Jinyang Liang

High-precision laser beam shaping using a binary-amplitude spatial light modulator

We have achieved high-precision laser beam shaping by using a binary-amplitude spatial light modulator, a digital micromirror device (DMD), followed by an imaging telescope that contains a pinhole low-pass filter (LPF). An error diffusion algorithm was used to design the initial DMD pixel pattern based on the measured input beam profile. This pattern was iteratively refined by simulating the optically low-pass filtered DMD image and changing DMD pixels to lift valleys and suppress peaks. We noted the gap between the experimental result of 1.4% root-mean-square (RMS) error and the simulated result for the same DMD pattern of 0.3% RMS error. Therefore, we deemed it necessary to introduce iterative refinement based on actual measurements of the output image to further improve the uniformity of the beam. Using this method, we have demonstrated the ability to shape raw, non-spatially filtered laser beams (quasi-Gaussian beams) into beams with precisely controlled profiles that have an unprecedented level of RMS error with respect to the target profile. We have shown that our iterative refinement process is able to improve the light intensity uniformity to around 1% RMS error in a raw camera image for both 633 and 1064 nm laser beams. The use of a digital LPF on the camera image is justified in that it matches the performance of the pinhole filter in the experimental setup. The digital low-pass filtered results reveal that the actual optical beam profiles have RMS error down to 0.23%. Our approach has also demonstrated the ability to produce a range of target profiles as long as they have similar spatial-frequency content (i.e., a slowly varying beam profile). Circular and square cross-section flat-top beams and beams with a linear intensity variation within a circular and square cross section were produced with similarly low RMS errors. The measured errors were about twice the ultimate limit of 0.1% RMS error based on the number of binary DMD pixels that participate in the beam-formation process.

1.5% root-mean-square flat-intensity laser beam formed using a binary-amplitude spatial light modulator

We demonstrate a digital micromirror device (DMD)-based optical system that converts a spatially noisy quasi-Gaussian to an eighth-order super-Lorentzian flat-top beam. We use an error-diffusion algorithm to design the binary pattern for the Texas Instruments DLP device. Following the DMD, a telescope with a pinhole low-pass filters the beam and scales it to the desired sized image. Experimental measurements show a 1% root-mean-square (RMS) flatness over a diameter of 0.28 mm in the center of the flat-top beam and better than 1.5% RMS flatness over its entire 1.43 mm diameter. The power conversion efficiency is 37%. We develop an alignment technique to ensure that the DMD pattern is correctly positioned on the incident beam. An interferometric measurement of the DMD surface flatness shows that phase uniformity is maintained in the output beam. Our approach is highly flexible and is able to produce not only flat-top beams with different parameters, but also any slowly varying target beam shape. It can be used to generate the homogeneous optical lattice required for Bose-Einstein condensate cold atom experiments.

Suppression of the zero-order diffracted beam from a pixelated spatial light modulator by phase compression

Phase compression is used to suppress the on-axis zero-order diffracted (ZOD) beam from a pixelated phase-only spatial light modulator (SLM) by a simple modification to the computer generated hologram (CGH) loaded onto the SLM. After CGH design, the phase of each SLM element is identically compressed by multiplying by a constant scale factor and rotated on the complex unit-circle to produce a cancellation beam that destructively interferes with the ZOD beam. Experiments achieved a factor of 3 reduction of the ZOD beam using two different liquid-crystal SLMs. Numerical simulation analyzed the reconstructed image quality and diffraction efficiency versus degree of phase compression and showed that phase compression resulted in little image degradation or power loss.

Grayscale laser image formation using a programmable binary mask

Abstract. 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.

High-precision laser beam shaping using binary-amplitude DLP spatial light modulators

Laser beams with precisely controlled intensity profiles are essential for many areas of optics and optical physics. We create such beams from real-world lasers: quasi-Gaussian beams obtained directly from a laser and beam-expanding telescope without spatial filtering. Our application is to form optical standing-wave lattices for Bose-Einstein condensates in quantum emulators. This requires controlled amplitude and flat phase, and that the beam be free of temporal modulation from either pixel dithering or refresh cycles. We describe the development of the pattern design algorithms and demonstrate the performance of a high precision beam shaper to make flattop beams and other spatial profiles with similarly low spatial frequency content. The digital micromirror device (DMD) was imaged through a telescope containing a pinhole low-pass filter. An error diffusion algorithm was used to design the initial DMD pixel pattern based on the input beam profile. This pattern was iteratively refined based on output image measurements. We demonstrate forming a variety of beam profiles including flattop beams and beams with 1-D linear intensity variation, both with square and circular cross-sections. Produced beams had less than 0.25% root-mean-square (RMS) error with respect to the target profile and nearly flat phase.

High-precision beam shaper for coherent and incoherent light using a DLP spatial light modulator

We designed a precision laser beam shaper using a Texas Instruments digital micromirror device (DMD) with a telescope system containing a pinhole low-pass filter. The performance of the beam shaper was measured by comparing the intensity and wave-front uniformity to the target function and by the energy conversion efficiency. We demonstrated flattop and other laser beam profiles with 1-1.5% root-mean-square (RMS) error for a raw camera image and nearly flat phase. A noise analysis of the system revealed that lower error is possible and that most of the error came from coherent speckle noise in the camera. A previous experiment using a 1064 nm single-mode fiber (SMF) laser produced around 7% beam power conversion efficiency. Here we report improvements in system automation and laser source flexibility that result in increasing both the speed of the system to calculate and produce a beam, and the beam uniformity and energy conversion efficiency. A LabVIEW program was written to accelerate the speed of the iterative process for beam profile refinement. A 760 nm super-luminescent light emitting diode (SLED) and a 781 nm Laser Diode (LD) were used as light sources in order to reduce the beam coherence and approach the ultimate performance of the shaper. Both sources greatly reduced the speckle noise and increased measured intensity uniformity. Experiments achieved less than 0.9% RMS error over the entire flattop area with a diameter of 1.32 mm. In addition, simulations were conducted to determine the optimized wavelengths for different types of DMDs. For the .7XGA DMD, the 5th diffraction order matches 750-800 nm. Matching the laser diode to this wavelength increased the power conversion efficiency (input beam to output beam) to 19.8%.

Suppression of the zero-order diffraction beam from computer-generated holograms produced by a DLP spatial light modulator

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.

Suppression of the Zero Order Diffracted Beam for Near Field Holographic Projection by Phase Compression

Phase compression technique is demonstrated to be capable of eliminating the near field on-axis zero order diffracted (ZOD) beam. Image quality including diffraction efficiency and root mean square (RMS) error is simulated and discussed.

Bandwidth-limited laser image projection using a DMD-based beam shaper

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 beam profiles. 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.

Evaluation of DMD-based high-precision beam shaper using sinusoidal-flattop beam profile generation

We evaluate system performance of a high-precision beam shaper using a digital micromirror device (DMD) followed by a telescope with an adjustable pinhole low-pass filter. Beam shaping quality was measured by comparing the intensity and wave-front conformity with respect to the target image, and by the energy conversion efficiency. We previously demonstrated various flattop beams with high-precision intensity and a nearly uniform wave-front by using both coherent and incoherent light sources at visible and infrared wavelengths. The diffraction efficiency analysis determined optimized operation wavelengths for different diffraction orders. This paper extends beam shaping experiments to target images of a series of 2-D sinusoidal functions. An iterative pattern refinement process, based on the point spread function (PSF) of a single DMD pixel, was used to improve the image quality and to seek the optimized DMD binary pattern. Sinusoidal-flattop profiles with different spatial carrier frequencies were chosen for the purpose of system evaluation. Experiments demonstrated RMS error ranging from 0.95% to 11.87% in the raw camera image as the sinusoidal period was decreased. The DMD-based beam shaper achieved 1% RMS error level at low system bandwidth (large sinusoid period) and maintained 5% RMS error performance for a wide bandwidth range. We analyzed the relationship between spatial intensity error and system bandwidth. The ultimate system performance had amplitude error of ±1 to ±1.5 PSFs. Iterative refinement made a significant improvement in error for low system bandwidth as compared to the simulation of a DMD pattern designed by the error diffusion algorithm.