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Entry  Mon Nov 7 17:24:12 2016, gautam, Update, Green Locking, Green beat setup on PSL table CurrentX.pdfCurrentY.pdfProposedShift_copy.pdf
    Reply  Wed Nov 9 23:21:44 2016, gautam, Update, Green Locking, Green beat setup on PSL table 
Message ID: 12603     Entry time: Mon Nov 7 17:24:12 2016     Reply to this: 12609
Author: gautam 
Type: Update 
Category: Green Locking 
Subject: Green beat setup on PSL table 

I've been trying to understand the green beat setup on the PSL table to see if I can explain the abysmal mode-matching of the arm and PSL green beams on the broadband beat PDs. My investigations suggest that the mode-matching is very sensitive to the position of one of the lenses in the arm green path. I will upload a sktech of the PSL beat setup along with some photos, but here is the quick summary.

  1. I first mapped the various optical components and distances between them on the PSL table, both for the arm green path and the PSL green path
  2. Next, setting the PSL green waist at the center of the doubling oven and the arm green waist at the ITMs (in vacuum distances for the arm green backed out of CAD drawing), I used a la mode to trace the Gaussian beam profile for our present configuration. The main aim here was to see what sort of mode matching we can achieve theoretically, assuming perfect alignment onto the BBPDs. The simulation is simplified, the various beam splitters and other transmissive optics are treated as having 0 width
  3. It is pretty difficult to accurately measure path lengths to mm accuracy, so to validate my measurement, I measured the beam widths of the arm and PSL green beams at a few locations, and compared them to what my simulation told me to expect. The measurements were taken with a beam profiler I borrowed from Andrew Wade, and both the arm and PSL green beams have smooth Gaussian intensity profiles for the TEM00 mode (as they should!). I will upload some plots shortly. The agreement is pretty good, to within 10%, although geometric constraints on the PSL table limited the number of measurements I could take (I didn't want to disturb any optics at this point)
  4. I then played around with the position of a fast (100mm EFL) lens in the arm green path, to which the mode matching efficiency on the BBPD is most sensitive, and found that in a +/- 1cm range, the mode matching efficiency changes dramatically

Results:

Attachments #1 and 2: Simulated and measured beam profiles for the PSL and arm green beams. The origin is chosen such that both beams have travelled to the same coordinate when they arrive at the BBPD. The agreement between simulation and measurement is pretty good, suggesting that I have modelled the system reasonably well. The solid black line indicates the (approximate) location of the BBPD

     

Attachment #3: Mode matching efficiency as a function of shift of the above-mentioned fast lens. Currently, after my best efforts to align the arm and PSL green beams in the near and far fields before sending them to the BBPD results in a mode matching efficiency of ~30% - the corresponding coordinate in the simulation is not 0 because my length measurements are evidently not precise to the mm level. But clearly the mode matching efficiency is strongly sensitive to the position of this lens. Nevertheless, I believe that the conclusion that shifting the position of this lens by just 2.5mm from its optimal position degrades the theoretical maximum mode matching efficiency from >95% to 50% remains valid. I propose that we align the beams onto the BBPD in the near and far fields, and then shift this lens which is conveniently mounted on a translational stage, by a few mm to maximize the beat amplitude from the BBPDs. 

Unrelated to this work: I also wish to shift the position of the PSL green shutter. Currently, it is located before the doubling oven. But the IR pickoff for the IR beat setup currently is located after the doubling oven, so when the PSL green shutter is closed, we don't have an IR beat. I wish to relocate the shutter to a position such that it being open or closed does not affect the IR beat setup. Eventually, we want to implement some kind of PID control to make the end laser frequencies track the PSL frequency continuously using the frequency counter setup, for which we need this change...

Attachment 1: CurrentX.pdf  72 kB  | Hide | Hide all
CurrentX.pdf
Attachment 2: CurrentY.pdf  68 kB  | Hide | Hide all
CurrentY.pdf
Attachment 3: ProposedShift_copy.pdf  37 kB  | Hide | Hide all
ProposedShift_copy.pdf
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