Tiny structures inscribed by femtosecond lasers for optical sensing

RESUMEN: En este trabajo se establece el procesado por láseres de femtosegundo como método fiable de producir diversos tipos de sensores de fibra óptica. Para ello se ha estudiado la inscripción en fibra, dando lugar a un interferómetro tipo Mach-Zehnder y a redes de difracción tanto de Bragg como de periodo aleatorio. Igualmente, se ha estudiado la inscripción en la punta de la fibra, generando diversos patrones difractivos en numerosos tipos de fibra óptica y aplicándolos tanto al sensado extrínseco como extrínseco.ABSTRACT: In this work, femtosecond laser processing is highlighted as an effective way to produce several types of optical fiber sensors. A deep study of in-fiber laser inscription has been performed, obtaining a Mach-Zehnder interferometer and both Bragg and random gratings as a result. Attention was also focused to the fiber end-face, where several fiber diffractive configurations were studied in several types of fiber optics. These patterns were employed for both intrinsic and extrinsic measurements.The following projects and grants supported the research performed in this PhD: Optical Fiber Sensors for Safety and Protection (TEC2013-47264-C2-1-R), Ministerio de Economía y Competitividad. Sensores fotónicos para la seguridad y protección (SENSA) (TEC2016-076021-C2-2-R), AEI/FEDER, UE. BES-2014-069736, Ministerio de Economía y Competitividad. EEBB-I-17-12409, Ministerio de Economía y Competitividad. From the participation on these projects, authors would like to acknowledge the contributions of the whole Photonics Engineering Group, especially Mauro Lomer and Pablo Varona Roldán. Jesús Mirapeix Serrano, Arturo Pardo Franco, Olga Ma Conde Portilla and Antonio Quintela Incera have also supported this work by thoughtful manuscript revisions or printing and binding management. This dissertation was improved by the fruitful collaborations with the group led by Prof. Manuel López Amo, particularly Rosa Ana Pérez Herrera performed measurements and montages that supported the results obtained in this work. Besides, authors would like to acknowledge the invaluable contributions that Bertrand Poumellec and Matthieu Lancry from the Institut de Chimie Moléculaire et des Matériaux d’Orsay have performed to this work

Slit beam shaping technique for femtosecond laser inscription of enhanced plane-by-plane FBGs

In this work, the development of plane-by-plane (Plb-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 ) and the slit width (s = D,) is determined in order to obtain a circular cross-section. In the experimental results carried out, it can be observed that D /D, = 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.This work was supported in part by the project TEC2016-76021-C2-2-R (AEI/FEDER) of the Spanish government and FEDER funds and in part by the Grant FPU2018/02797 from the Ministry of Science, Innovation and Universities of the Spanish government

In-fiber Mach-Zehnder interferometer inscribed with femtosecond laser for high temperature sensing

In this work, an In-fiber Mach-Zehnder Interferometer has been manufactured within a standard optical fiber using direct inscription with a femtosecond laser. Different geometries have been tested in order to enhance the sensitivity of manufactured devices to high temperature sensing, reducing the total device up to 300µm. The achieved results exhibit a remarkably good response for temperature sensing up to 650°C.This work has been supported by the projects TEC2013-47264-C2-1-R, TEC2016-76021-C2-2-R and grant BES-2014-069736 of the Spanish government and by the Parliament of Cantabria postdoc grant

Curvature sensor based on In-fiber Mach-Zehnder interferometer inscribed with femtosecond laser

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 1.1 × 10-2 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 9.49 nm/m-1 for curvature ranges from 0 to 14 m-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)m-1/-C up to 180 -C.This work has been supported by the pro-jects TEC2013-47264-C2-1-R, TEC2016-76021-C2-2-R (AEI/FEDER, UE) financed with FEDER funds, and grant BES-2014-069736 of the Spanish government and by the Parliament of Cantabria postdoc grant

Reflected power-based 2D bending sensor using femtosecond laser FBG inscription in multicore fiber

In this paper, we present research on the use of femtosecond lasers to develop a two-dimensional bending sensor by inscribing a 4 mm fiber Bragg grating (FBG) in each of the four cores of a multicore fiber (MCF) Fibercore SM-4C1500. The sensor located at the end of the fiber is spliced to a 50/125 multimode fiber (MMF). Due to the geometry of the MCF, part of its cores do not directly attach to the core of the multimode fiber, so that different curvatures cause variations in the reflected power. In this way, a reflection configuration and a commercial spectrometer are used to study its power response, simplifying the sensing, since it is not necessary to have WDM elements for the handling of wavelengths that vary tenths of nm in this type of sensors. Likewise, by carefully controlling the laser parameters and the motor stage position we are able to inscribe the FBGs by means of the point-by-point (PbP) method.This work has been supported by the project TEC2016-76021-C2-2-R (AEI/FEDER,UE) of the Spanish government and FEDER funds

Optical fiber sensors by direct laser processing: a review

The consolidation of laser micro/nano processing technologies has led to a continuous increase in the complexity of optical fiber sensors. This new avenue offers novel possibilities for advanced sensing in a wide set of application sectors and, especially in the industrial and medical fields. In this review, the most important transducing structures carried out by laser processing in optical fiber are shown. The work covers different types of fiber Bragg gratings with an emphasis in the direct-write technique and their most interesting inscription configurations. Along with gratings, cladding waveguide structures in optical fibers have reached notable importance in the development of new optical fiber transducers. That is why a detailed study is made of the different laser inscription configurations that can be adopted, as well as their current applications. Microcavities manufactured in optical fibers can be used as both optical transducer and hybrid structure to reach advanced soft-matter optical sensing approaches based on optofluidic concepts. These in-fiber cavities manufactured by femtosecond laser irradiation followed by chemical etching are promising tools for biophotonic devices. Finally, the enhanced Rayleigh backscattering fibers by femtosecond laser dots inscription are also discussed, as a consequence of the new sensing possibilities they enableThis research was funded by the Ministerio de Economía y Competitividad of Spain (TEC2016-76021-C2-2-R), the FEDER/Ministerio de Ciencia, Innovación y Universidades and Agencia Estatal de Investigación (PID2019- 107270RB-C21), and the Ministerio de Educación, Cultura y Deporte of Spain (PhD grant FPU2018/02797)

Astigmatism compensation for waveguide inscription in optical fiber by femtosecond lasers

The cylindrical geometry of optical fibers produces an astigmatic distortion in a wavefront focused within it. In the case of femtosecond lasers, this produces a fluence loss that decreases its processing performance. In this work, the phase change produced by an astigmatic femtosecond laser beam (direct exposition to the) and a corrected beam (applying a simple adaptive optics process) is compared. The astigmatic correction decreases the modification threshold by approximately a magnitude order and changes the sign of the refractive index change at low pulse energies.This work has been supported by the project TEC2016-76021-C2-2-R (AEI/FEDER, UE) and grants BES-2014-069736 from Ministerio de Economía y Competitividad

Hybrid Mach-Zehnder interferometer manufactured by femtosecond laser multiscan technique

In this paper, a 6 mm hybrid Mach-Zehnder Interferometer (MZI) has been manufactured within a standard optical fiber using multiscan inscription with femtosecond laser. This technique allows the employ of cladding waveguides (CWG) as sensing arms for the interferometer. Refracted Near Field (RNF) profilometry and Quantitative Phase Microscopy (QPM) consistently suggest that CWG exhibit a smooth Type I refractive index change (RIC) that increases with the number of scans. This makes the scan number a potential way to control the coupling and Free Spectral Range (FSR) of the manufactured MZI. Its combination with a fiber Bragg grating (FBG) inscribed in the core makes possible to discriminate between different parameters.This work has been supported by the project TEC2016-76021-C2-2-R (AEI/FEDER) of the Spanish government and FEDER funds

Femtosecond laser inscription of diffractive element in optical fiber end-face

In this work, the potential of femtosecond laser for optical fiber end-face inscription is highlighted by presenting a reliable setup for optical fiber longitudinal writing. This set-up has been used to write 1 dimension diffraction grating at the fiber end-face. Its far field intensity distribution has been measured and compared with simulations assuming rectangular air hole structure of same width and period. By comparing diffraction efficiencies, a first attempt of measuring hole deep is achieved.This work has been supported by the projects TEC2013-47264-C2-1-R, TEC2016-76021-C2-2-R (AEI/FEDER, UE) financed with FEDER funds, and grant BES-2014-069736 of the Spanish government and by the Parliament of Cantabria postdoc grant

High temperature distributed measurements using multimode gold-coated fiber and BOTDA sensor

RESUMEN: En este trabajo se presenta un sensor distribuido basado en la dispersión estimulada de Brillouin combinado con el empleo de una fibra multimodo con cubierta de oro que permite la realización de medidas distribuidas a alta temperatura. Con este sistema de medida se han realizado medidas de zonas calientes en la fibra hasta alcanzar una temperatura máxima de 600ºC sin que la fibra presente degradación en sus propiedades. La precisión de las medidas está en torno a los 10ºC.ABSTRACT: We present a distributed sensor based on stimulated Brillouin scattering with a gold-coated multimode fiber to carry out high temperature distributed measurements. With this sensor system, it is possible to measure hotspots in the optical fiber up to 600ºC ensuring that the optical fiber does not suffer any detrimental effect on its properties. The achieved accuracy of the measurements is about 10ºC.Este trabajo ha sido cofinanciadopor los proyectos TEC2013-47264-C2-1-R, TEC2016-76021-C2-2-R (AEI/ FEDER, UE), por la beca BES-2014-069736 del gobierno de España y por la beca postdoctoral del Parlamento de Cantabria