Figure 35: This figure shows an example of a Pound–Drever–Hall (PDH) signal of a two-mirror
cavity. The plots refer to a setup in which the cavity mirrors are stationary and the frequency of
the input laser is tuned linearly. The upper trace shows the light power circulating in the cavity.
The three peaks correspond to the frequency tunings for which the carrier (main central peak) or
the modulation sidebands (smaller side peaks) are resonant in the cavity. The lower trace shows the
PDH signal for the same frequency tuning. Coincident with the peaks in the upper trace are bipolar
structures in the lower trace. Each of the bipolar structures would be suitable as a length-sensing
signal. In most cases the central structure is used, as experimentally it can be easily identified because
its slope has a different sign compared to the sideband structures.