UChicago Instructional Physics Laboratories

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.
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.

Refer to Figure 2 below on the Day 1 Tasks page 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.

Refer to Figure 3 below on the Day 2 Tasks page while performing the following steps.

1. After recording your calibration spectra, block the beam entering the Interferometer half of your group board so that unwanted reflections do not make it into the spectroscopy optics.
2. Put the wedge beamsplitter back in its post holder and realign the pump beam to pass through the vaporcell and into photodetector III. You can refer back to the Day 1 instructions if needed.
3. Once you have the Probe Beam reestablished, carefully place the heated vaporcell with Helmholtz coils on your group board so that the Probe Beam passes cleanly through the vaporcell.
   1. Note that this vaporcell is recessed within the insulation and coils so that it is difficult to see. But it is easy to simply slide it back and forth until the Probe Beam clearly makes it through.

Refer to Figure 4 below on the Day 3 Tasks page while performing the following steps.

1. Put the wedge beamsplitter back in its post holder and realign the pump beam to pass through the vaporcell and into photodetector III. You can refer back to the Day 1 instructions if needed.
2. Once you have the probe beam reestablished, place the 50/50 beamsplitter at far edge of your table so that the probe beam passes through it.
   1. The 50/50 beamsplitter should be angled as shown in the figure.
   2. Make sure that the Probe Beam still goes into the photodetector and is not blocked by the beamsplitter holder.
3. Remove the vaporcell for the moment.
4. Place Iris 3 near the 50/50 beamsplitter as shown in the figure so that the Probe Beam passes through its center.
   1. To have enough clearance make sure that Iris 3 is ~1.5“ away from 50/50 beamsplitter.
5. Place Iris 4 close to the wedge beamsplitter. as shown in the figure, so that the Probe Beam passed through its center.
   1. To have enough clearance make sure that Iris 4 is ~1.5” from the wedge beamsplitter.
6. Place M4 as shown in the figure, and adjust it so that the beam which passes through the wedge beamsplitter hits the 50/50 beamsplitter. This beam will be called the Pump Beam.
7. Walk the Pump Beam util it passes through both irises in the opposite direction as the Probe beam.
8. Put the vaporcell back in place between Iris's 3 and 4 so that the Probe Beam passes cleanly through it.
9. Connect photodetector III to the scope, adjust the scope to see one full sweep of the laser frequency. You should now see the same doppler broadened spectrum as before, but now each of the four absorption peaks should show up to 6 doppler free dips.
10. At this point you will need to add neutral density filters and adjust the overlap between Pump and Probe beams to maximize the strength of the doppler free features and see all 6 of them.