The cause of the decrease was found and the problem was solved. We found this entry, which says
Yoich> We opened the MOPA box and installed a mirror to direct a picked off NPRO beam to the outside of the box through an unused hole.
Yoich> We set up a lens and a PD outside of the MOPA box to receive this beam. The output from the PD is connected to the 126MON cable.
We went to the PSL table and found the dc power cable for 126MOPA_AMPMON was clipping the 126MON beam.
We also made a cable stay with a pole and a cable tie.
After the work, 126MON went up to 161 which was the value we saw last night.
We also found that the cause of the AMPMON signal change by the DAQ connection, mentioned in this entry:
Jenne> 6. We teed off of the AMPMON photodiode so that we could see the DC values on a DMM.
Jenne> When we used a T to connect both the DMM and the regular DAQ cable, the DMM read
Jenne> a value a factor of 2 smaller than when the DMM was connected directly to the PD.
We found a 30dB attenuator is connected after the PD. It explains missing factor of 2.
Steve pointed this out to me today, and Koji and I just took a look at it together: The total power coming out of the MOPA box is constant, about 2.7W. However, the NPRO power (as measured by 126MOPA_126MON) has decreased from where we left it last night. It's an exponential decay, and Koji and I aren't sure what is causing it. This may be some misalignment on the PD which actually measures 126MON or something though, because 126MOPA_LMON, which measures the NPRO power inside the NPRO box (that's how it looks on the MEDM screen at least...) has stayed constant. I'm hesitant to be sure that it's a misalignment issue since the decay is gradual, rather than a jump.
Koji and I are going to keep an eye on the 126MON value. Perhaps on Monday we'll take a look at maybe aligning the beam onto this PD, and look at the impedance of both this PD, and the AMPMON PD to see why the reading on the DMM changed last night when we had the DAQ cable T-ed in, and not T-ed in.