European Space Agency


The project OSCAR (Optically-pumped Space Cesium Atomic Resonator) aims to develop a concept of industrial compact optically pumped cesium atomic beam standard exhibiting superior frequency stability as compared with its commercial counterparts, namely, magnetically pumped cesium clocks.

The frequency stability goal is set to 1x10-12 @ 1s (1x10-13 @ 100s) when using a single optical frequency architecture of the clock for light-atom interactions. The feasibility to industrially implement the single-frequency concept benefits from technological achievements in spectral line narrowing and reliability of semiconductor distributed feedback laser diodes.

The project is thus focused on identifying technical and technological solutions for the clock subsystems allowing to achieve the frequency stability goal.

Taking into account the advance in laser diodes and laboratory demonstrators of optically pumped cesium beam clock, the following objectives have been set:

  1. To review possible laser pumping schemes, designs and tests of optical subassembly for such clock.
  2. To design an optically-pumped atomic beam resonator capable to reach the frequency stability 1x10-12 @ 1s, taking into account the requirements for future space applications.
  3. To assemble and test the atomic clock.
  4. To conduct its performance assessment and to draw conclusions for a future flight model development.

Photographic image of the atomic clock OSCARino

click for larger image


The atomic resonator is based on the simplest optical scheme utilizing a single frequency laser to tailor all atom-light interactions in the atomic resonator: laser frequency locking, population inversion of atoms, and local oscillator frequency locking on the atomic signal.

To avoid population trapping and thus to improve the signal to noise ratio and stability of the clock, a depolarized laser beam is injected into optical interaction zones of the atomic resonator. As such, the magnetic C-field is made uniform across the microwave and optical interaction regions, further simplifying the design.

With this simple clock architecture, we thus avoid using of an acousto-optic modulator for laser frequency shifting and separated coils for intensifying the magnetic field at the optical interaction zones.


Simplicity of the proposed clock architecture opens a way to a first industrial single-frequency optically pumped cesium atomic clock for space and ground applications.

Frequency stability of such optically pumped clock is one order of magnitude better as compared to commercial magnetically pumped atomic frequency standards.

So far, laboratory demonstrators of this concept were either utilizing components that cannot provide a reliable clock operation or they had a cumbersome architecture unsuitable for an industrial implementation.

The single-frequency optically pumped cesium clock proposed here will enable long-term space missions (~ 12 years), offering an accuracy required for application at Galileo satellites and having a smaller size, weight and power consumption as compared to the currently envisioned hydrogen masers.


The architecture of the OSCARino includes three main subsystems:

  • The Atomic Resonator s/s (AR) delivers two electrical signals characterizing the Cs atomic beam, one to the Optics & Laser Control (OLC) for frequency locking the laser, and one to the Atomic Resonator Control (ARC) for frequency locking the local oscillator (LO),
  • The Optics & Laser s/s (O&L) delivers three optical beams to the AR for the laser diode locking and for the atomic preparation and detection processes,
  • The Supporting Electronics incorporates the Atomic Resonator Control (ARC) and the Optics & Laser Control (OLC) subsystems. ARC powers the AR, controls its functional parameters, locks the quartz LO to the atomic transition and delivers the LO signal to the user. OLC powers the OL, controls its functional parameters and lock the laser frequency.

Block diagram of the atomic clock OSCARino

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The development of an Optically-Pumped Space Cesium Atomic Resonator has been split in seven tasks distributed between two phases of the project:

Phase 1

  • Optical s/s definition
  • Optical s/s validation
  • Optical part extended characterization
  • Detailed design

Phase 2

  • S/s manufacturing, assembling integration and test
  • Modular Atomic Resonator Development
  • Overall Assessment

Current status

The project has been finished and the goal of the project has been achieved.

The frequency stability σy=1.51x10-12τ-1/2 has been demonstrated in the atomic clock resonator driven at D2:4-4’ transition of Cs atoms by an 852nm-wavelength Distributed Feedback Laser.

click for larger image


The results of the project confirm feasibility of a compact and reliable single-frequency optically pumped atomic beam clock. The frequency stability of the clock demonstrator already complaints to the ESA requirement.

The in-depth frequency stability analysis conducted in the project will allow an atomic frequency standard to be built in full compliance with the ESA specification for the frequency stability and clock operation lifetime. A further development towards a space-compatible clock is the next logical step of this activity.

Status date

Monday, November 3, 2008 - 12:05