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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?
==Daily Log==
'''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.
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.
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.
I've pretty much concluded the 2 photon experiment can't be done with the current setup. The way Josh did it previously was by broadening the cavity peaks and then tuning the frequency differences with the laser piezos. He could then scroll to a different broadened peak, which would be about 200 MHz away. We're at higher pressure and locking makes the frequency jump a lot and in a pretty unpredictable way.
I think we need to pre-stabilize the lasers with a low finesse cavity. This will 1). make locking to the high finesse cavity much easier. The lasers would potentially stay locked for a lot longer so that we could just focus on tuning them rather than keeping them locked. And 2). Give us another way to tune the laser frequency--we could lock to different peaks of the low finesse cavity to change frequencies by its free-spectral range.
Another way we could maybe do the experiment would be if we could recreate the peak broadening behavior. We used to turn off the slow feedback box and ramp the cavity or laser piezo. When the piezo hit the resonance position, the fast feedback would keep it resonant for a bit (even as the piezo kept moving), resulting in a broadened cavity peak. This doesn't work with the vescent box even when we unplug the piezo, which I think is because the minimum frequency the the current driver responds to is different from our previous setup. I'm waiting to hear back from vescent about this issue. In the mean time I tried making a high-pass filter so that I could manually adjust it (well, I couldn't decrease the minimum frequency, but I could effectively increase it though). I wanted it to be able to filter out frequencies below a couple hundred Hz, but it didn't really do anything though, which might be partly because I used a pot with wire coils which probably has a big impedance, but from talking to people it also sounds like you need to use an active filter at low frequencies. I probably should just buy something then, but I'll wait until I'm more convinced that will help.
The manometer also seems like it got calibrated somehow, since it was saying the cavity was pumping down to about 20 times lower pressures than it usually does and that seems unlikely. I unplugged it and reconnected it, and it gave more normal values, but it's a little bit offset from where it used to be, probably by 10-20 mV. That shouldn't really matter since most of the pressures we're at are in tenths of volts and I can't really get those accurate to 10 mV anyway.
There was also an issue with the 1064 laser giving unstable wavelength readings. Like fluctuations of about 500 MHz every second (I mean, it stayed withing about 1 GHz over pretty long times, but just bounced around within that). This turned out to be because of the multimode fiber we were using. The frequency wasn't actually doing this, but the fiber was supporting extra spacial modes which was confusing the wave meter. Using a single mode fiber gives readings stable to 10s of MHz, which makes more sense. I couldn't get enough of both 1064 and 155 coupled with a single mode fiber, so I might need to order one that will work better, or just keep in mind the deal with the 1064 one.
Vescent also put an updated manual for the servo controller up, and it has some helpful info. There are apparently some secret switches you can only get to by taking the side panel off, a few of which do interesting things. One secret switch lets you set the box to ramp in "master" mode rather than "slave". Previously we had to input an external ramp, which we could then turn on or off and adjust the amplitude of. There's nothing really wrong with this that I can see, but it's just another piece of equipment and nest of wires to complicate things. I switched to the internal ramp, but I might need to change back if we ever get the peak broadening working again, since you can't ramp while locking with just the vescent box. There's also a switch to change it so that the vescent box will only output a positive ramp. This is good since we don't want to put a negative voltage to the piezo. We normally have to manually put an offset on it each time and then take it off at the end of the day (since we don't want to leave a voltage on the piezo). This also means there's not really a reason to output to the Thorlabs piezo controller. All it basically does is serve as a buffer to prevent negative voltage going to the piezo, amplifies the signal, and lets you make small adjustments. The internal ramp seems strong enough, so I've switched to just using that for now. I'm not sure if it's better that way, but I'm hoping maybe it will make the lock better since it kind of has more direct control over the piezo now without having to go through another device with its own gain and phase delay.
'''11/19/14'''
Once again, 1555 locked at 0.05 atm with minimal adjustment. This is a very good sign. I've got a new procedure for getting 633 back if the 780 gets misaligned. No cameras or IR viewers required and it's quicker and easier--put a fair amount of gas in (maybe 1 V or so) so that there aren't many spacial modes without 633 and there's a lot of it if alignment is good. On the 1064 side of the cavity, turn down the power and use a card to overlap the 1064 and 780. Then turn the power back up, lock the 1064, and on the 780 side of the cavity use the other alignment mirror to overlap the transmitted 1064 with the 780. Repeating this 2-3 times will get 633 back depending on how badly aligned it was. Optimal alignment will change slightly when you switch back to whatever pressure you were working at, but it should still easily be detectable and so is not hard to fix.
I'm getting closer to being able to do the 2 photon experiment. Both lasers have been locking pretty well at this pressure, 633 is easily visible with just the 1064. Now I just need to practice tuning the laser frequencies and figure out how to keep them that way even when they lock and jump frequencies.
I think pre-stabilizing them with reference cavities would help a lot, but I'll keep trying this for a bit and see if I can make it happen without them. There's still probably a few weeks before the Moglabs wavemeter is ready, and the pulsed laser is still not set up either, so it's not really holding much up.
'''11/18/14'''
Hot damn! 0.05 atm might be the magic pressure. 1555 locked again today with only small adjustments. Good settings seem to be integrator at 5 kHz, differentiator at 500 kHz (this definitely seems the best setting for all pressures). Differentiator gain seems to give a trade off between power and stability. Prop. gain around 3.75. Optical feedback at 1.4 V (I didn't realize the scale on the oscilloscope had changed--I've normally been doing around 400 mV, but this actually worked well with different gain settings. Maybe more optical feedback, less gain is better if I can get it to work).
'''11/17/14'''
I've been mostly still trying different locking settings. The 1555 is amazingly inconsistent and I'm not sure what's really changing day to day. Both locks work better under lower pressure though, so I've been trying to find a good compromise between getting 633 generation and easy locking. 0.38 V (0.05 atm) seems like a good spot. There's 633 at a fair number of cavity modes, and it's easily visible in mirror or on the photodiode. I'm going to stick just at this pressure for a while and maybe I'll be able to learn more about locking just at this pressure.
It also occurred to me that we might be able to run the lasers are higher power. The limiting factor is the isolators, but the 1064 one is rated for 60 W and 500 W/cm^2. We're obviously under 60 W, and I think our intensity is ok too. At 30 W of input, we would need a beam larger than 1.4 mm. I'll measure it when I get a chance. The isolator's aperature is 4.7 mm, so we could always expand the beam and re-collimate it after. That would be a lot of trouble, but we could increase our single beam conversion efficiency by 50%.
'''11/5/14'''
I'm not sure what happened, but something must have gotten bumped and the 1555 was completely misaligned--no cavity peaks or anything. It was good practice at setting that back up at least. The best way seemed to be to turn the power way down and look at the incident beam and reflected beam with the card and try to overlap them in two places. Then I moved the transmitted photodiode and took off all the filters/covers and put it right at the transmitted side of the cavity. Using the HeNe was helpful for making sure the photodiode was pretty much aligned to where the 1555 beam would come out. Turning the power all the way back up and making small adjustments with the walking mirrors usually got some very small peaks which could them be optimized.
Locking is still inconsistent. It's not too bad under vacuum, but with gas settings that work one day don't work as well another, and the higher the gas pressure the worse it seems to be. I lowered the pressure only about 0.57 V (about 0.07 atm), and was able to achieve pretty good locking--~150 mW transmitted, with the best locks stable for a few minutes. I was able to simultaneously lock with 1064 too (the bandpass filter helped), and I think I can still get sideband generation at this pressure, although the 780 was misaligned and I'm not able to see any 633. Hopefully after fixing that, I'll be able to get some 633 generation and have consistent locking settings and performance at this pressure.
Current successful settings for 1555 lock at 0.07 atm:
course gian: 4 clicks
fine gain: bit over halfway
first integrator: 10 kHz
differentiator: 500 kHz
optical feedback: ~300 mV
'''11/3/14'''
The 1555 lock works pretty well under vacuum when it works. Getting it to work is a problem. It seems to need different settings everyday, so apparently is very sensitive to small fluctuations. When it is working well, I've gotten between 800-1000 mW that is pretty stable in power (often less than 20 mW fluctuations), that can last for over a minute. Again the necessary settings seem to differ everyday, but usually a good lock can be achieved just by adjusting the optical feedback, proportional gain, and auxiliary servo gain. The first integrator is set at 5 kHz, the differentiator is set at at 500 kHz (gain set somewhere in the middle), and the second integrator is off.
Try optical feedback somewhere in the 100-600 mV range, and the proportional course gain at 4 clicks (the necessary settings for these two seems to be loosely inversely proportional. Sometimes the piezo voltage will max out (158.6 V), which usually means the gain is too high. Sometimes lowering the proportional gain helps, but this can also cause the laser current feedback gain to be too low, so try adjusting the auxiliary servo gain independently. Sometimes it will still lock fine at very high power with the piezo maxed out, which is confusing. Maybe it is still actually making small adjustments, which just aren't visible, or is the piezo actually doing very little?
Also it basically refuses to lock by adjusting the cavity piezo and you have to use the DC offset on the lock-box instead. Who knows?
The necessary locking settings are very different even at just 0.1 atm. I have the first integrator all the way down at 2 kHz. The proportional gain and optical feedback I've had to adjust all over the place. I had it locking well briefly and was getting about 150 mW, but lost it pretty quickly when trying to optimize it. The window of "good" settings seems much smaller with gas. The laser piezo is especially difficult--with the auxiliary servo gain too high (or just the overall gain too high), it maxes out the piezo, but when just a bit lower it doesn't seem to do anything. In fact I would get locking signals that were reminiscent of ramping with just the fast feedback on back in the old days (suggesting that the slow feedback wasn't doing much).
I did finally figure out where I would get weird signals from the transmitted 1555 photodiode (sometimes negative, sometimes positive when locking; the signal would quickly jump up and then fall down to center around zero even with the output power fairly constant). They seemed like symptoms of AC coupling even though everything as set to DC, but it turned out the photodiode is premade to AC couple. That's slightly annoying because it makes it hard to tell how good a lock I have without measuring it, but a new photodiode it expensive (we don't have any extras at this wavelength), so I'll make do for now.
'''10/29/14'''
I don't think the cavity leak is a problem for now--don't usually run it at less than 0.1 atm, or 760 mV, so the leak is a pretty small fraction of that. I got both locks working individually, but when trying to lock together the 1064 beam seemed to interfere with the 1555 lock. I think it's the generated 807, since there would only be interference in cavity modes where 633 was being generated (and so 807 was also). This is probably fixable with just a notch or bandpass filter. I also noticed that the error signal on the 1555 was very noisy again, at least compared to the 1064. Not the 1 kHz noise I was getting for a while, but just generally very staticy. I'm not sure if this is new or I just didn't notice it before. I eventually tracked it down to the low-pass filter, which doesn't seem to be doing anything. Switching it with the 1064 one fixed it. It's not locking well now, but presumably I'll have to retune some parameters now that there is a working low-pass filter and that there is gas in the cavity.
'''10/24/14'''
I think the cavity has a small leak. At room temperature, it will pump down to ~12 mV, but will be up at ~30 mV by the next day. This is probably too small to be a big deal, but I should check with Deniz.
'''10/22/14'''
I'm not sure what was going on with the laser driver, maybe a loose cable? I haven't had that problem again in the last week. I got the 1555 lock working better and got to about 600-700 mW of power. However, when it was locking well and the noise was low, the transmitted signal looked like a sine wave with a frequency of 1 kHz. Clearly the noise I was having was causing a problem. It turned out that the RF generator was putting side bands on the signal at 1 kHz. I swapped RF generators which seemed like it should help a lot. I got up to ~850 mW, but I think there is more optimization to be done since the phase delay was changed. The lock is pretty inconsistent from day to day, but I decided to put it aside and get the 1064 working again and then add gas before trying to optimize the locks much more.
The 1064 was much easier to get locking, although the transmitted power is low. Only around 70 mW after playing with most of the parameters fairly extensively. I'm not sure why it is lower, but the lock is pretty stable.
I'm still confused as to why I don't see the changes in error signal that I would expect when I change the phase delay. There doesn't seem to be much difference on both the 1064 and 1555. This is definitely something to focus on because it seems like it could be having a big effect on the lock.
'''10/15/14'''
I tried to follow the various signals through all parts of the PDH circuit. I made slight adjustments here and there, and most notably added some BNC length to change the phase delay on the local oscillator. I also turned off the integrator on the lock-box and so was just using the proportional gain. The error signal should be linear near resonance, so do we even need differential and integral adjustments? I saw the best performance yet, with powers over 300 mW and fairly consistent.
'''10/9/14'''
I tried to track down where the noise was coming from. The DC error output on the vescent box only is noisy when it's connected to the error signal and the RF is on (doesn't matter whether the laser is on and there is actually an error signal). The full height is about 15 mV of noise. I checked the laser diode output using a photodiode and the noise doesn't seem to be coming from there. Most likely seems the RF amplifier in the circuit, although the low pass filter isn't cutting this as much as I'd think either. I tested the amplifier by itself though and it didn't show any sidebands, which would have been the obvious cause. It does have signals at integer multiples of the input RF signal, but those are all around 10 db lower. I checked the same amplifier on the 1064 setup and it showed similar behavior. The vescent box on the 1064 also shows similar noise with, although slightly less, so maybe nothing is broken and this has always been there.
'''10/7/14'''
'''8/8/14'''
I've abandoned the [[Thorlabs PDA10A Si Amplified Fixed Detector|Thorlabs PDA10A]] in favor of the [[Thorlabs PDA36A Si Switchable Gain Detector|Thorlabs PDA36A]]. It's fast enough to produce square waves at 400 Hz, and has a much larger sensor. It also has a lower minimum gain, which seems to work better when used in conjunction with the lock-in. I still should try to check if the response is linear at low powers. Ostensibly though, using
Accounting for the new factors, chopped 633 power is now:
'''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
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.
==Long-Term Ideas==
*TEC on EOMs
*Get higher power isolators and increase pump/stokes powers. Alternatively, expand beam before isolators since we are only pushing the intensity limit, not total power.
*Use laser diode instead of TA for the 780 beam? Thorlabs makes a 400 mW single mode one. Probably get as much power in the cavity and there would be way few optics that could become misaligned. The beam would also likely be better collimated and a telescope could work to shrink it.
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