In light of the discussion at today's meeting, Guantanamo and I looked at how the series resistance for the test mass coil drivers limits the amount of squeezing we could detect.
The parameters used for the following calculations are:
- 4.5 kΩ series resistance for the ETM's (this was 10 kΩ in the previous calculations, so these numbers are a bit worse); 15 kΩ for the ITM's
- 100 ppm transmissivity on the folding mirrors giving a PRC gain of 40
- PD quantum efficiency of 0.88
Since we need to operate very close to signal recycling, instead of the current signal extraction setup, we will need to change the macroscopic length of the SRC. This will change the mode matching requirements such that the current SRM does not have the correct radius of curvature. One solution is to use the spare PRM which has the correct radius of curvature but a transmissivity of 0.05 instead of 0.1. So using this spare PRM for the SRM and changing the length of the SRC to be the same as the PRC we can get
- 0.63 dBvac of squeezing at 205 Hz for 1 W incident on the back of PRM
- 1.12 dBvac of squeezing at 255 Hz for 5 W incident on the back of PRM
This lower transmissivity for the SRM also reduces the achievable squeezing from the current transmissivity of 0.1. For an SRM with a transmissivity of 0.15 (which is roughly the optimal) we can get
- 1 dBvac of squeezing at 205 Hz for 1 W incident on the back of PRM
- 1.7 dBvac of squeezing at 255 Hz for 5 W incident on the back of PRM
The minimum achievable squeezing moves up from around 205 Hz at 1 W to 255 Hz at 5 W because the extra power increases the radiation pressure at lower frequencies. |