Quantum communication is considered to be a key feature for secure communication e.g. between government organisations or other institutions with high security requirements. Therefore, the QuNET initiative was founded. It focuses on developing a quantum-secure German governmental agency network based on quantum key distribution (QKD). Free-space optical (FSO) links are a valuable part of infrastructure because they can be deployed temporarily, such as at summits or to bridge the last miles where there is no fiber infrastructure. In particular, high-throughput telescopes are of great importance as optical antennas for terrestrial networks or links between mobile nodes. The paper describes the development and manufacturing of an unobscured, afocal four-mirror metal telescope which is already tested for ground-based quantum communication. The off-axis system, operating with a full telescope aperture of 200 mm, a magnification of 20x, and a FOV (field of view) of 3.5 mrad and is designed to yield diffraction-limited performance for an operational wavelength of 810 nm and 1550 nm. The addressed wavefront error-target of the whole system amounts to 66 nm RMS (root mean square). The use case of the telescope implied an operational temperature range of -40 °C up to +50 °C. Therefore, an athermal system is realized using an aluminum-silicon alloy substrate material combined with a nickel-phosphorus polishing layer that allows to reach the required surface quality of the mirrors. To simplify the alignment of the telescope, its mechanical concept relies on a snap-together approach using two substrates with two optical mirrors on a common substrate, each. The manufacturing chain of these two so called mirror substrates is described in detail. That includes the CNC pre-manufacturing, ultra precision diamond turning and subsequent polishing steps. The resulting quality of the mirror substrates as well as of the telescope system is demonstrated by optical measurements using interferometric setups.
The usage of CubeSat platforms has seen a significant increase over the last decade. CubeSats are compact and cost-efficient. With it the need for free-space optical communication (FSOC) between satellites and optical ground stations did increase as well. Achieving a good ratio between the size and the performance of the optical payload is still a challenge. We propose a FSOC terminal to be implemented on a 16U CubeSat platform. A combination of a coarse pointing assembly (CPA) based on a dual Risley prism scanner in front of an all-metal freeform telescope, and a fine pointing assembly (FPA) with a fast-steering mirror (FSM) were developed. The Risley prisms have a smaller mechanical envelope compared to classical gimbal-based mirror mounts or periscopes but can still provide a suitable range of beam deflection. The use of such a Risley prism scanner has been a research topic in aerospace for quite some time. Especially the challenging driving and controlling of the nonlinear beam pointing behavior presents a challenge. To keep the payload on the CubeSat feasibly small, only a microcontroller with limited calculation power is used. Therefore, we propose a combined control scheme for the CPA and FPA based on simplified calculations and the use of classical digital control theory. Coarse and fine pointing are controlled in a closed loop pointing simultaneously.
In recent years, quantum key distribution (QKD) has seen the first proof-of-concept demonstrations from space. Next on the agenda towards a full-blown global quantum internet is to address the more practical aspects, such as efficiency, flexibility, and accessibility of QKD services. One of the main challenges that remains to be solved in this regard is to enable operation in the presence of daylight noise. Here, we present a complete framework for modelling daylight QKD from an orbiting satellite. We include the effects of atmospheric turbulence and adaptive optics correction at the receivers. We consider single- and multi-mode fibre coupling as a means of spatial filtering, for which we derived simple formulas for estimating coupling efficiencies of signal as well as noise. Using our framework, we identify the most critical system parameters for daylight operation and discuss the choice of signal wavelength and detection technology. Finally, we provide simulation results for various parameter combinations in a hypothetical daylight QKD between Berlin and Munich via a satellite in a low Earth orbit. The results show a clear advantage of 800 nm signal wavelength over 1550 nm with the currently available technology. Moreover, we show the relevance of single-mode fibre coupling and the importance of detectors with low timing jitters. We anticipate our work will provide valuable insight and tools to aid the future feasibility studies of daylight QKD in dual-downlink configurations. Additional presentation content can be accessed on the supplemental content page.
Even high-end optical components exhibit small amounts of imperfections, which can easily limit the performance of optical systems with respect to imaging contrast, optical throughput, imaging ghosts, and increased light scattering. Characterizing the scattering properties of optical components is thus an important step during the development of sophisticated optical systems as well as to identify and steadily improve materials as well as manufacturing and assembling steps. This is illustrated for different optical components as well as optical systems. Furthermore, different characterization concepts are discussed, which allow overcoming typical limits for angles resolved light scattering measurements, such as scattering very close to the specular beam directions (off specular scattering angles < 0.1°) or measurements in retro-reflection, which are important for gratings used in Littrow configuration or optical mirrors for laser-based communication
Secure communication networks are the critical infrastructure of the information age. To ensure secure communication between governmental institutions and other high-security environments, the German Federal Ministry of Education and Research (BMBF) initiated an ambitious project { the QuNET initiative. In a joint effort, the Max Planck Institute for the Science of Light (MPL), the German Aerospace Center (DLR) and the Fraunhofer Society aim to develop the technological basis of a German quantum key distribution (QKD) infrastructure. This paper describes the infrastructure used in a first link demonstrator within this project and how we achieved the transition from initial quantum transceiver concepts to first link experiments.
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.
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.
An active support for large reflective optics is suitable to compensate for manufacturing-induced deformations and (re-)positioning-induced deformations such as induced by slewing of earth-based telescopes.
KEYWORDS: Space telescopes, Mirrors, Telescopes, Deformable mirrors, Actuators, Wavefront sensors, Wavefronts, Monte Carlo methods, Active optics, Astronomical imaging
While ambitious plans are being developed for giant, segmented telescopes in space, we feel that a large monolithic mirror telescope would have several advantages in the near term. In particular, the risk involved in deploying the optics will be significantly reduced, and the telescope can provide excellent image quality without the need for precise segment alignment and phasing.
Large UVOIR (ultraviolet-optical-infrared) space telescopes that are going to be designed within the next decades are intended to answer the question about life on exoplanets [1], [2].
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.
KEYWORDS: Space telescopes, Wavefront sensors, Mirrors, Active optics, Deformable mirrors, Telescopes, Actuators, Wavefronts, James Webb Space Telescope, Infrared telescopes
The next generation of UVOIR space telescopes will be required to provide excellent wavefront control despite
perturbations due to thermal changes, gravity release and vibrations. The STOIC project is a response to an ESA
Invitation to Tender to develop an active optics correction chain for future space telescopes. The baseline space telescope
being considered is a two-mirror, 4m telescope with a monolithic primary mirror – we refer to this concept as Hypatia.
The primary mirror diameter could be extended, but is limited in the near future by launch vehicle dimensions. A
deformable mirror (pupil diameter 110mm) will be an integral part of the telescope design; it is being designed for high
precision and the ability to maintain a stable form over long periods of time. The secondary mirror of the telescope will
be activated to control tip-tilt, defocus and alignment with the primary. Wavefront sensing will be based on phase
diversity and a dedicated Shack-Hartmann wavefront sensor.
The project will develop a laboratory prototype to demonstrate key aspects of the active correction chain. We present the
current state of the preliminary design for both the Hypatia space telescope and the laboratory breadboard.
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.
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.
The speed of real-time adaptive optical systems is primarily restricted by the data processing hardware and computational aspects. Furthermore, the application of mirror layouts with increasing numbers of actuators reduces the bandwidth (speed) of the system and, thus, the number of applicable control algorithms. This burden turns out a key-impediment for deformable mirrors with continuous mirror surface and highly coupled actuator influence functions. In this regard, specialized hardware is necessary for high performance real-time control applications. Our approach to overcome this challenge is an adaptive optics system based on a Shack-Hartmann wavefront sensor (SHWFS) with a CameraLink interface. The data processing is based on a high performance Intel Core i7 Quadcore hard real-time Linux system. Employing a Xilinx Kintex-7 FPGA, an own developed PCie card is outlined in order to accelerate the analysis of a Shack-Hartmann Wavefront Sensor. A recently developed real-time capable spot detection algorithm evaluates the wavefront. The main features of the presented system are the reduction of latency and the acceleration of computation For example, matrix multiplications which in general are of complexity O(n3 are accelerated by using the DSP48 slices of the field-programmable gate array (FPGA) as well as a novel hardware implementation of the SHWFS algorithm. Further benefits are the Streaming SIMD Extensions (SSE) which intensively use the parallelization capability of the processor for further reducing the latency and increasing the bandwidth of the closed-loop. Due to this approach, up to 64 actuators of a deformable mirror can be handled and controlled without noticeable restriction from computational burdens.
KEYWORDS: Mirrors, Actuators, Deformable mirrors, Temperature sensors, Wavefronts, Sensors, Chemical elements, Copper, High power lasers, Wavefront sensors
Laser-induced mirror deformation and thermal lensing in optical high power systems shall be compensated by a
thermally-piezoelectric deformable mirror (DM). In our device, the laser-induced thermal lensing is compensated by
heating of the DM as previously described with compound loading. We experimentally show the capability of this mirror
for wavefront shaping of up to 6.2 kW laser power and power densities of 2 kW/cm2. The laser-induced defocussing of
the membrane is compensated by mirror heating. We introduce a new mirror setup with buried heater and temperature
sensor elements. Therewith, the compensation of laser-induced mirror deformation is possible within the same time
scale. The piezoelectric stroke of the single actuators depends on their position on the membrane, and is not affected by
the reflected laser power.
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.
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