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Entry  Sat Mar 26 17:39:50 2016, Koji, Electronics, Characterization, HQEPD dark noise PD_dark_current.pdf
    Reply  Sat Mar 26 18:22:24 2016, Koji, Electronics, Characterization, Baking / Contamination tests of the PDs 
       Reply  Tue Apr 5 18:22:40 2016, Koji, Electronics, Characterization, Baking / Contamination tests of the PDs QE_after_air_bake.pdf
          Reply  Tue Apr 5 21:20:15 2016, Koji, Electronics, Characterization, More dark noise measurement PD_dark_current.pdf
Message ID: 259     Entry time: Tue Apr 5 18:22:40 2016     In reply to: 257     Reply to this: 260
Author: Koji 
Type: Electronics 
Category: Characterization 
Subject: Baking / Contamination tests of the PDs 

Possible reduction of the QE was observed after air-bake at 75degC.

Yesterday I received Cage G from Bob for intermediate test of the PD performance after air bake but before vacuum bake.
This cage was prepared to be the production pair.

According to the ICS, https://ics-redux.ligo-la.caltech.edu/JIRA/browse/Bake-8047
the PDs were air baked at 75degC for 48 hours.

I took the PDs to my lab to check if there is any issue in terms of the performance.
- Dark current: No change observed
- Dark noise: No noise increase observed
- QE: Probably reduced by ~0.5%.

Here I attached the result of the QE measurement. I have measured the QEs of the baked ones (A1-23 and A1-25) and the reference. Since the reference PD has not been baked, this gives us the measure of the systematic effect. The reference showed the reduction of ~0.1%. Assuming this reduction came from the systematic effect of the measurement system, I observed at least 0.5% QE reduction (A1-23). Note that the previous measurement of 99.8% for A1-25 was too high and dubious. But both A1-23 and A1-25 showed ~0.4% lower QEs.

So I believe the air-baking process reduced the QE.

Another evidence was that now I could clearly see the beam spots on these air-baked-PDs with an IR viewer when the PDs were illuminated with a 1064nm beam. Usually it is difficult to see the spot on the PD. The spot on the reference PD was still dark. So this difference was very obvious. I was afraid that something has been deposited on the surface of the photosensitive element. The surface of the diodes looked still very clean when they were checked with a green LED flash light.

Attachment 1: QE_after_air_bake.pdf  174 kB  | Hide | Hide all
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