Quality improvement of unfiltered holography by optimizing high diffraction orders with fill factor
Computer-Generated Holography (CGH) and Spatial Light Modulators (SLMs) represent critical technologies and devices in contemporary optics. While deeply intertwined, each possesses unique features. Holograms generated through computation can be loaded onto an SLM to enable Holographic display of desired scenes. Employing the SLM as the recording medium for holograms provides significant convenience along with real-time modulation capabilities.
Paper Information:
Computer-generated holography (CGH) suffers from high diffraction orders (HDOs) due to the pixelated nature of spatial light modulators (SLMs), typically requiring bulky optical filtering systems. To address this issue, a novel unfiltered holography approach known as the high-order gradient descent (HOGD) algorithm was previously introduced to optimize HDOs without optical filtering, enabling compact holographic displays. However, this algorithm overlooks a crucial physical parameter of SLMs—the fill factor—leading to limited optical quality. Here, we introduce a fill factor-based HOGD (FF-HOGD) algorithm, specifically designed to improve the quality of unfiltered holography by incorporating the fill factor into the optimization process. The quality advantage of FF-HOGD is demonstrated through numerical simulations and Optical Experiments.
Experimental Procedure and Results
The optical setup used in the experiment is illustrated in the figure below.FSLM-2K70-P02 spatial light modulator (SLM) is utilized to display the computer-generated holograms. The SLM features a resolution of 1920 × 1080 pixels, a pixel pitch of 8 μm, and a fill factor of 87%. A 532 nm all-solid-state laser serves as the light source, with the propagation distance z from the SLM plane to the target plane set at 40 mm. The reconstructed images are directly photographed using a DSLR camera with a lens, eliminating the requirement for a 4f filtering system to remove higher diffraction orders (HDOs).
Figure 1. Schematic diagram of the optical experimental setup
Figure 2. Schematic diagram of holography without filtering. The phase map ϕ displayed on the spatial light modulator (SLM) generates a wavefront. This wavefront propagates in free space and eventually forms an unfiltered light field uunfilt (ϕ) at the target plane, which contains higher diffraction orders (HDOs) arising from discrete sampling. In contrast to previous studies that required a filtering step—typically implemented using a bulky 4f system—the present approach eliminates this necessity.
Figure 3. Schematic diagram of the pixelated structure of the spatial light modulator (SLM). By incorporating the fill factor α² and the pixel pitch p of the SLM, accurate modeling of higher diffraction orders (HDOs) can be achieved.
Figure 4. Schematic diagram of the FF-HOGD algorithm computation process. The amplitude (blue) and phase (yellow) are labeled for each wavefront. The resolution is denoted by the numerical value in the upper-right corner. The mean squared error (MSE) loss is computed between the target amplitude image atarget and the reconstructed amplitude | u’unfilt(ϕ)|, after which the error is back-propagated via gradient descent to iteratively optimize the phase pattern.
Figure 5. Accuracy comparison between the proposed model and the original higher diffraction orders (HDO) model. (a) Single-slit experiment; (b) Young's double-slit interference experiment; (c) Mean squared error (MSE) of Young's double-slit experiment under different conditions
Figure 6. Filter-free simulation results of three computer-generated holography (CGH) algorithms.
Figure 7. Filter-free experimental results of three computer-generated holography (CGH) algorithms.
Parameters of the phase-only spatial light modulator (SLM) used in this experiment are as follows:
| Model | FSLM-2K70-P02 | Modulation Type | Phase |
| LCOS Type | Reflective | Greyscale | 8bit, 256step |
| Liquid crystal mode | PAN | Drive mode | Digital |
| Resolution | 1920×1080 | Pixel Size | 8.0μm |
| Active area | 0.69" 15.36mm×8.64mm | Opening rate/Fill factor | 87% |
| Optical Utilization Efficiency | 70±5% @532nm | Wavefront correction | Yes (532nm/635nm) |
| Phase Modulation | ≥2π@532nm | Spectrum Range | 420nm-700nm |
| Response time | ≤16.7ms | Phase calibration | (450nm/532nm/635nm) |
| Gamma correction | Yes | Damage Threshold | 20W/cm2(No water cooling) 100W/cm2(water cooling) |
| Refresh Rate | 60Hz | Data interface | HDMI |
| Power Input | 12V 3A | LC alignment direction° | 0° |
Note: Different models vary in phase modulation range and optical efficiency. For specific requirements, please contact the sales manager for your region.
Additionally, we also offer a high-reflectivity, high optical efficiency version of the same series. The detailed parameters are as follows:
| Model | FSLM-2K70-P02HR | Optical Utilization Efficiency | 95%±5%@420nm/532nm/635nm |
Conclusion
As computing power continues to advance, deep learning algorithms evolve, and spatial light modulator technology progresses, the combination of computer-generated holography and SLMs will expand into broader applications. Looking ahead, we anticipate the development of higher-resolution, faster-response 3D display systems, along with more efficient and intelligent solutions for optical information processing and laser beam manipulation.
Paper Information: