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Entry  Wed Dec 17 13:54:19 2014, Zach, Laser, SiFi, Lasers mounted, energized, beat set up diagram.jpgsetup_with_beat.jpg
    Reply  Wed Dec 17 14:40:15 2014, Dmass, Laser, SiFi, Lasers mounted, energized, beat set up 
       Reply  Wed Dec 17 18:11:38 2014, Zach, Laser, SiFi, Lasers mounted, energized, beat set up 
Message ID: 1182     Entry time: Wed Dec 17 13:54:19 2014     Reply to this: 1183
Author: Zach 
Type: Laser 
Category: SiFi 
Subject: Lasers mounted, energized, beat set up 

On Monday, after I did some inventory of all the parts we have received from various companies, Dmass helped me mount the RIO lasers into their mounts so that I could get started with the optical setup. We cleaned the surfaces with methanol, applied a small layer of silver thermal compound, and then screwed them in.

I then borrowed the following to run the lasers:

  • The (separate) ThorLabs diode driver and temperature controller from Haixing's maglev setup
  • An integrated ThorLabs diode driver / temperature controller from the TCS lab

After finding the right cables, I powered up the lasers and verified the P-I curve for each as listed on the spec sheets.

I then built a quick (temporary) optical beat setup, combining the two beams on an 1811. I had the temperatures (actually, thermistor resistances) set to what was listed as the testing set point on the datasheet, and as soon as I overlapped the beams and focused them onto the PD, there was already a strong ~50 MHz optical beat.

diagram.jpg setup_with_beat.jpg

I have spent some time since then trying to lock various kinds of PLLs, both to interrogate the free-running frequency noise and to get used to controlling the lasers. Some things I've tried:

  • Locking a Marconi to the free-running beat, which I think might be an exercise in futility due to the relatively small range of the Marconi FM
  • Locking one laser to the other directly using a PLL, which I think might be an exercise in futility due to the bandwidth of the current actuation from the ThorLabs driver
  • With Dmass's help, locking a Zurich PLL to the free-running beat. This appeared to work, and we saw a preliminary frequency noise spectrum that looked about right, but I'm skeptical because the control signal doesn't seem to respond to my slewing one laser's frequency.
  • Briefly, locking one laser to the other at low frequencies using the Zurich PLL control signal as a frequency discriminator. This didn't work, adding to my suspicion.

The first two were not helped by the fairly basic loop shaping afforded by attenuators and an SR560.

I think my next step will be to simply use the I-Q demodulation method (like I did to measure the no-FM Marconi noise in ATF:1877) to measure the frequency noise. I'll compare that to what I get with the Zurich PLL.

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