Cindy Bellanger

Collective coherent phase combining of 64 fibers

A new architecture for active coherent beam combining of a large number of fibers is demonstrated. The approach is based on a self-referenced quadriwave shearing interferometer and active control with arrays of electro-optic ceramic modulators. Coherent phase combining of 64 independent amplified fibers is obtained. This is to our knowledge the highest reported number of combined fibers. A Strehl ratio degradation less than 2dB is achieved with a residual phase error

Phase and amplitude control of a multimode LMA fiber beam by use of digital holography.

Amplitude and phase control of the output beam of a multimode LMA fiber supporting 4 modes is demonstrated by digital holography in both continuous and ns pulsed regimes at 1064 nm. Our system allows dynamic compensation of beam pointing instabilities, external perturbations introducing low order aberrations and fluctuations of the relative phase of the modes supported by the fiber.

Design of a fiber-collimated array for beam combining

In this paper, we present the design of a very precise collimated fiber array that meets requirements for beam combining. Calculations permit to determine the tolerances toward key parameters and specify the components to manufacture. Thus, the collimated fiber array is composed of a high quality commercial microlens array and an especially dedicated fiber holder that we design and realize experimentally. Manufacture techniques for both the microlens and the holder are chosen to be collective and then compatible with a high number of fibers. With the collimated fiber array hence obtained, the individual beam quality was measured to be λ/10 and the pointing accuracy is under 0.6 mrad.

Coherent phase combining of 64 fibers

Fiber lasers provide an attractive means of reaching high output laser power because of their advantages in terms of compactness, reliability, efficiency, and beam quality. Even given these advantages, it is desirable to increase the system power or energy levels beyond what is possible with a single-mode fiber laser. A promising technique is coherent beam combining with active phase locking involving phase detection and active compensation of phase errors [1]. In this Communication, we present coherent phase combining of 64 independent fibers, selectively amplified by 4 optical amplifiers. This is to our knowledge the highest number of combined fibers ever demonstrated. In order to achieve this result, we have developed specific collective components for phase measurement of a large number of fibers, arrays of phase modulators, and an array of collimated fibers.

Phase measurement of a segmented wave front using PISton and TILt interferometry (PISTIL)

New architectures for telescopes or powerful lasers require segmented wave front metrology. This paper deals with a new interferometric wave front sensing technique called PISTIL (PISton and TILt), able to recover both piston and tilts of segment beams. The main advantages of the PISTIL technique are the absence of a reference arm and an access to the tilt information. An explanation of the principle, as well as an experimental implementation and the use of a segmented active mirror, are presented. Measurement errors of λ/200 for piston and 40 µrad for tilts have been achieved, well beyond performances requested for the above mentioned applications.

Piston and tilt interferometry for segmented wave front sensing

Segmented wave front phase measurement is a key technological topic for next generation of kHz lasers, which will be based on the combination of small-scale laser sources (fibers) before focusing on target, such as in Laser WakeField Acceleration (LWFA) [1].

Diffraction-Limited Operation from Multimode and Multi-Core Fibers Using Active Digital Holography Precompensation

We demonstrate beam pre-compensation shaping using digital holography allowing diffraction limited operation out of a multimode LMA fiber and a multi-core fiber in CW and pulsed regime.

Diffraction-limited operation from a multimode LMA fiber using active digital holography precompensation

We demonstrate for the first time beam pre-compensation shaping using digital holography allowing diffraction-limited operation out of a multimode fiber in CW and pulsed regime. Usually, coherent combining uses several separated single mode fibers to overcome the limitations of fiber lasers and amplifiers caused by nonlinear effects. These techniques imply the use of electronic feedback loops or iterative algorithms to correct the phase, which slows down the active phase correction. Recently, an interesting coherent combining solution based on digital holography has been proposed and demonstrated with three passive single mode fibers [1]. Here, we report on the active wavefront control of a multimode fiber (MMF) beam using the digital holography principle. Coherent combining of the spatial modes of a MMF seems an interesting alternative to separated emitters because of reduced relative thermal drifts, simpler and integrated architecture and the possibility to use standard MMF. Our technique allows the use of very large mode area fiber amplifiers without the need of beam cleanup at the output, while retaining a good spatial beam quality.