Table of Contents

Day 1 Pt. 1 - Build the beam path to observe the signal

Before you begin an instructor will provide an overview of the lasers and optical components you will be working with. After receiving this overview you can begin working on the exercise. The following tips and suggestions will help.

  • Use the low power alignment laser to do all of the initial alignment. The optical table is setup so that when you later switch to the infrared laser, its beam will follow the same path as that of the alignment laser.
  • Be careful handling the optical components. Do not touch the surfaces of the mirrors and beam splitters. Handle them by their mounts.

Setup the photodetectors and vapor cell

The goal is to send a pair of slightly diverging beams from the laser through a Rb vaporcell and into a pair of photodetectors. The outputs from the detectors will be observed on the scope while you tune the IR laser onto resonance with the Rb D2D2 stands for the second doublet transition. As with sodium, these lines are due to a single unpaired electron transitioning between S and P orbitals. D1 is for transitions to the P${}_{1/2}$ states, D2 is for transitions to the P${}_{3/2}$ states transitions.

Building The Interferometer

Refer to Figure 1 below while performing the following steps.

  1. Align the alignment laser to pass through both irises attached to the main board.
  2. Place your group board on the main board.
    1. You will notice that there are 4 through holes in your group table, one near each corner, which will line up with the tapped holes in the main table.
    2. Place your group table to the main table in the location shown above. Make sure that the beam path will pass over your group table.
    3. Attach your group table to the main table using the hex head bolts and allen wrench provided. Note that while you can secure all 4 corners to the main table, using 2 will suffice. Make a note of which holes on the main table you used so that you can return your setup to the same location on the main table on days 2 and 3 in the lab.
  3. Set heights of your two iris's to match the height of the alignment beam above your group board.
  4. Visually check level of beamsplitter and adjust it so that the top plate is not tilted relative to the bottom plate. This can be done by eye, using your judgement as to when the top and bottom plates are as parallel as you can get them.
  5. Place beamsplitter as shown in Fig 1 so that the incident alignment beam is reflected in the direction of M2 on on Fig 1.
    1. Check that beam does not hit top, bottom, or sides of the beamsplitter.
  6. Place Mirror M1 as shown.
    1. Check that the beam hits near center of mirror and does not clip the edge.
  7. Place Iris 1 between the beamsplitter and M1 as shown, so that the beam passes through the center of the iris.
    1. Attach the post holder to the table, do not leave it sitting unattached.
  8. The reflected beam from M1 should now be passing through the beamsplitter again, traveling from right to left, with some of the beam being reflected towards the photodetectors. Place Iris 2 on your group table as shown and adjust the beamsplitter so that the reflected beam from M1 goes through it.
  9. Position Photodetector I so that the reflected beam from M1 hits the active area of the photodiode.
    1. Note that the cap with a hole in the center can be helpful here. If the beam goes through the hole in the center of the cap, and the photodetector is pointed back at the beamsplitter, you can be confident that the beam is hitting the active area of the detector.
  10. Place M2 on your group table as shown, and adjust it do that the its reflected beam also passes through the center of Iris 2.
    1. There are internal reflections from within the beamsplitter which will also appear near Iris 2. Using a piece of paper or thin cardboard to block the reflected beams from M1 and M2 will help with ensuring you are aligning the correct beams, and not reflections.
  11. Switch to IR laser.
  12. Trace IR beam front to back, making sure it hits all of the optical components correctly.
    1. It is unlikely that the IR beam will follow the exact same path as the alignment beam. You will almost certainly need to realign some of the optics at least a little bit, but they should all be close.
  13. Connect photodiode I to the scope and trigger on the sync output of the electronics. Adjust the scope settings to show approximately one full frequency sweep of the laser. If you have performed the above steps carefully you should observe a small sinusoidal interference pattern on the photodetector signal.
    1. You may have to add some neutral density filters to the beam path to avoid saturating the photodetector.
    2. If you see what you suspect is an interference pattern, do a simple sanity check to ensure that the signal you see is in fact due to the combined beams from the interferometer. Other things which can create signals that look like an interference pattern include:
      1. Having the incandescent lights on in the room. Note that the fluorescent lights will not cause this effect, only the incandescents.
      2. Having unwanted reflections entering the photodetector.
      3. Having the beams only partially hitting the active area of the photodetector, resulting in a noisy signal that has to be zoomed in on a lot.
    3. Once you are convinced that you are able to see at least a hint of an interference pattern, make very small adjustments to M1 and M2 to try and maximize the amplitude of the interference by improving the overlap of the beams:
      1. Start by making small adjustments to both axes of M2. Note that your finger pressure on the mirror adjustment screws will be enough to alter the path length of the interferometer. So you need to make an adjustment then let go of the screw to see the effect. With a bit of practice this is not as difficult as it sounds.
    4. Once you have made the interference signal as large as you can by adjusting M2, repeat the same procedure on M1.
    5. Alternate back and forth between M2 and M1, making small adjustments to each until you are no longer able to make the interference pattern larger in amplitude.

IMPORTANT TIP

When doing beam alignment it is very helpful to ensure that all of the beams are the same height above the optical table. Doing so allows you to focus your attention on getting the beams to go where they need to be in only two dimensions as opposed to three.

This is easily accomplished by using an iris (you have one on your group optical table) as a height gauge. Before you start placing new optical components on your group board, set the height of your iris, in its pose holder, to the height of the main laser beam. Simply insert the iris in the beam path of the low power alignment laser and adjust its height so that the beam passes through its center.

From now on, whenever you put a new optical component on your group table, place your height gauge on the table where you want the beam off of the new optical component to go, note you do not need to attach the height gauge to the table for this purpose. Adjust the new optical component so that its beam goes through the center of the iris. Now you only have to worry about aligning the beam in two dimensions.

If you use your height gauge like this, for each optical component you put on the table, you will save yourself a lot of frustration as your optical layout progresses.

Note that the 1° wedge beamsplitter is designed to produce a pair of reflected beams with a 1° opening angle between them. This makes it possible to pass two beams through the same vaporcell and into two different photodetectors. For the purposes of this lab we only need one of the two beams from the 1° wedge beamsplitter passing through the vaporcell and into a photodetector. The other beam will not be used.

Perform the initial alignment using the alignment laser. Once you have the wedge beamsplitter, vaporcell and photodetectors in position and aligned using the alignment laser, have an instructor inspect it and provide instructions on how to use the IR laser.

Once you have switched to the IR laser, use the IR detection cards and the video monitor to verify that the IR beams are correctly passing through the vaporcell and into the photodetectors.

Day 1 Pt. 2 - Rb Spectrum Setup

Refer to the figure below while performing the following steps.

  1. Turn off the IR laser and switch back to the alignment beam.
  2. Block input to interferometer using a piece of cardboard. This prevents unwanted reflections from making their way into the optics you are aligning.
  3. Position wedge beamsplitter to reflect a portion of the alignment beam toward photodetector 3 as shown in the figure. We will refer to this beam as the Probe Beam from now on.
    1. Check that transmitted portion of the beam passes cleanly past mirror mount with out hitting it.
  4. Position Iris 3 as shown in the figure. Note that you will have to reuse one of the irises from the interferometer part of the setup. We suggest you remove Iris 2 from its post holder, leaving the post holder in place on your group table. Use a new post holder to position this iris as Iris 3.
    1. Note that Iris 3 in this new post holder needs to be the correct height above your group table. You can verify the height is correct by centering it on the alignment beam in front of the wedge beamsplitter.
  5. Adjust the wedge beamsplitter to direct one of the reflected beams through the center of Iris 3.
    1. The other beam from the wedge beamsplitter will not be used.
  6. Position photodetector III so that the Probe Beam hits the active area of the photodiode.
    1. Note that the cap with a hole in the center can be helpful here. If the beam goes through the hole in the center of the cap, and the photodetector is pointed back at the beamsplitter, you can be confident that the beam is hitting the active area of the detector.
  7. Place the standalone vaporcell into the path of the Probe Beam as shown in the figure.
  8. Switch to IR laser.
  9. Trace the IR beam, front to back, making sure it hits all of the optical components correctly.
    1. It is unlikely that the IR beam will follow the exact same path as the alignment beam. You will almost certainly need to realign some of the optics at least a little bit, but they should all be close.
  10. Connect the output of photodiode III to the scope and set it to display one full cycle of the frequency sweep of the laser.
  11. Use the CCD camera to make sure that the IR laser is tuned onto resonance by looking for the flashing of the beam as it passes through the vaporcell.
    1. You may have to add some neutral density filters to the beam path to avoid saturating the photodetector.
  12. At this point you should be able to observe the doppler broadened spectrum of Rb on the scope. Note that the optical alignment to see this spectrum is no where near as challenging to setup. Basically you just need to get one of the reflected beams from the wedge beamsplitter to pass through the vaporcell and into the photodetector.

Day 1 Pt. 3 - Tune the laser onto resonance

An instructor will assist you with this task.

Tuning the IR laser onto resonance with the Rb D2 transition is not difficult once you have seen it done, but it would be very difficult to explain in the Wiki.

The beam from the IR laser is hazardous and capable of causing permanent eye damage. Do Not Turn It On until you have been shown how to operate it by an instructor.

It is required that all people in the room wear appropriate eye protection goggles any time that the IR laser is on. Additionally the door to the lab should be closed.

Once you have been shown how to operate the laser and photodetectors, fine tune the laser diode current to obtain a clear and smooth spectrum which shows the absorption peaks for all four of the expected Rb transitions.

Day 1 Pt. 4 - Record a calibration spectrum

Once you have aligned the interferometer and obtained a clean interference pattern on the scope, use the cursor feature of the scope to measure the locations of the interference maxima (including an estimate on the uncertainty of these measured values) and record them in your lab notebook. You should also transfer a screenshot and the digitized data to the lab computer for inclusion in your out of lab assignment.

You will be expected to be able to articulate clearly and concisely how you performed this measurement, and how you estimated the uncertainties in the measured values.

Do not forget to measure and record the dimensions of the interferometer.

If you have time, we strongly recommend that you calculate the time difference to frequency difference conversion factor in the lab, and check with an instructor that the value you get is reasonable.

Day 1 Pt. 5 - Record the db absorption spectrum

Make sure the IR laser is tuned onto resonance with the Rb transitions, then save a screen shot of this spectrum and its digitized waveform to the lab computer.

Use the USB connection between the scope and the lab computer to transfer a screenshot of the scope trace and/or the digitized data. You will need this for your day 1 out of lab assignment, so have an instructor verify that your spectrum is good.

How do I save an image from the scope?

Your computers are already set up so that you can copy a screenshot or data from them by using your lab computer.

  • Open up the Open Choice Desktop program from the desktop
  • Press the 'Select Instrument“ button and select whatever starts with USB
  • Use the “Get Screen” button to capture a screenshot
    • Use the “Waveform Data Capture” if you want to pull numerical information from the scope

If this doesn't work for some reason, you can plug a usb drive into the front of the scope and press the save button (located just beside the multipurpose knob).

Day 1 Pt. 6 - Measure the FWHM of the doppler broadened (db) peak for 1 isotope

Choose one of the db absorption peaks for measuring the FWHM. You will compare your measured FWHM to the theoretical prediction at room temperature.

On the scope, zoom in on your chosen peak and use the cursor features to measure its FWHM and estimate the uncertainty in the measurement.

Note that there is some subjectivity in how you make this measurement. You will need to use your judgement to decide how to pick and measure the points which define the FWHM of the peak. You will also have to decide how to estimate the uncertainty in your measurements. We are not looking for a “correct” or “best” method. Part of the exercise is developing your ability to make scientifically plausible decisions regarding what and how to make measurements. To guide you in this process, consider the following points.

  • Given the definition of FWHM and the shape of the peak you are measuring, how can you reasonable define where to make your measurements. Note that you will need to clearly articulate both how you made this measurement and why you chose to measure these points.
  • You you should be using the scope cursor feature to make these measurements. Given the process you choose to use, what is the largest limitation on how well you are able to make the measurement? Assume that the scope is properly calibrated to give values which are accurate to the displayed number of sig figs. How much might you be off in locating the position of the cursors? What is the resolution limit of the display? How repeatable is your measurement?

Day 1 Pt. 7 - Measure the frequency difference between the two doppler broadened (db) peaks for each isotope and compare with their known values.

In your day 1 analysis you are asked to measure the energy difference between the two db absorption peaks for each isotope. This amounts to:

  • Identifying the db peaks associated with the each of the isotopes as shown here.
  • Measuring the time difference between the two peaks associated with each isotope using the scope cursor feature.
  • Converting these scope time differences into frequency differences of the laser light using the calibration factor you will determine in the next part todays work.

Day 1 assignment (20 points)

  • Present your calibration of the laser frequency sweep.(5 points)
    • Include a publication quality plot of the interference pattern used to calibrate the frequency sweep of the IR laser.
    • Show the full calculation of your calibration factor, including uncertainties.
    • Include all measured values with their associated uncertainties.
    • Briefly describe your procedure for obtaining your measured values. Your description should be clear and detailed enough that someone familiar with the apparatus could make these measurements the same way that you did in the lab.
  • Present your Doppler Broadened spectrum. (5 points)
    • Include a publication quality screen shot or plot of the doppler broadened spectrum that you used for measurements of the line width and energy differences.
    • The figure should be annotated to identify the individual doppler broadened transitions. Include annotations to help the reader understand what features you measured for the db line width and the peak separation.
  • Present your measurement of the doppler broadened line width for one peak.(5 points)
    • Include all measured values with associated uncertainties.
    • Briefly describe your procedure for obtaining your measured values and their uncertainties. Your annotated figure of the spectrum should be helpful here for making it clear what feature(s) you measured.
    • Show the full calculations of your measured DB line width and the theoretical prediction at room temperature.
    • Compare your measured and theoretical values and make a statement on the degree of agreement between the two in the context of demonstrating that the observed line width is plausibly consistent with being due to the effect of thermal doppler broadening.
  • Present your measurement of the energy differences.(5 points)
    • Include all measured values with associated uncertainties.
    • Briefly describe your procedure for obtaining your measured values and their uncertainties. Your annotated figure of the spectrum should be helpful here for making it clear what feature(s) you measured.
    • Show the full calculations of your measured energy difference.
    • Compare your measured and theoretical values and make a statement on the degree of agreement between the two.