TravisS, RickS

Today, we went down to address the reduced laser output power.

First, we assessed the beam pointing by looking at the entrance to the Rx-side integrating sphere. The beams were right on top of each other and well centered in the aperture (good news, the pointing seems quite stable).  See attached photo.

The laser output power was down to 1.67 W, from 2 W.  Given that this laser has been running continuously for about 5 years, this is not unexpected.  By tuning the diode temperature (W1 pot on the laser power supply), we increased the power to 1.72 W.  Then, by increasing the diode current slightly (5.135 V on pins 1 and 6 of the "Current and Temp Monitor" DB9 connector on the front panel of the Pcal interface module increased to the max level of 5.218 V) the power increased to 1.76 W.  We will check with the manufacturer to see if the current can (and should) be increased beyond this limit.

Note that there are issues with the current and temperature monitor channels on the MEDM screen.  The current monitor is railed at 10 and the temperature is at 4.8, but apparently responding to temperature changes.  Both should be investigated.

We then worked through sections 4, 5 and 6 of T1600436-V8 (which will soon be edited to address a couple of issues we discovered).  We found that the OFS servo was able to handle the maximum requested offset of 10.0 V.  Thus, we replaced a ThorLabs NE01A-B filter (OD 0.1) in front of the OFS PD with a NE02A-B filter (OD 0.2).  The max offset level in this configuration was 7.2 V. (Note that the calibration of the requested modulation amplitude is impacted by this change.)

The "95 % OFS Offset" is thus 6.8 V and the "Operating OFS Offset" is now 3.4 V.

The AOM input power was 1.66 W and the diffracted power was 0.959 W with 0.339 W un-diffracted power.  The AOM efficiency was only 58%.

By opening the OFS servo and driving the modulation input for maximum power (~1.5 V by running the OFS servo gain to max with the loop open), we got 1.11 W measured between M5 and WBS.  1.66 W in and 1.11 diffracted is only 67 %.  Should be 70-80%.

About 18 mW is rejected by the PBSC downstream of the AOM.

The measured 80 MHz drive level to the AOM with the modulation input at max. was 1.242 W (spec. is less than 1.3 W). 

We ran out of time to investigate further, but we might try replacing the AOM in the future.

The optimum OFS gain setting will have to be revisited too.