Non-contact detection of laser-induced acoustic waves from buried absorbing objects using a dual-beam common-path interferometer

The feasibility of a noncontact all-optical probe for surface detection of laser-induced acoustic waves generated in buried absorbing objects was investigated. The goal is to detect subsurface optically absorbing objects, such as hemorrhages or vascularized tumors, which generate acoustic waves when slightly heated by a Q-switched laser pulse transported to an internal object by light diffusion within a turbid tissue. A dual-beam common-path interferometer was constructed to provide sensitive time-resolved detection of surface movements. Arrival of a pressure wave at one surface beam irradiation site before arrival at the other beam site caused a differential surface movement and a pathlength difference detected by the interferometer. The dynamic range of linear measurement was about 20 mbar - 200 bar at 20 mV/bar. The sensitivity (20 mV/bar) and noise level (10-30 mbar) of the interferometer matched or exceeded the performance of a lithium-niobate piezoelectric transducer. The point spread function of response was studied in liquid phantoms which demonstrated a null plane of no response in the plane symmetrically between the two beams. The ability to image an absorbing object at a depth of 11 mm within an aqueous phantom medium was demonstrated illustrating sub-mm resolution.