Ultrasound Detection via Pulse Interferometry

Background: Ultrasound detection via optical resonators is commonly performed with a simple continuous wave (CW) interrogation setup. A CW laser is tuned close to the resonance wavelength and a photodetector measures the reflection (transmission) of the light from (through) the resonator, where ultrasound-induced wavelength shifts of the resonance lead to variations in the detected intensity. CW interrogation suffer from two major drawbacks: vulnerability to external disturbances and the lack of a scalable scheme for detector arrays, limiting this technique to single sensor systems. Pulse interferometry (PI) represents an alternative to CW interrogation, in which a wideband, pulsed source is used and the detection is performed by frequency demodulation. The use of a wideband source enables the interrogation of numerous resonators with a single laser and offer high stability against external disturbances. Methods and Results: PI was implemented in a fiber-optics setup including a pulse laser, a noise reducing free space Fabry-Perot (FP), and a demodulation scheme that is robust to external disturbances. The FP rejected incoherent spontaneous emission and enabled shot-noise limited detection, reducing the noise spectrum by 35 dB. A phase-modulation technique enabled the simultaneous interrogation of multiple ultrasound sensors with different resonance wavelength. Conclusions: PI overcomes the two major drawbacks of CW interrogation: robustness and scalability – the two properties required for the development of a detector array. Detector arrays simultaneously interrogated via PI will enable real-time optoacoustic imaging with high resolution, dynamic range, and sensitivity, beyond what is currently possible with piezoelectric transducers.