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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)

87Rb isoclinic point thermometry

N. Wells, Travis U. Driskell, James C. Camparo

Published Apr 1, 2015• 0 citations
Publisher

Abstract

The concept of atomic thermometry is dem onstrated in an experimental system consisting of two lasers frequency stabilized using FM spectroscopy. Utilizing the temperature insensitivity of the (su87)Rb D1 isoclinic point, a refrence laser is beat against a second laser locked to an atomic transition whose frequency is strongly dependent on the vapor temperature. The data presented here indicates that thermal fluctuations down to the mK level could potentially be sensed. This concept may be useful to obtain improved long term performance in Rb vapor cell atomic clocks.

DOI
10.1109/fcs.2015.7138786

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The concept of atomic thermometry is dem onstrated in an experimental system consisting of two lasers frequency stabilized using FM spectroscopy. Utilizing the temperature insensitivity of the (su87)Rb D1 isoclinic point, a refrence laser is beat against a second laser locked to an atomic transition whose frequency is strongly dependent on the vapor temperature. The data presented here indicates that thermal fluctuations down to the mK level could potentially be sensed. This concept may be useful to obtain improved long term performance in Rb vapor cell atomic clocks.
Experiments were carried out to evaluate the performances of a semiconductor laser pumped rubidium (87Rb) atomic clock. Two kinds of Rb gas cells were used and their performances were compared [gas cell A (natural rubidium (87Rb/85Rb =\frac{3}{7}) and buffer gases) and gas cell B (87Rb and buffer gases)]. The highest microwave frequency stabilities were estimated as3.4 \times 10^{-12} \tau^{-1/2}and2.7 \times 10^{-12} \tau^{-1/2}at the optimal gas cell temperatures of 60°C and 48°C for the gas cellsAandB, respectively (τ: integration time). The light shift, i.e., microwave frequency shift induced by laser light, was measured as -0.50 Hz/MHz and -0.11 Hz/MHz for the gas cellsAandBat their optimal operating conditions given above. As an improved experiment by utilizing high temporal coherence of the laser, a novel double resonance spectral line shape with a drastically narrower linewidth was demonstrated. A technique, similar to FM laser spectroscopy, was employed for this purpose by utilizing laser FM sidebands which are induced by microwave frequency modulation and nonlinear susceptibility of three-level87Rb atoms. The minimum linewidth obtained was 20 Hz, which can be used as a sensitive frequency discriminator for an improved87Rb atomic clock.
In the research presented here, we reconsider the problem of obtaining stable resonant features in linear absorption spectroscopy, which has application to ultraminiature atomic physics where sub-Doppler spectroscopic techniques are not always optimal. In particular, we consider the applicability of isoclinic points for precision atomic spectroscopy. These are defined as ``[a] wavelength, wave number, or frequency at which the first derivative of an absorption spectrum of a sample does not change upon a chemical reaction or physical change of the sample,'' and we demonstrate the existence of isoclinic points in the D1{D}_{1}D1​ spectra of I=3/2I=3/2I=3/2 alkali-metal isotopes, where III is the nuclear spin. We then consider the D1{D}_{1}D1​ isoclinic point of 87Rb^{87}\mathrm{Rb}87Rb in detail, showing that a slight 85Rb^{85}\mathrm{Rb}85Rb contamination in real 87Rb^{87}\mathrm{Rb}87Rb samples should have no significant effect on the frequency stability of the isoclinic point and that optical pumping by a linearly polarized laser should also not affect the isoclinic point's stability (i.e., the isoclinic point is insensitive to laser intensity). Finally, we perform an experiment demonstrating that the 87Rb^{87}\mathrm{Rb}87Rb D1{D}_{1}D1​ isoclinic point has a temperature shift more than an order of magnitude smaller than that of the most isolated transition in the 87Rb^{87}\mathrm{Rb}87Rb D1{D}_{1}D1​ spectrum.
We present a microwave frequency standard based on coherent population trapping (CPT) in the 87Rb D1 line. The CPT spectrum is obtained using two Raman lasers with a 6.8 GHz frequency offset by injection locking of a master laser to a slave laser. We have constructed an atomic clock employing a 5 cm long Rb vapor cell confined with 6.67 kPa neon buffer gas at 70 °C. Using this system, we improve the CPT contrast through the elimination of undesired off-resonant fields created by the direct modulation method. We measured the frequency shift of the CPT signal as a function of the temperature of the Rb cell and estimated it to be approximately 1.3 ×10−9/K. The frequency of a 10 MHz crystal oscillator has been stabilized to the CPT spectrum between the two ground states in 87Rb. The relative frequency stability is approximately 2.3×10−12 for an average time of 68 s.
To overcome the temperature sensitivity of lasers locked to Doppler-broadened transitions, we propose a scheme in which the laser is locked to an isoclinic point of the atom's spectrum: a frequency at which the first derivative of the absorption spectrum does not change upon a physical change of the sample. Here, we demonstrate that an isoclinic point exists in the Rb87D1spectrum at 795 nm, and that it is located midway between the Fg=2 → Fe=1 and Fg=2 → Fe=2 transitions. Locking a VCSEL diode laser to this isoclinic point, we have been able to demonstrate a very low sensitivity of the locked laser's frequency to temperature variations of the atomic vapor.
In the early 1980s, it was common to hear that Rb atomic clocks were limited to Sy(τ) = 10-11/τ1/2at best, and that their ultimate frequency stability could never be better than low-parts of 10-13. Twenty-first century Rb clocks put the lie to those pronouncements. Today, it is not uncommon for vapor-cell clocks to exhibit short-term stabilities at low-parts of 10-13/τ1/2, and ultimate stability levels in parts of 10-15. Here, we discuss what it will take to improve those clocks further (e.g., getting the long-term stability to break into the 10-16range). Specifically, it is our contention that next steps will require a better understanding of mesoscopic physics in vapor-phase systems: atomic phenomena occurring on a scale between the microscopic level of quantum dynamics and the macroscopic level of vapor temperature and pressure. Though at present we have some understanding of mesoscopic physics and its effects, much more needs to be understood. Further, while past and present advances have been driven by “one-off” phenomenological discoveries, it is our opinion that rapid advancement will be best facilitated by a more overarching vision of mesoscopic vapor-phase physics, which at present is lacking.
We describe the optimization of a Frequency Locked Loop (FLL) in an atomic clock which is based on Coherent Population Trapping (CPT) in (87)Rb vapor using the D(2) transition. The FLL uses frequency modulation (FM) spectroscopy and we study the effect of FM parameters (modulation frequency and index) on the sensitivity and the signal to noise ratio of the feedback signal in the FLL. The clock which employs a small spherical glass cell containing (87)Rb atoms and a buffer gas, exhibits a short term stability of 3x10(-11)/ radicaltau. The long term relative frequency stability of the 10 MHz output is better than 10(-10) with a drift of 10(-11) per day.