Planar waveguides can be written with a UV-laser into photosensitized silica to produce a wide range of optical devices. Careful modulation of two interfering beams allows Bragg gratings to be directly written into the channel (Figure 1.a). These Bragg gratings are inherently sensitive to temperature and strain. However etching away the surface exposes the mode within the grating to its surroundings. The corresponding observed shift in Bragg wavelength can be used to detect changes in this environment . Figure 1.b shows a large wavelength shift due to two analytes of markedly different refractive index. This demonstrates the apodised spectral response that in practice allows resolution of sub-picometer shifts in wavelength.
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