MODELING OF LONG-PERIOD FIBER GRATINGS AND THEIR APPLICATION ON LIQUID LEVEL SENSORS
I. Flores-Llamas and V. SvyrydIt is presented a mathematical model for non-uniform long-period fiber gratings which is based on the Coupled-Mode Theory. The model has the capability of considering the case of a surrounding medium with a refractive index higher than that of the optical fiber cladding, which gives advantages in a number of applications. As an application of the model a new liquid level sensor is proposed and the results of its simulation are compared with experimental data.
This paper describes a fiber-optic level indicator designed to detect the liquid interface in the liquid-hydrogen storage tanks. The considerations which led to the choice of the particular fiber-optic sensor and the construction of the measurement system are discussed. A fiber-optic refractometric transducer with the detection element of ellipsoidal form was developed which features a step-like response to the external refractive index in the narrow interval of this quantity between unity (gas) and 1.0974 (liquid hydrogen). The fiber-optic inputs to the storage system secure the reduced heat ingress in the liquid-hydrogen in comparison to sensors with electrical wires. The fiber-optic transducer features small dimensions, is strong mechanically, and supports multiple cycles of cooling to 20 K and heating to the ambient temperature. The device successfully indicated the level of liquid interface in a liquid-hydrogen storage tank under the maximum excess pressure in the tank of 4 MPa. The paper discusses a number of prospective applications of the developed fiber-optic transducer in the liquid-hydrogen storage systems.
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