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--[[User:Zjsimmons|Zjsimmons]] ([[User talk:Zjsimmons|talk]]) 19:52, 19 March 2015 (CDT)
*Reading a lot: papers, theses, etc. trying to understand the line broadening for our system. I really want to know the decay rate to be able to plug it into our simulations. More on that subject later.
*Got another isolator so i'm able to start working with the wide-tune laser. So far i see a mode-hop free tuning range of about 5GHz, that should be sufficient for our purposes, but i really want to get it working better. Initially didn't seem to work very well, but i'm hopeful that increasing the load is helping give more repeatable/reliable piezo scan performance.
--[[User:Zjsimmons|Zjsimmons]] ([[User talk:Zjsimmons|talk]]) 11:50, 11 March 2015 (CDT)
Quick Update:
*Wide-tune laser is designed, drawn and built. Seeing initial laser feedback. Now to see if I can get it performing well and to see if i can optimize the tuning range.
*Re: Laser linewidths: The interferometer has been set up for measuring linewidths at 1064, i think other Zach aligned it for us. I looked at two lasers we have on hand: the spare laser set up in our default way and the laser set up for rotating the grating about its face. Both had linewidths less than a MHz. Also looked at the experiment laser, initially the linewidth was quite large (few MHz), which may explain some difficulties with the crystal and why we haven't seen any narrow burned features. Attenuating the fast feedback gain to the diode and changing the laser driver seemed to bring the linewidth down to less than a MHz. When locked there was some evidence of further narrowing but the shape appears suspect and difficult to fit. We should probably look at this further.
*Re: EIT and experimental simulations: we looked at the delay between green and fluorescence but it was difficult to conclude when the fluorescence 'turned on'. Figuring out the effective gamma to use in simulations is still an unresolved question. We were able to measure AOM switching and observed times of 100ns or so, as expected. Another question is what effective density to use, have an estimate but this needs to be looked at more carefully as it contains a lot of assumptions. I don't think we're going to have much hope of seeing things until we have parameters really nailed down and a decent handle on a simulation.
*Re: In the crystal: trouble seeing anything when focused, but why?
--[[User:Zjsimmons|Zjsimmons]] ([[User talk:Zjsimmons|talk]]) 15:44, 2 March 2015 (CST)
Recent work, three main irons in the fire:
*wide-tune laser design: finally have Zemax working, need to figure out more exact cavity optical path length measurement to inform CAD laser design.
*green beam on-off: installed switches, have the NI-6002 putting out TTL pulses, doesn't seem very stable so far, perhaps pursue a clock-linked design, maybe we could get better stability. Built a TTL or-gate circuit to allow switches to be controlled by delay box or 6002, so far it doesn't work properly.
*EIT parameters: investigating with my simulation to try and decide on good parameters for seeing EIT in the experiment. So far I see evidence of EIT, but there are still a lot of parameters to play with.
--[[User:Zjsimmons|Zjsimmons]] ([[User talk:Zjsimmons|talk]]) 14:22, 16 February 2015 (CST)
Last few days: Little progress, having trouble digesting the new information. We have been making progress but there is still so much we want to do. Did order some parts to hopefully build a laser with a larger mode-hop-free freq sweep and as a very simple NI DAQ to use to turn RF switches on and off.
Ideally we will have evidence of EIT before the end of the month. Also would love to have a wider absorption scan of the overall (inhomogeneously broadened) absorption feature showing things like a spectral hole and side/anti- holes.
'''Things to better understand:'''
*How much power should we use? I think we need to know this if we want to know how long of EIT pulses to use as well.
*Spectral hole width: this is determined by hole-burning power, i.e. more power=larger width, but what exactly is the relationship? Does this even matter that much when talking about pumping?
*Fraction pumped back: When looking at transmission, it appears that a lot of population can be pumped back into the hole. However, this is somewhat misleading because we may have enough optical depth at that laser power to absorb our laser many times over in the crystal adjacent to that spectral hole in freq. Presumably, if we were to turn up the laser power of the probe scan to the point that it would be not totally absorbed, we would see that absolute contribution of the population pumped back from the other hyperfine ground states. Thus far, when turning up the probe power somewhat (perhaps a x10 factor) we don't see much of an intelligible change. I would expect the overall amount pumped back to appear smaller. Perhaps we just don't have enough probe sweep power, to see the difference.
Alternatively, we should look at the fluorescence. This could give us an observation that doesn't have the relative background. Fluorescence should relate directly to how much is there. We will i think want to understand this better for collecting data for our upcoming EIT experiments anyway.
'''Things to build:'''
*ULE cavity.. this could really help with our laser drift..
*Wide sweep laser, this would be great but will be some work.
*Implement switches on RF sources so we can turn them on and off through a combination of the delay box and computer.
--[[User:Zjsimmons|Zjsimmons]] ([[User talk:Zjsimmons|talk]]) 17:47, 9 February 2015 (CST)
OK, spent a little time today with the He. Some observations:
*spectral hole width depends on burning power. Green power of a few uW is sufficient to create noticeable features in the absorption. But what about burning all the way through, etc. hard to see as you don't really get much fluorescence when you try and scan with low power..
*We don't have our pump AOM in the right spot, we'll have to put it in after the control AOM in the control beam path if we want to get the correct spacing.
*Overall population MAY be decreasing, this would make sense if population was accumulating in the other hyperfine state, we should get the other AOM set up to investigate this.
Lots of things to get sorted out. Also, thinking about building a laser better suited to wide scanning...
--[[User:Zjsimmons|Zjsimmons]] ([[User talk:Zjsimmons|talk]]) 18:46, 6 February 2015 (CST)
Week in review. It's been an exciting week, we're finally doing some science! after spending a lot of time building things.
*Mon-put the ref cavity in a can; i should probably get some windows for that to further isolate it from the environment. We also did a little work with the crystal.
*Tue-more work with the crystal, hole-burning etc, temp in the lab spiked, destabilizing things.
*Wed-temp was wacked up, did some reading on spectral hole burning.
*Thur-Got back to the crystal. Spent a ton of time trying to get the laser int he exact right spot. The mode-hop-free tuning range is about the same size as the feature we want to see so it doesn't work very well for scanning, as you have to get it int he exact right spot and what you're looking for also has to be in the middle of the scan range. Remarkably, it cooperated for a little while. We were able to slowly ramp the laser piezo and lock the SHG cavity to the moving laser. This lets us scan the whole inhomogeneous linewidth and observe a burned feature 40MHz wide in the overall inhomogeneous linewidth. Needs to be improved if we are going get decent data, but we were able to see something!
*Fri- step back to document, try and digest what we've been looking at, some notes with scope shots:
[[File:2_2_data_summary.pdf|thumb|400px|]]
Note: the laser mode-hopping doesn't appear to cause a problem for the fiber amp, this is good news!
A few questions/issues:
*Need a laser with a larger mode-hop-free tuning range if we are going to be able to scan over the whole thing.
*Need to resolve why it appears we can pump so much back into our hole; need to reconcile abs and fluorescence observations and straighten that out.
--[[User:Zjsimmons|Zjsimmons]] ([[User talk:Zjsimmons|talk]]) 17:33, 30 January 2015 (CST)
Experimenting with spectral hole burning. Burning with a 10mW laser, looking with a sweep that is ~1-2uW.
Observations:
*Spectral hole is perhaps wider than we expect: We need to look at the laser linewidth, but i was thinking we would we see a hole ~1MHz wide and instead its more like ~10-15MHz wide.
*Temperature change-> ref cavity length-> laser wavelength->hole position: We heated the cavity with the heat gun for a fraction of a minute, see the transmission peak (spectral hole) fall out of the scan range. Also as the cavity length was changing and laser changing color, when we burn a hole the hole is large and shallow, this makes sens as the laser was effectively scanning.
*Hole persists for a long time: We see the hole minutes after its creation, as it drifts out of and into view.
*Big question: does burning a hole wipe out an adjacent hole?, no we can burn holes next to each other: two spectral holes
[[File:Two_spectral_holes.pdf|thumb|400px|]]
Also, i changed out the mirror for the invar ref cavity and it worked basically immediately, probably dumb of me to order that other cavity mirror. I wonder if that would have better stability performance? We should probably put it in a can and see.
--[[User:Zjsimmons|Zjsimmons]] ([[User talk:Zjsimmons|talk]]) 18:23, 29 January 2015 (CST)
We are finally back to doing science again! In the last week or so we have the laser locking well to a cavity and seeding the fiber amplifier. Fiber amplifier provides way more power to the SHG cavity. There is still ringing on the SHG output, but it seems to work OK; we could potentially mess with the lockbox parameters to get a cleaner lock. Nick has the AOM beams set up.
Today we finally went looking for hole burning again. We see evidence of it, but it doesn't look as narrow as we would hope, appears to be more like ~5-10MHz wide, when it should be more like the laser ~1MHz (locked actually more like 50kHz?- need to measure this). Perhaps this cavity is fluctuating significantly, (few MHz) or there is some other issue. Presumably, we could up the burning power but burn for less time, giving us a narrower feature? So far we have not observed such behavior yet. So far we have looked with about 1uW of power and burned with much more, up to ~1mW or so for a second.
We're doing science again! Next:
*look at inhomogeneous broadening again, might as well with the more stable laser.
*mess with lock parameters to get more stable/narrower laser?
*ref cavity change?
--[[User:Zjsimmons|Zjsimmons]] ([[User talk:Zjsimmons|talk]]) 17:30, 21 January 2015 (CST)
Added a pick-off to monitor laser on spectrum analyzer/wavemeter. Seems well behaved compared to the previous laser. This is quite encouraging. Have the fiber amp seed launch moved toward to the inside of the table, should protect it a little/keep it out of the way. Next is to mode-match the fiber laser output to the SHG cavity.
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