1,720,977 research outputs found
Power scaling concepts in fiber lasers and amplifiers
No full textFiber lasers and amplifiers have undergone rapid development in the past two decades, evidenced by the almost exponential rise in their output powers. This project investigates various concepts relating to high-power operation of fiber sources, focussing particularly on mode-area scaling, with the goal of developing a strategy for further power-scaling of fiber sources. The first concept examined is the phenomena of multimode interference (MMI), which occurs when fiber core areas are scaled to the extent that they support the propagation of more than one mode. MMI gives rise to self-imaging, whereby the initial phase relationship and spatial profile of an input beam is recovered at the output. This was investigated using a multimode ytterbium doped fiber amplifier, seeded with a wavelength tunable single mode ytterbium fiber laser. Tuning of the seed source showed that the amplifier underwent periodic self imaging of the fundamental mode beam incident from the seed, with a measured self-imaging wavelength period of 0.7 nm, consistent with predicted values. Despite significant higher order mode presence, self imaging yielded an excellent M2 value of 1.16. The laser beam M2 value in a non-self-imaging state reached a maximum value of 1.6. To illustrate the repercussions of these cyclical changes, the output of this amplifier was coupled into a single mode fiber. As the seed source was tuned the coupling efficiency underwent drastic changes from a maximum of 0.7 to a minimum of 0.04 due to changes in the amplifier’s beam pointing and quality as a result of MMI. A novel concept for mode selection was introduced which would preclude the negative effects of MMI by virtue of operating only on a single selected mode. The concept involves exploiting the mode-dependent spectral response of the reflectivity of fiber Bragg gratings. Experimental work with a multimode thulium fiber laser undertaken in collaboration with colleague Jae Daniel. Selected fundamental mode operation was successfully achieved, with an excellent M2 value of 1.1 compared to 3.3 for the same laser free-running without mode selection. Other higher-order modes could be selected by tuning the fiber laser. Analytical and numerical modelling showed that the reduced spatial overlap between the fundamental mode and the gain profile of the fiber would have minimal effect on the laser slope efficiency if operated at least 8 times above threshold. We speculate scalability of this novel technique to core diameters of up to 70 µm. To lay the groundwork for the transfer of the mode-selection technique to Q-switched multimode thulium fiber lasers, benchmarking experiments were conducted to probe the maximum achievable pulse energies and peak power from a thulium fiber. The highest pulse energy recorded was 618 µJ, with a corresponding peak power of 23 kW. However, the output pulse shape contained multiple peaks separated by one round-trip time (50ns). This multipeak phenomena was investigated numerically. It was found that the multipeak behaviour is initiated by the transient ASE wave injected into the cavity by the Q-switch as it is switched. A novel and elegant method for obtaining singly-pulsed, potentially high peak power output from a highly pumped Q-switch fiber source via regenerative amplification was proposed. This was proven experimentally in a Q-switched thulium fiber. We observed single-peaked pulses with a pulse width of < 14 ns. However, measured pulse energies and peak powers were low (20 µJ and 1.5 kW) due to high cavity losses and deleterious parasitic lasing. Inspection of the output spectrum confirmed that the Q-switched system was in fact a Q-switched ASE source. It is hoped that a more optimal experimental setup will yield better results in terms of the pulse energies and peak powers obtaine
Indirect measurement of LP01 effective area reduction in bent large-core step-index fibres using Raman scattering
No full textWe present an indirect measurement, using stimulated Raman scattering (SRS), of the fundamental mode effective area in several bent large-core step-index passive fibres used for industrial high-power high-brightness beam delivery. The increase of the SRS gain in bent fibres can lessen the benefit of such large-core fibres
Nonlinear effects in bent large-mode-area multimode fibres
No full textWe study the generation of SRS and FWM in bent LMA MM fibres with core diameters in the 25µm-50µm range. In contrast to SRS generation, the FWM power decreases with the bend radius
Fast and controllable beam switching in a pulsed fiber laser
No full textIn this letter, we demonstrate a fast fiber mode converter in a short piece of special passive fiber. The switching mechanism relies on selective wavelength resonance between the core and a high index surrounding ring in the fiber. Beam switching at 530 kHz from a Gaussian beam shape to an expanded beam with a ring with corresponding M2 from 1.4 to 3.5 is demonstrated in a modified industrial pulsed fiber laser at power levels in the excess of 15 W.</p
All-fibre bandwidth tunable filter for high power fibre lasers
No full textWe demonstrate an all-fibre broad bandwidth tunable filter that is high power capable. The fibre used for this device features high index Germanium rods arranged circularly within a fluorine-doped silica cladding. Tuning is achieved by bending the fibre, which increases cross mode coupling between the fundamental mode and the rods’ modes. A distinctive characteristic of this filter is that the short wavelength edge remains relatively unchanged while the long wavelength edge is tunable. Additionally, the filtering bandwidth ranges from tens to several hundreds of nanometers
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Fibre-delivered kilowatt petal beams with self-healing properties
No full textUsing a specially designed all-fibre mode converter we generate and deliver so-called “petal” beam with high-power output. We observe that under partial obstruction, the petal beam can be reconstituted through propagation
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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