This is an additional measurement of the laser PZT frequency response, obtained by monitoring the error signal of the locked PMC while injecting an excitation signal to the laser PZT by Moku:Lab.

The figure shows two transfer function measurements. The darker reference line was measured with the excitation amplitude of 2mV, and the bright line was measured with 20mV.

Below a few kHz, that is the UGF of the PMC loop, the TF is suppressed by the control since it was measured at the error point, so the data is available only above several kHz. The flat area between 4kHz and 40kHz corresponds to the former measurement in klog:35884 as 1.54 MHz/V.

One clear resonance can be seen just below 100kHz, but a very complex structure can be seen above 100kHz. First, I suspected that it might be noise, so I compared the different excitation amplitudes. Noise was improved at low frequency with the large excitation, but the structure at high frequency looks the same, so this structure does not come from noise.

This shape is very similar to the previous measurement using IMC, as in this original thread klog:35917. Measurement using PMC as a monitor is a kind of very direct measurement of PZT response using an optical cavity as a monitor. So, this structure above 100kHz is unlikely to originate from measurement noise.