MEMS deformable mirrors (DM) have many merits of low drive voltage, high response speed, small power consumption, low cost and small size. Its surface shape and displacement versus applied voltage are significant factors of MEMS DM. Phase-shifting interferometer (PSI) has many advantages such as non-contact, quickness and high precision. A phase-only liquid crystal spatial light modulator (LC-SLM), as a linear phase-shifter in PSI, is linear calibrated for its phase-shift characteristics. The PSI is set up to measure the static characteristic of MEMS DM. Five-step phase-shifting method is used to calculate the phase distribution from interference fringes, and Global phase unwrapping algorithm to solve the holes, noise and breakpoint of interfere images. Compared to the measurement results using Zygo instrument, these two experimental results are very close. The experiment results show, this measuring system is very reliable, convenient and cheap. Moreover, this test system need not stitch some fringe images to get the whole surface shape of the mirror like the Zygo instrument.
Wave-front sensor, as the main component of Adaptive optics system, detects light from the astronomic object or
reference sources. It aims to improve the utilization of light, especially for AO system work with the faint objects.
Compared with Shack-Hartmann sensor, pyramid wave-front sensor is a relatively new one with increased pupil
sampling and spatial resolution. Pyramid wave-front sensor uses a refractive element (the pyramid) to produce four
images of the entrance pupil. Usually, Single pyramid prototypes are made using the classical figuring and polishing
techniques. This approach, however, is not only very time consuming but also does not guarantee a uniform repeatability
of the optical characteristics of the pyramids. The loss of low frequency component increases due to the roofs existing on
its vertexes. Moreover, stray light is introduced in the four images. We therefore are investigating a modified pyramidal
optical components based on the binary optical concept. In this article we describe the diffractive pyramid prototypes
using the micro fabrication technique. The parameters of the pyramid are discussed.
A new pyramid wavefront sensor (PWFS), which utilizes a reflective pyramid mirror instead of a refractive pyramid prism at the focus of a telescope, is presented. As a key optical component in this PWFS, the pyramid mirror requires accurate microfabrication for excellent quality of the tip, the turned edges, and the surfaces. The moving mask lithography process is proposed for its economic, simple, and precise control to make the cross-sectional shape of the structure. The completed pyramid mirror has a square base of 1-mm length and four side facets inclined to the base at 3.7 deg. The sizes of the pyramid tip and turned edges are both about 6 µm, which show excellent aspects of sharpening-tip and knife-edges. The root mean square of four facets is approximately 70 nm, and the maximum profile deviation is 0.2 µm.
MEMS deformable mirror (DM) has yet to be incorporated into a facility AO instrument especially for atmospheric
compensation. Apart from these drawbacks such as limited stroke, reliability, its optical efficiency should
also be concerted. The wavefront corrector in AO system should existing high optical efficiency especially atmospheric
compensation under faint stars. However the MEMS DM fabricated by surface process must have etch
holes in the surface of mirror. The diffraction results from etch holes decrease its optical efficiency. An MEMS
deformable mirror is fabricated by commercial PolyMUMPs. There is array of etch holes to ensure that the
sacrifice layer is released fully. The far field intensity distribution was simulated. The result has been analyzed
and used to constructed a simple modal of the studied MEMS DM.
A homogeneous aligned nematic liquid crystal (LC) cell can be used to phase modulate light. It has a series of
attractive characteristics of compactness, high density integration, low cost and possibility of batch production
in adaptive optics. However a problem has long existed for such devices is that they may be used only to control
the phase of light polarized along the LC extraordinary axis since only the extraordinary light index can be
varied by the application of the electric field. For a liquid crystal adaptive optics system using for astronomical
imaging, low level un-polarized light is collected by the telescope. So the high optical efficiency is important
and key factor for an adaptive optics system using for compensate atmosphere turbulence. If a polarizer placed
before LC, 50% of incident un-polarized light is wasted. In this paper, a simple method is detailed described
for phase modulating un-polarized light. Un-polarized light can be thought of as the superposition of any two
orthogonal polarization states that are mutually incoherent.
Iceland-spar OE crystal split incident un-polarized
light into two polarized light, two same LCs modulated these two polarized light separately. After that, both
these two polarized light beam are combined using another
Iceland-spar OE crystal. These double LC adaptive
optics system can phase modulate all incident un-polarized light, no light intensity is wasted.
For many astronomical systems, Adaptive Optics (AO) plays an important role. Here, we report some preliminary studies
on MEMS (Micro-Electro-Mechanical-System) Project for micro actuators in AO applications at the Institute of Optics
and Electronics, Chinese Academy of Science. This paper presents a few MEMS actuators based on repulsive
electrostatic driven mechanism, which can achieve large out-of-plane strokes through eliminating the electrostatic pull-in
effect. Design principles, including the layout and the physical dimension of electrodes, and FEA models are illustrated;
it provides helpful guidance for designing electrostatic repulsive actuators for being implemented in Deformable Mirrors
(DMs). Some repulsive electrostatic driven micro actuators are given, the analysis focus on the displacement versus
applied voltage and resonant frequency. Repulsive electrostatic driven actuators can achieve large strokes and high
resonant frequencies, they meet the important requirements for DMs.
Liquid crystal spatial light modulator (LC-SLM) has a series of attractive characteristics as a wave-front corrector of
adaptive optical system such as compactness, high density integration, low cost and possibility of batch production.
However, it also has some limitations such as effective only on polarized light, chromatic aberration and low response
speed and limited phase stroke. In order to investigate these pros and cons in detail, a series of experiments and analyses
have been conducted using a LC-SLM Model P256 of Boulder Nonlinear System. The nonlinear phase response to
applied voltage was measured for 1different wavelengths. The coupling between neighboring elements and response
time were also measured. The error of phase wrapping for multi-wavelength wave-front of broad spectral band light was
studied. The fitting capability to Zernike polynomials was demonstrated. An adaptive optical system with this LC-SLM
and Shack-Hartmann sensor was close-looped. As a result, the applicability of LC-SLM for different applications will be
evaluated and discussed.
For some adaptive optics (AO) systems, one deformable mirror (DM) can not meet the need of large stroke and high
spatial frequency. In this paper, a double DMs way is present. In this system, a large stroke DM (LSDM) with low
spatial frequency corrects low order aberrations and a high spatial frequency DM (HSFDM) with small stroke corrects
high order aberrations. The decoupling algorithm of two DMs is essential for working properly. In this paper, a
decoupling algorithm and experimental results for a double DM AO system are presented. The result indicates that the
compensation result of double DMs AO system is almost the same as that of the conventional AO system using single
DM with ideal stroke and equivalent spatial frequency.