Abstract:

I compared the spectrum of CARM CMS TESTA1 signals between noisy and quiet timings.
Noise floor could increase at least by a factor of 3 and it might cause EOM path saturation.
So, it is ecessary to check the EOM path signals when laser frequency becomes noisy.

Detai:

I measured spectrum of CARM TESTA1 (test1 signals in the common path in CARM CMS) with Moku:Lab.
I compared the spectrum with two condition: one is IMC error signals are quiet (fig1 shows an example of the quiet IMC error signals) and the other is IMC error signals are noisy (fig2 shows an example of the noisy IMC error signals).
Figure 3 shows the spectra measured at CARM CMS TESTA1 when finishing INCREASING_LAS_POWER state.

Thin lines show the spectrum when IMC error signals are quiet and thick lines show the spectrum when IMC error signals are noisy.
The noise floor when IMC error signals are noisy is about 3 times larger than that when IMC error signals are quiet.
In addition, frequency noise excess happens not only at low frequency controlled by PZT but also high frequency controlled by EOM.

According to klog35081, voltage to EOM is roughly +- 40V in normal operation.
So, if the RMS becomes larger according with the noise excess, it might reach +-100V limit.
To confirm the EOM range would be necessary to stable operation of IFO.