Spectrometer using a tunable diode laser Ricardo Vasquez Department of Physics, Purdue University, West Lafayette, IN April, 2000 In the following paper the construction of a simple spectrometer using a diode laser, a Rubidium cell and a photodiode detector. The absorption spectra of a Rubidium atomic transitions: 85 Rb (F =3 F ) 87 Rb (F =1 F ) 85 Rb (F =2 F ) 87 Rb (F =2 F ) [4] can be analyzed. The absorption spectra spread agreed with the calculated Doppler broadening for the atoms and temperatures used. It s important to follow carefully the instructions since diode lasers are extremely sensitive to numerous factors. I. Introduction Spectroscopy is a technique that measures the response of material systems to radiation as a function of frequency. Through the relation E = hv (energy and frequency) observed resonance in the light matter interactions (e.g. fluorescence) give information about the atomic energy transitions of a certain material. [1] A spectroscopic transition may take an atom (in our case Rubidium) between one state to another. For example, one can describe the transition from an upper energy state, A, and a lower energy state, B. Thus A B implies emission of radiation, since the transition occurs from a higher to a lower state, and A B means absorption of radiation, since the transition goes from a lower to a higher state. [2] Since the laser built in the previous experiment is tunable to a very narrow frequency, we can sweep different frequencies around the needed frequency of an atomic transition just by changing finely the angle between the grating and the laser beam with a piezoelectric transducer. We know that the frequency modes that our laser enters have a discrete behavior (e.g. fig 1). Some modes are close to energy transitions of rubidium atoms (780nm). Therefore one can make the diode laser beam go through a Rb cell to analyze the spectra of absorbed beam at a certain atomic transition once the diode laser is adjusted to the correct mode. This mode hoping is dependent on temperature that is why it s extremely important that once an atomic transition is found, the temperature remains stable in all times. We used a photodiode detector to measure the intensity of the beam that makes it across the Rb cell. The absorption of radiation around an atomic transition doesn t occur in a very discrete manner. It basically spreads out like a Gaussian, slowly changing with respect to frequency. The spreading is due to relative velocity between the diode and the atoms in the cell, giving a Doppler effect in frequency. In other words, some atoms are moving towards the laser beam some others move away from it, therefore the atoms see some apparent frequencies different from the actual emitted by the laser. This is called Doppler broadening. The Doppler broadening is temperature dependent, so the hotter the gas the larger the broadening would be. [2]
II. Procedure Fig1.- Laser output wavelength vs. temperature of the laser. The short continuous segments indicate the tuning of the optical length of the cavity for a given longitudinal mode. When the peak of the gain medium has shifted too far, the laser jumps to another mode. (figure reproduce from reference [3] without permission) The optical setup is simple. A single beam has to go through the Rubidium cell.(see fig 2). On the other end of the cell we installed a photodiode detector that translated the radiation shinning on its aperture into a current signal. In order to find an atomic transition, the grating has to be carefully aligned. The alignment is not a trivial task. It s recommended installing a CCD television camera facing the rubidium cell. It is not essential, but doing the alignment in a dark room was helpful. First the temperature of the diode laser was lowered to about 10 o C. Observe the behavior of the beam going across the cell while the temperature drops. Three or four atomic transitions were found just by temperature changes. The track of the beam would appear bright visible on the screen when this happens (fluorescence). We recorded the temperature when these transitions happened and then we set the temperature controller to one of the temperatures where fluorescence occurred. It s important to have a very stable temperature to start working on the grating s position. It depends on the temperature controlling system how long it takes for the temperature to stabilize. We waited for the temperature readings to remain constant (it took us a couple of hours). Diode Laser Rubidium Cell Fig 2. Optical setup for the simple absorption of the diode laser beam. Photodiode
Then, again with the light off, the laser was tuned mechanically by adjusting the screws on the optical mirror mount: changing the angle of reflection of the grating. They weren t adjusted more than a complete turn since we did not want to loose optical feedback while the grating is positioned correctly. The diode laser is tuned until fluorescence is found. If no fluorescence is found at any grating angle the temperature should be changed ±0.5 or ±1.0 o C and then the search for the absorption line should be repeated. This process should be done iteratively until fluorescence is found. Patience is needed since it takes time for the diode to reach thermal equilibrium again. If fluorescence is not found after a few (4 or 5) iterations, it s recommended to check the gratings position once again and make sure that the laser is still tuning in the desired range, that the optical feedback is not lost with tuning.[4] Once fluorescence is found, the current on the laser can be adjusted to maximize fluorescence. The temperature should not be changed anymore. Then the piezoelectric transducer can be driven to start a scanning of the absorption of the beam at the atomic transition found. The photodiode signal was amplified using a 1k Ω resistor and then the signal was read by an oscilloscope. The piezoelectric was connected to a signal generator, a triangular signal of about ±10V was produced with 10-30Hz of frequency. The graph gives you the absorption spectra of 85 Rb (F =3 F ) a rubidium atomic transition.(see fig 3) Fig 3. Absorption of single beam in 85 Rb, F=3 F [4] It can be seen that the absorption is not a sharp deep peak it is a broad slowly changing reduction of intensity and slowly picking up again. This is due to the Doppler broadening. According to Whiffen D. H. [2] the doppler broadening can be obtained by ν=[2ν 2 kt(ln)/mc 2 ] 1/2. In our case, ν= 3.846e14Hz,T=300 o K, and M Rubidium =1.45e-25kg. The k is the Boltzmann Constant and c is the speed of light. The theoretical Doppler broadening is around 250Mhz. This result agrees with the Rb atomic transitions shown in in figure 4, reproduced from reference [4], which shows that the range that a transition cover is on the same order as the range calculated with the ecuation.
III. Results Our experiment gave us similar results as the ones shown in fig 3. The results observed on the oscilloscope had some discrepancies. The graph instead of showing a single deep, it had a couple of them, and instead of being stable on the screen it look more like the same graph was doubled and shifted one with respect to the other. This was thought to be due to the piezoelectric hysterissis. One of the graphs could have been the one going up in voltage across the piezoelectric and the other one could have been the
signal going down in voltage. It was hard to have complete control over the diode laser at the end of the experiment, more time was needed to understand and reproduce the diode laser s behavior thoroughly. IV. Recommendations One can actually install an optical lay out to build a saturated absorption spectrometer. That s means, one can get a Doppler-free reading of the absorption spectra of a single beam crossing the Rubidium cell. This would give a better understanding of what atomic transitions we are observing since there is no Doppler broadening that would erase traces of the absorbed signal. Acknowledgements To the support of Prof. Steve Durbin, Krystl Adams, Erick Dedrick, and Jacob Millspaw, and Ryan Pringer. References: [1] Suter, D.; The Physics of Laser-Atom Interactions; Cambridge University Press 1997 21 & 22. [2] Whiffen, D. H. ; Spectroscopy; John Wiley and Sons Inc. ; 1966 pp 1-22 [3] Wieman C. et al.; Using diode lasers for atomic physics; Rev. Sci. Instrum.62(1), Jan. 1991. pp 1-20. [4] MacAdam K.B., A narrow-band tunable diode laser system with grating feedback, and a saturated absorption spectrometer for Cs and Rb; Am. J. Phys. 60 (12), December 1992, pp 1098-1111.