User:Pranav Rathi/Notebook/OT/2010/09/29/CrystaLaser Vs CompassLaser


 * Our new laser looks completely different (Is it good or bad I do not know) in the old objective, and it’s been puzzling us for the last one week. So to search the answer of this question we replaced our beam expender lens system with old telescope; nothing changed. Next I removed the AOM and nothing changed. So the last thing left was the laser itself. So today I captured two movies: In first one we have crystalaser-telescope-old objective (mirrors in the middle) and in second we have compasslaser-telescope-old objective (mirrors in the middle). The difference is obvious; the trap with the new laser (1.9W) looks completely different. I just only see one bright spot at the center, as I go in and out of focus multiple reflections can be seen with diffraction pattern along x-axis of the video frame. I used the lower surface for the video. The trap with the old laser (1 W) looks as usual, 4 lobes can be seen clearly.I used somewhat the same beam expansion with 6-7mm beam diameter at the objective and 2-nd filter infront of the camera.Why the trap looks different (is it because of mode profile)? Which one is better (it looks like we were trapping the beads lot better with the new laser)? Is it possible that our old laser is multimode even at the low powers and because of that we see four lobes?


 * The next video is the trap with crystalaser-telescope-new objective. The outer periphery of the trap looks elliptical (Is it because of elliptical beam; I am going to investigate this issue today, I am planning to measure beam waist at two different points along z-direction, vertically and horizontally transverse to the z direction.) At the trap-center (when the focus is at the bottom surface of the cover slide) we see two reflections that is due to the water-glass interface between objective and lower surface of the cover slide (external reflection;) and glass-water interface between upper surface of cover slide and water around the specimen(internal reflection). Over all trap looks the way it should, because it is just a circular diffraction pattern made by a circular aperture (objective’s lens): A bright spot (Airy disc) at the center with circular dark and bright rings around it (no four lobes)


 * The last video is the trap with compasslaser-telescope-new objective. The outer periphery of the trap is somewhat circular with no problems. At the center we kind-off see the four lobes with no clarity because of two reflections overlapping. Everything else is the same as old objective.