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Nick Brewer: Difference between revisions
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We tried to see some absorption again yesterday. The data is [https://wiki.physics.wisc.edu/yavuz/images/4/47/Absorption_Data_2014-7-21.pdf here], and looks unconvincing. There is not a strong peak at 1055.076 nm like we are expecting. I think in order to get a better spectrum we will need to measure fluorescence from the crystal like in [http://www.opticsinfobase.org/DirectPDFAccess/B2A160D3-AED0-E02F-14B040E3D6801A62_7350/josab-11-4-591.pdf?da=1&id=7350&seq=0&mobile=no Shen's paper] instead of trying to directly measure absorption. [[Zjs log|Zach]] has been working on the locking circuit and we are now able to get a much less noisy signal. The fast feedback is fed directly to the diode instead of going through the current driver. Before the signal fluctuated ~25% or more when locked but now it is much less and we are able to get a clean signal out. There are still certain wavelengths where the error signal looks more like a peak than a dispersion graph and we are unable to lock, but we haven't yet determined the cause.
Today we found how to improve the error signal when it was mysteriously becoming distorted. Because the SHG cavity has a birefringent element, the error signal looks like two dispersion curves instead of just one. These two curves have opposite slopes when they cross zero and occur because the e- and o- polarized light each are resonant at different cavity lengths. If the cavity's birefringence changes, the two dispersion curves move relative to each other. When the two overlap the error signal you are trying to lock to is distorted and you can't get a good lock. By changing the birefringence of the cavity you can separate these two curves from each other. This can be done by changing the temperature slightly or by adding a half wave plate into the cavity. Adding the half wave plate adds a lot of loss to the cavity though so it probably isn't good for efficiently converting green light. Since the conversion efficiency is also strongly affected by the temperature, it might seem like a bad idea to change the temperature from the optimal temperature. I found the temperature bandwidth of the PPKTP crystal to be ~2.4 °C so we have a degree or so to work with before significantly affecting the conversion rate, which I think should be plenty in order to separate the error signals.
When only the fast feedback is on we can still see oscillations on the order of hundreds of Hz on the plateau of the cavity peaks. By changing the diode driver current we can make these oscillations appear and disappear (at the expense of slightly changing the wavelength). We still need to determine exactly what is causing these oscillations.
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