Fiber-optic acoustic pressure sensor based on large-area nanolayer silver diaghragm

Abstract

A fiber-optic acoustic pressure sensor based on a large-area nanolayer silver diaphragm is demonstrated with a high dynamic pressure sensitivity of 160 nm∕Pa at 4 kHz frequency. The sensor exhibits a noise limited detectable pressure level of 14.5 μPa∕Hz 1∕2 . Its high dynamic pressure sensitivity and simple fabrication process make it an attractive tool for acoustic sensing and photo-acoustic spectroscopy. . Although the pressure sensitivity of the sensor based on the graphene diaphragm can potentially be very high thanks to small thickness, it is usually difficult to prepare and process single-or few-layered graphene diaphragms with large diameters and transfer them onto the sensor head. In addition, optical reflectivity of the graphene layer is relatively small compared to metal films. In this Letter, we demonstrate an acoustic sensor based on a large area silver diaphragm 150 nm in thickness and 2.4 mm in diameter. While the thickness is comparable to the graphite-based diaphragms previously reported [6], the diameter is an order of magnitude larger. The acoustic pressure test shows that such a sensor based on a large area silver diaphragm has a sensitivity of up to 160 nm∕Pa at 4 kHz frequency. To our best knowledge, this is the highest acoustic pressure sensitivity ever reported for fiber-optic acoustic sensors based on diaphragms. The sensor is able to detect acoustic pressure as weak as 14.5 μPa∕Hz 1∕2 . The acoustic pressure sensing head based on the silver diaphragm comprises a standard polarization maintaining fiber (PM fiber), a ceramic split sleeve, and the silver diaphragm, as shown in Fig. 1(a). Figure 1(b) is the image of the proposed sensing head. A picture of the ceramic split sleeve, on which the diaphragm is attached, is also shown in the bottom inset of Fig. 1(b), and the inner diameter of this split sleeve is 2.4 mm. The PM fiber end facet was polished with an angle of 8°to reduce the Fresnel reflection from the fiber end surface. The fabrication process of the sensing head is similar to that reported in our previous paper [5]. The distance between the fiber end and the diaphragm is approximately 150 μm adjusted by a high precision linear stage (Newport ILS-250CC) with a displacement resolution of 0.5 μm.The silver thin film used in this sensor is approximately 150 nm thick with >2.4 mm diameter that covers one end of the split sleeve. The top inset of Fig. 1(b) is an SEM image of the silver diaphragm. This sensing diaphragm has several advantages, including good stability and high reflectivity over the near infrared wavelength range. In general, the pressure sensitivity depends both on the material and the thickness of the diaphragm. In fact, the pressure sensitivity of an acoustic sensor based on an edge-clamped circular diaphragm is proportional to R 4 ∕t 3 , where R is the radius and t is the thickness of the diaphragm [7]. Therefore a diaphragm with large

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