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Entry  Sat Jul 30 10:01:35 2011, Jenny, Update, PSL, PSL table work  modematchpic.pngmodematchinfo.pngLIGO_block_diagram.pngLIGO_block_diagram.pngmodematch_setup_pic.png
    Reply  Sun Jul 31 00:37:57 2011, kiwamu, Update, PSL, Re : PSL table work  
Message ID: 5069     Entry time: Sat Jul 30 10:01:35 2011     Reply to this: 5075
Author: Jenny 
Type: Update 
Category: PSL 
Subject: PSL table work  

I've been working on the PSL table to put together a setup so that I can measure the reference cavity's response to a temperature step increase at the can surrounding it. My first step was to mode match the beam coming from the AP table to the cavity.

I implemented my mode matching solution. I ended up using a different one from the one I last elogged about. Here is the solution I used:

Two lenses: f = 1016.7.6 mm at -0.96 m and f = 687.5 mm at -0.658 m. (I set my origin at the polarizing beam splitter--the spot where I want my beam to match the beam coming from the PMC, so all waists are behind that point). Below is what it should look like.

modematchpic.pngmodematchinfo.png

What I did on the table:

  • Before placing lenses I aligned the beam and added a 1/2-wave plate between the two polarizing beam splitters to change the polarization of the beam from S to P.
  • I aligned the beam so that it reflected off of the cavity opening (monitoring the reflected power with a photodetector connected to an oscilloscope and tweaking the alignment to maximize the reflected signal). 
  • I then placed the lenses at -0.93 and -0.64 mm because the exact spots were blocked by optics being used in another setup.
  • I reasoned that since the fitting for the initial waist is so uncertain, the lens position being off by a few cm will not produce the dominating source of error. I am now driving the laser frequency using a lock-in as a function generator to drive the laser temperature at ~1 Hz. I'm then monitoring the power transmitted by the reference cavity with a camera connected to a TV monitor. I will use this setup to improve my mode matching.

Here's a picture of the PSL table with the lenses and mirror I added. The beam is redirected by a mirror and then a polarizing beam splitter. Past the beam splitter is another lens (f=286.5 mm), which was already in place from the mode matching of the beam from the PMC to the reference cavity.

modematch_setup_pic.png

Here is a block diagram of my intended experimental setup:

LIGO_block_diagram.png

I am going to try to lock the laser to the cavity given my preliminary mode matching and then go back and improve it later. My next step is to find a frequency range for dithering the voltage sent to the PZT. To do this I will:

  • Measure the transfer function (amplitude response) of the PZT using a photodiode. The power outputted by the laser varies with driving frequency.
  • Find a frequency region in which the amplitude response is low.
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LIGO_block_diagram.png
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