Bots, Bureaucrats, Interface administrators, Suppressors, Administrators, Upload Wizard campaign editors
573
edits
David Notebook: Difference between revisions
Jump to navigation
Jump to search
→Daily Log
| (15 intermediate revisions by the same user not shown) | |||
|
==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:
| |||