Temperature-insensitive long-period gratings for strain and refractive index sensing

Long-period gratings have recently been demonstrated as highly versatile strain, temperature and refractive index sensors that can be implemented with simple demodulation schemes. The cross-sensitivity of long-period gratings to ambient temperature variations can be a limiting factor in applications that require their use as strain or refractive index sensors. The thermal-induced spectral shift in the grating transmission spectrum can be attributed to the material and waveguide changes in the grating characteristics. By designing special optical fiber refractive index profiles, these two effects can be counter-balanced to produce gratings that have an order of magnitude smaller temperature wavelength shift than conventional long-period gratings. We present experimental results for strain and refractive index sensors using these temperature-insensitive long-period gratings. Such transducers are shown to possess strain- and refractive index-induced wavelength shifts that are comparable to those of standard gratings. The cross-sensitivity to temperature is determined at different magnitudes of strain and ambient index to determine the effectiveness of these gratings in practical applications.