Observing the Doppler Absorption of Rubidium Using a Tunable Laser Diode System

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1 Observing the Doppler Absorption of Rubidium Using a Tunable Laser Diode System Ryan Prenger 5/5/00 Final Submission Purdue University Physics Department Abstract Using a tunable laser diode, Doppler absorption lines near 780 nm in rubidium are probed. By scanning a range of wavelengths using a triangle wave generator, a Doppler absorption curve is produced which can be used to test the stability of the laser wavelength and explain the characteristics of the laser system INTRODUCTION In order to build an optical trap for rubidium atoms, a very stable and very finely tuned laser beam is needed in order to excite the proper electron transitions. A tunable laser diode system can be used if the laser is properly tested and refined. The easiest and most efficient way to test the fine characteristics of the tunable laser diode is by observing Doppler, and saturated absorption lines of rubidium. When a laser beam is tuned, the wavelength of the emitted photons changes. At a certain point the photons from the laser beam will be at energies equal to the difference in the atoms, in this case rubidium's, electron's energy levels. At this point the rubidium will begin to absorb the light rather than letting it pass through. However, because certain atoms are moving toward the source of photons, they will experience a higher wavelength due to the Doppler shift. So these atoms will begin to fluoresce before the rest. The same thing happens as the frequency gets greater than the frequency of absorption, only then some atoms are moving away from the source and hence experiencing the proper wavelength due to the Doppler shift. This effect widens the gap of allowed wavelengths, and so it is called Doppler broadening. 1 To observe the Doppler absorption of a particular laser, the beam is passed through a sample of rubidium and then into a photodiode which measures the intensity of the exiting beam. The wavelength is then tuned towards Doppler absorption, in rubidium absorption begins to occur around 780 nm. As the Doppler absorption is achieved, the light that passing through the sample will decrease in intensity. By plotting this intensity versus the wavelength adjustment, in this case this will correspond to the voltage supplied to a piezo-electric element, the Doppler absorption curve

2 should result. Figure 1 is what a Doppler absorption curve should look like. 1 curve and fluorescence show us that the electrons are jumping to a higher energy state, however it tells us nothing about which hyperfine states we are exciting. Figure 2 shows the energy levels of 87 Rb and figure 3 shows the energy levels of 85 Rb. 2 Since both isotopes exist in the gas, and each isotope has two different F states to become excited from, there are 4 different absorption lines for rubidium near 780 nm. Figure 1. The reason this graph is so wide and not a sharp line at the one proper wavelength is because of Doppler broadening. For this observation of the Doppler absorption curve, a range of wavelengths that completely spans the absorption line will be continuously scanned. By continuously graphing this range of wavelengths on Figure 2. the x-axis with the observed intensities as the y-axis, a real time Doppler absorption curve will be produced. The way in which this graph behaves reveals how the laser is behaving. For instance, if the curve jumps from one end of the x-axis to the other as the laser cavity length is changed, it would indicate that a mode hop has occurred at that cavity length. In order to more precisely determine the characteristics of the light passing through the rubidium saturated absorption is utilized. The Doppler absorption Figure 3. In order to determine which of the hyperfine transitions is being excited, the beam is split and half is

the Doppler absorption curve where light has already excited the atoms and so the probe beam passed through APPARATUS While reading about apparatus refer to figure 6. to the detector.

3 used to excite the transition while half is used to probe the sample as before. Basically what happens physically is that excited states fill up as the probe beam goes through, and so at certain wavelengths there are spikes in Figure 5. the Doppler absorption curve where light has already excited the atoms and so the probe beam passed through APPARATUS While reading about apparatus refer to figure 6. to the detector. Figure 4 shows the ÒpumpedÓ Doppler absorption curve. The Doppler broadening shows up the same, since the pumping beam is coming from a different direction. However if figure 1 is subtracted from figure 4 then a characteristic curve, called the saturated absorption signal, appears. This not only reveals which F state the electron is jumping from, in that rubidium line, but also at what PZT voltage each hyperfine transition is occurring. Figure 5 shows the saturated absorption signal for the F=2 -> FÕ transition. This is the one needed for optical trapping of rubidium. In this experiment the Doppler absorption curve was found and examined. 1 Figure 6. A. Laser The used in this setup is a tunable pseudoexternal cavity laser diode. The wavelength is controlled using a thermoelectric cooler and an external cavity created with a diffraction grating. The laser is first cooled until the light is near the desired wavelength. After the temperature has reached equilibrium, the grating angle is changed using a screw for course tuning, and a piezo-electric for fine-tuning. By applying voltage to the piezo-electric the grating angle can be adjusted very slightly. A triangular wave voltage signal is sent to the piezo-electric to scan a small range of wavelengths. For a more detailed description of the Laser unit and it's Figure 4. components see reference 1.

4 B. Rubidium Sample The rubidium atoms are in the form of a gas inside of a 4-inch long, 1-inch diameter cylindrical glass cell. An optical flat is attached to each end of the cylinder. These windows are where the beam will pass through the cylinder. It is important that the beam passes through the windows in order to insure that no surface reflections that may be mistaken for fluorescence occur. Also it was found that difference in intensity caused by absorption through one rubidium cell is not enough for the photodiode used to detect sufficiently. To remedy this, a second cell was placed behind the first. This essentially doubles the amount of absorption and hence doubles the difference in the intensity due to absorption. C. Photodiode The photodiode used to detect the difference in intensity caused by absorption was a Thorlabs High- Speed Silicon Detector - DET 110. For this experiment it is necessary for the diode used to have high sensitivity in the range of 780 nm. Figure 7 is a graph of this photodiode's spectral response. As can be seen the spectral response at 780 nm is near the peak. It is also important that the diode is able to take 50 mw without being damaged. This photodiode has a damage threshold of 100 mw. Figure 7. EXPERIMENTAL PROCEDURE AND RESULTS First the laser was aligned with the rubidium sample and the photodiode. After cooling the laser, the course tuning screw was turned while the piezo was oscillating in a triangle wave at 20 Hz and with and amplitude of 30 volts. This was done until fluorescence was seen, an indication that the laser was near the desired wavelength. Then the voltage about which the piezo was oscillating was raised and lowered until a fluorescence line was between the maximum and minimum of one piezo oscillation. Once the setup was complete an oscilloscope was set on X-Y mode and the Intensity and piezo voltage were read in as the Y and X values respectively. The resulting graph was filled with discontinuous line segments. This was due to the large amplitude of the PZT voltage. By scanning too wide a range of wavelengths several different mode hops and fluorescence lines were all on the same graph. By decreasing the amplitude of the PZT voltage, the graph became more intelligible. By adjusting the voltage and

5 range of the oscilloscope an actual Doppler absorption curve was observed, but it was only observable for a short time, because the graph was unstable. The absorption lines seemed to change to be moving around ACKNOWLEDGEMENTS 1 K. B. MacAdam, A. Steinbach, and C. Wieman, Am. J. Phys. 60, 12 (1992) 2C. Wieman, G. Flowers, and S. Gilbert, Am. J. Phys. 63 (4), April (1995) as they were scanned. This was probably due to the historesis of the piezo and temperature stability.

Saturated Absorption Spectroscopy (Based on Teachspin manual)

Saturated Absorption Spectroscopy (Based on Teachspin manual) 1 Background One of the most important scientific applications of lasers is in the area of precision atomic and molecular spectroscopy. Spectroscopy