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 <λ/10 rms.

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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.

We present a novel interferometric technique dedicated to the measurement of relative phase differences (pistons) and tilts of a periodically segmented wavefront. Potential applications include co-phasing of segmented mirrors of Keck-like telescopes as well as coherent laser beam combining. The setup only requires a holes mask selecting the center part of each element, a diffracting component, and a camera. Recorded interferogram is made of many subareas with sinusoidal fringe pattern. From each pattern, piston is extracted from fringe shift and tilts from fringe frequency and orientation. The pattern analysis is simple enough to enable kilohertz rate operation. The λ ambiguities are solved by a two-wavelength measurement. This technique is compatible with a very high number of elements and can be operated in the presence of atmospheric turbulence.

We present a new configuration of quadriwave lateral shearing interferometer dedicated to phase detection for beam-combining purposes. Assuming that the fibers are disposed in a matrix arrangement, our scheme gives direct access to the phase step between adjacent fibers in two dimensions. Experimentally recorded interferograms are made only of two-wave interference fringes that scroll as the phase evolves in the fibers. This simplicity allows fast treatment by the spatial demodulation process, and the phase map from the fibers can be estimated in real time. No external reference is required, and the technique is fully compatible with a high number of fibers.

The PISTIL interferometry has been recently developed for the wavefront sensing of phase delays (pistons) and tilts of segmented surfaces, used in many domains such as astronomy, high-power lasers or ophthalmology. In this paper, we propose a two-wavelength version of this interferometer developed to bypass the dynamic range limitation of the ambiguous 2π phase wrapping. Principle of the technique is presented, along with experimental results obtained with a demonstration deformable mirror PTT-111 from Iris AO. Above wavelength pistons are measured with a precision and accuracy below λ/100, making the two-wavelength PISTIL interferometry a high-dynamic range technique. To prove these performances, we successfully compare the results in terms of precision and accuracy with those of a reference phase-shifting Interferometer, from a blind experimentation.

We present an original technique for coherent beam combining of an array of fiber amplifiers based on self-adaptive digital holography. In this method, the phase errors between the fibers of the array are compensated by using the diffracted phase-conjugated -1 order of a digital hologram. The proposed method is compatible with a large number of fibers and simply implemented with a CCD detector matrix and a spatial light modulator. This concept is analyzed and experimentally demonstrated with three polarization-maintaining passive fibers at 1.06 microm.

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.