High speed light focusing through dynamic turbid media
Controlling light propagation through scattering media at high speeds is critical for applications in biomedical imaging. As light propagates through biological tissue it becomes increasingly scattered, thus limiting the optical imaging depth to less than 1 mm. Recently introduced wavefront control techniques should allow for deeper imaging in biological materials. These techniques rely on the deterministic nature of multiple scattering to shape the incident wavefront and pre-compensate for the scattering effects of light propagation. However, living biological materials have structural changes which occur on the millisecond timescale, altering the path of light. This fast rate of change makes current methods of focusing through turbid media too slow. Most current methods use liquid crystal spatial light modulators, whose switching speed is typically in the 10s of Hz: much slower than the kHz rate required for the millisecond timescale of biological tissues. Thus, new high-speed techniques for optimizing wavefronts are required to implement focusing through biological samples. We have developed a new high-speed wavefront optimization technique, which utilizes off-axis binary-amplitude computer-generated holography. The computer-generated holograms are implemented via a deformable mirror device (DMD) based on micro-electro-mechanical technology, which can be updated at high data rates. We demonstrate this technique measuring a transmission matrix with 256 input modes and a single output mode in 33.8 ms and creating a focus with a signal to background ratio of 160. We also demonstrate focusing through a highly temporally dynamic, strongly scattering sample.