An alternative solution for the simultaneous measurement of temperature and refractive index is presented. A local micro-structured fiber Bragg grating （LMSFBG） is formed as the sensing head, in which a standard grating is etched by HF. According to the phase shift theory, the main spectral change of the LMSFBG is the formation of a narrow allowed band, which is strongly dependent on the etching features and the surrounding refractive index. As such, the temperature and refractive index measurements can be achieved by the shifts of the double peaks and narrow allowed band, and their fitting linearity coefficients are 0.996 and 0.994, respectively. Thus, the reflection and transmission peaks of the LMSFBG have a good linear relationshi~ with tem^era,ture and refractive index.
A kind of fiber-optic sensor for simultaneous measurement of refractive index of surrounding medium, temperature and strain is described. Based on core diameter mismatch, a multimode-single mode-multimode (MSM) structure is presented. It is demonstrated that the three parameters can be measured respectively by the interference of the core mode and cladding modes excited in the single mode fiber (SMF). Then combined with a polarization-maintaining fiber Bragg grating (PMFBG) which has different sensing properties from MSM structure, three parameters are measured simultaneously. The experimental results show that PMFBG is insensitive to the refractive index and the refractive index sensitivity of the MSM structure is 96.04 nm·RIU^(-1); the temperature sensitivities of the characteristic wavelength for MSM structure and the center wavelengths of fast and slow axes for PMFBG are 0.0911 nm °C^(-1), 0.00976 nm °C^(-1) and 0.0105 nm °C^(-1), respectively; the strain sensitivities of those are -0.013 nm με^(-1), 0.012 nm με^(-1) and 0.012 nm με^(-1), respectively.
