How to measure the distance from the moon to the earth? Maybe you need a "laser distance meter" ruler

In 1917, Einstein proposed the theory of laser generation, and 43 years later, Maiman invented the first laser. As the most outstanding artificial light source, laser is known as "the fastest knife, the most accurate ruler, and the brightest light".

Today, let’s talk about using laser as a “ruler”.

Part 1

Laser distance measurement, an intangible ruler

What if we want to know how far away we are from the moon?

The conventional method is this: take a tape measure and stretch it from the earth to the moon. But where can you find a tape measure that long, and who can stretch the tape all the way to the moon? Iron Man?

This is obviously not feasible, but of course there are always more solutions than problems. Scientists changed their thinking and focused their attention on "light".

The speed of light is: v = 299792 kilometers per second. Suppose we send a beam of light from the earth, and it takes t seconds for the light to reach the surface of the moon and reflect back. Then the distance traveled by the light is S = v×t, and the distance between the moon and the earth is half of S.

Now we can measure the distance between the Earth and the Moon without using a tape measure, but a new problem arises: what kind of light should we choose to ensure the completion of the task? First of all, the light needs to be bright enough and have good directionality so that it can reach the Moon and return and be received by us. So, just taking a flashlight will definitely not work.

I heard that laser is bright and directional? Then choose it!

In order to ensure that the laser can reach the moon smoothly and be reflected back to the earth, the astronauts deliberately left several laser reflecting mirrors when they landed on the moon. This makes the laser signal returning to the earth stronger when scientists use laser to measure the distance between the earth and the moon.

The last time humans landed on the moon was 50 years ago, and the mirrors placed on the moon at that time are still working. It is also through the use of laser ranging that we know that the moon is moving away from the earth at an average speed of 3.8 cm per year.

Astronaut Aldrin is placing the seismometer and mirror (full name of the mirror: Laser Ranging Retroreflector)

(Image source: project apollo archive)

Part 2

LiDAR: The core is laser ranging, the kind that surrounds 360 degrees

Human exploration never stops. After knowing the distance between the Earth and the Moon, scientists are busy exploring more secrets of the Moon, such as what the lunar surface looks like and how to choose a relatively flat area on the bumpy lunar surface to dock human spacecraft.

Obviously, it is not enough to just know how far a point on the moon is from us. So what should we do?

The solution is to expand the detection range, measure many points at a time, record the direction and distance information of each point, and then calculate the shape of the detected surface based on the collected information. This is exactly the working principle of lidar.

Laser measures the distance of many points around and senses the shape of the surrounding environment

(Source: Wikipedia)

The structure of the first generation of mechanical lidar is similar to the model above. On the basis of the laser rangefinder, a reflector that can change position quickly is added, which can reflect the laser in all directions.

When working, the laser radar will rotate and scan the surrounding environment, similar to a person holding a laser rangefinder in his hand, turning in circles while moving his hand up and down, so that the morphological information of the surrounding environment can be measured.

The laser radar installed on the car measures the distance information of the surrounding environment hundreds of thousands of times per second. Each distance measurement generates a data point, and a large number of data points constitute a point cloud that describes the three-dimensional environment.

(Image source: velodyne.inc)

Part 3

Large-scale application, cost reduction is the key

After the first generation of lidar came out, it made everyone feel that lidar is very powerful. It not only has strong detection capabilities, but also has the ability to empty people’s wallets.

Mechanical laser radar needs to rotate at high speed when working, and also needs a reflector that can swing quickly, which adds many difficulties to the manufacture of laser radar. Therefore, in the early days of laser radar development, the price of a laser radar was as high as hundreds of thousands.

But such a good thing cannot let cost limit its application, so engineers began to think about how to reduce the manufacturing cost of lidar.

First of all, there is the reflector of the mechanical laser radar. Its reflector is large and heavy. When the mirror is rotated, it is like swinging a sledgehammer, which is not only laborious but also expensive. So the researchers thought that if the mirror could be made lighter and smaller, it would be like replacing a sledgehammer with a small stick, which would be very comfortable to swing.

As a result, MEMS (micro-electromechanical system) laser radar appeared . The mirror in this laser radar is very small and light, and it is more flexible to move. The mirror can also be directly integrated on the chip, which makes the structure of the laser radar much simpler and reduces the manufacturing cost.

The mirror in the Mems lidar is only 1.5 mm wide.

(Image source: Reference 1)

Not only that, some engineers have even bigger ideas: Is it possible to do without mirrors and directly control the direction of laser emission to create a pure solid-state lidar without any moving parts?

Therefore, optical phased array technology came into being . Laser is an electromagnetic wave, which has the properties of waves and will produce interference. This is like two laser beams meeting and producing a phenomenon similar to two water waves overlapping each other. There are still water waves in some directions, while the water surface is very calm in other places.

Interference phenomenon when two water waves meet

(Image source: youtube)

Optical phased array technology generates electromagnetic waves in different directions

(Image source: Wikipedia)

With the help of the interference principle, as long as the emission time and other parameters of the multi-beam laser array are reasonably controlled, the synthesized light beam of multiple laser beams can be directed to a specific position.

The core component of phased array lidar, the laser phased array (actually an array of small lasers arranged together), can be manufactured on a large scale using semiconductor technology, which spreads the R&D and manufacturing costs.

Pure solid-state LiDAR integrated on a circuit board

(Image source: Quanergy.inc)

Thanks to the continuous efforts of researchers, the price of solid-state LiDAR has now dropped to a few thousand yuan. More and more LiDARs are appearing in practical scenarios, playing an important role in the fields of autonomous driving, mapping, archaeology, etc. We expect that the application scope of LiDAR will be even wider in the future.

References and websites:

[1] Wang D, Watkins C, Xie H. MEMS Mirrors for LiDAR: A Review[J]. Micromachines, 2020, 11(5):456.

[2] Amzajerdian F , Pierrottet DF , Petway LB , et al. Lidar systems for precision navigation and safe landing on planetary bodies[C]// International Symposium on Photoelectronic Detection and Imaging 2011: Laser Sensing and Imaging; and Biological and Medical Applications of Photonics Sensing and Imaging. International Society for Optics and Photonics, 2011.

[3] Goyer GG, Watson R. The Laser and Its Application to Meteorology[J]. Bulletin of the American Meteorological Society, 1963, 44(9):564-570.

[4] https://quanergy.com/products/s3/

[5] https://en.wikipedia.org/wiki/Phased-array_optics

[6]https://velodynelidar.com/products/puck-lite/

[7] https://www.hesaitech.com/zh/download

[8] https://www.zhihu.com/question/359079692/answer/920960280

[9] https://zhuanlan.zhihu.com/p/90852852

Produced by: Science Popularization China

Author: Salted Fish in the Sea

Editor: Wang Tingting

Producer: China Science Expo

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