8F-2 High-Frequency Low-Noise Ultrasonic Detection Arrays Based on Parallelly Probing an Etalon

For ultrasound detection systems using a thin polymer Fabry-Perot etalon, optically probing the etalon surface defines the acoustic array geometry and element size. We have previously demonstrated wide bandwidth signal detection (>40 MHz) and low noise-equivalent pressure (NEP) (2.1 kPa over a 50 MHz band) for an element on the order of 15 mum in diameter. By integrating an etalon into a photoacoustic imaging system, high-resolution 3D images were obtained. Nonetheless, the previous system is limited for clinical applications because the etalon surface is optically probed point by point and therefore a photodetector array cannot be used to increase frame rate. To move etalon detector technology toward real clinical applications, we have built and tested an ultrasound detection system with an optical end capable of parallel probing. A tunable laser (HP 8168F, Agilent Technologies) was used to drive an erbium-doped fiber amplifier (KPS-BT2-C-30-SLM-PM-PB-FA, Keopsys) to output light at 27 dBm at wavelengths between 1535 and 1565 nm. A lens collimated the light to probe a 6-mum thick SU-8 etalon in parallel. The -3 dB light spot on the etalon surface was 1.8 mm in diameter. The reflected light was guided onto a photodetector (818-BB-30A, Newport) with an active diameter of 100 mum using a two-lens system mapping the active area to an element 28 mum in diameter on the etalon surface. A 2D translation stage was used to carry the photodetector so that it could detect signals from different array elements. The photodetector output was amplified and then digitized using an oscilloscope (WaveSurfer 432, LeCroy). The NEP was estimated to be 7.0 kPa over a 50 MHz band and the detection bandwidth was estimated to exceed 70 MHz. Using a single probe wavelength of 1550.6 nm, a ID array with 81 elements and a 1.7 mm aperture length was formed to image a 25 mum silver wire photoacoustically with laser pulse fluence of 27 mJ/cm² at 532 nm (Surelite 1-20, Continuum). T- he final image shows an object size of 40 mum (axial) by 80 mum (lateral). According to the results, realizing high-frequency low-noise 2D ultrasonic detection arrays using an etalon is feasible.