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Laser Spectroscopy B3 Head of Experiment: James McGinty The following experiment guide is NOT intended to be a step-by-step manual for the experiment but rather provides an overall introduction to the experiment and outlines the important tasks that need to be performed in order to complete the experiment. Additional sources of documentation may need to be researched and consulted during the experiment as well as for the completion of the report. This additional documentation must be cited in the references of the report. 1 RISK ASSESSMENT Ensure that all relevant risks or hazards are addressed Remember, a HAZARD is an inherent property with potential to cause harm, A RISK is the likelihood of actually causing harm in the circumstances of its use. Please note: This is your assessment carried out on your work procedure. It is to help you to identify, then eliminate, reduce or control dangers that can harm. PHYSICS DEPARTMENT RISK ASSESSMENT CHECK LIST RESPONSIBLE PERSON: J. P. Cotter Group: 3rd Year Teaching Lab BEFORE WORK STARTS Head of Group: Prof J Marangos send completed assessment to DSO Date of Assessment: 26/10/10 Location: Room: 406 A PROCEDURE or WORK ACTIVITY: (Brief description) SAFETY CATEGORY Laser HAZARD Substance, equipment or process PERSONS AT RISK Home built infrared 780nm Class 3B laser max power 15mW All personnel Eye damage risk from these lasers Visitors, New workers, Emergency services, security, Cleaners, contractors, etc. RISK CONTROL ASSESSMENT REMARKS on ASSESSMENT Information, instruction, training provided? State precautions taken to meet legal requirement and comply with College Policy - All personnel entering this room are made aware of the hazard. - Laser goggles are available. - Beams are blocked when not in use. - Laser table has beam-blocks around it at optical path level - all jewelry watches are removed whilst laser is operated - All beams are contained within the surface area above the table 2 Electrical Electrocution risk from above laser power supplies All personnel - Supplies used for diode laser systems are no more dangerous than standard electrical power supplies within the laboratory. Therefore usual precautions must be taken for electrical equipment as shown below Mains powered electrical equipment All personnel - No live wires are exposed in electrical equipment, all items are fully cased - All equipment is raised above ground level incase of lab flooding. - For high power equipment requiring appropriate ventilation it is ensured this is available. Distribution boards All personnel - Boards are fixed securely in an appropriate position off the ground. - boards should not be “daisy chained” together Cables – electrocution All personnel Cables should have no bare insulation and exposed wires 3 Chemical Rb All personnel - Store Rb cell when not used in a well padded package to minimize risk of breakage. - Mount cell in an appropriate stable mount Mechanical Cables – trip hazard - Cables should not be trailed along the ground. - If possible cabling should be bundled together and should be transported via cable guttering - Remove all excess cables not being used Head level structures – head banging hazard All personnel - Padding should be placed around most prominent points and high visibility tape. 4 Laser spectroscopy: Doppler free saturated absorption of rubidium using a diode laser J. P. Cotter, Z. Diveki: July 2013, Version 0.1 Imperial College London, 3rd Year Physics Laboratory Introduction This experiment is designed to introduce some modern atomic and laser physics techniques currently used in research labs around the world. Many of these techniques are essential to laser cooling, Bose-Einstein condensation and precision measurements. You will familiarise yourself with a home-built grating stabilised diode laser. This will then be used to probe the internal structure of rubidium through Doppler free saturated absorption spectroscopy enabling you to investigate the hyperfine structure in more detail. The experiment consists of the following parts: Section 1: Send the laser beam in the rubidium cell and measure the absorption spectrum with a photodiode. Section 2: Set up the optical beampath and detectors for recording of Doppler broadened spectra. Section 3: Set up the optical beampath and detectors for recording of Doppler free spectra. Section 4: Determining the laser frequency with a Fabry-Peort interferometer. The amount of work required to complete each section is not equal; to that end, you should try not to become bogged down with the first sections. You are encouraged to leave around a week and a half to complete section 3, which should form the main body of your report. 5 LASER SAFETY Before you begin ensure that you have read and familiarised yourself in detail with the safety information outlined in this section and that you have signed the risk assessment documentation. Any student in contradiction of these safety notices will be removed from the lab immediately. • The laser you will be working with is class 3B. The output is therefore extremely hazardous to the eyes and can cause blindness if exposure occurs. • You will be working at a wavelength of 780 nm. The response of the human eye at this wavelength is poor (the visible spectrum is 700 nm). This means that the beam is far more powerful than it appears and additional precautions should be made to avoid exposure. Your blink reflex won't be able to save you at this wavelength. • Remove wristwatches and any other jewellery which may come into contact with the laser. (This includes rings, long necklaces, earphones etc.) • Wear the safety goggles provided when working with this apparatus. • Ensure that the external laser lamp is illuminated and that the laser screens are in place if the laser is on. • In order to observe your beams always use either an IR card or camera. • Ensure that all beams remain in the plane of the optical table. • Never lower your face anywhere near the optical table. If you have to pick something from the floor close your eyes to do so. • Only students who have signed the appropriate risk assessment forms are permitted to enter room 406A while the laser on light is illuminated. • In an emergency switch of the laser using the current controller and contact the nearest lab demonstrator or technician. 6 The Experiment Remember NOT to increase the current controller set point above 70 mA (this can damage the diode) and NEVER alter the temperature controller (this will significantly alter the wavelength and could cost you many hours work). Initial set up for fluorescence View from above with CCD Camera 1. 2. 3. 4. Set current & piezo modulation to zero Increase piezo modulation until you see mode hops Increase current modulation until mode hops disappear Fine adjust function amplitude and signal splitter ratio to get four dips 0.2 0.1 0 -0.1 -0.2 -0.3 -0.4 -0.5 Photodiode 1 Function Generator Isolator Signal Splitter Mirror 1 Rb Cell Laser diode Current Modulation 7 Mirror 2 Mirror 5 Photodiode 2 Probe Stage 2 50/50 Splitter Mirror 4 Substitute beam sampler for mirror and add two mirrors and beam splitter for pump and probe to reveal fine detail obscured by Doppler broadening Pump Pump & Probe Pump Isolator Rb Cell R Mirror 1 Laser diode Mirror 3 Mirror 2 Beam sampler wedge 8 Mirror 5 Photodiode 2 Mirror 6 Optics layout for Laser Diode Spectroscopy of Rubidium Probe Photodiode 1 Mirror 4 50/50 Splitter Pump Mirror 7 Mirror 8 Use target with central hole and mirrors 7&8 to align beam with etalon and adjust length for confocal cavity Pump Ref Pump & Probe F.P. Etalon Isolator R Rb Cell Mirror 1 Laser diode Mirror 2 Mirror 3 Beam sampler wedge 9 You will encounter many devices around the experimental setup that will help you to obtain results. The next layout shows the rather complicated connections between these devices. Do not worry when doing the experiment it will all be clearer. Layout of the applied devices 10 The Laser: The frequency of the laser depends on 3 parameters: laser temperature, laser current and the position and orientation of the grating. The laser temperature and current set the frequency range over which the diode laser will operate (coarse tuning). Within this range, the laser frequency can be continuously scanned using the position and orientation of the grating (fine tuning). Fine tuning of the laser frequency is accomplished by a piezoelectric transducer (PZT) located in the grating mount. The PZT expands in response to a voltage from the PZT controller. The voltage can be adjusted manually. As the laser cavity length varies, because the number of waves in the cavity will stay constant (for a while), the wavelength and therefore the laser frequency will vary as well. But if the cavity length change goes too far, the number of half-wavelengths inside the cavity will jump up or down (by one or two) so that the laser frequency will jump back into the range set by the temperature and current. These jumps are called mode hops. Continuous frequency tuning over the four hyperfine groups (¼ 8 GHz) without a mode hop can only be achieved if the settings for the temperature and current are carefully optimized. The picture below shows the layout of the laser itself. Inside the Laser Diode Box 11 Section 1: FL UORES CENE Synopsis: If the laser is tuned to precisely the correct wavelength then atoms within the cell can absorb photons and be excited to the 52P3/2 energy level. These excited atoms will then spontaneously decay back to the 52S1/2 ground state at a rate of , emitting a photon randomly through a solid angle of 4 sr as they do so. We will refer to this scattering process as fluorescence. FIG. 1: Fluorescence experiment. If your laser is tuned to the correctly then you should see bright lines your cell as the atoms which absorb light from the laser scatter photons in all directions. Method: To make sure the laser is close to resonance with an atomic transition let's begin by looking for fluorescence from rubidium atoms within the cell using the infra-red camera provided (remember your eyes can't see 780 nm light). Pass the beam reflected from the mirror through the rubidium cell as depicted in the initial setup diagram. If the laser is correctly tuned then the beams passing through the cell will appear as bright glowing lines on the camera, similar to those shown in FIG. 1(b). Because atoms can only absorb photons with very specific energies, and therefore wavelengths, it is unlikely that when you begin working with your laser it will be correctly tuned in order to see fluorescence. If you can't see anything try changing the wavelength of light by adjusting the piezo voltage. Piezo voltage adjustment: This device serves to control the incidence angle of the grating which is placed after the diode laser. By doing so one controls the wavelength of the outcoming beam. The adjustment consists of two devices: a) a box that controls the voltage U0 which determines the amplitude of the grating rotation b) signal generating box that provides an alternating voltage. The amplitude is given by U0 as an input. The frequency can be adjusted. If you scan the piezo voltage with a frequency of ∼ 1 Hz you should see the cell flashing as the laser sweeps through the atomic resonance. You may also need to change the current flowing through diode itself, though be careful to only change it by small amounts at a time. The current can be set with the control box of the laser. 12 You might like to begin looking for fluorescence with the following configuration: Piezo XXX V Current XXX mA When you see fluorescence you know that the laser is at the correct wavelength for the following experiments. Section 2: DO PPL E R B RO ADE NE D ABSO RPT IO N S PE CT RO SCO PY Synopsis: The next step is to record the absorption of the laser by rubidium atoms. The cell you have been provided with is filled with naturally occurring rubidium (this means it contains 72% 85Rb and 28% 87Rb atoms). Calculation 1: Calculate the FWHM of a Doppler broadened absorption line in both isotopes of Rb. Take the room temperature what you measure independently. You should find that the Doppler width, ΔDop, at room temperature is much larger than the excited state hyperfine splitting (typically ∼ 100 MHz ), and much smaller than the ground state hyperfine splitting for both isotopes (3 GHz in 85Rb and 6 GHz in 87Rb). This means that if Doppler broadening is present transitions to different excited states will be indistinguishable from one another, however transitions from different ground states should be easy to observe. Calculation 2: Why is it that the hyperfine structure is larger for the 52S1/2 than the 52P3/2? Provide simple, physical explanation and support it with mathematical methods as well. FIG. 2: You should be able to see four Doppler broadened dips in in your absorption spectrum using this set-up. 13 Method: Using the same set up from your earlier fluorescence experiment replace the mirror with the beam sampler as shown in stage 2 diagram. You should steer the path of the beam using the opto-mechanical mount for the slide so as to maximise the DC photodiode signal. Be careful not to let the photodiode saturate as this will stop you from taking meaningful data. If you find that the photodiode is saturating try using one of the neutral density filters provided to attenuate the laser beam. Because your cell contains two isotopes of Rb, each of which have two ground states, you should be able to observe four resolvable dips in the spectrum as you sweep the piezo voltage (two dips for each isotope and two dips for each ground state). FIG. 2(b) shows a typical rubidium spectrum. You probably won't be able to see all four dips in one scan as the laser doesn't have a large enough mode-hop-free range but try to identify them all independently. Using the scope provided, record and identify all four absorption dips. Once you're happy with your spectra, they should look something like those displayed in FIG. 2(b), use the matlab mfiles provided to fit a Gaussian line shape to each peak. Calculate 3: You can use this fit to measure the temperature, T. To do this you will need to record the scan voltage that you are applying to the piezo simultaneously with your spectrum. You will also need to know the calibration between the piezo voltage and change in optical frequency, this is stuck to the side of your laser. Does each dip return the same temperature and how do they compare to the temperature in the lab as measured by a thermometer? 14 Section 3: DO PPLER -FREE SATURATE D ABSORPTIO N SPECT ROSCO PY Synopsis: We're now going to try and remove the effect of Doppler broadening on our spectrum using a counter-propagating saturated absorption technique. FIG. 3: a) The set-up required in order to observe sub-Doppler features in your spectra. b) An example of the kind of spectrum you should see from PD2. Method: Begin by setting up the apparatus as shown in Optics layout for Laser Diode Spectroscopy of Rubidium. Monitor the output of PD1 and PD2 on the scope as you go. When the stronger beam (Pump) is aligned to be near anti-parallel with the weaker (Probe) beam small peaks in the Doppler broadened profile should appear. You should steer mirrors M1 and M2 in order to optimise the size of these peaks in your spectrum (they should look something like those displayed in FIG. 4. It's best to optimise your scans to observe one ground state transition at a time, recording each of your spectra as you go. 15 FIG. 4: The four Doppler broadened rubidium profiles exhibiting sub-Doppler features. Try to balance the DC voltage of both PD1 and PD2 to be the same (the Doppler broadened dips in both spectra should also be the same size). PD2 contains both Doppler broadened and Doppler-free information about the vapour while PD1 contains only Doppler broadened information. By looking at the difference between these signals we can isolate the Doppler free information. To begin with try looking at (PD2-PD1) using the MATH function on your scope. When you're happy your signals are well balanced try using the differential amplifier provided instead of the scope to take the difference between your signals. Your subtracted, Doppler free spectra should look something FIG. 5(b). 16 FIG. 5: An example of the F = 3 F’ transitions before and after Doppler subtraction. Use your scope to record each of the four Doppler free spectra you can see. Then using your voltage to frequency calibration try to determine the excited state hyperfine splitting for both 87Rb and 85Rb. It is easier to work with the 85Rb F = 3 spectrum as the signal is larger and less complicated so it is advisable to start with this one. You should check that your spectra make sense by comparing the splittings you measure by eye with the accepted values. Once you’re happy that your numbers make sense try fitting to the peaks to extract a more accurate measurement of the splittings. Be aware that some transitions are quite weak and therefore produce very small peaks so you may not be able to observe every one of them. Once you have measured the position of each observable peak try to identify them. Be careful not to confuse cross-over resonances in your spectra with real atomic transitions. How do your measured frequency splittings agree with the accepted values? Do the measured linewidths of your peaks agree with the accepted natural linewidth. If not what mechanism could be causing this and how could you overcome it? 17 Section 4: FREQUENCY CALIBRATIO N WITH A FABRY -PEROT ETALON Synopsis: In principle all the previous measurements results in an absorption spectra as the function of time (given by the signal generator). By recording the trace of the signal generator you can do the mapping between time-voltage (of the piezo). The Fabry-Perot etalon helps to do the mapping between voltage and frequency. Method: Insert the Fabry Perot Cavity as shown in Optics layout for Laser Diode Spectroscopy of Rubidium. Use Mirrors 7 and 8 and the florescent target provided to align the beam normal and coaxial and adjust the cavity length to make a confocal cavity. The focal length of both mirrors is 10cm. Connect the output of PD3 to the Ocilloscope. The oscilloscope will display discrete peaks as the function of time that corresponds to the constructive interference of the different laser wavelengths that fulfil the mλ=2L formula, where m is an integer and L is the length of the cavity. From this one can deduce the frequency separation, Δν, (Free Spectral Range) of the peaks on the oscilloscope: Δν=c/(2L), where c is the speed of light. By plotting the trace of the signal generator and Δν one can find the frequency-voltage mapping coefficient (G). To improve the determination of G, use the radio frequency generator to modulate the current of the laser diode that results in the appearance of sidebands next to the main laser peaks from the Fabry-Perot interferometer. The display of the radio frequency generator tells you the frequency difference between the sidebands and the main peaks. 18