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Message ID: 13900     Entry time: Thu May 31 02:04:55 2018
Author: johannes 
Type: Update 
Category: PSL 
Subject: AUX laser state of mind 

The AUX laser is down to 5.4 mW output power sad

What's worse, because we wanted those fast switching times by the AOM for ringdowns, I made the beam really small, which

  1. came with a severe tradeoff against conversion efficiency. I tried to squeeze the last out of it today, but there's only about 1.3 mW of diffracted light in the first order that reaches the fiber, with higher diffraction orders already visible.
  2. produced a very elliptical mode which was difficult to match into the fiber. Gautam and I measured 600 uW coming out of the fiber on the AS table. This per se is enough for the SRC spectroscopy demonstration, but with the current setup of the drive electronics there's no amplitude modulation of the deflected beam.

When going though the labs with Koji last week I discovered a stash of modulators in the Crackle lab. Among them there's an 80 MHz AOM with compact driver that had a modulation bandwidth of 30MHz. The fall time with this one should be around 100ns, and since the arm cavities have linewidths of ~10kHz their ringdown times are a few microseconds, so that would be sufficient. I suggest we swap this or a similar one in for the current one, make the beam larger, and redo the fiber modematching. That way we may get ~3mW onto the AS table.

I think I want to use AS110 for the ringdowns, so in the next couple days I'll look into its noise to get a better idea about what power we need for the arm ringdowns.

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