The west arm has been measured to be ~3cm shorter than the north arm (VIR-1085A-21). This causes the 56MHz TEM00 to resonate at different frequencies in the north and west arm. A question is if it causes issues with noise couplings.
Figure 1 shows the fitted coupling of CARM to DARM at a time of good sensitivity.
Figure 2 is the corresponding noise budget.
A 3cm length asymmetry over 3km length arm creates a 1e-5 coupling of CARM to DARM. This makes sense intuitively, and is confirmed by optickle simulations. So that asymmetry is comparable to what is measured once BS and SR are misaligned in LN3. Before misalignments the measured CARM to DARM coupling can easily be a factor 10 higher.
So the ~3cm arm length difference is not a major contribution to SSFS noise coupling. The other simple noise coupling is due to the BS AR coating, which creates a 3e-6 CARM to DARM coupling, the two could cancel each other if the west arm was ~1cm longer than the north arm. Something to keep in mind for the far future that we could use the difference in arm length to fine tune CARM to DARM couplings if we ever get to a situation where the coupling starts to be smaller than 1e-5.