An integrated optoacoustic transducer combining etalon and black PDMS structures

Abstract

An integrated optoacoustic transducer combining etalon and black polydimethylsiloxane (PDMS) structures has been designed and developed. The device consists of an 11-μm-thick black PDMS film confined to a 2-mm-diameter circular region acting as an optoacoustic transmitter, surrounded by a 5.9-μm Fabry-Perot polymer etalon structure serving as an optoacoustic detector array. A pulsed laser is focused onto a 30-μm spot on the black PDMS film, defining the transmit element, while a CW laser probes a 20-μm spot on the etalon for ultrasound detection. Pulse-echo signals display center frequencies of above 30 MHz with bandwidths of at least 40 MHz. A theta-array is formed for 3-D ultrasound imaging by mechanically scanning the generation laser along a 1-D array and the detection laser around an annular array. Preliminary images with 3 metal wires as imaging targets are presented. Characterization of the device’s acoustical properties, as well as preliminary imaging results, suggest that all-optical ultrasound transducers are potential alternatives to piezoelectric techniques for high-frequency 2-D arrays enabling 3-D high-resolution ultrasound imaging.

Fig. 1

Side view of the optoacoustic device’s structure. A 5.9-μm SU-8 etalon with a 2-mm-diameter window is fabricated on a glass substrate, followed by an 11-μm black PDMS film on top.

Fig. 2

(a) Optical resonance of the 5.9-μm-thick SU-8 etalon with 11-μm-thick black PDMS on top; (b) the etalon signal and the transducer pulse-echo signal; (c) spectrum of the etalon signal, the square root of the spectrum of pulse-echo signal of the piezoelectric transducer, and the frequency response of the etalon.

Fig. 3

(a) Experimental setup of the pulse-echo experiment; a 5-ns pulsed laser is focused onto the black PDMS for ultrasound generation, and a CW laser pulse is focused onto the etalon for ultrasound detection; (b) top view of generation and detection element geometry. Here d is the center-to-center distance between the generation and detection elements.

Fig. 4

Pulse-echo signal averaged 1000 times when (a) d = 1 mm; (c) d = 0.6 mm; (e) d = 0.2 mm; Spectrum of the pulse-echo signal averaged 1000 times when (b) d = 1 mm; (d) d = 0.6 mm; (f) d = 0.2 mm.

Fig. 5

(a) Illustration of the optoacoustic theta-array configuration: the transmission array consists of 8 elements with 200 μm separation between adjacent elements, and the detection array contains 400 elements along a circle with 2.5 mm diameter; (b) wavefield of recorded pulse-echo signals from the annular detection array when the generation element is aligned to the center.

Fig. 6

Geometry and experimental configuration used to image 3 metal wires.

Fig. 7

Wavefield plot of the detected acoustic field along the annular detection array for one of the generation spots.

Fig. 8

Reconstructed image from the x–y plane at (a) z = 1790 μm; (b) z = 2020 μm, image dynamic range is 10 dB; expected image from the x–y plane at (c) z = 1790 μm; (d) z = 2020 μm.