A holographic lidar concept established on the Digital Micromirror Mirror (DMD)-based hybrid light modulation is reported, which multiplexes coarse-steering by sawtooth phase modulation and fine-steering by binary amplitude modulation. The hybrid steering is achieved by overlaying displayed Computer-Generated Holograms (CGHs) with a sawtooth blazed grating phase mask, which the blaze angle programmed by synchronized short-pulse illumination of transitioning micromirrors creating the CGHs. The steering principle is demonstrated as a 2D beam steering scheme with a 532 nm visible pulse laser, and implemented into a 905 nm lidar system with a 44° field-of-view, 0.9°×0.4° angular resolution, 7.8 FPS video frame rate, and 1 m detection distance.
The “Angular Spatial Light Modulator” (ASLM) utilizes digital micromirror device (DMD) as a binary patterned programmable blazed grating to increase number of output pixels of a DMD by merging geometric and diffractive optical capabilities of the DMD. We demonstrate series of capabilities of the ASLM for beam and pattern steering. In particular, a single-chip beam steering lidar, an extended FOV display, a light-field projector, and a multi-view display which can be implemented into AR/VR systems. We also present our metrology results of wavefront distortion of DMD while micro mirrors are transitioning over between on and off states.
Laser beam steering technology is essential for modern consumer and scientific optical devices including displays, microscopy, and Light Detection and Ranging (LIDAR) systems. Along with mechanical and completely non-mechanical beam steering approaches, Micro Electro Mechanical Systems (MEMS) are emerging beam steering devices that are especially suitable for LIDAR systems due to their fast scan rate and large scan angle. A class of MEMS-based devices, the Digital Micromirror Device (DMD), has been demonstrated for beam steering too by synchronizing its mirror movement to laser pulse. The tilt movement of micromirrors synchronizes with multiple pulses from multiple laser sources that sequentially redirect the pulses to multiple diffraction orders within μs. Based on the beam steering principle, multi-beam and multi-pulse beam steering in single-chip DMD LIDAR architecture provides a pathway to fast distance range finding having over 1M samples/s scan rate by leveraging a commercially available DMD, laser diodes and drivers. As a proof of concept, 3.34kHz and 15 points of range finding is demonstrated by using three pulsed laser diodes operating at 905nm. Additionally, multi-pulse beam steering for 5 points with an increased scanning rate of 6.63kHz demonstrates further enhancement of the scanning speed. The approach opens up a pathway to achieve a LIDAR system with a scanning rate over 1M samples/s while leveraging a state of the art DMD and a moderate number of laser sources.
Spatial light modulators (SLMs) that operate in a phase modulation mode enable beam steering with higher diffraction efficiency compared to amplitude modulation mode, thus potentially be used for an efficient beam steering with no moving part. Currently, Twisted Nematic phase SLMs are widely adopted for phase modulation. However, their refresh rate is typically in the range below kilohertz. Recently, a new method for binary and spatial phase modulation using Digital Micromirror Device (DMD) was proposed by a research group in Germany. In the method, complemental self-images of DMD, corresponding to on- and off-pixels, are formed by two auxiliary optics while adding a pi phase shift between two images. The optics function as recycling of light in a coherent manner. The method enables over kilohertz refresh rate and higher diffraction efficiency in binary phase modulation mode to conventional amplitude binary modulation.
As alternatives to the binary phase modulation, we propose and experimentally evaluated high-speed beam steering by DMD based on light recycling. In our experiment, with binary phase modulation mode, system output efficiency reaches 8%. It can be doubled to 16% with light recycling method. Efficiency is still low compared to the reported value of 27% without light recycling. To further increase beam efficiency, system loss was analysed.
A novel method of beam steering, utilizing a mass-produced Digital Micromirror Device (DMD), enables a reliable single chip Light Detection and Ranging (LIDAR) with a large field of view while having minimum moving components. In the single-chip LIDAR, a short-pulsed laser is fired in a synchronous manner to the micromirrors rotation during the transitional state. Since the pulse duration of the laser pulse is substantially short compared to the transitional time of the mirror rotation, virtually the mirror array is frozen in transition at several discrete points, which forms a programmable and blazed grating. The programmable blazed grating efficiently redirects the pulsed light to a single diffraction order among several while employing time of flight measurement. Previously, with a single 905nm nanosecond laser diode and Si avalanche photo diode, a measurement accuracy and rate of <1 cm and 3.34k points/sec, respectively, was demonstrated over a 1m distance range with 48° full field of view and 10 angular resolution. We have also increased the angular resolution by employing multiple laser diodes and a single DMD chip while maintaining a high measurement rate of 3.34k points/s. In addition, we present a pathway to achieve 0.65° resolution with 60° field of view and 23k points/s measurement rate.
A novel Digital Micromirror Device (DMD) based beam steering enables a single chip Light Detection and Ranging (LIDAR) system for discrete scanning points. We present increasing number of scanning point by using multiple laser diodes for Multi-beam and Single-chip DMD-based LIDAR.