A laser ultrasound emitter based on high-power diode laser in overdrive operation mode for biomedical imaging applications.

Abstract

The most common transducers used to generate ultrasound in medical applications are based on short electrical pulses applied to piezoelectric transducers and capacitive micromachined ultrasound transducers. However, piezoelectric transducers have a limited frequency bandwidth, defined by their physical thickness, and capacitive micromachined ultrasound transducers have poor transmission efficiency. The high frequency cutoff limits the spatial resolution of ultrasonic images. The low frequency cutoff limits volumetric contrast of objects on ultrasound images so that typically only tissue boundaries are displayed. These limitations can be overcome with laser generated ultrasound. Laser ultrasound generation is based on the optoacoustic effect, which greatly increases the bandwidth of ultrasound signals. We show the generation of ultra-wideband ultrasound pulses using high power diode lasers operating in the overdrive regime, and thin composite films of candle soot in polydimethylsiloxane matrix as transmitters. We achieved a peak pressure of 228.59 kPa and a ultrawive bandwidth of 0.1 MHz-to-30 MHz (BW_6dB≈200%) at -6 dB level with an optoacoustic conversion efficiency of 6.27 × 10^-3 [Pa/(W/m²)] or 3.35 × 10⁶ [Pa/(mJ/cm²)]. We present a compact and low-cost ultra-wideband laser ultrasound emitter with the possibility to adjust the bandwidth of the transmitted frequency and the ability to generate ultrasonic images in ex-vivo tissues.