Zjs log

--Zjsimmons (talk) 18:00, 19 May 2014 (CDT) We observed some decent locking of the SHG cavity today. At highest, we saw intermittent power of about 9mW w/ about 65mW of 1055 incident, which is a pretty decent conversion ratio of ~14 or so. Observations/issues

• Piezo is oscillating which is lowering output power. When the error signal gain is turned down, we see higher intermittent power but the lock is not very stable and drops. With higher gain, the oscillation is very pronounced.
• Also put in place the fast feedback circuit to the laser driver, does not seem to help, in fact turning up the gain kills the lock. There could be many reasons for this. I am wondering if the phase is not correct. I would think it would be the same as the slow feedback branch and two inverting amplifiers shouldn't mess it up, but i would like to check this. Perhaps there is an issue with using long BNC and/or i would think that the bread-boarded circuit would be more susceptible to crappy performance at fast feedback frequencies. I do want to get this straightened out soon so i can order the board and get the permanent circuit built.
• Also, it's becoming increasingly clear we should use a different piezo, something with higher resonant frequency would be very nice/easier to work with. I don't think we need the travel that the current piezo stack provides anyway.

--Zjsimmons (talk) 18:45, 16 May 2014 (CDT) Not much going on in the lab the last few days as i've been working on a report for my microscopy class. Tried looking at improving the resolution of HELM/SIM using a couple schemes. It appears they probably will not work, but it was interesting and i learned some stuff. Perhaps there's something there that could be developed further.

• Locking circuit PCB layout is more or less ready to go. Want to test the circuit some more and make sure it works with the crystal in the cavity before getting it made. Also want to test the fast feedback. Nick has got the crystal in the cavity and we can see some single-pass green but it is definitely going to be more challenging to lock it. There's a lot more structure on the error signal and the cavity modes seem to jump around quite a bit.

--Zjsimmons (talk) 14:14, 13 May 2014 (CDT) Have seen a fragile lock:) Despite Multisim showing gain should be -20 dB at 400 Hz, it still seems prone to 420Hz (resonance) oscillation. Issues:

• Switch to put bypass resistor to kill integrator when not locked seems important. Otherwise integrator will chug away and rail at some point. I'm also thinking having a low noise op amp for that stage should help prevent gain on op amp error for example. -added the switch.
• Should we use a different piezo with a higher resonance frequency?
• Piezo DC offset should only be positive- I changed this.
• Lock seems ok. Also added a 100nF cap on the output/input to the sum with DC offset as a passive low pass filter. I'm sure this could be improved but i think we should try it for now and see what happens.

--Zjsimmons (talk) 22:51, 12 May 2014 (CDT) Spent today trying different locking gain stage configurations, tried adding an additional stage to increase gain. Discovered error signal was not being properly added to DC offset, this seems to be corrected but two problems have cropped up:

1. Integral gain has a relatively large DC offset, i guess this is not to be unexpected, but sort of fights the goal of zeroing in on the resonance point by adjusting the DC offset of the ramp and then turning on the gain, i.e. killing the ramp and adding the gained error signal to ramp DC offset. This can be suppressed but comes at the expense of gain.
2. Piezo is resonating. Despite lowering the gain and lowering the roll-off point to very low frequency, the piezo is now oscillating. Can this be eliminated? Perhaps we need a lighter mirror set-up to raise the mechanical resonance frequency? It may be a good idea to get more piezos anyway, in case of failure.

--Zjsimmons (talk) 18:44, 9 May 2014 (CDT)

• Cannot seem to get the SHG cavity tuned up. Very frustrating, stuff in between my peaks is only getting worse, not smaller. Polarization from fiber is also drifting, annoying.

• Trying to see locking. Not working so far.

• Saw some strange behavior with TA, output power seems to have decreased somewhat since we first used it. With 5mW seed power:

Current (mA)	ASE (mW)	output (mW)
1000	16.6	120
1501	126	377
2001	274	632

Compared to before (4-14-2014), where:

Current (mA)	ASE (mW)	output (mW)
1005	18.9	165
1501	165	430
2002	-	838
2074	434	1W

This seems to be down a fair amount from initial output and spec, degradation? are we doing something wrong?

• Shout-out to Jared, need to have power meter at 1064 not 780 when looking at 1064:P

• Note: weird power meter behavior. Old power meter head seems to behave a little different than the newer one. The old one gives about a 20% higher reading between 1.06um and 1.05um. The newer one gives about the same reading for both wavelengths, something is fishy with that. Perhaps this explains some of the discrepancy between the TA performance now and before, if we measured before at 1.06 and now at 1.05, still a bit different. We should keep an eye on this.

--Zjsimmons (talk) 18:25, 8 May 2014 (CDT)

• examined Josh's lockbox. It seems to short the positive rail momentarily when first connected to power and then recover. Very strange behavior. Occurs w/ ICs removed as well, so it doesn't seem to be the chips. Checked voltage ref pots, doesn't seem to be for example shorted diodes. curious.

• Having trouble tuning up the SHG cavity, could not get very nice clean modes, always some garbage in between peaks. Subsequently spun the fiber launch and totally misaligned the whole shebang. I guess it will be good practice tuning it up. Still unable to get as clean of 'troughs' between peaks, may have to pick Nick's brain about it.

--Zjsimmons (talk) 17:30, 7 May 2014 (CDT)

• Tried repairing the fault finder today. Turns out there are some optics inside, two elements in small brass rings, one attached to the laser. Looks like they were glued together at one time and had since broken loose. This was why we could not get light through fibers with the fault finder, even though it was on, it was not correctly launching into fibers. I aligned the two elements as best i could using the transmission through a multi-mode fiber and glued it back together. It sort of works, coupling to single mode fiber is not that great, but with a little fussing(rotating the fault finder and pulling the fiber out slightly) you can get a decent amount of light through a single mode fiber. We should probably get a new fault finder as I don't know how reliable this repair will be.

• re: mysterious polarization noise, could it be we are blocking the beam with our fingers/fiber and that's why the power goes to zero?

• Nick showed me how to tune up the SHG cavity today. It didn't seem too bad to mess with; it'll be good to not be afraid to adjust it.

--Zjsimmons (talk) 23:23, 6 May 2014 (CDT)

• Figured out that the flat polished fibers were the cause of power fluctuations after the TA in 1055nm set-up. Using an angle polished fiber fixed the issue.
• Power fluctuations are also not the reason the circuit is not locking the SHG cavity. Perhaps the gain roll-off needs to be at lower freq? as we are seeing some wiggles around 420Hz (piezo resonance freq). Maybe this is involved in failure to lock. We could also try adding fast feedback branch and see what happens.

4-6-2014 How about a log? Maybe this would work better that the paper lab notebook. Lab notebook seems ill-suited to many functions and i find myself not looking forward to using it, perhaps this will work better. Should be a more natural medium for computer-generated material. It's nice that it would be searchable as well. It could also host PDFs and reports and other documents. I kind of wish we would have started a wiki a while ago! -z