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==Daily Log==
'''1/15/15'''
Zach and I both kept having trouble getting much transmitted signal and any reflected signals. We thought maybe we weren't mode matched well, but we'd both tried a few times and that didn't seem to be it. It was also possible that the laser linewidth was much bigger than the cavity linewidth. I made a python thing [https://wiki.physics.wisc.edu/yavuz/images/0/05/Cavity_coupling_efficiency.zip here] that calculates the power of an incident beam that is coupled into a cavity based on the spatial and frequency profiles. The cavity mirrors listed a selectivity of about 99.98%, which would give a finesse of around 8,000. The cavity linewidth is just the FSR/finesse, so it would be ~190 KHz, so with our ~500 KHz linewidth laser, and even very rough spatial coupling, we should still have been getting plenty of power. We thought maybe the radius of curvature of one of the mirrors was wrong, but we took out the cavity mirror and found that it focused a collimated beam at ~25 cm, which would give the correct R of 50cm. In retrospect this probably wouldn't have mattered as much as we thought--the spatial coupling curve is quite forgiving.
Eventually we thought maybe the finesse was higher than we thought, and I measured the reflectivity of the plane mirror and found it to be about 99.998%. It's safe to say we'll never be buying German made optics again! Assuming a similar reflectivity for the curved mirror, this would give a finesse of 80,000. The linewidth would be very narrow then, and we'd only get about 3% power even ignoring spatial coupling. This is likely the issue. We ordered a back polished E03 mirror from Thorlabs which we'll swap in. That has a reflectivity closer to 99.6%, which would bring our finesse down to a more manageable 800 (too low?).
'''1/12/15'''
I briefly was getting the laser to lock for a couple second a few times over the last couple days, but couldn't improve it much. I think I'm seeing pretty good cavity peaks now though--stable and what appear to be a couple free spectral ranges. I want to figure out what kind of piezo is on the laser though, so I can know how much voltage change should give a free spectral range (so I know I'm not way zoomed in on some higher mode stuff).
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