Acoustics and Ultrasonic Sensors
The acoustic or ultrasonic parameters can be measured based on aforementioned dynamic FFPI pressure sensors with fast response time. Usually, the packaging of the FFPI acoustic or ultrasonic sensors was also similar to that of the pressure sensors, except the material and the geometric dimensions of the diaphragm . An FFPI acoustic sensor based on a multi-layer graphene diaphragm was developed by Ma et al. , as shown in Figure 4.8. Experimental results indicated a pressure-induced deflection of 1100 nm/kPa and a noise equivalent acoustic signal of ~60 |1 Pa/Hz1/2 at the frequency of 10 kHz. The sensor had a flat frequency response in the range of 0.2-22 kHz. By using a similar structure but with a gold-coated polyethylene tere- phthalate (PET) diaphragm, an FFPI acoustic sensor was developed, with a sensitivity of 40 mV/Pa at 1 kHz . The frequency response range was 100 Hz to 12.5 kHz and the signal-to-noise ratio was 60 dB. Another polymer material poly(phthalazinone ether sulfone ketone) (PPESK) diaphragm was also used for fabricating the FFPI acoustic sensor, and the sensing performance was investigated .
Figure 4.8 (a) Schematic structure and (b) a photo of the FFPI acoustic sensor based on a multi-layer graphene diaphragm. (c) The endface of the ferrule and (d) the multi-layer graphene film.
Figure 4.9 Fabrication process of FFPI acoustic sensor. (a) Fusion splicing and cleaving, (b)-(d) the silver film attached to the silica tube by UV glue, (e) the silver film deposited on the silica substrate, and (f) an optical microscopic picture of the fabricated sensor.
The fabrication process and the structure of an FFPI ultrasonic sensor are shown in Figure 4.9. The 300-nm-thick diaphragm was fabricated by thermal deposition method and then transferred onto the fiber tip with a hollow silica tube . By detecting the wavelength shift, a static pressure sensitivity of 1.6 nm/kPa and a resonant frequency of 1.44 MHz were obtained. Another earlier experiment demonstrated an acoustic noise floor of 2.3 kPa and a frequency response of 25 MHz .