1,721,014 research outputs found
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
High-power cladding-pumped Raman and erbium-ytterbium doped fibre sources
No full textRaman fibres and rare-earth doped silica fibres have been investigated for many years as gain media for the amplification of optical signals in telecommunications thanks to their broad gain bandwidth. They are now widely used for that application. It is only recently that power scaling of fibre laser sources has taken place with the development of double-clad fibres and high power laser diode pump sources. Cladding-pumped fibre lasers are now a rapidly expanding and emerging technology with a wide range of applications, where high-power and high-brightness laser sources are required. Nevertheless, so far, most high-power lasers have been based on highly efficient ytterbium-doped fibre, while progress to power-scale other rare-earth doped fibres and fibre Raman laser has been much more modest.This thesis can be divided into two main themes. The first concerns the power-scaling and study of erbium-ytterbium doped fibre laser sources for optical amplification or as laser sources. The second theme concerns the development and study of high-power Raman fibre lasers and amplifiers based on the novel concept of a cladding-pumped Raman fibre. The themes are jointed in that the cladding-pumped Er:Yb doped lasers, developed in the first theme, are used as pump sources in this second part for the Raman devices. They are also jointed in that they both concern power-scaling of “eye-safe” sources at around 1.6 mm.Firstly, in collaboration with co-workers, high-power, large core, erbium-ytterbium doped fibre laser sources are developed. Output powers in excess of 70 W are obtained. Good beam quality output is achieved thanks to a tapered fibre section. The taper is compatible with standard single-mode fibre which enabled the realization of tuneable fibre lasers free from bulk external grating. The laser’s tuning characteristics are investigated in the C- and L-band range. Subsequently, a master-oscillator power-amplifier (MOPA) based on large core Er:Yb doped fibres is developed for the generation of high-energy pulses. The details of the MOPA are studied and presented. With careful design considerations, pulses free from non-linear effects, with energy up to 1 mJ and peak powers up to 6.6 kW, with narrow spectral linewidth, are obtained at 1535 nm.Secondly, using a double-clad fibre, consisting of raised index, germanium doped, core and inner cladding, with a pure silica outer cladding, a high-power CW single mode Raman fibre laser, pumped by a multi-mode erbium-ytterbium doped fibre laser, is demonstrated for the first time. The laser slope efficiency is 67% and the output power is in excess of 10 W. An experimental and theoretical study of the laser is performed. Then, the pulse amplification in a cladding-pumped Raman fibre is studied in a single pass amplifier configuration. The effects on the laser performance of the pump and Stokes seed powers, fibre length and four-wave mixing are presented. The Stokes’s small-signal gain can be as high as 50 dB. Using this configuration, 700 ns long pulses are amplified up to 10 mJ which shows that, potentially, optical pulses could be amplified to much higher energy. Finally, these results together, let predict that, soon, cladding-pumped Raman fibre could be used as direct brightness converter
Brightness enhancement limits in pulsed cladding pumped fiber Raman amplifiers
No full textWe analyze theoretically limitations on the brightness enhancement of a multimode pump beam, to be efficiently converted into a diffraction-limited Stokes beam in a cladding-pumped fiber Raman amplifier. For a given minimum Raman pump absorption, parasitic 2nd Stokes generation limits the cladding-to-core area ratio, and thus the brightness enhancement. A W-type fiber acting as a spectral waveguide filter allows for nearly five times larger inner-cladding areas by suppressing the 2nd Stokes. We further analyze limits set by glass damage and indirectly propagation loss, as well as pulse walk-off. A well-designed fiber with 3.5 dB/km propagation loss allows for a pump-to-signal brightness improvement of up to 3600 times both in the pulsed and the cw regime
High power fiber lasers: a review
No full textIn this paper, we summarize the fundamental properties and review the latest developments in high power fiber lasers. The review is focused primarily on the most common fiber laser configurations and the associated cladding pumping issues. Special attention is placed on pump combination techniques and the parameters that affect the brightness enhancement observed in single-mode and multimode high power fiber lasers. The review includes the major limitations imposed by fiber nonlinearities and other parasitic effects, such as optical damage, transversemodal instabilities and photodarkening. Finally, the paper summarizes the power evolution in continuous-wave and pulsed ytterbium-doped fiber lasers and their impact on industrial applications
Suppression of stimulated Raman scattering in a high power Yb-doped fiber amplifier using a W-type core with fundamental mode cut-off
No full textWe demonstrate the suppression of stimulated Raman scattering in a high power, single-mode Yb-doped fiber amplifier using a W-type core structure. Raman-scattered light is not guided by the core. The amplifier consists of a master oscillator power amplifier (MOPA) system, seeded with 103 ps pulses at 32 MHz repetition rate in the final amplification stage. An average output power of 53 W, which corresponds to 13 kW of peak power, was achieved in the 23 m long W-type double-clad fiber without any significant loss of power due to transfer from the signal wavelength at 1060 nm to the Raman Stokes wavelength at 1114 nm and amplified spontaneous emission from Yb-ions at longer wavelengths (~1070 nm). The power conversion efficiency at 1060 nm was 80% with respect to the absorbed pump power
A fibre optic force sensing method based on the S2 imaging technique
No full textOptical fibres are the ideal sensing platform to measure strain, pressure, temperature, displacement, vibration, torsion, and humidity [1], thanks to their extreme environmental robustness. Since they are made of dielectric material, optical fibres are immune to electromagnetic interference and corrosion, are highly sensitive, and have a very wide operating temperature range. In addition, optical fibre-based sensors can be lightweight while providing easy integration into a wide variety of structures, while being low cost compared to an alternative approach. In particular, fibre Bragg gratings, polarization-maintaining fibres, and multicore fibres [2] are used as force or strain sensors in a broad range of industrial, medical, and military applications
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
Comparative modelling and design aspects of PANDA PM fiber
No full textIn this study, we investigate the design parameters of a high-birefringence (Hi-Bi) polarization-maintaining (PM) fiber. Our focus is on PANDA-type (Polarization-maintaining AND Absorption-reducing) fiber design that incorporates B2O3-doped silica stress rods. Utilizing the finite-element method (FEM) within the COMSOL Multiphysics software package, we conduct simulations to analyze the fiber's characteristics. We also provide a comprehensive review of common analytical expressions for birefringence and compare our simulation results with theoretical formulas. Our findings reveal a good agreement between simulation and one theoretical expression, while the other two exhibit discrepancies, each of a different nature
High-efficiency multi-spot beam generation with an all-fiber SMF-SCF structure
No full textWe propose an efficient, fully fiber-based method for generating multi-spot beam patterns using single-mode fiber (SMF) and square-core fiber (SCF) structures. This technique leverages multimode interference in the SCF to produce configurable square lattice beam patterns, such as 2×2, 3×3, and 4×4 arrays, by simply adjusting the SCF length. Both simulations and experiments at a wavelength of 1060 nm validate the approach, demonstrating high power uniformity, minimal loss, and excellent stability across beam configurations. Initial testing of multi-spot laser drilling on metal surfaces shows that increased beam spot counts can significantly enhance processing efficiency. This SMF-SCF structure offers substantial potential for scalable, compact, and high-precision applications in photonics, laser manufacturing, and biomedical imaging, where rapid processing and parallel operation are critical
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