Abstract

Optical fibre sensors have successfully entered several application fields and markets because they can overcome limitations of their conventional counterparts with an electrical readout. In particular, optical fibre sensors feature remote interrogation, electromagnetic interference-proof functionality and no direct electrical power supply. Concerning the latter, harsh environments as well as biomedical applications may greatly benefit from the replacement of an electric readout sensor with an optical fibre-based version. In this framework, a novel fibre optic flowmeter has been developed, relying on a single mode-multimode-single mode (SMS) interferometric structure [1]. The sensor is based on the hot-wire principle and the novelty lies in the SMS as a sensing element, which is easy to fabricate and more sensitive than other technologies, such as fibre Bragg gratings (FBGs). The schematic of the sensor is depicted in Fig. 1(a). The SMS structure is made by offset-splicing a 10 cm-long G.657A2 bend-insensitive fibre between two single mode pigtails at 1550 nm, to produce the excitation of high order modes in the bend-insensitive section. The input pigtail is actually a double cladding fibre that couples, by means of a feed-through signal/pump combiner, the radiation from a 9xx nm laser that acts as the heating source. Given the short length, the bend-insensitive section supports the propagation of two modes, which propagate at two different phase velocities and then interfere as in a Mach-Zender interferometer. The resulting spectral response is a periodic pattern in the frequency domain whose free spectral range (here selected to be about 20 nm) is inversely proportional to the length of the two-mode section.

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