This system will utilize a suspended optical resonator to stabilize the laser frequency in order to provide sufficiently frequency stabilized input light to the sub-SQL interferometer. The equivalent displacement fluctuations for the laser frequency fluctuations need to be suppressed down to 10^-16 to 10^-18 m/sqrt Hz from 20 to 1 kHz as shown in the figure below, labeled as Requirement.
We aim to suppress the sum of all the noise sources to the level shown as Total Noise in the figure. The level is 10 times better than the requirement, and in this way we will have an enough safety margin. The total noise will be dominated by seismic motions, thermal noises and the shot noise in the low, middle, and high frequency region, respectively. To isolate the seismic motions a triple suspension system will be utilized to suspend the cavity mirrors, as well as seismically isolated optical tables. Various suspension design parameters (e.g. resonance frequencies, wire thickness) as well as substrate material, are chosen to optimize the thermal noises. Optical design parameters (e.g. laser power inside the cavity, overall reflectivity of the cavity) are tuned to optimize the shot noise and the radiation pressure noise.
The reference cavity consists of three mirrors that form a triangular shape. Two mirrors will be placed on the central optical table and the other on the south table to achieve a long round trip distance of 25m, as sown in the figure below.
Since the tolerable displacement fluctuation scales as the cavity length, having a longer cavity is beneficial. A small amount of light(130mW) from a 35W laser will be picked off and injected into this system where the cavity finesse is high (7300) so that the circulating power is enough to have optimized shot noise signal to noise ratio, but small enough that the radiation pressure will not cause problems. The rest of the power will be injected into the pre-mode cleaner, placed upstream of the reference cavity, for mode cleaning purpose. The laser light is transmitted through the optical fibre before entering into the pre-mode cleaner and it provides additional mode cleaning process. Separating the two functions of spatial cleaning and frequency stabilizing enables us to avoid radiation pressure issues. A triple suspension system will suspend the 850g mirrors to sufficiently filter out the ground motion.
A Pound-Drever Hall scheme is applied to control the cavity length, laser frequency and angular motions. At frequencies well above the pendulum resonance, the laser frequency is controlled so that it stays on resonance, and below that frequency the cavity length is controlled.