Novel fabrication technique for planar glass waveguides

A novel technique has been developed for the deposition of low-loss planar glass waveguides by directly spin coating from the liquid, thus overcoming the problems of reproducing glass stoichiometry when depositing from the vapour

Towards a 1.39 µm planar neodymium doped fluoride glass waveguide amplifier

This thesis describes the steps towards the fabrication of a planar neodymium doped fluoride glass waveguide amplifier operating in the 1.3µm band. Approximations of the amplifier performance, combined with more detailed models of an integrated neodymium doped fluoroaluminate glass amplifier based on spectroscopic measurements from the bulk glass (Nd:ALF70), show that channel waveguides with propagation losses below 0.5dB/cm at the pump wavelength are needed to produce a device with a pump requirement of 200mW or less. In the absence of any fabrication technique available for fluoride glasses with this level of performance, the novel process of hot dip spin coating was developed for the fabrication of single mode Nd:ALF70 planar waveguides. The process is based on the inverted spin coating of molten glass onto a solid glass substrate and currently holds the record for the minimum propagation losses of less than 0.1dB/cm @ 1048nm in a fluoride glass waveguide measured in a 5 micron thick film. In order to capitalise on such exceptional waveguide performance, a new method for fabricating channel waveguides in fluoride glass thin films was developed based on direct UV writing of a negative index change using photothermal expansion. The process is based on generating a negative index change, via exposure to laser radiation at a wavelength of 244nm. The negative index change forms the lower refractive index 'cladding' on either side of the desired guiding region and is induced by a thermal mechanism generated by the large absorption of UV radiation by cerium ions doped into the slab guiding region. A maximum index range of close to 0.01 was produced for a 40µm thick unclad waveguide doped with 0.5mol% Ce3+ and an index change of approximately 2x10-3 was estimated for a 2.5mol% Ce3+ doped, 6 micron thick buried waveguide. Propagation losses of 0.1±0.1dB/cm @ 1048nm were determined by the Findlay-Clay technique for a 90µm wide multimode channel waveguide laser whilst an average value of 0.3±0.1dB/cm @ 1048nm was obtained for less multimode guides with an average width of 20µm. The best device performance has been characterised in a 1mol% Nd3+ doped 90µm wide and 6µm thick waveguide. Laser action at 1048nm has been observed with a slope efficiency of 27% for a 56% output coupler and a threshold pump Power of just 4mW. The threshold for laser action at 1317nm was 32mW for a 0.3% output coupler giving a slope efficiency of 2%. A peak internal gain of 1dB at 1317nm was achieved in this waveguide for a pump power of 100mW and suggests that the performance of optimised single mode guides should be close to that predicted by theory. <br/

Towards a 1.3#mu#m planar neodymium doped fluoride glass waveguide amplifier

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Evaluation of neodymium doped fluoride glass films deposited by pulsed laser deposition

Pulsed laser deposition (PLD) has been evaluated as a technique for the realisation of neodymium doped fluoride glass waveguides. In contrast to other high energy techniques such as sputtering and molecular beam epitaxy, pulsed laser deposition appears to reliably reproduce the bulk stoichiometry of the doped glass in thin film form. However, characteristically for this deposition technique, the film topography is dominated by micron size particulates generated during fabrication. Film uniformity appears to be improved by depositing the film with a laser beam fluence close to the ablation threshold of the material. Unfortunately, this is at the cost of a significantly reduced film thickness which ultimately limits the usefulness of PLD for depositing waveguides from this material

A 1317nm neodymium doped fluoride glass waveguide laser

Laser emission at 1317nm has been observed for the first time in a neodymium doped fluoroaluminate glass waveguide fabricated by a combination of hot dip spin coating and direct UV writing

Effect of core-cladding interface on thermal poling germano-silicate optical waveguides

We report on the effect of the interface between silica and germano-silicate glass during thermal poling of optical waveguides. By measuring the depth of the nonlinear region, using the Maker's fringes technique combined with etching in hydrofluoric acid, we conclude that the germania-doped layer acts as a barrier for the migration of cations at standard poling temperature and voltage. It is also found that the problem can be circumvented by poling at higher temperature and voltage, e.g. 320°C and 8 kV

Fabrication of high numerical aperture fluoroaluminate fibres

A family of fluoroaluminate glasses has been identified as the ideal host for doping with Nd3+ for use in the development of lasers and amplifiers in the second telecom window. For optimum performance, a fibre with high numerical aperture (NA), fibre length of 5cm and loss of 10dB/m is required. The high NA implies a core diameter of 3µm is needed and thus the core-to-clad interface represents greater than 50% of the core area. Crystallisation in the interface introduces large scattering losses. The critical step in the fabrication process is preform fabrication. We will present results comparing the different techniques employed and thermal and mechanical fabrication process modelling consistent with our experimental observations and detail the advantages and limiting parameters for the different processes. Rod-in-tube is the simplest technique of preform fabrication and works exceptionally well for oxide glasses. Fluorides are highly prone to surface crystallisation. We have looked at the fibre losses as functions of polishing material, polishing time, and etching. It is a good first step to test the optical properties and performance in a fibre. Rotational casting is usually employed in fluoride glasses. The high Tg(450°C) and large expansion coefficient ( &gt;150x10-7/°K. We will compare the results of rotational casting with built-in-casting and the influence of processing time, thermal history and mould material (thermal conductivity). Extrusion as a procedure for preform fabrication is specially suitable for fluoraluminates. Extrusion allows one to operate at temperatures nearer Tg than Tx. Preforms can be prepared with small cores and thus avoid the thermal cycling involved in sleeving to get the singlemode fibre. To date our losses for fibres with 5-8µm cores vary from 100dB/m using rod-in-tube to 20dB/m using extrusion

A study on the effect of the core-cladding interface on thermal poling in GeO₂-SiO₂ optical waveguides

GeO2-SiO2 multilayer glass samples are thermally poled. The interface between the layers strongly affects the thickness of the second order nonlinearity as well as its overlap with the core of the waveguide

Remarks on the Johan Dahl Land hydropower project in the light of recent model calculations.

The advantages of gallium lanthanum sulphide (GLS) based glass over other competing glasses for active and infrared applications are evident through its low-phonon energy, high rare-earth solubility, high transition temperature and non-toxicity. However this glass often devitrifies during fibre drawing due to a small separation between the crystallisation and fibre drawing temperatures. Improving GLS fabrication technology may hold the key to achieving practical optical waveguide devices. In this paper, we describe the current GLS research status, methods of improving glass purity and our directions toward alternatives to traditional fibre technology, in particular planar channel waveguides and holey or microstructured fibres

Optical amplifiers and lasers in infrared fibres

We are currently investigating two infrared glasses for active applications. Gallium lanthanum sulphide (GLS) glass is investigated as a potential host material for rare-earth doped mid-infrared fibre lasers. We have fabricated gallium lanthanum sulphide glass by melt quenching and drawn into fibres using the rod-in-tube technique. Fluoroaluminate glasses (ALF) are being prepared in planar form by spin coating and clad waveguides have been achieved. The quality of waveguides from both these materials is gradually being improved as methods to eliminate transitions metals and other impurities, understand crystallization and reduce imperfections at the core/clad interface are developed. Although initially motivated by the demand for a practical 1310nm amplifier, interest has now extended further into the infrared. We describe recent progress in these glasses, their properties and applications