We report on the characterization of Cs vapor microfabricated cells filled with a Ne-He buffer gas mixture using coherent population trapping (CPT) spectroscopy. The temperature dependence of the Cs clock frequency is found to be canceled at the first order around a so-called inversion temperature higher than 80°C whose value depends on the buffer gas partial pressure ratio. This buffer gas mixture could be well-adapted for the development of miniature atomic clocks devoted to be used in specific applications such as defense and avionic systems with high operating temperature environment (typically higher than 85°C). This solution suggests an alternative to buffer gas mixtures generally used in optically-pumped vapor cell atomic clocks.
© 2015 Optical Society of America
Vapor cell atomic clocks use interactions of resonant light with alkali atomic vapor in a cell filled with buffer gas. The presence of buffer gas allows to slow down the diffusion of alkali atoms in the cell and to increase the time for the atoms to collide against the cell walls. In this so-called Dicke regime , the clock microwave transition resonance is narrowed, allowing to improve the relative frequency stability performances of the atomic clock. At the same time, a drawback of the presence of buffer gas is to induce a temperature-dependent frequency shift Δνbg of the clock transition that can limit the clock mid or long term frequency stability. This frequency shift Δνbg can be approximated in a limited temperature range by :
Experimental measurements of buffer gas coefficients are difficult and delicate. In general, they are performed with an atomic clock experimental setup by measuring the change of the clock frequency with the cell temperature for a given buffer gas-filled cell. Experimental data are then fitted by a second order polynomial function as reported in Eq. (1) to extract buffer gas coefficients. These measurements are usually realized using cm-scale glass-blown cells. For such cell dimensions, optical thickness of the vapor becomes relevant as soon as the cell temperatures surpasses 55 – 60°C, inducing a strong reduction or even disappearance of the atomic signal. Consequently, most of the literature only reports measurements of linear coefficients due to a lack of resolution of frequency measurements in the experimental setup or to a limited temperature range studied [3,4]. Reported values by different authors often suffer from dispersion and lack of accuracy, making difficult the realization of buffer-gas filled vapor cells with predetermined characteristics. For Cs atom, Kozlova et al.  reported very recently for the first time values of the quadratic coefficient γ for Ne, N2 and Ar buffer gases.
A standard way to reduce the temperature coefficient of the clock frequency in optically-pumped vapor cell atomic clocks is to use a mixture of two buffer gases, each of them shifting the clock frequency in opposite directions [5–8 ]. When a mixture of two buffer gases 1 and 2 is used, Eq. (1) becomes:9]. For Cs atomic clocks, Kozlova et al.  reported that Ar-N2 buffer gas mixture is convenient to obtain an inversion temperature in a large temperature range.
Over the last decade, outstanding progress in microelectromechanical systems (MEMS) technologies and semi-conductor lasers has allowed the development of high-performance miniature atomic clocks [10, 11] based on coherent population trapping (CPT) physics . Such frequency references provide the base for a number of mobile and embedded applications including network synchronization, new-generation mobile telecommunication systems, satellite-based navigation systems, secure banking data transfer, military and avionic systems or underwater sensor systems for seismic research and oil exploration. The heart of a chip-scale atomic clock consists of a micro-fabricated cell generally made of an alkali-buffer gas filled Si-etched cavity sandwiched between two anodically bonded glass wafers. In clock operation, the mm-scale vapor cell needs to be heated and stabilized at a typical temperature of 80 – 100°C, mainly in order to increase the alkali density, i. e. the clock signal.
In the frame of the MAC-TFC european project , we developed a microcell technology where Cs vapor is generated after complete sealing of the cell through localized laser activation of a Cs pill dispenser . Unfortunately, we demonstrated in  that N2 is absorbed by zirconium compound of the Cs pill dispenser, preventing the use of N2-Ar buffer gas mixture. However, it has been demonstrated that the temperature-dependence of the Cs clock frequency can be canceled around 79 – 80°C using a cell with a simple single Ne buffer gas configuration [16, 17]. In this case, it has been reported that the inversion temperature point does not depend on the buffer gas pressure . This allows to relax strongly the constraints on the buffer gas pressure accuracy during the microcell fabrication. Nevertheless, this solution is not well-adapted for miniature atomic clocks devoted to operate in environments with elevated temperatures (> 85°C typically). This requirement is of relevant importance for specific defense systems, avionic systems or even cell-phone base stations implemented in locations where high temperature can be reached in sunny days. Using the Cs-Ne microcell solution, a first proposal to operate the clock at high external temperature would be to cool down the Cs-Ne cell temperature at 80°C with a Peltier element but at the expense of the power consumption budget. A second solution would be to stabilize the Cs-Ne cell temperature at 90 – 95°C. However, in this case, the cell temperature operation point would be far from the inversion temperature point, increasing the clock frequency sensitivity to temperature variations and giving more stringent specifications on the cell temperature control.
To address this issue, we report in this article successful tests of a new buffer gas mixture solution (Ne-He) that allows to detect an inversion temperature point at temperatures higher than 80°C in Cs vapor microfabricated cells . Direct measurements of the Cs clock frequency temperature-dependence in Cs-He-Ne vapor microcells are reported using a table-top CPT-based clock experimental setup. Basic characterizations of the CPT resonance (linewidth, contrast) are also reported.
2. Experimental set-up
Figure 1 shows the experimental set-up used to detect coherent population trapping resonances in buffer-gas filled Cs vapor microfabricated cells. The laser source is a distributed-feedback (DFB) diode laser tuned on the Cs D1 line at 894.6 nm . The D1 line of alkali atoms is known to be a better candidate than the D2 line for CPT interaction [21,22]. An optical isolator is placed at the output of the diode laser to prevent optical feedback. The laser light is injected into a polarization-maintaining (PM) fibered Mach-Zehnder intensity electro-optic modulator (MZ EOM - Photline NIR-MX800-LN-10). The EOM is actively temperature-controlled. The EOM is driven at 4.596 GHz in order to generate two first-order optical sidebands frequency-split by 9.192 GHz for CPT spectroscopy. The frequency reference that drives the EOM consists of a 10 MHz quartz-crystal oscillator frequency-multiplied to 4.596 GHz by a low noise synthesis chain . A direct digital synthesis (DDS) allows to tune finely the output signal frequency. An active carrier suppression stabilization technique described in  is implemented onto the EOM. At the output of the EOM, the light is sent into a Michelson-based delay-line system to produce a bichromatic optical field that alternates between right and left circular polarization at the clock frequency. This optimized CPT pumping scheme, pioneerly proposed by Jau et al. in 2004 , allows to detect high-contrast resonances on the 0-0 magnetic field insensitive clock transition . The 2-mm diameter output collimated beam is sent through a 1.4 mm-long and 2 mm-diameter Cs vapor microfabricated cell filled with a mixture of Ne and He buffer gases. Different cells (cell 1 to cell 4) coming from the same wafer were measured. The cell can be temperature-stabilized at the mK level in the 70 – 100°C temperature range. The cell is located in a static magnetic field parallel to the laser beam to raise the Zeeman degeneracy and to isolate the clock transition. The cell is protected from external magnetic perturbations with a double layer mu-metal shield. The transmitted optical power through the cell is detected by a low noise photodiode. With the 4.596 GHz signal applied to the EOM, the laser is frequency locked to the position of maximum optical absorption by modulating sinusoidally its current at 60 kHz, followed by demodulation of the absorption signal on the photodiode with a lock-in amplifier (LA1) and finally a servo amplifier that feeds back a correction signal onto the injection current. The 4.596 GHz signal can be locked to the CPT clock resonance by feeding an error-signal voltage to the local oscillator. The clock output frequency is measured by comparison with a reference Cs beam clock.
3. Experimental results
3.1. Basic CPT spectroscopy
Figure 2 reports an example of a CPT resonance detected in a Cs-Ne-He microcell (cell 2) heated at 87°C. The laser power at the input of the cell is 82 μW. The CPT resonance is well-fitted by a lorentzian function with a full-width at half maximum (FWHM) of 7.5 kHz. The contrast of the resonance, defined as the ratio between the height and the dc background is measured to be 9.6 %. Figure 3 shows the contrast of the CPT resonance versus the cell temperature for the cell 2. The laser power at the input of the cell is 55 μW. As observed in [7,18], the CPT signal is increased with atomic density up to a maximum before decreasing because of the increased optical thickness of the atomic vapor . The CPT contrast is found to be maximized for a temperature of about 87°C. A similar optimum temperature for CPT contrast was found for other cells. Figure 4 plots the CPT linewidth and the CPT contrast/FWHM ratio versus the laser power Pl for the cell 2 heated at 87°C. The linewidth-laser power dependence is well fitted by a linear function such as FWHM = 2.8 + 0.05 Pl, in kHz. The ratio contrast/FWHM is found to be maximized for Pl < 35 μW. This indicates that the clock short-term frequency stability should be optimized for low laser power as already observed in numerous references [7,18].
3.2. Frequency shift versus cell temperature
Figures 5(a) to 5(c) show the measured frequency shift of the clock transition (from the unperturbed Cs atom frequency = 9.192631770 GHz) versus the cell temperature for cells 2 to 4. Results for all cells are summarized in Table 1. All tested cells exhibit a turnover temperature point higher than 85°C at which the Cs clock frequency temperature dependence is canceled at the first order. The inversion temperature Ti is measured to be 89.7°C, 89.6°C, 91.0°C and 94.6°C for cells 1 to 4 respectively. The frequency shift at the inversion temperature is measured to be 30260 Hz, 30129 Hz, 31250 Hz and 31072 Hz for cells 1 to 4 respectively. From these data, we estimated the actual total buffer gas pressure in the cells and partial pressures. The temperature dependence of the frequency shift vanishes at the inversion temperature Ti for the pressure ratio a = P 2/P 1 such that :3] for He and in  for Ne were used.
Results from Table 1 show that the homogeneity of the cells contents among the wafer is correct. For information, cells 1 to 3 were close to each other on the wafer whereas cell 4 was placed at the periphery of the wafer. This could explain that contents of the cell 4 is slightly different from other cells. It is important to note that inversion points at higher temperatures could be obtained by increasing the partial pressure of helium in the cell.
For information, we observed in the cell 4 an important drift of the clock resonance frequency of about −2 Hz/hour. This drift is huge compared to drift values generally reported in gas cell clocks . This phenomenon could be explained by losses of He gas or Ne gas from the cell induced by microleaks, imperfect anodic bonding at the silicon-glass interface or gas permeation through the cell windows. If attributed to a loss of He gas, the measured frequency drift would correspond to a change of He pressure of about −1.6 mTorr/hour. We point out that the use as a buffer gas of He, known to be extremely light, volatile and to exhibit high permeation rate through glass [27,28], will require a cell fabrication process and technology that ensures a long-term stable cell inner atmosphere. This stands also for Ne buffer gas. The loss of Ne gas due to its permeation through borofloat glass windows of a micro-fabricated cell was recently found to be the cause of the frequency drift of a microcell-based clock . However, it is important to note that other glass materials, such as alumino-silicate glass (ASG) [30, 31] or pyrex , are expected to be alternative solutions to reduce greatly gas permeation in microfabricated cells. In that sense, researchers from NIST have recently identified and demonstrated successfully the use of a suitable source of ASG, fabricated in wafer form and which can be anodically bonded to silicon, with a permeation rate a factor 100 lower than that of pyrex at 90°C .
We developed Cs vapor microcells filled with Ne-He buffer gas mixture and studied them in a laboratory-prototype CPT clock setup. Basic spectroscopy of the CPT resonance (linewidth, contrast) was reported. We pointed out that this buffer gas mixture allows to cancel the temperature dependence of the Cs clock frequency at temperatures higher than 80°C. The presence of such an inversion temperature point is very important for the development of a CPT atomic clock with high mid and long-term frequency stability. Compared to single Ne buffer gas filled Cs vapor microcells, this buffer gas mixture solution is interesting for miniature atomic clocks operating in high-temperature external environment, as specified in defense systems for example. The observation of a clock frequency drift, potentially due to buffer gas losses out of a cell, was noted and briefly discussed.
E. Kroemer PhD thesis is funded by Thales Avionics. V. Maurice PhD thesis is co-funded by Délégation Générale de l’Armement (DGA) and Région de Franche-Comté. M. Abdel Hafiz PhD thesis is co-funded by Région de Franche-Comté and LabeX FIRST-TF. The authors thank C. Rocher, P. Berthelot and P. Abbé (FEMTO-ST) for their help with experimental stuff.
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