J. Oberling, P. King

LHO WP #5810.

Short Version

IT'S ALIVE!!!

Long Version

Today we began the HPO turn on procedure.  First things first, we put a water cooled 200W power meter head in the main beam path after the PMC (see attached picture 200W_power_meter_20160404.jpg).  This way no light is getting out of the PSL enclosure until we are ready for it, and once we have a recovered PMC we simply need to plug in a power meter so we can set the available power with IO_MB_HWP1. 

In an abundance of caution we began by repeating the HPO Laser Head turn on test we did back on March 22nd (see LHO alog 26193).  Everything came up without issues, just as in the previous test.  We then removed the HR mirror that sat in front of the HPO Output Coupler (OC); it is currently left in the HPO box, set to the side and out of the beam path (see attached picture OC_removed_20160404.jpg).  This taken care of, we set all the laser heads to 30 A of pump diode current and turned on the HPO.  As the currents were coming up we saw the HPO go through the first, low power stability range, but nothing at the high power stability range (which is what we expected, not enough pump power yet).  We repeated this at 35 A, 40 A and 45 A with the same results.  Getting a little discouraged we input the previous operating currents (from Peter's alog here) and turned on the HPO.  This time the laser came on!  It started at ~120 W of total power (~70 W in the forward direction and ~50 W in the backward direction).  We then increased the diode currents individually until they all read ~100% power and then let it sit for ~10 minutes so everything could come to thermal equilibrium; the output power settled at ~145 W.  As a final check we moved the pump currents up and then down slightly (~0.4A in both directions) to check that we were on the correct side of the stability range; we are.

Now with a running HPO we decided to try injection locking the 35 W FE.  Following the instructions in T1200259 we powered up the FE laser and pressed the "Lock" button on the PSL Beckhoff computer control screen.  It locked, without issue.  Total output power now read 208 W.  This seemed really high, and looking at the HPO diode currents it seemed the diodes for head 4 were suddenly delivering more power without changing current; power changed from 100% to 105% at the same diode current.  This is very odd and we currently cannot explain it.  We lowered the current back to 100% power and the total power out of the full system was reading 183 W.

It was at this point that we realized we had forgot to align mirror M5, which controls the beam out of the HPO.  This is important as this mirror needed to be tweaked when the HR mirror previously in front of the OC was installed back in 2014.  In the process of trying to realign this mirror we lost the alignment entirely and spent the afternoon trying to recover it.  We eventually turned the HPO off, reinstalled the HR mirror and used it and M5 to mostly recover the alignment.  We are currently at a state where spending more time recovering this alignment is a waste as we are going to have to do alignment work with the HPO on anyway.

We left the system with the HPO OFF and the lid on; as a safety measure we wrote down the last used HPO diode currents and set the current input fields to 0 A.  We the left the FE laser ON but shuttered, so there is no laser light available to the IFO.  We left the environmental controls (HEPA fans, AC, and Make-Up air) ON.

Tomorrow

Tomorrow we plan on once again removing the HR mirror and then beginning the recovery of the PSL subsystems downstream of the HPO; PMC, ISS, FSS, DBB (likely in that order).