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David Notebook: Difference between revisions
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Zach made some cavities with a finesse of a few thousand. They're made of a special steel that has a very low expansion coefficient, and so they should be pretty stable. I didn't realize mode-matching was so important for cavities, but it is and so I had to learn a lot about that and Gaussian beam optics. Nick has a helpful MATLAB problem for calculating the coupling parameters to the cavity, which I've [https://wiki.physics.wisc.edu/yavuz/images/5/54/Gaussian_Cavity_Beams.zip uploaded here]. I also found [https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0CB8QuAIwAA&url=http%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3D5bR6wkZGa18&ei=k82uVKa-JcWTyASbn4CICA&usg=AFQjCNG3uid7LOjd2hwqP27orz6CvYjhCA&sig2=YcBQYbJ0TKODETkQYVxheQ&bvm=bv.83134100,d.aWw this youtube video pretty helpful] and the few chapters about lasers in the Pedrotti optics book.
The cavity is 10 cm with one plane mirror and one with a 50 cm radius of curvature, for a <math>g_1=1</math> and <math> g_2=0.8 </math>. I used the spinning camera for measuring the Gaussian beam parameters to figure out what kind of lens was needed to mode match. With the initial beam, I could only match the x or y component unless I got some cylindrical lenses or a prism pair, which seemed like too much work, so I decided to couple to a fiber to get a mostly circular output. I found the correct mode matching lens to then by an f=750mm about 10 cm from the fiber output and then another 55 cm to the cavity. The lens is on a 1-d stage for small translation adjustments.
I was having difficulty finding a signal from the cavity for a while, but looking at the reflected signal rather than the transmitted proved useful. The reflected signal is made with a quarter wave plate and a beam cube to provide some isolation from the back reflections and to allow the power to be adjustable. The center of the cavity is at a height of ~3.375 inches, so I marked that on the fluorescent stone and walked the beam to that height before putting the cavity in. Then by overlapping the horizontal back reflection with the incident beam, I could usually see peaks on either the reflected or transmitted photodiode. The reflected signal looks very weird though--very jagged without any obvious periodic dips, but it seems like it has to be result from the cavity transmission since it only is visible with the ramp on. The transmitted signal looks more normal, except it is extremely unstable--just barely touching a mirror without even adjusting it will make the peaks jump around, and they fade in and out just over a few seconds by themselves. I've gotten it aligned where it looks pretty good except for how unstable it is. Maybe I'm actually locking to a of axis-eignemode or whatever and these are just the biggest peaks I've seen so far and I'm mistaking them for a 0,0 mode free spectral range. Maybe the cavity mirrors are slightly misaligned.
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