Coherent diffraction imaging (CDI) is a kind of computational technique seeking to reconstruct a complex wavefront from the measured intensity. To solve the phase retrieval problem, phase modulation is introduced to be a physical constraint. In this work, the alternative structured phase modulation (ASPM) and orbital angular momentum (OAM) are used to be the physical constraints. The ASPM can act as the phase grating. The modulated intensities are concentrated, which can be captured with a high signal-to-noise ratio (SNR). OAM is recently employed as a type of reliable modulation. The topological charge is robust during propagating in the atmosphere. The maximum value of the spectrum intensity is reduced by a factor of 170 under OAM modulation when topological charge is 50. The stagnation can be avoided using multiple ASPM and OAM modulations. The complex wavefront can be robustly reconstructed from redundant measurements without a priori knowledge.
Reliable phase-only spatial light modulators (SLMs) are in demand for accurate phase modulation. However, the nonlinear optical response of liquid crystals and the limited manufacturing process can lead to the spatial nonuniformity of the phase modulation of the SLM. The transfer from the grayscale to the modulated phase can be different from the lookup table (LUT) shown in the SLM manual. The SLM should be measured for calibration. We propose a calibration method based on digital holography to calibrate the spatial nonuniformity of phase modulation of the SLM. Using a self-generated grating, the SLM involved system is converted to the calibration system based on the principle of digital holography. The in-situ strategy for low cost and efficient calibration was demonstrated with optical experiments using a 4K (3840 × 2160 pixels) phase-only SLM. The spatial nonuniformity was calibrated to decrease by more than 75% using only a beam splitter and an imaging sensor.
Reliable phase-only spatial light modulators (SLMs) are in demand for accurate phase modulation in a wide range of fields. Due to the nonlinear optical response of liquid crystals and the limited manufacturing process available, the spatial nonuniformity of the phase modulation by the pixels should be measured and/or calibrated. We propose an in situ calibration method based on digital holography to calibrate the spatial nonuniformity of phase modulation of the SLM. The SLM panel is divided into blocks composed of pixels. The differential phase on hundreds of blocks can be reconstructed through the holograms. The distribution of modulated phase can then be derived after eliminating statics phase anomalies. The spatial nonuniformity of the panel can be measured for calibration with high efficiency. A modulated phase step on the SLM was calibrated to increase linearly. The spatial nonuniformity was calibrated to decrease by more than 75% using only a beam splitter and an imaging sensor. The in situ strategy for low cost and efficient calibration was demonstrated with optical experiments using a 4K (3840 × 2160 pixels) phase-only SLM.
We propose a self-reference interferometric method for phase calibration of spatial light modulator (SLM) based on two blazed gratings. Compared with traditional methods, the proposed method yields more stable results by generating fringes with lower spatial frequency, thus making it possible for accurate, low cost measurement of the phase modulation of an SLM.
Phase-only liquid crystal spatial light modulator (SLM) has been widely used for accurate phase-manipulation in holographic display, optical tweezers, lithography, etc. Due to the nonlinear optical response of liquid crystal and manufacturing defects, the grayscale-phase response could be different for every single SLM device. The calibration for phase modulation is needed for improving the performance of the SLM involved system. The principles of phase-only SLM are reviewed. The main phase calibration methods for SLM are discussed and compared.
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