Christoph Affolderbach

A compact laser head with high-frequency stability for Rb atomic clocks and optical instrumentation

We present a compact and frequency-stabilized laser head based on an extended-cavity diode laser. The laser head occupies a volume of 200cm3 and includes frequency stabilization to Doppler-free saturated absorption resonances on the hyperfine components of the Rb87D2 lines at 780 nm, obtained from a simple and compact spectroscopic setup using a 2cm3 vapor cell. The measured frequency stability is ⩽2×10−12 over integration times from 1 s to 1 day and shows the potential to reach 2×10−13 over 102−105 s. Compact laser sources with these performances are of great interest for applications in gas-cell atomic frequency standards, atomic magnetometers, interferometers and other instruments requiring stable and narrow-band optical sources.

Experimental demonstration of a compact and high-performance laser-pumped rubidium gas cell atomic frequency standard

The authors present a compact high-performance laser-pumped Rubidium atomic frequency standard exploiting the advantages of laser optical pumping for improved stability. The clock is based on an industrial Rb clock with the lamp assembly removed and optically pumped by light from a compact frequency-stabilized laser head. The modification of the buffer gas filling in the clock resonance cell reduces instabilities on medium-term timescales arising from the ac Stark effect and temperature variations. The frequency stability of the demonstrator clock was measured to be better than 4/spl times/10/sup -12//spl tau//sup -1/2/ up to 10/sup 4/ s, limited by the local oscillator (LO) quartz and RF loop electronics. Long-term drifts under atmosphere amount to 2-6/spl times/10/sup -13//day only, comparable to or lower than that for lamp-pumped clocks under similar conditions. Typical signal contrasts lie at around 20%, corresponding to a shot-noise limit for the short-term stability of 2/spl times/10/sup -13//spl tau//sup -1/2/. The results clearly demonstrate the feasibility of a laser-pumped Rb clock reaching <1/spl times/10/sup -12//spl tau//sup -1/2/ in a compact device (< 2 L, 2 kg, 20 W), given the optimization of the implemented techniques. Compact high-performance clocks of this kind are of high interest for space applications such as telecommunications, science missions, and future generations of satellite navigation systems [GPS, global orbiting navigation satellite system (GLONASS), European satellite navigation system (GALILEO)].

Microfabricated rubidium vapour cell with a thick glass core for small-scale atomic clock applications

This paper presents a new fabrication method to manufacture alkali reference cells having dimensions larger than standard micromachined cells and smaller than glass-blown ones, for use in compact atomic devices such as vapour-cell atomic clocks or magnetometers. The technology is based on anodic bonding of silicon and relatively thick glass wafers and fills a gap in cell sizes and technologies available up to now: on one side, microfabrication technologies with typical dimensions <= 2 mm and on the other side, classical glass-blowing technologies for typical dimensions of about 6-10 mm or larger. The fabrication process is described for cells containing atomic Rb and spectroscopic measurements (optical absorption spectrum and double resonance) are reported. The analysis of the bonding strength of our cells was performed and shows that the first anodic bonding steps exhibit higher bonding strengths than the later ones. The spectroscopic results show a good quality of the cells. From the double-resonance signals, we predict a clock stability of approximate to 3 x 10(-11) at 1 s of integration time, which compares well to the performance of compact commercial Rb atomic clocks.

Compact microwave cavity for high performance rubidium frequency standards

The design, realization, and characterization of a compact magnetron-type microwave cavity operating with a TE(011)-like mode are presented. The resonator works at the rubidium hyperfine ground-state frequency (i.e., 6.835 GHz) by accommodating a glass cell of 25 mm diameter containing rubidium vapor. Its design analysis demonstrates the limitation of the loop-gap resonator lumped model when targeting such a large cell, thus numerical optimization was done to obtain the required performances. Microwave characterization of the realized prototype confirmed the expected working behavior. Double-resonance and Zeeman spectroscopy performed with this cavity indicated an excellent microwave magnetic field homogeneity: the performance validation of the cavity was done by achieving an excellent short-term clock stability as low as 2.4 × 10(-13) τ(-1/2). The achieved experimental results and the compact design make this resonator suitable for applications in portable atomic high-performance frequency standards for both terrestrial and space applications.

Microfabricated alkali vapor cell with anti-relaxation wall coating

We present a microfabricated alkali vapor cell equipped with an anti-relaxation wall coating. The anti-relaxation coating used is octadecyltrichlorosilane and the cell was sealed by thin-film indium-bonding at a low temperature of 140 °C. The cell body is made of silicon and Pyrex and features a double-chamber design. Depolarizing properties due to liquid Rb droplets are avoided by confining the Rb droplets to one chamber only. Optical and microwave spectroscopy performed on this wall-coated cell are used to evaluate the cells relaxation properties and a potential gas contamination. Double-resonance signals obtained from the cell show an intrinsic linewidth that is significantly lower than the linewidth that would be expected in case the cell had no wall coating but only contained a buffer-gas contamination on the level measured by optical spectroscopy. Combined with further experimental evidence this proves the presence of a working anti-relaxation wall coating in the cell. Such cells are of interest for applications in miniature atomic clocks, magnetometers, and other quantum sensors.

Demonstration of a high-performance pulsed optically pumped Rb clock based on a compact magnetron-type microwave cavity

We demonstrate a high-performance pulsed optically pumped (POP) Rb vapor-cell clock based on a magnetron-type microwave cavity of only 44 cm3 external volume. Using optical detection, an unprecedented 35% contrast of the Ramsey signal has been obtained. Both the signal-to-noise ratio (of 30 000) and the estimated shot-noise limit of 1.7 × 10−14 τ−1/2 are at the same level as those found with a bigger cylindrical TE011 cavity (100 cm3 inner volume) and are sufficient for achieving excellent clock stability. Rabi oscillations are measured and indicate a sufficiently uniform microwave magnetic field distribution inside the cavity. The instability sources for the POP clocks performance are analyzed. A short-term stability of 2.1 × 10−13 τ−1/2 is demonstrated which is consistent with the noise budget.

Low-temperature indium-bonded alkali vapor cell for chip-scale atomic clocks

A low-temperature sealing technique for micro-fabricated alkali vapor cells for chip-scale atomic clock applications is developed and evaluated. A thin-film indium bonding technique was used for sealing the cells at temperatures of ≤140 °C. These sealing temperatures are much lower than those reported for other approaches, and make the technique highly interesting for future micro-fabricated cells, using anti-relaxation wall coatings. Optical and microwave spectroscopy performed on first indium-bonded cells without wall coatings are used to evaluate the cleanliness of the process as well as a potential leak rate of the cells. Both measurements confirm a stable pressure inside the cell and therefore an excellent hermeticity of the indium bonding. The double-resonance measurements performed over several months show an upper limit for the leak rate of 1.5 × 10−13 mbar·l/s. This is in agreement with additional leak-rate measurements using a membrane deflection method on indium-bonded test structures.

Laser-pumped paraffin-coated cell rubidium frequency standard

We have realized and studied a rubidium atomic frequency standard based on a paraffin-coated cell, exhibiting a short-term frequency stability <3 × 10−12 τ−1/2 between τ = 1 and 100 s. Characterization of the wall-coating is performed by measuring the T1 and T2 relaxation times. Perturbations of the medium- to long-term clock stability, due to variations in the laser-intensity, laser frequency, the microwave power shift, and the shifts due to temperature variations are measured and analyzed. A method for reducing the intensity light-shift by detuning the laser frequency and the resulting improvement in clock stability is demonstrated. This work is of relevance for further improvements on Rb cell standards using anti-relaxation wall-coating technology.

Imaging Microwave and DC Magnetic Fields in a Vapor-Cell Rb Atomic Clock

We report on the experimental measurement of the dc and microwave magnetic field distributions inside a recently developed compact magnetron-type microwave cavity mounted inside the physics package of a high-performance vapor-cell atomic frequency standard. Images of the microwave field distribution with sub-100-μm lateral spatial resolution are obtained by pulsed optical-microwave Rabi measurements, using the Rb atoms inside the cell as field probes and detecting with a CCD camera. Asymmetries observed in the microwave field images can be attributed to the precise practical realization of the cavity and the Rb vapor cell. Similar spatially resolved images of the dc magnetic field distribution are obtained by Ramsey-type measurements. The T2 relaxation time in the Rb vapor cell is found to be position dependent and correlates with the gradient of the dc magnetic field. The presented method is highly useful for experimental in situ characterization of dc magnetic fields and resonant microwave structures, for atomic clocks or other atom-based sensors and instrumentation.

Compact high-performance continuous-wave double-resonance rubidium standard with 1.4 × 10 −13 τ −1/2 stability

We present our studies on a compact high-performance continuous wave (CW) double-resonance (DR) rubidium frequency standard in view of future portable applications. Our clock exhibits a short-term stability of 1.4 × 10-13 τ-1/2, consistent with the short-term noise budget for an optimized DR signal. The metrological studies on the medium- to longterm stability of our Rb standard with measured stabilities are presented. The dependence of microwave power shift on light intensity, and the possibility to suppress the microwave power shift is demonstrated. The instabilities arising from the vapor cell geometric effect are evaluated, and are found to act on two different time scales (fast and slow stem effects). The resulting medium- to long-term stability limit is around 5.5 × 10-14. Further required improvements, particularly focusing on medium- to long-term clock performance, are discussed.