Acoustic and Ultrasonic Detection With Radio-Frequency Encoded Fiber Laser Sensors

Fiber lasers have been exploited as sensors for the detection of extremely weak mechanical parameters, by monitoring the radio-frequency beat signal between orthogonal polarized lasing modes. This sensor scheme has presented advantages including advanced measurement capability, low cost and convenience in signal demodulation, resistance to environmental disturbances, and intrinsic multiplexing capability. Here, we demonstrate the capability of the fiber laser sensors for the detection of low-frequency (<;2 kHz) acoustic waves and ultrasound signals at frequency ranges >1 MHz (denoted as “high frequency” in this paper) and around 200 kHz (denoted as “medium frequency”), toward different applications. For low-frequency acoustic sensing, the detection limit reaches 74 μPa/Hz
^1/2
at 1 kHz which is comparable to the zeroth-order sea noise. For high-frequency ultrasound detection, the laser sensor presents a detection capability of 40 Pa over 50 MHz bandwidth, which allows its application as an ultrasound detector in photoacoustic (PA) imaging. Preliminary three dimensional PA imaging is presented, by means of time-to-space reconstruction. Ultrasound detection with a polymer-packaged fiber laser at 100 kHz range is demonstrated, which could find applications in underwater search and targeting. Its imaging capability is also exhibited.