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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:
The power of the 780 reaching the cavity must first be maximized. Use the magnetic mirror on the 780 side of the cavity to send the beam towards the middle of the table and measure the power. This is the only place where the power meter can be easily put in.
Adjust the alignment of the seed laser to the TA to maximize the power measured at the cavity--usually just one mirror is sufficient, and the horizontal adjustment tends to have a much larger affect than the vertical. Then re-walk to the isolator, and then finally to the fiber. These adjustments have a small enough effect on the overall position of the beam that it is not necessary to adjust the placement of the power meter. Repeating these alignments once more usually gives an additional 5% or so in power since the aligning to the TA and isolator will slightly affect the coupling to the fiber.
Assuming that these adjustments have been small and that the 780 is still mostly aligned to the cavity, center two irises on the 1064 side of the cavity on the 780 beam with the irises placed a foot or two apart. Then maximize the HeNe power through these irises (close them all the way and walk it to get maximum power after the second iris) to ensure that it is overlapped with the 780 beam. This is usually a small enough adjustment that the HeNe is still centered on the photodiode, but occasionally re-centering it or even re-aligning it to the PVC pipe is necessary. Then align the 1064 beam as usual (cool the cavity, perform threshold tests, walk beam to get good free-spectral ranges). 633 should be visible in the flip mirror past the grating now when ramping the cavity. A signal may be detectable with the lock-in/photodiode at this point when locking the 1064, but note that it's possible that the 633 can be visible in the mirror and be aligned to the photodiode and still produce no signal on the Thorlabs PDA10A detector. This photodiode is better for getting an absolute power measurement of the 633 since it has a faster response time and produces good square waves, but the UDT Sensors PIN 5DP is less noisy and can be used at higher gain settings. The power of the 633 can easily vary by a factor of 10 with little change in perceived brightness in the mirror, so rather than try to re-align to the grating/PVC pipe/photodiode at this point, it's better to first switch out the photodiode, quickly recenter it using the HeNe (adjusting the photodiode position, not the HeNe alignment), and then maximize whatever signal is found by walking the 780. The alignment change by walking the 780 is small enough that it should still be overlapped well with the HeNe, but if the 780 alignment was very off it might be necessary to use the irises to overlap the HeNe again and check that it is still centered on the photodiode.
I measured conversion efficiency vs pump power--it is fairly linear as expected. See [https://wiki.physics.wisc.edu/yavuz/images/8/85/633conversionefficiency.PNG graph]
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