HOM GW spectrometer response, photon polarization angular rotation in function of time (estimated from LIGO numerical waveform data for GW150914), with the spectrometer large signal arm lengths set at Ls  =  1.000 km, Ls  =  5.000 km, and Ls  =  10.000 km as shown in insets (a)–(c), respectively. These outputs are in line with the respective interferometers normalized transfer functions ploted in figure 4. Insets (a) indicate a configuration of the interferometer tuned on short wavelengths adequate to investigate the ring down phase. Inset (b) shows one large envelope enclosing signals of varying frequencies with maximum crest corresponding to a GW wavelength equal to λg=2×Ls=10.000 km. Inset (c) reveals two envelopes with maximum amplitude for GW wavelengths of λg=2Ls=20.000 km and λg=2Ls/3=6.667 km, the minimum between the two envelopes correspond to the GW wavelength λg=Ls=10.000 km (which is coherent with inset (b)).