We have investigated the effect of the frequency shift associated with an acousto-optic (AO) interaction on the spectral quality of acousto-optically tuned external cavity semiconductor lasers. A ring-cavity was built and the wavelength controlled by a frequency compensating acousto-optic tunable filter pair, which allowed user control over the frequency shift of the interacting light. Laser spectra were recorded for a range of frequency shifts, and a 30 kHz driving frequency gave the optimal spectral response where the output spectral shape did not vary as a function of operating wavelength and resulted in <0.1 nm linewidths.
KEYWORDS: Metals, Coating, Fiber lasers, Fiber coatings, Laser scattering, Scattering, High power lasers, Cladding, Optical fibers, High power fiber lasers
We investigate the effects of coating thickness on the scattering losses of metal coated active fibre. A range of low numerical aperture metal coated optical fibres are placed in etchant solutions whilst measuring propagation loss as a function of time. By utilising concurrent coating diameter measurements, we are able to correlate propagation losses with coating thickness. Experimentally we find a monotonic dependence on coating thickness and scattering loss. We present the results of this work, providing useful parameters for high power metal coated fibre laser designs.
CO2 laser processing facilitates contamination free, rapid, precise and reproducible fabrication of devices for high power fibre laser applications. We present recent progress in fibre end-face preparation and cladding surface modification techniques. We demonstrate a fine feature CO2 laser process that yields topography significantly smaller than that achieved with typical mechanical cleaving processes. We also investigate the side processing of optical fibres for the fabrication of all-glass cladding light strippers and demonstrate extremely efficient cladding mode removal. We apply both techniques to fibres with complex designs containing multiple layers of doped and un-doped silica as well as shaped and circularly symmetric structures. Finally, we discuss the challenges and approaches to working with various fibre and glass-types.
For the first time, an electronically-controlled, wavelength-agile tuneable holmium-doped fibre laser is presented. A narrow-band acousto-optic tuneable filter was characterized and used as the wavelength selective element to avoid any inertial effects associated with opto-mechanical tuning mechanisms. We demonstrate operation over a 90 nm wavelength range spanning 2040 – 2130 nm. The laser produced >150 mW over this entire range with a signal-to-noise ratio of >45 dB and line-width of ~0.16 nm. Switching times of ~35 μs and sweep rates of up to 9 nm/ms were also demonstrated.
We present a novel metal coated triple clad active fibre design, utilising an all glass inner cladding structure and aluminium outer coating. This metal coated active fibre enables a number of benefits to high power laser design, such as increase robustness and extended operating temperature range. As a demonstration of the advantages of this design a passively cooled ytterbium fibre laser is presented. A 20 m length of active fibre was coiled into a planar arrangement and mounted onto a high emissivity heatsink. Up to 405 W of output power was achieved without the need for active water or forced air cooling. The slope efficiency of this source was 74 % and maximum outer heat sink temperature was ~140°C. This arrangement allowed for significant weight and size savings to be achieved with the active fibre laser head weighing less than 100 g. We will discuss the design choices and trade-offs of metal coated active fibre on high power fibre laser systems as well as the prospects for further power scaling to the kW level.
Optical Coherence Tomography has been successfully applied to the non-invasive imaging of subsurface microstructure
of a variety of materials from biological tissues to painted objects of art. One of the limitations of the technique is the
low depth of penetration due to the strong scattering and absorption in the material. Previous studies found that for paint
materials, the optimum window for large depth of penetration is around 2.2 microns. This is also true for many other
materials with low water content. We have previously demonstrated OCT systems in this wavelength regime for imaging
with improved depth of penetration. In this paper, we present an improved 2 micron high resolution Fourier domain OCT
system using a broadband supercontinuum source. The system achieved a depth resolution of 9 microns in air (or 6
microns in paint or any polymer).
Very high temperature operation of a rare earth doped fibre laser is investigated. An all-glass ytterbium-doped fibre laser is demonstrated, operating at heat sink temperatures ranging from 25°C to 400°C and output powers of > 40 W. Over this temperature range only a small reduction in laser output power was observed. The emission spectrum of this source was also measured for heat sink temperatures up to 400°C. We discuss the advantages of operating at such extended temperatures such as weight savings from reduced heat sink size, extended long wavelength operation and suppression of detrimental non-linear effects.
Optical Coherence Tomography has been successfully applied to the imaging of painted objects in recent years. However, a significant limitation is the low penetration depth of OCT in paint because of the high opacity of paint due to either scattering or absorption. It has been shown that the optimum spectral window for OCT imaging of paint layers is around 2.2μm in wavelength. In this paper, we demonstrate a 1950nm OCT for imaging painted objects using a superfluorescent fiber source at low power.
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