Large sensitivity-range product of liquid level enabled by constrained multimode interference

A long-length hollow-core fiber-based few-mode interferometer is developed, aiming to overcome the limitation between the sensitivity and dynamic range of liquid level measurement. The characteristics of cladding-mode distribution are investigated and the constrained multimode interference is achieved and validated experimentally, with the visibility up to 13.8 dB. The results demonstrate the simultaneous achievement of a high wavelength response of 10.1 nm/cm over a large linear range of 9 cm. The sensitivity-range product reaches approximately 91 nm and corresponds to an improvement of more than 300%. Additionally, the measured error from the ambient refractive index is assessed and the accuracy is guaranteed within less than 0.1 mm by an empirical third-order polynomial-based solution.