Specialty Fiber Lasers and Novel Fiber Devices

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

Comparison of higher-order mode suppression and Q-switched laser performance in thulium-doped large mode area and photonic crystal fibers

We report the influence of higher order modes (HOMs) in large mode fibers operation in Q-switched oscillator configurations at similar to 2 mu m wavelength. S-2 measurements confirm guiding of LP11 and LP02 fiber modes in a large mode area (LMA) step-index fiber, whereas a prototype photonic crystal fiber (PCF) provides nearly single-mode performance with a small portion of light in the LP11 mode. The difference in HOM content leads to a significant difference in Q-switched oscillator performance. In the step-index fiber, the percentage of cladding light increases by 20% to \u3e 40% with increasing pulse energy to similar to 250 mu J. We accredit this degradation to saturation of the gain in the fundamental mode leading to more light generated in the HOMs, which is eventually converted into cladding light. No such degradation is seen in PCF laser system for \u3e 400 mu J energies

Monolithic Fiber Lasers Combining Active Pcf With Bragg Gratings In Conventional Single-Mode Fibers

Novel monolithic fiber laser architectures utilizing large mode area (LMA) photonic crystal fiber (PCF) and fiber Bragg gratings (FBG) in conventional single-mode fibers (SMF) are presented. The main challenge is to address high cavity losses arising from the intrinsic 18-fold mode-field mismatch between the SMF and the active LMA PCF. Employing an all-fiber, robust and reproducible mode-field matching approach based on graded-index multimode fibers, we numerically and experimentally demonstrate that the SMF-to-LMA PCF coupling can be more than three-fold improved. This MFA approach is further implemented in monolithic fiber laser cavities combining FBGs in SMF and active LMA PCF. We demonstrate that cavity losses can be significantly mitigated when using appropriate MFAs resulting in a substantial increase of the laser output performances

Detailed Characterization Of Optical Fibers By Combining S\u3csup\u3e2\u3c/sup\u3e Imaging With Correlation Filter Mode Analysis

Spatially and spectrally resolved imaging (S2 imaging) and correlation filter technique (CFT) are two very different, widespread fiber mode analysis techniques. Both techniques have been successfully employed to decompose few-modes and multimode beams respectively. In this study, we present a novel experimental tool combining S2 imaging and CFT mode analyses in a unique system. We demonstrate that both methods are complementary with the ability to fully resolve scalar and vector-valued transverse modal fields. Using results from the combined experiment, mode powers (o2) evaluated from CFT analysis and S2 imaging are directly compared for a wide range of fiber beams (from single-to multi-mode). As a result, we experimentally identify the mode detection limit of each mode analysis and prove that S 2 imaging accuracy range can be considerably increased employing an analytical mode evaluation method. The conclusion contains a table summarizing the expertise of each mode analysis. © 1983-2012 IEEE

Strong And Robust Bragg Gratings In Photo-Thermo-Refractive Glass Fiber

We demonstrate strong and robust fiber Bragg gratings in the first low-loss optical fiber made from photo-thermo-refractive (PTR) glass. A high grating strength of 20 dB is maintained even at 12 hour exposure to temperatures above 400 °C. © 2013 The Optical Society