Table of Contents
Hyperfine Spectroscopy of Rubidium - Winter 2026
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| An oscilloscope trace, showing the raw spectrum (blue), the Doppler-free spectrum (yellow), their difference (red), and an interferometer showing the laser's frequency shift (pink) |
In this experiment you will use a narrow bandwidth, tunable diode laser to probe the hyperfine structure of natural rubidium (Rb). The technique of Doppler-free saturated absorption spectroscopy will be used to resolve the hyperfine structure which is otherwise masked by the effect of thermal Doppler broadening of the spectral lines.
From creating Bose-Einstein condensates to quantum encryption, the techniques and optical components used to precisely control and align laser beams are a common element of current physics research. This lab aims to provide you with some hands-on experience working with lasers and optics, and is the only lab where you have to design, build and run an experiment yourself. The experimental goal is to measure the magnitude of the energy splitting due to the hyperfine effect in atomic rubidium using a technique known as Doppler Free Saturated Absorption Spectroscopy (DFSAS).
The teaching goals are as follows:
- Learn about laser beam conditioning and alignment.
- Understand the function of (and how to use) common optical components such as mirror mounts, irises, polarizers, and polarizing and non-polarizing beam splitters.
- Learn how to safely align laser beams.
- Learn how to design an optical layout to build a DFSAS experiment.
- Measure the energy of the hyperfine splitting in atomic rubidium.
- Understand atomic energy level structure and the hyperfine interaction.
- Understand the effects of Doppler broadening.
- Understand how the DFSAS technique works.
Overview
Laser Safety
The IR laser operates at wavelengths near 780 nm and the power output can be as high as 120 mW. This power level makes it a Class 3B laser, which means the laser can cause permanent damage to the eye, but will not cause damage to skin or set objects on fire. In order to protect yourself, appropriately-rated eye goggles must be worn whenever the laser is on, and the laser must only be operated with the door to the room closed. (For this laser, “appropriately-rated” means that the goggle offers at least OD 3 protection at 780 nm.)
The mostly likely time for a laser accident is during alignment; when placing and adjusting components, laser beam paths can go in unexpected places, including being reflected upward out of the plane of the table and toward eye level. Therefore, as an added level of protection, you will build and align your optics using a separate low power alignment light laser (which has a power of less than 5 mW and is roughly equivalent to the type of laser pointer used in presentations.) Nevertheless, you should still practice proper laser safety with the low power laser and avoid reflections and stray beams.
Theory and apparatus
The following are links to our wiki pages discussing some of the concepts that you need to be familiar with. Search the internet for additional information if you come across concepts or terms you are not familiar with.
- A description of the Hyperfine interaction as it applies to atomic rubidium: Rb Hyperfine
- A description of thermal doppler broadening of atomic emission/absorption lines for atoms in a gas: Thermal Doppler Broadening
- A description of the technique of Doppler Free Saturated Absorption Spectroscopy: Doppler Free Saturated Absorption Spectroscopy
- A description of the optical components available for your use in the lab: Description of Optical Components
- A description of how an interferometer works: interferometer-details
There is one IR laser and three photodetectors available for each room. As such, you will need to share these components with other groups (who will be working on their own version of the experiment on days when you are not in lab). The laser will stay put on the main optical table (and should not be adjusted), but the rest of your optical setup will be built on a small 300 mm x 600 mm optical bread board which can be attached to the main optical table while you are working on it (and subsequently removed and stored at the end of the lab period). When you return to the lab, you simply reattach your optical setup to the main table and continue your work. (Do not take components off of other groups' boards.)
1. Prelab exercises (5 points)
This exercise must be completed before coming to lab.
First, do a little research into the following questions. Most of what you need to know can be found in the lab wiki and Wikipedia.
Make sure you are familiar with the following concepts:
- absorption spectroscopy;
- atomic emission and absorption spectrum;
- spectral linewidth; and
- thermal Doppler broadening.
Use this information to do the following:
- Calculate the doppler broadened line width for an atomic emission line whose wavelength is 780.0 nm. Assume the gas is in thermal equilibrium at room temperature T = 300 K. Express the Doppler broadened linewidth in units of frequency (Hz).
- Sketch a plot which illustrates how broad this line is on a scale of 2000 MHz. Your y-axis should represent the intensity of the emission (in arbitrary units) and the x-axis should represent frequency (in units of MHz).
- Note that this is a back of the envelope sort of exercise to give a qualitative sense of how wide or narrow the line is on a scale of 2000 MHz. Don't worry about the exact functional form that describes the shape of the line or the values of the intensity. Just use the definition of full width half maximum. Your plot can be hand drawn, or you could assume a Gaussian form for the line shape and use a program like Mathmatica or Python to generate the plot. What matters is that you get the overall qualitative features correct.
- Sketch a plot of what the emission spectrum would look like for an element which had two such lines whose energy difference is equivalent to 2000 MHz. Assume each of these emission lines has the same linewidth you calculated above.
- From looking at your sketch for #3, answer the following question: Would you expect to be able to resolve both lines as separate features in an emission spectrum?
- Sketch a plot of an emission spectrum containing 4 lines where:
- The second line is 75 MHz higher in energy than the first line.
- The third line is 150 MHz higher in energy than the second line.
- The fourth line is 250 MHz higher in energy than the third line.
- From this sketch, answer the following question: Would you expect to be able to resolve all four lines as separate features in an emission spectrum?
The point of this prelab question is to illustrate what is meant when we say that the spectral features we wish to measure are unresolvable due to the effect of thermal doppler broadening. This concept is central to why we need to use DFSAS to measure these features as opposed to ordinary absorption spectroscopy.
In-Lab Exercises
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| A beamsplitter illuminated with a Helium-Neon (HeNe) laser |
For this experiment you will position and align your own optics in order to measure the hyperfine splitting in atomic Rb. The lab is structured so that you will build up to the DFSAS optical setup in pieces, each day in lab building on the work of the previous day. At the end of the day you should leave the optics you installed on your personal optical table for use the next day.
There is one IR laser and three photodetectors available for each room. As such, you will need to share these components with other groups (who will be working on their own version of the experiment on days when you are not in lab). The laser will stay put on the main optical table (and should not be adjusted), but the rest of your optical setup will be built on a small 300 mm x 600 mm optical bread board which can be attached to the main optical table while you are working on it (and subsequently removed and stored at the end of the lab period). When you return to the lab, you simply reattach your optical setup to the main table and continue your work. (Do not take components off of other groups' boards.)
The figure above shows the layout of the optical table you will be working with for this lab.
There is a large optics table (main table) with two lasers, a pair of mirrors and a pair of irises already mounted to it. With the exception of the area labeled Photodetector Area, you should not move or add optical components to the main table. The mirrors and lasers have been prealigned to direct the beam from either the IR Laser or the Alignment Laser through the center of the two irises.
The Photodetector Area is where you will place the photodetectors for making measurements.
Your group has its own smaller optical table (group table) and a set of optical components which an instructor will point out during your initial introduction to the lab. Your optical table can be placed and bolted on top of the main table. This is where you will build your own optical setup for the various parts of the lab. The two irises define a precise beam path across your group table that you will use as a starting point for building your own optical setups.
Day 1 - Doppler Broadened Spectrum
Your objectives for Day 1 in the lab are the following.
- Build an optical path to send some of the laser light through a Rb vapor cell and into a pair of photodetectors.
- Learn how to operate the IR laser.
- Build an interferometer for calibrating the frequency sweep of the IR laser.
- Collect data necessary to calibrate the frequency sweep of the IR laser.
- Collect data to show the full absorption spectrum for the Rb.
- Collect data and measure the doppler broadened (db) line width of one absorption peak.
The rest of the instructions for Day 1 are found here.
Day 2 - Thermal doppler broadening vs temperature
Your objectives for today are the following.
- Restore optical paths from Day 1.
- Collect a calibration spectra.
- Measure the FWHM of the db absorption peak for one transition as a function of temperature.
When you first turn on the Laser Diode Controller you should change the temperature set point to 25°C following the the procedure found here. Doing this at the start of the lab period will let the vapor cell cool down (takes ~45min) while you are getting setup for the measurement.
The rest of the instructions for Day 2 are found here.
Day 3 - DFSAS Measurement
You are now ready to move on to building the full Doppler Free Saturated Absorption Spectroscopy setup. Your objectives for today are the following.
- Restore your day 2 optical setup.
- Record a calibration spectrum.
- Add the pump beam to your setup.
- Optimized pump and probe overlap and power ratio to obtain and record DFSAS spectra.
Before doing so however you need to familiarize yourself with the physics involved in both the phenomena you are studying, and the technique of DFSAS. While none of the physics involved is more advanced than PHYS235 level quantum mechanics, there are a number of processes at work which can take some thought to sort out to the point where you truly understand what you are trying to accomplish with the optics and laser beam.
We highly recommend that you review the material in these two links PRIOR to coming to lab on day 2 of the experiment. By the end of day 2 you are likely to be ready to begin on this check point exercise. Regardless of whether or not you have read the material, you should go over it with an instructor before proceeding.
Doppler Free Saturated Absorption Spectroscopy
The rest of the instructions for Day 3 can be found here.
Notes on experimental procedure
Main optical table
The above photo shows the main optical table on which you will be working; it is shared by all groups working on the experiment.
Mounted on this table are the main 780 nm laser which will be used to perform the spectroscopy experiment. Two mirrors are used to direct the beam from this laser through a pair of irises. Your group's smaller optical table will bolt to the main optical table in the location shown, and then you can place and align all of your components on your table which can then be removed from the main table and stored in between lab sessions. The two optical tables are made with sufficient precision that you can easily return your table to the main table and all of your previous optical alignment will still be intact. The two irises are there to ensure that the laser beam always passes over the same path across the main table.
There is also a low power HeNe laser which will you can use to align your optics. You do not want to build your optical setup using the main laser for the following two reasons:
- The beam from the main laser is hazardous and can cause serious eye injury. Most accidents with laser beams occur during alignment work when you are moving optical around on the table.
- The beam from the main laser is almost invisible to the human eye and is difficult to see without the aid of beam cards and CCD cameras which are sensitive in the near infrared part of the spectrum.
The beam from the alignment laser is also directed through the same two irises that the main beam passes through. This is accomplished by using a flip mirror which can be flipped in and out of the beam path without changing its alignment. Using the flip mirror you can determine whether the main laser beam or the alignment beam is going through the irises.
Only one of the two lasers should ever be on at any given time – either the main laser or the alignment laser. The main laser should only be turned on after you have completely aligned the optics using the alignment laser and you are ready to take data.
There are three photodiodes on in moveable mounts which can be positioned to detect the beams.
There is an inexpensive CCD camera which can be used to view the beam from the main laser.
There will also be a set of bolts and an Allen wrench for attaching your optical table to the main table.
Other apparatus shown in the photo but now actually on the optical table include:
- The electronics for controlling the main laser.
- A computer.
- A couple of scopes.
- A monitor connected to the CCD camera.
Individual group components
The photo below shows four smaller optical tables, each with a complete set of optics. Each lab group will use one of these tables and optics sets. Each group has an identical set of optical components which are more than sufficient to do the experiment.
Use only your optics, do not borrow or use components from any other groups set… even if they are not using them. If you think you need additional components, or if one of yours is damaged, talk to a member of the lab staff about obtaining what you need.
Under no circumstances should you disturb any of the other optical setups with belong to other groups. Be careful when retrieving and returning your table to the storage area so as not to bump someone else's setup.
Do not work on your table in the setup area. Take your table to the main optical table or to the free standing table at the far wall to work on it.
Each group has the following optical components to work with in designing and building your experiment:
| Component | Qty |
|---|---|
| Silver Mirror | 3 |
| Cube Beam Splitter | 1 |
| Linear Polarizer | 1 |
| Iris | 2 |
Tips
Aligning optics is a skill which requires practice to get good at. If this is your first time working with laser optics, you may find it a frustrating experience at times. Here are some tips to keep in mind while you work.
- Beam alignment in three dimensions is more challenging that doing it in two dimensions (and is unnecessary for this experiment). Keep all of your beams the same height off the table to simplify things.
- Have a detailed sketch of your optical layout on hand before you start placing components on the optical table. It is a lot easier to make changes on paper than it is to repeatedly tear down and rebuild parts of your setup.
- Work front to back. Start with the first component which the beam will encounter on your portable optics table. Once that component is in place and properly aligned, move on to the second component, then the third, etc. Remember, making even a slight adjustment to the alignment of a component will throw off everything “downstream”.
- Watch for unwanted reflections of the beam off of vertical surfaces. (For example, when the beam enters your vapor cell some of it will be reflected at the air to glass surface.) Pay attention to where that reflected beam is going to go and make sure it is not going to interfere with anything else, or create a dangerous situation where it might scatter into someone's face.
Video on Aligning Optics.
Once you have your optical paths setup and aligned, it is time to turn on the Toptica laser and use the flip mirror as shown in the video to switch from the guide laser to the main beam. Remember, everyone in the room must be wearing goggles appropriate for the 120 mW 780 nm laser before the beam is turned on. The door should be closed and anyone entering the room will need to put on goggles before entering or the beam should be shutoff. You should also have all of the detectors on and connected to the scope and the scope should be on and triggering on the sync pulse from the Toptica laser before turning its beam on.
Use the IR card to follow the Topica beam and make sure that it is aligned the same as your guide beam was. At this point, you should not have to do more than minor tweaking to the Toptica beam in order to have its alignment in good order.
Now you should find the signals from the photodetectors on the scope and tune the laser onto resonance with the Rb hyperfine transitions as shown in the video. Don't spend more than 20 minutes trying to tune the laser onto resonance. If you cannot get it to the point where you see at least three of the four doppler broadened peaks you should find the TA or one of the lab staff. Sometimes the laser itself needs a slight adjustment which is not something you can do yourself.
Video on tuning the laser onto resonance.
Once you have the Toptica laser sweeping over the hyperfine transitions you can begin the process of optimizing the signal and and recording spectra. Part of the optimization process involves maximizing the overlap of the probe and pump beams. Another significant part of the process involves setting the power in the probe and pump beams.
Finding the correct beam powers is accomplished through the process of trial and error. To help you in this process we provide a power meter with which you can measure the approximate power in the laser beam at any point on the table. In order to control how much of the laser power passes through the first iris on the main table we have placed a combination of a half-waveplate and a polarizing beam splitter (PBS) in the main laser beam as shown in the following image. The PBS has been aligned to pass horizontally polarized light and reflect vertically polarized light. The half-waveplate in front of the PBS allows you to rotate the plane of polarization of the light incident on the PBS, thus you can control how much of the laser light is horizontally polarized so that it passes through the PBS.
To get the beam powers in the correct range, use the half-waveplate and the power meter to set the power of the pump beam to be somewhere between 0.5mW and 1.5mW. The intensity of the probe beam should be about a factor of 5 or 6 less than the pump beam.
When you leave the lab
Since there will be other groups working on the apparatus, it is your responsibility to ensure that everything in the lab is in order with the next group arrives. The room should be tidy and everything should be either put away or reset to its default. If the lab room was in disarray when you arrived, you are still responsible for leaving it in the appropriate state for the next group.
Here are some general tips on things to check before you leave.
- The lasers should off and the Toptica electronics rack should be off as well.
- Your optical breadboard should be removed from the main optical table, labeled, and stored in its proper location.
- Any and all optical components which are in use on your breadboard or permanently mounted on the main optical table should be put back where they belong.
- The IR sensor cards should be laid out on the main optical table.
- Photodetectors should be turned off and IR filters should be in place if you removed them.
- All tools should be returned to the tool box.
- The CCD camera and monitor should be turned off.
- Scopes should be turned off and their inputs disconnected.
- Any applications on the computer should be closed. Be sure to sign out of any accounts you may have logged into.
- The computer can be allowed to go into sleep mode and does not have to be shut down.
Related Applications
- The same technique is used to measure hyperfine splitting in positronium, which may help resolve discrepancies between measurements and Quantum Electrodynamic(QED) calculations.
- Follow-up precision measurements seem to indicate that there may be agreement with theory after all
- Doppler-free spectroscopy may be used to non-invasively probe the temperature of a flame