Towards a 1.39 µm planar neodymium doped fluoride glass waveguide amplifier

Abstract

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.

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