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Entry  Wed Nov 13 18:32:04 2013, Nic, Evan, Update, ISS, SR560 ISS loop psl_aom_overhead.jpgaom_driver.jpgloop_on_settings.jpgfxn_gen.jpg40m_iss.pdf
    Reply  Wed Nov 13 19:41:55 2013, Jenne, Update, ISS, ISS AOM 
    Reply  Wed Nov 13 20:02:12 2013, Nic, Evan, Update, ISS, SR560 ISS loop ISS_560_rot.pdfISS_560cal.pdf
       Reply  Fri Nov 15 10:31:45 2013, Steve, Update, ISS, SR560 ISS loop connection ISS.png
       Reply  Thu May 8 02:03:51 2014, rana, Update, ISS, ISS: fuse was blown, repaired, loop back on ISS.pdf
Message ID: 9380     Entry time: Wed Nov 13 20:02:12 2013     In reply to: 9376     Reply to this: 9392   9929
Author: Nic, Evan 
Type: Update 
Category: ISS 
Subject: SR560 ISS loop 


We have implemented an SR560-based ISS loop using the AOM on the PSL table. This is a continuation of the work in 40m:9328.

We dumped the diffracted beam from the AOM onto a stack of razor blades. This beam is not terribly well separated from the main beam, so the razor blades are at a very severe angle. Any alternatives would have involved either moving the AOM or attempting to dump the diffracted beam somewhere on the PMC refl path. We trimmed the RF power potentiometer on the driver so that with 0.5 V dc applied to the AM input, about 10% of the power is diverted from the main beam.

We ran the PMC trans PD into an AC-coupled SR560. To shape the loop, we set SR560 to have a single-pole low- pass at 300 Hz and an overall gain of 5×104. We take the 600 Ω output and send it into a 50 Ω feed-through terminator; this attenuates the voltage by a factor of 10 or so and thereby ensures that the AOM driver is not overdriven.

The AOM driver's AM input accepts 0 to 1 V, so we add an offset to bias the control signal. The output of the 50 Ω feedthrough is sent into the 'A' input of a second SR560 (DC coupled, A − B setting, gain 1, no filtering). Using a DS345 function generator, a 500 mV offset is put into the 'B' input (the function generator reads −0.250 V because it expects 50 Ω input). The 50 Ω output of this SR560 is sent into the AOM driver's AM input.

A measurement of suppressed and unsuppressed RIN is attached. We have achieved a loop with a bandwidth of a few kilohertz and with an in-loop noise suppression factor of 50 from 100 Hz to 1 kHz. This measurement was done using the PMC trans PD, so this spectrum may underestimate the true RIN.

 A small followup measurement. Here are spectra of the MC trans diode with and without the ISS on. The DC value of the diode (in counts) changed from 17264.2 (no ISS) to 17504.3 (with ISS), but I didn't account for this change in the plot.

There is a small inkling of benefit between 100Hz and 1kHz. Above about 100Hz, the RIN is suppressed to about the noise level of this measurement. Below 100Hz there is no change, which probably means that power fluctuations are introduced downstream of the AOM, which argues for an outer-loop ISS down the road.

Atm #2 is in units of RIN.

Attachment 1: ISS_560_rot.pdf  49 kB  | Hide | Hide all
Attachment 2: ISS_560cal.pdf  49 kB  Uploaded Thu Nov 14 00:35:27 2013  | Hide | Hide all
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