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projects:optics:log [2026/07/24 13:27] – [Day 2026-07-23 (rahix, guest)] 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 2026-08-29 (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.
 +
 +Also set up the interferometer and produced some tiktok brainrot: [[https://youtube.com/shorts/R46GZN6YByc]]
 +
 +Next step will be improving alignment of the fiber.
 +==== Day 2026-07-24 (rahix) ====
 +Briefly checked the system again. Alignment didn't degrade since yesterday, output seems qualitatively similar. Assuming either the isolation helped or the current alignment sits further from the beam waist and thus is less sensitive to position change.
  
 ==== Day 2026-07-23 (rahix, guest) ==== ==== Day 2026-07-23 (rahix, guest) ====
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 {{:projects:optics:pxl_20260723_151056119.jpg?600|}} {{:projects:optics:pxl_20260723_151056119.jpg?600|}}
 +
 +Also designed a mount for the microscope, so it is directly attached to the frame of the alignment jig.
  
 Aligned the fiber again a few more times. Backlash in the OpenFlexure is quite noticeable with us moving across the focus spot quite a few times. Aligned the fiber again a few more times. Backlash in the OpenFlexure is quite noticeable with us moving across the focus spot quite a few times.
 +
 +Replaced the 15cm fiber with one about a meter long.
  
 Mounted the collimator onto an optics breadboard to send the beam through a michelson interferometer. Didn't have enough time to check stability yet. Mounted the collimator onto an optics breadboard to send the beam through a michelson interferometer. Didn't have enough time to check stability yet.
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 Used this light to further adjust the position of the fiber using the DeltaStage. First with 250 units step size, then 50. With 50, the backlash of the steppers in the stage got quite noticeable. Used this light to further adjust the position of the fiber using the DeltaStage. First with 250 units step size, then 50. With 50, the backlash of the steppers in the stage got quite noticeable.
  
-Installed a temporary testing FC connector on the end of the fiber. Here's the exiting mode:+Installed a temporary testing FC connector ([[https://de.aliexpress.com/item/1005011973192476.html|AliExpress]]) on the end of the fiber. Here's the exiting mode:
  
 {{:projects:optics:pxl_20260722_192219500.jpg?400|}} {{:projects:optics:pxl_20260722_192219500.jpg?400|}}
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 With this longer design, the TEM₀₀ beam diameter at the waist is 3.66µm which matches the MFD (mode field diameter) of the selected fiber.  With the shorter design, we are at 2.2µm. With this longer design, the TEM₀₀ beam diameter at the waist is 3.66µm which matches the MFD (mode field diameter) of the selected fiber.  With the shorter design, we are at 2.2µm.
 +The rayleigh length (long version) thus is  z<sub>R</sub> = 16.6µm.
  
 On the laser diode side, we capture 23° (half angle) of the emitted light (Compared to 18° with the shorter design). On the laser diode side, we capture 23° (half angle) of the emitted light (Compared to 18° with the shorter design).