TECHNICAL FIELD

The present disclosure relates to an optical beam-steering device, such as a microelectromechanical system (MEMS) mirror used in light detection and ranging (LiDAR) systems, and more particularly to, systems and methods for detecting resonant frequencies of the MEMS mirror using an acoustic detection system.

BACKGROUND

LiDAR systems have been widely used in advanced navigation technologies, such as to aid autonomous driving or to generate high-definition maps. For example, a typical LiDAR system measures the distance to a target by illuminating the target with pulsed laser light beams and measuring the reflected pulses with a sensor. Differences in laser light return times, wavelengths, and/or phases can then be used to construct digital three-dimensional (3D) representations of the target. Because using a narrow laser beam as the incident light can map physical features with very high resolution, a LiDAR system is particularly suitable for applications such as sensing in autonomous driving and high-definition map surveys.

Optical components, e.g., a MEMS mirror, used in LiDAR systems includes a micro mirror that can be moved (e.g., rotated) by an actuator to reflect (and steer) incident laser beams a range of directions in order to scan a field of view (FOV). The MEMS mirror can be a single micro mirror, or an array of micro mirrors integrated into a micromachined mirror assembly. When operating under a certain frequency, a MEMS mirror oscillates and rotates for a certain tilting angle, thus reflecting light beams at a reflection angle. Such a frequency is known as the resonant frequency of the MEMS mirror. Therefore, it is important to know the resonant frequency and its corresponding mirror tilting angle in order to effectuate the desired tilting movements of the MEMS mirror during operation.

However, due to fabrication process variations, resonant frequency of each of the MEMS mirror of the same array of micromachined mirror assembly(s) may vary. For example, a structure etched on a single crystalline wafer may suffer from greater fabrication process variations (e.g., the lower part of the structure is narrower than the design due to the etching process) when the structure is closer to the edge of the wafer compared to if the structure is located in the center of the wafer. Thus, MEMS mirrors in the same array of micromachined mirror assembly may have different resonant frequencies. Measuring the resonant frequency of each MEMS mirror becomes especially important when multiple MEMS mirrors need to be synchronized to operate in the same MEMS array.

Conventionally, resonant frequencies of MEMS mirrors are detected using optical detection methods. For example,FIG. 1illustrates a schematic diagram of a prior art resonant frequency detection system100(hereafter, “prior art system100”) for detecting the resonant frequency of a MEMS mirror. Prior art system100shown inFIG. 1includes a laser source102and a light detector103for detecting the resonant frequency of a MEMS mirror110in a micromachined mirror assembly101(also referred to as mirror assembly101). At its resonant frequency, MEMS mirror110can oscillate and rotate around an axis109for a certain tilting angle (e.g., tilting angle (3), thereby directing incident laser beams from a laser source towards a plurality of directions.

Prior art system100uses a driving circuit to conduct a frequency sweeping over a range of frequencies to drive MEMS mirror110to oscillate. When the sweeping frequency is the resonant frequency, MEMS mirror110will resonant and rotate around axis109to a corresponding tilting angle. Prior art system100then uses the optical sensing system to sense the tilting angle. For example, laser source102emits a laser beam to a reflective surface of MEMS mirror110. The laser beam may be reflected by the reflective surface of MEMS mirror110and then received by light detector103. The tilting angle of MEMS mirror110can be determined based on a position of a light spot on the surface of light detector103where the reflected laser beam is received. Prior art system100then associates the determined titling angle with the resonant frequency.

However, resonant frequency detection systems relying on optical sensing, such as prior art system100has various drawbacks. In particular, the performance of these detection systems is limited at large tilting angles. For example,FIG. 1illustrates a scenario that light detector103fails to receive a reflected laser beam104. The failure may be caused by the small surface dimensions of light detector103, insufficient to capture the reflected beam at large tilting angle of MEMS mirror110. As a result, resonant frequencies for mirrors with large mirror tilting angles will go undetected by prior art system100.

On the other hand, misalignment of the optical detection system and the mirror assembly may also contribute to errors of the detection. However, aligning the optical detection system to a MEMS mirror for each setup is unpractical for large batch wafer level testing.

Embodiments of the disclosure address the above problems by providing an acoustic method for detecting the resonant frequency of the MEMS mirror.

SUMMARY

Embodiments of the disclosure provide a method for detecting a resonant frequency of an optical beam-steering device. The method may include driving the optical beam-steering device with a driving signal oscillating at a plurality of frequencies. The method may also include detecting, by an acoustic detector, an acoustic signal caused by a movement of the optical beam-steering device. The method may further include analyzing a spectrum, by a controller, of the acoustic signal. The method may additionally include determining, by the controller, the resonant frequency of the optical beam-steering device based on the spectrum.

Embodiments of the disclosure also provide a system for detecting a resonant frequency of an optical beam-steering device. The system may include an acoustic detector configured to detect an acoustic signal caused by a movement of the optical beam-steering device when driven by a driving signal oscillating at a plurality of frequencies. The system may also include a controller operatively coupled to the acoustic detector. The controller may be configured to analyze a spectrum of the acoustic signal. The controller may also be configured to determine the resonant frequency of the optical beam-steering device based on the spectrum.

Embodiments of the disclosure further provide a method for detecting resonant frequencies of MEMS mirrors. The method may include placing an acoustic detector in a proximity of a first MEMS mirror. The method may also include driving the first MEMS mirror with a driving signal oscillating at a plurality of frequencies. The method may further include detecting, by the acoustic detector, a first acoustic signal caused by a movement of the first MEMS mirror. The method may additionally include moving the acoustic detector into a proximity of a second MEMS mirror. The method may further include driving the second MEMS mirror with the driving signal. The method may additionally include detecting, by the acoustic detector, a second acoustic signal caused by a movement of the second MEMS mirror. The method may further include determining, by a controller, the resonant frequencies of the first and second MEMS mirrors based on the first and second acoustic signals, respectively.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.

