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.
Attach your group table to the main table
Place your group table on the main table as shown in the above diagram. 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.
- 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.
- 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.
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 D1 transitions.
Figure 1.1
Use the 1º 9:1 wedge beam splitter to direct the beam from the laser through the vaporcell and into the two photodetectors as shown in the figure above. The 1º 9:1 wedge beam splitter is labeled with orange tape, as shown in figure 1.2, and should be sitting in the Photodetector Area of the main table when you arrive to lab. If you cannot find it ask an instructor for assistance. Since you will have to return the wedge beamsplitter to the main table at the end of the day, you should use one of the empty post holders supplied. This post holder can be left on your group table while the wedge beamsplitter is returned to the main table.
For todays tasks you will be working with a stand alone vaporcell of natural Rb. However, the next two days in lab will make use of a different vaporcell and Helmholtz coil assembly which is physically much larger than the stand alone vaporcell. The optical path you will build today will be used for the next two days of the lab. We provide a template of the boundary of the vaporcell and Helmholtz coil assembly which you can use to make sure that the components you place today will not be blocked when you switch vaporcells.
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.
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 - Tune the laser onto resonance
An instructor will assist you with this task.
Tuning the IR laser onto resonance with the Rb D1 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. 3 - Build the interferometer
You now need to build a Michelson Interferometer in order to calibrate the frequency sweep of the IR laser. Why we need to perform this calibration should have been covered when you were shown how to operate the laser. To recap…
The IR laser outputs light with a specific frequency, and sweeps that frequency over a small range. When you “tune” the laser onto resonance what you are doing is adjusting the operating parameters of the laser so that the frequency of light associated with the energy of the Rb transitions falls within the sweep range of the lasers output.
What you observe on the scope is the output of the photodetector as the frequency of the laser light “sweeps” back and forth through the Rb transitions. The peaks you see in the output of the photodetector occur when the frequency of the IR laser light matches one of these transition energies. If we know the rate at which the frequency of the IR laser is changing during a sweep, we can measure how far apart those peaks are in time on the scope, and then convert that time difference into a frequency difference. Since the frequency of light is directly related to its energy, if you know the frequency difference between two peaks you also know their energy difference.
We can measure the rate at which the frequency of the IR laser is changing using an interferometer. The details of how the interferometer works can be found here.
It is crucial that you understand the logic and the physics associated with the need for this calibration. This is a very common point of confusion for students, so make sure that you fully understand this part before you leave lab today.
Using your set of optics, place mirrors M1, M2, M3, M4, a beam splitter and a third photodetector to create the beam path shown in figure 1.3.
Figure 1.3
IMPORTANT TIP
There are a lot of ways to get two beams to overlap at a point. But for an interferometer to work it is also necessary for the two beams to overlap along their entire path from the beam splitter to the photodetector. This can be challenging. This procedure will make the task considerably easier.
- Place the beam splitter on the table and use the iris to set the height of its reflected beam.
- Place M3 on the table with your height gauge halfway between it and the beam splitter such that the beam traveling towards the mirror goes through the center of the iris.
- Adjust M3 so that its reflected beam travels back through the iris on its way back to the beam splitter.
- When the return beam from M3 reaches the beam splitter, part of it will be reflected towards the photodetectors on the far side of the main table. Place photodetector #3 so that this beam hits the active area of the sensor.
- Now you can remove the height gauge from your group table.
- Place M4 close to the beam splitter and align it so that its reflected beam overlaps the reflected beam from M3 at the photodetector.
If you follow this procedure carefully and systematically, your two beams from the interferometer will be co-aligned enough that when you switch to the IR laser you will be able to see at least some degree of interference, enough to begin optimizing by making very small adjustments to M3 and M4.
Aligning an interferometer to the degree of precision needed to see fringes is the most challenging alignment task you will need to perform for this lab. It is not trivial and can be frustrating. This step must be completed before you leave lab on day 1 in order to do your out of lab assignment. After completing all of the other day 1 tasks you should have at least 2 hours for this part. Keep an eye on the clock, if you are not at the point of taking data by the last half hour of the period, find an instructor to assist you. It is suggested that you get some advice and pointers from an instructor before starting to build the interferometer.
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 Desktopprogram 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 1 isotope
In your day 1 analysis you are asked to measure the energy difference between the two db absorption peaks for one isotope. This amounts to:
- Identifying the two db peaks associated with the hyperfine transitions from the excited state to each of the hyperfine ground states for one isotope.
- Measuring the time difference between these two peaks using the scope cursor feature.
- Converting this time difference into the frequency difference of the laser light using the calibration factor you will determine in the next part todays work.
- Converting the frequency difference of the laser light into an energy difference.
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 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.