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XCAN

 Jean-Christophe Chanteloup

Jean-Christophe Chanteloup received his Ph.D. in laser physics from the Ecole polytechnique (Palaiseau, France) supervised by A. Migus in 1998 after graduation as an optics engineer at Institut d’Optique Graduate School (IOGS, Palaiseau, France). He then join the group of S. Payne at the Lawrence Livermore National Laboratory (LLNL, DoE, Livermore, USA) as a postdoc on the Mercury Diode Pumped Solid State Laser (DPSSL) program. In 2000, he was appointed tenure researcher by the French national science agency CNRS at the Laboratoire Utilisation des Lasers Intenses (LULI) at the Ecole polytechnique where he worked on DPSSL laser physics (Lucia laser program). In 2003, he co-founded Phasics (www.phasicscorp.com) and, in 2008, received the French Physics Society (SFP) Yves Rocard Award for inventing and achieving the successful technology transfer of an optical sensing technique. Since 2015, he is in charge of the Ecole polytechnique-IZEST XCAN research program dedicated to coherent beam combination of dozens of fiber amplifying channels. He is the author or co-author of 200+ communications in international conferences or papers in scientific reviews.

Ultrashort and high-peak-power laser pulses find applications in the fields of fundamental physics, metrology or industry. Until now, solid state lasers are the current solution to provide high-peak powers. However, their repetition rate is limited due to thermal management. On the contrary, fiber-based laser systems, thanks to a very large surface/volume ratio, exhibit a good thermal management leading to high average power capability. However, they are limited in terms of high peak power due to mode confinement in the core of the fiber. To overcome this limitation, the concept CAN [1-2] (Coherent Amplifying Network) where fiber amplifiers provide an attractive means of reaching both high peak and high average powers by scaling up the available energy while keeping the intrinsic advantages of fibers like beam quality, reliability, robustness and compactness. Certain applications such as particle accelerations will require the coherent combining of up to 10000 fibers.

The XCAN project aims at developing a laser system based on the coherent combination of laser beams produced through a network of amplifying optical fibers as illustrated on the synoptic chart below.

Figure 1. XCAN synoptic chart.The architecture has to be compatible with very large number of fibers (1000-10000). The goal of XCAN is to overcome all the key scientific and technological barriers to the design and development of an experimental laser demonstrator. The coherent addition of 61 individual phased beams is aimed to provide more than 10 mJ in 350 fs pulses at 50 kHz.

XCAN was proposed by IZEST in 2014 and is carried out jointly by Thales and Ecole polytechnique. It is funded by both entities and hosted by Ecole polytechnique’s LULI laboratory and Thales Research & Technologies (TRT). The official start of the program was January 1st 2015.

Figure 2. Signature of the partnership agreement on the 9th of February 2015 at Thales headquarter in Paris La Défense. Left to right: Jacques Biot, President of École polytechnique, Marko Erman, Chief Technology Officer at Thales and Denis Levaillant, head of Thales's laser activities.

During this first year, the global architecture was derived as illustrated on the following chart, where, after being stretched in time, the pulses see its repetition rate reduced at 1 MHz with an average power of 100 mW. The pulse train is then spatially demultiplexed in 61 channels for further amplification to reach an expected energy level of about half a mJ before coherent addition followed by temporal compression.

Figure 3. XCAN laser system layout

Figure 4. XCAN laser system front-end picture

Two amplifying channels based on a Large Mode Area (LMA) fiber were assembled and used for a first interferometric measurement dedicated at noise level estimation. A very satisfying value of l/50 rms was recorded as illustrated on figure 5.

Figure 5. Phase noise spectra recorded at LULI laboratory where the XCAN 61 channel laser system will ultimately be implemented. The 100 HZ peaks are related to environmental noise and will not be an issue for the laser to operate.

Whereas these experiments where performed at LULI premises, a second set of experiments was carried out at TRT labs. The interferometric phase measurement technique is particularly well suited to phase-lock large number of fibers. In a first pre-demonstrator, the phase locking of three fibers was successfully demonstrated in the femtosecond regime. A dedicated algorithm was developed to measure both the phase and the delay between the fibers. A servo-loop was setup to control piezo-electric fiber stretchers that ensure compensation of time-varying phase and delay variations. The measured phase shift errors between the fibers are below λ/80rms.

Figure 6. Left: scheme of the experimental set-up of the phase locking of three fibers in pulse regime with the interferometric method. Right: Evolution of the three fibers phase shift in closed loop or opened loop.

References :

[1] G. A. Mourou, D. Hulin and A. Galvanauskas, ¡°The road to High Peak Power and High Average Power Laser: Coherent Amplification Network (CAN), AIP Conference Proceedings, Third International Conference on Superstrong Fields in Plasmas, vol. 827, Dimitri Batani and Maurizio Lontano,  152-163 (2006).

[2] Gerard Mourou,  Bill Brocklesby, Toshiki Tajima, and  Jens Limpert, The future is Fiber Accelerators, Nature Photonics, Vol.7, 258-261 (2013)