High contrast holography through dual modulation

TL;DR

This work tackles the contrast limitations of short-distance holographic imaging in compact head-mounted displays by introducing dual modulation: a low-resolution amplitude SLM placed near a high-resolution phase SLM to achieve high-contrast holography without bulky optical relays. The authors derive design guidelines, including a bound on amplitude-pixel size $\Delta_a$ relative to the maximum diffactive displacement $d$, and demonstrate through simulations (up to 31 dB PSNR gain) and a benchtop prototype (up to 6.5 dB PSNR gain) that near-ocular formats can deliver high-contrast images. A comprehensive calibration framework is developed, combining a physics-inspired forward model with learnable lookup tables, cross-talk modeling, TPS-based inter-SLM alignment, and patch-based aberration correction, followed by camera-in-the-loop fine-tuning to reconcile model and hardware. The approach enables practical, compact high-contrast holography suitable for head-mounted displays, preserving étendue while leveraging transmissive amplitude modulation and flexible deployment geometries. Overall, the work expands design space for holographic displays by showing that coarse amplitude modulation can substantially improve contrast at short propagation distances, enabling next-generation near-eye holographic systems.

Abstract

Holographic displays are a promising technology for immersive visual experiences, and their potential for compact form factor makes them a strong candidate for head-mounted displays. However, at the short propagation distances needed for a compact, head-mounted architecture, image contrast is low when using a traditional phase-only spatial light modulator (SLM). Although a complex SLM could restore contrast, these modulators require bulky lenses to optically co-locate the amplitude and phase components, making them poorly suited for a compact head-mounted design. In this work, we introduce a novel architecture to improve contrast: by adding a low resolution amplitude SLM a short distance away from the phase modulator, we demonstrate peak signal-to-noise ratio improvement up to 31 dB in simulation and 6.5 dB experimentally compared to phase-only modulation, even when the amplitude modulator is 60$\times$ lower resolution than its phase counterpart. We analyze the relationship between diffraction angle and amplitude modulator pixel size, and validate the concept with a benchtop experimental prototype. By showing that low resolution modulation is sufficient to improve contrast, we open new design spaces for high-contrast holographic displays.

Figures (17)

• Figure 1: Dual modulation system architecture. Dual modulation holography combines a low resolution amplitude spatial light modulator (SLM) with a phase SLM to produce high-contrast images at short propagation distances. (a) Traditional phase-only modulation produces images with low contrast at short propagation distances (20 mm). These images have intensity errors and variations across large uniform regions. Increasing the propagation distance (80 mm) can improve contrast. (b) Our dual modulation system places a low resolution amplitude SLM at a small distance ($\delta z$) in front of or behind a phase SLM in order to produce images with high contrast at short propagation distances (20 mm). The amplitude SLM contributes to low-frequency and uniform intensity regions, while the phase SLM prioritizes high-frequency details and transitions between light and dark regions.
• Figure 2: Contrast in holographic displays. We simulate holograms where the target image is a white square of varying size. We visualize the intensity cross-section as a function of propagation distance, noting that light cannot be bent beyond $\theta_d$. With phase-only modulation (a-c), light at the edges of the SLM cannot reach the white square and therefore adds background intensity and reduces contrast. In addition, for the large square size (c), light cannot be moved from outside the square to the center, resulting in undesirable intensity oscillations. Finally, phase-only modulation creates an unnatural bright spot at longer propagation distances, which is not desirable if natural defocus cues are needed. In comparison, with our dual modulation approach (d-f), light that cannot be moved into the square is blocked at the amplitude SLM, which improves contrast and uniformity over all square sizes and creates natural defocus.
• Figure 3: Dual modulation (simulation). (a) Even in an ideal simulation, phase-only modulation has difficulty creating large uniform regions, like in this image of vertical bars. Our dual modulation approach can correctly create the target image even when the amplitude pixel size ($\Delta_a$) is large. (b) Dual modulation also improves contrast on a natural scene, although in this case there is some degradation in PSNR for the largest value of $\Delta_a$, which is outside the recommended bound in Eq. \ref{['eq:maximum_pixel_size']}. (c) Although the amplitude SLM is low resolution, fine details in the image are correctly created even when they are significantly smaller than $\Delta_a$, which is depicted by the dashed red square. (d) Intensity cross-sections highlight the improvement in contrast provided by dual modulation.
• Figure 4: Focal stacks (simulation). We use dual modulation to generate focal stacks with natural defocus cues, propagating from $20$ to 25 mm with $5$ jointly optimized frames per color channel. Dual modulation, here with $\Delta_a = 480$ µ m amplitude pixel size, outperforms phase-only modulation. PSNR and CE calculated over the full focal stack are shown on the left side of each image.
• Figure 5: Experimental setup schematic. Our dual modulation prototype uses two reflective SLMs with equal pixel pitch, allowing us to vary the relative pixel size by binning at the amplitude modulator. This requires a 4$f$ system between the SLMs to make room for beamsplitters, but we anticipate our dual modulation approach is compatible with a transmissive amplitude modulator that could enable a compact setup.