Chip-scale Atomic Clocks Research Articles

Dynamic Continuous-Wave Spectroscopy of Coherent Population Trapping at Phase-Jump Modulation

A method of dynamic continuous-wave spectroscopy of coherent population trapping (CPT) resonances using phase modulation of the jump type is developed. The time evolution of the spectroscopic signal is investigated. A method for the formation of an error signal for frequency stabilization is proposed. We show that our approach has a reduced sensitivity to the lineshape asymmetry of the CPT resonance. The experimental results are in good qualitative agreement with theoretical predictions based on a mathematical model of a three-level $\Lambda$ system in a bichromatic field. This method can be used in atomic frequency standards (including chip-scale atomic clocks).

Reducing frequency drift caused by light shift in coherent population trapping-based low-power atomic clocks

We propose a model to reduce the influence that the light-shift induced frequency drift has on the long-term stability of coherent population trapping (CPT)-based low-power atomic clocks. We experimentally validated the proposed model using the chip-scale atomic clock architecture. The model considers both the drift of the dc-bias current to compensate for the wavelength aging of the vertical-cavity surface-emitting laser (VCSEL) and the variation in the modulation index of the light field generated by the VCSEL. We investigated the dc-bias-current-dependency of the frequency drift in an atomic clock module both theoretically and experimentally, confirming that the frequency drift can change as a function of the VCSEL basic parameters. When the atomic clock module was operated over a period of five months at the dc-bias current affording a zero-crossing of the clock frequency drift, the Allan standard deviation and clock frequency drift for long-term stability were 8.0×10−12 for averaging times of 4×106 s and 2.4×10−13/day, respectively. Our approach is promising to improve the long-term stability of CPT-based low-power atomic clocks.

Distributed multi‐GNSS timing and localization for nanosatellites

Recent advances in distribution and miniaturization among spacecraft and expansion of global navigation satellite systems (GNSS) motivate the Distributed Multi-GNSS Timing and Localization system. The DiGiTaL system is intended to provide nanosatellite swarms with unprecedented centimeter-level relative navigation accuracy in real time and nanosecond-level time synchronization through the integration of a multiconstellation GNSS receiver, a chipscale atomic clock, and an intersatellite link. This paper describes DiGiTaL's hardware and software design, architecture, and navigation software in detail. The swarming spacecraft are grouped into subsets, where differential GNSS is performed by a precise orbit determination module with integer ambiguity resolution in real time. The resulting precise orbits are then exchanged and fused by a swarm determination module, creating full-swarm knowledge. Finally, the DiGiTaL architecture is integrated into CubeSat avionics and tested with key hardware in the loop using Stanford's GNSS navigation testbed. For the first time, this paper shows the capability to perform centimeter-level precise relative navigation using commercial-off-the-shelf CubeSat hardware for spacecraft swarms.

Sealing of MEMS Atomic Vapor Cells Using Cu-Cu Thermocompression Bonding

MEMS atomic vapor cells have a large variety of applications in chip-scale atomic devices such as clocks, gyroscopes or magnetometers. These cells are usually hermetically sealed by anodic bonding of borosilicate to silicon, a proven and reliable method, yet showing specific limitations such as inherent oxygen contamination of the cavity or limitation to a few combinations of materials. As an alternative, we report on the successful wafer-level fabrication of cells sealed by Cu-Cu thermocompression bonding. This alternative technique allows to overcome the constraints of anodic bonding while allowing a new variety of other materials. Characterization of the bonding quality is presented as well as a spectroscopic characterization in view of use for chip-scale atomic clocks (CSACs).

ULPAC: A Miniaturized Ultralow-Power Atomic Clock

This article introduces a chip-scale ultralow-power atomic clock (ULPAC) in the microwave frequency region. A new suspended quantum package architecture along with a fully integrated frequency probing and locking loop implemented in CMOS technology results in a compact package and ultralow-power consumption. In addition, dedicated low-noise magnetic field and temperature control loops are incorporated for isolating and mitigating the internal and external factors affecting the frequency stability. The output frequency of a crystal oscillator is continuously compensated and stabilized by locking to the peak of a coherent population trapping signal, thereby inheriting the superior frequency stability of the atomic resonance. The proposed ULPAC system achieves a frequency stability of $2.2 \times 10^{-12}$ at an averaging time of 105 s, while consuming 59.9 mW. The frequency probing and locking circuits implemented in a standard 65-nm CMOS process node occupy an area of 2.55 mm2. The prototype of this atomic clock occupies a volume of 15 cm3.

Underwater Acoustic Modems with Synchronous Chip-Scale Atomic Clocks for Scalable Tasks of AUV Underwater Positioning

Accurate time synchronization of the nodes of digital underwater acoustic (UWA) networks is necessary for the effective use of information obtained from various kinds of underwater sensors over a vast water area. The sensors of autonomous underwater vehicles (AUV) are widely used to address new challenges. One of them is time synchronization of sensors on mobile carriers, as well as synchronization of the clocks on mobile nodes of UWA networks, for example, teams of AUVs that perform coordinated and/or cooperated operations. The paper presents theresults of the experiments on using UWA modems for AUV positioning, namely, UWA modems with chip-scale atomic clocks (CSAC) that allow accurate measurements of delays in propagation of UWA signals and, thus, accurate estimation of ranges to their georeferenced sources. In addition, the accuracy of CSAC operation in various situations as well as positioning accuracy of AUVs having UWA modems with integrated CSACs are analyzed based on the experimental results. Also given are practical recommendations on how to “discipline” (steer) CSACs and provide their phase synchronization with a source of timekeeping signals.

An all-sapphire Cs gas cell for a chip-scale atomic clock

We realized a Cs gas cell for a chip-scale atomic clock fabricated from single-crystal sapphire chips, which have extremely-low gas penetration coefficients. The coherent population trapping (CPT) resonance spectrum was evaluated to validate the developed sapphire Cs gas cell. A CPT resonance with a contrast of 1.5%, a frequency shift (from the Cs hyperfine transition frequency) of 55.9 kHz, and a full width at half maximum of 2.2 kHz was obtained at an incident light intensity of 40 μW cm−2. The frequency shift was mainly caused by the collision between Ne sealed in the gas cell and Cs atoms. From the experimental results, the pressure of Ne was estimated to be 10.7 kPa.

Research on a chip scale atomic clock aided vector tracking loop

With the development of Micro-Electro-Mechanical-System (MEMS) technology, size and accuracy of Chip Scale Atomic Clock (CSAC) gradually improve, and a commercial CSAC products available, which makes it possible to integrate a CSAC to a handheld GNSS receiver. Published results have demonstrated that CSAC could better the Scalar Tracking Loops (STL) navigation results. However, the Vector Tracking Loop (VTL) has a different architecture compared with STL, which allows it meaningful to investigate CSAC driven VTL. In this study, two problems are discussed in a classic VTL employing CSAC as frequency reference: (i) how much improvement in navigation solutions initialed by substituting TXCO with CASC; (ii) whether the CSAC driven VTL can work excluding clock bias or clock drift as state variables, in other words, whether the VTL can provide navigation solutions at a moderate accuracy with only three satellites in view. Two comparisons have been done in this investigation and demonstrated that CSAC could improve the VTL navigation solutions, and the clock drift variable could be removed from the VTL model with no influence on the VTL performance.

A reflection-type vapor cell using anisotropic etching of silicon for micro atomic clocks

This paper reports the design, fabrication and evaluation of a reflection-type optical Rb vapor cell for chip-scale micro atomic clocks. To reduce the physical package height, the reflection-type vapor cell is developed, in which optical components can be mounted on one side of the vapor cell. A (100)-oriented Si wafer with a cut-off 9.74° toward [011] direction is used to make 45° mirrors by anisotropic wet etching. 90° mirrors are fabricated by Si deep reactive ion etching and surface planarization using H2 annealing. Following the detection of D1 optical absorption for Rb atoms, coherent population trapping resonance was observed.

Iodine Frequency Reference on a Sounding Rocket

Frequency-stable laser systems are a key technology in precision experiments and have recently become applicable in space. Optical frequency references based on cavities, atoms, or molecules allow for precise laser frequency stabilization and thus enable, for example, high-precision laser ranging in space missions. Examples are the gravity recovery and climate experiment-follow-on (GRACEFO) mission [1,2] that was recently put in operation, the planned laser interferometer gravitational wave observatory LISA [3], and future global satellite navigation systems based on optical clocks [4,5]. In the context of these and other missions, frequency references based on optical cavities [6,7] and unequal-arm-length interferometers [8,9] have been developed as compact, ruggedized instruments for application in space, featuring good short-term instability. One way to also realize low long-term instability is the stabilization to atomic or molecular transitions, providing an advantage in accuracy over relative frequency references. Together with optical frequency combs, such absolute references can further be operated as optical clocks with instability relevant for global navigation satellite systems (GNSS). Several such systems are under *klaus.doeringshoff@physik.hu-berlin.de investigation today at various wavelengths, based on thermal calcium beams [10–12] or hot vapor cells using single- [13] or two-photon transitions in rubidium [14,15]. Another, already matured frequency reference is based on saturation spectroscopy of molecular iodine using the second harmonic of the narrow linewidth Nd:yttriumaluminum- garnet (YAG) laser at 1064 nm. These systems rely on the modulation-transfer spectroscopy (MTS) technique applied to the rovibronic transition R(56)32- 0, featuring narrow transitions with a natural linewidth of 300 kHz [16]. The hyperfine spectrum of this transition was studied in detail [17], the absolute frequency was accurately measured with an uncertainty of 1.1 kHz by Nevsky et al. [18], and Ye et al. demonstrated operation of a molecular iodine optical clock over the course of a full year [19]. In the past decade, several groups realized compact and portable iodine references with fractional frequency instability in the low 10−15 regime [20,21] and subjected their setups to environmental tests [22]. We believe that, using available technology at 1064 nm, developed in the context of the LISA and the GRACE-FO mission and established in satellite laser communication terminals, such systems can be developed for spaceborne instruments on relatively short time scales. Here, we present a stand-alone iodine frequency reference at 1064 nm, named JOKARUS, that is based on a microintegrated extended-cavity diode laser (ECDL) 2331-7019/19/11(5)/054068(9) 054068-1 © 2019 American Physical Society KLAUS DORINGSHOFF et al. PHYS. REV. APPLIED 11, 054068 (2019) [23,24] in a master oscillator plus power amplifier (MOPA) configuration. JOKARUS was built to demonstrate the maturity of our technology and its applicability in space missions. To this end, we operate JOKARUS on a sounding rocket mission and thereby prove the autonomous operation of an optical iodine frequency reference.We compare the optical frequency to a chip-scale atomic clock (CSAC) via a self-referenced frequency comb on the ground and in space. This paper is organized as follows. Section II describes the optical iodine frequency reference and its autonomous operation, as well as the optical frequency comb used to verify the frequency instability of the iodine reference aboard the sounding rocket. Section III presents results on the characterization of the frequency instability of the iodine frequency reference obtained on the ground and from the operation in space. Finally, in Sec. IV, we summarize the results and give a conclusion.

Micro-fabricated alkali vapor cells sealed at low temperature using asymmetric Au–In transient liquid phase (TLP) bonding

Vapor cells as the key part of chip-scale atomic clocks determine their volume stability and power consumption. Micro-fabrication, especially the cavity packaging, is challenging. In this paper, micro-fabricated rubidium vapor cells are fabricated and the Au-In TLP bonding with asymmetric metal configuration is proposed and designed to package wafer-scale cell chips at a low processing temperature of 200 °C. Tests show that the designed intermetallic compounds AuIn2 and AuIn are uniformly formed in the asymmetric TLP bonding interlayer, indicating that the bonded interlayer can endure a post-processing temperature as high as 490 °C. The TLP bonded surface exhibits shear strength of 31.68 MPa, and the upper limit for the leak rate of the sealed vapor cell is 5 × 10−10 mbar · L s–1 which remains stable after undergoing a high temperature post-treatment. Meaning the proposed Au-In TLP bonding technique is applicable at low-temperature, hermetic and reliable wafer-scale packaging for micro-fabricated alkali vapor cells.

Experimental study of the application feasibility of a novel chip-scale atomic clock scheme.

We report on research into a microfabricated 87Rb vapor cell with differential detection. Elliptically polarized light is used to interact with alkali atoms to generate coherent population trapping (CPT) resonance, and the CPT signal is obtained by detecting the Faraday rotation effect with differential detection technology. To move closer to an actual chip-scale atomic clock (CSAC), we reduce the volume of the experimental apparatus and use a divergent laser beam to interact with the atoms. We obtain the short-term frequency stability of the CSAC based on the differential detection scheme and compare it with that of a conventional CSAC. The results show that the frequency stability is more than two times better than that of current commercial CSAC devices with the same power consumption and volume.

Research on a chip scale atomic clock driven GNSS/SINS deeply coupled navigation system for augmented performance

Chip scale atomic clock (CSAC) is a recently developed low-profile atomic frequency reference device. Researchers have demonstrated CSAC was able to improve the accuracy of Global navigation satellite system (GNSS) receiver positioning results. However, the work was conducted using scalar tracking loops (STL). It is meaningful to investigate CSAC augmented vector tracking loop (VTL), since the VTL has different architecture with STL. Thus, in this paper, the authors investigated a CSAC driven VTL_DI system for exploring CASC impact on VTL. Firstly, the authors explored 1) whether VTL_DI could operate without regarding clock offset and drifts as VTL_DI integration filter state variables, and 2) whether VTL_DI could provide reliable position with only three satellites in view. Secondly, side-by-side comparative field tests were conducted to evaluate performance improvement: by solution A: VTL_DI supported by CSAC and solution B: a temperature compensated crystal oscillator (TXCO) with more than four satellites and three satellites in view scenarios. It was discovered that the CSAC could enhance the position performance, even if amount of satellites were fewer than four in view. However, VTL_DI system could not guarantee its positioning accuracy without clock offset as state variable, which was different from STL_DI system.

Micro-fabrication process of vapor cells for chip-scale atomic clocks

As the key part of chip-scale atomic clocks (CSACs), the vapor cell directly determines the volume, stability, and power consumption of the CSAC. The reduction of the power consumption and CSAC volumes demands the manufacture of corresponding vapor cells. This overview presents the research development of vapor cells of the past few years and analyzes the shortages of the current preparation technology. By comparing several different vapor cell preparation methods, we successfully realized the micro-fabrication of vapor cells using anodic bonding and deep silicon etching. This cell fabrication method is simple and effective in avoiding weak bonding strengths caused by alkali metal volatilization during anodic bonding under high temperatures. Finally, the vapor cell D2 line was characterized via optical-absorption resonance. According to the results, the proposed method is suitable for CSAC.

Technological Assessment of MEMS Alkali Vapor Cells for Atomic References.

This paper is a review that surveys work on the fabrication of miniature alkali vapor cells for miniature and chip-scale atomic clocks. Technology on microelectromechanical systems (MEMS) cells from the literature is described in detail. Special attention is paid to alkali atom introduction methods and sealing of the MEMS structure. Characteristics of each technology are collated and compared. The article’s rhetoric is guided by the proposed classification of MEMS cell fabrication methods and contains a historical outline of MEMS cell technology development.

Noise reduction by a single photodiode in coherent population trapping atomic clocks

We present a digital differential detection scheme using a single photodiode to suppress the diode laser frequency noise and improve the short-term stability in atomic clocks based on coherent population trapping (CPT) resonance. One laser light is divided into two beams via modulation of an acousto-optic modulator. One beam induces CPT resonance with circularly polarized light, and the other beam induces only absorption with linearly polarized light. A differential algorithm counteracts common-mode noise from the two beams. A stability of 6.86×10−12 at 100 s and 2.26×10−12 at 1000 s has been demonstrated. The short-term stability is improved by a factor of 2.4 at 100 s. The scheme promises to eliminate laser-related noise and explore potential applications in chip-scale atomic clocks.

Chip-scale atomic devices

Chip-scale atomic devices combine elements of precision atomic spectroscopy, silicon micromachining, and advanced diode laser technology to create compact, low-power, and manufacturable instruments with high precision and stability. Microfabricated alkali vapor cells are at the heart of most of these technologies, and the fabrication of these cells is discussed in detail. We review the design, fabrication, and performance of chip-scale atomic clocks, magnetometers, and gyroscopes and discuss many applications in which these novel instruments are being used. Finally, we present prospects for future generations of miniaturized devices, such as photonically integrated systems and manufacturable devices, which may enable embedded absolute measurement of a broad range of physical quantities.

High-Precision Heading Determination Based on the Sun for Mars Rover

Since the American Mars Exploration Rover Opportunity landed on Mars in 2004, it has travelled more than 40 km, and heading-determination technology based on its sun sensor has played an important role in safe driving of the rover. A high-precision heading-determination method will always play a significant role in the rover’s autonomous navigation system, and the precision of the measured heading strongly affects the navigation results. In order to improve the heading precision to the 1-arcminute level, this paper puts forward a novel calibration algorithm for solving the comparable distortion of large-field sun sensor by introducing an antisymmetric matrix. The sun sensor and inclinometer alignment model are then described in detail to maintain a high-precision horizon datum, and a strict sun image centroid-extraction algorithm combining subpixel edge detection with circle or ellipse fitting is presented. A prototype comprising a sun sensor, electronic inclinometer, and chip-scale atomic clock is developed for testing the algorithms, models, and methods presented in this paper. Three field tests were conducted in different months during 2017. The results show that the precision of the heading determination reaches 0.28–0.97′ (1σ) and the centroid error of the sun image and the sun elevation are major factors that affect the heading precision.

An on-chip fully electronic molecular clock based on sub-terahertz rotational spectroscopy

Mobile electronic devices require stable, portable and energy-efficient frequency references (or clocks). However, current approaches using quartz-crystal and microelectromechanical oscillators suffer from frequency drift. Recent advances in chip-scale atomic clocks, which probe the hyperfine transitions of evaporated alkali atoms, have led to devices that can overcome this issue, but their complex construction, cost and power consumption limit their broader deployment. Here we show that sub-terahertz rotational transitions of polar gaseous molecules can be used as frequency bases to create low-cost, low-power miniaturized clocks. We report two molecular clocks probing carbonyl sulfide (16O12C32S), which are based on laboratory-scale instruments and complementary metal–oxide–semiconductor chips. Compared with chip-scale atomic clocks, our approach is less sensitive to external influences and offers faster frequency error compensation, and, by eliminating the need for alkali metal evaporation, it offers faster start-up times and lower power consumption. Our work demonstrates the feasibility of monolithic integration of atomic-clock-grade frequency references in mainstream silicon-chip systems. The sub-terahertz rotational transitions of polar molecules can be used as frequency bases to create low-cost, low-power miniaturized clocks.

Molecular clock redux in miniature

Microwave transitions in the rotational spectrum of carbonyl sulfide molecules provide a timing reference that can be used to develop chip-scale atomic clocks.