A temperature-insensitive Bragg grating sensor—Using orthogonal polarisation modes for in situ temperature compensation

Abstract

An exposed Bragg grating incorporated into a planar waveguide forms an optical device that acts as a refractive index sensor. The exposed evanescent field causes the Bragg peak to be sensitive to the refractive index of its surroundings and can be used to detect changes in this environment. The method reported is able to provide accurate temperature compensation by applying a scaling factor derived from measurement of the birefringence of the transverse electric (TE) and transverse magnetic (TM) modes of the Bragg reflection. For a fluid sensor, fluctuations in ambient temperature can be both accounted for and compensated to produce a temperature-insensitive sensor.

Using the photoinduced reversible refractive-index change of an azobenzene co-polymer to reconfigure an optical Bragg grating

“…The relative insensitivity of the TM mode could be used for in situ temperature referencing through differential studies of the two modes allowing for the fabrication of an athermal optical device. 14 It should be noted that the birefringence of the waveguide was consistent with the modelled system previously reported 15 and further that no evidence for optical anisotropy in the co-polymer film was observed.…”

Monolayer detection of ion binding at a crown ether-functionalised supramolecular surface via an integrated optical Bragg grating

“…The Bragg gratings used here are inherently sensitive to temperature and strain, with a change of 1 C corresponding to a shi of only $10 pm. 17 With removal of the cladding layer the evanescent eld of the optical mode within the grating is exposed, allowing changes in the refractive index of the environment to alter the effective refractive index, n eff of the Bragg grating; with the corresponding shi in the peak Bragg wavelength used to spectrally detect these changes. Sputtering a thin lm of the high-index material, tantalum pentoxide, onto the sensor surface combined with precise monitoring of the Bragg wavelength of the grating allows for changes in the refractive index of the analyte to be monitored to 10 À6 RIU.…”

An investigation into dispersion upon switching between solvents within a microfluidic system using a chemically resistant integrated optical refractive index sensor

“…These Bragg gratings are inherently sensitive to temperature and strain, with a change of 1 uC corresponding to a shift of y10 pm. 24 If the cladding is etched away, the evanescent field of the optical mode within the grating is exposed, allowing changes in the refractive index of the environment to alter the local index of the Bragg grating. The corresponding shift in the peak Bragg wavelength, Dl B can be used to detect these changes in refractive index.…”

“…This can be achieved either by incorporation of an additional local Bragg grating written near to the etched region or through differential studies of the two orthogonal polarisations of the waveguide. 24 For bulk changes in analyte, such as switching between fluids in a microfluidic system, it was found that the former approach was sufficient. In this way the Bragg response to changes in analyte refractive index within the etched region could be separated from thermal effects by a simple first order approximation.…”