Journal

and power supply works!

(but did not eliminate the fluctuations, as Metcalf predicted)

July 20, 2004

No more writing class! finally! Laser teaching center presentations for the REU kids today; I was the first one up since I had to run and submit my portfolio afterwards. My presentation was really short and incomplete, a) because I was in a hurry to hand in my work, b) because I did not go to sleep last night and was feeling loopy and nauseous, and c) I was distracted by Azure trying to take sneak pictures of me. My research paper turned out really good I think, and much longer than was required since I had so much to say. The highlight of my day was that I got my free bike from the Freewheel Collective, and she's a beaut.

July 19, 2004

Research paper for writing class due tomorrow...I missed the BNL trip since I was working on my "U.S. Role in the Uselessness of the United Nations" paper.

July 16, 2004

yay power supply! yay keg! I learned a lot in group meeting, apparently temp fluctuations are independent of voltage fluctuations, because a configuration of zener diodes in the temperature regulator keeps the temp stable. Unfortunately this means that my new power supply isn't going to solve the temperature problem. Some other issues I brought up in group meeting today were: the method preferable to locking the laser (absorption spectroscopy, because it is easier), how one gets rid of mode hops (search for the right combination of grating voltage to current, PZT can handle up to a kV of voltage), why the Fabry-Perot isn't spaced equally (to which Hal said something I didn't understand about how I shouldn't expect the peaks to be equally spaced...why not? Look into this...but he also mentioned what Dr. Noe said about the nonlinear behavior of the PZT). Also, right before group meeting I saw the clearest signal ever; a noiseless one with THREE lamb dips! very sharp, very intense. Unfortunately I ruined it by changing the grating position on the laser, in an attempt to save my signal from being destroyed by a mode hop. I should've taken a picture first (not that you can lock the laser to a jpeg, but it still would have been nice to display on my website here). I am a little disconcerted about building a diode laser now, after speaking with Claire. She told me about how she built one when she was an undergraduate and how tedious it was, not the construction itself but the collimation. This was something I didn't even consider, because the diode goes into a collimation tube, the end of which contains a movable lens. How hard can it be to move a lens along one axis? Apparently though, it takes a long time if you want the laser collimated over a distance of 5 room lengths. I don't know if we need our collimation that good however, since we only span the distance of an optical table plus a few reflections. I should ask someone about this.

July 15, 2004

Shout-outs to Brendan Wyker (now a Grad student at UMD-College Park), who built a MOT and successfully tested it today! Our MOT race is over, we have a winner.

Still no power supply. I asked Mr. Lee about power broadening today and he said that although the desired intensity ratios can be looked up, the best way is to use trial and error with neutral density filters. (but still need to find out the ideal ratios to get ballpark figures). Also, Dr. Noe and I figured out the intricacies of the signal generator and high voltage amplifier today. To keep the voltage to the PZT positive and non-clipped, the amplitude applied must be twice the offset. Also, I also replaced the 15 kOhm resistor in the high voltage divider with a 2.5 kOhm resistor (2.5:250 = 1:100 voltage divider ratio), so that the oscilloscope reads 1% of the voltage as opposed to the 5% it did previously. This makes life a lot easier, since now when I want to know how much voltage the PZT is actually getting when I turn the knob on the high voltage controller, I can just multiply whatever the oscilloscope says by 100. Therefore, 500V to the PZT will display 5V on the oscilloscope, instead of a clipped 25V signal. Oh and clipping occurs when the divided voltage from the high voltage amplifier is smaller than the applied signal on the generator. It's really great to finally understand all this stuff because now I can figure out a systematic way to increase the scan range, rather than trial and error with the hope that I don't get negative voltages or clipping.

July 14, 2004

I electrocuted myself today! 400 volts that was amazing, my elbows started twitching as if someone were shaking it. I went on a tour with Dr. Noe and saw the van de graf accelerator as well as the MOT used by Orozco's group with a Ti-Sapphire laser (very pretty I must say), learned about arcs formed from plasma by ionizing air. I aligned the fabry perot again since when we redid our setup we had to take it out altogether. However, the peaks don't seem to be perfectly equal in their spacing. This is a problem because we have been using the Fabry as a ruler to measure the frequency range between peaks. I wanted to use these distances and compare to the known energy levels in order to figure out which transitions we are looking at. Dr. Noe said it was probably because the fabry was not reacting linearly with the applied ramping voltage. I guess for now I will just have to go by pictures. Then I had machine shop for 3 hours, it was tiring but fun...the plumb bob is coming along very nicely.

July 13, 2004

Let the temperature drift and saw three of the 4 spectra. I took pictures of the transitions we are looking for, which are now on my website. However, I was not able to compare the spectra to the pics (or to measured values of the energy spectrum for rubidium) since the spectra would flash so quickly and disappear due to the drifting temperatire. So my plan now is to record the temperatures at which i saw the different spectra and then try to go back to them, take pictures and measure them with the Fabry-Perot. However, before I even start all that, I first need to be able to resolve our first signal enough to see the peaks very clearly. Our Doppler-free peaks are also not intense enough (compare our Doppler-free pics to Mr. Lee and Matt's pics on James's website). I thought this was due to the fact that the probe was not weak enough; right now we have it so that the probe is 16% of the pump, but I either need to make that ratio smaller or attenuate the whole beam because the pump might be too strong. I need to find out the limits for power broadening, so far I have just used the variable neutral density filter to eliminate the power broadening (if that is what it is) but that has not given me a better signal. Also, the temperature still fluctuates a little bit so I keep going off fluorescence, which makes it really difficult to troubleshoot.

July 12, 2004

some quantum stuff, more answered questions:

-How do fine structure and hyperfine structure arise?

Fine structure is a result of relativistic correction for kinetic energy, and spin-orbit coupling. Fine structure is found in all atoms, but the interaction that gives rise to fine structure depends on the particular atom. For rubidium, we ignore the relativistic terms in the Hamiltonian, so it is spin-orbit coupling that causes fine structure splitting. Spin-orbit coupling results from the fact that in the rest frame of the electron, the proton is orbiting it. The orbiting charge induces a magnetic field that exerts a torque on the electron and aligns the electron's magnetic moment with the field. Hyperfine structure is due to the magnetic interactions between the dipole moments of the electron and proton. The proton's dipole moment is much smaller than the electron's since its mass is larger (and in the denominator) (WHY) But there are two terms in the Hamiltonian for hyperfine structure of a hydrogenic atom. The 2nd term is the electric quadrupole hyperfine interaction term.

Schroedinger equation for Fine structure, hyperfine, and Lamb shifts can all be solved using time-independent perturbation theory.

n quantum number is degenerate, is made up of a bunch of L's (orbital angular momentum). Fine structure, or the lifting of degeneracy in L's arise from K+V: interactions between kinetic energy of the single electron and the the Coulomb potential between the electron and the nucleus and shielded electrons. Higher L ==>higher angular momentum, more kinetic energy. Fine structure breaks the degeneracy in L: different L's give different energies instead of same like they are supposed to.

More on perturbation theory from Griffith's (as long as I'm reading the entire book right now anyway):

If we want to study transitions between one energy level and another, must allow for quantum dynamics, or time-dependent potentials. Problems involving time-dependent potentials generally cannot be solved, but if the time-dependent part of the Hamiltonian is small compared with the time-independent part, the time-dependent part can be treated as a perturbation. The most important application of perturbation theory is the emission and absorption radiation by an atom. (dependent because atoms are at different states at different times-need initial conditions, find prob of being in a different state)

July 8, 2004

some questions that are now answered:

-resolution is limited by linewidth? What does that mean and how big is this linewidth. Is the linewidth just the width of the Doppler-free peak?

yes, the limitation comes from the uncertainty principle, which says we cannot know a transitions energy or frequency exactly if we know the lifetime. the width is 6MHz, and is indeed the width of a Doppler-free peak.

-which isotope is preferable to lock to? Rb 85 since it is more abundant?

Nope, look at the spacings between the transitions of each isotope, and choose the one for which the spacings are more far apart, so as to ensure not to excite two transitions at once.

-what is the Sharp LT25 diode (the one in our laser) made of?

GaAlAs.

-we verify whether what we see is actually the 87b and 85b by subtracting the signals and measuring the height of their peaks and checking to see if the ratio is correct. Should the ratios just be the ratio of the isotopes?

yes.

-why 4 Doppler-broadened peaks?

There are two peaks for each isotope, the two peaks for each come from the hyperfine splitting of the ground states

-why 2 lasers for MOT?

the second one is detuned from the first (meaning detuned from transition) and is the light that is used in the MOT to trap and cool atoms.

-what is the difference between linear and nonlinear spectroscopy?

Maxwell's equations are linear: they assume that electric (P) and magnetic (M)susceptibilties of materials are independent of the strengths of the applied fields. Sat spec however, exhibits nonlinear optical effects, where P and M are not directly proportional to E and H. If there were only one laser beam going through the sample and the intensity was measured from a photodiode on the opposite side, the measured intensity would be a linear function of the incident light. However, if there is a counterpropagating beam that intersects the first one, then the intensity of outgoing light is dependent on not only the incident beam but also the counterpropagating beam, hence nonlinear spectroscopy.

-What do the F's mean?

total angular momentum = J = L+S (orbital + spin).

I = nuclear angular momentum, proportional to Mu, which is nuclear magnetic dipole moment.

F = I+J = total angular momentum quantum number, includes nuclear spin.

July 7, 2004

I gave my presentation on Spatiotemporal Chaos today, it went pretty well I think except for my occasional stuttering. After my talk, James and I continued working on Sat Spec. Our new optical flat arrived today and solved all our problems! The reference and pump/probe beams are now almost a centimeter apart so they no longer saturate each other. We also removed two of the mirrors in our setup since we are no longer using the double photodiode to subtract our signals. Our setup looks really simple and elegant now, I'm so proud of it. We got a beautiful subtracted signal too, pics are in the Pictures link of this website. The subtracted signal is the red one (I can't emphasize how much I love that digital oscilloscope). The signal, though a lot more stable than before (and less noisy - we had to troubleshoot every single connection to figure out what the problem was, turned out to be a [faulty] two-prong connector from the photodiode to the oscilloscope), kept going slightly off fluorescence. This is because the temperature is still fluctuating...starting from 17.1 degrees celcius and going to 17.6 within two hours. I suspect this is due to the uncovered makeshift power supply that sits ungracefully on the table with us. The problem should go away once we get our new supply though, and then we can lock the signal. Still looking into how to lock the signal...also, we need to figure which of the four transitions we are observing on the oscilloscope. So far we have only seen Lamb dips for what we think is the Rb 87 F=1-->F=0, 1, 2 transition. All this tomorrow. It should also be made known that Brendan and I are officially competing to build a MOT.

July 6, 2004

Could not work today, the floor in the LTC was getting waxed and it was too much of a health hazard to work there, plus, all the furniture was piled up in the room with our experiment.

July 2, 2004

Matt Eardley is leaving on Sunday for a month!! This is terrible, since he helps us with sat spec and eats Green Cactus with us...well I can still hunt him down in emails. Also terrible was that the keg was cancelled. But on a less devastating note, I brought up the plans for the next diode laser in group meeting today. It may be possible to buy a used one with the controllers if we can find it cheap, if not, we can even use the graduate students' laser and controller since their Sat Spec setup is not currently in use. Mr. Lee also generously offered us a beamsplitter from his lab, which I will test on Tuesday. Today's lesson was on locking the laser, and the schematic and locking mechanisms seem pretty straightforward. However, Dr. Noe mentioned some other aspects or ways of locking I should look into, such as circular polarization and AOMs for locking to a detuned signal. I don't think I have seen the lock-in amplifier or a dither or an AOM on our electronics shelf, so I may have to look around for these things (once I know what exactly what they all are). I have not read that much on locking yet, but I will have more info on this by Tuesday. Off to the papers it is.

July 1, 2004

Things i learned today: there is a distinction between lamb dip and lamb shift!! we see dip, which refers to hyperfine structure, not shift, which cannot be observed in sat spec. The Doppler-broadened signal shows the fine structure, while the Lamb dip reflects the hyperfine structure. further elaborating on terminology, "Doppler-free signal" refers to the subracted signal that only shows the Lamb dips, and not the probe beam signal that shows the dip as well as the rest of the frequency spectrum.

A nice thing about the subtracted signal is that even if there is noise in our setup that appears in both the Doppler-broadened and Sat Spec signals, it will be eliminated when we subtract them, yielding a cleaner signal . Of course, since the probe beam is much weaker than the pump beam (16% of pump), the pump needs to go through an additional neutral density filter in order to equalize the intensities of the Doppler-broadened part of the two signals.

Experiment is running again, problems we had with our signal were noise and that there were lamb dips on both the reference as well as the probe beams. the noise was fixed by taking the electronics off the optical table (the table wasnt grounded!) and the saturation was a result of the reference beam overlapping with the pump (the dots were too close together-the beams were saturating each other). We will try to fix this by ordering a new chunk of glass tomorrow, to serve as the beamsplitter. The glass we found was a circular fused silica optical flat, with 1/20 wave accuracy on both sides. It is 25mm diameter by 21mm thick. 21mm is perfect, we tested it by using the thick side of our old glass which happens to be 22mm, and it split the beam about a cm apart. The circular optical flat will also be really easy to mount on a lens mount.

June 30, 2004

By an entirely haphazard sequence of events, I got into machine shop today. The student who was orginally supposed to take the class did not show up, which led Mark Jablonski to seek out Dr. Noe, who happened to see me in the Sinc Site in passing even though I had not been in the LTC all day (I was doing some internet research). 2pm is apparently a crucial time to be around and available in the Physics building. Machine shop seems like it will be fun. It reminds me of the woodshop class I took in high school, where I made a mining cart for my final project (when you moved the wooden guy's arms up and down, the cart rolled). In this class we make a plumb bob.

June 29, 2004

James and I spent the evening reading and looking up specs and prices for the construction of the diode laser. I have a checklist for all the components we will need; the running list includes: a baseplate, diffraction grating, soft rubber cushions, a box to prevent air currents, a PZT disk, current source, temperature stabilizer, Peltier disk, Laser Diode, Collimation tube with lens, laser mount, aluminum block, and mirror mount. Can't work on experiment yet since the floor needs to be waxed because of the drippings from the ceiling.

June 28, 2004

More high school students today and experiment still out of commission, so I spent most of the day reading and taking notes in my ever-growing email draft of facts and questions. I also attended Tim Chupp's (University of Michigan) talk today on the Radon EDM (Electron Dipole Moment) Experiment. It's pretty cool stuff, EDM is forbidden by the Standard Model (or at least, observable levels of the SM), because it violates the PCT theorem (invariance under parity, charge conjugation, and time reversal for any circumstance), and implies that the electron is not a point charge. Baryon asymmetry, which is result of there being more matter than anti-matter and occurs from baryogenesis, is another violation of PCT. (on a side note, the Supreme Court ruled today that detainees at Guantanamo Bay have the right to challenge their detention in court and "a state of war is not a blank check for the president when it comes to the rights of the nation's citizens"...woot)

June 25, 2004

Couldn't work on the experiment today - there was a guy there fixing the the drippy ceiling right over our optical table! I had lunch at group meeting though - some really great chocolate cake that Matt Eardley baked for Matt Partlow's 29th birthday. Hal was elated that we finally achieved Sat Spec and that we learned how to put together a makeshift power supply. We looked up a Thorlabs Laser Diode Current Controller as well as a Temperature Controller for our second laser. The temp. controller looks pretty impressive; it keeps the temperature stable to <.002 degrees C! Our current controller only keeps it stable to .1 degrees C. However, these items come out to about $2400, so we also need to start discussing a budget with Hal and Dr. Noe. Gotta work hard next week for a good signal and better pictures.

June 24, 2004

Today was busy. During break from my writing class, I met a new high school student named Yaagnik Kosuri who is interested in BECs. After class I went on a field trip with Dr. Noe to the Linac supply room, and found all sorts of interesting machine parts. We went there to return the Linac Resonator Controller we took on Monday, after I dissected the last power supply out of it. Dr. Noe suggested that we build a panel in the electronics shelf to keep all the electronics hidden (since we have so many boxes and switches now), so we looked at some old ones that were used in the Linac experiment 20 years ago. We also looked at some power supplies online and decided to order a 6 amp supply instead of the 3 amp one sufficient for the temperature regulator, in order to power other things in the setup. I also put our makeshift power supply and cooler back in the circuit to see if we still get fluorescence and a Sat Spec signal, and indeed we do; everything is back to normal. When James got here, we designed an electronics shelf with two panels and made a list of all the devices we will need for the MOT, which will all be eventually hardwired to the power supply. Among the electronics we hope to power with our new supply are: a second cooler, a second current controller, two voltage amplifiers, Helmholtz coils, and the summing amplifier (do we have a lock-in amplifier?). Since most of the electronics operate on only milliamps of current (for the Op Amps), and the controllers are less than 3 amps, I don't think powering all these things with one supply would be a problem. We also printed out some device specs in order to get an idea of how much current each one draws, so that they don't use up more current than the power supply outputs. We might also be able to build our own custom-made panels in the machine shop so that all our bulky electronics fit, since the ones that are commercially available are inconvenient sizes. James and I designed a fancy panel with a built-in ohmmeter...till 2:30 in the morning!

June 23, 2004

Today was a good day. There's a good chance of me getting into machine shop in August, so I won't have to wait until school starts. I'm also learning more and more about the components of this experiment. Today's lesson was on PID (Proportional, Integral, Derivative) Controllers, used in feedback loops; the temperature transducer we use in our cooler is the AD590. The basic principle is to keep the desired parameter at a constant, by comparing the measured value to a set-point value. The difference between these two values is called the error. The proportional band mode causes the controller output to be proportional (but still not equal...there is an offset) to any change in measurement or set point, and optimizes the response time of the feedback loop. "Integral action" eliminates the offset by giving the controller a large gain at low frequencies (like a low-pass RC filter...an integrator), and causes the controller output to be proportional to the amount of time that the error is present. The consequence is a phase lag, which is turn is eliminated by "derivative action." This provides a phase lead of the same amount, thus stabilizing the loop and optimizing the dynamic response of the feedback loop in order to account for rapid changes in the thermal load. It is also used to avoid overshoot, by slowing the controller output down when it has almost reached the set point.

I noticed overshooting today while taking voltage and current readings of the power supply, and then an oscillatory damping back to the set point equilibrium. I wanted to use the RS Meter program to record voltages and currents automatically but the RS Meter is only designed to work with one COM port. Since I could only record one variable at a time so it wasn't worth it, although it would have been easier to show the overshoot. Oh well, the computer is too far away from me anyway. The stabilizer is working really well; whenever there were small fluctuations in voltage, the current's fluctuations were inversely proportional, and the temperature was stable to a tenth of a degree. Finally, we can continue on with Sat Spec, although I do think I've developed a newfound appreciation for electronics. Tomorrow: sodder the yellow 5V wires back together, and work on getting a nice Doppler-free subtracted signal.