Miniature Atomic Clock Research Articles

Pulsed Laser Links Simulation for Small Satellite Clock Model Estimation

The expansion of satellite constellation applications brings attention to the need for responsive, reliable satellite communication. For preflight technology assessment of missions, a simulation of two spacecraft in low Earth orbit has been created to estimate clock synchronization and precision orbit determination based on measured instrumentation performance. A novel MATLAB-based numerical simulator was developed to model spacecraft-to-spacecraft laser time-transfer and estimate the offset between the spacecraft clocks over time. This simulation includes timing errors associated with laser pulse detection, as well as non-Gaussian clock drift models. Two on-board clocks are modeled: a cesium-based chip-scale atomic clock and a rubidium-based miniature atomic clock. The positions and velocities of the spacecraft at a reference epoch and the constant coefficients of a polynomial clock model are estimated. Results compare the estimated clock model of a mission operation that only uses GPS measurements and one that uses both GPS and laser pulse time-of-flight measurements between spacecraft referenced to their on-board clocks. Including lasing measurements reduces the root-mean-square clock model error to approximately 80% of the RMS of the cases with only GPS measurements. This simulation tool can be used to optimize the lasing operations schedule based on mission timing performance objectives.

VCSEL quick fabrication of 894.6 nm wavelength epi‐material for miniature atomic clock applications

A vertical cavity surface emitting laser (VCSEL) quick fabrication (VQF) process is applied to epitaxial materials designed for miniature atomic clock applications (MACs). The process is used to assess material quality and uniformity of a full 100 mm (4-inch) wafer against the stringent target specification of VCSELs for MACs. Target specifications in optical power (>0.6 mW) and differential efficiencies (<0.5 W/A) are achieved over large portions of a wafer; however, the variation in the oxide aperture diameter is shown to limit the yield. The emission of the fundamental mode at 894.6 nm at 70 ℃ $^{\circ}\mathrm{C}$ is met over a significant area of the wafer for ∼4 μ $\mu $ m aperture multi-mode devices. The consideration of the on-wafer variation reveals that further optimisation is required to increase the device yield to levels required for volume manufacture.

Alkali Vapor MEMS Cells Technology toward High-Vacuum Self-Pumping MEMS Cell for Atomic Spectroscopy.

The high-vacuum self-pumping MEMS cell for atomic spectroscopy presented here is the result of the technological achievements of the author and the research group in which he works. A high-temperature anodic bonding process in vacuum or buffer gas atmosphere and the influence of the process on the inner gas composition inside a MEMS structure were studied. A laser-induced alkali vapor introduction method from solid-state pill-like dispenser is presented as well. The technologies mentioned above are groundbreaking achievements that have allowed the building of the first European miniature atomic clock, and they are the basis for other solutions, including high-vacuum optical MEMS. Following description of the key technologies, high-vacuum self-pumping MEMS cell construction and preliminary measurement results are reported. This unique solution makes it possible to achieve a 10−6 Torr vacuum level inside the cell in the presence of saturated rubidium vapor, paving the way to building a new class of optical reference cells for atomic spectroscopy. Because the level of vacuum is high enough, experiments with cold atoms are potentially feasible.

Microfabricated vapor cells filled with a cesium dispensing paste for miniature atomic clocks

A method for filling alkali vapor cells with cesium from a dispensing paste is proposed and its compliance with miniature atomic clock applications is evaluated. The paste is an organic-inorganic composition of cesium molybdate, zirconium-aluminum powder, and a hybrid organic-inorganic binder. It is compatible with collective deposition processes such as micro-drop dispensing, which can be done under ambient atmosphere at the wafer-level. After deposition and sealing by anodic bonding, cesium is released from the consolidated paste through local heating with a high power laser. Linear absorption signals have been observed over one year in several cells, showing a stable atomic density. For further validation of this technology for clock applications, one cell has been implemented in a coherent population trapping clock setup to monitor its frequency stability. A fractional frequency aging rate around –4.4 × 10−12 per day has been observed, which is compliant with a clock frequency instability below 1 × 10−11 at one day integration time. This filling method can drastically reduce the cost and the complexity of alkali vapor cell fabrication.

Characterization of commercially available vertical-cavity surface-emitting lasers tuned on Cs D1 line at 894.6 nm for miniature atomic clocks.

We report on the metrological characterization of novel commercially available 894.6nm vertical-cavity surface-emitting lasers (VCSELs), dedicated to Cs D1 line spectroscopy experiments. The thermal behavior of the VCSELs is reported, highlighting the existence of a minimum threshold current and maximum output power in the 55°C-60°C range. The laser relative intensity noise, measured to be -108 dB/Hz at 10Hz Fourier frequency f for a laser power of 25μW, is reduced with increased power. The VCSELs frequency noise is 108 Hz2/Hz at f=100 Hz. The spectral linewidth of the VCSELs is about 30MHz. VCSELs injection current can be directly modulated at 4.596GHz with microwave power in the range of -10 to +0 dBm to generate optical sidebands. A VCSEL was used in a microcell-based Cs atomic clock based on coherent population trapping. A preliminary clock short-term fractional frequency stability of 8×10-11τ-1/2 up to about 100s is reported, demonstrating the suitability of these VCSELs for miniature atomic clock applications.

An Ultra-Miniature Cell-Type Rb Atomic Clock Based on a Novel Waveguide Cavity

We introduce a novel waveguide cavity for ultra-miniature cell-type Rb atomic clock. In the cavity, a coupling ring shaped to be a semi-circle imports an rf signal from electronics, a screw regulator acts as a medium coupler to transmit the microwave signal into the absorption cell, and both the parts serve as a filter to suppress useless components except 6.8 GHz. Furthermore, the waveguide cavity can be used to design a miniature Rb atomic clock, and spread it to a chip-scale atomic clock. We have completed the design of the smallest cell-type Rb atomic clock in the world based on this waveguide cavity. It has not only a small size but also obviously better key performances than those of other miniature Rb atomic clocks. This ultra-miniature cell-type Rb atomic clock aging is 4 × 10−13/d, which is obviously slower than that of other miniature Rb atomic clocks.

The Microloop-Gap Resonator: A Novel Miniaturized Microwave Cavity for Double-Resonance Rubidium Atomic Clocks

Nowadays mobile and battery-powered applications push the need for radically miniaturized and low-power frequency standards that surpass the stability achievable with quartz oscillators. For the miniaturization of double-resonance rubidium ( <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">87</sup> Rb) atomic clocks, the size reduction of the microwave cavity or resonator (MWR) to well below the wavelength of the atomic transition (6.835 GHz for <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">87</sup> Rb) is of high interest. Here, we present a novel miniaturized MWR, the μ-LGR, for use in a miniature DR atomic clock and designed to apply a well-defined microwave field to a microfabricated Rb cell that provides the reference signal for the clock. This μ-LGR consists of a loop-gap resonator-based cavity with very compact dimensions (<;0.9 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ). The μ-LGR meets the requirements of the application and its fabrication and assembly can be performed using repeatable and low-cost techniques. The concept of the proposed device was proven through simulations, and prototypes were successfully tested. Experimental spectroscopic evaluation shows that the μ-LGR is well-suited for use in an atomic clock. In particular, a clock short-term stability of 7 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-12</sup> τ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1/2</sup> was measured, which is better than for other clocks using microfabricated cells and competitive with stabilities of compact Rb clocks using conventional glass-blown cells.

Shrinking Atomic Clocks

Physics Atomic clocks housed in national laboratories around the world provide a precise time signal on which our present high-technology lives depend. Precise synchronization allows for colossal amounts of data to be reliably transferred around networks of optic fibers at ever-increasing rates, as well as providing accurate navigation systems. With increasing demand for mobile devices, there is a requirement for these atom-based time pieces to shrink. Present chip-scale atomic-clock (CSAC) devices based on alkali atoms in a buffer gas are lightweight (<50 g) and consume little power (<150 mW) but tend to show frequency drift over time that requires frequent recalibration. Clocks based on trapped single atoms or ions should be more stable because the atoms are vacuum-packed and do not interact. However, these tend to be relatively large and power-hungry. Jau et al. have developed a miniature atomic clock based on trapped Yb ions that has size and power requirements similar to those of existing CSAC technology but also offers to match the long-term stability expected of the much larger trapping systems. Appl. Phys. Lett. 101 , 253518 (2012).

In situ dissolution or deposition of Ytterbium (Yb) metal in microhotplate wells for a miniaturized atomic clock

Current atomic clocks are burdened by size, weight, power and portability limitations to satisfy a broad range of potential applications. One critical need in the fabrication of a miniaturized atomic clock is small, low-power metallic sources. Exploiting the relatively high vapor pressure of ytterbium (Yb) and its dissolution in anhydrous ammonia, we report two independent techniques for depositing Yb inside a well micromachined into a microhotplate. Subsequent in situ evaporation of Yb from the microhotplate well serves as a low-power metallic source suitable for atomic clocks. The deposition and evaporation of Yb were confirmed using a variety of physicochemical techniques including quartz crystal microbalance, scanning electron microscopy, energy dispersive X-ray spectroscopy, and laser fluorescence. We also describe the fabrication of the microhotplate device, an integral component of our Yb-based miniature atomic clock. The Yb deposition/evaporation on a microhotplate well is thus useful as a low power Yb source during the fabrication of a miniaturized atomic clock, and this technique could be used for other applications requiring a vapor of a metal that has a moderate vapor pressure.

AC Stark-shift in CPT-based Cs miniature atomic clocks

We report on studies on the light-shift in caesium miniature atomic clocks based on coherent population trapping (CPT) using a micro-fabricated buffer-gas cell (MEMS cell). The CPT signal is observed on the Cs D1-line by coupling the two hyperfine ground-state Zeeman sublevels involved in the clock transition to a common excited state, using two coherent electromagnetic fields. These light fields are created with a distributed feedback laser and an electro-optical modulator. We study the light-shift phenomena at different cell temperatures and laser wavelengths around 894.6 nm. By adjusting the cell temperature, conditions are identified where a miniature CPT atomic clock can be operated with simultaneously low temperature coefficient and suppressed light-shift. The impact of the light-shift on the clock frequency stability is evaluated. These results are relevant for improving the long-term frequency stability of CPT-based Cs vapour-cell clocks.

Thermal Characterization of an LTCC Module for Miniature Atomic Clock Packaging

This paper presents the design and thermal characterization of a 15 × 22 mm2 LTCC module dedicated to the packaging of the elements of a miniature atomic clock. This module acts as a carrier for the components of the clock as well as temperature controller; this is particularly attractive since each component of the atomic clock requires precise and well defined working temperatures. For the thermal characterization of the designed module, various thermal simulations have been performed, by finite-element modelling using the software ANSYS; in each simulation a different experimental heating configuration was hypothesized, in order to determine the power required for achieving the desired temperature in the ideal case (module in perfect vacuum with no losses rather than conduction towards the external “cold” zone, through two small bridges) , and also for estimating the thermal losses that convection introduces into the system. The simulations were then experimentally validated by realizing and characterizing the different configurations, yielding results that were consistent with the simulations. The best experimental configuration was found, in which the heating performance was fairly close to the ideal one. The results show that LTCC is a very attractive technology for atomic clock packaging also in terms of dissipated power, provided the system is efficiently insulated.

Effects of Thermal Losses on the Heating of a Multifunctional LTCC Module for Atomic Clock Packaging

An innovative multifunctional LTCC module has been designed for miniature atomic clock packaging. Efficient packaging and interconnection of the atomic clock packaging is a critical issue and a precise temperature control is required for some components, such as mini-cell and light source. The great advantage of using LTCC technology for this application is that it allows the integration of different functions, such as heaters and PTCs resistors for temperature measurement and control, and optionally other active elements. In this research, a platform for measuring the thermal conductivity of materials has been developed in order to perform precise thermal studies on the packaging. The relationship between achieved temperature and power dissipated for the heating of the LTCC module has been calculated in different experimental configurations, in order to determine the effects of conduction and convection on the heating and estimate the thermal losses that they introduce into the system.

Study of laser-pumped double-resonance clock signals using a microfabricated cell

We present our microwave spectroscopic studies on laser–microwave double-resonance (DR) signals obtained from a micro-fabricated Rb vapor cell. This study focuses on the characteristics and systematic shifts of the ground-state ‘clock transition’ in 87Rb (| Fg = 1,mF = 0⟩ → |Fg = 2, mF = 0⟩) used in Rb atomic clocks, and represents a first step toward a miniature atomic clock based on the DR scheme. A short-term clock instability below 2 × 1011τ−1/2 is demonstrated, staying below 10−11 up to τ = 104 s.

Polarization Control and Dynamic Properties of VCSELs for MEMS Atomic Clock Applications

We report the polarization stability and small-signal characteristics of single-polarization single-mode vertical-cavity surface-emitting lasers (VCSELs) emitting at 894.6-nm wavelength for Cs-based miniature atomic clock applications. Polarization control is achieved by integration of a semiconducting surface grating in the top Bragg mirror. The VCSELs are polarized orthogonal to the grating lines with a peak-to-peak difference between the dominant and the suppressed polarization modes reaching 29 dB even at substrate temperatures up to 65 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$^{\circ}\hbox{C}$</tex> </formula> . A modulation bandwidth of more than 5 GHz is reached at only 0.5-mA bias. Modulation current efficiency factors larger than 12 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$\hbox{GHz}/\sqrt{\rm mA}$</tex></formula> are achieved. Moreover, the intrinsic modulation characteristics of the VCSELs are investigated. A <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$K$</tex></formula> -factor of less than 0.30 ns and a maximum 3-dB bandwidth exceeding 30 GHz are obtained.

Coherent-population-trapping resonances with linearly polarized light for all-optical miniature atomic clocks

We present a joint theoretical and experimental characterization of the coherent population trapping (CPT) resonance excited on the D1 line of 87Rb atoms by bichromatic linearly polarized laser light. We observe high-contrast transmission resonances (up to 25%), which makes this excitation scheme promising for miniature all-optical atomic clock applications. We also demonstrate cancellation of the first-order light shift by proper choice of the frequencies and relative intensities of the two laser field components. Our theoretical predictions are in good agreement with the experimental results.

Measuring Transistor Large-Signal Noise Figure for Low-Power and Low Phase-Noise Oscillator Design

This paper presents an experimental method for determining additive phase noise of an unmatched transistor in a stable 50-Omega environment. The measured single-sideband phase noise is used to determine the large-signal noise figure of the device. From the Leeson-Cutler formula and a known oscillator circuit with the characterized transistor, the phase noise of the oscillator can be predicted. The method is applied to characterization of several bipolar devices around 3.4 GHz, the frequency of interest for miniature rubidium-based atomic clock voltage-controlled oscillators.