David Notebook

Things to Do/Check

• Use beam chopper on 780. Pump down cavity and put in new D2. Translate iris in front of PVC pipe to see if signal is from a beam or scatter. Try different filter combinations. Check characteristics of 633 filter--does it let through 1555? Calculate angular positions of second order diffracted beams--are they close to where 633 should be?

• Why is 1064 current driver fluctuating?

• Get a really big lens before the 633 detection photodiode.

• Measure 780 power before and after cavity; see changes with cavity piezo

• Why is reflected 1064 signal fluctuating in power so much?

• What's wrong with the IR viewer?

Daily Log

7/8/14 Pumped down the cavity in case some nitrogen had leaked in over the last few weeks. Re-added deuterium to about .3 atm. Noted that the .5 nW signal when locking the cavity is present even when the path from the diffraction grating to the photodiode is blocked. The locking signal looks about the same size whether or not the path is blocked, although the offset is different. There is no noticeable difference with the 780 on or off. I tried blocking off part of the optical table at beam level, but this didn't affect the .5 nW signal. It seems that the signal is almost definitely due to scattering. The 633 bandpass filter we use is not rated at 1555, so it may be letting through significant amounts of light at this wavelength. This could explain why the signal was present when locking even with the filter in place if the signal is from scattering. Putting in the cavity window in front of the PVC pipe reduced the locking signal by about a factor of 5 both with the beam path to the detector blocked and unblocked.

7/7/14 Installed a flip mirror for the HeNe, following Nick's advice. Switching between HeNe and 780 beams is much easier now and I am more confident in the alignment of the 780. Using a mirror as we have previously done, I was able to see 633 past the PVC pipe just before the lens, so it is almost certainly hitting the photodiode given that it is getting through the pipe and that I centered it on a photodiode with a lens using the HeNe as a guide. Still though, there is no clear change in signal between having the 780 on or off when locking the 1064. Josh, Deniz, and I tried to come up with explanations as to what could cause this and alternate methods to make the measurement (See Things to Try). We thought maybe a second order beam from the diffraction grating could be angularly close to the 633, but using the grating equation with G=1800 lines/mm, no second order diffracted light should exist above 475 nm, so that seems unlikely. Two 89 THz shifts and one 8.9 THz shift could produce light in the 630 nm range, but this would probably be a weaker signal than the .5 nW we are seeing.

7/4/14 It still seems that there is no difference in signal whether the 780 is blocked or unblocked, and there is a clear signal of around .5 nW when locking the 1064. The HeNe is not staying very well aligned--I hoped only a horizontal adjustment would be necessary, but it seems unscrewing it from the pedestal creates too much of a vertical change.

7/3/14 Had some difficulty with the 1064 beam today, see 1064 laser notes. I also realized that even after getting the HeNe laser going through the cavity and hitting the diffraction grating, there was still a fair amount of play with the mirror walks which ultimately could move the beam at the photodiode by a couple inches. The generated 633 beam might be missing the detector then. I put two irises in the 780 beam path after the cavity and separated by a couple feet of path length. After lining up the iris to the 780 beam, I swapped in the HeNe and used its mirror walk (which is independent of the 780 path) to align through both irises and then put the photodiode and lens in this beam path. Hopefully the generated 633 beam will be overlapped with the HeNe path now and is hitting the photodiode. Maybe try using a very large diameter lens before the photodiode so there's more tolerance?

7/2/14 Decided to try propagating the beams across the lab instead of making a PVC maze in hopes of getting good spacial separation. We borrowed a breadboard from the optics lab and aligned the HeNe through a PVC pipe about 15 feet away from the diffraction grating. The beam then goes through a lens and onto the photodiode mounted on a translation stage. There is still a signal when locking the 1064 beam, even with the 780 blocked. The signal does not change unblocking the 780 when the 1064 is not locked, so there is no 780 beam getting to the detector. Whatever it is seems to be from the 1064 or a sideband as we previously believed.

7/1/14 Realigned 1064 to the cavity and 780 beam after they had been left alone for a few weeks, and got 633 visible again by eye through the flip mirror. Re-overlaped the HeNe beam with the 780 path so that I can see where the generated 633 beam should go. USA is out of the world cup.

Business School/Vacation

6/13/14 We set up a diffraction grating over the past two days (G=1800; reflective in visible light region; borrowed from Saffman group), which gives better angular separation between wavelengths than the prism. Using the grating equation,

$Gm\lambda =sin(\alpha )+sin(\beta )$

(page 22 in Newport Diffraction Grating Handbook), 780nm and 633nm should be separated by about 18.6°. We overlapped a HeNe (λ=632.8 nm) with the 780 laser path, and then used this to figure out where the generated 633 should be. We set up a 1 inch flip mirror for viewing by eye and had the photodiode behind it. This worked very well and we were immediately able to see the 633 in the mirror, which was aided by the fact that it was much smaller than the 4 inch mirror we used before and we could set up the irises using the bright HeNe signal. We could still not see a definitive signal for 633 with the photodiode though, although we are confident we are hitting it or are extremely close. We mounted the photodiode on a translation stage, which we will try adjusting next.

There was scattered light getting to the photodiode , which is mostly 1064 or one of the generated sidebands, since there is an offset on the photodiode signal that changes while adjusting the 1064 beam power. At its worst, we saw 1-2 nW, and this dropped to a few tenths of a nW after better covering the photodiode. There was also a change in signal corresponding to ramping and locking the 1064 beam, even with the 780 off. The 780 also caused a slight offset on the photodiode, but a much weaker one than the 1064. We're going to try making a non-reflective container to put the photodiode in to get more isolation. Otherwise we'll go with a long path through PVC pipes, which shouldn't be too difficult using the HeNe.

6/11/14 We aligned once more with the camera today, and also noticed that the waveplate after the 780 TA was not set optimally to match the polarization of 1064, which would reduce 633 generation efficiency. Fixing this and getting a better alignment of the 1064 beam with the cavity today let us see 633 through the prism again, with about the same level of brightness as previously. We could still not see it through the fiber, but this is likely just a coupling problem so we aren't concerned. We looked at the signal from the photodiode after the prism, and also a pickoff from the 780 TA and found them both to be very correlated with the cavity piezo movement. The 780 seed laser still seems stable though. We also found some correlation with the prism photodiode with only the 1064 beam on, so 1064 or a sideband must also be getting through to the photodiode.

The power fluctuations in the 780 TA are small and we don't think they are a problem beyond the difficulty they cause in detecting the 633 signal. We're going to ignore them for now and try to better isolate the 633 signal. We're switching out the prism for a diffraction grating and are going to use two PVC pipes in a "V" shape. Since there's only scattered light getting through and not any beams besides 633, this should greatly reduce noise. We're also going to remove the lens before the photodiode, since this is focusing scattered light too, and instead put the photodiode on a translation stage.

6/10/14 Re-aligned the 780 and 1064 with the camera, but this did not make the 633 visible through the fiber. We re-walked the beams into the fiber, since it was fairly misaligned after yesterday's adjustments to the isolators. We still couldn't see 633 flashing though when ramping the cavity and with filters in place. The alignment to the prism was off after re-aligning to the fiber, so we set up a separate walk to the fiber and prism. We can't see any 633 now, but hopefully decoupling the fiber and prism will make it easier to optimize the signal once we find it again.

6/9/14 We realized just before DAMOP that we were getting feedback to the 780 laser. A lot of it seems to be coming from the cavity mirror, since the frequency/mode of the 780 seed changes when adjusting the cavity piezo. After retuning the isolators, the seed laser appears stable both on the OSA and wavemeter when adjusting the cavity piezo. We noticed there is still some change in signal on the photodiode when adjusting the piezo, and this occurs even with the 1064 off, so it seems there are still small back reflections affecting the 780. This is a much smaller effect than previously though, and since it isn't causing the laser to jump frequencies, we think it shouldn't be a problem.

The 633 light was much weaker today than when we were working on it before the conference. It was still visible through the prism, although dimmer, and it was unclear whether it could be seen through the fiber.

DAMOP Twenty-Fourteen!!!!!

Poster (PDF PowerPoint)

5/23/14 The last few days were spent trying to improve the 633 signal and better isolate it. We used a Thorlabs PDA36A photodiode and pushed it right up against the fiber, but could only get a very weak signal. However it did seem to correspond with locking the laser. The laser locked well enough to try walking the fiber connection and the signal was improved, but eventually it was discovered that the 780 beam was blocked at the source. This means that some other light was getting to the fiber, even past all of the filters we had set up. 1064 seems a likely candidate. We added in an old cavity window, which reflects almost all 1064, realigned the 780 beam and found a signal we believe comes at least mostly from 633. We also switched to the much more sensitive PIN 5DP Photodiode and SR570 Preamplifier combination. While the previous signal was responsive mostly to adjusting the fiber coupling and showed little response to the 780 beam alignment, this signal was responsive to both. We were able to improve the signal by adjusting both the fiber coupling and 780 alignment. We also tried using a power meter, and while the results were not entirely clear, we believe we might have seen a signal of a few tenths of nW when locking.

We wanted to be more sure that the signal we were looking at was entirely 633, which can't be done with our current method even with numerous filters in place. We instead sent the cavity beams through a prism and aligned an approximately 2 foot long piece of PVC pipe and a small iris until we could only see the 633 beam through it by eye. Further adjustments greatly reduced the amount of scattered ASE that was previously visible. We then put in a 30mm -B lens at the end of the pipe and lined it up to the power meter. Measuring with the power meter proved too difficult even when covering the setup and blocking most external light to the room, but using the preamp and photodiode, we could detect a signal. There was some background noise, likely from light leakage, but when locking the laser we saw spikes between 100-300 mV from the photodiode. At 633 nm, the photodiode has a sensitivity of about $0.4$ $Amps/Watt$. The preamp was set at $10^{9}$ $V/Amp$. Calculating the beam power, we found the 300 mV signal to be 0.75 nW, which was consistent with our previous observations, although we expected this to be somewhat higher now that this was not coupled through a fiber.

Next time maybe try removing filters to reduce 633 losses, since we will spatially separate it from the other beams with the prism.

5/20/14 We continued to try to measure the 633 beam using the OSA and a power meter. We still couldn't detect anything, and the power meter measurements put an upperbound on the power in the nW range. The 780 seed laser hadn't been staying single mode very well, which could be reducing 633 generation or spreading it out to nearby wavelengths. We decided to try to fix this before continuing the search for 633. We tracked the 780 problem down to ASE feedback from the TA getting through the isolator and to the seed laser. This was eventually solved (see 780 laser notes). We're hoping this will increase 633 generation, and the nW power upperbound might no longer be accurate. We will look using the OSA and power meter again tomorrow and also using a photo-diode with high gain, which might be more sensitive. We swapped the ________ photodiode and unplugged the 1555 reflected signal, which we will try to remember in a couple weeks when it doesn't work.

Bread time was especially good today. Josh added one banana to the mix, which seemed to result in a slightly more flavorful and substantially moister bread. Nick was notably agitated by having a pineapple cored on his desk, but settled down once it became apparent the mess wouldn't be too bad.

5/19/14 After initial alignment of 1064, the 633 beam could easily be seen when ramping the cavity without any adjustment. It seems very repeatable, but also suggests that the power isn't close to being optimized yet since there was no noticeable difference in brightness after having a weekend to drift. We spent most of the day trying to increase the 633 beam power so that we can see it on an OSA. First we worked on the 780 alignment and adjusted it while looking at the 633. A full "beam-walk" was too difficult without some sort of numeric measure of 633 power, but by only adjusting the vertical and horizontal of one mirror we made the 633 substantially brighter. At one point, the 633 signal disappeared (well after the walking process) and moving to different cavity modes didn't help. I'm not sure why this happened, but this was solved by slightly translating and rotating the prism and then adjusting the beam blocks for the 807/780/1064 beams. This was an easy process to get the 633 beam back, which is encouraging.

We also doubled the power of the 780 as measured directly after the TA (see 780 laser notes). We still couldn't see a signal on the OSA though, even with lots of averaging and with the beam locked. Looking at the coupling fiber when ramping the cavity, however, we could clearly see the 633 beam flashing above a constant background of 780. Putting in an 780 notch filter and a 633ish bandpass filter (borrowed from 1st floor John), we now have what seems to be a flashing 633 signal with very good contrast (i.e. against very little background). The fact that this filter works makes it even more definitive that what we're looking at is 633. We hope that this will be easier to optimize and finally detect with the OSA. It's confusing that the 633 signal is so visible even through the fiber and when not looking directly at it--is the eye that sensitive to 633, or could we be having equipment issues?

We also looked at the 780 beam on the OSA after the TA and saw that there is a large spread in wavelength due to the amplified spontaneous emission (ASE) of the TA. (The 780 region contains far more power than the other regions though--the signal looks somewhat Gaussian with a delta function in the middle). This explains why were still see light from the 780 beam even after two notch filters. The 633 filter should help.

5/16/14 1064 laser was notably better today--easier to align, better peaks, and less prone to drift. We slightly translated and rotated the prism until the 807 beam coming out seemed more defined. It looked much better than the previous day, but this might be due to the improvement in the 1064 beam. We built a fort (what's took Jared so long with his?) to block most external light in the lab and blocked the 1064 and 807 beams from the prism, as well as the small amount of 780 that was getting through the notch filters. Any 633 should have greater angular separation from the other beams than the 780, so we were careful to only put the beam blocks right to the edge of the 780 so as to avoid blocking any 633 that might be generated. We then used the camera to overlap the 780 and 807/1064 beams on the 1064 side of the cavity. This worked well today and was an easy process once we made sure no beams were clipping.

We then looked for 633 generation by eye by sending the prism output to a mirror and then perpendicular off the table at eye level. We still wore goggles as they should have little effect on a 633 beam but would help with 780 or 1064 if something went wrong. We immediately saw a red beam, but believe it to be a small amount of 780 internally reflecting in the prism and scattering. Further searching revealed another red beam (perhaps of slightly different color) that flashed when we were ramping the piezo. It was highly angularly dependent, but once we found it the first time and knew where to look it was fairly easy to find again. This beam also changed intensity when we changed the 1064 or 780 beam intensities. Additionally it disappeared when turning off the RF signal (we were using the old locking circuit and had it set to ramp so that it would exhibit peak broadening), and would disappear when moving the cavity piezo to a "bad" position. This is the exact behavior we would expect from a 633 beam, but we couldn't find a signal with the OSA yet, which will be our next step. We want to try slightly adjusting the 780 alignment as well to see how this affects the candidate 633 beam intensity.

Jared and Nick were somewhat impressed when we showed them the 633 beam, but Nick was quick to point out that he had a green laser while ours was only red. Blocking all external light in the lab along with the whirring from the machines creates a very spooky atmosphere, and Josh and I were quick to note to the potential marketability of the setup. We are thinking of turning the lab into a full-fledged fun house style attraction. Follow us on twitter (@HauntedLaserLab)!

5/15/14 1064 laser still seems somewhat different than usual, but aligned more easily than yesterday. It was still more prone to drift than was typical. Attempted to overlap 780 and 1064/807 beams on the 1064 side of the table using the camera in two locations. However, the beams were clearly not overlapping on the 1555 side after this, so we did a visual alignment on both sides of the cavity. We later realized the 780 was slightly clipping and we were aligning to a scattered beam with the camera. The 1064 is too dim to see on the 1064 side past the dichroic, so we aligned to the 807 since they should be in the same place. We added in two 785 notch filters before the prism to make it easier to look for 633 generation. The 780 beam was slightly clipping the prism (and so any 633 might be also), so we adjusted the prism. The very defined 807 sidebands we saw yesterday were worse afterwards and the number of sidebands we saw didn't match the number seen with the OSA when immediately switching between the two. No beams are clipping on any optics as far as we can tell, so the angle must not be quite right with the prism and we will try to find a better position tomorrow.

Got ice cream for employee appreciation day. Josh, Nick, and I were more than willing to wait in the 10 minute line for unlimited free ice cream, but Jared and Zach quickly left. "Captain No-Fun" now redirects to Jared's page. Nick believed sherbet and sorbet to be synonymous, which was proven incorrect.

5/14/14 1064 laser was more difficult than usual to align, was very prone to drift, and only locked passably well. Gas pressure is about 0.3 atm. Josh says it's been like this for a few days, although it seemed to show slight improvement in the afternoon. We set up a mirror on the 1064 side so that we can send 807 from the cavity to a fiber launch or a prism. Despite having the dichroic as one of the coupling mirror to the cavity, on the Saffman group's OSA, we could see all 3 beams with rotational sidebands (should only by 807, but 1064 and 1555 were about the same power as 807 instead of much stronger). It seemed there were some only vibrational modes too, but we haven't tried to repeat this yet.

Putting in a filter dropped the 807 signal by about 10 dBm and made the 1064 and 1555 no longer visible on the OSA, however 1064 could still be seen very faintly with the IR viewer after the prism. We're confident we're seeing pure 807 after the prism due the to OSA signals and since we can clearly see 1064 and 807 diverge. Sending the 807 about 8 feet away to the wall, the separation between 807 and 1064 is about a foot. Depending on the mode, we see 1-2 or 5+ sidebands on 807 over about an inch.

Froze snack banana for 10-15 seconds in nitrogen. Ice clearly formed on the peel, but the interior was only somewhat cold. Will try for 30 seconds next time.

780 Laser Notes

Tapered Amplifier (TA) is a 3 micron chip 1W 780 nm from Eagleyard Photonics. TPA-0780-01000-3006

Specified maximum rating is 3 amps of input current when seeded, but should be able to output 1W with 2 amps.

There should be about 2 mW of power going into the TA, although 5-10 mW should be fine if the output power still isn't at 1W with the current around 2 amps.

6/9/14 Re-retuned isolators both directly before and after TA. Feedback seems reduced. Re-aligned to isolators and TA in the process, seed laser is now set to 70 mA, with about 3.3 mW going to the TA. At 2000 mA, the TA now outputs about 1000mW.

5/20/14 Seed laser for the TA was very multi-mode. We eventually tracked the issue down to feedback from ASE due to the TA. We tuned the isolator and brought feedback down from a few hundred μW to about 20 μW, and the seed laser is now very stable. It should be possible to reduce this further with additional tuning, but it doesn't seem necessary at this point. We also reduced the TA current from 2000 mA to 1800 mA to reduce ASE, and had to increase the seed power. Seed current is now at 70 mA and the waveplate was adjusted to give about 6 mW to the TA. Output power from the TA is now around 900 mW.

5/19/14 Having difficulty seeing 633 nm generation from the 780 TA beam, but realized that the TA can actually take more current than we previously thought. Increased current to 2 amps and adjusted waveplate to give about 3.3 mW of seed power. Current for the seed is still at 65 mA. Output from the TA increased from about 500 mW to 1000 mW.

5/15/14 Increased 780 Laser current from 43 mA to 65 mA and adjusted the waveplate to continue sending 2 mW to the TA and the rest of the power to a fiber. Locked the waveplate at the right position--do not adjust without decreasing laser current.

1064 Laser Notes

Threshold tests Usually get around 100 μW at 20 mA of current. A small adjustment of the diffraction grating (~a quarter turn total) will usually show little adjustment in output power, except for a narrow spike and drop off near the optimal grating feedback setting.

7/3/14 Threshold test seemed off yesterday and was very off today. It took about 26 mA today to get to 100 μW and the laser was very multimode. Adjusting the grating position ultimately resolved the problem. The current reading on the driver is sometimes fluctuating, usually just by .1 mA, but sometimes by up to 5 mA. This doesn't seem to be causing any problems right now, but it is unclear why this is happening.