Automotive Light Detection and Ranging (LiDAR) modules, wearable augmented reality display engines, and field-deployable free-space optical communication systems all require fast and robust solid-state beam and image steering solutions with a wide 2-dimensional field of view, as mechanical laser beam scanning is prone to mechanical failure. Diffractive beam steering with a digital micromirror device provides a robust solid-state beam steering solution to these problems and has been show to increase the field of view in 1-dimension for LiDAR and display systems. By extension, two Digital Micromirror Devices arranged orthogonally can be synchronized with a pulsed laser to diffractively steer a beam arbitrarily in 2-dimensions. This technique enables all-solid-state 2-dimensional beam steering solutions for beam steering and image steering applications.
An ideal Near-to-Eye display (NED) requires high-resolution images, a large field of view (FOV) and depth cues. Sometimes, those performances are degraded due to optical aberrations of optics. To correct for aberrations, in this work, we utilize digital phase conjugation (DPC) with a Texas Instruments phase light modulator (TI-PLM) to generate a 3D image with TIR/geometrical image guide. TI-PLM is a type of MEMS device that modulates the phase of the incoming light by moving the micromirrors in a piston motion, thereby modulating the phase. To measure aberration induced by the image guide combiner, we employed an off-axis holography, capturing the off-axis fringes using a camera sensor. Subsequently, image processing on the captured fringes, involving Fourier transform and cropping of +1st order, to extract the final field information while reducing low-frequency noise. Computer-generated hologram (CGH) was generated to negate the phase aberration, which is then displayed on the PLM. Through phase conjugation, we reconstruct the wavefront, resulting in a series of point sources displayed at different depths, and producing a 3D point images. This method allows us to generate multiple point sources with different depths, contributing to the 3D image in our Near-to-Eye display even via aberrated medium.
To facilitate Augmented Reality (AR) displays suitable for all-day-long usage, technologies are anticipated to realize compact form factor, low power consumption without compromising key aspects such as field-of-view (FOV), brightness, resolution, and uniformity. The balance among those requirements is crucial for creating a better AR experiences that are both comfortable and visually appealing for extended use. One of the challenges, FOV-resolution-form factor trade-off is addressed by using a MEMS based micro display panel, Digital Micromirror Device (DMD) in diffractive image steering mode. The DMD is paired with synchronized and short laser pulse illumination and a prism array expands FOV. This setup facilitates steering images into one of the multiple diffraction orders in a time sequential manner. The FOV is horizontally increased by fivefold. The vertical FOV is increased by twofold by incorporating illumination multiplexing. The resulting FOV reaches 30 degrees horizontally and 12 degrees vertically, all while sustaining a resolution of 1.86 arc minutes per pixel. This method increases FOV without sacrificing resolution nor increasing the form factor of micro display panel.
Beam and image steering by Micro Electro Mechanical System (MEMS) Spatial Light Modulators decouples trade-offs between resolution, field of view, and size of displays and optics that are a common challenge found in optical designs. We overview solid state lidar and augmented reality display engine employing MEMS SLMs, Texas Instruments Digital Micromirror Device and Phase Light Modulators.
We demonstrated a real-time lidar system applying a Digital Micromirror Device (DMD) as a field of view (FOV) expander of a lidar receiver employing a 2D Multi-Pixel Photon Counter (MPPC). By temporally synchronizing the transitional state of micromirrors with returning photons from lidar, receiver FOV is diffractively steered to the targets’ direction enabled by nano-second pulse laser. With a nanosecond 905nm laser transmitter, time-of-flight (ToF) lidar images were captured across seven diffraction orders with the expanded 35 degrees full field of view lidar scanning range.
Micro Mechanical Electronics System based Spatial Light Modulators (MEMS-SLM) enables unique capability “Just in time photon delivery” or steering beam images to where and when they are needed. The beam and image steering solves challenges commonly found in both lidar and AR optical engines dominated by classical tradeoffs, such as image FOV, resolution and SLM size or form factor of optical engine. As a novel beam and image steering device, we transformed Texas Instruments Digital Micromirror Device (TI-DMD) into a diffractive beam and image steering device. TI-DMD is known as a binary spatial light modulator. Micromirros’ tilt re-directs light into on- or off-states. Without modifying TIDMD, but with employing a nano-second pulse illumination synchronized to the transitional movement of micromirrors between the of- and off-states turns DMD into a diffractive beam and image steering device.
We demonstrated a real-time lidar system that utilizes a Digital Micromirror Device (DMD) as a field of view (FOV) expander and a 2D Multi-Pixel Photon Counter (MPPC) as a lidar sensor. By synchronizing the dynamic transition of DMD micromirrors between on- and off-states with the MPPC and a nanosecond pulse laser, the receiver FOV is diffractively steered to expected direction enabled by timing the delay of micromirrors transition to the laser. The DMD-MPPC lidar can capture up to 7 diffraction orders of high-resolution geospatial data. By applying the laser beam steering technique, this system is able to span over 35 degrees FOV, which is 10 times expansion of FOV compared to the single lidar detector FOV. In this work, as a preliminary demonstration towards diffractive FOV expansion, we presented the high resolution lidar images while DMD is switching between on and off state. Also, we performed distance resolution testing to validate the functionality of DMD-MPPC flash lidar system.
In this work, we investigate the ultrafast charge carrier dynamics in the double-layered architecture mixed halide (DHA) perovskite photovoltaic devices by ultrafast pump-probe transient absorption spectroscopy (TAS). The measured TAS results show the perovskite solar cell consists of SnO2/(FAPbI3)1-x(MAPbBr3)x/HTAB has stronger transient absorbance with photoinduced bleaching at 750 nm and photoinduced absorption in the range of 550-700 nm. The lifetime of DHA perovskite observed from TAS is approximately 46 µs in conjunction with the electron injection discovered within the first 150 fs, indicating the charge carriers would be easily extracted. Besides, we measured a high power-conversion-efficiency of the DHA perovskite solar cell (PSC) of 21%. Hence, understanding the ultrafast charge carrier dynamics in PSC by pump-probe TAS provides detailed insights into the advanced working mechanism. The results open a door for the development of high-performance perovskite photovoltaics.
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