High frequency optoacoustic arrays using etalon detection

Abstract

Two-dimensional phased arrays for high frequency (>30 MHz) ultrasonic imaging are difficult to construct using conventional piezoelectric technology. A promising alternative involves optical detection of ultrasound, where the array element size is defined by the focal spot of a laser beam. Element size and spacing on the order of a few microns are easily achieved, suitable for imaging at frequencies exceeding 100 MHz. We have previously shown images made from a receive-only, two-dimensional optoacoustic array operating at 10 to 50 MHz. The main drawback of optical detection has been poor sensitivity when compared with piezoelectric detection. In this paper, we explore a different form of optical detection demonstrating improved sensitivity and offering a potentially simple method for constructing two-dimensional arrays. Results from a simple experiment using an etalon sensor confirm that the sensitivity of etalon detection is comparable with piezoelectric detection. This paper concludes with a proposal for a high frequency optoacoustic array system using an etalon.

Optically excited nanoscale ultrasonic transducers

“…This means that the slope of the response for each wavelength present in the pulse is essentially the same, and so the sensitivity is unaffected. However, if the finesse was higher, e.g., in the 100-300 range as is more typical for lower frequency ultrasound detection, 19 then this would no longer hold as the different wavelengths would see gradients of differing size and more importantly of opposite signs, thus the sensitivity would drop off rapidly.…”

“…17,18 Using optical resonances to improve the detection of ultrasound has been known for some time, for example, the trade-off between sensitivity, ultrasonic detection bandwidth, and the structural properties of an etalon was discussed by Ref. 19, Fabry-P erot detectors 20 have been used in photo acoustic measurements with an etalon substrate for the detection of low frequency acoustic waves 21,22 and at high frequencies with an air gap cavity formed between a reflector and the sample being studied. 23 The transducers presented here are small structures that form zero-order Fabry-P erot cavities.…”

Opto-ultrasonic communications in wireless body area nanonetworks

“…Finally, the receiver acoustic sensitivity, i.e., the minimum pressure detectable by the receiver, can be expressed as [22] We observe that the sensitivity of the receiver increases with higher finesse or with longer cavity. Moreover, the finesse depends only on the mirror reflectivity, and not on the cavity length.…”

“…As a result, the incident beam goes through several reflections in the resonant cavity, each time producing a reflected signal, i.e., a signal emitted in the direction opposite to the incident beam direction, whose intensity depends on the optical path length within the resonator. The displacement produced by the incident acoustic wave changes the cavity length, and hence modulates the intensity of the reflected signal [22].…”

“…Using a first order approximation, for a small change in length δL, i.e., a small ultrasonic pressure reaching the receiver surface, the reflected light intensity variation is given by [22] …”