Fig. 3
From: Demonstration of a length control system for ALPS II with a high finesse 9.2 m cavity
Length noise measurement. Amplitude spectral densities of green control (solid blue trace) and error (solid red trace) signal of the PDH control loop calibrated in length noise as well as the beatnote frequency measurement (solid purple trace) representing the differential length noise. For a comparison with the ALPS II requirements the error signal and the beatnote frequency measurement are filtered by a cavity pole frequency of 6 Hz in post processing and its corresponding integrated RMS is shown with the dashed lines