Michael and Yuhang

When aligning OPO, we found that the TEM00 mode position has quite obvious drift on the oscilloscope time axis. Therefore, we monitored this effect for about 10min for the old OPO in TAMA. This starting and end positions of this drift are marked by blue curcouses 'a' and 'b', as shown in the attached figure. According to this drift, we estimate the OPO optical length drift assuming the OPO optical length drift is the only cause. Also the estimation here is just a rough estimation.

1. From the attached figure, we can see that the drift of TEM00 corresponds to a time of 2.686ms.

2. The scan speed of triangular ramp signal is 0.088 V/ms. Therefore, the TEM00 movement corresponds to a PZT driving voltage change of 0.236 V.

3. We have a calibration of PZT driving coefficient of around 1.2GHz/V. So the TEM00 movement corresponds to 284 MHz. This corresponds to a length of lambda/2 * (284/4000) = 38 nm. Here lambda is 1064nm, 4000 is FSR of OPO (4GHz).

4. If we assume the cavity length drift has the same speed, we have a cavity length change of 2.3 um in 10hours. 

This number seems a bit large for me since the PZT seems to be able to control only 3um according to a datasheet in our wiki.