On the spectral structures in DARM at 100-300Hz. As already reported, we firstly suspected CARM direct contamination but it would not the case (24169).

As shown by some bruco results, IMMT1 trans QPDs seem to have large coherence with those spectral structures, so another senario might be due to angular jitter of input beam. If you assume that the angular jitter in the light beam incident on PRC might be converted to intensity noise, which would be also seen in TMSX/Y IR PDs. So I checked them but unfortunately may be the ADC noise would dominate these channels in the frequency range concerned... Can we apply whitening filters for these channel??