Josh Notebook

5/22/2014

Still unable to definitively isolate and detect 633.

Coming from the cavity, we have the following filtering:

1000nm shortpass dichroic two 780 notch filters 1 cavity mirror (reflects (thus blocking) 1064 and 1555 extremely well) John Boffard's 633 10nm-wide bandpass filter

After that, we have the light coupled to a multimode fiber.

We still can't see anything on the OSA. We maybe can see about 0.2nW on the power meter if we put the fiber right up against it, and then cover everything with dark cloth and cardboard and turn all the lights off. This is definitely reaching the lower limits of what the detector can see, though, so we're not sure. Also, there's no way to be sure that this is 633.

We can definitely see this same signal if we put the same fiber up to one of our "bare" photodiodes (not the pre-boxed kind, the kind where it's just a diode that we soldered up). We send the output of the photodiode to an SRS preamp - (SR 570) - and then we send that signal to the scope. The pre-amp is used to filter out any DC offset of the signal, and it also provides the needed gain. We can easily see this signal on the scope, though we're not sure how much comes from 633. We're sure that it's not all of it because we can see a small portion even when 780 is blocked. However, it is very obviously coordinated with the lock, and the signal is by far (by a factor of 5 maybe) the largest when we have 1064 locked to the cavity and 780 on.

I set up 1/2" apertures after the prism set-up (mentioned 5/16) to make sure we know exactly where the 633 is going. You can see the 633 through the two apertures. I think that I will put a lens in and try to see if I can measure any power there.

5/20/2014

Generating the (probable) 633 seems repeatable. David found it yesterday morning without having to realign the 780 to the 1064. He lost it later in the day, but was able to realign things and find it again. It seems brighter than ever. (He turned up the 780 tapered amp some.)

David coupled the beam that goes through the dichroic through a multimode fiber, and when the modulation is on, you can see some modulation in frequency in the light that it is transmitted. (It's not really flashing in the sense that it never goes to low brightness; the intensity definitely fluctuates visibly, though.) We can see this by eye, but we are still unable to detect it with the power meter or OSA.

We borrowed a filter centered at 630nm from John Boffard from CC Lin's group, and with that filter in front of the fiber coupler, the flashing through the fiber is more pronounced. The flashing light seems to be a larger fraction of the light, and the constant background seems to be cut down.

Looking at the spectrum analyzer, it seems that the "other red" we saw on 5/16 was probably ASE from the 780 tapered amplifier that has a wavelength outside of the range of the notch of the 780 notch filters.

5/16/2014

The stability of the 1064 laser was really good today. I think it is back to normal. I'm not sure what changed; we didn't do anything to correct it.

We set up a bunch of cardboard and fabric to shield light so that we could do a visual search for 633 (785 anti-Stokes) after the prism. Even though the light reaching the prism had already gone through a 1000nm shortpass dichroic and 2 785 notch filters, we still carefully blocked the 785, 807, and 1064 beams on the output side of the prism. The beam propagates maybe 90 cm after going through the prism.

We almost certainly saw 633! We couldn't see it on the spectrum analyzer yet, so I'm not 100% sure, but I'm about 98% sure it's real.

We were able to see a bright red beam that did all of the following:

--flashed when we had only the modulation on, but no lock

--faded and came back when we turned down and back up the 1064 power

--seemed to correspond with the resonance peaks of the cavity

--seemed to correspond with the turning on and off of the modulation RF

It was a bit confusing, though, because in addition to the (probable) 633 that has the above characteristics, you can still see other red light coming from the prism. This other red light is constant in intensity, in contrast to the 633. I'm not sure exactly what is from, but it must be from the 785 beam. Maybe it is diffuse scattered light from the reflections of the (small amount of) 785 that reaches the prism. At its best, the 633 was seen to be at least as bright as this other constant beam, if not brighter. The 633 seemed to be coming from the same location on the prism as this other beam, though it was visible when the prism was viewed at a slightly different angle.

We initially overlapped the transmitted 785 (through the dichroic) and the generated 807 that also is transmitted through the dichroic. We did this with the Newport camera.

We forgot to see how the beam overlap affected the light. We'll do this on Monday.

This was with 1/3 atm of D2.

David, Jared, Nick, and I all saw it. Once we found it, we easily saw it multiple times, and it seems easy to reproduce.

5/15/2014

The 1064 stability seems to be better, but not great.

Got the 785 beam going back through the cavity. We tried to overlap the 785 with the 1064 in two different ways: -using the Newport camera to look at two places after the dichroic coupling mirror on the 1064 side --comparing the transmitted 785 and the reflected 1064, which we assume should match up with the resonant 1064 in the cavity ---we may have messed this part up, confusing a reflection of the 785 for the main 785 beam

-roughly overlapping the two beams on either side of the cavity --we did this by eye/IR viewer ---due to the different propagation directions, it's tough to do this with the camera. --on the 1064 transmitted side, we used the IR card, using the transmitted 1064 on the front and the 785 showing through from the back. --on the 1064 reflected side, we used a notecard and the IR view to see the beams in the same location as for the first method above.

added 2 785 notch filters in the path of the beam transmitted through the dichroic on the 1064 reflected side so that we have a better chance of seeing the 785 sidebands (at about 633 and 1020)

couldn't see any sidebands on OSA, but we're not very confident about overlap

5/14/2014

The 1064 laser is still not behaving very well. David also thought that it was behaving weird and was hard to couple to the cavity. We proceeded anyway.

We saw pretty solid solo 1064 generated spectra. Our OSA isn't working (it can't see low power stuff), but with Saffman's OSA we easily saw 807 and up to 5 (maybe 8?) rotational Stokes sidebands off of 807. We also could see many rotational sidebands off of 1064, and we saw 1555 and one rotational Stokes beam off of that.

We also saw all of these sidebands (except the 1555's rotational Stokes) using the IR viewer after a prism. For the most part, the beams all looked like well-defined gaussians.

We installed a removable magnetic mirror in the path of the 807 that is transmitted through the dichroic coupling mirror. We have it so that without the mirror, it goes to a fiber coupler to the OSA, and with the mirror we can send it to a prism for visual searching.

The dichroic coupling mirrors seem like a great idea; we are considering removing the prism that we use with the 1064 reflected signal because the dichroic works so much better and it's easier.

5/13/2014

More 1064 laser problems today. Can't quite describe problem. The cavity peaks (without modulation) are very sensitive to cavity piezo position. Something doesn't look right about the peaks. They almost look like the laser is multimode, but checking with the OSA, it doesn't appear to be. I'm not sure what the problem is. I had a ton of trouble getting the peaks to be single mode, and when I did, it wasn't a very stable single mode. A slight change in the cavity piezo or in the alignment made a bigger-than-normal difference in the single-modeness.

5/12/2014

Today I discovered the internet.

Also, I aligned the 807 (anti-Stokes from 1064) that goes through the dichroic mirror on the 1064 side. It is now coupled to a multimode fiber that we can send to the OSA. The laser was behaving pretty weirdly, so it's only kind-of aligned. (The resonant peaks were extremely sensitive to the piezo positions, and there were many multimode regions.) This was done using the old (home-made) locking circuits with only the fast modulation on (laser flashing).