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 present first, promising experiments with a novel, compact and simple Nd:YVO4 slab laser with 12 W of 1.06 μm optical output power and a beam quality factor M2 ∼ 2.5. The laser is made of a diffusion-bonded YVO4/Nd:YVO4 composite crystal that exhibits two unique features. First, it ensures a one-dimensional heat removal from the laser crystal, which leads to a temperature profile without detrimental influence on the laser beam. Thus, the induced thermo-optical aberrations to the laser field are low, allowing power scaling with good beam quality. Second, the composite crystal itself acts as a waveguide for the 809 nm pump-light that is supplied from a diode laser bar. Pump-light shaping optics, e.g. fast- or slow-axis collimators can be omitted, reducing the complexity of the system. Pump-light redundancy can be easily achieved. Eventually, the investigated slab laser might be suitable for distortion-free high gain amplification of weak optical signals.
Future LIDAR and range-finding missions in space require high-energy pulsed lasers. Typical concepts require a pump laser in the NIR spectral range, which is frequency-converted to the requested wavelength by external measures. We propose a novel master-oscillator power-amplifier (MOPA) concept capable to deliver 500- mJ pulses of about 10 ns pulse width at a pulse repetition frequency of 100 Hz. The output at 1064 nm is singlefrequency, linearly polarized, and exhibits high beamquality.
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
GSI Darmstadt currently builds a high-energy petawatt Nd:glass laser system, called PHELIX (Petawatt High-Energy Laser for Heavy-Ion Experiments). PHELIX will offer the world-wide unique combination of a high current, high-energy heavy-ion beam with an intense laser beam. Aberrations due to the beam transport and
due to the amplification process limit the focusability and the intensity at the target. We have investigated the
aberrations of the different amplification stages. The pre-amplifier stage consists of three rod-amplifiers which
cause mainly defocus, but also a small part of coma and astigmatism. The main amplifier consists of five disk
amplifiers with a clear aperture of 315 mm. These large
disk-amplifiers cause pump-shot aberrations which occur
instantly. After a shot, the disk amplifiers need a cooling time of several hours to relax to their initial state.
This limits the repetition rate and causes long-term aberrations. We will present first measurements of the
pump-shot and long-term aberrations caused by the pre- and the main amplifier in a single-pass configuration.
In this context, we will present the adaptive optics system which is implemented in the PHELIX beam line and
discuss its capability to compensate for the pump-shot and long-term aberrations.
Adaptive laser resonators with deformable MOEMS mirrors under closed-loop control are discussed and experimental results are presented. The requirements for deformable mirrors and for closed-loop control systems of these mirrors are analyzed. Several deformable mirrors have been characterized and the results are presented. Currently available membrane mirrors deform under laser load and need further development before they can be used for aberration correction of solid state lasers above some tens of Watts. Nevertheless, the results are encouraging and the requirements are within reach of currently available technology. Finally, we demonstrate an Nd.YVO4-laser with a closed-loop adaptive resonator and more than 6 W of output power. The closed-loop system was able to compensate artificially introduced aberrations from a phase plate.
We present experimental and theoretical results on aberration control in solid state laser amplifiers and resonators. In lasers with diffraction-limited beam quality, aberrations cause diffraction losses that reduce the output power. In laser amplifiers, aberrations in the active medium degrade the beam quality of the amplified beam. Adaptive optics can be used to correct for the aberrations and thus increase output power and beam quality, respectively.
The required precision of the adaptive aberration correction can be estimated with a simple mode expansion model in which an aberrated TEM00 mode is expanded into Gauss-Hermite modes. Apart from these theoretical results we will present experimental results for a MOPA (Master-Oscillator-Power-Amplifier) laser system consisting of a Nd:YVO4 master oscillator and two Nd:YAG power amplifiers. A micromachined deformable mirror was used in a closed-loop system to correct for the thermo-optic aberrations of the amplifiers. A beam with a beam quality of M2= 2.5 at an out power of 80 W was obtained. The deformable mirror was controlled by a genetic algorithm.
The practical aspects of measuring and defining the beam parameter products of laser beams from high-average-power solid state lasers with unstable resonators are presented. In particular, measurements at kW-class Nd:YAG slab laser are discussed. Emphasis is on experimental techniques, measurement errors, and the influence of thermo-optical aberrations. Numerical Kirchhoff-integral calculations of the unstable resonator modes of a slab laser with thermo-optical aberrations are presented and discussed.
In this talk the affects that determine the beam quality of high power Nd:YAG lasers are discussed. Suggestions are made on how the beam quality can be improved. Experimental results concerning the relationship of output power and beam quality are presented. The different sources of thermo-optical aberrations in rod and slab lasers that influence the beam quality of stable resonators are discussed. Design criteria for rod and slab lasers with improved beam quality are presented and the direction of future work is outlined.