Research Reports: Active and adaptive optics

The Fraunhofer IOF offers for customers from industry and research the development and testing of application-specific deformable mirrors and complete AO systems for active beam shaping. Active and adaptive optics show their potential in laser material processing, space telescopes, or laser and quantum communication, among other things.

Could adaptive or active optics are a solution for your beam shaping problem?

Find out about our technologies, licenses, and range of services for customers from industry and science by clicking here:

Active and adaptive optics

Below you will find research reports on our recent AO developments:

Adaptive optics box for quantum communication ground station

Together with Synopta GmbH (Switzerland), Fraunhofer IOF has developed an adaptive optics module for a quantum communication ground station for the Institute for Quantum Optics and Quantum Information (IQOQI), Austria. The “Quantum Communication Adaptive Optics Box” (Q-AO-box) will be mounted on the flange of an 80 cm Ritchey-Chrétien telescope and will correct the atmospheric turbulence-induced phase aberrations of the incoming satellite-to-ground quantum signal, enabling coupling of the signal into a single-mode optical fiber.

A satellite source emits a quantum signal and a brighter “beacon” beam at two different wavelengths. On the ground, the Q-AO-box measures the atmospheric turbulence effects on the beacon using a wavefront sensor. This is then used to control a tip-tilt- and multi-actuator deformable mirror which corrects both the beacon and quantum signals in real-time, regardless of the wavelength difference. This enables single-mode fiber coupling of the quantum signal, which then can be directly measured or shared further via a fiber network.

For maximum flexibility in the choice of future experiments, the Q-AO-box has been designed for a wavelength range of 650 – 730 nm for the beacon and 770 nm – 860 nm for the quantum signal. It has the critical property for quantum experiments; it is polarisation independent and has very high transmission over the wavelength range.

The optical system is integrated into an extremely stable, transportable, temperature-regulated, and dustproof box. The fiber-coupling unit is accessible via the separately isolated “User-box” (bottom right of the unit) which includes a mini-breadboard for the flexible integration of additional optics.

Authors: Aoife Brady, Andreas Kamm, Teresa Kopf, Edgar Fischer (Synopta GmbH), Rupert Ursin (IQOQI - Vienna, Austrian Academy of Sciences)

Download

Data sheet: AO-box for quantum communication

Deformable mirrors for highly dynamic beam oscillation

The selective coupling of energy into the workpiece plays a crucial roll in laser material processing. In conventional laser processing systems, highly dynamic scanners are used for beam manipulation in the X- and Y-axes. The focus shift in the Z-direction is realized by sliding lens systems or deformable mirrors which are activated either hydraulically or pneumatically. These systems impose a significant limitation on the dynamics of the overall system. In the fields of laser cutting and welding, in particular, preliminary investigations by colleagues at the Fraunhofer IWS show that the use of highly dynamic beam oscillation in the Z-direction significantly increases processing speed and improves process stability.

To increase the system dynamic range, an active deformable mirror was developed as a Z-axis within the scope of the Zwanzig20 project "PISTOL³". Figure 2 shows the principle design of the deformable mirror and its schematic integration into the construction of a laser processing head. The critical component of the deformable mirror is a membrane, the special design of which allows the correction of astigmatic aberration due to 90° beam deflection on a curved mirror with simultaneous surface deformation. The required toric surface is generated with a prestressed piezoelectric actuator. Accordingly, the central deflection of the mirror is determined by the applied voltage. In the non-deformed or plane state, a collimated laser beam is deflected by the deformable mirror and focused by a lens at the distance of the lens’s nominal focal length. When the mirror is activated, it acts as a convex mirror and thus the laser beam diverges after it has passed the mirror. This changes the focal length of the system and, as a result, the focus position of the beam shifts. The resulting focal length changes depending on the deformation or effective focal length of the mirror surface.

The developed deformable mirror (Fig. 1) has an elliptical aperture of 60 mm x 30 mm. In combination with a focusing lens with a focal length of 200 mm, a focus shift of 18 mm with an oscillation frequency of up to 8.5 kHz is achieved. In addition, the mirror has a high-performance coating whose suitability has been established in tests with up to 4 kW CW laser power. Initial experimental investigations show an increase of the processing speed in laser cutting by up to 60 % with consistently high edge quality. In the application of laser beam welding, an increase of the process stability and a reduction in the porosity of the weld seam could be demonstrated.

Authors: Claudia Reinlein, Paul Böttner

back to top

Dynamic focus-shifting mirror for laser processing

The dynamic focus-shifting mirror enables fast focal length changes in optical systems and can be used, for instance, to adapt the focus position in laser processing machines (Fig. 2). The mirror setup is based on a unimorph concept using a piezoelectric disc bonded to a thin glass substrate with a highly reflective multilayer coating (Fig. 1). An integrated copper layer improves the heat dissipation and thus increases the mirror’s laser damage threshold. When a voltage is applied to the piezoelectric disc, it bends the mirror substrate, which in turn changes the radius of curvature of the mirror (Fig. 2). This specific mechanical design enables the mirror surface to bend down to a radius of curvature of 3.4 m.

In a verification test, the focus mirror was combined with a commercial processing head. A collimated beam from a multi-mode laser source was reflected at an angle of 45° by the focus mirror and afterwards focused by the processing head. Through the actuation of the mirror, a focus shift of 3.6 mm with stable optical beam quality was achieved. The focal length range is defined by the nominal focal length of the processing head and the mirror’s radius of curvature. If the mirror is combined with a processing head of 200 mm focal length, an even larger focal length change of 20 mm is achievable.

The mirror’s step response was characterized and optimized by modulating the excitation signal. In doing so, a short response time of less than 2 ms for full deflection is possible, which is significantly faster than commonly known state of the art solutions for focus shifting.

Thanks to the actuator’s short response time, it reduces the laser processing time compared to commercial solutions for focus shifting. Based on the mirror’s extensive focal length change and its convincing thermal properties, we see application fields primarily in laser cutting and welding. As proven by our measurements, the mirror can be used for high-power applications up to 6 kW laser power (continuous wave) at 1064 nm wavelength. The mirror’s surface only has a small and non-power-related influence on the beam quality factor M², which was slightly increases from M² = 2 (only laser source) to M² = 3 by using the focus mirror.

The mirror is designed for a beam diameter of 1” and can easily be integrated into processing heads using commercially available 3” mirror mounts. Application-specific customizations such as coatings for further laser wavelengths, beam aperture or the adaption of the mirror as 90° deflection mirror are possible.

Authors: Teresa Kopf, Claudia Reinlein

back to top

Adaptive optical system for earth-space laser communication

Within an ESA StarTiger project, a breadboard has been developed which emulates adaptive optics (AO) assisted laser communication between an optical ground station (OGS) and a geostationary satellite. Figure 2 depicts the baseline scenario between ESA‘s OGS on Tenerife and a laser communication satellite. The satellite emits a laser beam downlink that is diffraction limited until it passes through the atmosphere. Atmospheric turbulences impose wavefront aberrations on the downlink signal leading to beam wander, broadening, and higher-order aberrations. In our scenario the uplink beam travels from the OGS to the satellite, passing through the atmosphere in such an equivalent manner that a distorted beam arrives at the satellite with deteriorated efficiency of the communication link. In addition, a point-ahead angle (PAA) must be considered for the uplink beam due to the finite speed of light and the relative velocity of OGS and satellite. The OGS must point ahead of the satellite trajectory to ensure that the uplink beam hits satellites telescope. Figure 1 depicts the developed AO system assembled from a deformable mirror, tip/tilt mirror and Shack-Hartmann sensor working in a closed loop to compensate for the aberrations caused by an etched aberration emulator. In contrast to the realized systems, this 1 kHz fast AO system uses the downlink to measure atmospheric-like wavefront aberrations and then pre-compensates the uplink beam sent back to the satellite.

Accordingly, the uplink beam quality at the receiver, which can be quantified by the Strehl ratio S, is improved from S = (4 ± 4) % without compensation to S = (28 ± 15) % with compensation and applied PAA.

In the limited time frame of only 9 months, Fraunhofer IOF, together with TU Ilmenau and FSU Jena, developed a bread-board with a real-time closed loop system that emulates the described scenario providing the proof-of-concept for efficient pre-compensation. In fact, uplink and downlink are compensated for in this setup. Further, the setup is diffraction limited at 1064 nm and 1550 nm in order to investigate this two typical laser communication wavelength. Despite the impact of the PAA on the compensation, efficiency may be flexibly studied. The developed InGas-based wavefront sensor works for both wavelengths and with the lowest light intensities in order to be integrated in the real application later.

Authors: Claudia Reinlein, Nina Leonhard, René Berlich, Stefano Minardi (FSU Jena), Alexander Barth (TU Ilmenau)

back to top

Further information

Emerging technologies

Get an overview of the entire range of services offered by our scientific department "Emerging Technologies" at Fraunhofer IOF.

Overview

Further information on optical quantum technologies