Message ID: 16129
Entry time: Mon May 10 18:19:12 2021
In reply to: 16127
Reply to this: 16132

Author:

Anchal, Paco

Type:

Update

Category:

LSC

Subject:

IMC WFS noise contribution in arm cavity length noise, Corrections

A few corrections to last analysis:

The first plot was not IMC frequency noise but actually MC_F noise budget.

MC_F is frequency noise in the IMC FSS loop just before the error point where IMC length and laser frequency is compared.

So, MC_F (in high loop gain frequency region upto 10kHz) is simply the quadrature noise sum of free running laser noise and IMC length noise.

Between 1Hz to 100 Hz, normally MC_F is dominated by free running laser noise but when we injected enough angular noise in WFS loops, due to Angle to length coupling, it made IMC length noise large enough in 25-30 Hz band that we started seeing a bump in MC_F.

So this bump in MC_F is mostly the noise due to Angle to length coupling and hence can be used to calculate how much Angular noise normally goes into length noise.

In the remaining plots, MC_F was plotted with conversion into arm length units but this was wrong. MC_F gets suppressed by IMC FSS open loop gain before reaching to arm cavities and hence is hardly present there.

The IMC length noise however is not suppresed until after the error point in the loop. So the length noise (in units of Hz calculated in the first step above) travels through the arm cavity loop.

We already measured the transfer function from ITMX length actuation to XARM OUT, so we know how this length noise shows up at XARM OUT.

So in the remaining plots, we plot contribution of IMC angular noise in the arm cavities. Note that the factor of 3 business still needed to be done to match the appearance of noise in XARM_OUT and YARM_OUT signal from the IMC angular noise injection.

I'll post a clean loop diagram soon to make this loopology clearer.