency continuous-wave probe. The power of each pulse at a distinct frequency is lower than the threshold for nonlinear effects, while the sensor resp...

Multicore optical fibre can be used to implement multidimensional optical fibre sensors. In this paper, an array of four multiplexed FBGs is inscribed in a multicore optical fibre in order to obtain a multipoint curvature sensor. An improved FBG inscription technique is used in order to mitigate the several issues that arise during the inscription...

The use of microwave radiation for curing carbon-fiber reinforced polymer materials (CFRP) can solve the nonhomogeneous heating problems when using conventional techniques based on the use of catalysts and can reduce the processing times. Optical fiber sensors have well-known advantages for Fiber Reinforced Composites (FRC) monitoring. In this pape...

Tunable FBG at 1.3 microns has been employed to improve the characterization of QD compared to conventional grating monochromators. Using this technique, single metamorphic InAs/GaAs QDs has been characterized by time-resolved photoluminescence.

A novel scheme for the control of microwave signals carried at optical wavelengths by use of PbS colloidal quantum dots embedded in PMMA waveguides is presented. When these structures are pumped at wavelengths where PbS has efficient absorption (980 or 1310 nm), a phase shift in a signal carried at 1550 nm is induced. Optimal conditions have been a...

Photoluminescence and time resolved photoluminescence from single metamorphic InAs/GaAs quantum dots (QDs) emitting at 1.3 μm have been measured by means of a novel fibre-based characterization set-up. We demonstrate that the use of a wavelength tunable fibre Bragg grating filter increases the light collection efficiency by more than one order of m...

A novel technique for interrogating photonic sensors based on long fiber Bragg gratings (FBGs) is presented and experimentally demonstrated, dedicated to detect the presence and the precise location of several spot events. The principle of operation is based on a technique used to analyze microwave photonics (MWP) filters. The long FBGs are used as...

A novel scheme for controlling microwave signals in the optical domain is proposed by means of Integrable Microwave Photonic Phase-Shifters based on Colloidal Quantum Dots embedded in PMMA waveguides. A phase shift at 1550 nm can be induced when the structures are pumped at 1310 nm or 980 nm due to the refractive index changes provoked by the absor...

Regenerated Fibre Bragg Gratings have the potential for high-temperature monitoring. In this paper, the inscription of Fibre Bragg Gratings (FBGs) and the later regeneration process to obtain Regenerated Fiber Bragg Gratings (RFBGs) in high-birefringence optical fiber is reported. The obtained RFBGs show two Bragg resonances corresponding to the sl...

In this work we developed new integrated microwave photonic phase shifters on polymer waveguides. A phase shift at 1550 nm can be induced in PMMA waveguide when it is pumped at 1310 nm due to the refractive index changes provoked by heating. Such a shift can be controlled with the length of the waveguide or the frequency of the microwave signal; or...

We present the fabrication and characterization of regenerated fiber Bragg gratings (RFBG) in high-birefringence optical fiber for high-temperature sensing. The RFBG presents two Bragg resonances corresponding to the slow and fast axis of the optical fiber. The distance between these two Bragg resonances can be used to reduce the cost of the interr...

A technique for liquid-level sensors based on a long fiber Bragg grating (FBG) is presented and experimentally demonstrated. The measurement system is based on the measurement of the central frequency distribution of the FBG based on time-frequency domain analysis. A short optical pulse is injected into a 10-cm long FBG mounted in a container. The...

A new technique to investigate the spatial distribution of the reflection spectrum along fabricated long weak fiber Bragg gratings (FBG) is experimentally demonstrated, together with its potential applications for distributed fiber sensing and broadband signal processing. A short pulsed coherent light signal is launched into a FBG and the signal fr...

Recent events such as the fires on the Channel tunnel (1996), St. Gotthard (2001), the Twin Towers in New York (2001), the Windsor Building in Madrid (2005) and the collapse of the MacArthur Maze in Oakland (2007) show the importance and the necessity of a monitoring strategy for fire vulnerable structures. Through this strategy, the assessment of...

The following paper presents a bend sensor that consists of three Bragg Gratings inside a Standard Fiber Optic for the characterization of the reflected wavelengths with respect to the radius of curvature in conditions of compression and extension, and it is used to determinate the distance between the bend sensor and the neutral axis. This necessi...

We report on temperature sensors based on regenerated fiber Bragg gratings (RFBGs) for measurements up to 1100. The annealing process required to regenerate such gratings makes the optical fiber too delicate, thus making an adequate packaging necessary. Prior to the grating regeneration the optical fiber is protected with a ceramic tube which in tu...

We present the results of a real fire test using optical fiber sensors embedded in concrete samples. The temperature curve used in this experiment is described in the Spanish/European standard UNE-EN 1363-1 temperature profile for normalized concrete resistance to real fire tests, reaching temperatures of more than 1000°C inside the fire chamber an...

In this paper we have studied the performance of a high-temperature packaged sensor based on Regenerated Fiber Bragg Gratings (RFBGs). The packaging consists of a two-bore ceramic tube and a 1.5 mm-thick metal casing. The optical response of packaged and unpackaged RFBG sensors for temperature measurements up to 1100°C have been evaluated and compa...

Optical fiber sensors based on Fiber Bragg Gratings (FBG) have been embedded in concrete samples for temperature measurement. Three different types of gratings have been used in this experiment: FBGs inscribed in photosensitive germanium-boron codoped fiber and Regenerated Fiber Bragg Gratings (RFBG) inscribed in germanium doped and in germanium-bo...

This work addresses important issues of photonic crystal fiber (PCF) interferometers built by fusion splicing: high insertion loss, low mechanical strength, and complex multiplexing schemes. We have found that by decreasing the length of the collapsed region in these interferometers, the overall insertion losses diminish (down to 0.7 dB) without co...

We have implemented a dynamic strain sensor using a Polymer Optical Fiber Bragg Grating (POFBG). In this paper, we have investigated an approach for making such systems cheaper through the use of easy to handle multimode fiber. A Vertical-Cavity Surface-Emitting Laser is used to decrease the cost of the interrogation system and a photodetector conv...

We have successfully created Chemical Composition Gratings (CCGs) into two different types of optical fiber: standard telecommunications Germanium doped fibers and photosensitive Germanium/Boron co-doped fibers. We have performed temperature cycles for analyzing the sensing properties and degradation or hysteresis with respect to the CCG sensors. T...

We present in this paper the results of monitoring the construction process of a steel incrementally launched bridge located at the Kadagua Valley in Bilbao (Spain) with FBG sensors. The installation of FBG strain and temperature sensors was done in order to obtain deformation and temperature variations during the launching operation. The deflectio...

We have successfully created Chemical Composition Gratings (CCGs) into two different types of fiber: standard telecommunications Germanium doped fibers and photosensitive Germanium/Boron co-doped fibers. We will present results on the regeneration process, the sensing properties and also the high temperature decay and maximum operative temperature...