Jenne, Matt, Patrick, Sheila, Stefan, Kiwamu,
We tried two different differential CO2 settings today at 20W to see if we can improve the imbalanced RF sidebands (29535). One of the two tests was already reported by Jenne et al. on 29553.
According to the tests, the addition of a 10 uD single-pass substrate defocus to the X compensation plate seems to balance the amplitude of the upper and lower 45 MHz sidebands.
[Settings]
- Locked at 20 W
- CO2X 200 mW -> 1 W
- CO2Y stayed at 0 W
The interferometer arrived at a 20 W lock at 22:53 local. We then added a 800 mW of laser power on to CO2X, resulting in a 1 W CO2 power at 22:55:40. As opposed to the previous test (29535), we did not change CO2Y this time and therefore it stayed at 0 W.
The measurement finished at 23:15 and we set CO2X to 700 mW. We then increased the PSL power to 50 W while maintaining the same lock.
[Results]
As expected from the previous test, the imbalance of the 45 MHz RF sidebands, as seen by the optical spectrum analyzer, was improved by this action. After roughly 20 min, we reached a point where the upper and lower sidebands almost had the same amplitude. HWSX was functioning at the beginning of the test (for 10 minutes or so), but then unfortunately started showing some meaningless signals. We have no idea why. Nevertheless, we were able to extrapolate where we would be in terms of the substrate defocus based on the initial rise. It looks like that an additional 20 uD double-pass (= 10 uD single pass) is the good defocus value for ITMX in order to get balanced upper and lower 45 MHz sidebands. A 10 uD single-pass defocus corresponds to a 400 mW CO2 laser on the X compensation plate in equilibrium [assuming a 25 uD/W actuation coefficient (single-pass), 28799].
The attached is a screenshot of OSA outputs in which scans from different time are overlaid. It is clear that the imbalance improved as time elapsed.
On the other hand, the frequency noise coupling became worse by a factor of a few comparing to the initial coupling value at 900 Hz. It did not experience a sign change. Not sure what this means.
Here is a plot of the 45 MHz sideband powers durring this test. At 0, the IFO is locked at 20 W, at 5 minutes Kiwamu changed TCS, at around 18 the power is increased to 50 W, then the modulation depth drops at around 22.
