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projects:optics:log [2026/02/22 18:33] – [Day 2026-02-21 (k8ik, rahix)] rahixprojects:optics:log [2026/07/18 12:47] (current) – [Day 2026-07-18 (rahix, guest)] rahix
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 ====== Lab Notes for Optics Experiments ====== ====== Lab Notes for Optics Experiments ======
 +These are the lab notes for our optics experiments.  Check the [[projects:optics:start|Laser Optics & Interferometry]] project page for an overview of the project.
  
 +> //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 2026-07-18 (rahix, guest) ====
 +Cases are ready:
 +
 +{{:projects:optics:2026-07-18-cases.jpg?600|}}
 +
 +==== Day 2026-07-17 (rahix, guest) ====
 +Since we do not have a FAFO laser driver design yet (see techtree [[https://git.fa-fo.de/fafo/techtree/issues/85|#85]]), we are going to use an ''LD1100'' ([[https://www.thorlabs.com/oem-laser-diode-driver-constant-power?tabName=Overview|Thorlabs]]) module. We set up the module for the diode and verified everything works:
 +
 +{{:projects:optics:20260717_diode-test.jpg?600|}}
 +
 +Selected gain for the ''LD1100'' is max I<sub>mon</sub>=260µA. The diode monitoring current is specified for 150µA typ., but up to 400µA. With the selected gain and this diode, it works out well and the diode is easily driven at about 30mA.
 +
 +Wired up the diode socket to a 3-pin dupont header. The wires are labelled with 1, 2, 3 rings for pin 1-3 of the diode. For pin code A diodes like the one we are using, this means:
 +
 +{{:projects:optics:pin-code-a-diode-description.png?nolink&600|}}
 +
 +And it lines up with the pinning of the ''LD1100'' so you can connect it directly, as visible in the picture.
 +
 +----
 +
 +In the meantime, somewhere else, we machined the case(s) for the diode modules:
 +
 +{{:projects:optics:2026-07-17-diode-module-cases.jpg?600|}}
 +
 +The CAD really doesn't do it justice how small this is!
 +
 +==== Day 2026-07-16 (rahix, guest) ====
 +We decided to build two versions of the module case. Increasing the distance between lens and fiber end, we can much improve the theoretical coupling efficiency. But this comes at the cost of being much more sensitive to lens alignment (0.1mm shift of the lens moves the focus spot by 1.6mm). So we will need to see if we can pull it off.
 +
 +{{:projects:optics:2026-07-17_long_diode_module.png?400|}}
 +
 +[The longer module with more ideal coupling conditions]
 +
 +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.
 +
 +On the laser diode side, we capture 23° (half angle) of the emitted light (Compared to 18° with the shorter design).
 +==== Day 2026-07-15 (rahix, guest) ====
 +Continued working on the diode module design. It lives here: [[https://git.fa-fo.de/rahix/FiberCoupledLaserModule]]
 +
 +{{:projects:optics:2026-07-15_diode-module.png?400|}}
 +
 +Main changes of our assembly against the original design:
 +
 +  * Slightly larger to accommodate larger lens with longer focal distance
 +  * Diode is not press-fit into the housing but rather clamped from the back – we will also use the block clamping the diode to provide thermal control (Using a TEC + NTC)
 +  * Mounting solution for fixing the assembly to an optics breadboard
 +
 +Short list of selected components:
 +
 +^ **Component** ^ **Part** ^ **Specifications** | **Link** ^
 +| Laser Diode | ''L635P5'' | 635nm, 5mW, Ø5.6mm | [[https://www.thorlabs.com/item/L635P5]] |
 +| Diode Socket | ''S7060R'' | for Ø5.6mm diodes | [[https://www.thorlabs.com/item/S7060R]] |
 +| Aspheric Lens | ''CAY046'' | Acrylic, Ø7.40mm, f=4.60mm, 0.40 NA | [[https://www.thorlabs.com/item/CAY046]] |
 +| Single-Mode Fiber | ''SM600'' | 633-780nm, MFD=3.6-5.3µm@633nm | [[https://www.thorlabs.com/single-mode-fiber?pn=SM600]] |
 +==== Day 2026-07-12 (rahix, guest) ====
 +We want to build a fiber-coupled laser diode module like the one shown by //Diffraction Limited// on YouTube: [[https://www.youtube.com/watch?v=l26sCJn0sB4|Building Fiber Coupled Laser Modules]].
 +
 +Our module will include a few modifications to better suit the metrology applications we have in mind:
 +
 +  * TEC for temperature stabilization
 +  * NTC for temperature stabilization
 +  * A different lens
 +  * Breadboard-mounting option
 +
 +Our plan is to use an [[https://openflexure.org/projects/deltastage/|OpenFlexure Delta Stage]] for performing the fiber alignment.
 +==== 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) ==== ==== Day 2026-02-21 (k8ik, rahix) ====
  
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 Ordered a cheap fiber + collimator optics in hopes that we can use it to improve our beam profile (see [[#day_2025-09-06_rahix_q3k_hugo|2025-09-06]] notes). Ordered a cheap fiber + collimator optics in hopes that we can use it to improve our beam profile (see [[#day_2025-09-06_rahix_q3k_hugo|2025-09-06]] notes).
 ==== Day 2026-01-18 (k8ik, rahix) ==== ==== Day 2026-01-18 (k8ik, rahix) ====
-Built up two more mirror mounts and a special mount for the [[projects:stm|STM]] head (we want to measure piezo displacement).+Built up two more mirror mounts and a special mount for the [[projects:stm:start|STM]] head (we want to measure piezo displacement).
  
 Placed the camera into the interferometer to take a look at the interference pattern digitally.  Hello world: Placed the camera into the interferometer to take a look at the interference pattern digitally.  Hello world: