Optical feeder-links play crucial role in closing the global broadband coverage gaps. Point-to-point GEO satellite links provide advantage of uninterrupted availability limited only by ground conditions at the cost of higher SWaP to be able to close the link budget. By increasing field-of-view of the on-board optical terminals to approximately 1◦ × 4◦ , coverage over large enough area for exploitation of site diversity with more than ten optical ground stations and thus network availability above 99.9% can be achieved. We discuss design challenges and constraints together with trade-off evaluation towards the final design. We present optical and mechanical design of a payload prototype, including telescope with 250mm aperture, capable of tracking multiple optical ground stations over entire Mediterranean region. The tracking system concepts are presented to show potential compensation of the orbital effects that arise due to platform vibrations and orbital inclination. The preliminary results of the breadboard´s verification and acceptance process are presented. Insight into manufacturing, assembly, integration and testing stage of the individual prototype sub-assemblies will be given. Finally, system trade-off between various concepts as well as between the traditional use of multiple optical payloads and the presented baseline will be shown and discussed. The goal is to demonstrate the practical application potential of the multiple optical receive systems for future optical GEO feeder-links.
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.
Low-stress bonding using the laser-based soldering technique Solderjet Bumping is presented for the assembly of optical systems. Selected systems include SQ1 and INVAR 36 (baseline system), CaF2 and 1.4371 stainless steel, BBO and Dilver P1, and KTP and Super Invar, the first two proposed as lens demonstrators and the other two as laser crystal demonstrators. Solder parameters found for the SQ1 and INVAR using the DoE method using 400μm SAC305 alloy with the desirability of 0.655, for BBO and Dilver P1 and CaF2 and stainless steel 1.4371 systems, parameters must be changed due to damage observed in the optical components. KTP and Super Invar system parameters are still under investigation. In the case of BBO, the parameters were successfully found while CaF2 is still under investigation, changing the solder bump size due to the constant presence of damage in the samples. Finite elements simulations show that excessive stress occurs locally at the singularity of the soldered interface, but is still below the critical value, nevertheless, the survival of the demonstrators under environmental loads (Δ60 K, 600 g) is expected.
Current satellite optical communications systems, e.g. EDRS, include a single optical terminal for point-to-point links. In case of a hand-over to a next available location, the optical terminal must actively repoint and reacquire the signal. We investigate a novel satellite telescope design covering multiple optical ground stations within its field-of-view.
Additionally, orbital perturbations, mainly the ones due to inclination, distort the received optical field over the period of one day and must be compensated for each link individually. We present preliminary design of the space telescope and focal-plane-array and plan of the following breadboarding activity.
A laser based soldering technique – Solderjet Bumping – using liquid solder droplets in a flux-free process with localized thermal impact demonstrates the all inorganic, adhesive free attachment of optical components and support structures made of heterogeneous materials for a high-resolution optical filter under harsh environmental conditions. Space applications demand an attachment technology which maintains the precise alignment of bonded components and overcomes challenges of common adhesives such as being more radiation resistant and appropriate for vacuum environments. Besides, stress and strain induced into optical components can deteriorate the wavefront of passing light and therefore reduce the system performance significantly. The presented case study shows the mandatory changes in the design of an optical filter instrument according to the boundary conditions of Solderjet Bumping for different bonding issues. First, a filter window made of N-BK10, covering the optical sensor beneath, is soldered into a frame of DilverP1®. Second, this sub-assembly is aligned w.r.t. to fiducials on a support structure and is attached in this state by soldering as well. The process chain of Solderjet Bumping including cleaning, wettable metallization layer, handling, soldering and inspection is discussed. This multi-material approach requires well-defined reflow energies to melt the spherical shaped solder preforms to create a media-fit joint and to prevent damages on the fragile filter window simultaneously. The findings of process parametrization and environmental testing are presented. The optical performance with respect to stress/strain before and after soldering as well as the alignment state are evaluated using non-contact optical techniques.
A high-precision opto-mechanical breadboard for a lens mount has been assembled by means of a laserbased
soldering process called Solderjet Bumping; which thanks to its localized and minimized input
of thermal energy, is well suited for the joining of optical components made of fragile and brittle
materials such as glasses. An optical element made of a silica lens and a titanium barrel has been studied
to replicate the lens mounts of the afocal beam expander used in the LIDAR instrument (ATLID) of the
ESA EarthCare Mission, whose aim is to monitor molecular and particle-based back-scattering in order
to analyze atmosphere composition. Finally, a beam expander optical element breadboard with a silica
lens and a titanium barrel was assembled using the Solderjet Bumping technology with
Sn96.5Ag3Cu0.5 SAC305 alloy resulting in a low residual stress (<1 MPa) on the joining areas, a low
light-depolarization (<0.2 %) and low distortion (wave-front error measurement < 5 nm rms) on the
assemblies. The devices also successfully passed humidity, thermal-vacuum, vibration, and shock tests
with conditions similar to the ones expected for the ESA EarthCare mission and without altering their
optical performances.
Solder joining is an all inorganic, adhesive free bonding technique for optical components and support structures of advanced optical systems. We established laser-based Solderjet Bumping for mounting and joining of elements with highest accuracies and stability. It has been proven for optical assemblies operating under harsh environmental conditions, high energetic or ionizing radiation, and for vacuum operation. Spaceborne instrumentation experiencing such conditions and can benefit from inorganic joining to avoid adhesives and optical cements. The metallization of components, necessary to provide solder wetting, mainly relies on well-adhering layer systems provided by physical vapor deposition (PVD). We present the investigation of electroless Ni(P)/Pd/Au plating as a cost-efficient alternative under bump metallization of complex or large components unsuitable for commercially available PVD. The electroless Ni(P)/Pd/Au plating is characterized with respect to layer adherence, solderability, and bond strength using SnAg3Cu0.5 lead-free solder alloy.
A miniaturized diode-pumped solid-state laser (DPSSL) designed as part of the Raman laser spectrometer (RLS) instrument for the European Space Agency (ESA) Exomars mission 2020 is assembled and tested for the mission purpose and requirements. Two different processes were tried for the laser assembling: one based on adhesives, following traditional laser manufacturing processes; another based on a low-stress and organic-free soldering technique called solderjet bumping technology. The manufactured devices were tested for the processes validation by passing mechanical, thermal cycles, radiation, and optical functional tests. The comparison analysis showed a device improvement in terms of reliability of the optical performances from the soldered to the assembled by adhesive-based means.
A waveguide image slicer with resolutions up to 270.000 (planned: 300.000) for the fiber fed PEPSI echelle spectrograph at the LBT and single waveguide thicknesses of down to 70 μm has been manufactured and tested. The waveguides were macroscopically prepared, stacked up to an order of seven and thinned back to square stack cross sections. A high filling ratio was achieved by realizing homogenous adhesive gaps of 3.6 μm, using index matching adhesives for TIR within the waveguides. The image slicer stacks are used in immersion mode and are miniaturized to enable implementation in a set of 2x8. The overall efficiency is between 92 % and 96 %.
Advanced optical systems of telescopes and scientific instrumentation require high accuracy mounting and joining of components. Applications for deep UV, under high energetic radiation, for vacuum operation, or assemblies subjected to environmental loads (e.g. humidity and temperature) require a replacement of organic adhesives or optical cement by a more robust bonding agent. Soldering allows the bonding of different materials with an inorganic filler material. We present the optimization of the laser-based Solderjet Bumping for the mounting of optical components and the parameters of the bonding process for fused silica and LAK9G15 (radiation resistant glass) with thermally matched metal mounts. The investigation covers the experimental determination and optimization of solder wetting to the respective base materials and the bond strengths achieved.
Soldering using metallic solder alloys is an alternative to adhesive bonding. Laser-based soldering processes are especially well suited for the joining of optical components made of fragile and brittle materials such as glass, ceramics, and optical crystals. This is due to a localized and minimized input of thermal energy. Solderjet bumping technology has been used to assemble a lens mount breadboard using specifications and requirements found for the optical beam expander for the European Space Agency EarthCare Mission. The silica lens and a titanium barrel have been designed and assembled with this technology in order to withstand the stringent mission demands of handling high mechanical and thermal loads without losing the optical performance. Finally, a high-precision optomechanical lens mount has been assembled with minimal localized stress (<1 MPa) showing outstanding performance in terms of wave-front error and beam depolarization ratio before and after environmental tests.
Solder joining using metallic solder alloys is an alternative to adhesive bonding. Laser-based soldering processes are especially well suited for the joining of optical components made of fragile and brittle materials such as glasses, ceramics and optical crystals. This is due to a localized and minimized input of thermal energy. Solderjet bumping technology has been used to assemble a lens mount breadboard taking as input specifications the requirements found for the optical beam expander for the European Space Agency (ESA) EarthCare Mission. The silica lens and a titanium barrel have been designed and assembled with this technology in order to withstand the stringent mission demands; handling high mechanical and thermal loads without losing its optical performances. Finally a high-precision opto-mechanical lens mount has been assembled with a minimal localized stress (<1 MPa) showing outstanding performances in terms of wave-front error measurements and beam depolarization ratio before and after environmental tests.
A novel laser-based soldering technique – Solderjet Bumping – using liquid solder droplets in a flux-free process with only localized heating is presented. We demonstrate an all inorganic, adhesive free bonding of optical components and support structures suitable for optical assemblies and instruments under harsh environmental conditions. Low strain bonding suitable for a following high-precision adjustment turning process is presented, addressing components and subsystems for objectives for high power and short wavelengths. The discussed case study shows large aperture transmissive optics (diameter approx. 74 mm and 50 mm) made of fused silica and LAK9G15, a radiation resistant glass, bonded to thermally matched metallic mounts. The process chain of Solderjet Bumping – cleaning, solderable metallization, handling, bonding and inspection – is discussed. This multi-material approach requires numerical modelling for dimensioning according to thermal and mechanical loads. The findings of numerical modelling, process parametrization and environmental testing (thermal and vibrational loads) are presented. Stress and strain introduced into optical components as well as deformation of optical surfaces can significantly deteriorate the wave front of passing light and therefore reduce system performance significantly. The optical performance with respect to stress/strain and surface deformation during bonding and environmental testing were evaluated using noncontact and nondestructive optical techniques: polarimetry and interferometry, respectively. Stress induced surface deformation of less than 100 nm and changes in optical path difference below 5 nm were achieved. Bond strengths of about 55 MPa are reported using tin-silver-copper soft solder alloy.
Solder-joining using metallic solder alloys is an alternative to adhesive bonding. Laser-based soldering processes are especially well suited for the joining of optical components made of fragile and brittle materials such as glasses, ceramics and optical crystals due to a localized and minimized input of thermal energy. The Solderjet Bumping technique is used to assemble a miniaturized laser resonator in order to obtain higher robustness, wider thermal conductivity performance, higher vacuum and radiation compatibility, and better heat and long term stability compared with identical glued devices. The resulting assembled compact and robust green diode-pumped solid-state laser is part of the future Raman Laser Spectrometer designed for the Exomars European Space Agency (ESA) space mission 2018.
Miniaturization of photonic devices is required by various applications such as data storage and processing, optical
communications, and metrology. This request can be met by new optical designs, miniaturized components, and
advanced packaging technologies. Design, assembly, and characterization of a miniaturized photonic wavelength-division
multiplexing (WDM) device for optical measurements are presented. The device features the use of gradient
index lenses (GRIN-lens) and the utilization of an adhesive free, laser-based joining technology. Solderjet Bumping
offers flux-free soldering in a localized inert nitrogen atmosphere with minimized input of thermal energy, thus allowing
for the joining of fragile materials such as glass or brittle ceramics. The proposed system design consists of a system
platform made of borofloat BF33 with a footprint of approx. 30x20 mm2. Mechanical stops also made of borofloat glass,
fiber-ferrules with a length of approx. 5 mm, and GRIN-lenses with a length of 4.05 mm are attached to the base-plate by
solder joints. The solder process uses tin-silver-copper (Sn3Ag0.5Cu) solder spheres with a diameter of 200, 400, and
760 μm. A fiber-to-fiber coupling efficiency of 72 % is demonstrated using uncoated components.
Laser beam soldering is a packaging technology alternative to polymeric adhesive bonding in terms of stability and
functionality. Nevertheless, when packaging especially micro optical and MOEMS systems this technology has to fulfil
stringent requirements for accuracy in the micron and submicron range. Investigating the assembly of several laser
optical systems it has been shown that micron accuracy and submicron reproducibility can be reached when using
design-of-experiment optimized solder processes that are based on applying liquid solder drops ("Solder Bumping") onto
wettable metalized joining surfaces of optical components. The soldered assemblies were subject to thermal cycles and
vibration/ shock test also.
Laser based solder bumping is a highly flexible and fast approach for flux-free soldering of micro-optical components in
complex 3D geometries with localized and time restricted energy input. Solder joints provide superior mechanical
strength, higher radiation stability, humidity resistance and a good thermal and electrical conductivity compared to
adhesive bonding. Due to the good long term stability solder joints are feasible for the integration of optical, mechanical,
electronic, and MEMS/MOEMS devices in multi functional hybrid optical assemblies. Comparative studies of solder
bumping of optical components with sputtered thin film metallization on platforms made of Alumina (Al2O3) and Low
Temperature Cofired Ceramics (LTCC) with both Au and AgPd thick film metallization were carried out using design of
experiment methods (DoE). The influence of the system parameters, laser pulse energy and duration, distance, incidence
angle and nitrogen pressure on targeting accuracy and bond strength were evaluated. The jetting of liquid solder spheres
within a localized nitrogen atmosphere improves wetting on the respective wetting surfaces and simplifies the joining
process due to integration of solder alloy preform handling and reflowing, thus showing great potential for a high degree
of automation.
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