Table of Contents
Hyperfine Spectroscopy of Rubidium
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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.
Overview
In the lab, we provide you with everything you need to do the experiment. You will select from the optical components available, arrange them on the table to create the beam geometries that you need, and align the beams so that they all go precisely where they need to go.
The heart of the experiment is a Toptica DL100 laser which produces a very narrow bandwidth beam that can be tuned over an appropriate frequency range for the measurement you need to make. The 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.
There are only one Topitica laser and two photodetectors available for the entire class. 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.)
If you think you need something which is not provided, ask the lab staff.
Teaching points
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:
- Lean 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.
A note about uncertainties and fitting the data.
In this experiment most of your data will be from measurements which can be made straight from the scope. While you could attempt to fit the data from the scope, doing so is NOT recommended. The signals which you will be working with do not have simple functional forms and the uncertainties on the fit parameters are challenging to interpret; you can easily waste hours on this. Instead, you are encouraged to make measurements directly off of the scope using its built in cursor features, and use your judgement to estimate the uncertainties.
Laser safety
Details about how to safely operate the laser will be given in the lab by the instructor.
- Always wear the protective laser goggles provided when the high-power laser is on.
- Note that the beam from the high-power laser will cause permanent damage if it strikes your eye. If the laser is on and you are in the room, you must wear the protective goggles provided. Additionally, anyone entering the room while the laser is on must be wearing goggles as well.
- The laser beam is not harmful to the skin, so it is safe to operate with your hands in the beam. However, you should remove any reflective objects – such as watches and rings – from your hands and wrists before working on the optics table to prevent accidental deflection of the beam out of the plane of the table.
You will be using the technique of Doppler Free Saturated Absorption Spectroscopy (DFSAS) to measure the energy splitting of the hyperfine effect in natural rubidium. That sentence contains a mouthful of technical terms and physics concepts which you would not likely be familiar with unless you had worked in an atomic and molecular optics (AMO) research lab. You are, however, likely familiar with the underlying physics concepts. The prelab question and subsequent in-lab exercises will walk you through the the processes of understanding the physics involved in the DFSAS technique as well as how to use a laser and some common optical components to build an experiment to perform the measurement.
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
1. Prelab exercises (4 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 need to design, build and align your own optics in order to measure the hyperfine splitting in atomic Rb. The following exercises will familiarize you with the optical components you will be using and provide hands on experience placing and aligning them.
The laser which you will use to measure the hyperfine splitting emits light at 780 nm, which is in the infrared part of the electromagnetic spectrum and is nearly invisible to the eye. It is also sufficiently high power that it will cause permanent damage to the eye if you are struck by the unattenuated beam. These two factors make beam alignment more challenging, so you will begin with a low power, visible light laser.
The optical layout that you need to build and align for this experiment does not require any special skills. However it is involved enough that jumping straight into building the final spectroscopy layout without prior experience working with these types of optical components can be very challenging.
The series of alignment exercises which you will work through are designed to build up your understanding of the optical components you will be working with, and give you helpful practice working with them in smaller, more accessible chunks. By the time you have finished all of the tasks you will have gained experience with all of the beam alignment and manipulation which you will have to do to assemble the full doppler free saturated spectroscopy setup.
Check Point 1 - Beam Steering and Laser Operation (7 points)
The optical path that you will have to construct and have properly aligned for the final measurement is not overly complex. However if you do not have experience with beam steering and optical alignment, it can be challenging. This first exercise will provide some experience working with optical components while also allowing you to become familiar with the operation of the laser and the photodetectors you will be making use of later. The goal of this exercise is to:
- Design and build an optical layout which:
- Directs the beam from the laser through a vapor cell containing Rb, and then into a photodetector.
- Independently controls the intensity of the laser light passing through the vapor cell and at the photodetector.
- Safely operate the laser to tune it onto resonance with the atomic transitions of interest in the Rb vapor.
- Record the resulting absorption spectrum on the scope and save the digitized waveform so that it can be read into a Python script for plotting and analysis.
- Make measurements from the scope to measure the doppler broadened width of the observed absorption lines.
- Create a professional plot of the spectrum.
To receive credit for this exercise you need to produce the plot of the recorded absorption spectrum, and report the measured value for the FWHM of the observed absorption lines. The plot must be of professional quality, with properly scaled and labeled axes. The measured line widths have to include uncertainties.
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.
- Before you begin working with the actual optical components, produce a sketch of the optical path you intend to build. Include all of the optical components you will need to accomplish the task at hand and show their layout with respect to one another. This sketch should be done in your lab notebook. Have an instructor check your plan before beginning to assemble it in order to be sure that it will work.
- 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.
An upcoming Check Point exercise will go into more detail on the hyperfine effect and the features of the spectrum you will be working with. However to help orient you and set some context for this check point exercise figure X shows a screenshot of a scope trace when laser has been properly tuned onto resonance. The absorption peaks for the 4 transitions we expect to see are labeled in the figure and are identified as:
- $^{87}Rb (F=2)$
- $^{85}Rb (F=3)$
- $^{85}Rb (F=2)$
- $^{87}Rb (F=1)$
The meaning of this notation will be explained by an instructor in lab.
Figure X. Doppler broadened absorption spectrum for natural rubidium. This spectrum was obtained using the Toptica Laser in the lab.
In order to receive credit for this checkpoint you need to produce the following.
- Using the data saved from the scope, produce a professional quality plot of the doppler broadened spectrum that you obtained in lab.
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).
- Calculate the FWHM for each of the peaks in your spectrum. Before leaving the lab you should verify with an instructor that you have successfully tuned the laser to cover the peaks of interest. This calculation can be done using data which was read off the scope in lab using its built in cursor function. Alternatively you can use the digitized waveform which you saved, but this can be more time consuming than using the scope cursor features. DO NOT try to fit the peaks to any functional form! This is a more complicated fit, can be very time consuming, and does not produce a better result for the purpose of this exercise than reading data points off the curve itself. Show your measured values with uncertainties. Be prepared to discuss what criteria you used to make this measurement.
Note that your measurements from the scope data will be in units of time, not frequencies corresponding to the energy of the transition. The time axis on the scope is a measurement of the sweep of the laser frequency, a calibration must be done in order to convert these sweep times into photon frequencies. This calibration will be the focus of another Check Point exercise.
For the purposes of this Check Point exercise use the following approximate calibration factor. $\frac{\Delta f}{\Delta t} = 97 MHz/ms$
Note that this is intended to be a rough order of magnitude estimate to make sure that you understand the concepts. You are not being tested in this check point to see if you use some “right” method of determining the FWHM of the peaks or the correct value for the FWHM. You will be evaluated on how well you understand what you choose to do and that your method and results were at least plausible. For the FWHM in this case, being within a factor of two is plausible and shows that you understand what you are doing.
Check Point 2 - Calibration of the Laser Sweep (7 points)
As was mentioned in the first check point exercise the time base of the scope needs to be calibrated in terms of the frequency sweep of the laser. Our laser is designed to sweep back and forth over a small range of photon frequencies that correspond the the frequencies of the Rb transitions we wish to study.
You will determine the calibration factor by building a Michelson Interferometer on your optical board. By directing the laser beam into the interferometer, and recording the intensity of the output on the scope, you can determine the calibration factor needed. Ultimately what we want to measure are the energy differences between hyperfine levels, so all we need are frequency differences between features in the spectra. We can accomplish this by using a Michelson interferometer to measure the change in frequency of the laser as a function of time.
The geometry of the Michelson interferometer is shown in Fig. 10. The beams from the two arms of the interferometer will combine at the photodetector with varying degrees of constructive interference depending on their phase difference $\Delta\phi$. It can be shown that the phase difference depends on the difference in lengths of the two arms of the interferometer and the frequency of the light as
| $\phi_1 - \phi_2 = \Delta \phi = \dfrac{4\pi f}{c}(L_1 - L_2)$ | (7) |
where $f$ is the frequency of the light, $L_1$ and $L_2$ are the path lengths of the two arms of the interferometer, and $c$ is the speed of light. From this relation, it can be shown that the frequency spacing of the interference maxima at the output of the interferometer is
| $\Delta f = \dfrac{c}{2(L_1 - L_2)}$ | (8) |
Using Fig. 10 as a guide, set up the optical path for the interferometer. Try to make $L_1 - L_2$ as large as possible. This results in closer fringe spacing which allows you to better characterize the frequency sweep rate.
Some tips for aligning the interferometer.
- Always build your optical layouts front to back. Meaning begin with the first optical component the beam will interact with, carefully align it, then move on to the next component leaving the previous one alone.
- In order to have the two output beams come together properly at the photo detector:
- Choose one arm of the interferometer to build first.
- Place an iris approximately halfway between the beam splitter and the mirror as shown.
- Adjust the beam splitter to send the beam through the center of the iris.
- Then setup the mirror to reflect the beam back on itself and through the iris. This will ensure that the beam is well aligned.
- This return beam determines where you place the detector, do not make any further adjustments to the mirror until you get to the fine tuning stage.
- Now when you setup the second mirror, all you have to do is adjust it so that its return beam overlaps the first one at the detector. Do not make adjustments to the beam splitter or the first mirror, work only with the second mirror. This will ensure that the two beams are nearly perfectly square to one another which is what you need to see the interference fringes.
Aligning a Michelson interferometer can be tedious. Be patient and you will succeed!
Once you have aligned the interferometer with the alignment laser you can switch to the infrared laser and view the signal on the scope. If you did the initial alignment well you should see at least a hint of an interference pattern on the signal. At this point it is a matter of making very minute adjustments to the two mirrors, one at a time, in order to maximize the amplitude of the interference pattern.
- Make a tiny adjustment to one of the mirrors and take your hand off the mount(your finger pressure is enough to change the path length of the light at this point).
- If the interference pattern improved, make another adjustment in the same direction, if it got worse adjust the mirror in the other direction.
- Once you have maximized the amplitude of the interference signal by adjusting one mirror axis, repeat the process for the other axis of that mirror.
- Repeat for the other mirror, going back and forth between the two mirrors until you can no longer improve the interference pattern.
In order to receive credit for this part of the lab you need to do the following.
- Capture the digitized scope trace of the interference pattern.
- Measure and record the lengths of the two arms of the interferometer. Include uncertainties.
- Measure the position of the interference maxima using the scope cursor feature. Alternatively you can do this outside of lab with the digitized data you saved from the scope.
- Calculate the conversion coefficient to convert time differences on the scope into laser frequency differences. Show your calculations.
- Apply your calibration to the FWHM measurements that you made in the earlier check point exercise, in place of the approximate value we provided for you.
Note that you will have multiple interference maxima to work with. This gives you the opportunity to take multiple readings and average, or check for linearity of the sweep. You do not necessarily need to do this for the purpose of this check point exercise, but you will want to more carefully consider these factors for your measurement of the hyperfine energy splitting.
Check Point 3 - DFSAS signal (7 points)
You are now ready to move on to building the full Doppler Free Saturated Absorption Spectroscopy setup.
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.
Building up the DFSAS configuration
Your goal for this Check Point Exercise is to build a Doppler-Free Saturated Absorption Spectroscopy (DFSAS) layout and optimize it until you can see six doppler-free dips in the ${}^{87}$Rb(F=2) peak and in the ${}^{85}$Rb(F=3) peak.
Do not proceed with this section until you have discussed the principles behind the DFSAS technique. Placing and aligning the optics for the DFSAS measurement can be tricky. While there are a lot of ways to accomplish the goal, there are a great many ways to make the whole task much more difficult and time consuming than it has to be. Not understanding why the optics have to be laid out the way they do makes it even more difficult.
We will walk you through the process in steps. It is important that you work systematically, following the path of the beam.
You are going to build your DFSAS setup as an extension of your interferometer. You should not move or adjust the components of the interferometer as you will need to use it after collecting your absorption spectra.
The full setup is shown in the figure below. By placing the vapor cell between the beam splitter and mirror which makeup the long arm of your interferometer you automatically have overlapping pump and probe beams. The beam (shown in red) which leaves the beam splitter and passes through the vapor cell, moving left to right on the diagram, becomes your pump beam. The retro reflected beam from the mirror (shown in blue), which travels right to left on the diagram, passes back through the vapor cell before reflecting off of the beam splitter and then entering PD1 is your probe beam. The neutral density filter in between the vapor cell and the retro reflecting mirror can be used to attenuate the probe beam if necessary.
Obtain the DFSAS signal
For now ignore the beam path shown in orange on the diagram. Begin by focusing on obtaining the DFSAS signal from the red and blue beam paths.
Subtracting off the Doppler Broadened Profile (if time permits)
The reflected beam from the beam splitter (shown in orange on the figure) is directed through the vapor cell at an angle with respect to the pump/probe beams, and into a second photo detector (PD2). PD2 will give you the doppler broadened profile without the doppler free features. If you connect PD1 to channel 1 on the scope and PD2 to channel 2 on the scope, and trigger the scope externally from the scan controller, you can use the math mode of the scope to subtract one from the other. If you balance the intensity of the signals in the two detectors the resulting subtraction signal will will be a flat line with just the doppler free features. This part is not necessary for a complete analysis of the data, but is nice to include if you have the time to set it up. If however you are running out of time on day 3 of the lab, you can omit this part without adversely impacting your grade.
Fine Tune And Record The Signals
From this point forward you will want to focus on the first two doppler broadened features, the 87Rb(F=2) and 85Rb(F=3) transitions. You can zoom in on these features on the scope.
You will still need to do some fine tuning of the pump/probe beam overlap to maximize the visibility of the doppler free features. You will also need to adjust the total power in the probe and pump beams as well as the ratio of pump beam power to probe beam power. This is necessary in order to be able to see the weaker doppler free features.
Using the laser power meter (ask an instructor for it, there is only one meter for both rooms unfortunately) set the power in the pump beam to be approximately 0.2 mW, and the probe beam power to about 0.02mW. since the laser beam is polarized you can adjust the power by inserting a linear polarizer into the beam path at the appropriate location.
You will need to spend some time adjusting the overlap and power of the probe and pump beam in order to see all 6 of the expected doppler free features on each of the doppler broadened peaks.
To receive credit for this Check Point exercise you will need to produce the following.
- For the first peak, Rb87(F=2), make a detailed sketch of the DFSAS peak in your lab notebook. Include notes on the voltage and time base settings. Try to include all of the doppler free features which you were able to find. You should have at least 5 of the 6.
- Identify which of the doppler free features correspond to the hyperfine transitions, and which are cross over features. If you can see 3 or more of the doppler free features, and you understand the origin of the cross over features, you can determine which are which. Make sure that you are able to explain your reasoning.
Collecting A Full Data Set
By optimizing the overlap of the probe and pump beams and their power ratio, you should able to see 5 or 6 of the doppler free features within each of the doppler broadened peaks for 87Rb(F=2) and 85Rb(F=3). Once you have achieved this, zoom in on the scope to one of the doppler broadened features and adjust the vertical scale on the scope and the gains of the photodetectors to make the signals in channel 1 and channel 2 about the same amplitude. Channel 1 should be connected to the PD that is recording the probe beam and channel 2 should be connected to the PD that records the doppler broadened signal. Once the two signals are about the same amplitude, use the scopes math mode to subtract the two signals from one another. The resulting math mode signal will be a mostly flat line as the doppler broadened component of both signals cancel each other out, while leaving just the doppler free features. It is some times easier to see and measured the doppler free features in this manner.
Once you are happy with the signal on the scope save the data from all there channels (1,2 and math) plus a screen shot of the scope display. This is the data you will work with for your full analysis. We strongly recommend that you use the scopes cursor feature to measure the separations of the doppler free peaks. Include your estimate of the uncertainty of these measurements.
Repeat the above process for the other isotope of Rb, optimizing its doppler free features, zooming in on the scope, balancing the signals on channels 1 and 2 and using math mode. Record the time differences between peaks and save this data as you did for the first isotope.
At this point you have all of the data you need for the spectral analysis except for the calibration factor needed to convert from time differences on the scope into photon frequency differences. If none of the laser settings have changes, the calibration you performed using the interferometer for Check Point exercise 2 would work for this data set. However if you have time after completing your collection and measurements of the doppler free spectra, use your interferometer which should still be setup on your optics board, to get another calibration data set.
Final data analysis
You have one week to perform a full and complete analysis of the data you collect in lab and submit the following assignments.
Laser frequency calibration (25 points)
Show all of your data, plots, calculations and uncertainties for the calibration of the laser frequency sweep. Include data and analysis related to systematic effects which you evaluated. Plots should be publication quality with properly labeled and scaled axes and annotations.
Do the following:
- Calculate the laser frequency sweep calibration, with uncertainties, from your interferometer.
- Include a diagram or photo of your interferometer setup along with measured arm lengths with uncertainties.
- Include plots of the scope traces used in your calibration.
- Show how you determined values of maxima locations from the scope data. If you fit sections of the scope data to obtain these values, the fit functions and plots of all fits should be included.
- Include all measured values from the scope traces including uncertainties.
- Show details of calculations, including error propagation. If the same calculation is performed multiple times, it is sufficient to detail one example.
Hyperfine splitting analysis (25 points)
Show all of your data, plots, calculations and uncertainties for your analysis of the hyperfine splitting for ${}^{87}$Rb(F=2) and ${}^{85}$Rb(F=3).
Do the following:
- Calculate the energy differences between hyperfine states for both isotopes of Rb.
- Include plots of the spectra from the scope. Relevant features such doppler free transitions and crossover dips should be identified.
- Show detailed examples of relevant calculations including frequency calibrations, error propagation, etc.
- Show how you determined values of peak locations, peak widths, etc. from the scope data. If you fit sections of the scope data to obtain these values, the fit functions and plots of all fits should be included.
Conclusions and comparison with literature (25 points)
You will need to write a complete and persuasive conclusion that includes a comparison of your results to expectations/literature and puts the results in proper context. Even more than in the assignments above, this assignment leans heavily on your ability to write clearly and correctly. You may want to look over our page on Drawing Conclusions.
Do the following:
- Compare your measured values for the energy difference between hyperfine states for all both isotopes with literature values.
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 | 5 |
| 50/50 Cube Beam Splitter | 2 |
| Linear Polarizer | 2 |
| 780nm Half-Waveplate | 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