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projects:optics:log [2026/01/25 18:06] – [Day 2026-01-18 (k8ik, rahix)] rahixprojects:optics:log [2026/04/14 08:22] (current) – [Day 2026-04-04 (k8ik, rahix, many guests)] rahix
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 ====== Lab Notes for Optics Experiments ====== ====== Lab Notes for Optics Experiments ======
 +
 +==== Day 2026-04-04 (k8ik, rahix, many guests) ====
 +We presented our low-cost optics and the interferometer at EH23!  Watch our talk here: [[https://media.ccc.de/v/eh23-budget-beams-laser-experiments-in-a-hacker-lab|Budget Beams – Laser Experiments in a Hacker Lab]]
 +
 +{{:projects:optics:2026-04-04-interferometer-eh23.jpg?600|}}
 +
 +During the event, our laser diode broke (well, rahix fed it too much voltage really) so we scrambled to disassemble a DVD drive and extract the red 650nm diode from it. We kept the collimation optics from the DVD drive and hot-glued the entire subassembly to an optics mount.  It took 3 attempts (3 DVD drives) to not immediately kill the new diode again.
 +
 +We managed to demonstrate the fringe movement from the piezo quite a few times.  We borrowed a different power supply to drive it.  This one seems to lead to a slow ramp-up of voltage (fringes move slowly when turning on) and an abrupt cutoff (fringes fall back very quickly when turing off).
 +
 +Stability was quite alright with the reduced isolation setup as long as nobody touched the table.
 +==== Day 2026-04-01 (k8ik, rahix) ====
 +Set up the interferometer experiment again, but with "analog" readout (i.e. just a viewing screen where the pattern is enlarged).
 +
 +Tried driving the Z-axis piezo of the STM head with +20V.  We can see movement!  When turning on the voltage, the pattern immediately jumps.  When turning it back off, it slowly drifts back to its original state.
 +
 +Eyeballed the movement to be somewhere below λ/2, so <300nm.  But for any proper measurement, we need to do digital readout again.
 +
 +Getting the laser stable enough to see the movement without noise is not easy. The mechanics hold up plenty fine.
 +==== Day 2026-02-21 (k8ik, rahix) ====
 +
 +We successfully coupled our shitty Aliexpress laser into the also high quality single mode fiber. However, we do see a beautiful beam but also what could be higher modes in the output, meaning the fiber is either broken at the end, the collimator has some issues or the fiber is indeed a multimode fiber. Most likely explanation right now: The fiber is a "broadband single mode fiber" for infrared (not specified) meaning it could allow for higher modes at 650nm.
 +
 +{{:projects:optics:screenshot_from_2026-02-21_21-31-28.png?400|Ground mode of laser without collimator}}
 +{{:projects:optics:signal-2026-02-21-212805_003.jpeg?400|Weird mode}}
 +
 +We put a collimator on the other fiber end and aimed it at our camera to take some beam pictures.
 +
 +Wrote a little script which outputs the average intensity across the camera frame as a single number so we can tune the fiber coupling against it [[https://git.fa-fo.de/fafo/k8ik-stm/src/commit/3ecbe0b53f74b1d71ca9628accaadcc8cf0977f9/Calculations/live_intensity.py|live_intensity.py]].
 +
 +Even after adjustment, only very little power coming out of the fiber.  After tuning, we readjusted the exposure of the camera to try imaging full beam pictures (but this only partially worked because the auto exposure setting of the camera is completely broken).
 +
 +First, here is a capture of a higher mode:
 +
 +{{:projects:optics:2026-02-22-fiber-beam-multimode.png?400|}}
 +
 +With a bit more tuning of the fiber coupling, we end up seeing some much nicer beam shapes:
 +
 +{{:projects:optics:2026-02-22-fiber-beam-01.png?400|}}
 +{{:projects:optics:2026-02-22-fiber-beam-02.png?400|}}
 +
 +The curve does not look gaussian which is a bit worrying. Current assumption is that this is still a higher transversal mode that just looks close enough.
 +==== Day 2026-01-31 (k8ik) ====
 +Aliexpress quality fiber and collimators arrived:
 +
 +Fiber:
 +[[https://de.aliexpress.com/item/1005003117758993.html?spm=a2g0o.order_list.order_list_main.5.240e1802Ts08Gj&gatewayAdapt=glo2deu|Fiber Link]]\\
 +Aliexpress description: SC To SC LC ST FC To FC Fiber Patch Cord Jumper Cable SM Simplex Single Mode Optic Drop FTTH Cable For Network 1m
 +
 +Collimators:
 +[[https://de.aliexpress.com/item/1005010797142816.html?spm=a2g0o.order_list.order_list_main.11.240e1802Ts08Gj&gatewayAdapt=glo2deu|Collimators Link]]\\
 +Aliexpress description: Collimator Aspherical fiber focusing light spot adjustable size FC interface 
 +
 +Rebuild our setup (removed the cube) to test the fiber. The fiber will enable us to get a nice Gaussian beam profile with which we can work in the future. Built in the fiber in a very adventurous setup because we do not yet have a mount printed. Tried to couple in the laser but did not succeed yet. The fiber is built very close to the first mirror and quite far to the second to decouple place and angle of the beam entering the fiber. As detector tried the bare eye (without collimating optics at the fiber end). Need complete darkness to get first signal to optimize on next time.
 +
 +{{:projects:optics:optics_setup_fiber_coupling_20260131.jpeg?400|}}
 +{{:projects:optics:optics_setup_fiber_coupling_2_20260131.jpeg.jpeg?400|}}
 +
 +Way forward: We need a proper mount for the fiber collimator, such that we can use a pinhole to pre-align the beam as good as possible for getting a first signal. With unclear exact mounting angle, it will get very difficult to get a first signal. Back tracking the backreflection did not yet lead to success.
  
 ==== Day 2026-01-19 (k8ik, rahix) ==== ==== Day 2026-01-19 (k8ik, rahix) ====