Over the past 5 years we have developed a new type of unimorph deformable mirror. The main advantages of
this mirror technology are
· very low surface scattering due to the use of superpolished glass
· excellent coatings, even suitable for high power lasers, can be applied
· active diameter of the mirrors can be between 10 mm and 100 mm
· large strokes can be achieved even for small mirror diameters
· integrated monolithic tip/tilt functionality based on a spiral arm design
We have modeled these mirrors by analytical models as well as by the finite element method. This allows us
to quickly design new mirrors tailored to specific applications. One example is a mirror for laser applications
that has a diameter of 10 mm and can achieve a stroke in defocus mode of 5 μm. The stroke for these mirrors
scales as the square of the mirror diameter, meaning that we can achieve, for example, a stroke of 125 μm for a
mirror of 50 mm diameter. We will present design criteria and tradeoffs for these mirrors. We characterize our
mirrors by the maximum stroke they can deliver for various Zernike modes, under the boundary condition that
the Zernike mode has to be created with a certain fidelity, usually defined by the Maréchal criterion.
With a view to future large space telescopes, we investigate image-based wavefront correction with active optics. We use an image-sharpness metric as merit function to evaluate the image quality, and the Zernike modes as control variables. In severely aberrated systems, the Zernike modes are not orthogonal to each other with respect to this merit function. Using wavefront maps, the PSF, and the MTF, we discuss the physical causes for the non-orthogonality of the Zernike modes with respect to the merit function. We show that for combinations of Zernike modes with the same azimuthal order, a flatter wavefront in the central region of the aperture is more important than the RMS wavefront error across the full aperture for achieving a better merit function. The non-orthogonality of the Zernike modes with respect to the merit function should be taken into account when designing the algorithm for image-based wavefront correction, because it may slow down the process or lead to premature convergence.
Active optics is an enabling technology for future large space telescopes. Image-based wavefront control uses an image-sharpness metric to evaluate the optical performance. A control algorithm iteratively adapts a corrective element to maximize this metric, without reconstructing the wavefront. We numerically study a sharpness metric in the space of Zernike modes, and reveal that for large aberrations the Zernike modes are not orthogonal with respect to this metric. The findings are experimentally verified by using a unimorph deformable mirror as corrective element. We discuss the implications for the correction process and the design of control algorithms.
We have developed a new type of unimorph deformable mirror, designed to correct for low-order Zernike modes. The mirror has a clear optical aperture of 50 mm combined with large peak-to-valley Zernike amplitudes of up to 35 μm. Newly developed fabrication processes allow the use of prefabricated super-polished and coated glass substrates. The mirror’s unique features suggest the use in several astronomical applications like the precompensation of atmospheric aberrations seen by laser beacons and the use in woofer-tweeter systems. Additionally, the design enables an efficient correction of the inevitable wavefront error imposed by the floppy structure of primary mirrors in future large space-based telescopes. We have modeled the mirror by using analytical as well as finite element models. We will present design, key features and manufacturing steps of the deformable mirror.
Concepts for future large space telescopes require an active optics system to mitigate aberrations caused by thermal deformation and gravitational release. Such a system would allow on-site correction of wave-front errors and ease the requirements for thermal and gravitational stability of the optical train. In the course of the ESA project "Development of Adaptive Deformable Mirrors for Space Instruments" we have developed a unimorph deformable mirror designed to correct for low-order aberrations and dedicated to be used in space environment. We briefly report on design and manufacturing of the deformable mirror and present results from performance verifications and environmental testing.
We have developed, manufactured and tested a unimorph deformable mirror for space applications based on
piezoelectric actuation. The mirror was designed for the correction of low-order Zernike modes with a stroke of several
tens of micrometers over a clear aperture of 50 mm. It was successfully tested in thermal vacuum, underwent lifetime
tests, and was exposed to random vibrations, sinusoidal vibrations, and to ionizing radiation. We report on design
considerations, manufacturing of the mirror, and present the test results. Furthermore, we discuss critical design
parameters, and how our mirror could be adapted to serve recently proposed space telescopes such as HDST and TALC.
Astronomy is driven by the quest for higher sensitivity and improved angular resolution in order to detect fainter or smaller objects. The far-infrared to submillimeter domain is a unique probe of the cold and obscured Universe, harboring for instance the precious signatures of key elements such as water. Space observations are mandatory given the blocking effect of our atmosphere. However the methods we have relied on so far to develop increasingly larger telescopes are now reaching a hard limit, with the JWST illustrating this in more than one way (e.g. it will be launched by one of the most powerful rocket, it requires the largest existing facility on Earth to be qualified). With the Thinned Aperture Light Collector (TALC) project, a concept of a deployable 20 m annular telescope, we propose to break out of this deadlock by developing novel technologies for space telescopes, which are disruptive in three aspects:
• An innovative deployable mirror whose topology, based on stacking rather than folding, leads to an optimum ratio of collecting area over volume, and creates a telescope with an eight times larger collecting area and three times higher angular resolution compared to JWST from the same pre-deployed volume;
• An ultra-light weight segmented primary mirror, based on electrodeposited Nickel, Composite and Honeycomb stacks, built with a replica process to control costs and mitigate the industrial risks;
• An active optics control layer based on piezo-electric layers incorporated into the mirror rear shell allowing control of the shape by internal stress rather than by reaction on a structure.
We present in this paper the roadmap we have built to bring these three disruptive technologies to technology readiness level 3. We will achieve this goal through design and realization of representative elements: segments of mirrors for optical quality verification, active optics implemented on representative mirror stacks to characterize the shape correction capabilities, and mechanical models for validation of the deployment concept. Accompanying these developments, a strong system activity will ensure that the ultimate goal of having an integrated system can be met, especially in terms of (a) scalability toward a larger structure, and (b) verification philosophy.
We have developed a new type of unimorph deformable mirror for the correction of low-order Zernike modes. The
mirror features a clear aperture of 50 mm combined with large peak-to-valley amplitudes of up to 35 μm. Newly
developed fabrication processes allow the use of prefabricated, coated, super-polished glass substrates. The mirror's
unique features suggest the use in several astronomical applications like the compensation of atmospheric aberrations
seen by laser beacons and the use in woofer-tweeter systems. Additionally, the design enables an efficient correction of
the inevitable wave-front error imposed by the floppy structure of primary mirrors in future large space telescopes. We
have modeled the mirror by using analytical as well as finite element models. We will present design, key features and
manufacturing steps of the deformable mirror.
We have developed a new type of unimorph deformable mirror for real-time intra-cavity phase control of high power
cw-lasers. The approach is innovative in its combination of super-polished and pre-coated highly reflective substrates,
the miniaturization of the unimorph principle, and the integration of a monolithic tip/tilt functionality. Despite the small
optical aperture of only 9 mm diameter, the mirror is able to produce a stroke of several microns for low order Zernike
modes, paired with a residual static root-mean-square aberration of less than 0.04 μm.
In this paper, the characteristics of the mirror such as the influence functions, the dynamic behavior, and the power
handling capability are reported. The mirror was subjected to a maximum of 490 W of laser-light at a wavelength of
1030 nm. Due to the high reflectivity of over 99.998 % the mirror is able to withstand intensities up to 1.5 MW/cm2.
We present a new type of unimorph deformable mirror with monolithic tip-tilt functionality. The tip-tilt actuation is
based on a spiral arm design. The mirror will be used in high-power laser resonators for real-time intracavity phase
control. The additional tip-tilt correction with a stroke up to 6 μm simplifies the resonator alignment significantly. The
mirror is optimized for a laser beam footprint of about 10 mm. We have modeled and optimized this mirror by finite
element calculations and we will present design criteria and tradeoffs for this mirrors. The mirror is manufactured from
a super-polished glass substrate with very low surface scattering and excellent dielectric coating.
We present a novel unimorph deformable mirror with a diameter of only 10 mm that will be used in adaptive resonators
of high power solid state lasers. The relationship between applied voltage and deformation of a unimorph mirror depends
on the piezoelectric material properties, layer thicknesses, boundary conditions, and the electrode pattern. An analytical
equation for the deflection of the piezoelectric unimorph structure is derived, based on the electro-elastic and thin plate
theory. The validity of the proposed analytical model has been proven by numerical finite-element modelling and
experimental results. Our mirror design has been optimized to obtain the highest possible stroke and a high resonance
Using electronic isolators as substrates for organic thin films one has to take care of long living surface charges which might influence or even control the film deposition. Unknown charges may be the origin of poor reproducibility. On the other side these charges may be used for controlled patterning as we will demonstrate in this paper. The described phenomenon might be of interest for electronic displays or patterned OLED's. Presented are different methods to deposit surface charges. The molecular orientation is investigated by measuring the angular dependence of the absorption coefficient.
Evaporated films of the azo-material DR 1 have been investigated. In the as-grown state partly crystalline films with low transmittance are obtained. Using homogeneous exposure transparent regions may be formed. The recording of holographic gratings in thin films (< 1 μm) of the azo dye is investigated for the case of more-dimensional
light-intensity patterns. The mechanism of the photo-isomerisation of the azo-compounds is used to form dual gratings with a refractive index grating and a surface-relief grating. The grating-formation is investigated in case of 1D-gratings first. The time dependent diffraction efficiency is discussed in a model of 2 processes with different time-constants. A material transport process is involved in the formation of relief patterns. The enhancement of the modulation-depth of the surface-relief gratings is investigated for the application of a Corona discharge and a thermal treatment after the holographic recording. 2D-gratings are formed using either a 3-beam holographic set-up or a consecutive method. The resulting light patterns are simulated. Diffraction patterns and AFM-measurements are used to confirm these simulated structures. The modulation of the surface-relief gratings can be enhanced by thermal treatment after the holographic recording.