Next-generation free-space laser links use the phase of the optical carrier for scientific measurements, for example to compare distant clocks, or to measure distances with very low noise (gravitational wave detection, optical Doppler ranging). In addition, the modulation of the optical phase allows the transmission of data at very high speed, as already done in optical fibers.

These measurements are subsequently used to, for example, determine the gravitational field difference between the locations of two distant clocks by measuring their relativistic frequency shift (gravitational redshift). This new method, made possible by the spectacular advancement of atomic clocks, is known as "chronometric geodesy" [1,2] and requires an ultrastable frequency link between remote clocks.

An ongoing project TOFU (Transfert Optique de Fréquence Ultrastable) in collaboration between SYRTE/CNES and the University of Western Australia (UWA) has realized a first free space optical link between two buildings at CNES premises in Toulouse [3]. Since then the project has focused on extending the link to an airborne platform. First tests with a captive balloon at the CNES test site in Aire sur Adour took place in 2022-2025. They have shown world class results [4]. Those tests have been continued (PhD thesis finishing 2025) and will be followed by tests on higher altitude platforms (up to stratospheric) and using an active optical relay.
The thesis concerns the continuation of this activity, together with permanent staff and following the current PhD student (2022-2025). The candidate will participate in the realization and improvement of the ground and airborne terminals (servo-controls, calibration) and tests in the lab, between buildings and to airborne platforms. He/she will be at the center of the design and implementation of the equipment, and its operation on a balloon. This experimental component encompasses the mastery of the link as a whole (ground stations and airborne terminals) with active optics systems and fine pointing as well as phase measurements by heterodyne detection. The final goal of the thesis will be the development and implementation of the world’s first coherent optical link via an airborne relay and the characterization of its performance in terms of phase noise and amplitude fluctuations.

The PhD student will work mostly at CNES (Toulouse) with occasional trips to Paris Observatory. He/she will be able to rely on the instruments, methods and simulations that have been developed during the previous theses, in order to successively address the technical and scientific challenges that are the fine pointing and the experimental validation of phase noise compensation on a bidirectional operational free space transmission system. 

The work will be mainly experimental in free-space and fibre optics, adaptive optics, control and command, phase measurements, with activities that cover the entire chain: design, production and tests in the laboratory and on balloons. We are looking for a highly motivated student, eager to integrate into an existing team to learn to design, develop and operate new and unprecedented technology and methods for ultimate precision measurements in real-life conditions in the field.

References :

[1] Lion, G.; Panet, I.; Wolf, P.; Guerlin, C.; Bize, S. & Delva, P. Determination of a high spatial resolution geopotential model using atomic clock comparisons, Journal of Geodesy, 2017, 91, 597.

[2] Denker, H.; Timmen, L.; Voigt, C.; Weyers, S.; Peik, E.; Margolis, H. S.; Delva, P.; Wolf, P. & Petit, G. Geodetic methods to determine the relativistic redshift at the level of 10-18 in the context of international timescales: a review and practical results, Journal of Geodesy, 2017, 92, 487.
[3] Dix-Matthews, B. P.; Schediwy, S. W.; Gozzard, D. R.; Savalle, E.; Esnault, F.-X.; Leveque, T.; Gravestock, C.; D'Mello, D.; Karpathakis, S.; Tobar, M. & Wolf, P., Point-to-Point Stabilised Optical Frequency Transfer with Active Optics, Nature Communications, 2021, 12, 515.
[4] Maron, N.; Fernandez, S.; Esnault, F.-X.; Lévèque, T.; Muzeau, T. & Wolf, P.,Free space optical link to a tethered balloon for frequency transfer and chronometric geodesy,Opt. Express, 2024, 32, 4267-4276.

Contact: Peter Wolf (peter.wolf@obspm.fr), François-Xavier Esnault (Francois-Xavier.Esnault@cnes.fr).

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For more Information about the topics and the co-financial partner (found by the lab!); contact Directeur de thèse - peter.wolf@obspm.fr

Then, prepare a resume, a recent transcript and a reference letter from your M2 supervisor/ engineering school director and you will be ready to apply online before March 13th, 2026 Midnight Paris time!