In recent years, Light Detection and Ranging (LiDAR) has been drawing extensive attention both in academia and industry because of the increasing demand for autonomous vehicles. LiDAR is believed to be the crucial sensor for autonomous driving and flying, as it can provide high-density point clouds with accurate three-dimensional information. This review presents an extensive overview of Microelectronechanical Systems (MEMS) scanning mirrors specifically for applications in LiDAR systems. MEMS mirror-based laser scanners have unrivalled advantages in terms of size, speed and cost over other types of laser scanners, making them ideal for LiDAR in a wide range of applications. A figure of merit (FoM) is defined for MEMS mirrors in LiDAR scanners in terms of aperture size, field of view (FoV) and resonant frequency. Various MEMS mirrors based on different actuation mechanisms are compared using the FoM. Finally, a preliminary assessment of off-the-shelf MEMS scanned LiDAR systems is given.

MEMS Mirrors for LiDAR: A review.

In recent years, light detection and ranging (LiDAR) technology has gained huge popularity in various applications such as navigation, robotics, remote sensing, and advanced driving assistance systems (ADAS). This popularity is mainly due to the improvements in LiDAR performance in terms of range detection, accuracy, power consumption, as well as physical features such as dimension and weight. Although a number of literatures on LiDAR technology have been published earlier, not many has been reported on the state-of-the-art LiDAR scanning mechanisms. The aim of this article is to review the scanning mechanisms employed in LiDAR technology from past research works to the current commercial products. The review highlights four commonly used mechanisms in LiDAR systems: Opto-mechanical, electromechanical, micro-electromechanical systems (MEMS), and solid-state scanning. The study reveals that electro-mechanical scanning is the most prominent technology in use today. The commercially available 1D time of flight (TOF) LiDAR instrument is currently the most attractive option for conversion from 1D to 3D LiDAR system, provided that low scanning rate is not an issue. As for applications with low size, weight, and power (SWaP) requirements, MEMS scanning is found to be the better alternative. MEMS scanning is by far the more matured technology compared to solid-state scanning and is currently given great emphasis to increase its robustness for fulfilling the requirements of ADAS applications. Finally, solid-state LiDAR systems are expected to fill in the gap in ADAS applications despite the low technology readiness in comparison to MEMS scanners. However, since solid-state scanning is believed to have superior robustness, field of view (FOV), and scanning rate potential, great efforts are given by both academics and industries to further develop this technology.

/pdf/a-survey-on-lidar-scanning-mechanisms-51aocuuik2.pdf

A Survey on LiDAR Scanning Mechanisms

Accurate three-dimensional (3D) imaging is essential for machines to map and interact with the physical world1,2 Although numerous 3D imaging technologies exist, each addressing niche applications with varying degrees of success, none has achieved the breadth of applicability and impact that digital image sensors have in the two-dimensional imaging world3–10 A large-scale two-dimensional array of coherent detector pixels operating as a light detection and ranging system could serve as a universal 3D imaging platform Such a system would offer high depth accuracy and immunity to interference from sunlight, as well as the ability to measure the velocity of moving objects directly11 Owing to difficulties in providing electrical and photonic connections to every pixel, previous systems have been restricted to fewer than 20 pixels12–15 Here we demonstrate the operation of a large-scale coherent detector array, consisting of 512 pixels, in a 3D imaging system Leveraging recent advances in the monolithic integration of photonic and electronic circuits, a dense array of optical heterodyne detectors is combined with an integrated electronic readout architecture, enabling straightforward scaling to arbitrarily large arrays Two-axis solid-state beam steering eliminates any trade-off between field of view and range Operating at the quantum noise limit16,17, our system achieves an accuracy of 31 millimetres at a distance of 75 metres when using only 4 milliwatts of light, an order of magnitude more accurate than existing solid-state systems at such ranges Future reductions of pixel size using state-of-the-art components could yield resolutions in excess of 20 megapixels for arrays the size of a consumer camera sensor This result paves the way for the development and proliferation of low-cost, compact and high-performance 3D imaging cameras that could be used in applications from robotics and autonomous navigation to augmented reality and healthcare A compact, high-performance silicon photonics-based light detection and ranging system for three-dimensional imaging is developed that should be amenable to low-cost mass manufacturing

A universal 3D imaging sensor on a silicon photonics platform

Lidar, the acronym of light detection and ranging, has received much attention for the automotive industry as a key component for high level automated driving systems due to their high resolution and highly accurate 3D imaging of the surroundings under various weather conditions. However, the price and resolution of lidar sensors still do not meet the target values for the automotive market to be accepted as a basic sensor for ensuring safe autonomous driving. Recent work has focused on MEMS scanning mirrors as a potential solution for affordable long range lidar systems. This paper discusses current developments and research on MEMS-based lidars. The LiDcAR project is introduced for bringing precise and reliable MEMS-based lidars to enable safe and reliable autonomous driving. As a part of development in this project, a test bench for the characterization and performance evaluation of MEMS mirror is introduced. A recently developed MEMS-based lidar will be evaluated by various levels of tests including field tests based on realistic scenarios, aiming for safe and reliable autonomous driving in future automotive industry.

/pdf/mems-based-lidar-for-autonomous-driving-n0tlemfx4m.pdf

MEMS-based lidar for autonomous driving

The SEMATECH High-NA Actinic Reticle review Project (SHARP) is a synchrotron-based, EUV-wavelength microscope, dedicated to photomask imaging, now being commissioned at Lawrence Berkeley National Laboratory. In terms of throughput, resolution, coherence control, stability and ease of use, SHARP represents a significant advance over its predecessor, the SEMATECH Berkeley Actinic Inspection Tool (AIT), which was decommissioned in September 2012. SHARP utilizes several advanced technologies to achieve its design goals: including the first Fouriersynthesis illuminator on a zoneplate microscope, EUV MEMS mirrors, and high-efficiency freestanding zoneplate lenses
with numerical aperture values up to 0.625 (4×). In its first week of operation, SHARP demonstrated approximately 150 times higher light throughput than AIT and a spatial resolution down to 55-nm half-pitch with 0.42 4×NA (i.e. the smallest feature size on our test mask.) This paper describes the current status of the tool commissioning and the performance metrics available at this early stage.

/pdf/commissioning-an-euv-mask-microscope-for-lithography-2xeccfhyj6.pdf

Commissioning an EUV mask microscope for lithography generations reaching 8 nm

Firstly, we demonstrated a wirelessly controlled MEMS scan module with imaging and laser tracking capability which can be mounted and flown on a small UAV quadcopter. The MEMS scan module was reduced down to a small volume of <90mm x 60mm x 40mm, weighing less than 40g and consuming less than 750mW of power using a ~5mW laser. This MEMS scan module was controlled by a smartphone via Bluetooth while flying on a drone, and could project vector content, text, and perform laser based tracking. Also, a “point-and-range” LiDAR module was developed for UAV applications based on low SWaP (Size, Weight and Power) gimbal-less MEMS mirror beam-steering technology and off-the-shelf OEM LRF modules. For demonstration purposes of an integrated laser range finder module, we used a simple off-the-shelf OEM laser range finder (LRF) with a 100m range, +/-1.5mm accuracy, and 4Hz ranging capability. The LRFs receiver optics were modified to accept 20° of angle, matching the transmitter‟s FoR. A relatively large (5.0mm) diameter MEMS mirror with +/-10° optical scanning angle was utilized in the demonstration to maintain the small beam divergence of the module. The complete LiDAR prototype can fit into a small volume of <70mm x 60mm x 60mm, and weigh <50g when powered by the UAV‟s battery. The MEMS mirror based LiDAR system allows for ondemand ranging of points or areas within the FoR without altering the UAV‟s position. Increasing the LRF ranging frequency and stabilizing the pointing of the laser beam by utilizing the onboard inertial sensors and the camera are additional goals of the next design.

UAV-borne lidar with MEMS mirror-based scanning capability

In 2016, we presented a low SWaP wirelessly controlled MEMS mirror-based LiDAR prototype which utilized an OEM laser rangefinder for distance measurement [1]. The MEMS mirror was run in open loop based on its exceptionally fast design and high repeatability performance. However, to further extend the bandwidth and incorporate necessary eyesafety features, we recently focused on providing mirror position feedback and running the system in closed loop control. Multiple configurations of optical position sensors, mounted on both the front- and the back-side of the MEMS mirror, have been developed and will be presented. In all cases, they include a light source (LED or laser) and a 2D photosensor. The most compact version is mounted on the backside of the MEMS mirror ceramic package and can “view” the mirror‟s backside through openings in the mirror‟s PCB and its ceramic carrier. This version increases the overall size of the MEMS mirror submodule from ~12mm x 12mm x 4mm to ~15mm x 15mm x 7mm. The sensors also include optical and electronic filtering to reduce effects of any interference from the application laser illumination. With relatively simple FPGA-based PID control running at the sample rate of 100 kHz, we could configure the overall response of the system to fully utilize the MEMS mirror‟s native bandwidth which extends well beyond its first resonance. When compared to the simple open loop method of suppressing overshoot and ringing which significantly limits bandwidth utilization, running the mirrors in closed loop control increased the bandwidth to nearly 3.7 times. A 2.0mm diameter integrated MEMS mirror with a resonant frequency of 1300 Hz was limited to 500Hz bandwidth in open loop driving but was increased to ~3kHz bandwidth with the closed loop controller. With that bandwidth it is capable of very sharply defined uniform-velocity scans (sawtooth or triangle waveforms) which are highly desired in scanned mirror LiDAR systems. A 2.4mm diameter mirror with +/-12° of scan angle achieves over 1.3kHz of flat response, allowing sharp triangle waveforms even at 300Hz (600 uniform velocity lines per second). The same methodology is demonstrated with larger, bonded mirrors. Here closed loop control is more challenging due to the additional resonance and a more complex system dynamic. Nevertheless, results are similar - a 5mm diameter mirror bandwidth was increased from ~150Hz to ~500Hz.

Closed-loop control of gimbal-less MEMS mirrors for increased bandwidth in LiDAR applications

A high brightness Head-Up Display (HUD) module was demonstrated with a fast, dual-axis MEMS mirror that displays
vector images and text, utilizing its ~8kHz bandwidth on both axes. Two methodologies were evaluated: in one, the
mirror steers a laser at wide angles of <48° on transparent multi-color fluorescent emissive film and displays content
directly on the windshield, and in the other the mirror displays content on reflective multi-color emissive phosphor plates
reflected off the windshield to create a virtual image for the driver. The display module is compact, consisting of a
single laser diode, off-the-shelf lenses and a MEMS mirror in combination with a MEMS controller to enable precise
movement of the mirror’s X- and Y-axis. The MEMS controller offers both USB and wireless streaming capability and
we utilize a library of functions on a host computer for creating content and controlling the mirror. Integration with
smart phone applications is demonstrated, utilizing the mobile device both for content generation based on various
messages or data, and for content streaming to the MEMS controller via Bluetooth interface. The display unit is highly
resistant to vibrations and shock, and requires only ~1.5W to operate, even with content readable in sunlit outdoor
conditions. The low power requirement is in part due to a vector graphics approach, allowing the efficient use of laser
power, and also due to the use of a single, relatively high efficiency laser and simple optics.

High Brightness MEMS Mirror Based Head-Up Display (HUD) Modules with Wireless Data Streaming Capability

In this work we demonstrate a highly flexible laser imaging system for 3D sensing applications such as in tracking of VR/AR headsets, hands and gestures. The system uses a MEMS mirror scan module to transmit low power laser pulses over programmable areas within a field of view and uses a single photodiode to measure the reflected light. User can arbitrarily select the number of pixels to scan over an area and can thus obtain images of target objects at arbitrarily fast rates. The work builds on the previously presented “MEMSEye” laser tracking technology which uses a fast steering MEMS scan module with a modulated laser, and a tuned photosensor to acquire and track a retroreflector-marked object. To track VR/AR headsets, hands and similar objects with multiple markers or no markers at all, a single-point tracking methodology is not sufficient. Cameras could be more appropriate in such multi-point imaging cases but suffer from low frame rates, dependence on ambient lighting, and relatively low resolution when without zooming and panning capability. A hybrid method can address the problem by providing a system with its own light source (laser beam), and with full programmability of the pixel locations and scans such that frame rates of >100 Hz are possible over specific areas of interest. With a modest 1 Mpixel rate of measurement, scanning a sub-region of the field of view with 64 x 64 pixels results in ~200Hz update. Multiple such modules can be used to scan and image or track objects with multiple markers and fully obtain their position and attitude in a room with sub-5ms updates. Furthermore the room itself could be imaged and measured with wall markers or in conjunction with a camera for a total 3D scanning solution. Proof of concept demonstrator is presented here with pixel rates of only 30k-50k per second due to limitations of the present prototype electronics, resulting in refresh rates that are significantly lower than possible with the MEMS mirror scan modules.

Veljko Milanovic

Papers

UAV-borne lidar with MEMS mirror-based scanning capability

Commissioning an EUV mask microscope for lithography generations reaching 8 nm

Closed-loop control of gimbal-less MEMS mirrors for increased bandwidth in LiDAR applications

High Brightness MEMS Mirror Based Head-Up Display (HUD) Modules with Wireless Data Streaming Capability

A fast single-pixel laser imager for VR/AR headset tracking