Figure 1

Left: Simplified noise budget for Advanced LIGO, showing the relative contribution of quantum noise, mirror thermal noise (Brownian and thermo-optic) as well as everything else to the design sensitivity. Right: Example noise budget for a modified LIGO interferometer with the 4.5 spot resonant delay line shown in Fig. 4. The design uses existing Advanced LIGO mirror coatings. To improve the quantum noise the test masses were increased from 40 to 160 kg, and a broadband quantum noise reduction of 6 dB through the use of nonclassical states of light was assumed. Finally all mirror reflectivities were then retuned to optimize the quantum noise in this configuration. Both plots were calculated using the Gravitational Wave Interferometer Noise Calculator (GWINC) tool, a software developed by the gravitational wave community.Reuse & Permissions

Figure 2

Fabry-Perot cavities configured in standing wave (left) and traveling wave geometries (right).Reuse & Permissions

Figure 3

Example of a positive branch (left) and negative branch (right) folded Fabry-Perot cavity with four bounces. Both represent a traveling wave configuration.Reuse & Permissions

Figure 4

Standing wave cavity with an elliptical beam orbit, five spots on the input test mass, and four spots on the end test mass. Left: side view; right: frontal view with the elliptical beam orbits indicated in yellow. The g-factor was chosen to be equal to Advanced LIGO ($g=0.8303$), corresponding to $N_b=7.4$ bounces per orbit, and a beam size of $w=57.3\mathrm{mm}$. The optics have a radius of 40 cm, and a thickness of 15 cm, corresponding to a mass of 160 kg if made out of fused silica. This design provides a beam clearance of about $2.5w$, limiting clipping losses to about 1 ppm (lower on the end test mass). The coating Brownian thermal noise of this configuration is 2.1 times below Advanced LIGO in amplitude. (The arm length is shrunk by a factor of 1000 for illustration purposes.)Reuse & Permissions

Figure 5

The geometry used to calculate the mirror size required to support a delay line with $N_b$ bounces per mirror.Reuse & Permissions