Low-Power Chip-Scale Rubidium Plasma Light Source for Miniature Atomic Clocks

We present the development, testing and characterization of a low-power chip-scale Rubidium (Rb) plasma light source designed to serve for optical pumping in miniature atomic clocks. The technique used is electrodeless capacitively coupled plasma (CCP) discharge, driven in a microfabricated Rb vapor cell. The device is electrically driven at frequencies between 1 and 36 MHz to emit 140 μW of stable optical power while coupling < 6 mW of electrical power to the discharge cell. To our knowledge this is the first reported Rb plasma emitted from a chip-scale device

MICROFABRICATION AND PACKAGING OF A RUBIDIUM VAPOR CELL AS A PLASMA LIGHT SOURCE FOR MEMS ATOMIC CLOCKS

We report on the micro-fabrication and characterization of a chip-scale plasma light source based on a Rubidium (Rb) vapor cell. The Rb plasma light source is intended for use as an integrated optical pump-light source in miniature double-resonance Rb atomic clocks [1, 5]. The RF plasma is capacitively coupled using external electrodes, and the light source is impedance matched to the source for frequencies between 1 and 36 MHz. Rb vapor cells have been previously developed as reference cells for atomic clocks but not as light sources. This is the first reported Rb plasma emitted from a chip-scale device. Stable light emission is observed for over 18 days

Research on MEMS-Based Rb-85 Filter in Miniaturized Passive Rubidium Atomic Clock

研究并提出一种分析设计rb气泡原子钟微型rb-85滤光泡的方法。从量子物理出发,通过为rb灯的发射光谱引入lOrEnTzIAn线形函数,得到一个滤光泡内的光强与入射光强的关系式,其中包含了跃迁系数,频移,谱线宽度等参数,通过研究确定这些参数并最终建立一个具有高吸收效率的滤光泡的理论模型。基于这种方法,我们设计了MEMS滤光泡,rb-87灯射出的光谱经过该滤光泡后,90%多的α线被吸收,而β线则只衰减不到3%,因此,MEMS滤光泡不仅可以大幅度减小体积与功耗,其滤光效果也更为优越。This paper presents an analysis and design of a micromachined Rb-85 filter for passive rubidium atomic clock.By introducing the Lorentzian shape functions into the derived output light intensity equation of the filter for a monochromatic light,a formula of light intensity in a filter with a light spectrum input is set up,and parameters in this formula involving transition coefficient,frequency shifts and linewidth are studied and determined.Based on these,a micromachined Rb-85 filter with high filtering performance is designed,in which more than 90% of α-line component in the light spectrum input from the Rb-87 lamp is filtered but β-line component is weakened less than 3%.Compared the MEMS filter with the conventional filter,the MEMS filter not only reduces the size and power consumption,but also acquires the better filter effect.福建省重大科技项目前期研究项目资助(2005HZ1021);厦门大学引进人才科研启动基金资助(0000-X07191

CONCEPTION, TECHNOLOGIE ET PACKAGING DE CELLULES À VAPEUR DE CÉSIUM POUR LES HORLOGES ATOMIQUES DE TYPE MEMS

Atomic clocks are nowadays among the most accurate time and frequency standards, and are used, e.g.,for international time distribution service or in global navigation satellite systems. During the last severalyears, based on reliable and well-stabilized microelectromechanical systems (MEMS) technology and onthe availability of lasers on chip (single-mode vertical-cavity surface-emitting laser; VCSEL),considerable work has been performed by different groups around the world to develop miniaturizedversion of atomic clocks, called MEMS atomic clocks (MAC) or chip scale atomic clocks (CSAC).The goal of this thesis was to design and develop the technology of Cs vapor cells along with thermalanalysis for the thermal management of fully packaged Cs vapor cell for MEMS atomic clock. This workhas been carried-out in the framework of European project “MEMS atomic clock for timing, frequencycontrol and communication” (MAC-TFC). Two different architectures of Cs vapor cell have beenconsidered. The first one operates on the transmission of light through the cell and thus the Cs-vapor, andis called transmissive cell or T-cell. Such T-cell is made of silicon based deep-cavities sandwichedbetween two borosilicate glass wafers. For their fabrication, deep reactive ion etching (DRIE) process hasbeen optimized in order to produce smooth enough side walls of silicon cavities. In addition, specificanodic bonding process has been developed to fill the cavities with buffer gas at the required pressure.Second version of Cs vapor cell is based on the reflection of laser light inside the KOH etched siliconcavity sealed by one borosilicate glass wafer and is called reflective cell or R-cell. R-cells, as an advantageover the T-cells, allow e.g. a longer interaction of light/atom inside the Cs cavity, whereas location ofoptical source and detection elements on the same side of cell leads to better clock compactness. For theirfabrication, wet KOH etching, employed to realize the cavities inside the silicon with near mirror like(111) planes, has been studied and optimized. Further, diffraction gratings for routing of circularlypolarized light have been designed, fabricated and integrated on top of the Cs vapor R-cell. In bothversions of Cs vapor cells, our goal was to simplify the related clock assembly by doing maximumintegration and alignment at the wafer level, thanks to refractive and diffractive micro-optical componentswhile thermal analysis has been also performed for the thermal management of fully packaged Cs vaporcell (transmissive one) based on the Low temperature co-fired ceramics (LTTC) packaging.Les horloges atomiques sont de nos jours parmi les normes de temps et de fréquences les plus précises etsont utilisées, par exemple, pour les services de distributions travaillant à l'heure internationale ou pour lessystèmes de navigation globaux par satellite. Au cours des dernières années, un travail considérable a étéaccompli par différents groupes à travers le monde pour développer une version miniaturisée des horlogesatomiques, basée sur la technologie des systèmes microélectromécaniques qui est fiable, bien stabilisée etsur la disponibilité de lasers sur puces (diode laser monomode à cavité verticale émettant par la surface ouVCSEL : Vertical-Cavity surface emitting Laser). Ce type d'horloge atomique est appelé horloge atomiqueMEMS ou horloge atomique sur puce (CSAC : Chip Scale Atomic Clocks).L'objectif de cette thèse était de concevoir et développer la technologie des cellules à vapeur de Césium(Cs) ainsi qu’une analyse thermique pour sa gestion thermique lorsqu’elle est complètement packagéepour les horloges atomiques MEMS. Ce travail a été réalisé dans le cadre du projet européen "MEMSatomic clock for timing, frequency control and communication" (MAC-TFC). Deux conceptionsdifférentes de la cellule à vapeur de Cs ont été considérées. La première est basée sur la transmission de lalumière à travers la cellule et donc au travers de la vapeur de Cs et est appelée cellule transmissive ou T-cell. Ces T-cells sont réalisées à base de cavités profondes générées dans du silicium et prises en sandwichentre deux wafers de verre borosilicaté. Pour leur fabrication, le processus de gravure profonde par ionsréactifs (DRIE-Deep reactive ion etching) a été optimisé afin de produire des cavités dans le silicium dontles parois soient suffisamment lisses. De plus, le procédé de soudure anodique a été développé pourremplir les cavités avec du gaz tampon à la pression requise. La deuxième version de la cellule à vapeurde Cs est basée sur la réflection de la lumière du laser à l'intérieur des cavités gravées dans le silicium parKOH et scellées par un wafer de verre borosilicaté. Cette cellule est appelée cellule réfléchissante ou R-cell. Les R-cells permettent, par rapport aux T-cells, une interaction lumière/atome plus longue dans lescavités contenant du Cs, tandis que la localisation de la source optique et des éléments de détection dumême côté de la cellule permet la réalisation d’une horloge plus compacte. Pour leur fabrication, lagravure humide par KOH, employée pour générer les cavités à l'intérieur du silicium avec des parois dontla surface est proche de celle d’un miroir (111), a été étudiée et optimisée. De plus, les réseaux dediffraction pour le guidage de la lumière polarisée circulairement ont été conçus, fabriqués et intégrés surla partie supérieure de la R-cell à vapeur de Cs. Pour les deux versions des cellules à vapeur de Cs, notreobjectif était de simplifier l'assemblage relatif à l'horloge en faisant un maximum d'intégration et d'alignement à l'échelle du wafer, grâce à des composants micro-optiques réfractifs et diffractifs. Une analyse thermique a aussi été effectuée pour la gestion thermique de la cellule à vapeur de Cs complètement packagée (T-cell) à base de céramiques cofrittée à basse température (LTCC : Lowtemperature Co-fired ceramics)

Integrated packaging solutions and hotplates for a miniature atomic clock and other microsystems

This thesis aimed at developing innovative packaging solutions for a miniature atomic clock and other microsystems in the cm-scale, i.e. somewhat larger than what is practical for full "chip-scale" device-package integration using clean-room technologies for fabrication of microelectromechanical systems (MEMS). Besides well-defined and robust mechanical attachment, such packaging solutions must provide reliable electrical interconnection with the other system components, and, if needed, additional functions such as local temperature control, insulation from electrical magnetic or temperature perturbations, chemical separation (hermeticity). In order to accomplish this objective, different packaging technologies and modules were developed, fabricated and characterized in the frame of this thesis, with particular emphasis on the packaging of a miniature double-resonance (DR) rubidium atomic clock, which is an ideal demonstration platform given the associated large variety of requirements. First, the possibility of encapsulating the reactive Rb metal in ceramic / glass substrates using soldering was explored, with the aim to achieve simple and reliable fabrication of miniature atomic clock elements such as the reference cell and the Rb lamp. After a thorough literature review investigation of the metallurgical interactions between rubidium and materials used in packaging such as solder (Sn, Pb, Bi..) and thick-film metallizations metals (Ag, Pd, Au, 2 Pt...), an innovative design for a Rb reference cell (dimensions 10 × 12 mm ) is presented. The cell is based on a multifunctional low-temperature cofired ceramic (LTCC) spacer, closed by two glass windows allowing light transmission and acting as lids. Bonding is achieved by low-temperature soldering, avoiding exposing Rb to high temperatures. The use of LTCC as the main substrate material for Rb vapor cells in principle allows further integration of necessary functions for the Rb lamp and reference cell, such as temperature regulation, excitation / microwave resonator electrodes, impedance-matching passive components (lamp), and coil for static magnetic field generation (reference). In this work, to test the hermeticity of the bonding, a pressure sensor was integrated into the cell by replacing one of the glass windows by a membrane comprising an integrated piezoresistive Wheatstone bridge. In this frame, a new lamination technique for LTCC is proposed. The technique consists in applying a hot-melt adhesive on top of the LTCC green tape, and allows good bonding of the tapes even at low lamination pressure. This technique is particularly attractive for the lamination of LTCC microfluidic devices or membrane pressure sensors, because the low pressure applied during lamination does not affect the shape of the channels in a microfluidic device, or the membrane of the sensor. The resulting cells are shown to be hermetic, and a Rb response could be measured by the project partners. However, heating resulted in loss of this response, indicating Rb depletion by undesired reactions between Rb and the sealing metals or contaminants. This result is somewhat in line with studies made in parallel with the present work on low-temperature indium thermocompression bonding. Therefore, although the results are promising, further optimisation of metallizations, solders and package design is required. An important generic function that may be integrated into LTCC is temperature control. In this frame, a multifunctional LTCC hotplate was designed, fabricated and studied. This device allows controlling the temperature of any object in the cm-scale, such as the abovementioned Rb vapor cells (reference or lamp) and other temperature-sensitive elements used in miniature atomic clocks such as lasers and impedance-matching passive components. Full thermal analysis, mathematical calculations, finite-element simulations and laboratory experiments were performed. The excellent structurability and modest thermal conductivity of LTCC make it much better suited than standard alumina for integrated hotplates, resulting in conduction losses in the LTCC structure being small compared to surface losses by conduction and convection. It is therefore concluded that insulation and/or vacuum packaging techniques are necessary to achieve optimized low-power operation. Although we have seen that LTCC is an excellent integrated packaging platform, there are some limitations for carrying relatively massive components such as the DR atomic clock resonator cavity structure, which in general is a solid metal part. Therefore, an alternative hotplate technology platform, was developed, based on the combination of standard fiberglass-reinforced organic-matrix printed-circuit board (PCB), combined with thick-film alumina heaters. The PCB acts as high-strength, low-cost and readily available mechanical carrier, electrical interconnect and thermal insulator, and the thick-film heaters provide local temperature regulation, with the high thermal conductivity of alumina ensuring good local temperature uniformity. Therefore, such a hybrid PCB-Al2O3 platform constitutes an attractive alternative to LTCC hotplates for benign operating conditions. In conclusion, this work introduced several innovative packaging solutions and techniques, which were successfully applied to various dedicated modules carrying the elements of miniature atomic clocks. Beyond this application, these developments allow us to envision efficient packaging of a wide variety of new miniature devices. Also, new areas for further investigations are suggested, such as long-term metallurgical interactions of alkali metals with solders, hermeticity, optimization of temperature distribution and thermal insulation techniques, as well as reliability at high-temperatures and under severe thermal cycling.This thesis aimed at developing innovative packaging solutions for a miniature atomic clock and other microsystems in the cm-scale, i.e. somewhat larger than what is practical for full "chip-scale" device-package integration using clean-room technologies for fabrication of microelectromechanical systems (MEMS). Besides well-defined and robust mechanical attachment, such packaging solutions must provide reliable electrical interconnection with the other system components, and, if needed, additional functions such as local temperature control, insulation from electrical magnetic or temperature perturbations, chemical separation (hermeticity). In order to accomplish this objective, different packaging technologies and modules were developed, fabricated and characterized in the frame of this thesis, with particular emphasis on the packaging of a miniature double-resonance (DR) rubidium atomic clock, which is an ideal demonstration platform given the associated large variety of requirements. First, the possibility of encapsulating the reactive Rb metal in ceramic / glass substrates using soldering was explored, with the aim to achieve simple and reliable fabrication of miniature atomic clock elements such as the reference cell and the Rb lamp. After a thorough literature review investigation of the metallurgical interactions between rubidium and materials used in packaging such as solder (Sn, Pb, Bi..) and thick-film metallizations metals (Ag, Pd, Au, 2 Pt...), an innovative design for a Rb reference cell (dimensions 10 × 12 mm ) is presented. The cell is based on a multifunctional low-temperature cofired ceramic (LTCC) spacer, closed by two glass windows allowing light transmission and acting as lids. Bonding is achieved by low-temperature soldering, avoiding exposing Rb to high temperatures. The use of LTCC as the main substrate material for Rb vapor cells in principle allows further integration of necessary functions for the Rb lamp and reference cell, such as temperature regulation, excitation / microwave resonator electrodes, impedance-matching passive components (lamp), and coil for static magnetic field generation (reference). In this work, to test the hermeticity of the bonding, a pressure sensor was integrated into the cell by replacing one of the glass windows by a membrane comprising an integrated piezoresistive Wheatstone bridge. In this frame, a new lamination technique for LTCC is proposed. The technique consists in applying a hot-melt adhesive on top of the LTCC green tape, and allows good bonding of the tapes even at low lamination pressure. This technique is particularly attractive for the lamination of LTCC microfluidic devices or membrane pressure sensors, because the low pressure applied during lamination does not affect the shape of the channels in a microfluidic device, or the membrane of the sensor. The resulting cells are shown to be hermetic, and a Rb response could be measured by the project partners. However, heating resulted in loss of this response, indicating Rb depletion by undesired reactions between Rb and the sealing metals or contaminants. This result is somewhat in line with studies made in parallel with the present work on low-temperature indium thermocompression bonding. Therefore, although the results are promising, further optimisation of metallizations, solders and package design is required. An important generic function that may be integrated into LTCC is temperature control. In this frame, a multifunctional LTCC hotplate was designed, fabricated and studied. This device allows controlling the temperature of any object in the cm-scale, such as the abovementioned Rb vapor cells (reference or lamp) and other temperature-sensitive elements used in miniature atomic clocks such as lasers and impedance-matching passive components. Full thermal analysis, mathematical calculations, finite-element simulations and laboratory experiments were performed. The excellent structurability and modest thermal conductivity of LTCC make it much better suited than standard alumina for integrated hotplates, resulting in conduction losses in the LTCC structure being small compared to surface losses by conduction and convection. It is therefore concluded that insulation and/or vacuum packaging techniques are necessary to achieve optimized low-power operation. Although we have seen that LTCC is an excellent integrated packaging platform, there are some limitations for carrying relatively massive components such as the DR atomic clock resonator cavity structure, which in general is a solid metal part. Therefore, an alternative hotplate technology platform, was developed, based on the combination of standard fiberglass-reinforced organic-matrix printed-circuit board (PCB), combined with thick-film alumina heaters. The PCB acts as high-strength, low-cost and readily available mechanical carrier, electrical interconnect and thermal insulator, and the thick-film heaters provide local temperature regulation, with the high thermal conductivity of alumina ensuring good local temperature uniformity. Therefore, such a hybrid PCB-Al2O3 platform constitutes an attractive alternative to LTCC hotplates for benign operating conditions. In conclusion, this work introduced several innovative packaging solutions and techniques, which were successfully applied to various dedicated modules carrying the elements of miniature atomic clocks. Beyond this application, these developments allow us to envision efficient packaging of a wide variety of new miniature devices. Also, new areas for further investigations are suggested, such as long-term metallurgical interactions of alkali metals with solders, hermeticity, optimization of temperature distribution and thermal insulation techniques, as well as reliability at high-temperatures and under severe thermal cycling

MEMS Components for NMR Atomic Sensors

This dissertation introduces a batch fabrication method to manufacture Micro-Electro-Mechanical System (MEMS) components for NMR atomic sensors, such as NMR gyroscope (NMRG) and NMR magnetometer (NMRM). The introduced method utilized a glassblowing process, origami-like folding, and a more traditional MEMS fabrication. We developed an analytical model of imperfections, including errors associated with micro-fabrication of MEMS components. In light of the developed error model and experimental evaluation of components, we predicted the effect of errors on performance of NMRG and NMRM. We concluded that with a realistic design, a 5mrad angular misalignment between coils and folded mirrors and a 100um linear misalignment between folded coils, it would be feasible to achieve an NMRG with ARW 0.1deg/rt-hr and an NMRM with sensitivity on the order of 10fT/rt-hz using MEMS technology. A design process for miniaturized atomic vapor cells using the micro-glassblowing process was presented in this dissertation. Multiple design considerations were discussed, including cell geometry, optical properties, materials, and surface coating. The geometry and the optical properties were studied using experimentally verified analytical and Finite Element Models (FEM). The cell construction material and surface coating were the focus of our experimental study on factors that affect the transverse relaxation time (T2) of nuclear spins. We showed that the developed wafer-level coating process with Atomic Layer Deposition (ALD) of Al2O3 increased the relaxation time (T2), which is projected to reduce the ARW of NMR gyroscopes and the sensitivity of NMR magnetometers by four times. Complementary to the developed atomic sensors components, an analog emulator for NMR atomic sensors was developed. The emulator represents the spin dynamics of atoms in an applied magnetic field that are governed by Bloch equations. Characterization of atomic sensors' components using the emulator was achieved by including one or more of those components with the emulator in a hardware-in-the-loop (HIL) configuration. Finally, we presented a comparison of the response between the NMR emulator and an actual NMR system, showing similarities of responses of the two systems and feasibility of using HIL configuration in development of micro-scale NMR sensors

Towards optimised portable quantum technologies via additive manufacturing

Cold atom experiments open wide prospects for applications in metrology, quantum computing, quantum information processing, and many other fields. They also present a promising avenue for experiments in fundamental physics, such as testing new states of matter like Bose-Einstein condensates, the precision probing of gravity and general relativity, and simulations of condensed matter. The first step of most cold atom-based experiments and applications is to form magneto-optical traps (MOT). Typically, these experiments are bulky, complicated, high-cost, and usually occupy room-size benches of optical elements. Nevertheless, because these experiments serve as the foundation for future generations of quantum technologies, there is growing interest in utilising them in real life applications. We exploit the method of additive manufacturing to build a portable and compact cold atom system that outperforms conventional apparatus in terms of size, weight, power, and cost (SWAP-C), all of which are critical criteria for portable quantum technologies. By demonstrating the successful operation of the first AM UHV chamber, and AM optical frameworks for frequency stabilising and MOT beams’ power distribution, we prove how the AM approach, when combined with optimisation algorithms, enables radical mass reduction and offers superior stability and performance. Our compact device described herein has a volume of less than 5% of the total volume of conventional cold atom systems. The device is 55×60×45 cm3 in size, and weighs ∼ 3.2 kg (excluding commercial components). Using the permanent magnetic field generated purely from an array of neodymium magnets, the apparatus is capable of creating magneto-optical traps of > 2×108 85Rb atoms in the AM UHV chamber. By characterising the response of the system to changes in environmental temperature between 10 and 30 ◦C and exposure to vibrations between DC to a few tens of kilohertz, the reliability of our system to operate in outdoor applications is proven. To sum up, the miniaturisation of cold atom setups is critical for various experiments and applications, as existing conventional arrangements are large, complicated, and extremely sensitive to even minor environmental fluctuations. By leveraging the advantages of the AM approach, our system addresses these issues and complies with SWAP-C requirements. The results presented in this thesis will be of great interest to the community of quantum technology, UHV, and a broader range of atomic and optical physics researchers