LiDAR: The War Between 905 and 1550

As market competition gradually enters the deep water zone, the words "mechanical rotation", "semi-solid", "rotating mirror", "prism" and " MEMS " that are used to describe scanning methods are no longer very new and cannot even arouse the interest of others in participating in the discussion. The laser transmitting and receiving system is becoming a new focus of the market.

According to the different system integration levels, the laser emission systems of TOF laser radars that have entered the mass production stage can be divided into EEL and VCSEL, and the receiving systems can be divided into APD and SPAD; and according to the different wavelengths of the light source, the laser transceiver systems are mainly divided into two categories: 905 and 1550. Because it touches on key points such as "eye safety", "detection distance", "power consumption" and "cost", the classification of 905 and 1550 is worth discussing.

905nm and 1550nm are both wavelengths of light. Each wavelength has its own characteristics, such as the characteristics of reception, transmission, interference, and impact on the human eye. In fact, there is no difference between the two in themselves, but when applied to lidar products , they will show their own "advantages and disadvantages" in different aspects.

Currently, both 905 and 1550 lidar products have entered the mass production and delivery stage. As for which technology route is "better", lidar manufacturers naturally have different opinions. Moreover, if we only look at it from a single perspective, the manufacturers' statements are often "absolutely true". However, according to the person in charge of a lidar manufacturer, "automakers are not particularly satisfied with both 905 and 1550 now."

From a third-party perspective, it is irresponsible to simply and rashly say "who is better than who". Compared with this, it is more meaningful to clarify many misunderstandings and doubts by unraveling the issues layer by layer.

In order to further explore the "truth", in the process of writing this article, Jiuzhang Zhijia interviewed not only several manufacturers focusing on 905 laser radars, but also three manufacturers focusing on 1550 lasers, with a total of more than 15 people interviewed. This article has in fact become a "debate platform" for the two factions.

In order to prevent any technical route from being accidentally harmed by the "private agenda" remarks of individual interviewees, and of course to avoid being misled by the "one-sided words" of any interviewee, the author has submitted the first draft of the article to several lidar manufacturers with different technical routes for confirmation. This not only gives the "attacked party" an opportunity to "defend itself", but also makes the content of the article as objective as possible.

Note 1: In FMCW, 1550 is a better choice than 905. This has become a consensus in the industry. Therefore, the subsequent comparison between 1550 and 905 in this article refers specifically to the TOF technology route.

 

Note 2: The interviewees for this article include but are not limited to Hesai Technology Director and Tudatong CEO Bao Junwei, Onna Technology Automotive Business Director Dang Na, Wanji LiDAR Chief Engineer Hu Panpan, Eagle Semiconductor Liang Renqiao, Fushi Technology Chairman Assistant Wang Lizidong, Ouster China Marketing Director Liu Zhigang, Hamamatsu Photonics Wu Jiazhe, Zonghui Xingguang Business Manager Huang Xinfeng, Xiante Technology Li Dan, Yingxun Xingguang Liu Shaodong, and optical communication expert Kuang Guohua.

 

Note 3: In some places, this article only quotes the views of a certain interviewee, but this view may not be agreed by other interviewees, and there may even be cases where other interviewees "completely disagree". Similarly, in some cases, the author believes that most interviewees have "reached a consensus" on a certain view, so the specific source of the view is not noted. For such a view, there is still the possibility that some interviewees "completely disagree".

1. The advantages of 1550 are not limited to "detection distance"

When it comes to 1550 laser radar, everyone's first reaction is that compared with 905, the most obvious advantage of 1550 is that it can operate at a higher luminous power while ensuring the safety of human eyes, thus achieving a longer detection distance.

Tudatong CEO Bao Junwei believes that for vehicle safety, a laser radar detection distance of 250 meters is the "passing line", but at present, "only 1550 can reach this level."

Bao Junwei said:

"Usually, when people discuss the detection range of lidar, they are talking about targets with a reflectivity of 10%. But in fact, in some extreme cases, small targets with a reflectivity of 5% should also be visible - not only should they be visible, but they should also not be just a dot."

"According to the actual test results of us and some customers, although some 905nm LiDARs have a detection distance of 150 meters for very large targets with a reflectivity of 10%, and a detection distance of about 100 meters for relatively dark large targets with a reflectivity of 5%, due to the divergence angle and other reasons, its detection distance for small targets with a reflectivity of 5% is only about 40 meters. Tudatong's 1550 LiDAR has been extensively tested on targets with a reflectivity of 5% and a size of 20-30cm."

So, what else?

Before answering this question, we need to popularize a basic knowledge that is easily overlooked - the application of the 1550 band in the field of lidar is earlier than 905. For example, the lidar used in the field of high-end surveying and mapping is 1550, and this started two or three decades ago.

The reason why high-end surveying and mapping fields prefer to use 1550 is not the "detection distance", but the fact that the beam divergence angle of 1550 is much smaller than that of 905.

A large divergence angle means that the farther the light beam propagates, the larger the light spot will be, which may result in insufficient energy returned after the light beam hits a small object in the distance, resulting in failure to measure. Secondly, it will also affect the accuracy of the point cloud in complex scenes. If the light spot is so large that it hits two objects that are very close to each other at the same time, and the laser radar cannot decode and distinguish the two objects, a "false point" may be formed between the two objects; when encountering objects that are separated by height, such as zebra crossings or checkerboards, "wavy lines" may even be measured.

According to the technical director of Hesai, the luminous surface of the 905 laser is several hundred microns, while the luminous surface of the 1550 fiber laser is only about 10 microns. The smaller the luminous surface, the smaller the far-field divergence angle of the light spot can be suppressed. This means that compared with the 905, the 1550 can have better measurement of small objects at a long distance in terms of laser divergence angle.

In addition, many companies believe that the 1550 has much stronger anti-interference capabilities than the 905.

1550 is a fiber laser, and the light energy is concentrated in the waveform head of a very thin single-film fiber laser, so the light emitted has high density and extremely high brightness. The light emitted by the laser radar needs to overwhelm other external light sources in brightness to resist the interference of the latter. Therefore, the light emitted by the 1550 fiber laser has an advantage in brightness over the 905, which also means that it has a stronger ability to resist interference such as sunlight.

However, Hesai said that this view is one-sided. Whether the detector is resistant to ambient light interference is mainly determined by the size of the detector. Therefore, it cannot be concluded that "1550 is more resistant to interference."

2. Is there a solution to the problem of 1550 being easily absorbed by water?

Here’s the answer first: This problem exists objectively and has no solution, but it is not serious.

Last September, when I was researching FMCW companies, most manufacturers mentioned that 1550-nanometer light waves are easily absorbed by water mist (a water layer a few millimeters thick can absorb all the energy of a 1550-nanometer laser; the professional term in the field of optical communications is "water-air absorption") and are difficult to be reflected back, making it difficult to work normally on rainy days.

At the time, an unnamed FMCW lidar company founder said:

"People in our industry are lying with their eyes open, saying that the 1550 has 'good penetration and strong anti-interference capabilities'. I have done a test, pouring water on the wall. In the eyes of the 1550 laser radar, the wall is black, but it becomes clear after the water flows down.

"Many experiments have shown that our 1550 LiDAR will be blinded by a two-millimeter thin film of water. There is no solution to this problem. Therefore, the 1550 LiDAR can only be used on cloudy days, sunny days, and light rainy days, but not on heavy rainy days."

At the same time, the person in charge of a TOF lidar manufacturer also said that the reason why 1550 has little impact on human eye safety is that it is easily absorbed by liquid water - the laser is absorbed by the water in the eyeball before it hits the retina; and being easily absorbed by liquid water will indeed cause the lidar using 1550 nanometer lasers to be unable to measure when there is water on the ground lane line, or its detection capability will be reduced.

However, the technical VP of a commercial vehicle driverless company said that the 1550 lidar is not "invisible" when encountering rain, but the detection distance will become shorter after some of the energy is absorbed.

According to a 1550 LiDAR manufacturer, the problem of 1550 being easily absorbed by water can be overcome by increasing the power of the laser, "You are absorbed by water and attenuated by half, so I just double the power." However, increasing the power often also means more difficult heat dissipation, reduced reliability, and shortened lifespan.