High-dynamic range segmented mirror metrology by two-wavelength PISTIL interferometry: demonstration and performance.

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.

Holography with Photochromic Diarylethenes.

Photochromic materials are attractive for the development of holograms for different reasons: they show a modulation of the complex refractive index, meaning they are suitable for both amplitude and phase holograms; they are self-developing materials, which do not require any chemical process after the light exposure to obtain the final hologram; the holograms are rewritable, making the system a convenient reconfigurable platform for these types of diffractive elements. In this paper, we will show the features of photochromic materials, in particular diarylethenes in terms of the modulation of a transparency and refractive index, which are mandatory for their use in holography. Moreover, we report on the strategies used to write binary and grayscale holograms and their achieved results. The outcomes are general, and they can be further applied to other classes of photochromic materials in order to optimize the system for achieving high efficiency and high fidelity holograms.

Editorial for the Special Issue on Optical MEMS.

Optical micro-electro-mechanical systems (MEMS), micro-opto-electro-mechanical systems (MOEMS), or optical microsystems are devices or systems that interact with light through actuation or sensing at a micron or millimeter scale [...].

Operation of a MOEMS Deformable Mirror in Cryo: Challenges and Results.

Micro-opto-electro-mechanical systems (MOEMS) Deformable Mirrors (DM) are key components for next generation optical instruments implementing innovative adaptive optics systems, both in existing telescopes and in the future ELTs. Characterizing these components well is critical for next generation instruments. This is done by interferometry, including surface quality measurement in static and dynamical modes, at ambient and in vacuum/cryo. We use a compact cryo-vacuum chamber designed for reaching 10⁻6 mbar and 160 K in front of our custom Michelson interferometer, which is able to measure performance of the DM at actuator/segment level and at the entire mirror level, with a lateral resolution of 2 µm and a sub-nanometer z-resolution. We tested the PTT 111 DM from Iris AO: an array of single crystalline silicon hexagonal mirrors with a pitch of 606 µm, able to move in tip, tilt, and piston (stroke 5⁻7 µm, tilt ±5 mrad). The device could be operated successfully from ambient to 160 K. An additional, mainly focus-like, 500 nm deformation of the entire mirror is measured at 160 K; we were able to recover the best flat in cryo by correcting the focus and local tip-tilts on all segments, reaching 12 nm rms. Finally, the goal of these studies is to test DMs in cryo and vacuum conditions as well as to improve their architecture for stable operation in harsh environments.

Optical characterization of two-dimensional array of 2,048 tilting micromirrors for astronomical spectroscopy.

A micromirror array composed of 2048 silicon micromirrors measuring 200 × 100 µm² and tilting by 25° was developed as a reconfigurable slit mask for multi-object spectroscopy (MOS) in astronomy. The fill factor, contrast, and mirror deformation at both room and cryogenic temperatures were investigated. Contrast was measured using an optical setup that mimics a MOS instrument, and mirror deformation was characterized using a Twyman-Green interferometer. The results indicate that the array exhibited a fill factor of 82%, a contrast ratio of 1000:1, and surface mirror deformations of 8 nm and 27 nm for mirrors tilted at 298 K and 162 K, respectively.