Abstract
A route to monitor external refractive indices greater than the core index of the waveguide is presented. Initial application utilizes an integrated optical fibre (IOF) platform due to its potential for use in harsh environment sensing. IOF is fabricated using a bespoke flame hydrolysis deposition process to fuse an optical fibre to a planar substrate achieving an optical quality, ruggedized glass layer between the fibre and substrate was fabricated. The presented refractometer is created by direct UV writing of multiple fibre Bragg gratings into an etched (22 μm diameter) optical fibre post fabrication. Linear regression analysis is applied to quantify propagation loss by monitoring each FBG's back reflected power. The device operates with a sensitivity of approximately 350 dB/cm/RIU at a refractive index of 1.451 at 1550 nm. Numerical simulations using a transfer matrix method are presented and potential routes for development are discussed.
Highlights
As optical devices are not subject to interference from external electromagnetic fields, they have been of great interest in recent decades for use in sensing technologies
One of the paths of the beam passes through a computer controlled electro-optic modulator (EOM), enabling a rolling fringe pattern as the laser is translated along the fibre
The fibre Bragg gratings (FBG) linear regression analysis method for quantifying propagation loss in an optical waveguide may be applied in a planar structure, such as those presented by Holmes et al [18] if the upper clad is sufficiently thin
Summary
As optical devices are not subject to interference from external electromagnetic fields, they have been of great interest in recent decades for use in sensing technologies. FBGs may be used as sensors in several configurations: Wet-etched, D-shaped, microstructured and side-polished optical fibre all present methods of chemical or biological sensing via interrogation of an FBG’s response to the surrounding-medium refractive index (SRI). The presented refractometer operates as a leaky waveguide in which propagation loss measurements are used to quantify the device’s response to the SRI Propagation loss in this device is characterized by FBGs which are direct UV written into the fibre in the final stages of fabrication, combining the advantages of leaky mode and FBG devices. This work builds on a previously demonstrated IOF refractometer which was based on thinned FBGs for evanescent field sensing [17] and was characterised by means of a shift in Bragg wavelength. Initial results are presented along with a theoretical analysis of the device and means for improvement
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