This is a list of reviewers who served the Journal of Optical Microsystems in 2021.

Guest editor Yuzuru Takashima introduces the Special Section on MEMS Lidar.

High performance distributed Bragg reflectors (DBRs) are key elements to achieving high finesse MEMS-based Fabry–Pérot interferometers (FPIs). Suitable mechanical parameters combined with high contrast between the refractive indices of the constituent optical materials are the main requirements. In this paper, Germanium (Ge) and barium fluoride (BaF₂) optical thin-films have been investigated for mid-wave infrared (MWIR) and long-wave infrared (LWIR) filter applications. Thin-film deposition and fabrication processes were optimised to achieve mechanical and optical properties that provide flat suspended structures with uniform thickness and maximum reflectivity. Ge-BaF₂-Ge 3-layer solid-material DBRs have been fabricated that matched the predicted simulation performance, although a degradation in performance was observed for wavelengths beyond 10  μm that is associated with optical absorption in the BaF₂ material. Ge-Air-Ge 3-layer air-gap DBRs, in which air rather than BaF₂ served as the low refractive index layer, were realized to exhibit layer flatness at the level of 10 to 20 nm across lateral DBR dimensions of several hundred micrometers. Measured DBR reflectance was found to be ≳90  %   over the entire wavelength range of the MWIR band and for the LWIR band up to a wavelength of 11  μm. Simulations based on the measured DBR reflectance indicates that MEMS-based FPIs are able to achieve a peak transmission of ≳90  %   over the entire MWIR band and up to 10  μm in the LWIR band, with a corresponding spectral passband of ≲50  nm in the MWIR and <80  nm in the LWIR.

Lens-assisted beam steering (LABS) has emerged as a promising solution for compact chip-based optical beam steering for light detection and ranging (LiDAR) applications. In a LABS system, light is steered within an integrated optical chip and emitted at a desired location. This emitted light is focused out into the scene with a lens, analogous to a camera operating in reverse. LABS systems offer many advantages compared to competing technologies such as solid-state reliability, simple control, compactness, and fast random access scanning. Different methods for LABS systems are described and compared. Most LABS systems demonstrated thus far have small arrays, and therefore, only offer a relatively small number of possible beam locations. It is important to understand how these systems will scale to the much larger arrays needed for a practical LiDAR system.

We present the design and system integration of a hybrid MEMS scanning mirror (MSM) array developed for real-time three-dimensional imaging with a panoramic optical field of view (FOV) of 360  deg  ×  60  deg (horizontal  ×  vertical). The pulsed time-of-flight light detection and ranging (LiDAR) system targets a distance measurement range of 100 m with a video-like frame rate of 10 Hz. The fast vertical scan axis is realized by a synchronous scanning MSM array with large receiver aperture. It increases the scanning rate to 3200 Hz, which is four times faster in comparison with state-of-the-art fast macroscopic polygon scanning systems used in current LiDAR systems. A hybrid assembly of frequency selected scanner elements was chosen instead of a monolithic MEMS array to guaranty high yield of MEMS fabrication and a synchronous operation of all resonant MEMS elements at 1600 Hz with large FOV of 60 deg. The hybrid MSM array consists of a separate emitting mirror for laser scanning of the target and 22 reception elements resulting in a large reception aperture of D_eff  =  23  mm. All MSM are driven in parametric resonance to enable a fully synchronized operation of all individual MEMS scanner elements. Therefore, piezoresistive position sensors are integrated inside the MEMS chip, used for position feedback of the driving control. We focus on the MEMS system integration including the microassembly of multiple MEMS scanning elements using micromechanical self-alignment. We present technical details to meet the narrow tolerance budgets for (i) microassembly and (ii) synchronous driving of multiple MEMS scanner elements.

In many applications, there is a great demand for reliable, small, and low-cost three-dimensional imaging systems. Promising systems for applications such as automotive applications as well as safe human robotic collaboration are light detection and ranging (lidar) systems based on the direct time-of-flight principle. Especially for covering a large field of view or long-range capabilities, the previously used polygon-scanners are replaced by microelectromechanical systems (MEMS)-scanners. A more recent development is to replace the typically used avalanche photodiodes with single-photon avalanche diodes (SPADs). The combination of both technologies into a MEMS-based SPAD lidar system promises a significant performance increase and cost reduction compared with other approaches. To distinguish between signal and background/noise photons, SPAD-based detectors have to form a histogram by accumulating multiple time-resolved measurements. In this article, a signal and data processing method is proposed, which considers the time-dependent scanning trajectory of the MEMS-scanner during the histogram formation. Based on known reconstruction processes used in stereo vision setups, an estimate for an accumulated time-resolved measurement is derived, which allows to classify it as signal or noise. In addition to the theoretical derivation of the signal and data processing, an implementation is experimentally verified in a proof-of-concept MEMS-based SPAD lidar system.

Piezoelectrically actuated resonant micromirrors were designed to meet the light detection and ranging (LiDAR) system requirements. Key features were a 3-mm mirror aperture, a 40-deg field of view, and a 50-Hz refresh rate. The presented micromirror provides biaxial symmetrical beam steering with ±12.7  deg mechanical tilt angle, resulting in a 50-deg field of view with an adjustable Lissajous XY-scanning pattern for a forward-looking LiDAR system. The mirrors were fabricated using silicon on insulator wafers, and actuation was based on piezoelectric aluminium nitride thin film. The mirrors were vacuum packaged for high-quality factor resonator operation. The device design contained eight separate piezoelectric aluminium nitride elements arranged as differential pairs for each axis, where each actuator was equipped with a sensing element providing a mechanically coupled electrical feedback signal. The piezoelectric elements connected as actuators required only minimal power and were directly compatible with CMOS low-voltage logic, which eases integration to driving digital systems. The sense elements are used to monitor phase, amplitude, and frequency. A digital control system connected to each of these elements provides accurate frequency and phase control of independent orthogonal resonators, permitting control of the X and Y amplitudes and the refresh rate of the Lissajous pattern.

We describe the verification of a long-range one-dimensional (1D) scanning micro-electro-mechanical systems (MEMS) lidar specifically considering the robustness against external vibration influences. The 1D scanning MEMS lidar exploits a multichannel horizontal line laser to scan the scene vertically for a 10  deg  ×  11  deg horizontal and vertical field of view at a frame rate of up to 29 Hz. To evaluate the robustness against vibrations, a vibration evaluation setup is developed to apply a wideband vibration based on the automotive standard LV124. The vibration tests are performed in three conditions open loop without control and two phase-locked loops (PLLs) with default and high gain settings. The test results demonstrate that vibration can cause wobbly distortion along the scan angle in the open loop case and the PLLs can suppress effectively this influence in the mean and standard deviation of the standard point to surface error up to 69.3% and 90.0%, respectively. This verifies the benefits of the MEMS mirror control, ensuring stable point cloud measurements under vibrations in harsh automotive environments.