Meta-optics allow the realization of new optical functions that are increasingly complex to realize and characterize locally. This is why we propose an interferometric method of systematic wavefront metrology of the meta-elements constituting a metasurface. This technique will allow the design of a library of nano-antennas, characterized in phase and amplitude. Once constituted, this library will allow the design of more complex optical functions. Tested for MIM (Metal-Isolating-Metal) metasurfaces, this technique can be applied to all metasurfaces.
PISTIL (PISton and TILt) interferometry is a segmented wavefront metrology technique that can fulfill the role of being an independent phase analyzer for tiled laser arrays used in coherent beam combining (CBC). It presents a plug-and-play characteristics enabling others research or industrial applications such as metrology of segmented mirrors, MOEMS or measurement standards. It can operate onto complex optical benches. Alongside the PISTIL concept, we developed methods for phase extraction and meta-analysis, with best accuracy to rightfully address an end user needs in term of segmented wavefront diagnosis. We demonstrate those functionalities onto the HIBISCUS optical testbed equipped with a segmented mirror, specifically designed test data analysis pipelines and improve the control-command based on PISTIL wavefront analysis. In the current configuration, it can emulate CBC near field piston and tilt variations.
PISTIL (Piston and Tilt) is a recent interferometric system that computes the absolute piston and tip/tilt map of a segmented wavefront. Its high precision makes it usable as a metrology tool for wavefront sensing of coherently-combined laser arrays for example. This interferometer needs to correctly address high dynamic piston sensing, while dealing with fringes wrapping that leads to ambiguous phase estimations. We derived a mathematical combination for two measurements at different wavelengths and did a technical demonstration of it, using a IRIS-AO PTT111 Deformable Mirror as a segmented wavefront generator. We have verified that the loss of accuracy is slightly increased for a larger piston compared to a previous study, and we got a standard error of λ/160 with a Peak-to-valley of λ/50. This technique could be extended to a broader spectrum.
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.
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. In order to obtain much higher output power than it is possible
from a single fiber, beam-combining techniques have been investigated. In this communication, we present a new
technique of coherent fiber combining, based on self adaptive digital holography that does not require any phase error
measurement. A low power plane reference beam is first launched into the fiber amplifier array. The interference pattern
between the beams with phase φ(x,y) issued from the fiber array and a plane reference beam is recorded on a digital
camera and directly transferred to a Spatial Light Modulator (SLM) which acts as a programmable digital hologram. This
hologram is read out simultaneously and a phase conjugate beam with phase -φ(x,y) is generated in order -1 of the
diffraction pattern. This beam is then injected in the fiber amplifier array. At the output of the fiber amplifier array, the
phase of each elementary beam are locked. Experimental demonstration of coherent beam combining by digital
holography is demonstrated with polarization maintaining fibers operating at 1 μm. Digital holography is realized thanks
to a CCD/CMOS camera and a liquid crystal SLM. Owing to the high resolution of existing SLMs and cameras, this
technique could be applied to phase lock a large number of fiber amplifiers.
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