Matthias Goy

Performance of a thermal-piezoelectric deformable mirror under 6.2 kW continuous-wave operation

The thermal-piezoelectric deformable mirror (TPDM) is a device employed to compensate for laser-induced mirror deformation and thermal lensing in high-power optical systems. The TPDM setup is a unimorph deformable mirror with thermal and piezoelectric actuation properties. Laser-induced thermal lensing is compensated for by heating of the TPDM. We show that this mirror can be applied to high-power laser systems of up to 6.2 kW laser power and high power densities of up to 2  kW/cm2. The piezoelectric stroke of the single actuators is between 1.5 and 4 μm and is not reduced by either the absorbed laser power or mirror heating.

Temporally-stable active precision mount for large optics.

We present a temporally-stable active mount to compensate for manufacturing-induced deformations of reflective optical components. In this paper, we introduce the design of the active mount, and its evaluation results for two sample mirrors: a quarter mirror of 115 × 105 × 9 mm3, and a full mirror of 228 × 210 × 9 mm3. The quarter mirror with 20 actuators shows a best wavefront error rms of 10 nm. Its installation position depending deformations are addressed by long-time measurements over 14 weeks indicating no significance of the orientation. Size-induced differences of the mount are studied by a full mirror with 80 manual actuators arranged in the same actuator pattern as the quarter mirror. This sample shows a wavefront error rms of (27±2) nm over a measurement period of 46 days. We conclude that the developed mount is suitable to compensate for manufacturing-induced deformations of large reflective optics, and likely to be included in the overall systems alignment procedure.

Real-time adaptive optics testbed to investigate point-ahead angle in pre-compensation of Earth-to-GEO optical communication

We explore adaptive optics (AO) pre-compensation for optical communication between Earth and geostationary (GEO) satellites in a laboratory experiment. Thus, we built a rapid control prototyping breadboard with an adjustable point-ahead angle where downlink and uplink can operate both at 1064 nm and 1550 nm wavelength. With our real-time system, beam wander resulting from artificial turbulence was reduced such that the beam hits the satellite at least 66% of the time as compared to merely 3% without correction. A seven-fold increase of the average Strehl ratio to (28 ± 15)% at 18 μrad point-ahead angle leads to a considerable reduction of the calculated fading probability. These results make AO pre-compensation a viable technique to enhance Earth-to-GEO optical communication.

Mounting with compliant cylinders for deformable mirrors.

A method is presented to mount large aperture unimorph deformable mirrors by compliant cylinders (CC). The CCs are manufactured from a soft silicone, and shear testing is performed in order to evaluate the Youngs modulus. A scale mirror model is assembled to evaluate mount-induced change of piezoelectric deformation, and its applicability for tightly focusing mirrors. Experiments do not show any decrease of piezoelectric stroke. Further it is shown that the changes of surface fidelity by the attachment of the deformable mirror to its mount are neglectable.

Cryogenic testing of a unimorph-type deformable mirror and theoretical material optimization

Abstract. The testing of a lightweight unimorph-type deformable mirror (DM) for wavefront correction in cryogenic instruments is reported. The presented mirror manufactured from the titanium alloy TiAl6V4 with a piezoelectric disk actuator was cooled to 86 K and characterized for thermally induced deformation and the achievable piezoelectric stroke between room temperature and 86 K. Through a finite element analysis, we obtained a first approximation in determining the exact temperature-dependent coefficient of thermal expansion (CTE) of the piezo material PIC151. Simulations were based on dilatometer measurements of the CTE of the TiAl6V4 mirror base between room temperature and 86 K. These investigations will enable the improvement of the athermal design of a unimorph-type DM.

Long-term stable active mount for reflective optics

We report on the development of an active mount with an orthogonal actuator matrix offering a stable shape optimization for gratings or mirrors. We introduce the actuator distribution and calculate the accessible Zernike polynomials from their actuator influence function. Experimental tests show the capability of the device to compensate for aberrations of grating substrates as we report measurements of a 110x105 mm2 and 220x210 mm2 device With these devices, we evaluate the position depending aberrations, long-term stability shape results, and temperature-induced shape variations. Therewith we will discuss potential applications in space telescopes and Earth-based facilities where long-term stability is mandatory.

Unimorph-type deformable mirror for cryogenic telescopes

Deformable mirrors can be used in cryogenic instruments to compensate for temperature-induced deformations. A unimorph-type deformable mirror consists of a mirror substrate and a piezoelectric layer bonded on substrates rear surface. A challenge in the design of the deformable mirror is the lack of knowledge about material properties. Therefore, we measured the coefficient of thermal expansion (CTE) of the substrate material TiAl6V4 between 295 K and 86 K. The manufactured mirror is characterized by an adaptive optical measurement setup in front of a test cryostat. The measured mirror deformations are feedback into a finite element model to calculate the CTE of the piezoelectric layer. We compare our obtained results to other published CTE-values for the piezoelectric material PIC151.

An active optics system for large UVOIR space telescopes

The next generation of large monolithic mirror space telescopes will use aberration correction to ensure resolution performance is maintained throughout their mission. This is due to the use of ever thinner and lighter primary mirrors, which are susceptible to deformation due to a range of effects such as mechanical stress and thermal changes. In this work, we outline our space-telescope design and its corresponding active optics system to correct for these aberrations. We also describe our laboratory system for testing the wavefront sensing and aberration correction capabilities of the active optics components, along with some preliminary experimental results.

Design of an active metal mirror for large space telescopes

Manufacturing telescopes with 4, 8 or 16 meter apertures is the most effective way to gather the light of faint exoplanets or look back towards the Big Bang. However, ultra-high optical quality large mirrors drastically increase the mass of such instruments - if made conventionally. Thinner, and hence lighter, primary mirrors suffer from gravity release, temperature changes and misalignment during launch. The resulting surface distortions as well as inherent surface errors which arise during manufacturing can be reduced by the implementation of active optics. We designed an active metal mirror as a key element for active optics in space. Our goal was to develop an ultra-stable, set-and-forget, lightweight active mirror with good wavefront correction performance. A simulation routine was developed to investigate the dependency between geometric parameters of the Deformable Mirror (DM) and the residual surface error after correction of Zernike modes. With the final 25 actuator mirror design we can achieve residual errors of less than 10 nm RMS for individual Zernike modes for an optical pupil of 103 mm diameter. It is able to withstand quasi-static launch loads, is insensitive to temperature changes and we can limit the overall weight to 2500 g including actuators and mirror mount.

Adapting the axial focus in high-power laser processing machines within mm-range

The High-Power Focus Mirror we present in this paper gives access to dynamic focus position adaptation along 3.6 mm in high-power laser manufacturing. We developed and tested a new thermo-mechanical design for a unimorph deformable mirror that provides an extensive focal length range down to -2 m focal length. Moreover, the mirror’s unique thermal characteristics enable high-power applications up to 6.4 kW (2000 W/cm²) with stable optical beam quality as thermal lensing is successfully suppressed. Thus, the laser’s optical beam quality M² is stable over the entire actuation and thermal range. We will describe the design and the characterization of the High-Power Focus Mirror. The mirror setup is based on a unimorph concept using a piezoelectric actuator and a thin glass substrate with a highly reflective multilayer coating. An integrated copper layer improves the heat dissipation. Providing maximum stroke, as well as excellent dynamic properties, the deformable mirror substrate is mounted by our established compliant cylinders [1]. Furthermore, we investigate the incorporation of the High-Power Focus Mirror into a commercial laser-cutting system. We set up a laser-cutting test bench including a multimode laser source, the focus mirror, a commercial laser processing head, and measuring instruments. In this assembly, we measure the achievable focus position range as well as the laser beam quality. With this focus mirror, we want to encourage new, innovative high-power application fields in 3D laser processing such as laser cutting, welding, and structuring.