Koji came down to the lab to have a look at the pre-cavity BN dector. With a might tigher lens f = 37 mm he was able to get a DC voltage of - 2.59 V for an total input power of 577 µW. We should expect about 4.00 V for this power, but this is what we get. We couldn't see a beat note until Koji started changing the laser crystal temperature on the south laser controller. The set point temperature was changed from 34.4961 C to 47.0749 C. Slow laser voltage offset channels were set to 0.6901 V for the south path and 3.8294 V for the north path. With these settings we were able to see a BN at 216 MHz of -50 dBm. This is sufficient for the diagnostic purpose of this detector.
For reference, here is a configuration summary of the lasers and controls at present:
BN = -50 dBm @ 216 MHz
South slow frequency voltage offset = 0.6901 V
South slow frequency voltage offset = 3.8294 V
Todo: make a BN laser north vs laser south map similar to what koji did in this this post 40m:3759. This should make the good operating parameter regiem obvious in the future.