July 26, 2004
I really hate mode hops. They keep ruining my beautiful signal. I found a cryptic note I wrote to myself in my lab book saying to read Chap 5 in Griffiths to understand mode hops...didn't find anything specifically relevant but there is a little bit on band structure that I should know in order to understand the solid state physics of a diode laser...will get on that soon. I also need to read the thesis by Mary Stone on the stabilized external cavity diode laser.
July 22, 2004
I keep losing fluorescence now, and for some reason I can only obtain a good fluorescence near a frequency that is right between modes, so I get mode hops...this means I need to start tinkering with the settings again, possibly find a new temperature to work with. In the process I discovered a broken bnc cable...those things'll get ya everytime, in fact it was the same cable that was making me think that the new power supply wasn't working yesterday when I first tested it, though it was. Well I just taped that hooligan right up and put it away for good.
July 21, 2004
Last day of machine shop!! Had a double session, from 9:30-12:30 and 2-3:30. I finished my plumb bob and can't get over how professionally done it looks. I love that machine shop...After playing low budget virtual-reality games outside the SAC I also worked on the new power supply and soddered the +5V, +15V, and -15V wires to some lugs on it. I am always wary when it comes to soddering, ever since Dr. Lathrop at Maryland last summer told me to always wash my hands after handling lead otherwise my babies will come out defective (well not in those words).
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)
-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?
-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?
-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.
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.
June 22, 2004
Didn't go to lab today, I had a doctor's appointment in the infirmary for a checkup, which took up my whole morning, and then class in the afternoon, and then I continued reading journal articles on diode lasers. Will make up for lab work tomorrow.
June 21, 2004
Today was by far the most fulfilling day of Sat Spec. The temperature regulator works! When I asked Dr. Noe a few days ago what we would do about the system, he said to "learn electronics and fix it." So that's what James and I set out to do this morning. We took the whole thing apart, learned what all the components were, and troubleshooted by taking voltage measurements at various points. None of the resistors or capacitors seemed to be burned out, and we measured 0, 5, and 15 Volts from the power supply, so we deduced that there must be something wrong with the op amps. Art, from the electronics shop was able to lend us a couple of OP27's, but we still needed the other components. After speaking to Radio Shack representatives who didn't know what an Op Amp was, and searching the physics building far and wide (Frank Chin's office) for electronic devices, we finally found a place that that had the OP177GP: Active Electronics, which had incidentally been renamed "A&P" (this set us back slightly). However, just as James and I were about to leave to buy this long-coveted operational amplifier, Dr. Noe reminded us that Op Amps are powered by both positive and negative voltages...meaning that the power supply, which gave 0 volts instead of -15, was in fact not working correctly. Well this was when the fun began; now the search was for a three-output power supply (+15 and -15 V powers the circuit, while the 5 volts supplies the Peltier cooler). So Dr. Noe brought in a junked Resonator Controller from the Linac Accelerator and James and I dissected it and took out the ?15V supply. After testing, we discovered that the +15V on our original supply only worked independent of the cooler, but once the load was connected the voltage output dropped to +6V. So we stole another power supply from the optical tweezers setup (that poor experiment?we already took its camera lens) since it isn?t currently in use. Now we have two external parts connected to our original power supply, which only outputs 5V. The next step is to measure how much current the cooler gets in so that we can order another power supply for our electronics. The new power supply will also be used to power our forthcoming duplicate set of electronics for the second diode laser.
June 18, 2004
Spent the morning reading some papers about Sat Spec and Diode Lasers, stuff I did not have time to get to last semester. I am working on a summary of everything I learned during the course of this research project (including many, many questions) and will put it all up soon. At noon Dr. Noe took the LTC group out to lunch at Ichi's, a Japanese restaurant nearby. Since James was driving, we almost ended up eating at the Pet store =P. After that, did some more reading. going to see the band Phish tonight!
June 17, 2004
We observed lamb dips today! Sat Spec seems to be finally working, except we keep losing our signal. This would be so much easier if we could just keep the diode at a stable (and cooler) temperature, but the temperature regulator isn't working, and we still need to find the schematics but don't think we even have them anywhere. We will next test the Peilter cooler in the diode laser to see if it is actually getting any power. We decided not to use the double photodiodes to subtract the reference beam from the probe beam, since we can just use the math feature on our digital oscilloscope (the awesome one with different colored signals) to subtract the reference beam from the probe beam with normal photodiodes. This way, we can also get rid of the signal booster for the double diode since the single photodiodes have a built-in booster. We shall continue investigating the temperature problem tomorrow. If nothing works, we might just resort to blowing on the laser to keep it cool, as per Mr. Lee's suggestion. Mr. Lee is the man.
June 16, 2004
Continued working on Sat Spec today. We've had a nice Doppler Broadened signal for a while (pics to come up soon) but as of last night it got a little messy when the temperature of the diode increased (25 degrees C rather than our usual 17 degrees C) and we had to then readjust all our other parameters. Fluorescence now occurs at different currents as well: around 90 and 117 mA. This worries me because the cooler doesn't seem to be cooling, and turning the voltage knob on the cooler did not change the voltage output on a voltmeter. We will probably have to take it down to the electronics shop soon. I'm going to spend this week reading papers and trying to clean up our signal. Matt suggested that we change our Sat Spec setup to the more conventional one (the one he used in grad lab) to get a better signal, so we might resort to that. I need to figure out what that third knob does!
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