Chapter 477 Seems Simple, But Actually Very Troublesome

"Engineer Mo, did you finally choose He-Ne laser?" Although this answer was as Gao Zhendong expected, after all, helium-neon laser has many advantages in this matter, Gao Zhendong still praised their technical sensitivity.

They were not cheating on their own. Helium-neon lasers had not yet been invented at that time. They made this choice by analyzing the properties of materials.

"Yes, Mr. Gao, this is how we think about it. Helium has a low specific heat capacity, which is good for heat dissipation, while neon has a high energy level. At the same time, the collision between helium atoms and neon atoms can effectively improve the excitation efficiency of the laser. These are all beneficial to increasing laser power. Based on the principle you gave, it is beneficial to improving accuracy and sensitivity." said a comrade from the original research institute.

Gao Zhendong nodded: "Yes, and the low density of these two things is conducive to reducing the inertial error of the laser gyroscope. And this is a gas laser, which is good for the layout of the laser."

Engineer Mo and the other two rubbed their heads in admiration. They were still struggling with the principle, but CEO Gao had already thought of the impact on the actual results. "Ah, I see. We hadn't thought of this."

Gao Zhendong was more interested in their choices than which path they wanted to choose.

"So which specific solution for the laser gyroscope do you prefer?"

In principle, Gao Zhendong gave several solutions, but he did not mention any one related to optical fiber. There is no optical fiber now, and this application has very high requirements for the single mode and polarization of optical fiber. Even if optical fiber is available, it will not be able to meet these requirements in a short time.

"We still want to develop an active ring resonator, but it is difficult to decide which specific solution to use." Engineer Mo and his colleagues have thought a lot and deeply.

"Tell me."

"Chief Engineer Gao, we have calculated based on the theory you gave us, and it is just as you said. The interference fringes of the annular dual-beam interferometer gyroscope are too weak to be practically applied."

In fact, this principle is not unusable, but its use must be based on fiber optic devices, because its measurement sensitivity is based on the area enclosed by the circular optical path. For the interferometer gyroscope, this area requirement is still very large.

"We used a closed loop area of ​​1 square meter and a wavelength of 633nm to simulate and measure the Earth's rotation speed. The calculation results showed that at a rotation speed of 15 degrees per hour, the interference fringe displacement was only a little over 1μm, which was really difficult to interpret in the system and had no practical significance."

The speed of the Earth's rotation is relatively slow, but not extremely slow, so this sensitivity is far from being sufficient.

Moreover, the closed-loop area of ​​1 square meter is too large before optical fibers are used to form multiple optical paths. This basically means that the length and width of the optical path will not be less than 1 meter * 1 meter, and few equipment can accommodate this thing.

"Well, the conditions for the ring laser interferometer are not mature yet and have not yet reached the conditions for application." Gao Zhendong nodded in agreement.

Engineer Mo was very curious about what he said. This sensitivity basically meant a complete farewell to practical engineering applications, but Chief Engineer Gao only said that the conditions were not mature, and did not sentence him to death?

Maybe the boss's ideas are always different from others. Mr. Mo did not stop to ask, but continued to report his ideas.

"As for the passive ring resonator, we always feel that it is neither high nor low. We have to make the most troublesome ring resonator, but because of the passive characteristics, the accuracy and reliability are not very good. It is like getting none of the advantages and all the disadvantages."

A passive annular resonant cavity is an optical resonant cavity (annular optical path of specific structure and size) that injects two positive and negative laser beams. The rotation condition can then be obtained by detecting the oscillation frequency difference between the two laser beams in a rotating state.

Gao Zhendong smiled. Mr. Mo’s analogy was absolutely correct.

The accuracy of the passive annular resonant cavity depends on the cavity bandwidth and peak light intensity. When the cavity bandwidth is determined, the "passive" characteristic greatly hinders its performance. Although passivity does not cause errors related to active gain, in practical applications, this advantage is completely offset and not apparent.

Like the circular double-beam interferometer, the laser gyroscope with passive resonant cavity structure also has immature technical conditions. It also has to wait for the development of optical fiber technology.

Gao Zhendong nodded again: "Well, your analysis is correct. This thing is not mature either."

Why is it still immature? It seems that in the eyes of the big guys, there is no bad technology, only immature technology.

Before Engineer Mo could speak, Gao Zhendong said, "Since you want to develop an active resonant cavity, then tell us about your ideas and difficulties."

The basic principle of the active resonant cavity is the same as that of the passive one. The biggest difference is that a helium-neon laser gain tube is inserted into the optical path. This laser tube plays the role of emitting laser and gaining laser, which can effectively increase the light intensity.

This is also the reason why we choose helium-neon laser, which is a transparent gas laser. Compared with solid-state laser, it causes much smaller changes to the optical path and is easier to design the resonant cavity structure.

The change in the gain tube has brought great improvements to the laser gyroscope, even though it introduces gain errors and disadvantages such as frequency difference locking effect, but overall, it still greatly improves the measurement accuracy. However, what is the price behind all this?
"The primary problem is the material of the optical resonant cavity. This problem cannot be avoided no matter which path we take."

An optical resonant cavity is not something that can be created by enclosing a glass plate and then drawing a vacuum. The resonant cavity here is different from the resonant cavity of a laser, and refers to the annular optical path of the entire laser gyroscope.

The first thing that Engineer Mo racked his brains for was, what should he use to build this resonant cavity?

Gao Zhendong laughed when he heard this. This was really a coincidence. He happened to know a material that could be used to make an optical resonant cavity, and someone was already doing it.

"Don't worry about that. I'll help you solve it."

"As for the characteristics of the resonant cavity material...ah?!" Engineer Mo was still talking about the materials, but he didn't expect Gao Zhendong to interrupt him in an instant.

Hmm? Is that the answer? At least let me finish what I want to say? You know what I want? Mo Gong felt a strong sense of discomfort as if he hadn't finished speaking.

But think about it, he was originally a materials scientist, and even his first laser was, to some extent, a victory for materials, so I can rest assured.

"That's great, what material is it?" Engineer Mo was very curious about this.

"Go back to the Northeastern Institute of Optics and tell them that I want you to go. They are working on a material called fused quartz. Ask them for it and tell them your requirements. Ask them to cooperate with you and make corresponding adjustments."

It is clearly written in the book on laser gyroscopes that fused quartz can be used to make optical resonant cavities.

"Good! Good!" Engineer Mo was extremely happy when he heard that this unit, the Northeast Optical Institute, was professional! As for how Chief Engineer Gao knew there was such a thing and was sure that he could use it, that was not something he could ask.

"Another problem, which is also a common problem, is the problem of helium-neon lasers. We have calculated based on your theory. Due to the need for frequency stabilization, the mirror-image holes of the reverse mode pair will overlap, which will cause serious mode competition."

In simple terms, the frequency stabilization operation performed to obtain better measurement preconditions will cause the laser power (light intensity) tuning result to be seriously degraded due to the interaction between the positive and negative laser beams, thus affecting the measurement.

In short, if you don’t do this, you can’t measure it accurately. If you do this and still can’t measure it accurately, it will be very difficult to deal with.

For Gao Zhendong, this is not a big problem. He has a solution: "Use isotopes, neon gas in helium-neon lasers, and use a 20:22 mixture of neon 1 and neon 1."

Isotopes? Mo Gong's eyes lit up immediately. Is this even possible?
Without caring about Gao Zhendong being there, he and his colleague immediately took out paper and pen and started writing and calculating.

The comrades in the original research institute have really solid basic skills. After calculating for a while, they looked up with surprise and said, "Mr. Gao, your method is amazing! The peak gain of the mixture avoids the Doppler center frequency of the mode pair, and the mode competition is also avoided!"

Calculations show that the difference in atomic weight caused by the different number of neutrons will lead to a series of changes, and ultimately the result of using this mixed gas can avoid the above-mentioned defects without affecting the frequency stability.

Engineer Mo can be said to be very excited. Chief Engineer Gao is simply a treasure trove here. He always answers questions and fills gaps.

"Chief Engineer Gao, after much thought, we think it would be more appropriate to use the gyroscope based on the mechanical vibration frequency deviation principle you mentioned."

For any laser gyroscope that uses an active resonant cavity, frequency deviation is necessary to overcome the frequency locking problem. There are many specific frequency deviation solutions, and the one chosen by Engineer Mo happens to be the simplest one.

But, it just seems that way.

Gao Zhendong could probably guess what he was thinking: "Your consideration is that the mechanical dithering frequency deviation has no moving optical components, a simple structure, and no optical components in the resonant cavity. It is relatively easy to manufacture. You only need to control the mechanical dithering stage to shake the entire resonant cavity, right?"

This is a very interesting thing. The laser gyroscope based on this principle was the first to be put into practical use, and its performance seemed to have once maintained a leading position.

However, our earliest practical laser gyroscope took a different path and has been following that path ever since, with basically no mechanical jitter offset gyroscope used. As to why, Gao Zhendong estimated that it was related to processing technology and control technology.

The principle of this thing sounds simple, but when it comes to how to shake it specifically, that becomes a problem.

(End of this chapter)