In the battle for the route of autonomous driving lidar, at this stage, MEMS galvanometer scanning + ToF ranging is the biggest winner. In fact, MEMS and mechanical rotating mirror scanning structures are also the only LiDAR products that can be mass-produced and are small enough to be installed on passenger cars.
However, on the road to pure solid-state LiDAR in the future, some people in the industry believe that OPA (Optical Phased Array) scanning + FMCW ranging LiDAR will be the ultimate best solution. The only problem is that although OPA is the most efficient form of scanning in all aspects, so far no practical products have been implemented. Compared with other technologies such as Flash and FMCW that realize pure solid-state lidar, the progress of the implementation is Obviously a long way behind.
New progress of OPA lidar
Recently, Hao Hu and Yong Liu, two members of the research team at the Technical University of Denmark, published an article in the journal Optics, introducing a new chip-based beam steering technology. Hao Hu said their results will lay the foundation for a low-cost, compact lidar based on OPA.
In lidar, beam control is one of the most critical technologies. At present, mechanical beam control systems such as MEMS and rotating mirrors have some limitations. Due to the existence of mechanical components, they are more sensitive to vibration and have limited scanning speed. The chip-based OPA lidar can achieve fast and precise control of light without a mechanical structure. However, due to technical reasons, most of the current OPA devices have poor beam quality, and the field of view is difficult to exceed 100°.
The principle of OPA lidar is to form an emission array through multiple laser emission units, and to change the emission angle of the laser beam by adjusting the phase difference of each unit in the emission array, and to generate mutually reinforcing interference in the set direction to achieve high intensity the pointing beam to complete the scan.
All along, an important reason for the slow progress of OPA lidar is that the sidelobe effect is difficult to solve. The side lobe effect is a proximity effect due to optical diffraction. On the OPA lidar, the beam synthesis after the beam passes through the OPA device is actually formed by the mutual interference of light waves, so it is easy to form array interference, so that the laser energy is dispersed, and eventually optical artifacts and other problems occur. It is for this reason that it is difficult to balance the OPA field of view and beam quality.
So this time the research team designed a new type of OPA structure that replaces the traditional OPA multiple emitters with flat gratings. Since the adjacent channels in the flat grating can themselves be very close, interference and beam formation can be performed in close proximity to a single emitter, the coupling between adjacent channels does not interfere in the flat grating, and aliasing errors can be eliminated. At the same time, the researchers also employed other optical techniques to reduce background noise and reduce optical artifacts such as sidelobe effects.
In the actual measurement, this new OPA system can achieve ±70° beam steering without aliasing, but there is also some beam attenuation. The beam control in the vertical direction was tested by tuning the wavelength of the light source from 1480nm to 1580nm, and finally a tuning range of 13.5° in the vertical direction was achieved. The tests were carried out with a beam width of 2.1°, and the research team is also working on reducing the beam width, hoping to achieve higher resolution and beam steering over a longer range.
How are the major manufacturers progressing?
Overseas, Quanergy is the first company to enter the OPA lidar field, founded in Silicon Valley in 2012. In May, Quanergy announced that its OPA lidar achieved a detection distance of 250 meters, compared to 100 meters 15 months ago. According to Quanergy’s official introduction, its OPA technology is based on CMOS process-compatible silicon photonic chips, which can achieve mass production. But so far, Quanergy’s S-series OPA lidars are still difficult to achieve mass production and meet customer requirements at the same time. In addition, foreign OPA lidar players such as Analog Photonics, Voyant Photonics, and Scantinel Photonics are expected to achieve mass production after 2025.
There are also many domestic lidar manufacturers and scientific research institutions investing in OPA, including Luowei Technology, Wanji Technology, Lice Technology and so on. On the key silicon optical chip of OPA lidar, in July this year, Yangzhou Qunfa Heat Exchanger Co., Ltd. cooperated with the University of Michigan, saying that after years of research and development, it has mastered the new patented technology of OPA 2D/3D lidar, and completed multiple rounds of iterations in China Among them, the technical indicators of the OPA 2D lidar trial chip have met the design requirements, and will soon enter the stage of industrialization development. At the same time, the development of OPA 3D lidar chip is also in progress.
Another leading OPA lidar company in China, Luowei Technology, completed the tape-out of the second-generation silicon photonics FMCW SoC and OPA lidar silicon photonic chip in September last year. As mentioned earlier, may FMCW+OPA be the ultimate form of lidar? Luowei Technology also established the development direction of pure solid-state lidar using FMCW and OPA technology at the beginning of its establishment. According to the official website of Luowei Technology, the D series pure solid-state lidar using the LuminScan beam control system has been launched. Based on the self-developed silicon photonic chip, the detection distance is up to 30m, the field of view angle is 120°×90°, and the angular resolution is 0.3° ×0.3°.
In 2019, Lice Technology stated that it will achieve complete mass production of OPA lidar in 2020. At present, a long-distance OPA lidar has appeared on the official website, the model is LT-X. The LT-X scanning field angle is 60°×60°, using a light source with a wavelength of 905nm, the maximum measurement distance can be greater than 200m, and the sidelobe suppression is <30db.
Overall, OPA lidar may be 2-3 years later than other pure solid-state lidars, such as products using Flash, FMCW and other technologies. However, it is chip-based, highly integrated, low-cost after mass production in the future, and has the advantages of strong anti-interference ability, long detection distance, and high scanning frequency. OPA technology has the potential to unify the autonomous driving lidar industry in the future. With the improvement of silicon photonic chip technology and the further improvement of beam quality, it is believed that OPA technology will be chosen by more lidar manufacturers.