부산대학교 도서관

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. The multimode section, which is sensitive to strain and temperature, is coated with a graphite film that converts the 9xx nm radiation into heat, to increase the temperature and hence produce a red shift of the spectral response. On the other hand, a blue shift occurs when air or a fluid flows across the sensing surface, removing the optically induced heat. The SMS structure is terminated to provide sufficient reflectivity for the sensor to be monitored in reflection with a commercial FBG interrogator and be used as a probe. The SMS sensor was embedded in a 3D-printed fixture and characterized in comparison with a reference commercial sensor [2], demonstrating its sensing capability. Fig. 1(b) shows the shift of the spectral response of the SMS flowmeter when 1.5 W optical heating is applied. The wavelength shift of 6 nm corresponds to an increment of temperature up to 40 °C, which was also verified with a thermal camera. Fig. 1(c) depicts the measurement of an air flow (generated by an air pump) that simulates a respiratory activity. The SMS sensor readout was compared with that of the reference sensor, whose outcome is expressed in standard litres per minute (slm). The calibration factor was found to be 0.33 nm/slm, and the response at different flows exhibited good linearity. While the reference sensor has a quick response time (the datasheet reports a value $< 3$ ms), the SMS sensor exhibited a transitory with response time of seconds. However, the transitory is expected to be greatly reduced by optimizing the packaging (e.g., by replacing the 3D-printed plastic fixture with a metallic enclosure for a fast dissipation of the heat). These preliminary results demonstrate the feasibility of the design and suggests remarkable advantages in biomedical applications where remote and powerless operation are required.