Our group is involved in the study and development of Cs vapor cell atomic clocks based on coherent population trapping (CPT). This domain involves laser spectroscopy, atomic physics, low phase noise microwave generation, MEMS-based devices (microfabricated alkali vapor cells, MEMS-based oscillators), time and frequency metrology.
Our activities are divided into two main sections:
- Miniature Cs vapor cell atomic clocks: With the help of microfabrication techniques, VCSEL lasers and integrated electronics, we aim to develop miniature atomic clocks combining a low power consumption (100-150 mW), a small volume (15 cm3) and a fractional frequency stability better than 10-11 at 1 hour and 1 day integration (1 μs per day). Such battery-powerable “pocket-size” atomic time keepers could be an alternative to quartz-crystal oscillators in numerous applications including satellite-based navigations systems on-earth receivers, synchronization of networks and telecommunication systems, defense systems, etc...These activites are performed in strong collaboration with FEMTO-ST MOEMS Group (Leader: C. Gorecki) [http://projects.femto-st.fr/MOEMS-Group/fr].
- High-performance compact Cs vapor cell atomic clocks: We aim to explore the ultimate performances that can be achieved using the CPT technique for atomic clocks applications. Typical objectives are a fractional frequency stability of a few 10-13 τ-1/2 for averaging times up to 10 000 s. These performances are 10-50 times better than commercial Rb clocks and could be competitive with passive hydrogen masers. Our laboratory-prototype clock exhibits currently a short-term frequency stability at the level of 2 10-13 τ-1/2 up to 100 s. Efforts are under progress to improve the clock mid and long-term frequency stability.
A/ Basics on Coherent Population Trapping-based vapor cell clocks
In CPT physics, atoms generally in a vapor cell, are trapped through a destructive quantum interference process into a non-interacting coherent superposition of two long-lived ground state hyperfine levels. Under ideal conditions (isolated three-level scheme, no ground-state decoherence), a so-called dark state, obtained by simultaneous action of two resonant optical fields exactly frequency-split by the atomic ground-state hyperfine frequency (Raman resonance), is fully decoupled from the excited state. In this particular state is observed, through electromagnetically induced transparency (EIT), a narrow peak in optical transmission. The CPT resonance linewidth is ultimately limited by the microwave coherence relaxation time and can be measured to be of a few tens to hundreds of Hz.
Figure 1: CPT interaction in Cs atom represented by a 3-energy level scheme interacting with a bi-chromatic optical field. Both optical lines are frequency-split by 9.192 GHz (ground-state splitting), the CPT Cs clock transition. When the frequency difference between both optical lines exactly equals 9.192 GHz, atoms are optically-pumped in a coherent superposition of both ground states. In this so-called dark state, the transparency of the atomic vapor is increased, as shown on the right inset. The CPT resonance, with narrow linewidth, can be used for the development of an atomic clock. In a CPT clock, the microwave interrogation signal is directly optically carried, preventing the use of a resonant cavity.
B/ Miniature Atomic clocks
We started studies on CPT-based miniature atomic clocks in FEMTO-ST in 2005 thanks to fundings from CNES, Région de Franche-Comté and ANR. From 2008 to 2012, we piloted (coordinator: C. Gorecki, FEMTO-ST) a challenging European project called MAC-TFC (EC funding), combining the expertise of 10 industrial and academic partners (www.mac-tfc.eu). This project led to the demonstration of a first European miniature atomic clock prototype. Main highlights and scientific achievements of this project were the development of custom-designed 894 nm (Cs D1 line) VCSEL diode lasers (Ulm University), the demonstration of 15-mW and low phase noise 4.596 GHz ASIC synthesizer devoted to be used as local oscillator (EPFL-IMT), and the development of an original Cs vapor microcell technology (FEMTO-ST/SAES/Wroclaw University). This stimulating and successful project led in France to the beginning of new projects (fundings by DGA, LabeX FIRST-TF, ANR, BPI). An industrial platform is now installed to push the industrial transfer of the Cs vapor microcell technology and to achieve at horizon 2020 a novel commercial miniature atomic clock product.
Basic blocks of a miniature atomic clock (MAC) are shown below on figure 2:
Figure 2: Schematic of CPT-based miniature Cs vapor cell atomic clock
The heart of a MAC is a microfabricated cell that contains alkali vapor, generally diluted with a pressure of buffer gas in order to slow down alkali atoms. Slight collisions between alkali atoms and buffer gas allow to increase the time for the atoms to reach the cell walls. This allows the detection of narrow CPT resonances (a few kHz linewidth in mm-scale cell dimensions). The cell is typically heated at 80-95°C. A static magnetic field, parallel to the laser beam propagation direction, is applied to raise the Zeeman degeneracy. Magnetic shieldings are used to prevent perturbations from the environment. Atoms in the cell interact with a laser beam coming from a VCSEL diode laser whose injection current is directly modulated at 4.596 GHz by a local oscillator to produce two first-order optical sidebands frequency-split by 9.192 GHz for CPT interaction. The laser beam is shaped, routed and polarized circularly with a quarter-wave plate. The light transmitted through the cell is detected by a photodiode. The output signal of the photodiode is used both for laser frequency stabilization and local oscillator frequency stabilization using lockin amplifiers and standard synchronous modulation-demodulation techniques.
B.1/ Microcell technologies
We have developed in FEMTO-ST (MOEMS Group) an original Cs vapor microcell technology [1,2,3,4,14]. This basically consists of a glass-silicon-glass sandwich with two Si-cavities etched by deep reactive ion etching (DRIE) and anodically-bonded glass wafers on each face of the Si wafer. Both cavities (dispenser cavity and CPT cavity) are connected through filtration channels. The originality of this cell technology is that the Cs vapor is generated after complete sealing of the cell by local laser activation of a Cs pill dispenser placed in a cavity neighbor to the CPT cavity. Figure 3 shows a photograph of Cs microfabricated cells developed in FEMTO-ST. This cell technology is called T-cell.
Figure 3: Photographs of Cs vapor microfabricated cells. The Cs dispenser is visible in the square cavity. Courtesy from FEMTO-ST MOEMS Group.
Figure 4: Example of a CPT resonance detected in a Cs-Ne microfabricated cell.
We have recently reported a method for filling alkali vapor cells with cesium from a dispensing paste, compatible with collective deposition processes. This filling method could reduce the cost and the complexity of miniature cells fabrication. Optical linear spectroscopy was performed over one year on such cells, showing a stable atomic density. One of these cells has been implemented in a lab-prototype CPT clock. A fractional frequency aging rate of about –4.4 10-12 per day has been observed, compliant with typical objectives of MACs .
We have developed in FEMTO-ST (MOEMS Group) another original architecture of microfabricated alkali vapor cell, named R-cell, designed for miniature atomic clocks. The cell combines diffraction gratings with anisotropically etched single-crystalline silicon sidewalls to route a normally-incident beam in a cavity oriented along the substrate plane .
Figure 5: Photograph of the R-cell architecture. Courtesy from FEMTO-ST MOEMS Group.
B.3/ MEMS-based oscillators
In general, the local oscillator in a MAC is a voltage controlled oscillator (VCO) phase locked to a 10 MHz quartz crystal oscillator using a fractional phase locked loop (PLL). Different technologies could represent an alternative to be used as local oscillators in miniature atomic clocks. In this domain, we’re interested in the study, development and characterization of MEMS-based oscillators using bulk acoustic wave (BAW) resonators such as solid mounted resonators (SMR) or high-overtone bulk acoustic wave resonators (HBAR). Such resonators exhibit high Q-f factors and low phase noise performances, miniaturized dimensions and low power consumption. They could be used to probe directly at microwave frequencies (4.596 GHz for Cs atom) atomic transitions.
We demonstrated in 2011, in collaboration with IEMN and CEA-LETI, the development and the use of a SMR-based oscillator for miniature atomic clocks applications .
In 2014-2015, we've developed and performed the metrological characterization of 2.3 GHz oscillators based on high-overtone bulk acoustic wave resonators (HBARs). The residual phase noise of HBARs has been measured. A HBAR oscillator has been used as a local oscillator in a microcell-based atomic clock based on CPT [23,27].
Figure 6: Photograph of a AlN-Sapphire HBAR resonator (designed by CEA-LETI).
Figure 7: Phase noise performances of a 2.3 GHz HBAR oscillator (in red). Performances compared to those of a conventional MAC LO or to state-of-the-art ideally frequency-multiplied 100 MHz OCXO.
VCSEL laser sources combine a low threshold current, low power consumption and high-modulation bandwidth capabilities and are well-adapted to be used in miniature atomic clocks. In the frame of the MAC-TFC project, in collaboration with Ulm University and LTF-UNINE, we reported the metrological characterization of custom-designed 894nm VCSELs . We have also investigated the chacterization of commercially-available 894 nm VCSELs .
B.5/ Laboratory-prototype clocks
We developed, in collaboration with MAC-TFC partners, a microcell-based compact atomic clock as shown on figures below.
Figure 8: Picture of the MAC-TFC clock physics package.
Figure 9: Picture of the MAC-TFC clock. The physics package is connected to electronics to operate the clock. Electronics were developed by EPFL-IMT.
We have recently developed a novel MAC prototype, based on a Cs vapor microcell from FEMTO-ST. The physics package, as well as electronics of the clock has been developed by FEMTO-ST (J. Rutkowski, V. Maurice, C. Rocher). This prototype could be used in a near future in lab works in ENSMM school or EFTS seminar.
Figure 10: Miniature clock prototype. Courtesy of J. Rutkowski and V. Maurice.
C/ High-performance Cs cell CPT atomic clock
We have started in 2011, in collaboration with LNE-SYRTE (S. Guérandel and E. de Clercq), the development of a high-performance CPT-based Cs cell atomic clock. Funded in the beginning by LNE, these activities have been pursued from 2013 to 2016 in the frame of a European project called MClocks, funded by EURAMET, and piloted by INRIM (coordinator: S. Micalizio). This project is now supported since 2017 by LabeX FIRST-TF.
C.1/ A Cs cell atomic clock based on push-pull optical pumping
We've developed a high-performance Cs cell microwave CPT clock based on the push-pull optical pumping (PPOP) technique, pioneerly proposed by Y. Y. Jau et al. in 2004 (Happer's group, Princeton, Phys. Rev. Lett. 93, 16, 2004). This technique consists to make the atoms interact with a bichromatic optical field that alternates between right and left circular polarization. We demonstrated in Cs vapor cells the possibility to detect high-contrast CPT resonances with this technique. This optimized pumping scheme can be combined with a pulsed Ramsey-like interrogation technique to allow the detection of high-contrast Ramsey-CPT fringes.
Our laboratory-prototype clock exhibits currently short-term frequency stability performances at the level of 2 10-13τ-1/2 up to 100 s [22,25,29]. These performances are among the best performances ever reported for a CPT-based clock.
Figure 11: Schematic of the CPT clock based on PPOP
Figure 12: Example of CPT-Ramsey fringes
C.2/ Low phase noise microwave frequency synthesizers
We have developed, in collaboration with INRIM (C. Calosso/S. Micalizio) and LNE-SYRTE, ultra-low phase noise microwave frequency synthesizers at 4.596 GHz or 9.192 GHz dedicated to be used as local oscillators in high performance Cs (or Rb) vapor cell atomic clocks [17,21]. Such microwave sources allow to reduce greatly the Dick effect contribution (~ 3 10-14 at 1 s) to the short-term frequency stability of the atomic frequency standards.
Figure 13: Photograph of a microwave frequency synthesizer developed by FEMTO-ST / INRIM [B. Francois, C. Calosso, R. Boudot].
Figure 14: Absolute phase noise performances of the microwave frequency synthesizer shown above. The pilot is a low noise 100 MHz OCXO (perfs in red at 100 MHz). The green data shows the absolute phase noise at 4.596 GHz. The green line is the phase noise at 4.596 GHz of the ideally frequency-multiplied 100 MHz OCXO.
C.3/ Dual-frequency sub-Doppler spectroscopy
Saturated absorption technique is an elegant method widely-used in atomic and molecular physics, for high-resolution spectroscopy, laser frequency standards and metrology purposes. We have recently discovered that a saturated absorption scheme used with a dual-frequency laser can lead to a significant sign-reversal of the usual Doppler-free dip, yielding a deep enhanced-absorption spike . We reported detailed experimental investigations of this phenomenon , together with a full in-depth theoretical description, thanks to the precious help of D. Brazhnikov (ILP, Novossibirsk) and E. de Clercq (LNE-SYRTE). It is shown that several physical effects can contribute or oppose to the formation of the high-contrast central spike in the absorption profile. These effects are the creation or destruction of dark states in the ground state and velocity selective optical pumping effects. Beyond their theoretical interest, results obtained in this manuscript are of great interest for laser spectroscopy, laser frequency stabilization purpose, with applications in laser cooling, matter-wave sensors, atomic clocks or quantum optics.
Figure 15: Dual-frequency sub-Doppler spectroscopy setup.
The figure below reports high-contrast sub-Doppler absorption spikes in a hot atomic vapor cell exposed to a dual-frequency laser field.
Figure 16: High-contrast sub-Doppler absorption spikes in a hot atomic vapor cell exposed to a dual-frequency laser field.
This setup has allowed a significant improvement (one order of magnitude) of laser frequency stabilization, compared to a conventional single-frequency saturated absorption scheme .
C.4/ Wall coated Cs vapor cells.
We have reported the realization and characterization using coherent population trapping (CPT) spectroscopy of an octadecyltrichlorosilane (OTS)-coated centimeter-scale Cs vapor cell . The dual-structure of the resonance lineshape, with presence of a narrow structure line at the top of a Doppler-broadened structure, was clearly observed. The linewidth of the narrow resonance was compared to the linewidth of an evacuated Cs cell and of a buffer gas Cs cell of similar size. The Cs-OTS adsorption energy was measured to be (0.42+/-0.03) eV, leading to a clock frequency shift rate of 2.710-9/K in fractional unit. A hyperfine population lifetime, T1, and a microwave coherence lifetime, T2, of 1.6 and 0.5 ms were reported, corresponding to about 37 and 12 useful bounces, respectively.
Figure 17: Photograph an OTS-coated cell.
Figure 18: CPT resonance in a Cs-OTS cell (compared to a pure Cs cell of similar dimensions).
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