YuhangZhao, MatteoLeonardi - 23:10, Monday 30 October 2017 (576)
Instability of green light
Last week, I and Tomura-san found that the output of SHG was pretty unstable. So we tried to find out what's the reason.
Today, I made some measurements about the green light. The process is
Firstly, the green light was stable. Then I sent message to Matteo, and Matteo thought it's saturation. Actually this is what I and Tomura-san found last week. So I decided to use the filter to check it. And I found Matteo was right.
Secondly, the green light became unstable. I measure the noise spectrum at this moment. I want to see what will happen after a while.
Thirdly, after a while, the green light becomes stable again. I measure the noise spectrum again. This is different from the previous one certainly. Then I want to see what will happen after a while again.
Fourthly, after a long-while, the green light becomes unstable again. And I told it to Matteo, Matteo thought it could be cause by beat of two signals. So I checked it on the oscilloscope, it turns out that we can see slow variation of signal on oscilloscope(3rd picture). Indeed, it looks like a beat signal.
The main difference between unstable and stable signal is a peak around 2.7kHz. Matteo suggested to remove the thermal dissipator. After removing it, we measure noise spectrum again. It's almost the same with the stable spectrum we measured before.
Above all, we find that a reason of instability is the resonance of thermal dissipator. Actually, we can see from noise spectrum, the only difference is the peak around 2.7kHz. So there is also at least another reason of instability, it's caused by the peak around 650Hz. And we can see its harmonic peak. (But we cannot see this frequency fluctuation on oscilloscope clearly)
(Sorry for not using matlab to make plot. It's because of my saving of wrong format data.)
The first picture, it's the unstable noise spectrum.
The second picture, it's the stable noise spectrum.
The third picture, it's the long term signal in the view of oscilloscope(unstable).
The fourth picture, it's thermal dissipator.
The fifth picture, the same time scale with the 3rd picture of oscilloscope(stable). Actually, it just doesn't have something like beat or resonance. As you can see it's not stable for a long time.
Today, I made some measurements about the green light. The process is
Firstly, the green light was stable. Then I sent message to Matteo, and Matteo thought it's saturation. Actually this is what I and Tomura-san found last week. So I decided to use the filter to check it. And I found Matteo was right.
Secondly, the green light became unstable. I measure the noise spectrum at this moment. I want to see what will happen after a while.
Thirdly, after a while, the green light becomes stable again. I measure the noise spectrum again. This is different from the previous one certainly. Then I want to see what will happen after a while again.
Fourthly, after a long-while, the green light becomes unstable again. And I told it to Matteo, Matteo thought it could be cause by beat of two signals. So I checked it on the oscilloscope, it turns out that we can see slow variation of signal on oscilloscope(3rd picture). Indeed, it looks like a beat signal.
The main difference between unstable and stable signal is a peak around 2.7kHz. Matteo suggested to remove the thermal dissipator. After removing it, we measure noise spectrum again. It's almost the same with the stable spectrum we measured before.
Above all, we find that a reason of instability is the resonance of thermal dissipator. Actually, we can see from noise spectrum, the only difference is the peak around 2.7kHz. So there is also at least another reason of instability, it's caused by the peak around 650Hz. And we can see its harmonic peak. (But we cannot see this frequency fluctuation on oscilloscope clearly)
(Sorry for not using matlab to make plot. It's because of my saving of wrong format data.)
The first picture, it's the unstable noise spectrum.
The second picture, it's the stable noise spectrum.
The third picture, it's the long term signal in the view of oscilloscope(unstable).
The fourth picture, it's thermal dissipator.
The fifth picture, the same time scale with the 3rd picture of oscilloscope(stable). Actually, it just doesn't have something like beat or resonance. As you can see it's not stable for a long time.
Images attached to this report
