8F-3 High Frequency Transducer Characterization with a Sagnac Interferometer

Optical techniques achieve extremely fine spatial resolution and broad detection bandwidth for high frequency transducer characterization. We demonstrate that a Sagnac interferometer can map the pressure field from a spherically focused high frequency lithium niobate transducer. The common path nature of the Sagnac interferometer provides immunity to perturbations, a significant advantage for practical applications. The optical spot size on the sensing surface is measured to be 8 mum, and the photodetector bandwidth is 150 MHz. Single-shot waveforms have a signal- to-noise ratio of 23 dB. The measured -3 dB transmit beam width of the transducer is 40 mum along both x and y axes. The sidelobe level is -21 dB. The Sagnac and pulse echo data have peak frequencies of 80 MHz and 95 MHz, respectively. The corresponding -6 dB bandwidths are 75 MHz and 70 MHz, respectively. The slight difference is expected since the pulse echo spectrum is shaped by the two-way response of the transducer. Based on a comparison to a calibrated PVDF hydrophone at lower frequencies, the single-shot noise equivalent pressure of the Sagnac interferometer is 38 kPa. These results demonstrate the potential of the Sagnac interferometer for high frequency transducer characterization..