Computer Generated Holography Achieves 99% Zero-Order Diffraction Suppression with New Method

Scientists have long sought effective methods to eliminate the unwanted zero-order diffraction that plagues computer generated holography, hindering its use in display technologies. Now, Alessandro Cerioni, Samuele Trezzi and Marco Astarita, all from the Physics Department at Politecnico di Milano, alongside Tommaso Ongarello, Anna Cesaratto and Giulio Cerullo et al. from EssilorLuxottica Smart Eyewear Lab, present a novel interferometric technique to tackle this persistent problem. Their research demonstrates up to 99% suppression of zero-order diffraction using a ‘camera-in-the-loop’ calibration procedure, crucially without compromising image quality or field of view. This advancement removes a significant obstacle to creating compact and high-fidelity holographic engines, paving the way for practical applications in augmented and mixed reality displays.

The research introduces a method relying on destructive interference between the unwanted zeroth-order light and a carefully crafted suppression beam, achieved in a plane optically conjugated to the spatial light modulator (SLM). A key innovation is the implementation of a camera-in-the-loop (CITL) calibration procedure, which retrieves an optimal pixel-wise phase map designed to cancel the ZOD component with high precision, crucially preserving the full modulation depth of the SLM. Experimental demonstrations utilising both point-cloud and 2D/3D holograms have achieved up to 99% suppression of ZOD intensity, without compromising image quality or field of view.

The team achieved this breakthrough by engineering the SLM’s phase pattern to reshape modulated light into a corrective optical field, effectively mirroring the ZOD in both intensity and phase. This allows for destructive interference, suppressing the central bright spot commonly observed in holographic reconstructions without impacting the desired image. The process involves a four-stage iterative approach, beginning with an initial estimation of the corrective beam amplitude and progressing through a “piston sweep” to probe the corrective beam phase. A pixel-resolved phase map is then retrieved using the camera, followed by precise mapping of the corrective phase from camera coordinates to the SLM, with recursive optimisation until convergence.
This sophisticated calibration process ensures accurate ZOD suppression across a range of holographic patterns. A dedicated optical setup was employed, featuring a 4f telescope conjugating the SLM plane to a camera (Cam1) and a second camera (Cam2) for Fourier plane imaging without spatial filtering. A pinhole positioned in the Fourier plane acts as a low-pass filter, isolating the ZOD for precise calibration. Crucially, accurate phase calibration of the SLM, ensuring a linear phase response across the full 0 to 2π range, was a prerequisite for successful implementation. Once calibrated, the corrective phase map can be applied to any subsequent hologram without recomputation, enabling real-time operation and robust performance over time, a significant advancement for practical applications.

This research removes a major barrier to the practical deployment of full-field holography, paving the way for the development of compact, high-fidelity holographic engines for augmented and mixed reality displays. By eliminating the need for bulky spatial filtering systems, the method facilitates miniaturisation and enhances the potential for integration into near-eye displays. The ability to achieve near-complete ZOD suppression without sacrificing image quality or field of view represents a substantial step towards realising the full potential of holographic technology in next-generation display systems, particularly for applications demanding realistic 3D virtual images and eliminating the vergence-accommodation conflict.

Zero-order diffraction suppression via CITL calibration is crucial

Scientists developed a novel interferometric technique to suppress zero-order diffraction (ZOD) in phase-only generated holography, addressing a long-standing limitation in full-field holographic displays. The research pioneered a destructive interference approach, creating a suppression beam that interferes with the zeroth-order light in a plane conjugated to the spatial light modulator (SLM). This innovative method directly tackles the unwanted bright spot commonly observed in holographic reconstructions, improving image fidelity and enabling more compact designs. A camera-in-the-loop (CITL) calibration procedure was central to the study, retrieving an optimal pixel-wise phase map designed to cancel the ZOD component with high precision.

The team employed this CITL process to meticulously characterise the system and generate a corrective phase map, preserving the full modulation depth of the SLM throughout the calibration. Experiments involved projecting both point-cloud data and 2D/3D holograms onto the SLM, allowing for comprehensive evaluation of the ZOD suppression capabilities across diverse holographic content. The experimental setup harnessed the principles of interferometry, carefully aligning the suppression beam to achieve destructive interference with the ZOD at the designated plane. Researchers measured the resulting intensity distribution using a high-resolution camera, quantifying the degree of ZOD suppression achieved by the calibrated phase map.

Demonstrations consistently achieved up to 99% suppression of the ZOD intensity, crucially without compromising image quality or reducing the field of view. This performance represents a significant advancement over existing methods, which often trade off suppression for reduced image clarity or limited viewing angles. Once the system was calibrated, the corrective phase map could be applied to any subsequent hologram without requiring recalculation, enabling real-time operation and ensuring robust performance over extended periods. The technique reveals a pathway towards practical deployment of full-field holography, facilitating the development of compact, high-fidelity holographic engines suitable for augmented and mixed reality displays. This method overcomes limitations of previous approaches, such as bulky 4f systems or reductions in diffraction efficiency, offering a streamlined and effective solution for ZOD mitigation.

Zero-order diffraction suppression via interferometric calibration is crucial

Scientists have developed a new interferometric technique to effectively suppress the zero-order diffraction (ZOD) that commonly occurs in phase-only generated holography. This method utilises destructive interference between the unwanted zeroth-order light and a specifically designed suppression beam, achieved in a plane conjugated to the spatial light modulator (SLM). A camera-in-the-loop (CITL) calibration procedure optimises a pixel-wise phase map, cancelling the ZOD component with high precision while maintaining the SLM’s full modulation depth. Experimental results, using both point-cloud and 2D/3D holograms, demonstrate up to 99% suppression of ZOD intensity without compromising image quality or field of view.

Once calibrated, this correction can be applied to any hologram without recalculation, allowing for real-time operation and consistent performance. The researchers also extended this approach to handle the three primary colours (red, green, and blue) by retrieving independent RGB phase maps through the CITL procedure. They intend to implement this RGB extension using the Holoeye GAEA device, integrating it with dynamic augmented reality content and maintaining spatial resolution. This research represents a significant advancement in holographic beam control, as zero-order suppression is achieved directly during the phase encoding process, removing the need for external filtering optics.

By integrating interference engineering into the computation of computer-generated holograms (CGH), a long-standing gap between laboratory holography and compact display integration has been bridged. The authors acknowledge that the technique’s performance is contingent upon precise calibration and alignment during the CITL process. However, the demonstrated ability to perform full-field ZOD cancellation in a compact and alignment-free configuration signifies progress towards deployable holographic engines for near-eye and wearable displays, potentially broadening the scope of holographic display technology. Furthermore, the underlying camera-in-the-loop interferometric principle could be adapted to compensate for optical aberrations in various diffractive systems.