Research On Light Focusing And Imaging Through Disordered Media Based On Photoacoustic Wavefront Shaping

When the laser is incident on a disordered medium such as biological tissue,paper,or the atmosphere,light absorption and light scattering occur.Light scattering causes the laser to be unable to focus and image through the disordered medium,while light absorption can produce a photoacoustic effect.The emergence of wavefront shaping technology makes it possible to perform light focusing and imaging inside a disordered medium.However,traditional wavefront shaping technology cannot achieve non-invasive focusing and imaging in a disordered medium because of the need of CCD camera as feedback.Since the scattering of sound waves inside biological tissues is two to three orders of magnitude weaker than the scattering of light,the photoacoustic effect produced by light absorption can achieve non-invasive wavefront shaping inside biological tissues,thereby making damage to living biological tissues.Detection and imaging.Therefore,it is of great significance to study the research of light focusing and imaging in disordered media based on photoacoustic effect.In this paper,the photoacoustic effect is applied to the field of laser wavefront shaping,and the light focusing through the disordered medium and imaging of the target are realized.Due to the damage threshold of living biological tissues,the pulsed laser energy incident into biological tissue is generally low.In this paper,the digital micromirror device(DMD)with a low-energy binary amplitude modulation on the surface is used to modulate the wavefront of the incident light.The photoacoustic waves generated by the absorber after collecting the scatterers by the ultrasonic transducer are used as feedback of the genetic algorithm to achieve focusing.Due to the limitation of the incident laser energy,the generated photoacoustic signal is very weak.And due to the influence of static electricity and amplifier noise in the experimental equipment,the initial signal-to-nois ratio of the photoacoustic signal is very low.This paper proposes a photoacoustic signal extraction scheme combining wavelet denoising and correlation detection algorithms.Not only the signal-to-noise ratio of the photoacoustic signal is increased from 3.76 to 25.4,and the influence of the clutter on the photoacoustic signal is effectively eliminated.Using the photoacoustic signal extraction scheme proposed in this paper,we achieve a faster focusing at a lower pulse laser repetition rate.After the focus is completed,we scanned the absorber and the contrast of the photoacoustic image was increased from 1.51 to 5.30 compared to before unfocused.Then we apply the photoacoustic signal extraction scheme combining wavelet denoising and correlation detection algorithm to the scanning imaging after the focus is completed.An improved photoacoustic signal extraction scheme for adaptive time window correlation detection is proposed,which further improves the contrast of photoacoustic images to 9.57.