High-fidelity off-axis digital optical phase conjugation with transmission matrix assisted calibration.

The spatial information carried by light is scrambled when it propagates through a scattering medium, such as frosted glass, biological tissue, turbulent air, or multimode optical fibres. Digital optical phase conjugation (DOPC) is a technique that 'pre-aberrates' an illuminating wavefront to compensate for scatterer induced distortion. DOPC systems act as phase-conjugate mirrors: they require a camera to holographically record a distorted wavefront emanating from the scatterer and a spatial light modulator (SLM) to synthesize a phase conjugate of the measured wavefront, which is sent back through the scatterer thus creating a time-reversed copy of the original optical field. High-fidelity DOPC can be technically challenging to achieve as it typically requires pixel-perfect alignment between the camera and SLM. Here we describe a DOPC system in which the normally stringent alignment criteria are relaxed. In our system the SLM and camera are placed in-line in the same optical path from the sample, and the SLM is used in an off-axis configuration. This means high-precision alignment can be achieved by measurement of the transmission matrix (TM) mapping optical fields from the SLM to the camera and vice-versa, irrespective of their relative position. The TM also absorbs and removes other aberrations in the optical system, such as the curvature of the SLM and camera chips. Using our system we demonstrate high-fidelity focussing of light through two ground glass diffusers with a peak-intensity to mean-background ratio of ∼700. We provide a step-by-step guide detailing how to align this system and discuss the trade-offs with alternative configurations. We also describe how our setup can be used as a 'single-pixel camera' based DOPC system, offering potential for DOPC at wavelengths in which cameras are not available or are prohibitively expensive.