10.28.09

Here's a crappy video of the total, tilt removed phase as its corrected by the PI controller.


Quite amazing considering the crap I was getting before. Most of the bias error is concentrated in the focus mode, and its important that note only is the phase largely corrected, but the actuators don't saturate at all.

Of course adding tilt screws things up. Even though the DM clearly attempts to correct those errors, some of the actuators quickly saturate. Manually removing it is difficult since the large distances in my setup make it extremely sensitive to even someone touching a mirror. There are three options for dealing with this:

1. Ignore tilt completely. My preference, and not entirely unrealistic since it is often handled by a separate tracking loop anyway.

2. Add a steering mirror. Probably tougher than it sounds, would require another beamsplitter and probably a month of playing around.

3. Use the disturbance DM. The disturbances are largely free of tilt and probably don't saturate the mirror on their own, thus the tilt modes on the disturbance mirror could be used to correct for some of the overall tracking error of the system.

For now #1 is the only option. Word on the grapevine is that another package is on its way, so hopefully it contains my second mirror. In the mean time, here's what I want done tomorrow:

- Finish implementing an RLS adaptive equalizer example in Simulink
- Test out/modify the existing Simulink files to run the PI loop
- Work on my prospectus
- Look into committee possibilities.

Harder, Better, Faster, Stronger

Finally got the faster DM control code working, only 3 months late! It does make quite a difference, as you can see comparing the response to the original function:

Here's the norm of the wavefront error averaged over 100 different random commands applied with both versions. Its startling how slow the old script was; a pause of 0.5 seconds wouldn't be too conservative. Even with a new script though it looks like some pause is necessary since, although commands can be sent around 100 Hz, actuation doesn't happen until at least 0.1 sec.

You can also see this when looking for actuator decay. Here's the max displacement of each actuator. Note the ~0.1 sec rise time is the same regardless of the actuator unlike before:

Compare this with the similar plot from 10/22. Ultimately I might be able to get away with a pause less than 0.1 sec depending on the application, since it does put a limit on the speed of the overall system. I'm still amazed just how accurately the DM is able to reproduce the modes, here's mode 30 for instance, lookin snazzy:
Mmmm, deformable mirror pr0n...I bet no one's ever said that before.

Ludicrously Good

The refurbished DM came in late Monday, and I finally got all the glass aligned and working today. I've switched to using Galilean telescopes to resize the beam to keep the overall path length small, and the results so far seem to be pretty good. I tested the DM itself for the first time today and the results seem to be almost absurdly good compared to the crap I was getting before.

Here are the actual influence functions for actuators 1-31 (there are 31 total) taken from the columns of the poke matrix:



Each one is almost suspiciously clean. There was a significant pause between applying the command and capturing the wavefront, so it doesn't look like I'm having the problem with actuator decay like I was before. The columns of the modal poke matrix are even more impressive:



Even the high frequency modes are recognizable and look almost simulated. I haven't had the balls to look at the reconstruction error yet, maybe tomorrow. Even better, there doesn't seem to be any actuator decay, shown by looking at the plots of the peak displacement




Compared to the results from Sept. 9 with the old DM this is golden. Note how higher numbered actuators take longer to respond). Right now the WFS is using a lot of subapertures to capture the wavefront in relatively high resolution. After the DM is vetted I'll swap out the optics to go back to a reduced frame and higher frame rates. Hopefully the higher resolution DM for generating disturbances will show up soon as well.

Tomorrow more testing. I'd also like to get the new (faster) DM control code working, which was the original motivation for everything that's gone on the last 3 months.

10.15.09

I finally got the new laser late Monday, just as I was about to call and see what the dillio was. Tues. was mostly dedicated to getting the spatial filter aligned, usually quite the bitch apparently, but some online resources were really helpful and I can do it in a few minutes now. Since then I've been going slowly with the realignment, especially since a package (with mysterious contents) was dispatched from AOS today. If the new DM's arrive then I can finally assemble the (hopefully) complete system. How many times have I said that? So far this laser is much easier to work with. I think the asymmetric divergence of the diode laser was really screwing things up.

Other than that I spent most of last week learning LaTeX while working on my prospectus. Just like shaving and wearing matching socks, its something every graduate engineer has to learn to do at some point.

Beware the time vampire

If your boss bought some equipment from ebay with no obvious utility to "play around with," what's the worst thing you could imagine? How about an impenetrable metal box the size and weight of a small fridge?

Oh wait, it's not impenetrable, there's a 1 inch slot on the top.

10.1.09

Shit I can't believe its October already. I feel like I've basically been a waste of precious government resources the last year, but I guess that's just the pace research moves. This blog has helped though...helped me realize how much time I've wasted on certain ::cough::spgd::cough:: algorithms.

I finally ordered a new HeNe laser like I always wanted. Its going to require some substantial changes to the glass in my experiment so I've decided not to waste any time messing around with my current setup until I get it. The diode laser spot was just too messed up by the time it reached the WFS to provide useful data. The problem, I think, is that the spot divergence is inversely related to the spot magnification, and diode laser spots diverge asymmetrically. The result was far from circular after passing through so many optical elements, and as a result the accuracy of a measured wavefront was seriously suspect.

I hoping it arrives next week. I'm using the down time to get outside of the lab and reacquaint myself with some long rusty controls material. I'm really interested in eventually trying some adaptive Q parameterization, but that means I have to know such riveting concepts as coprime factorization and Bezout identities.

Plus, the ratio sucks in the engineering buildings. Outside classes are in session, the sun is shining, people are out and about...its no time to be stuck indoors with a bunch of lasers and postdocs.