Toyed around with the modematching some more today.
The outermost glass elements of the OSA are about 28cm apart.
With the OSA beeing a confocal cavity that should mean that the ROC of every mirror is 28cm on the cavity side. If the input surface is flat we need a 28cm focusing lens for good MM. If it's not we shouldn't need any MM.
Tried a f=250mm lens on different positions first. Got at best about 570mV (PD gain=10) in transmission of the OSA.
Then tried a f=1000mm lens. Best transmission 1.22V (7.2% transmission). SB were (PD gain =100) 11MHz: 87.2mV (m=0.17) , 55MHz: 59.2mV (m=0.14)
Lost the position while toying around. Left it then at 1.0V transmisison at 15:15 local time. Let's see how much it drifts. SBs for this were 11MHz: 52.8mV (m=0.17), 55MHz: 73.8mV (m=0.14)
[Ed by KA: If the carrier transmission was really 1.22V and 1.0V the modulation depths calculated are inconsistent with the measurement.]
Spacing between carrier 11MHz and 55MHz SBs seems consistent, and leads to a FSR measurement of 1.5GHz, also fine.
Update: After 90mins no change in carrier transmitted power. Next morning: Carrier transmission down by 10%.