Temperature insensitive high-precision refractive-index sensor using two concatenated dual-resonance long-period gratings

In this Letter we report on fabricating and analyzing a temperature insensitive refractometer based on two concatenated dual-resonance long-period gratings (LPGs) with an appropriate inter-grating space (IGS) in between. The IGS provides a temperature-dependent extra phase difference between the core and cladding modes, making the refractometer similar to a Mach–Zehnder interferometer with its arms phase shifted. We demonstrate that an appropriate IGS can produce temperature-insensitive resonance wavelengths. The interferometer is highly stable over a wide range of temperature (20°C–100°C). The measured refractive index sensitivity for aqueous solutions (1.333–1.343) is $\sim 2583\mathrm{nm}/\mathrm{RIU}$, which is the highest reported so far for biological samples. The interferometer can be used for various other temperature-immune sensing applications also.