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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Towards a simple and performing CPT based magnetometer: optimization of experimental paramaters (Invited Paper)

G. Bevilacqua, V. Biancalana, Evelina Breschi, Yordanka V. Dancheva, C. Marinelli, E. Mariotti, L. Moi, C. Andreeva, Todor S. Karaulanov, S. Cartaleva

Published Apr 22, 2005• 5 citations
Publisher

Abstract

A simple set-up for observation of CPT effect at single hyperfine ground state excitation has been examined for application in measurements of non-vanishing magnetic fields. The CPT is prepared using low-frequency modulated (in the kHz range) diode laser. Magnetic field measurements down to 500 pT sensitivity and reproducibility were obtained for integration time of 30 ms. One of the important characteristics of the proposed methodology is a proper Cs excitation in order to overcome effectively the huperfine optical pumping effefct. This is possible working in narrow ranges of laser beam intensity, buffer gas pressures and Cs vapor densities. The best conditions have been experimentally determined and discussed.

DOI
10.1117/12.617473

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A simple set-up for observation of CPT effect at single hyperfine ground state excitation has been examined for application in measurements of non-vanishing magnetic fields. The CPT is prepared using low-frequency modulated (in the kHz range) diode laser. Magnetic field measurements down to 500 pT sensitivity and reproducibility were obtained for integration time of 30 ms. One of the important characteristics of the proposed methodology is a proper Cs excitation in order to overcome effectively the huperfine optical pumping effefct. This is possible working in narrow ranges of laser beam intensity, buffer gas pressures and Cs vapor densities. The best conditions have been experimentally determined and discussed.
A fully automated magnetometer suitable for long lasting measurement under stable and controllable experimental conditions has been implemented. The device is based on Coherent Population Trapping (CPT) effect produced by a multi-frequency excitation. It has an all optical sensor connected only by an optical fiber to the laser head. This allows truly remote sensing and minimization of the field perturbation. The CPT resonance is observed when a frequency comb, generated by a diode laser current modulation, excites a sample of Cs atoms confined in a cell with 2 Torr of buffer gas. In differential sensor configuration, allowing for common-mode noise cancellation, the sensitivity of 45pT/square root of Hz is obtained.
An automated magnetometer suitable for long lasting measurement under stable and controllable experimental conditions has been implemented. The device is based on coherent population trapping (CPT) produced by a multifrequency excitation. CPT resonance is observed when a frequency comb, generated by diode laser current modulation, excites Cs atoms confined in a π/4×(2.5)2×1 cm3, 2 TorrN2 buffered cell. A fully optical sensor is connected through an optical fiber to the laser head allowing for truly remote sensing and minimization of the field perturbation. A detailed analysis of the CPT resonance parameters as a function of the optical detuning has been made in order to get high sensitivity measurements. The magnetic field monitoring performances and the best sensitivity obtained in a balanced differential configuration of the sensor are presented.
We have experimentally investigated the potential of narrow coherent population trapping (CPT) resonances for precision applications like magnetometry or atomic frequency standards, using the D lines in thermal Cs or Rb vapor. The magnetometer operates by monitoring the position of the Zeeman-shifted outermost resonance component. The central Zeeman component is well suited for frequency standard applications because its position is shifted by magnetic fields only in second order. We derive the required pair of laser fields form a special diode laser by direct modulation of the injection current. Magneto metric sensitivity down to a picotesla and 10-12 relative instability for a finger-sized clock have been achieved in this way. Use of a magnetic gradiometer allows to cancel fluctuations of ambient magnetic fields to a large degree, making possible sensitive measurements even outside magnetically shielded rooms.
Summary form only given. We have experimentally investigated the potential of narrow coherent population trapping resonances for precision applications like magnetometry or atomic frequency standards. CPT can be observed in /spl Lambda/ systems where two ground states are coupled to a common excited state by two near-resonant light fields. Effective formation of ground state coherences takes place when the frequency difference of the light fields precisely matches the ground state splitting. This process leads to reduced absorption in the medium (electromagnetically induced transparency) and thus reduced fluorescence intensity (dark resonance) at the optical resonance frequency. Extremely narrow dark resonances can be observed when the two light fields have fixed phase and frequency difference and time-of-flight broadening is suppressed by the use of a buffer gas, making this type of system a promising candidate for precision applications. The /spl Lambda/ systems under study here are the D/sub 2/ lines of Cs, /sup 87/Rb and /sup 85/Rb where the ground state hyperfine splittings are 9.2 GHz, 6.8 GHz, and 3.0 GHz, respectively. Thus in a magnetic field the CPT resonance splits into several Zeeman components whose frequency positions are determined by the Breit-Rabi formula.
Based on a Λ-type three-level system consists of cesium (Cs) 6S1/2 (Fg = 3) and 6S1/2 (Fg = 4) long-lived ground states and 6P3/2 (Fe = 4) excited state, we have experimentally measured and theoretically analyzed the characters of coherent population trapping (CPT) resonances in Cs vapor cells with different partial pressure of neon (Ne) as buffer gas around room temperature. The impact of some experimental parameters, such as the relative intensity ratio between two phase-locked laser beams, the laser intensity, with or without buffer gas and the partial pressure of Ne, the temperature and the longitudinal magnetic field, on the CPT resonance was studied in details. With the optimized parameters, we got typical CPT signal with the full-width half-maximum (FWHM) linewidth as narrow as ~ 181 Hz in a Cs vapor cell filled with 30 Torr of Ne as buffer gas.
Most optically pumped magnetometers based on alkali atom vapor cells and pumped by a single narrow-band laser suffer from a loss of signal since atoms become trapped in the ground hyperfine states that are not coupled to the laser beam. This can be counteracted by additional optical repumping of these ground-state levels. We study hyperfine repumping using cesium vapor cells with partly overlapped ground-state splitting due to their nitrogen buffer-gas filling. We implement two methods of repumping and compare them to the conventional case of F=4 pumping: F=3 repumping with an additional repumper laser and combined pumping/repumping in the light-narrowing mode, where a single high-power laser is tuned near the F=3 transitions. All these modes are investigated for two different methods of spin-phase synchronization: the Mx and intensity modulation methods. For both methods, any kind of repumping results in a clear improvement of the magnetometer sensitivity compared to no repumping, but in the Mx mode it is more pronounced (about 50 versus 200 fT/√Hz for a 50 mm3 Cs vapor cell). The mechanisms responsible for the distinct results in the different working modes are discussed.