Does anyone make a coherent CWDM 40G QSFP? I thought so, but the first couple places I checked, I struck out at. This would be for a passive mux\MROADM.
Not aware of ACO/DCO in QSFP form factor. Inphi is doing 100G QSFP28 PAM4
DWDM for MS. Probably the best you will see for a while.
Apparently I just remembered the big transport platforms using coherent 40G and 100G and assumed there was a cheap variant, but there isn't.
True 40 gigs is rare period. Most of the time it is bandwidth limiting on a 100 gig wave.