Bots, Bureaucrats, Interface administrators, Suppressors, Administrators, Upload Wizard campaign editors
573
edits
David Notebook: Difference between revisions
Jump to navigation
Jump to search
→Daily Log
|
==Daily Log==
'''1/8/15'''
'''Pre-stabilizing cavity'''
Wow, I've really let this get out of date. We've moved forward with the pre-stabilizing cavity idea. We needed to buy a lot of optics, but realized we could get away without another EOM by just picking off after the EOM on the main experiment. Even when we feedback to the same current driver, the error signals shouldn't interfere with each other, because their frequency components ultimately depend not just on the EOM modulation frequency, but on the cavity length too.
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 and 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.
We thought the instability might be caused by the non-angle polished fiber I was using, so I switched that out but it didn't seem to do anything. It doesn't seem to be back reflections feeding back to the laser either, since looking at the laser on the spectrum analyzer shows nothing weird even when the cavity peaks are drifting in and out. The power reaching the cavity is pretty constant too, so I don't think it's the fiber being weird.
That mostly brings us up to today
We got a new IR scope from the surplus store. We were having trouble with it for a while, but we traced it back to just a bad connection with the battery case and swapped it for the case from the old IR scope. It works great now.
We were thinking of trying to modulate the output from the pulsed laser before we broaden it with the PCF as a preliminary experiment while we're still figuring out all the PCF stuff. The wavelength would be too close to the pump beam though, so the optics wouldn't work for us to be able to pass it through the cavity. We thought we could maybe frequency double it first and then send it in, but Deniz thinks we'd need about 10 mW of green to do it, and I'm not getting more than a few hundred μW from the doubling crystal. I tried changing the temperature of the crystal some since that's supposed to affect the wavelength it responds to, but that didn't seem to do much. I might try expanding and more tightly focusing the beam to increase the intensity if I get stuck on the cavity stuff.
'''12/3/14'''
Been swamped with classwork lately, but a few lab things worth noting.
| |||