During Yuhang's Christmas visit we attempted to replace the TAMA OPO. It was aligned but could not take a scan signal - the high voltage driver would just zero out then turn off, suggesting that the PZT on the input mirror is broken. I have to disassemble the "new" OPO and replace the PZT. The connectors for the PZT are soldered to our Dsub box - the OPO was kept in a zip lock bag during soldering. The Dsub box connects the OPO PZT and temperature actuator wires and puts them on a Dsub port, which then plugs into the controls of TAMA electronics.

I measured the output power vs current of the Coherent Mephisto laser in the ATC cleanroom used for the Taiwan and Korea experiments, using both standard Thorlabs (up to 500 mW) and thermal (several W) power meters. I fixed the laser temperature at 24.21 C. The output of the laser is about 47 mW at 0.8 A, up to 1.17 W at 1.5 A (it can go higher but I stopped here).

In the leadup to the Taiwan/Korean Christmas visit I designed the input beam path to: 1) inject collimated infrared p-polarized light into the Taiwan fiber SHG (~2.3mm diameter), and 2) send a small amount of s-polarized light to the Korea Faraday isolator. The actual beam profile was measured by Yi Ru Chen. There is a HWP and PBS setup which transmits p-pol to Taiwan (SHG) and reflects s-pol to Korea (Faraday). I set up to reflect a small amount of s-pol to Korea. At 0.800 A laser current, 30 mW reaches the HWP, with 3.8 mW s-pol going to Korea and 25.1 mW p-pol going to Taiwan. 3.8 mW is reasonable for our OPO characterization, it's about what we used last time. There is plenty of room to adjust up or down anyway, and since the beam size going to the OPO is around 20 micron the intensity is quite dangerous even at 4 mW.

I constructed a beam path to travel across the indicated line on the breadboard, cutting off from the Korean FI path (they aren't going to come back until October). The beam travels horizontally at 75mm, the height of the OPO crystal. In a previous iteration of this test we had a rotation stage below the OPO for rotation alignment but we found it to be unnecessary. It is better to set a beam at constant height, rough align the OPO case by hand then fine align with steering mirrors. We also did it this way in TAMA twice during the replacement/re-replacement episode and it works fine, could achieve > 90 % mode matching quickly. The beam path should be collimated at 2.3mm for quite some distance, so based on this I can set up a system to send the beam through an EOM. The lenses we originally used to mode match to the OPO (40 and 75 mm) are in the ATC unused.

I took a soldering iron from the elec shop and the Beam Master beam profiler from birefringence compensation. For other instruments the ATC KOACH cleanroom now has the 8 GHz function generator, PZT high voltage driver, Thorlabs temperature controller, resonant EOM, oscilloscope, old TAMA spectrum analyzer, PDA05CF2 RFPD, DET10N/M biased PD. We can scan the PZT of the reassembled OPO to test mode matching but to lock I would need another function generator or mokulab. The CCD and TV screen were taken to speed meter but maybe I don't need them. Locking is probably unnecessary though. It seems based on previous calculations that, due to the short length and low finesse, the storage time is too short to determine internal loss from cavity ring-down, even with a nanosecond scale off-switch. We will need to wait until the sqz/antisqz measurement to really see the optical loss.