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International Conference on Space Optics (ICSO)

Optical design of PHARAO

M. Saccoccio, Philippe Laurent, P. Lemonde, Jacques Loesel, Jacques Berthon, Nathalie Raimbault, Yvan Aubry, André Clairon

Published Nov 21, 2017• 1 citation
Publisher

Abstract

The purpose of the PHARAO project is to develop a new atomic clock generation in space. This clock takes advantage of the very low atomic velocities obtained by laser cooling techniques and the microgravity environment. Designing the PHARAO optical bench, which provides all the laser tools for the atomic manipulations, is a difficult task. In this paper we will give a global overview of the optical bench in term of functions, interfaces and performances. After establishing the optical parameters, which have an impact on the atomic clock performance, we present the model and software, which are used for the design and analysis of the optical system, taking into account the Gaussian laser beams. Some critical functions have been experimented and characterized to prove the model’s accuracy.

DOI
10.1117/12.2307894

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The purpose of the PHARAO project is to develop a new atomic clock generation in space. This clock takes advantage of the very low atomic velocities obtained by laser cooling techniques and the microgravity environment. Designing the PHARAO optical bench, which provides all the laser tools for the atomic manipulations, is a difficult task. In this paper we will give a global overview of the optical bench in term of functions, interfaces and performances. After establishing the optical parameters, which have an impact on the atomic clock performance, we present the model and software, which are used for the design and analysis of the optical system, taking into account the Gaussian laser beams. Some critical functions have been experimented and characterized to prove the model’s accuracy.
PHARAO (Projet d'Horloge Atomique par Refroidissement d'Atomes en Orbite), which has been developed by CNES, is the first primary frequency standard specially designed for operation in space. PHARAO is the main instrument of the ESA mission ACES (Atomic Clock Ensemble in Space). ACES payload will be installed on-board the International Space Station (ISS) to perform fundamental physics experiments. All the sub-systems of the Flight Model (FM) have now passed the qualification process and the whole FM of the cold cesium clock, PHARAO, is being assembled and will undergo extensive tests. The expected performances in space are frequency accuracy less than 3.10-16 (with a final goal at 10-16) and frequency stability of 10-13 τ-1/2. In this paper, we focus on the laser source performances and the main results on the cold atom manipulation.
The technique of laser cooling of atoms gives an opportunity to improve the performance of atomic clocks by using laser-cooled atoms. The most successful cold atom clock, called the atomic fountain, is now widely used as the primary frequency standard in many labs. The cold atom clock for satellite applications, however, has not been reported so far due to special requirements of space applications. Here, we report the development of an engineering model of a satellite-borne cold atom clock, which satisfied all requirements of in-orbit operation. The core of the clock’s principle is the laser cooling of atoms by diffuse laser lights inside the microwave cavity. The structure of the physics package is presented, and its main parameters are also given. The principle and design of the optical bench are described. The initial test results are presented, and the possible improvements are also discussed.
The Space Optical Clocks project aims at operating lattice clocks on the ISS for tests of funda-mental physics and for providing high-accuracy comparisons of future terrestrial optical clocks. A pre-phase-A study (2007-10), funded partially by ESA and DLR, includes the implementa-tion of several optical lattice clock systems using Strontium and Ytterbium as atomic systems and their characterization. Subcomponents of clock demonstrators with the added specification of transportability and using techniques that are suitable for later space use, such as all-solid-state lasers, low power consumption, and compact dimensions, have been developed and are being validated. The talk will give a brief overview over the achieved results and outline future developments.
The work described in this thesis was done at the Istituto Nazionale di Ricerca Metrologica (I.N.Ri.M.), in the optic division, within the Ph.D. of Metrology of the Politecnico di Torino. My work consisted in developing a new generation optical frequency standard based on Ytterbium atoms: the goal of this project is to realize a clock whose performance overcomes the ones of the standard which actually realizes the Second, the atomic Cesium fountain. This means realizes an apparatus which shows stability and accuracy better then the fountain ones: a possible way adopted in several new generation clock is using optical transitions, because they offer an higher quality factor of the clock transition itself, which means a better stability and the possibility to reach the 10^-17 level in the global accuracy of the clock. The fundamental idea of a clock is to realize as close as possible the basic idea under the actual definition of the Second, that is to fix the unit if time with a particular atomic transition, when the atom is completely unperturbed. Accomplishing this prescription means a lot of work in cooling down the atoms until the 10^-6 K regime using laser cooling techniques, developing a trap where atoms can be excited by the clock laser (the optical lattice) and of course a very careful frequency and amplitude stabilization of the clock laser itself is needed to reach our goal. The work described in this thesis regards the development of the two cooling stages of the Yb atoms and it is presented as follows. Chapter 1 explains with more details why an optical clock is needed to improve the accuracy of the actual realization of the Second, then describing the theoretical framework in which the clock will be developed. Chapter 2 contains a description of the experimental apparatus used, with more attention on the instrumentation related to the laser cooling of the atoms. Chapter 3 is about the characterization of the first cooling stage, both in a static and in a dynamic way, with some attention on the theoretical model developed to perform both the characterizations. Chapter 4 focuses on the development of the second cooling stage and it describes the actual state of the art of this stage
The Space Optical Clocks project aims at operating lattice clocks on the ISS for tests of fundamental physics and for providing high-accuracy comparisons of future terrestrial optical clocks. A pre-phase-A study (2007- 10), funded partially by ESA and DLR, included the implementation of several optical lattice clock systems using Strontium and Ytterbium as atomic species and their characterization. Subcomponents of clock demonstrators with the added specification of transportability and using techniques suitable for later space use, such as all-solid-state lasers, low power consumption, and compact dimensions, have been developed and have been validated. This included demonstration of laser-cooling and magneto-optical trapping of Sr atoms in a compact breadboard apparatus and demonstration of a transportable clock laser with 1 Hz linewidth. With two laboratory Sr lattice clock systems a number of fundamental results were obtained, such as observing atomic resonances with linewidths as low as 3 Hz, non-destructive detection of atom excitation, determination of decoherence effects and reaching a frequency instability of 1×10-16.
Optical atomic clocks demonstrate a better stability and lower systematic uncertainty than the highest performance microwave atomic clocks. However, the best performing optical clocks have a large footprint in a laboratory environment and require specialist skills to maintain continuous operation. Growing and evolving needs across several sectors are increasing the demand for compact robust and portable devices at this capability level. In this paper we discuss the design of a physics package for a compact laser-cooled 88 Sr + optical clock that would, with further development, be suitable for space deployment. We review the design parameters to target a relative frequency uncertainty at the low parts in 10 18 with this system. We then explain the results of finite-element modelling to simulate the response of the ion trap and vacuum chamber to vibration, shock and thermal conditions expected during launch and space deployment. Additionally, an electrostatic model has been developed to investigate the relationship between the ion trap geometrical tolerances and the trapping efficiency. We present the results from these analyses that have led to the design of a more robust prototype ready for experimental testing.