There are a few problems with the locking now:
1.The power of the TEM00 transmission drops suddenly
When we try to scan the cavity by changing the output voltage of PZT driver by hands slowly, we can observe the slowly increasing part of the TEM00 power both in the PD and CCD, but then when it reaches its maximum(or near) it drops suddenly. On the oscilloscope connected to the PD, we can see that the power suddenly drop from several tens of mV to several mV, and from the CCD, the diameter of the beam and also the brightness suddenly get smaller.
When I try to lock the cavity, I did as what I will describe below, which I am not sure it's the right way or not. The power of the TEM00 will change from weak to strong and go back to weak when I scan the cavity, which means there is a range of the voltage that gives to the PZT, like from 15V to 17V, I tried to lock the cavity at different voltage. If I chose to lock the cavity at the maximum power, the lock will last for less than 5 mins, and from the CCD I can see it get weaker and move to next modes. And if I locked the cavity at the range where the power is slowly increasing, the lock will last longer, like more than one hour, but the power still slowly increase to the maximum and start to reduce, finally it moves to the same 'next' modes as in last situation. And if I start in the 'suddenly drop' range, it seems much more stable then the other situations, I wait for more than one hour, the power increase to some point at the first beginning then it stops there and does not have any obvious change after. I am not very clear about what this means, and I am also not sure about this result can be repeated since I tried the last situation only once. So maybe if it has some meaning,I can try to do it tomorrow again.
2.The strange shape of the green beam
If we look at the green beam when the cavity is locked(the green is much more powerful and easy to see the detail)and the temperature is at the 'good' temperature, the beam is not a spot, there are many irregular dark lines in it which cut the green like into many strange shape pieces. At the beginning of this week, when we see the green light under room temperature, it almost a spot with a circle which is caused by the Laguerre mode. But now even when we turn the temperature back to the room temperature, it is not as good as what we saw before, but because it's very weak so we can not see it very clearly what exactly the pattern is.
3.The TEM00 is 20 times less in cavity-locking compare to mode-matching-finding and the error signal is only several mV
In my Tuesday logbook entry I gave the picture how I lock the cavity, the PD is connected to the 'RF' port of the demodulation board and also the CH1 of oscilloscope. Also in this picture, from the small picture of the oscilloscope screen,(the yellow is from the PD), it shows that the magnitude of TEM00 (the highest peak of the CH1) is about 50mV. But when we did the mode matching(CH1: PD, CH2: the triangle wave), the TEM00 is more than 1V. Then we found out if disconnect the demodulation board 'RF' port and the PD, the TEM00 goes back to it used to be (>1V). It may caused by the input resistance of the demodulation board,so I will check the PD output resistance and the demodulation board input resistance, to make sure this problem is caused by them or not.
4. The output voltage of the PZT driver does not change
When the loop is closed, if there is some fluctuation, then the feedback should work and the PZT driver will give out the voltage to push or pull the PZT, make it back to the resonance. And in my situation there is a screen on the PZT driver shows the output voltage of it, but it does not change during any lock process until now. My guess is the error signal is too small (couple of mV), the magnification of the PZT driver is 30 times, so even after the signal is amplified, it's couple of tens mV, which cannot show on the display of PZT driver whose digit limit is 0.1V.
I will check the following things later:
1. The properties of the PD and the demodulation board (resistances, the cutting frequency of low pass filter on the demodulation board)
2. Which PD we used in last locking with Matteo
3. The properties of the dichroic mirror, since it should be reflect infrared and transmit green, and now the reflection green beam is really powerful.
4. The exact power sent to the cavity when using the maximum output of the laser
5. The polarization we are using now