During a cavity scan, a second modulation is applied to the laser frequency. The response of a Fabry-Perot cavity to laser frequency modulation contains information about the cavity parameters, including cavity length, free spectral range, modal spacing, and the radius of curvature of the cavity optics. The goal of cavity scans is to measure cavity characteristics and characterize their time evolution in response to heating of the optics.

While the 532 nm ALS laser is locked to the arm cavity, laser frequency modulations are injected into the Innolight Prometheus frequency-doubled laser through the laser frequency servo (Common Mode A) with an SR785 signal analyzer. These frequency modulations are transmitted into the 1064 nm beam used for PSL phase locking and the 532 nm beam used for arm cavity locking. The PSL phase locking beam does not interact with the arm cavity, and the signal from the RF photodiode is used as a proxy for the signal injected into the arm cavity. The arm cavity reflection photodiode gives the output signal, which the SR785 divides by the input signal to produce a transfer function.

Automation of SR785 measurements is necessary to perform and store transfer functions in quick succession over a period of hours. A Prologix GPIB-Ethernet controller is used for remote control and retrieval of data from the SR785. Scripts for performing transfer functions and retrieving data using the GPIB-Ethernet interface were created for use at the 40m interferometer, and were used for cavity scan transfer functions here. The Python script exttt{TFSR785.py} performs a single transfer function.

A bash script, exttt{autoTF}, was used to repeatedly call this Python script. This script was originally set to perform one scan from 30kHz to 80kHz, alternate smaller scans around the first-order modes (from 46-47 kHz and from 65.75-66.75 kHz) three times, then repeat until 12 hours elapses or the script is terminated. During the scan, it was found that the first-order peaks moved further than anticipated, so the script was altered to scan from 45-46 kHz and 66.75-67.75 kHz. In future scans, the smaller scans will be run from 45-47 kHz and 65.75-67.75 kHz for the duration. For all scans, the amplitude of the excitation was 10 mV, and 10 averages were used.

The ALS laser output power is 100 mW, and the SR785 excitation output current is 100 mA. Therefore, the modulation depth for these cavity scans is 0.1, or 1% of power in the modulation sidebands.

The attachment cavity_characterization.pdf contains a more thorough explanation of the motivation behind cavity scans, and some preliminary results. The attachment cavityshift.pdf plots a cold cavity scan and a cavity scan after 2.5 hours of heating on the same axes, showing the shift in modal spacing with ITM heating.