This is a quick summary of today's TCS fun, but in short, we are confused. Look at the attached pictures.
The goal of the day was to change the common CO2 setting while checking the interferometer properties such as carrier's recycling gain, mode matching and contrast defect. At the beginning, we did a quick test where we let CITF freely swing. This test showed something unexpected to us -- the recycling gain decreased as we increased the common CO2 power. Later, we further investigated this by fully locking CITF on the carrier. We will perform careful analysis tomorrow, but it seems that CITF consistently drops the recycling gain as we increase the common CO2 power. By the way, it would be extremely helpful if we could get a power sensor for each CO2 beam so that we don't have to go through an offline calibration.
In addition, one thing we immediately noticed was the beam shape. The size of the beam resonating in CITF significantly increased which is opposite of what we have thought. This needs further theoretical investigations. We are leaving the interferometer without the CO2 lasers and both arms locked.