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Long Period Grating Sensors

Richard Carter, Robert Maier, Jim Barton

PhD research has concentrated on the investigation and characterisation of a hydrogen sensor based on a palladium (Pd) coated long period grating (LPG). LPGs have similar structures to the better known fibre Bragg gratings but has a period several orders of magnitude larger. This excites significantly different optical phenomena as the LPG couples light from the core into the cladding modes. This coupling is wavelength selective and as such individual cladding modes are represented by losses in the transmission spectrum of the LPG.

Pd is known to absorb large quantities of hydrogen (up to 900 times its own volume). This strains the Pd lattice resulting in changes to the optical and electrical properties. Since the spectral position of LPG loss bands is dependent on the fibre cladding-external boundary, a layer of Pd on the outer surface of the fibre acts as a hydrogen sensor where the concentration is encoded as a shift to the loss band position.

Example LPG spectrum (theory red, experiment blue)

This work has concentrated on the characterisation of this system. Considerable efforts have gone into characterising the Pd thin films used for the sensor elements including surface quality (XPS and electron microscopy), refractive index (ellipsomety) and index changes induced by hydrogen (surface Plasmon resonance). In particular the refractive index of these thin films has been found to differ significantly to previous published data highlighting the necessity to thoroughly characterise films used for these types of sensor system.

Comparative data for the refractive index of thin film Pd

This has been combined with a robust model of the LPG system (coupled mode theory) which has been expanded to include a metallic layer. This allows for not only the characterisation of the gratings but also theoretical analysis of the responsivity to hydrogen through the phase matching conditions. This work has led to several designs for optimal, high responsivity, gratings operating around the phase match turning point; the subject of continuing research.

Phase matching conditions for higher order, more sensitive, LPGs

 

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