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projects:optics:log [2026/08/03 20:51] – [Day 2026-08-01 (rahix, guests)] rahixprojects:optics:log [2026/08/29 22:20] (current) – [Day 2026-08-29 (leah, rahix)] rahix
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 > //This page is a lab notebook. It documents the work done in the lab and our thoughts, theories, and findings related to it. Everything you read here is work in progress and not final results. We believe it is best to have our lab notes out in the open, because it gives more detailed insight into what worked for us and what didn't. Of course you can also use this page to follow our progress!// > //This page is a lab notebook. It documents the work done in the lab and our thoughts, theories, and findings related to it. Everything you read here is work in progress and not final results. We believe it is best to have our lab notes out in the open, because it gives more detailed insight into what worked for us and what didn't. Of course you can also use this page to follow our progress!//
  
-==== Day 2026-08-01 (rahix, guests) ====+==== Day 2026-08-29 (leah, rahix) ==== 
 +Investigating whether we can just buy an ND filter for our specific microscope. There is a small cover glass on the front with ø=36mm. Seems to have a M34x0.5 thread? There seem to be some "37mm" ND filters available that seem they could fit. 
 + 
 +Mom, we have ND filter at home (2 pieces of polarization filter foil): 
 + 
 +{{:projects:optics:20260829_nd-filter-at-home.jpg?400|}} 
 + 
 +But we are still not seeing much improvement. 
 + 
 +Removed the brass rod for now as it is just in the way for experimentation. 
 + 
 +After lots of trial and error, we achieved //some// coupling again. But we never got it really good and unfortunately the magnification of our microscope is just not good enough to see the focus spot on the end of the fiber. 
 + 
 +Clamped the fiber with a protective sleeve now so we don't damage it again. Sometimes, this leads to the fiber being angled. This is a problem as even a few degrees already lead to much much worse coupling efficiency. 
 + 
 +---- 
 + 
 +With the light output we got, coupled it into the interferometer setup. We get really nice fringes, though unfortunately hard to take a picture of... 
 + 
 +{{:projects:optics:20260829_213134967.night.jpg?600|}} 
 + 
 +Added some 26cm path length difference to the interferometer to see if we get some amount of coherence length. Result: We do! Fringe contrast has not visibly decreased. 
 + 
 +Going back to the topic at hand: 
 + 
 +The assembly is probably not stiff enough as it stands to meaningfully perform alignment. We removed the vibration isolation again to get some more stiffness from resting directly on the table surface and this helps quite a bit... Achieved better coupling than in the earlier attempts today. Should probably place the whole assembly onto a breadboard or metal base plate. 
 + 
 +Also we need to investigate what we can use to better check focal plane alignment of the fiber end, with the microscope not working out. 
 + 
 +Considering to just automate the focus search entirely instead of manual stepping. But backlash of the openflexure is considerable which may make this rather difficult. 
 +==== Day 2026-08-07 (rahix, guest) ==== 
 +Remounted the microscope to get a better view of the fiber end. It still overexposes which currently makes it impossible to see where the beam waist is. Thinking about strapping an ND filter to the microscope objective... 
 + 
 +{{:projects:optics:pxl_20260807_195958627.jpg?400|}} 
 + 
 +During realignment, we noticed that our mounting bracket that holds the fiber to the open flexure has crushed the coating of the fiber quite severely. It looks like the core is still intact inside, but maybe we want to figure out a more gentle mount in the future. 
 + 
 +{{:projects:optics:2020_0116_235529_012_crop.jpg?400|}} 
 + 
 +[squished fiber under the microscope] 
 + 
 +Played a bit with aligning the fiber, bit didn't get too far since we ended up with a very unadvantageous angle of the mounted fiber in relation to the diode casing. 
 + 
 +In any case, have some pretty microscope images of the business end of the diode module setup: 
 + 
 +{{:projects:optics:2020_0116_233431_003.jpg?400|}} 
 +{{:projects:optics:2020_0116_233451_004.jpg?400|}} 
 + 
 +[for scale, the brass rod beneath the fiber is ø2mm] 
 +==== Day 2026-08-01 (rahix, anuejn) ====
 Started setting up the diode module experiment again. Ran into some issues due to not being able to ground the DC supply... Pretty sure we had it grounded last time? But it makes sense, the laser diode will short the supply to VCC in that case. Got it working to the point of setting up the laser diode for 28mA operation. Started setting up the diode module experiment again. Ran into some issues due to not being able to ground the DC supply... Pretty sure we had it grounded last time? But it makes sense, the laser diode will short the supply to VCC in that case. Got it working to the point of setting up the laser diode for 28mA operation.