Automotive applications require: high performance and mass production capability
For LiDAR to be used in autonomous vehicles, it must meet two fundamental requirements: firstly, high performance, including long range and wide field of view; and secondly, mass production feasibility to enable manufacturing and installation in millions of vehicles. LiDAR manufacturers have devised a range of solutions to address these challenges. Mechanical LiDAR systems are currently the most common, with motors rotating to drive beam deflection units. While mechanical LiDAR offers a wide field of view and long-range detection, its mechanical components require regular maintenance, and it is also bulky, heavy, and costly to produce. Overall, mechanical LiDAR only meets the high performance requirement.
Another approach to addressing these challenges is MEMS (Micro-Electro-Mechanical Systems) technology. This technology, where components are manufactured using silicon, offers the advantage of mass production: thanks to years of application and testing, all components can be mass-produced in a cost-effective manner.
How does MEMS-based lidar address the challenge of achieving high performance?
Choosing a suitable laser source enables long-distance detection.
For autonomous vehicles to drive safely at high speeds, they must be able to "see" and perceive the world around them—not just nearby, but also at greater distances. This is especially important on highways, where vehicles travel at high speeds and must reliably detect distant objects, curves, and other vehicles to react promptly. Therefore, sensors need to detect objects at long distances for autonomous driving to be possible on highways.
To achieve this detection range, both the transmitter and detector of a lidar sensor need to be specifically optimized for this application. The first thing that might be adjusted is the laser source. Typically, there are two wavelengths of laser source used in lidar sensors. Some lidar manufacturers use fiber lasers with a wavelength of 1550 nanometers. The human retina does not focus laser light of this wavelength, thus meeting eye safety standards even at high energy levels. The higher the energy of the laser source in this type of lidar, the longer the detection range. However, this type of laser source also has a fatal drawback: 1550nm lasers are large and complex to manufacture, resulting in a large lidar housing and high cost.
Therefore, many lidar applications use laser diodes with a laser pulse emission wavelength of 905nm. Their significant advantages are their small size and their long-standing widespread use in other fields. In fact, these diodes are inexpensive and readily available in the market. However, eye safety regulations require the beam intensity of the diodes to be lower than that of a 1550nm laser, thus limiting optimization on the emitter side.
Finding the right size for the lens
So how do we optimize the detector? Aperture plays a crucial role in achieving long-distance detection. In MEMS-based designs, aperture corresponds to mirror size. To capture as much light as possible, a large aperture is needed—in other words, a mirror that is as large as possible. However, the size of the mirror is also limited by certain factors, so it is necessary to consider these factors and calculate the optimal size of the mirror. These factors include: the number of photons received, collimation, deflection angle, and resonant frequency.
Number of photons
On one hand, the size of the lens used in lidar depends on how many photons must be emitted to return a sufficient number to detect the target. The minimum number of photons can be accurately calculated based on link estimation. This measurement needs to consider the number of photons lost over the distance and through low-reflectivity surfaces, uniform scattering of light, and detector efficiency. Using this method, it becomes possible to calculate how many photons must be emitted, or what aperture size, to detect the minimum number of photons again. Furthermore, the sensor employs a coaxial design, meaning that only light returning from the same direction of emission can be recaptured. This is advantageous for the sensor, preventing the capture of other interfering light signals.
Laser collimation
To obtain high-resolution data and reliably identify small targets, the laser must be collimated to the target. This is achieved by placing a lens in front of the laser. Now, the size of the lens comes into play again: the lens must be large enough to deflect all the light collimated by the lens.
Resonance frequency
MEMS mirrors oscillate at a specific resonant frequency. They are triggered by integrated actuators, thus eliminating the need for motors or any other mechanical devices. This is a significant advantage, as motors and moving parts wear out quickly and require regular maintenance. These issues are avoided if the oscillation is triggered by an integrated actuator. The resonant frequency of the mirror depends on its size and mounting method. To address this, we have developed a proprietary technique for embedding mirrors, enabling the use of exceptionally large mirror sizes. Due to the very large diameter, a large number of photons directly enter the surrounding environment and return to the detector. This allows the lidar sensor to achieve precise long-range detection. Furthermore, due to their larger size, these mirrors are more durable than traditional products with diameters of only a few millimeters. The mirrors used in Hongke lidar, due to their lightweight structure and high resonant frequency, ensure that as many photons as possible return to the detector: if the mirror oscillates too fast or too slow, the detector can still receive a sufficient number of photons due to the coaxial structure.
MEMS technology specifically designed for LiDAR applications
In summary, the size of a lens is determined by a series of factors. To manufacture high-performance MEMS-based lidar, it is essential to study the composition, size, and embedding method of the reflector. Only by combining MEMS technology with lidar applications can the requirements of long range, wide field of view, and high resolution be achieved.
