Remote picometric acoustic sensing via ultrastable laser interferometry

Acoustic detection has many applications across science and technology, from medical to imaging and communications. However, most acoustic sensors have a common limitation in that the detection must be near the acoustic source. Alternatively laser interferometry with picometer-scale motional displacement detection can rapidly and precisely measure sound induced minute vibrations on remote surfaces. Here we demonstrate the feasibility of sound detection up to 100 kHz at remote sites with ~ 60 m of optical path length via laser homodyne interferometry. Based on our ultrastable Hz-linewidth laser with 10-15 fractional stability, our laser interferometer achieves 0.5 pm/Hz1/2 displacement sensitivity near 10 kHz, bounded only by laser frequency noise over 10 kHz. Between 140 Hz to 15 kHz, we achieve a homodyne acoustic sensing sensitivity of sub-nm/Pa across our conversational frequency overtones. The minimal sound pressure detectable over 60 m of optical path length is ~ 2 mPa, with dynamic ranges over 100 dB. With the demonstrated standoff picometric distance metrology, we successfully detected and reconstructed musical scores of normal conversational volumes with high fidelity. The acoustic detection via this precision laser interferometer could be applied to selective area sound sensing for remote acoustic metrology, optomechanical vibrational motion sensing and ultrasensitive optical microphones at the laser frequency noise limits.