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==Things to Do/Check==
*Upload power meter data....meh
*circularly polarize everything for preferential anti-stokes generation
*finish mode matching python program and upload
*mount the power strip
==Questions for Deniz==
*Do photons from a laser with a broad linewidth have a larger an uncertainty in energy, or is it just that that laser contains a wider energy distribution?
==
'''5/10/2016''' Swapping back to short cavity. Long cavity is at 46x5. Walk mirrors at 40x3, 39x7 (pedestal location). 250 mm lens at 39x4, 700mm lens at 41x23. Pickoff at 35x3 (pedestal). Other mirrors at 31x3, 31x23.
'''12/16/15'''
Swapping back to the long cavity. Short cavity showed a lot of success: 10e-6 CW efficiency, 10e-4 when ramping. Mode matching for 1064 27 cm cavity is: Cavity at 57x2 (0,0 is corner near the computer, cavity position is marked by closest screw to 0,0 holding it down). Walk mirrors at 50,3.5 and 49,7. 250mm lens at 45,4 700mm lens at 32,18 500mm at 47,23.
'''11/9/15'''
The bright flashes turned out to just be from the ramp peaks greatly expanding when the ramp was slowed down all the way--effectively the duty cycle was higher without the peak efficiency being any more. We've abandoned the two photon experiment again for now. There's nothing guaranteed the independent and generated 1555 will be in phase so maybe there's just always a lot of interference?
We got lower finesse mirrors from ECI. They came out to be around 3500 finesse and seem very promising so far. Locking is way easier, although the efficiency is still around 10e-6 for 807 generation. The linewidth when locking actually broadens some to about 0.5 MHz so we still need to figure that out. The exciting part though is that the peak efficiency is around 10e-4 when ramping! We're working on optimizing the locking performance and increasing the pressure to a more optimal place. 8 atm should be ideal due to broadening/narrowing effects. I ordered a new regulator that could handle that but am still having problem and haven't seen anything good yet.
'''9/25/15'''
We got our vacuum chamber and have the 10 cm cavity ready, but haven't set it up yet. We've looked more into the efficiency calculations and it actually seems like huge gains might come from using lower finesse mirrors--somewhat counter-intuitive. The theory breaks down a little in some of these parameter spaces (for instance getting more than 100% transmitted power), but it does suggest that lower finesse could help a lot. It's a difficult problem since the optimal finesse depends on the intra-cavity intensities, which in turn depends on the locking performance and the finesse. After more calculations and investigations we decided ~5000 at both pump and stokes would be a good balance and so we've ordered those. Calculation and mirror purchase details coming soon. Meanwhile, we've been working on the Ti:Sapph. We have all the mirrors and parts, but no luck yet aligning it. We'll right up a procedure once we figure out the best way to do it.
With the Raman cavity, we're still using the 27 cm setup and have gotten the 1555 beam going again. We've decided that locking both beams for the 2 photon experiment might be too hard right now, but we've discovered that we can get 1555 generation from 1064 (and then 807) just from ramping slowly. This suggested we might be able to do the 2 photon experiment just by ramping, which is a lot easier. It seems like it might have worked! I tuned the 2 lasers to approximately the right frequency separation, ramped the cavity and overlapped the 1555 and 1064 peaks on the scope using a combination of laser piezos and cavity piezos. The fact that both beams are aligned to the cavity guarantees spatial overlap. With a free spectral range of ~550 MHz, we'd expect only one peak overlap would work, since the transition linewidth is ~600 MHz. So I looked with an IR viewer where 807 should appear from a prism and tried overlapping different peaks. One combination gave bright flashes of light as the peaks moved back and forth over each other! Unfortunatenly I haven't been able to repeat this yet after a couple weeks of trying. We're still trying to figure out what might be different, but we'll all ready to measure the peak power when it happens.
Locking is working better than it used to. It's very helpful to look at the servo output from the box. If it doesn't go to 0 when locking, then the aux output (piezo) is most likely railed. So finding a locking area that locks around 0 is very useful. It also turned out that one of the boxes was broken--the servo output sat at 10V even when in "unlock", which basically meant that the laser current and cavity piezo were trying to drive the resonance to two different locations. We sent it back to Vescent and both boxes are working now.
'''6/29/15'''
I got the mode-matching working on that cavity. The cavity was slightly too long causing it to be unstable. I added a few O-rings, which pushed it into the stable region and then mode-matching was easy. Profiling the beam is a bit of a pain though--you can't just turn down the amplifier power with the beam cube because we'll get only the "bad" polarization which is pretty much in the 1,1 mode. Instead you have to send in mostly full power (which requires having it mostly aligned to the cavity) and then use a pick-off to look at a small portion of the beam. Locking worked reasonably well, but at .1 and .3 atm of pressure I saw no stokes generation--surprising, but we aren't cooling the cavity this time. In a previous paper we put out though we had plenty of stokes power at these pressures with no cooling in a similar cavity. I'm a little concerned and we wanted to look at much higher pressures to see what would happen, but we ran out deuterium and that's taken a few weeks to get in. Hopefully though we'll find the pressure threshold is just a little higher than before or maybe the gas was contaminated somehow and it just needs a fresh batch of deuterium. The gas should be in this week and then we'll know.
I've been working on more detailed calculations for the mixing efficiency of a cavity redesign--[[File:Modulation_Efficiency.zip|here's some code and generated plots]]. Current limitations of this calculation are that we assume constant intensity inside the cavity (i.e. the waist is very close to the spotsize on the mirrors--this is not a bad assumption for the geometries we're interested in since these tend to be more stable, but could be improved. It's not good for longer cavities especially. This could be corrected by doing some sort of total integrated intensity like in the previous entry). Additionally, the mode-overlap factor between the pump and the stokes and a potentially separate mixing beam is only valid at short cavity lengths (this term should depend on cavity length but we just use a constant multiplicative factor for all lengths). We also have limited knowledge of how locking performance will change with pressure and cavity length--the most we do is in one set of plots just introduce a multiplicative factor for the laser linewidth and cavity linewidth overlap.
The results still suggest a shorter cavity is probably the way to go, but it's unclear how much improvement we'll get. It seems unlikely to be a huge change though--the mixing efficiency scaling--(pressure*length)^2*Pump_intensity*Stokes_intensity--is misleading since increasing the pump power won't linearly increase the pump or stokes intensity and increasing length causes the intensities to drop as does pressure. So the efficiency changes from most of those terms scales more like a square root rather than the square like we hoped.
But we're moving ahead with it. I'm in the process or ordering a 10 inch vacuum chamber in which we'll probably mount a 10 cm cavity and see what happens to us.
'''6/3/15'''
Ok, so here's the deal in more detail. A couple weeks ago I started trying to modulate the HeNe instead of the orange laser, since it's single frequency and easier to detect. It's also comparably low power with about 0.5 mW inside the cavity compared to the 3 mW of the orange laser (but inferior spatial mode) Also since it's CW, I could safely look for the modulated green light. I was hoping to put some sort of bound on what would be detectable since I hadn't seen a hint of single from the modulated orange laser yet. I got the HeNe well mode matched to the cavity and got it polarized correctly (borrowing a beamcube from Nick and just rotating the HeNe barrel). After some work I was able to see green after a grating and iris and some interference filters. I'm pretty confident it was nearly maximized since it was well overlaped with the 780 beam, and that is relatively easy to optimize since I can get a large signal from the modulated beam. I wasn't able to detect an actual signal from the modulated HeNe light though despite trying a few different methods. Some rough calculations showed the noise on the lock-in amplifier was about 10 times as large as the signal I'd expect from the HeNe. That suggests it's unlikely we'll be able to see a modulated signal from the orange laser. Even if we could, we wouldn't be able to bin the power into more than a few wavelength groups, so it wouldn't really do what we wanted.
All this led us to decide it was time for a cavity redesign, since <math>10^{-6}</math> seems about the best efficiency we're able to get with the current one. That's just too low for most applications and measurements. The modulation efficiency is proportional to the <math>(P*I*L)^2</math>, with P as gas pressure, I as pump intensity and L as cavity length. Some rough calculations of the I<math>^2</math> suggest a shorter cavity might help. See result [https://wiki.physics.wisc.edu/yavuz/images/5/5e/Coherence.png here] and code [https://wiki.physics.wisc.edu/yavuz/images/0/00/Raman_cavity.zip here]. The plot is the total integrated intensity (both radial and longitudinal) of the pump beam. It assumes we get the same lock efficiency and stay at the same pressure of gas. The dots show the proposed change from our current cavity (L=.75, ROC=1) to the new cavity (L=.27, ROC=.3). Note though that this doesn't account for the cavity length factor. Including this factor, the plot looks like [https://wiki.physics.wisc.edu/yavuz/images/d/d3/Coherence-length.png this]. So actually going to a shorter cavity might hurt us slightly in the overall intensity-length factor. But the changes aren't very big. What we're really hoping is that the lock-performance will be substantially better with a shorter cavity. Right now, with about 12 Watts of incident power, we only get about 200mW out under vacuum and about a couple mW out with gas. A shorter cavity might be much easier to lock to and the performance increase could more than overcome the decrease in the intensity-length factor. We could then operate at higher pressure as well, which could greatly increase our efficiency.
We're not quite sure if or how well this would work, so before designing a new cavity we're trying out an old one we had (L=.27 with mirror radius of curvature=.3)
So that's where I'm at now. I've got the new cavity in but have been having a bit of trouble with the mode matching. It seems doable though and I'm not going to worry about the mode-matching being perfect (it probably wasn't with the old cavity). I just want to see if there's a big difference in lock-performance or not.
'''5/28/15'''
Changing out the old Raman cavity! More to come, just recording it's position here. From the corner of the table near the computer, the corner of the cavity (nearest the computer) is 45 by 6 holes away. (i.e. the corner is 0x0)
Also need to move the walking mirrors for mode-matching purposes. The Dichroic was at 36x7 and the other is at 36x3.5. The pickoff is at 41x7, and the prism is at 41x10
'''5/18/15'''
Nick wants to borrow a vescent setup and so I'm gonna do him a solid and loan it out for a bit. Here's the settings for the 1064 pre-lock, which hopefully someday will be useful:
gain: 2.75
first integrator: 1kHz
second integrator: off
differentiator: 20 kHz
diff gain: 23 turns
aux gain: '--', 'low'
Saw green from the HeNe!!!
I'm getting about 0.71 mW incident (chopped) on the cavity and about 0.56 mW inside. I couldn't detect a signal from the green, but it was very faintly visible after the diffraction grating. This is good, but the orange laser is not a whole lot more powerful. If I can't detect the modulated narrow frequency light from the HeNe, the orange laser isn't looking too good. I'm getting ~12mW chopped power incident on the cavity from the orange laser (with the thermal head centered at 1024) and about 2mW inside :-/
'''4/28/15'''
Eventful last couple weeks. I finally ordered the new computers and Nick and I spent a productive day building them. We also won a SWAP auction for 30 monitors for only $375. Every computer in the lab will have 2 monitors minimum now. I spent a while writing a python program to calculate what lenses to put where to mode match a beam to the cavity (or to any shape). The orange laser was just so big by the time it got to the table it was hard to just eyeball it with lenses. I'll probably still make a few edits but I'll put the program up eventually---it worked great and the beam is pretty well focused through the cavity now.
I ordered a fast 2 GHz photodetector from Thorlabs--the DET025AL. Since the rep. rate of the orange laser is 100 MHz, I was hoping we'd see pretty high peak intensity on this, but so far I can't see any AC signal. I think maybe this one is just defective. In the meantime, I'm going to not try to detect individual spectral components and just the whole modulated beam. So no monochromator, just short pass filters to let through light under 800 nm. Using the same settings as when I did this for 780, I get fluctuations of around 75 mV out of the lock-in. Roughly 1 nW=1.77 V, so with only 10 mW incident on the cavity, it might be a little rough seeing a signal.
'''4/17/15'''
The beam quality is somewhat lacking with the orange laser, which makes it hard to match to the cavity mode--it's currently very poorly matched which is at least part of why I can't detect anything. It doesn't really behave as you'd expect when it's collimated. I checked how well it would focus--with a 40mm lens, I got to to about 30x60 micron diameter. The beam was about 5 mm before, so the diffraction limited spot should be closer to 10 microns. Still, we only need to focus to about 800 in the cavity, so no big deal hopefully. I'm vaguely trying to write a python thing so that I can say what initial beam I have and what I want to be and it tells me the best lenses to put where.
'''4/15/15'''
Been working on modulating the PCF output of the orange laser. Got the 1064 beam up and running again and locking as well as it used to. 780 was very easy to get modulated again, though it's always a little hard to actually detect with the lock-in. The 780 diode burned out--started outputting only a few mWs even at high current, but I replaced it and it's working fine again. I now have the 780, orange laser, and the HeNe aligned to the cavity using 2 flip mirrors. Each beam has two separate mirrors, but they're not all independent. Still, it was the cleanest setup I could come up with and after the initial difficult alignment it's not much of a problem. I'm trying to use a monochromator instead of the diffraction grating setup like before. I'm still chopping the beam and looking at it on the lock-in It works well enough for the 780 at least. My hope is that it will give me some sort of spectrum-resolving ability. I couldn't detect anything from the orange laser yet though--I think there's just too much unmodulated light getting through so that when I change its alignment to the cavity I'm mostly affecting how much light makes it through the monochromator. I got some low-pass filters though that should cut out everything but the modulated light though which will hopefully help. I haven't tried to make the beam-profile better match the 1064 yet either which will help increase conversion efficiency.
I've also been trying to modulate a HeNe mostly just to see if I can. It should shift to 532nm and be pretty visible, but I couldn't see anything yet. The beam profile probably isn't good though or the polarization and it's so low power, so I'm not surprised. They have some 20 mW HeNes in the optics lab maybe I should borrow. I don't want to waste too much time on this though since it doesn't really show anything different from what we've done before--it was mostly just something to do while waiting for the filters for the orange laser.
I was hoping the orange laser would be easier to detect since it's pulsed, but it seems like the pulse width is too narrow--I tried looking at it on a photodetector and it didn't seem any easier to detect than a CW beam.
'''3/16/15'''
Major findings of the last month:
We got the interferometer set up again and, surprise, there's no linewidth narrowing from the ebay mirrors. So I switched back to the layertec mirrors (the two curved ones--again it's unclear why it doesn't work well with one curved and one flat). The linewidth looks around 20 KHz, which is at the limit of what the interferometer can measure, so the narrowing is potential less. Some sources suggest you need a substantially longer fiber delay than the coherence time of your last (like 6 times as long) but I've seen it quoted that being equal is fine too. It's clearly narrowing substantially though.
The layertec mirrors never locked very well. I attributed this to bad slow feedback for a while (all the piezo would ever do is rail). It seems like the actual problem though was etaloning between the fiber launch and some other element. Essentially the pre-locking cavity peaks were inside a larger envelope of some much lower finesse cavity. This explains the strangely high sensitivity the walking mirrors seemed to show. I put an isolator in which mostly fixed the problem. It seemed to substantially reduce the error signal and transmitted signal though--way more than the 20% or so lost through the isolator. I'm unsure why since it the shape of the beam shouldn't be affected. But the cavity locks okayish for now--upwards of 10 minutes when working well. Improvement can certainly be made, but for now I'm working on the Raman cavity since the basic features I want from the pre-locking cavity are there (much longer lock times than the Raman cavity and substantial linewidth narrowing).
Little luck with locking the Raman cavity. I've been working on making a slow-feedback locking circuit (see Zach's description under equipment list) but am still having a few issues. I borrowed on of his completed circuits though and haven't had luck, but still have a few things to try. I had thought our old slow box was broken since the power supply always went a little crazy when I plugged it in, but I realized today that the manometer is actually drawing way more current than it should. Plugging the slow box in just always pushed the power supply over the edge. The -15 V line was actually running closer to -11 V. I'm not sure how long this has been going on. I've just unplugged the manometer for now since I don't really need it when I'm just operating under vacuum.
The low pass filter after the mixer on the PDH setups seems to be greatly lowering the error signal. Like most things, I'm not sure why. I'm taking them out for now on the grounds the added signal is pretty much too high frequency to do anything in the servos, and any detrimental effect it has is probably greatly outweighed by the decrease in SNR.
I'm getting worried this isn't going to work. I guess that was a risk when I started this, but I really thought if we got the laser linewidth on the order of the cavity linewidth we could lock with just slow feedback. I don't really see how that's different from what Nick does with the SHG cavity. At least I've learned a ton, and I've really only been trying something that might potential work for a day or two. I can always table this whole things for a bit and come back to it in a few months. I think I might be starting to become frustrated with it to the point where I'm not working effectively. Probably in another week or so I'll start trying to set things up to modulate the pulsed orange laser, which is something I think I can do.
'''2/11/15'''
Got very poor locking by using the old 1555 slow feedback box on the Raman cavity while the laser was locked to the low finesse cavity. I was having trouble sending two error signals to the Vescent box since they seemed to interfere with each other, so I'm trying this method for now. Having the laser locked to the low finesse cavity seemed to help stability with the Raman cavity, but not for transmitted power. I think we're not getting any or enough linewidth narrowing, so I'm going to swap the cavity mirrors again. I think the problem with the Layertec mirrors wasn't that they were too high fiensse, but that we weren't impedance matched (I still don't think we understand that very much though). So I'm going to use the two curved mirrors which should be pretty identical and presumably more reflective than the ebay mirrors.
Good locking settings for the ebay mirrors are:
1st integrator: 100 Hz
2nd integrator: 1 KHz-1MHz or off
differentiator: off
aux gain: who cares, can lock without it
prop gain: 4.0 turns--sometimes turn up to get it to lock then turn back down a bit to stabilize.
'''2/5/15'''
The eBay mirrors ended up working. I'm using two curved ones with R=75 cm, so the waist is in the middle of the cavity now. The size was only about 10 microns different, so I basically just moved the cavity forward 5 cm, and what do you know--huge transmitted and reflected peaks. I'm guessing it's better because the mirrors must be very close in reflectivity, but it's weird because the impedance matching equations (see the python script) suggest that it shouldn't be a huge deal assuming the reflectivities differ by a couple tenths of a percent (seems likely for any cavity mirrors). Maybe there was something about the plano-concave configuration? We don't quite understand something here, but sometimes you just gotta move on.
With a bigger reflected signal it wasn't too hard to get the laser locking to the cavity. It was easier to lock to non 0,0 peaks. Even though the signal was smaller than the 0,0 mode ones, they were more stable in transmitted power, which made it easier to lock to and the lock more robust. I don't want to send to much power to this cavity though since we need most for the fiber amplifier, so for now I'm locking to the 0,0 mode but I might switch back. The cavity seems much more responsive than before. When I adjust the MML, I can see the cleanness of a FSR change. It looks about as good as the Raman cavity.
The lock still isn't perfect, but it's much better than the Raman lock--I've gotten a lock maintained for ~1 hour, which is already good enough. I think I can probably still improve it. In addition to the locking circuits, putting the cavity in a padded tube might help damp low frequency noise which seems to be the dominant error signal frequencies.
So now is the hard part of the whole locking 1 laser to 2 cavities thing. There's a lot of ways I can see to do this. We generate a second error signal from the Raman cavity--we can try sending this to a lockbox and feeding back just to the cavity piezo (maybe the laser line is sufficiently narrowed from the low-finesse cavity). We could send both error signals to one lockbox (it turns out you can add two signals just be using a splitter backwards! Who knew? No one in lab, that's for sure). But that could be tricky to try to get it to lock to two things at once, even though the frequencies are pretty different. After locking one, trying to adjust the DC offset to get the other signal locked would probably disrupt the first lock. I could probably lock the low-finesse cavity with just the fast feedback and then try using the aux servo output on the same vescent box to lock the Raman cavity--this has the same issue though of having to adjust the DC offset, but would have less competition with the fast feedback.
What I think is most likely to work is to use a second lockbox and feedback either directly to the laser diode or combine error signals and send both to the current driver (I don't see any advantage of feeding to the diode, and that would be difficult/invasive to setup). I tried the combining error signals and feeding back to the current driver (slow-feedback to the separate piezos today). One problem I had was with the RF signals. At first I used a second RF generator for the Raman error signal, but I noticed a lot of noise. I eventually realized it was at 5 Hz and tracked it down to the fact that the RF generators aren't super accurate and the signals differ by about 5 Hz even when they are both set to exactly 50 MHz. The problem got a lot better when I reduced the frequency of 1 by 5 Hz, but it was still a little off and I didn't have the resolution to improve it more. So it seems like all the signals need to come from one RF generator. But without some crazy combination of splitters, I can't send the same power to all components. '''I eventually upped the power of the 1064 RF generator to 13 dBm''' (this is now the normal power that should be run at), split the output to send 10 to the Raman error signal, which leaves 10 to be split into two 7 dBm signals for the EOM and low finesse locking signal as usual. But this means that the Raman cavity error signal is getting 10 dBm instead of the usual 7. It probably doesn't really matter, but this is different than before. Maybe I can find an attenuator that'll work?
Anyway, once that was all worked out I wanted to try the double locking thing. In short, it didn't work yet but it seems like it could be possible--I had two error signals going into the current driver and nothing crazy happened and both Vescent boxes seemed like they were almost locking. I was having trouble with the low-finesse lock, which I think it just because the RF power is probably slightly different, and more importantly the summing signal thing (backwards splitter) probably introduces a phase delay that I haven't properly accounted for. I want to check with RF or function generators tomorrow and see if I can figure out how much it delays things. Hopefully there's no dispersion because I don't know how I could fix that.
It would also be good to check the linewidth when locking to the low-finesse cavity once new Zach gets the interferometer running.
'''1/22/15'''
Still haven't heard back from Layertec. Thorlabs says they can do it, but it'll be ~7 weeks, so I'm holding off on that for now. Deniz says Lambda Research is usually pretty fast. I requested a quote from them but haven't heard back yet. Zach found some mirrors on eBay that don't give a ton of info, but seem like they might work. I ordered them and hopefully with that we can make something happen.
'''1/20/15'''
I put an arrow on the backpolished E03 mirror pointing towards the side that was facing upwards in the packaging. I think this is the "back" side that is less well polished. The mirror didn't seem to help the signal strength though, and in fact made it worse. I flipped the mirror around too in case I got it in backwards, but I didn't notice any change. The problem now is probably that it is poorly impedance matched because the reflectivities are so different, which apparently is a thing. I updated that python program to account for this--we're probably only getting a few percent of the full height of the reflected signal for resonance dips just because of the impedance, so that's probably the issue now. I'm seeing if Thorlab can make us a custom curved mirror (2 E03s would give a finesse of ~800) and I'm talking to Layertec about making a pair in the 2000-3000 range. We want the highest finesse we can get away with that will still be easy to lock and stay locked, but we're not sure what value that will end up being.
'''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/index.php/File: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 16,000. The cavity linewidth is just the FSR/finesse, so it would be ~95 KHz, so with our ~500 KHz linewidth laser, and even very rough spatial coupling, we should still have been getting some 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 160,000. The linewidth would be very narrow then, and we'd only get about 2% 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 1600.
'''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).
I can't see anything for a transmitted signal with the PDA10A, without using an amplifier which would reduce the bandwidth too much. I don't see dips on the reflected signal, so this might be part of the locking problem. Maybe I'm not mode matched well enough and aren't getting enough power. I thought it a different lens/cavity position configuration I was seeing peaks on the PDA10A, but that seemed like it had worse mode-matching. Well it was pretty spot on in one dimension, but not even close in the other. Right now it should be pretty close in both, but maybe the last little bit matters a lot and so it was still better overall before. I'm going to re-profile the beam tomorrow and see how close I am and maybe see if I can improve it.
'''1/8/15'''
Nick had the really good idea that my peaks might be so unstable because I wasn't ramping at a high enough frequency (James from Saffman said this can be a problem), and I don't think I was at a high enough amplitude either. I switched to the external ramp and it looks much better now. I think I'm seeing a few free spectral ranges with not much non-0,0 modes, but the reflected signal is very weak (I can't really see it). I'm going to try locking though and see if I can get anything. I'm just using the vescent box for the 1064 cavity beam, so here are the current locking parameters for when I need that to work again:
First integrator: 10 KHz
Second integrator: off
Differential: 500 KHz
Differential gain: 22 turns
Auxiliary servo gain: 4.5 turns
Proportional gain: 3.3 clicks
Phase delay line: ~9'2"
'''Winter Break Update'''
'''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.
'''7/31/14'''
There's about 150 pW of noise on the lock-in even with all lasers and lights off. This is more than before, since I was able to easily measure a 60 pW signal. I'm not sure what caused the change. Maybe the old photodiode was less noisy and I just haven't looked at such small scales on the new one. It shouldn't be a problem if I'm generating nWs of 633, but maybe the generation is way down again since I haven't optimized everything after switching the 780 to a fiber. If I still can't see a signal today, I'll try switching back to the old photodiode
Found a weak 633 signal using the old photodiode. Quickly improved it and switched back to the new photodiode (so that I could get accurate power measurements). The signal was now visible on the new photodiode. I thought for a long time I must just be missing the photodiode, but apparently the
New efficiency measurement is <math> 5.5*10^{-7} </math> In addition to walking the 780 beam, I also slightly adjusted it's polarization and the focus of the a-sphere on the output fiber launch, both of which slightly improved efficiency.
Deniz can calculate the maximum theoretical conversion efficiency of 780 if we know the amount of 1064 and 1555 in the cavity. After a silvered mirror and a prism, I measured 0.55 mW maximum of 1064 when locking, and 0.50 μW of 1555.
The prism (Thorlabs PS853) has a listed transmittance of ~28% for both 1064 and 1555 and a silver mirror should be ~97% reflective at both wavelengths, so exiting the cavity there should be 2.0 mW of 1064 and 1.8 μW of 1555.
The cavity mirror transmits 32 ppm and 38 ppm of 1064 and 1555 respectively, so that puts the powers at 62.5 W and 47 mW inside the cavity.
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