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Entry  Wed Mar 30 19:43:33 2011, Bryan, Configuration, Green Locking, The wonderful world of mode-matching 
    Reply  Wed Mar 30 19:54:19 2011, Bryan, Configuration, Green Locking, The wonderful world of mode-matching laser_output_non_circular.png
       Reply  Wed Mar 30 20:08:34 2011, Bryan, Configuration, Green Locking, The wonderful world of mode-matching Beam_Matching_02c_Vertical.pngBeam_Matching_02c_Horizontal.png
          Reply  Wed Mar 30 20:14:17 2011, Bryan, Configuration, Green Locking, The wonderful world of mode-matching 
             Reply  Wed Mar 30 20:31:30 2011, Bryan, Configuration, Green Locking, The wonderful world of mode-matching 
                Reply  Wed Mar 30 20:50:43 2011, Bryan, Configuration, Green Locking, The wonderful world of mode-matching After_Faraday.pngAfter_Faraday_and_Rotated_Lens.png
                   Reply  Wed Mar 30 21:21:15 2011, Bryan, Configuration, Green Locking, The wonderful world of mode-matching Oven_Lens_Solution_66mm.pngMatching_Into_Green_Oven_zoomed_out.pngMatching_Into_Green_Oven_zoomed_in.png
                      Reply  Thu Mar 31 02:59:49 2011, Koji, Configuration, Green Locking, The wonderful world of mode-matching 
                         Reply  Thu Mar 31 14:10:00 2011, Bryan, Configuration, Green Locking, The wonderful world of mode-matching P3310028.JPGP3310029.JPG
                         Reply  Thu Mar 31 15:23:14 2011, Bryan, Configuration, Green Locking, The wonderful world of mode-matching Zoomed_out_fitting01.pngZoomed_in_fitting01.png
                            Reply  Mon Apr 4 14:20:32 2011, Bryan, Configuration, Green Locking, The wonderful world of mode-matching Waist_Fits_from_laser.pngWaist_Fits_Bench_Position.png
Message ID: 4470     Entry time: Wed Mar 30 21:21:15 2011     In reply to: 4469     Reply to this: 4473
Author: Bryan 
Type: Configuration 
Category: Green Locking 
Subject: The wonderful world of mode-matching 

Step 4: Matching into the oven

 

 

Now that the astigmatism is substantially reduced, we can work out a lens solution to obtain a 50um waist *anywhere* on the bench as long as there's enough room to work with the beam afterwards. The waist after the Faraday and lens is at position 22.5 on the bench. A 50 mm lens placed 18 cm after this position (position 14.92 on the bench) should give a waist of 50 um at  24.57 cm after the waist (position 12.83 on the bench). This doesn't give much room to measure the beam waist in though - the Beamscan head has a fairly large finite size… wonder if there's a slightly less strong lens I could use…

OK. With a 66 mm lens at 23 cm (position 13.45 on the bench) after the waist we get a 50 um waist at 31.37 cm after the waist (position 10.15 on the bench). 

 

Oven_Lens_Solution_66mm.png

 

Closest lens I found was 62.9mm which will put the 50um point a bit further towards the wall, but on the X-arm the oven is at position 8.75 ish. So anything around there is fine.

 

Using this lens and after a bit of manual fiddling and checking with the Beamscan, I figured we needed a close in, fine-grained measurement so set the Beamscan head up on a micrometer stage Took a whoie bunch of data around position 9 on the bench:

 

 

Position A1_13.5%_width A2_13.5%_width

(mm) (um mean) (um mean)

-15 226.8 221.9

-14 210.9 208.3

-13 195.5 196.7

-12 181.0 183.2

-11 166.0 168.4

-10 154.0 153.1

-9 139.5 141.0

-8 127.5 130.0

-7 118.0 121.7

-6 110.2 111.6

-5 105.0 104.8

-4 103.1 103.0

-3 105.2 104.7

-2 110.9 110.8

-1 116.8 117.0

0 125.6 125.6

0 125.6 125.1

1 134.8 135.3

2 145.1 145.6

3 155.7 157.2

4 168.0 168.1

5 180.5 180.6

6 197.7 198.6

7 211.4 209.7

8 224.0 222.7

9 238.5 233.7

10 250.9 245.8

11 261.5 256.4

12 274.0 270.4

13 291.3 283.6

14 304.2 296.5

15 317.9 309.5

 

Matching_Into_Green_Oven_zoomed_out.pngMatching_Into_Green_Oven_zoomed_in.png

 

And at this point the maximum power available at the oven-waist is 298mW. With 663mW available from the laser with a desired power setting of 700mW on the supply. Should make sure we understand where the power is being lost. The beam coming through the FI looks clean and unclipped, but there is some stray light around.

 

Position A1_13.5%_width A2_13.5%_width

(bench) (um mean) (um mean)

7 868.5   739.9

6 1324 1130

5 1765 1492

4 2214 1862

 

The plot looks pretty good, but again, there looks to be an offset on the 'fitted' curve. Taking a couple of additional points further on to make sure it all works out as the beam propagates. I took a few extra points at the suggestion of Kiwamu and Koji - see the zoomed out plot.  The zoomed in plot has by-eye fit lines - again, because to get the right shape to fit the points there appears to be an offset. Where is that coming from? My suspicion is that the Beamscan doesn't take account of the any background zero offsets when calculating the 13.5% and we've been using low power when doing these measurements - very small focussed beams and didn't want to risk damage to the profiler head.

 

Decided to take a few measurements to test this theory. Trying different power settings and seeing if it gives different offset and/or a changed width size

 

7 984.9 824.0 very low power

7 931.9 730.3 low power

7 821.6 730.6 higher power

7 816.4 729.5 as high as I'm comfortable going

 

Trying this near the waist…

 

8.75 130.09 132.04 low power

8.75 106.58 105.46 higher power

8.75 102.44 103.20 as high as it can go without making it's saturated

 

So it looks like offset *is* significant and the Beamscan measurements are more accurate with more power to make the offsets less significant. Additionally, if this is the case then we can do a fit to the previous data (which was all taken with the same power setting) and simply allow the offset to be a free parameter without affecting the accuracy of the waist calculation. This fit and data coming to an e-log near you soon.

 

Of course, it looks from the plots above (well... the code that produces the plots above) that the waist is actually a little bit small (around 46um) so some adjustment of the last lens back along the beam by about half a cm or so might be required.

 
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