Impact of aberrations in SLM-based far-field holography

TL;DR

This work addresses how aberrations in SLM-based far-field holography affect reconstruction quality. It employs camera-in-the-loop calibration with a physically interpretable neural-network digital twin to model key boundary conditions and aberrations, including phase modulation nonlinearity, fringing field, phase aberrations, illumination amplitude, and fill factor. The study shows that including all modeled aberrations yields the best PSNR and SSIM with reduced speckle, while phase aberrations exert the strongest influence; fringing-field and illumination effects are present but secondary. The findings highlight the potential of CITL calibration for robust hologram generation and suggest avenues for isolating aberration sources and improving phase-retrieval techniques in practical imaging systems.

Abstract

We use camera-in-the-loop calibration to calibrate a phase-only spatial light modulator (SLM) in a far-field hologram setup. The recorded intensity distributions achieve a high degree of consistency with the calculated results, indicating a precise calibration and sufficient modeling of the most prominent aberrations. In this work, we discuss the modeled aberrations and examine the improvement or loss in image quality and diffraction efficiency that is obtained by including or excluding the modeled aberrations in the calibration. We further show the influence of aberrations on speckle-reduced holograms and evaluate the speckle contrast.

Figures (12)

• Figure 1: Optical setup with the sketched optical path. The SLM is illuminated with collimated light and the sensor is placed in the back focal plane of lens 2. Lens 1 has an internal aperture, which limits the beam diameter to a size which is slightly larger than the SLM. The polarizers and the quarter wave plate ensure the correct polarization for the SLM and allow for adjusting the intensity of the illumination.
• Figure 2: Visualization of the structure of the digital twin and the schematic of the optical setup in the middle. The values from the phase-only hologram are converted to the actual phase delay introduced by the SLM. Next, these values are altered by the fringing field effect. Phase aberrations are added and the phase is converted to a complex wave field by adding the illumination amplitude. The wave field is zero-padded to maintain a uniform pixel size after propagation using a fast Fourier transform. The far field is attenuated by a sinc distribution and finally the intensity is calculated and the target area is cropped from the far field.
• Figure 3: Comparison of the image on the sensor with the prediction as it is calculated from the calibration with the full model. The close ups show a low deviation between the recording and the prediction with a good match of the speckle positions. The difference between prediction and recording are the largest in the upper right corner (e and f), which has the largest distance to the optical axis.
• Figure 4: Comparison of the recorded intensities in the far field for the different calibration models. The upper row contains a motif where speckle suppression was applied during calculation while the lower row displays a motif without speckle suppression.
• Figure 5: Comparison of the learned 3x3 kernels to model the fringing field effect.