Quantitative phase imaging (QPI) is a label-free technique providing both morphology and quantitative biophysical information in biomedicine. However, applying such a powerful technique to in vivo pathological diagnosis remains challenging. Multi-core fiber bundles (MCFs) enable ultra-thin probes for in vivo imaging, but current MCF imaging techniques are limited to amplitude imaging modalities. We demonstrate a computational lensless microendoscope that uses an ultra-thin bare MCF to perform quantitative phase imaging with microscale lateral resolution and nanoscale axial sensitivity of the optical path length. The incident complex light field at the measurement side is precisely reconstructed from the far-field speckle pattern at the detection side, enabling digital refocusing in a multi-layer sample without any mechanical movement. The accuracy of the quantitative phase reconstruction is validated by imaging the phase target and hydrogel beads through the MCF. With the proposed imaging modality, three-dimensional imaging of human cancer cells is achieved through the ultra-thin fiber endoscope, promising widespread clinical applications.

The generation of tailored complex light fields with multi-core fiber (MCF) lensless microendoscopes is widely used in biomedicine. However, the computer-generated holograms (CGHs) used for such applications are typically generated by iterative algorithms, which demand high computation effort, limiting advanced applications like fiber-optic cell manipulation. The random and discrete distribution of the fiber cores in an MCF induces strong spatial aliasing to the CGHs, hence, an approach that can rapidly generate tailored CGHs for MCFs is highly demanded. We demonstrate a novel deep neural network—CoreNet, providing accurate tailored CGHs generation for MCFs at a near video rate. The CoreNet is trained by unsupervised learning and speeds up the computation time by two magnitudes with high fidelity light field generation compared to the previously reported CGH algorithms for MCFs. Real-time generated tailored CGHs are on-the-fly loaded to the phase-only spatial light modulator (SLM) for near video-rate complex light fields generation through the MCF microendoscope. This paves the avenue for real-time cell rotation and several further applications that require real-time high-fidelity light delivery in biomedicine.

[ES] La fibra óptica ha supuesto una gran revolución en el mundo de las telecomunicaciones debido a su alta capacidad de transmisión y sus bajas pérdidas. Hoy en día no sería posible transportar la cantidad de tráfico que se genera en internet si no fuera por sis- temas de comunicaciones basados en fibras ópticas. Sin embargo, el número de dispo- sitivos conectados a internet es cada vez mayor, por lo que la capacidad de la fibra óptica estándar de un solo núcleo se puede ver limitada en un futuro no muy lejano. Una forma de aumentar dicha capacidad es utilizar fibras ópticas con varios núcleos. Actualmente existe un gran interés sobre la investigación en este tipo de fibras para aplicaciones de telecomunicaciones, por lo que no es difícil encontrar fibras multinú- cleo comerciales. Aunque el uso más común de la fibra óptica es para telecomunicaciones, también se puede utilizar como sensor. Uno de los métodos más comunes para la implementa- ción de sensores es la inscripción de redes de difracción en fibras ópticas de un solo núcleo. Sin embargo, la inscripción de redes de dirección en fibras de múltiples núcleos abre nuevas líneas de investigación para el desarrollo de sensores avanzados. En esta tesis se ha estudiado distintos tipos de redes de difracción inscritas en una fibra de siete núcleos para su aplicación en la implementación de sensores. En primer lugar, se describe el sistema de fabricación que permite inscribir distintos tipos de redes de difracción en la fibra multinúcleo de forma selectiva, es decir, permite seleccionar en que núcleos se va a inscribir la red. Mediante este sistema se han inscrito redes de periodo largo y posteriormente se han caracterizado como sensor de deformación, tor- sión y curvatura. Después, se han inscrito redes de Bragg inclinadas para aumentar de forma intencionada la diafonía entre los núcleos de la fibra mediante el acoplo de luz entre ellos. Además, se ha demostrado experimentalmente que esta diafonía es sensible a la deformación de la fibra, a la curvatura, a la temperatura y al índice de refracción que rodea la fibra. Por otro lado, se ha demostrado que las redes de Bragg inscritas en fibras multinúcleo se pueden utilizar para implementar sensores de curvatura capaces de operar en entornos radioactivos. Finalmente se han fabricado redes de Bragg rege- neradas capaces de operar a altas temperaturas, estas redes se han caracterizado como sensor de temperatura, deformación y curvatura. / [CAT] La fibra òptica ha suposat una gran revolució en el món de les telecomunicacions a causa de la seua alta capacitat de transmissió i les seues baixes pèrdues. Hui en dia no seria possible transportar la quantitat d'informació que es genera en internet si no fos pels sistemes de comunicacions basats en fibres òptiques. No obstant això, el nombre de dispositius connectats a internet es cada vegada més gran, per la qual cosa la capacitat de la fibra òptica estàndard d'un sol nucli es pot veure limitada en un futur no gaire llunyà. Una manera d'augmentar aquesta capacitat es utilitzar fibres òptiques amb diversos nuclis. Actualment existeix un gran interès sobre la investigació en aquesta mena de fibres per a aplicacions de telecomunicacions, per la qual cosa no es difícil trobar fibres de múltiples nuclis comercials. Encara que l'ús mes comú de la fibra òptica es per a telecomunicacions, també es pot utilitzar com a sensor. Un dels mètodes més comuns per a la implementació de sensors es la inscripció de xarxes de difracció en fibres òptiques d'un sol nucli. No obstant això, la inscripció de xarxes de difracció en fibres de múltiples nuclis obri noves línies d'investigació per al desenvolupament de sensors més complexos. En aquesta tesi s'ha estudiat diferents tipus de xarxes de difracció inscrites en una fibra de set nuclis per a la seua aplicació en la implementació de sensors en fibra òptica. En primer lloc, es descriu el sistema de fabricació de xarxes de difracció que permet inscriure diferents tipus de xarxes de difracció en la fibra de múltiples nuclis de manera selectiva, es a dir, permet seleccionar en que nuclis s'inscriurà la xarxa. Mitjançant aquest sistema s'han inscrit xarxes de període llarg i posteriorment s'han caracteritzat com a sensor de deformació, torsió i curvatura. Després, s'han inscrit xarxes de Bragg inclinades per a augmentar de manera intencionada la diafonia entre els nuclis de la fibra mitjançant l'acoblament de llum entre ells. A més d'això, s'ha demostrat experimentalment que aquesta diafonia es sensible a la deformació de la fibra, a la curvatura, a la temperatura i a l'índex de refracció que envolta la fibra. D'altra banda, s'ha demostrat que les xarxes de Bragg inscrites en fibres múltiples nuclis es poden utilitzar per a implementar sensors de curvatura que poden operar en entorns radioactius. Finalment s'han fabricat xarxes de Bragg regenerades que suporten altes temperatures, aquestes xarxes s'han caracteritzat com a sensor de temperatura, deformació i curvatura. / [EN] Optical fiber has been a great revolution in the world of telecommunications due to its high transmission capacity and low attenuation. Today it would not be possible to transport the amount of traffic that is generated on the Internet without communication systems based on optical fibers. However, the number of devices connected to the Internet is increasing, so the capacity of standard single-core fiber optics may be limited so far in the future. One way to increase this capacity is to use multi-core optical fibers. Nowadays is a great interest in research in this type of fibers for telecommunications applications, so it is not difficult to find commercial multicore fibers. Although the most common use of fiber optics is for telecommunications, it can also be used as a sensor. One of the most common methods for sensor implementation is the inscription of diffraction gratings on single-core optical fibers. However, the enrollment of steering networks in multi-core fibers opens new lines of research for the development of advanced sensors. In this thesis, different types of diffraction gratings inscribed in a seven-core fiber have been studied for their application in the implementation of sensors. In the first place, the diffraction grating manufacturing system is described that allows to inscribe different types of diffraction gratings in the multicore fiber selectively, that is, it allows to select in which cores the grating is going to be inscribed. By means of this system, long-period networks have been inscribed and subsequently they have been characterized as a deformation, torsion, and curvature sensor. Then, slanted Bragg gratings have been inscribed to intentionally increase the crosstalk between the fiber cores by coupling light between them. Furthermore, this crosstalk has been experimentally shown to be sensitive to fiber deformation, curvature, temperature, and the index of refraction surrounding the fiber. On the other hand, it has been shown that Bragg networks inscribed in multicore fibers can be used to implement curvature sensors capable of operating in radioactive environments. Finally, regenerated Bragg nets capable of operating at high temperatures have been manufactured. These nets have been characterized as a temperature, deformation, and curvature sensor. / Agradezco a la Universitat Politècnica de València la beca FPI (PAID-01-18) que me fue concedida para realizar está tesis. / Madrigal Madrigal, J. (2022). Diseño, fabricación y caracterización de sensores basados en fibras ópticas de múltiples núcleos [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/180806 / TESIS

At the Dawn of the 21st century, the field of specialty optical fibers experienced a scientific revolution with the introduction of the stack-and-draw technique, a multi-steps and advanced fiber fabrication method, which enabled the creation of well-controlled micro-structured designs. Since then, an extremely wide variety of finely tuned fiber structures have been demonstrated including novel materials and novel designs. As the complexity of the fiber design increased, highly-controlled fabrication processes became critical. To determine the ability of a novel fiber design to deliver light with properties tailored according to a specific application, several mode analysis techniques were reported, addressing the recurring needs for in-depth fiber characterization. The first part of this dissertation details a novel experiment that was demonstrated to achieve modal decomposition with extended capabilities, reaching beyond the limits set by the existing mode analysis techniques. As a result, individual transverse modes carrying between ~0.01% and ~30% of the total light were resolved with unmatched accuracy. Furthermore, this approach was employed to decompose the light guided in Large-Mode Area (LMA) fiber, Photonic Crystal Fiber (PCF) and Leakage Channel Fiber (LCF). The single-mode performances were evaluated and compared. As a result, the suitability of each specialty fiber design to be implemented for power-scaling applications of fiber laser systems was experimentally determined. The second part of this dissertation is dedicated to novel specialty fiber laser systems. First, challenges related to the monolithic integration of novel and complex specialty fiber designs in all-fiber systems were addressed. The poor design and size compatibility between specialty fibers and conventional fiber-based components limits their monolithic integration due to high coupling loss and unstable performances. Here, novel all-fiber Mode-Field Adapter (MFA) devices made of selected segments of Graded Index Multimode Fiber (GIMF) were implemented to mitigate the coupling losses between a LMA PCF and a conventional Single-Mode Fiber (SMF), presenting an initial 18-fold mode-field area mismatch. It was experimentally demonstrated that the overall transmission in the mode-matched fiber chain was increased by more than 11 dB (the MFA was a 250 ?m piece of 50 ?m core diameter GIMF). This approach was further employed to assemble monolithic fiber laser cavities combining an active LMA PCF and fiber Bragg gratings (FBG) in conventional SMF. It was demonstrated that intra-cavity mode-matching results in an efficient (60%) and narrow-linewidth (200 pm) laser emission at the FBG wavelength. In the last section of this dissertation, monolithic Multi-Core Fiber (MCF) laser cavities were reported for the first time. Compared to existing MCF lasers, renown for high-brightness beam delivery after selection of the in-phase supermode, the present new generation of 7-coupled-cores Yb-doped fiber laser uses the gain from several supermodes simultaneously. In order to uncover mode competition mechanisms during amplification and the complex dynamics of multi-supermode lasing, novel diagnostic approaches were demonstrated. After characterizing the laser behavior, the first observations of self-mode-locking in linear MCF laser cavities were discovered.

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