ABSTRACT
Piezoelectric technology is the backbone of most medical ultrasound imaging arrays; however, signal transduction efficiency severely deteriorates in scaling the technology to element size smaller than 0.1 mm, often required for high-frequency operation (>20 MHz). Optical sensing and generation of ultrasound has been proposed and studied as an alternative technology for implementing sub-millimeter size arrays with element size down to 10 µm. The application of thin polymer film Fabry-Perot resonators has been demonstrated for high-frequency ultrasound detection; however, their sensitivity is limited by light diffraction loss. Here, we introduce a new method to increase the sensitivity of an optical ultrasound receiver by utilizing a waveguide between the mirrors of the Fabry-Perot resonator. This approach eliminates diffraction loss from the cavity, and therefore the finesse is only limited by mirror loss and absorption. By applying this method, we have achieved noise equivalent pressure of 178 Pa over a bandwidth of 30 MHz or 0.03 Pa/Hz1/2, which is about 20-fold better than a similar device without a waveguide. The finesse of the tested Fabry-Perot resonator was around 200. This result is 5 times higher than the finesse measured in the same device outside the waveguide region.
Subject(s)
High-Energy Shock Waves , Polymers/chemistry , Transducers , Ultrasonography/instrumentation , Equipment DesignABSTRACT
Optical resonators are used in a variety of applications ranging from sensors to lasers and signal routing in high volume communication networks. Achieving a high quality (Q) factor is necessary for higher sensitivity in sensing applications and for narrow linewidth light emission in most lasing applications. In this work, we propose a new approach to achieve a very high Q-factor in polymeric Fabry-Perot resonators by conquering light diffraction inside the optical cavity. This can be achieved by inducing a refractive index feature inside the optical cavity that simply creates a waveguide between the two mirrors. This approach eliminates diffraction loss from the cavity and therefore the Q-factor is only limited by mirror loss and absorption. To demonstrate this claim, a device has been fabricated consisting of two dielectric Bragg reflectors with a 100 µm layer of photosensitive polymer between them. The refractive index of this polymer can be modified utilizing standard photo-lithography processes. The measured finesse of the fabricated device was 692 and the Q-factor was 55000.
