The lasers are not Gaussian but Lorenztian. This is a detail that’s not terribly important. The lasers generate frequencies in C-band at ~194THz. The O-band

The lasers are not Gaussian but Lorenztian. This is a detail that’s not terribly important. The lasers generate frequencies in C-band at ~194THz. The O-band photon source is ~229THz. Classically these fields do not interact. For passive beam splitting, there is also no quantum interference. The photo detectors are broadly sensitive to both frequency bands. So if the C-band light hits the photodiode you won’t be able to count photons. The filters are there to reduce the background “noise” of the C-band light hitting the photodiode that’s being used for photon counting. The use of filters for this purpose is very common and really not that interesting.