Rare earth ion doped antimony-based chalcogenide fibres for biologically relevant wavelengths in the mid-IR.
Publication Date
December 13, 2022
Creator
Abstract
The wide mid-IR transmission window and rare earth dopant solubility of chalcogenide glasses have identified them as a potentially novel medium to facilitate mid-IR (Mid-IR) fibre lasing. These glasses, with their comparatively low phonon energy matrices, are capable of transmitting and accommodating radiative transitions at wavelengths not accessible by other doped fibre systems such as ZBLAN (ZrF4-BaF2-LaF3-AlF3-NaF) or indium fluoride (InF3-ZnF2-SrF2-BaF2). Reported instances of mid-IR fibre lasing are few, with extrinsic oxygen and hydrogen impurities increasing fibre losses and potentially conferring a high phonon energy local environment to the active rare earth ions, reducing the probability of mid-IR radiative transitions. Therefore, the work here aims to develop high purity, devitrification resistant, optically active antimony-based chalcogenide fibres.
Initial work focused on the development of a Ge-Sb-Se-Ga glass composition that was suitable for rare earth ion doping. This was to be a lower phonon energy alternative to the more commonly reported Ge-As-Se-Ga doped glasses. A 1000 ppmw Sm3+ Ge20Sb10Se65Ga5 at. % preform documented in the literature was replicated, but extensive crystallisation resulted from the processing methods used. A second composition, Ge19.4Sb9.7Se67.9Ga3 at. %, was designed and demonstrated resistance to devitrification during fibre drawing up to doping concentrations of 1000 ppmw. A 1000 ppmw Sm3+ doped Ge19.4Sb9.7Se67.9Ga3 at. % fibre exhibited lowest losses of 2.15 dB/m at 6.04 µm wavelength with photoluminescence measured between 3.25-4.25 µm, and 6-9 µm wavelengths when pumped at 1.3 µm, and 1.47 µm, respectively. A 500 ppmw Dy3+ doped Ge19.4Sb9.7Se67.9Ga3 at. % fibre exhibited lowest losses of 3.2 dB/m at 6.63 µm.
With the aim to achieve fibre lasing, a small core, step-index fibre is required. Therefore, the next step was to design an accompanying cladding glass for the Ge19.4Sb9.7Se67.9Ga3 at. % host glass composition. A straight substitution of 2.9 at. % selenium for sulfur (Ge19.4Sb9.7Se65Ga3S2.9 at. %) was trialled, but excessive devitrification prevented this composition from being drawn. A series of glasses with compositions Ge19.4Sb9.7Ga1-2Se67.9-(Ga+S)S1-3 at. %, and a standalone Ge21Sb10Se69 at. % composition were produced with the target of Tg (glass transition temperature) being ± 10 °C of the Tg of the core glass, Ge19.4Sb9.7Se67.9Ga3 at. % (Tg 232 °C). Mid-IR variable angle spectroscopic ellipsometry (VASE) was used to measure refractive index values of these glasses finding values between 2.5218-2.5615 at 2.8 µm and 2.5264-2.5565 at 4.3 µm, wavelengths chosen based on the Dy3+ radiative emissions. An ellipsometry model comprised of one pole (Sellmeier) oscillator centred in the electronic absorption edge, a second pole oscillator centred in the vibrational fundamental absorption bands, and a Gaussian oscillator around the fundamental absorption of Se-Se homobonds (~33 µm) was found to consistently provide a good fit to the measured ellipsometry data. An assumed (lower refractive index) oxide surface layer was also incorporated into the model using a surface roughness feature. This added a layer between 5.52 – 14.45 nm thick that assumed air(void)-glass was in a 50:50 ratio. Ge21Sb10Se69 at. % was proved to match the Tg criteria (Tg 227 °C) and had a refractive index to provide a suitable numerical aperture (NA) at Dy3+ photoluminescent wavelengths (0.3438 at 2.8 µm, 0.3638 at 4.3 µm).
A 500 ppmw Dy3+ doped Ge19.4Sb9.7Se67.9Ga3 at. % core glass was successfully fibre drawn to a small core, step-index fibre with a Ge21Sb10Se69 at. % cladding. This novel antimony-based cladding glass was also successfully fibre drawn to a small core fibre with a 500 ppmw Ce3+ doped Ge15As21Se63Ga1 at. % core glass. This Ce3+ small core fibre was optically pumped at 4.15 µm (NA: 0.4704) to excite the Ce3+: 2F7/2 → 2F5/2 radiative transition. Three laser peaks at 5.12 µm (NA: 0.4757), 5.17 µm (NA: 0.4759), and 5.28 (NA: 0.4764) were observed. This is the first recording of mid-IR fibre CW (continuous wave) lasing in a chalcogenide glass fibre host.
An open vacuum distillation method for purification of anhydrous, powdered DyCl3 in silica glass tubing housing was trialled. After 0.5 hours heat treatment above 850 °C, DyCl3 had distilled into a large translucent crystalline form and a fine colourless powder form. A Ge15As21Se63Ga1 at. % host glass was distilled with 1000 ppmw TeCl4 (hydrogen getter) and 700 ppmw aluminium (oxygen getter). This was then sectioned into two batches with one doped with 250 ppmw of the distilled translucent DyCl3 crystals and the second batched with 500 ppmw dysprosium foil that had undergone a mechanical surface exfoliation. The resulting fibres measured their lowest loss of 4.86 dB/m at 6.085 µm (DyCl3) and 2.07 dB/m at 6.13 µm (Dy foil), respectively.
A moth eye patterning at the chalcogenide glass surface designed to be antireflective in the infrared, aimed to increase incident pump power into rare earth ion doped fibres. A silicon mould was fabricated via electron beam lithography (EBL) and dry plasma etching the features into the silicon wafer (negative mould). Initial attempts to transfer the pattern into bulk chalcogenide glass samples proved unsuccessful and so focus was switched to fibre pressings. A manually controlled hot-pressing rig, with real time force sensing feedback, was designed and produced moderate pattern transfer with poor replicability due to its strong dependence on the operator. To circumvent this, an automatic feedback-loop based rig was designed which controlled the process, based on pre-selected parameters to reduce inter-pressing variability.
Not only did this Project lead to the realisation of the first CW chalcogenide mid-IR fibre laser operating beyond 5µm, this was the first glass fibre laser worldwide to operate beyond 4 µm wavelength. In addition, substantial progress to the continuing development of these fibres was made with the development of rare earth ion doped, antimony-based step-index fibres, potentially allowing future lasing at biologically relevant wavelengths for new precision cutting or welding in laser medical surgery.
Item Type
ethesis
Thesis Type
PhD
Supervisors
Associated Schools / Departments
Department of Mechanical, Materials and Manufacturing Engineering (UK)
eprints ID
71842
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