We present a method to encode complex values into three or four quantized complex values for wavefront modulation using two digital micromirror devices (DMDs). This encoding offers advantages to eliminate the twin image or suppress the zero order diffraction as well to improve hologram fidelity. The optical architecture utilizes a Michelson interferometer with a DMD in Littrow configuration replacing the mirrors to combine the two holograms with the desired phase shift. System performance was examined using numerical simulations and experimental measurements to explore different encoding methods for hologram reconstruction. Both ZOD and conjugate image suppression were demonstrated for different encoding schemes.
We present grayscale laser image formation from a programmable binary mask using a digital micromirror device (DMD) followed by a telescope with an adjustable pinhole low-pass filter. System performance was measured by comparing the intensity conformity with respect to the target image and by the energy conversion efficiency. A theoretical analysis of image precision proved high-precision image formation and inspired the iterative pattern refinement process based on the point spread function of a single DMD pixel to seek optimized image quality. We derived the diffraction efficiency formula of the DMD and discussed the overall system energy efficiency with operation wavelengths. Actual image precision performance was evaluated by measuring the root-mean-square (RMS) error of a series of sinusoidal-flattop profiles with different system bandwidths. We produced grayscale laser images with different spatial spectral content using intensity profiles of Laguerre-Gaussian, Hermite-Gaussian, and Lena-flattop beams. Measured RMS errors of all examples of various bandwidths were consistent with the image precision of the sinusoidal reference patterns. The ripple effect caused by the sharp-edged pinhole was the major contributor to the residual error in the output images. Error histograms had a zero-mean Gaussian distribution with standard deviation equal to the value of the RMS error.
We present a new optical technique to suppress the unwanted zero-order diffraction (ZOD) in holograms produced by
the digital micromirror device (DMD). The proposed optical architecture consists of two light beams illuminating the
DMD in an interferometer configuration. The two beams are incident from different angles, +24° and -24°, in order to
utilize light diffracted from all the pixels to produce a binary Fresnel hologram. The relation between these two beams
diffracting from the DMD was found to be complementary, and they both generated the same reconstructed image
pattern. With π phase difference between the two beams, the diffracted beams had their ZOD components out of phase
while the reconstructed holograms were identical and in phase. Experiments were conducted to demonstrate ZOD
suppression by destructive interference and simultaneous hologram enhancement by constructive interference. The
method was shown to suppress the ZOD by a factor of 2.9 in a Fresnel hologram.
A digital micromirror device (DMD) laser beam shaper was implemented for projecting spatial bandwidth-limited laser
images with precisely controlled intensity. A telescope images the binary DMD pattern with an adjustable pinhole low-pass
filter that controls the system bandwidth and converts the binary pixelated image back to grayscale. Images with
arbitrary but bandwidth-limited spatial frequency content are formed. System performance was evaluated by examining
the spatial frequency response in terms of RMS intensity error by generating sinusoidal-flattop beam profiles with
different spatial periods. This system evaluation was used as a reference to predict the error level of arbitrary output
In addition, we demonstrated band-limited laser image projection for different spatial bandwidths using a grayscale
image superimposed on a flattop laser beam profile. Optimized system bandwidth was simulated by considering the
tradeoff between image precision and spatial resolution. Experimental results demonstrated that the RMS error of output
beam profiles was consistent with the system evaluation reference. The major residual error in the output beam profile
came from the sharp-edged pinhole low-pass filter. Error histograms had a Gaussian distribution with mean value of zero
and standard deviation equal to the value of the RMS error. We plan to apply this technique to generate programmable
optical trap shapes in ultracold atom experiments.