Chapter 477 seems to be very simple, but is actually very troublesome
"Mo Gong, what did you choose in the end was the He-Ne laser?" Although the answer was not what Gao Zhendong expected, after all, the helium-neon laser played an important role in this matter. There are many advantages, but Gao Zhendong still praised their technical sensitivity.
They were not themselves, just driving around. Helium-neon lasers had not yet been born at this time. They made this choice by analyzing the characteristics of matter.
“Yes, Mr. Gao, we think about it this way. The low specific heat capacity of helium is conducive to heat dissipation, while the energy level of neon is high. At the same time, the collision of helium atoms and neon atoms can effectively improve the excitation efficiency of the laser. , these are all good for increasing laser power, and based on the principles you gave, they are good for improving accuracy and sensitivity," said a comrade from the original research institute.
Gao Zhendong nodded: "Well, and the low density of these two things is beneficial to reducing the inertial error of the laser gyroscope. And this is a gas laser, which is good for the layout of the laser."
Mo and Gong touched their heads and admired them. They were still working on the principles. Mr. Gao had already directly thought of the impact on the actual results: "Ah, I understand. We really didn't expect this."
Gao Zhendong was more interested in their choices, which path they wanted to choose.
“As for the specific solution for laser gyroscope, which one do you prefer?”
In principle, Gao Zhendong gave several solutions, but none of them related to optical fiber. Not to mention, there is no optical fiber now, and this application has very high requirements for the single mode and polarization of the optical fiber. Even with optical fiber, it will not be able to meet these requirements in a short time.
“We still want to build an active ring resonator, but it’s still hard to decide which solution to use.” Mo Gong and the others thought a lot and deeply.
"Tell me about it."
"General Engineer Gao, based on the theory you gave us, we conducted calculations. As you said, the interference fringe changes of the annular double-beam interference gyroscope are too weak. In fact, they are very weak. Difficult to apply. ”
In fact, this principle is not impossible, but its use must be based on fiber optic devices, because its measurement sensitivity is based on the area enclosed by the ring light path. For interference For gyroscopes, 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 show that at a rotation speed of 15 degrees/hour, the interference fringe displacement is only more than 1μm, which is really difficult. Interpreting it in the system is meaningless. "
The earth's rotation speed is relatively slow, but it is not particularly slow, so this sensitivity is far from being sufficient.
Moreover, the closed-loop area of 1 square meter was too large before there was no optical fiber to form a multi-circle optical path. Basically, it means that the length and width of the optical path will not be less than 1 meter * 1 meter, and few pieces of equipment can fit it. Get this thing.
"Well, the conditions for ring laser interferometer are not yet mature and have not yet reached the application conditions." Gao Zhendong nodded in agreement.
Gong Mo was very curious about his words. This sensitivity basically meant saying goodbye to practical engineering applications, but Chief Engineer Gao just said that the conditions were not mature and did not impose a death sentence?
Maybe the boss’s ideas are always different. Mo Gong did not stop to ask, but continued to report his thoughts.
“As for the passive ring resonant cavity, we always feel that the most difficult thing is to build a ring resonator. However, due to the passive characteristics, the accuracy and reliability are not very good. The advantages are not reaped, but the disadvantages are taken advantage of.”
The ring passive resonator is to inject two positive and negative laser beams into an optical resonant cavity (a ring optical path with a specific structure and size), and then by detecting the oscillation frequency difference between the two laser beams in the rotating state, the rotation situation can be obtained .
Gao Zhendong smiled, Mo Gong’s metaphor was really accurate.
The accuracy of the ring passive resonant cavity depends on the cavity bandwidth and peak light intensity. When the cavity bandwidth is determined, the "passive" characteristic greatly hinders it. Although passive It will not bring errors related to active gain, but in practical applications, this benefit is completely invisible and is completely offset.
Like the annular double-beam interferometer, the laser gyroscope with a passive resonant cavity structure also has immature technical conditions. This thing has to wait for the development of fiber optic technology.
Gao Zhendong nodded again: "Well, your analysis is correct, this thing is not mature either."
Why are you still immature? It seems that in the eyes of the big guys, there is no bad technology, only immature technology.
Before Mo Gong could speak, Gao Zhendong said: "Since you want to build an active resonant cavity, let's talk about your ideas and difficulties."
The basic principles and principles of active resonant cavity The passive ones are the same. 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 gain laser, and can effectively increase the light intensity.
This is also the reason why a helium-neon laser is used. This is a transparent gas laser. Compared with a solid laser, the change in the optical path is much smaller, and it is easier to design the resonant cavity structure.
This change in the gain tube has brought great improvements to the laser gyroscope, even if it introduces gain errors and frequency difference locking effects, etc., but in summary, in fact, It still greatly improves the accuracy of measurement. However, what is the price behind all this?
"The primary problem is the material of the optical resonant cavity. No matter which way we go, we cannot avoid this problem."
The optical resonant cavity is not surrounded by glass plates. A vacuum will do. The resonant cavity here is different from the resonant cavity of the laser, and refers to the entire ring-shaped optical path of the laser gyroscope.
The first thing that Mo Gong scratched his head about was, what to use to build this resonant cavity?
Gao Zhendong laughed when he heard this. It was a coincidence. He happened to know a material that could be used to build an optical resonant cavity, and someone happened to be doing it.
"Don't worry about this, I'll help you solve it."
"The characteristics of the resonant cavity material... ah?!" Mo Gong was still talking about the material, but he didn't expect Gao Zhendong to finish the sentence all at once Pinch it back.
Huh? Is this solved? Anyway, let me finish, okay? Do you know what I want? Mo Gong felt a strong sense of discomfort that he could not explain.
But considering that this person is originally a materials person, even the first laser he built was a victory for materials to a certain extent, so you can rest assured.
“That’s great, what kind of material is it?” Mo Gong was very curious about this.
"You go back to the Northeast Institute of Optics and tell them that I asked you to go. They are working on a material called fused quartz. You ask them for it, tell them your requirements, and ask them to cooperate with you in making corresponding adjustments. . ”
Fused quartz can be used as an optical resonant cavity. This is clearly written in the laser gyroscope book.
"Okay! Okay!" Mo Gong was extremely happy when he heard that this unit, Northeast Optics Institute, is professional! As for how Chief Engineer Gao knew that this thing was there 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, it will cause hole burning in the mirror image of the reverse mode pair. Overlapping, serious mode competition will occur.”
In human terms, the frequency stabilization operation to obtain better measurement preconditions will be caused by the interaction between the forward and reverse laser beams. , causing serious deterioration of laser power (light intensity) tuning results and affecting measurements.
In short, if you don’t do this thing, the measurement will not be accurate. If you do this thing, the measurement will still be inaccurate, and it will be difficult to handle.
For Gao Zhendong, this problem is not a big problem. There is a solution: "Use isotopes, neon in helium-neon lasers, use a 1:1 mixture of neon 20 and neon 22."
Isotopes? Mo Gong's eyes suddenly lit up. How could he do this?
Ignoring Gao Zhendong’s presence, he and his colleagues immediately took out pen and paper and started writing and calculating on the paper.
As for the comrades from the original research institute, their basic skills are really solid. After calculating for a while, they raised their heads with surprise: "Mr. Gao, your method is amazing! The peak gain of the mixture avoids the model Correct Doppler center frequency, mode competition is also avoided ”
Calculations show that due to the difference in atomic weight caused by the difference in the number of neutrons, a series of changes are triggered. The final result of using this mixture can avoid the above defects without affecting frequency stability.
Gong Mo can be said to be very happy. Chief Engineer Gao’s place here is simply a treasure trove. All questions must be answered and any pitfalls must be filled in.
"General Engineer Gao, after much thought, we feel that a gyroscope based on the mechanical jitter bias principle you mentioned is more suitable."
Any laser gyroscope that uses an active resonant cavity, In order to overcome the frequency locking problem, frequency offset is necessary, and there are various specific frequency offset solutions. The one Mo Gong chose happened to be the simplest one among them.
However, this is just a look.
Gao Zhendong could probably guess what he was thinking: "Your consideration is because the mechanical dither bias has no moving optical parts, the structure is simple, and there are no optical parts in the resonant cavity. It is relatively easy to manufacture. You only need to control the mechanical dither stage to control the entire Just shake the resonator, right? "
This is a very interesting thing. The laser gyroscope based on this principle was the first to be practical, and its performance seems to have been leading.
But our earliest practical laser gyroscope took a different path, and has been taking that path. It basically did not engage in mechanical jitter and offset gyroscopes. As for why, Gao Zhendong estimates that it has to do with processing. Technology is related to control technology.
The principle of this thing sounds simple, but when it comes to how to shake it, it becomes a problem.
(End of this chapter)