Update, beat measurement after several optics replacement. Peaks around 10 Hz, 35 Hz show up this time.
Optics that we replaced are:
The problem with the curve mirror from last entry has not been fixed yet. It turns out that the mirror we borrow from 40m is worse than the one we have (surface is more milky), so we leave the original mirror as it is.
Note: The beat measurement was done when the air springs were inactive. Noise at high frequency goes down a bit.
The power input to each cavity is 1mW, setup on PLL is 1kHz input range, with gain = 200.