“…The thermal noise limit is calculated to be 1.3 × 10 -15 for the cavity with 6-μm dielectric-coated FS mirrors, a radius of curvature of 1 m, and bore-aperture of 15 mm [22,26]. It can be reduced by increasing the cavity length L [27, 28], operating at cryogenic temperature [29-32], increasing the beam radius w 0 [25,33], and using materials with low mechanical loss φ [34,35]. Considering miniaturization and non-laboratory operation, only the latter two work.…”