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

In 1917, Einstein proposed the theory of laser, 43 years later, Meman invented the first laser. Laser is the one who stands for the most crowded chicken group in the artificial light source. It is known as: "the fastest knife, the most accurate ruler, and the brightest light."

Today, we will talk about the "ruler" with laser.

Laser ranging, a ruler that can't be touched

If you want to know how far we are from the moon?

The conventional method is like this: take a leather tape from the earth to the moon. But where can I find such a long leather ruler, and who can I find the ruler all the way to the moon? Iron Man?

This is obviously incomprehensible. Of course, there are always many methods than difficulties. Scientists focus on "light" to "light".

The speed of light is: V = 299792 kilometers per second. Suppose we launch a beam of light from the earth, reaches the surface of the moon and reflected it back. The distance between the moon and the earth is half of S.

At this point, the distance between the earth and the moon without the use of a ruler can also be measured, but the new problems appear again. What light to choose to ensure the completion of the task? First of all, this light needs to be bright enough and the direction is good enough to reach the moon and then return and receive it by us. Then it is definitely not possible to take a flashlight.

I heard that the laser is bright and the direction is good? Then choose it!

So in order to reach the moon smoothly and reflect back to the earth, the astronauts deliberately left a few mirrors with reflected laser when landing on the moon, which made scientists use laser measurement to measure the moon distance, and the laser signal returned to the earth became more powerful.

The last moon landing on the moon was 50 years ago, and the mirrors on the moon at that time were still at work. It is also using laser ranging that we know that the moon is far from the earth at an average of 3.8 cm per year.

Astronaut Ortyl is placing a seismic and mirror (full name of the mirror: laser distance to reflector)

(Source: Project Apollo Archive)

Lidar: The core is the laser ranging, the type surrounded by 360 degrees

The footsteps of human exploration are never stopping. After knowing how far the distance between the earth and the moon, scientists are busy to detect more secrets of the moon. For example, what are the appearance of the moon? How do we choose a relatively flat place on the surface of the potholes on the surface of the potholes?

Obviously, it is not enough to know how far it is to know how far from the moon. What should I do?

The method is -expand the scope of the detection, measure many points at one time, record the direction and distance information of each point, and then solve the appearance of the detected surface according to the collected information, and this is exactly the work of the laser radar principle.

Laser measurement of many points around and perceive the appearance of the surrounding environment

(Figure source; wiki encyclopedia)

The structure of the first -generation mechanical laser radar is similar to the above model. Based on the laser rangefinder, the reflector that can quickly change the position can reflect the laser to all directions.

Lidar will rotate the surrounding environment when working, similar to the laser rangefinder on the person's hands, and the hand is still moving up and down at the same time, so that the appearance information of the surrounding environment can be measured.

The laser radar on the car, measure the distance information of hundreds of thousands of surrounding environment per second, a data point is generated for each ranging, and a large amount of data point constitutes a point cloud describing the three -dimensional environment.

(Source: velodyne.inc)

Large -scale application, cost reduction is the key

After the first generation of lidar was released, it made everyone feel that the laser radar was very strong.

Mechanical lidar needs to rotate at high speed while working, and also requires a reflex mirror that can be swinged quickly, which adds many difficulties to the manufacturing of laser radar. Therefore, in the early days of laser radar, the price of a lidar was as high as hundreds of thousands.

But such a good "gadget" cannot restrict its application, so engineers began to think about how to reduce the manufacturing cost of laser radar.

The first is the reflex mirror of mechanical lidar. Its reflex mirror is large and heavy. When the mirror rotates, it is like a big hammer. It is not like changing the big bells with a small stick, and it must be very comfortable to pick up.

As a result, MEMS (micro -electrical system) lidar appeared, and the mirror in this laser radar was very small and light, and it was more flexible to move. It can also integrate the mirror directly on the chip, so that the structure of the lidar is much simpler, and the manufacturing cost is reduced.

The mirror in the MEMS lidar, its width is only 1.5 mm

(Source: References 1)

Not only that, some engineers have a bigger brain hole: Can you not want to control the direction of laser output, create a pure solid -state laser radar without any activity components?

As a result, optical phase control arrays came into being. Laser is an electromagnetic wave with a wave of waves and can cause interference. This is like the two laser meets, and the phenomenon similar to the overlapping two rows of water waves is produced. Some directions still have water waves, while some places are calm.

The interference phenomenon that occurs in two columns of water waves

(Source: YouTube)

Optical phase control arrays generate electromagnetic waves in different directions (picture source: wikipedia)

With the help of interference principles, as long as the transmission time of the multi -beam laser array is reasonably controlled, the multi -beam laser synthesis light beam can be moved towards a specific location.

The core component laser phase control arrays of the phase control array laser radar (in fact, the small laser array arranged together) can be manufactured by large -scale semiconductor technology, which shares the cost of R & D and manufacturing.

Pure solid -state lidar integrated on the circuit board

(Picture source: quanergy.inc)

With the continuous efforts of scientific researchers, at present, the price of solid -state lidar has been explored to thousands of yuan, and more and more laser radars have appeared in a practical scenario. effect. We expect that the use of lidar in the future will be more widely used.

[1] Wang D, Watkins C, xie H. MEMS Mirrs for lidar: A Review [J]. Micromachnes, 2020, 11 (5): 456.

[2] Amzajerdian F , Pierrottet D F , Petway L B , 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. 2011.

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

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

[5] https://en.wikipedia.org/wiki/phased-Rray_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: Popular Science China

Author: salted fish in the sea

Edit: Wang Tingting

Producer: China Science Popularization Expo

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