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David Notebook: Difference between revisions
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==Daily Log==
'''7/29/14'''
I tried a variety of lens combinations again for a telescope, and adjusted the lens positions over a large fraction of the focal distance of the second lens, but they all failed to collimate the beam at the wall (~20 feet) better than just the a-sphere by itself. I've decided to go with just a single lens for now and if this doesn't improve the 633 generation efficiency enough, I'll maybe switch to checking other variables like gas pressure. Calculating the proper focal length for the lens doesn't seem practical since there is about 34 cm of propagation distance before the lens (a distance over which the beam diverges a non-negligible amount) and then 20 cm after the lens before the beam enters the cavity. Finally the cavity mirror will also adjust the focus somewhat. I modeled this setup with a spare cavity mirror and just tried different lenses to see what works best. A focal length of 400mm was a good compromise, giving a beam size of around 930 at the start of the cavity, 630 in the middle, and 1075 at the end. While the 1064 is 810, this is actually only at the center and is somewhat larger at the front and back of the cavity--these sizes for the 780 beam might be alright then.
'''7/28/14'''
Further measurements on Friday showed that the beam actually still diverged quite a bit after the telescope, to the point where it won't be useful. The performance seems limited by the laser/a-sphere which will not fully collimate out of the fiber. I'm switching out the 110 a-sphere on the output for a 230. It doesn't collimate quite as well, but with a shorter focal length, I'm hoping it will be smaller over a propagation distance of about 1 meter.
Looking at the fiber with the fiber scope, I saw that it was damaged. This explains some of the weird structures in the beam when looking at it against the wall and will maybe improve the generation efficiency of the 633. I changed to a 810 nm single mode fiber, which seems to work fine for 780. The beam quality if much better, although it still won't collimate very well. I tried a 150mm and 75mm lens again and looked at it against the wall, but the smallest I could focus it to by moving the 75 mm lens was still a couple inches. My best bet at this point might be to go back to a single long focal length lens, but I will make one more try tomorrow.
'''7/24/14'''
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