Post-processing of optical fibers using femtosecond lasers

RESUMEN: Resulta incontestable que la fotónica empieza a tener una importancia significativa en la sociedad, como indica su presencia en los diseños tecnológicos más avanzados de la actualidad. La fotónica, o ciencia y tecnología de la luz, es una de las seis tecnologías clave de futuro que la Comisión de la Unión Europea considera claves para su relanzamiento y crecimiento económico sostenido. Es considerada “esencial” in EE.UU., tecnología “facilitadora” en los planes Horizonte 2020 y en el de I+D+i vigentes en la UE y en el Estado español. Dentro de la fotónica, el post-procesado de materiales ópticos mediante láseres de alta intensidad se presenta como uno de los campos con mayor aplicabilidad práctica. El presente trabajo se enmarca en dicha cuestión, en tanto en cuanto el procesado láser sirve como base para generar nuevos sensores, dispositivos, y subsistemas fotónicos. En muchos casos, estos dispositivos se conciben y realizan para detectar, monitorizar y medir parámetros relevantes en la salud humana por un lado y, por otro, para combatir el cáncer mediante terapias activadas por luz. Las temáticas mencionadas anteriormente se consideran vitales tanto en varios grupos de trabajo de la Plataforma Europea Photonics21 y en los retos científico-técnicos (societal challenges) que se deben afrontar y, además, las prospectivas de organismos públicos y privados les identifican grandes oportunidades de mercado. Este Trabajo de Fin de Máster recoge las contribuciones aportadas en el campo de los sensores de fibra óptica. Realizando un estudio previo de los conocimientos teóricos necesarios, se acomete el diseño y la fabricación de dos sensores in-fiber de dimensiones milimétricas. Asimismo, con el tratamiento adecuado de las medidas realizadas con ambos, se establecen las novedades generadas con respecto al estado del arte, fundamentalmente en relación a la mejora que supone emplear óptica adaptativa desde el punto de vista de fluencia en el volumen focal, así como la correlación que presentan las variaciones del índice de refracción con la temperatura para guías de onda basadas en propagación por volumen focal o por filamentación.ABSTRACT: It is unquestionable that photonics begins to have a significant importance in society, as indicated by its presence in the most advanced technological designs of today. Photonics, light science and technology, is one of the six Key Enabling Technologies (KETs) for the future that the European Commission (EC) considers key to its re-launch and sustained economic growth. It is considered “essential” in the USA, “facilitator” technology in the Horizon 2020 plans and in the R+D+i in force in the European Union (EU) and in the Spanish State. Within photonics, the post-processing of optical materials using high intensity lasers is presented as one of the fields with greater practical applicability. The present work is framed in this question, insofar as the laser processing serves as a basis to generate new sensors, devices, and photonic subsystems. In many cases, these devices are conceived and performed to detect, monitor and measure relevant parameters in human health on the one hand and, on the other hand, to combat cancer through light-activated therapies. The above-mentioned topics are considered vital both in several working groups of the European Platform Photonics21 and in the scientific-technical challenges (societal challenges) that must be faced and, furthermore, the prospects of public and private organizations identify them great opportunities of the market. This M.Sc Thesis reflects the contributions made in the field of optical fiber sensors. By carrying out a preliminary study of the theoretical knowledge required, the design and manufacture of two in-fiber sensors of millimetric dimensions is undertaken. Likewise, with the adequate treatment of the measurements made with both, the novelties generated with respect to the state of the art are established, fundamentally in relation to the improvement that supposes using adaptive optics from the point of view of fluence in the focal volume, as well as the correlation of the refractive index variations with the temperature for waveguides based on propagation by focal volume or by filamentation.Máster en Ingeniería de Telecomunicació

Reflection-based lab-in-fiber sensor integrated in a surgical needle for biomedical applications

Recently, lab-in-fiber (LIF) sensors have offered a new paradigm in many different scenarios, such as optofluidics, due to their ability to integrate different multiphysics sensor elements in a small space. In this Letter, the design and manufacture of a multiparameter sensing device is proposed, through the combination of an in-fiber air microcavity and a plane-by-plane fiber Bragg grating (FBG). The reflection-based sensor, with a length of less than 300 µm, is located at the end of a single-mode fiber and integrated into a surgical needle for exploitation in biomedical applications. Here we present the first (to our knowledge) ultra-short LIF sensor reported under the "touch and measure" approach. In this first prototype, the detection of axial tensile strain (6.69pm/µe in air cavity) and surrounding refractive index (11.5 nm/RIU in FBG) can be achieved simultaneously.Agencia Estatal de Investigación (PID2019-107270RB-C21, TEC2016-76021-C2-2-R); Ministerio de Educación, Cultura y Deporte (FPU2018/02797)

Slit beam shaping technique for femtosecond laser inscription of symmetric cladding waveguides

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.This work has been supported by the Ministerio de Ciencia, Innovación y Universidades and Agencia Estatal de Investigación (PID2019-107270RB- C21), the Ministerio de Educación, Cultura y Deporte of Spain (PhD grant FPU2018/02797), and FEDER funds

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

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

Light technology for efficient and effective photodynamic therapy: A critical review

Photodynamic therapy (PDT) is a cancer treatment with strong potential over well-established standard therapies in certain cases. Non-ionising radiation, localisation, possible repeated treatments, and stimulation of immunological response are some of the main beneficial features of PDT. Despite the great potential, its application remains challenging. Limited light penetration depth, non-ideal photosensitisers, complex dosimetry, and complicated implementations in the clinic are some limiting factors hindering the extended use of PDT. To surpass actual technological paradigms, radically new sources, light-based devices, advanced photosensitisers, measurement devices, and innovative application strategies are under extensive investigation. The main aim of this review is to highlight the advantages/pitfalls, technical challenges and opportunities of PDT, with a focus on technologies for light activation of photosensitisers, such as light sources, delivery devices, and systems. In this vein, a broad overview of the current status of superficial, interstitial, and deep PDT modalities - and a critical review of light sources and their effects on the PDT process - are presented. Insight into the technical advancements and remaining challenges of optical sources and light devices is provided from a physical and bioengineering perspective.This work was supported by Ministerio de Ciencia e Innovación and Agencia Estatal de Investigación (PID2019-107270RB-C21/AIE/10.13039/501100011033

Recent advances in biomedical photonic sensors: a focus on optical-fibre-based sensing

In this invited review, we provide an overview of the recent advances in biomedical pho tonic sensors within the last five years. This review is focused on works using optical-fibre technology, employing diverse optical fibres, sensing techniques, and configurations applied in several medical fields. We identified technical innovations and advancements with increased implementations of optical-fibre sensors, multiparameter sensors, and control systems in real applications. Examples of outstanding optical-fibre sensor performances for physical and biochemical parameters are covered, including diverse sensing strategies and fibre-optical probes for integration into medical instruments such as catheters, needles, or endoscopes.This work was supported by Ministerio de Ciencia e Innovación and Agencia Estatal de Investigación (PID2019-107270RB-C21/AEI/10.13039/501100011033), and TeDFeS Project (RTC-2017- 6321-1) co-funded by European FEDER funds. M.O. and J.F.A. received funding from Ministerio de Ciencia, Innovación y Universidades of Spain under Juan de la Cierva-Formación and Juan de la Cierva-Incorporación grants, respectively. P.R-V. received funding from Ministerio de Educación, Cultura y Deporte of Spain under PhD grant FPU2018/02797