Cryogenic liquid level sensor based on long period grating

We propose a novel cryogenic liquid level sensor based on a long-period grating inscribed in one of the external cores of a multicore optical fibre. The proposed device has a small grating period to improve the sensitivity to external refractive index changes while reducing the temperature sensitivity. We compare the results obtained with an LPG inscribed in a single-core fibre, demonstrating that the optical spectrum of our device is not divided into two different resonances and shifts smoothly towards shorter wavelengths as the relative liquid level increases. The sensitivity of the proposed level sensor when measuring liquid nitrogen is -0.772 nm/cm.