Coherent Beam Combining of Four Semiconductor Optical Amplifiers for High-Peak-Power Acousto-Optic Imaging

Acousto-optic imaging (AOI) aims at producing in-depth images in highly scattering biological tissues, by tagging scattered photons with ultrasound (US) waves as they propagate through the sample. By selectively detecting photons traversing the insonified region, AOI generates images of the local optical absorption [1]. To approach video-rate imaging of in-vivo samples, a single-shot acquisition protocol is necessary. This requires laser sources in the 650 nm ≤ <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\lambda$</tex> ≤ 900 nm spectral range (corresponding to low absorption of haemoglobin and water) with both a high peak power and a narrow spectral linewidth (<20 MHz). The penetration depth is currently limited by the optical power available from single-frequency diode laser devices. Coherent beam combination ─ based on the constructive interference of multiple coherent laser beams ─ offers a promising solution as it enhances optical power while preserving beam quality and spectral purity.