All-Optical Phase Conjugation Using Diffractive Wavefront Processing
This provides a compact, passive solution for optical phase conjugation applications like turbidity suppression and aberration correction, especially in parts of the electromagnetic spectrum lacking cost-effective wavefront engineering, but it is incremental as it builds on existing nonlinear techniques with a novel method.
The researchers tackled the problem of counteracting wavefront distortions in optical systems by designing a diffractive wavefront processor using deep learning to approximate all-optical phase conjugation, and they experimentally validated it with terahertz radiation, achieving successful operation on unseen phase distortions through a compact, shallow structure spanning tens of wavelengths.
Optical phase conjugation (OPC) is a nonlinear technique used for counteracting wavefront distortions, with various applications ranging from imaging to beam focusing. Here, we present the design of a diffractive wavefront processor to approximate all-optical phase conjugation operation for input fields with phase aberrations. Leveraging deep learning, a set of passive diffractive layers was optimized to all-optically process an arbitrary phase-aberrated coherent field from an input aperture, producing an output field with a phase distribution that is the conjugate of the input wave. We experimentally validated the efficacy of this wavefront processor by 3D fabricating diffractive layers trained using deep learning and performing OPC on phase distortions never seen by the diffractive processor during its training. Employing terahertz radiation, our physical diffractive processor successfully performed the OPC task through a shallow spatially-engineered volume that axially spans tens of wavelengths. In addition to this transmissive OPC configuration, we also created a diffractive phase-conjugate mirror by combining deep learning-optimized diffractive layers with a standard mirror. Given its compact, passive and scalable nature, our diffractive wavefront processor can be used for diverse OPC-related applications, e.g., turbidity suppression and aberration correction, and is also adaptable to different parts of the electromagnetic spectrum, especially those where cost-effective wavefront engineering solutions do not exist.