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Entry  Tue Dec 4 19:34:22 2018, awade, DailyProgress, WOPO, Diabolo 532 nm power drop and retuning 18-12-04_DiaboloInside_annotate.jpegplot20181204_DiaboloSHGRaligned1064nm.pdfplot20181204_DiaboloSHGRErrSig.pdf20181204_RetuneDiaboloSHGAlignment.zip
    Reply  Thu Dec 6 20:04:50 2018, awade, DailyProgress, WOPO, Diabolo 532 nm power drop and retuning 
       Reply  Fri Dec 7 20:31:06 2018, awade, DailyProgress, WOPO, Diabolo: cleaning dust out of SHG unit, self heating issues 18-12-07_20-06-46_4774.jpg18-12-07_20-06-58_4776.jpg18-12-07_20-07-44_4780.jpgIMG_4785.JPGIMG_4766.MOV
          Reply  Thu Dec 13 14:01:13 2018, awade, DailyProgress, WOPO, Diabolo: contacted coherent 
             Reply  Thu Dec 13 16:24:06 2018, awade, DailyProgress, WOPO, Diabolo: baseline numbers for coherent plot20181218_PhaseMatchingCurveFastScanDiaboloSHG.pdf20181213_SHGScanWithLNTempTuning.zip
                Reply  Wed Dec 19 16:48:18 2018, awade, DailyProgress, WOPO, Diabolo: baseline numbers for coherent 
                   Reply  Thu Dec 20 19:20:50 2018, awade, DailyProgress, WOPO, Diabolo: trialing SHG oven set temperature points to see if there is a maxima when locked plot20181220_SHGLockedOutputVsOvenSetpointTemperature.pdf20181220_SHGLockedPowerVsSetPointTemp.zip
                      Reply  Fri Dec 21 15:33:26 2018, Koji, DailyProgress, WOPO, Diabolo: trialing SHG oven set temperature points to see if there is a maxima when locked 
                         Reply  Fri Dec 21 15:51:27 2018, awade, DailyProgress, WOPO, Diabolo: trialing SHG oven set temperature points to see if there is a maxima when locked 
                      Reply  Mon Dec 24 10:08:49 2018, awade, DailyProgress, WOPO, Diabolo: back to power with deliberate pre-misalignment 18-12-21_22-04-30_4812.jpg18-12-21_22-04-54_4813.jpg18-12-21_22-07-55_4814.jpg
                         Reply  Mon Dec 24 12:07:18 2018, Koji, DailyProgress, WOPO, Diabolo: back to power with deliberate pre-misalignment 
Message ID: 2289     Entry time: Mon Dec 24 10:08:49 2018     In reply to: 2284     Reply to this: 2290
Author: awade 
Type: DailyProgress 
Category: WOPO 
Subject: Diabolo: back to power with deliberate pre-misalignment 

I think I've managed to bring the power of the Diabolo back to a usable level for WOPO.

More power with misalignment

I had another look at the alignment into the SHG.  This time I systematically trialed a range of cavity misalignments to deliberately set the pointing into the cavity to be wrong for the fast scanning mode of the control unit (cold) but to allow for the locked (hot) cavity to expand into correct alignment.  This video shows the SHG output as the cavity is locked IMG_4766.MOV,  this seems to show the expansion affecting mostly the horizontal axis. However, eventually I found that the locked cavity had a preferred misalignment on the vertical axis (in terms of the nobs that I needed to turn).  In reality its a mixture of horizontal and vertical once all the DOF are traced through the lenses and mirrors.

I was able to get a steady 160 mW of power with this new misalignment/alignment strategy.  The photos below shows the 1064 nm transmission peaks and error signal for a combination of (mostly) vertical and (a little) horizontal misalignment​ that gave greater power. 

SHG transmission peaks for alignment that delivered 160 mW
PDH error signal corresponding to above misalignment (202 mVpp amplitude)
Steady state power, looks like it peaks at much higher power
(823 mW, but couldn't find a optimal alignment and
oven temperature point that exceeded about 165 mW)

The oven operating temperature was at 110.14 C with a laser diode current of 2.102 A.  PDH gain was set to 0.56 and offset was set to 5.1.

Upping the oven temperature limit

The Diabolo SHG oven set point temperature range doesn't put the ideal phase matching temperature at the center of the available values.  In fact, the optimal power for the above-mentioned power boost was achieved pretty much of the edge of the available range (top of range is 110.26).  I popped the lid on the controller box and traced through all the PCBs to find out how the temperature range was trimmed. The front panel potentiometer (1k) is trimmed with a small set screw (blue) potentiometer located directly to the low left of the temperature knob (when viewed from the component mounting side).  I trimmed the set point temperature value so that the upper value was 111.00 C, giving an extra 0.75 C of headroom above the ideal phase matching set point.  

I found that at the very upper edge of the new range the thermal control loop started to loose some stability.  I limited the increase of temperature to the 111.00 C point and will use some caution when adjusting temperatures at these upper ranges to make sure I don't get oscillations in power.

 

 

 

 

Quote:

The strategy locking at lower laser power and optimizing phase matching temperature by slowly walking the temperature and laser power up won't work: the autolocker mode of the Diabolo SHG won't engage at lower laser power. Instead I tried a brute force approach of stepping temperature and re-locking the cavity in 0.05 C increments of set point temperature.  I've plotted the result below.  Error bars are based on standard deviation on 32 averaged measurements over 10 seconds.

I stopped at 106.00 C as  power output was not improving beyond that point. Maximum temperature is clipped at the controller at 110.26 C. Peak output of 66 mW is actually  at 110.20 C. This is counterintuitively higher than the previous set point but might be good enough to work with, as long as its stable over longer periods.

Still not clear what has changed to shift the ideal SHG over set point to 1.4 C higher than it was before.  Maybe the SHG oven needs to go even a little hotter, but we can only access tempertures up to 110.26 C. I'll take what I can get.

Some other laser settings: 

  • Injection current: 2.077 A
  • Laser crystal temperature: 26.32 C
  • SHG PDH gain: 0.5
  • PDH offset: 4.95
  • Noise eater: off

Data and plotting notebook are attached in a zip below: 20181220_SHGLockedPowerVsSetPointTemp.zip

 

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