J. Oberling, P. King

After Peter's check of the PSL this morning, we noticed that the power out of the HPO was down to ~154W; it was at ~165W before yesterday's PSL shut-down for the water leak fix.  We began adjusting the currents and temperatures of various pump diodes to see if we could find where the power loss came from.  All work was done with the ISS OFF.

The operating temperature for pump diode 1 in HPO Diode Box (DB) 1 was adjusted from 24.0 °C to 20.0 °C.  This increased the relative output of the diode box from 89.0% to 89.5%, but only increased the total power out of the HPO by ~1.0W.  Recall that diode 1 in HPO Diode Box 1 has been on a steady downward trend.  I then noticed that the 35W FE was reading 33.9W, so I proceeded to adjust the FE pump diode operating currents.  The change is summarized in the table below.

This only increased the power out of the FE to 34.1W; adjusting the pump diode operating temps did not change the output power.  Our theory here is that the NPRO output power is low enough that the MOPA is unable to increase the power anymore than this.

We then began adjusting the HPO pump diode currents.  In the interest of preserving DB1, we began by increasing the current to only DBs 2, 3, and 4.  Interestingly, increasing these pump diode currents lowered the output power of the HPO.  We returned the currents to their original value and increased the current for DB1 only; same result, lower HPO output power.  We then decreased the currents for all DBs and, lo-and-behold, the HPO pump power increased.  The currents were decreased by 0.4 A in total, and the HPO is now outputting ~157.4 W.  The theory here is that the decay of DB1, specifically pump diode 1 in DB1, has caused the thermal lens in Head 1 to change, thereby changing the location of the stability range of the HPO.  The stability range moved lower, which resulted in the HPO output power being on the opposite side of the range (hence why increasing pump diode currents lowered the HPO output power).  This then required a decrease in pump diode currents to increase HPO output power.  A summary of the new and old HPO pump diode currents is shown in the table below.

I have attached a screenshot showing the new values of both the HPO DBs and the FE DB for future reference.  The above behavior is indicative of a DB that is near the end of its useful life.  I have spoken with LLO (the holder of all spare HPO DBs) and they are shipping 2 HPO DBs our way, hopefully out the door this week.

This closes FAMIS 8423.