FMCW LiDAR Popular Science (Part 2): Lasers, Detectors, and Scanning

This article is the second in a series of FMCW LiDAR popular science articles. Click on the hyperlink to read the previous article: FMCW LiDAR popular science (Part 1): basic concepts, technical routes, advantages, controversies and misunderstandings.

FMCW LiDAR is mainly composed of laser, detector, coherent optical path and scanning components. Among these key parts, the most noteworthy ones are the following:

1. Laser wavelength: 1550

1.Why 1550?

At present, the lasers used in TOF LiDAR are mainly 905 nanometers, while those used by Luminar, Tudatong and Yijing Technology are 1550 nanometers, while the lasers used in FMCW LiDAR are all 1550 nanometers. So, what is the reason?

Luowei CTO Andy Sun said: When we make any product, we first look at the maturity of the supply chain, because you can't make all the parts yourself.

Specifically for FMCW LiDAR, in principle, the laser does not have to use 1550nm, 905nm can also be used, and there is no essential technical difference between using 1550nm or 905nm. It’s just that in the field of optical communications, FMCW is paired with devices based on the 1550nm band. The supply chain of these devices is relatively mature and the cost is controllable. In comparison, the cost of FMCW based on 905nm is too high.

In addition, considering the eye safety requirements of continuous light, the upper limit of the optical power of 1550nm is 40 times greater than that of 905nm. Under the coherent amplification principle of FMCW, theoretically, 1550nm can be tested at a distance 40 times farther than 905nm.

The founders and technical leaders of several FMCW lidar companies agree with the term "supply chain maturity".

Among them, a technical director said: FMCW must ensure that the return light can interfere with the local light. Only when there is interference can the target be detected. Therefore, the "coherence" of the laser is very high (even if there is a delay of several microseconds between the return light and the reflected light, there is still good coherence). In the industry, lasers with better coherence are called "narrow linewidth lasers", and at this stage, narrow linewidth lasers are only based on 1550 nanometers.

In addition, the person in charge also said that FMCW lidar needs to improve its integration, but highly integrated multi-channel products are usually based on silicon photonics, which can only be used with 1550.

However, Wu Lei, IC design director of Zhigan Photonics, said that the laser they use is 1310 nanometers, "this type of laser is used in large quantities in communications and is very cheap."

By the way, the laser used in FMCW lidar and the 1550nm laser used in TOF lidar are made of the same material (indium phosphide), but the structure is different - FMCW requires narrow linewidth (to ensure self-coherence) and low noise, which leads to completely different performance, yield, cost, etc.

A practitioner said that at present, the 1550 laser for FMCW lidar is much more expensive than the 1550 laser used for TOF, and it has not yet been mass-produced in China.

2. Advantages of 1550 nm lasers

The advantages of 1550 nanometer lasers have been mentioned a lot in the introduction to TOF lidar. The most critical point is that it will not affect the safety of human eyes, so the power can be made very high, thereby achieving a longer detection distance than 905 nanometer lasers.

In addition, even in TOF, the 1550 is better than the 905 in overcoming sunlight noise.

3. Disadvantages and Controversies of 1550 nm Lasers

(1) The cost is too high and difficult to reduce in the short term

The material used in 1550nm lasers is not silicon CMOS, but indium phosphide, and it also requires an external power supply and complex electronic control devices, so the cost remains high. In addition, 1550nm lasers need to be paired with expensive indium gallium arsenide APDs.

An architect of a LiDAR manufacturer believes that although the usage of 1550 in the field of optical communications is already considerable, the cost has not come down, so it is difficult to expect it to be reduced in LiDAR.

The architect said: "When you have millions of units, the cost will certainly be much lower than the current hundreds or thousands of units, but it will still be much more expensive than the 905. It's like, you can't say that if I buy a lot of gold rings, its price will drop to the level of copper rings, right? This is determined by the physical properties of the material."

(2) Easily absorbed by water mist and cannot work properly on rainy days

Most LiDAR manufacturers mentioned in their communications with Jiuzhang Zhijia that 1550-nanometer lasers 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.

A founder of an FMCW lidar company who wishes to remain anonymous said: “People in our industry are lying with their eyes open, saying that 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-nanometer laser, the wall is black, but it becomes clear after the water flows down.

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

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 lead to the situation that when there is water on the ground lane line and the clothes of pedestrians are wet, the lidar using 1550 nanometer laser will not be able to measure, or its detection ability will be reduced.

However, Yao Jian, CEO of Guangshao Technology, does not fully agree with the above statement. Yao Jian said: "There are two types of FMCW coherence, one is frequency modulation and the other is phase modulation. If it is frequency modulation, the 1550nm laser is indeed greatly affected by rain. For example, the detection distance is attenuated from the usual 500 meters to 50 meters. However, the phase modulation technology we use can still achieve about 50% of the usual detection distance in heavy rain."

However, Mr. Xi, CEO of an FMCW LiDAR company, said: "In TOF, 1550nm lasers are easily absorbed by rain and fog, which will greatly reduce the detection distance and generate serious noise. In FMCW, coherent detection is used, and the signal-to-noise ratio is higher. Therefore, when using the same 1550nm laser, the LiDAR will have a stronger ability to penetrate rain and fog to produce a stable point cloud."

In addition, Luowei CTO Andy Sun believes that it is incomplete to only look at the absorption rate of water in different bands, because when the laser encounters rain, absorption is only part of the situation. There are also many changes related to scattering and refraction. In these two parts, different wavelengths perform differently. For example, short waves have stronger scattering, so comprehensive consideration is required.

Andy Sun said that in 2019, the French research team conducted a lot of theoretical analysis and experiments, and found that in fact, the performance of 905nm and 1550nm lasers in rainy and foggy days is similar - the overall absorption rate of 1550nm is only less than 10% higher than that of 905nm. This is completely overshadowed by the advantage of 1550nm in providing greater optical power for eye safety considerations.

Mr. Xi, CEO of a FMCW lidar company, also mentioned this set of experiments from France.

But in any case, Volvo, Audi and other very conservative car companies are planning to use Luminar's 1550-based TOF lidar. This may mean that in the eyes of these car companies, the failure of 1550 to work properly on rainy days is not a serious problem?

(3) “Eye safety” is not absolutely guaranteed

During the interview, the author mentioned a question: If several self-driving vehicles equipped with lidar gather together and emit lasers at the same time, the lasers will converge together and face the human eyes. The instantaneous power will be very high, which should also affect human eye safety?

In response to this, the person in charge of a TOF lidar manufacturer replied: If the power of 905 nanometers is high, it will damage the retina, but the retina is similar to a pixel of the photosensitive image on a mobile phone. The laser hits different places and affects different pixels on the retina. Therefore, even if the power is relatively high, it will not burn out these tens of millions of pixels; 1550 is different. If it burns, the entire area will be burned out. The human eye is sensitive to the 905 nanometer band because of the energy it hits a certain pixel, and is sensitive to the 1550 nanometer band because of its total energy.

In short, 1550 nanometer lasers actually also have eye safety issues - the laser does not hit the retina, but is absorbed by the cornea. If the power is too high, the cornea will also be damaged.

2. Chip-based and integrated lasers and detectors

1. Single photon, number of channels and spot frequency

During the interview, Jiuzhang Zhijia repeatedly asked a question: Will FMCW LiDAR have many lasers like some TOF, or only one laser? Most people answered: In theory, there can be dozens or hundreds of lasers, but in practice, manufacturers often only use one laser for cost considerations. FMCW LiDAR is also called "single-photon LiDAR".

However, despite the small number of lasers, with the help of the beam splitter, the actual number of lines emitted can be very large, such as Mobileye's FMCW lidar, which claims to have 184 lines. The number of lasers may be more than one, but it will definitely not be dozens or hundreds.