2.1

Design Considerations

The starting point of the optical and mechanical design is the OSIRIS4CubeSat payload, which is shown as a CAD model in Figure 1. It consists of two main functional systems: The transmitter system (Tx-system) and receiver system (Rx-system). In the Tx-system, light is coupled from a fibre into a free space collimated beam, which is magnified with an afocal telescope after being reflected by a beam splitter and a Fast Steering Mirror (FSM). The Rx-system receives a laser beacon signal sent from an OGS and compresses the incident beam with the same telescope used in the Tx-system. It also passes the FSM, transmits through the beam splitter and is directed to a four quadrant detector (4QD). This system enables the detection of the angular offset between the boresight of the satellite optical sytem and that of the OGS.

Figure 1.

CAD model of the OSIRIS4CubeSat payload. The numbers are indicating the main components; 1: main body, 2: aperture lens, 3: mainboard, 4: laser, 5: fibre collimator, 6: heat sink for laser driver, 7: 4QD.¹

The assigned numbers in Figure 1 are indicating main components of the system and shall be briefly summarised in the following in order to understand the boundary conditions of the optical design for further developement: Two assembled solid aluminium blocks provide a rigid and stiff basis (1) for the whole system. This includes the aperture lens (2) and the rest of the optical assembly which is coloured in red. The mainboard PCB (3) is directly attached to the blocks. The laser (4) is connected via FC/APC plug to the collimator (5), which enables angular adjustment of the Tx-system to the Rx-system (Tx/Rx-alignment). The laser driver is located next to the laser and must be passively cooled with a heat sink (6). Since the accuracy of the satellite body pointing is limited and cannot always fulfill the requirements of the needed precision for an optical terminal, an optical tracking system is implemented. This consists of the FSM and the 4QD (7) working in a closed control loop. In order to minimise the effort for a redesign and qualification, several design considerations must be taken into account:

Even though some of the above listed design criteria were already accounted for in the optical design of the OSIRIS4CubeSat payload, the performance has to be maintained and some new considerations appeared due to the additional wavelength and are interdependently. Criterion #1 is the most design-influencing one, as it defines the maximum allowed size of the new optical system, as well as the beam paths for the Tx- and Rx-system. Criterion #2 remains the same, but is stronly related to criterion #3, since the Tx- and Rx-systems partly use the same optical path. Therefore the Rx-system also depends passively on the new wavelength. There, precise and continous tracking is only possible, if the spot shape at the 4QD is radial symmetric, the optical power homogenously distributed and the spot size appoximately between 300μm and 600μm in diameter. Furthermore, criteria #4 and #5 are important for multiple optical downlinks at the same time, i.e. tracking is performed with a L-band wavelength and the downlinks are performed with one or both of the C-band wavelengths as well as with 850nm.

For an unaberrated system using ideal optical elements, the optical paths from inside the satellite to earth would be the same for all wavelengths, even if all beams were transmitted at an angular offset to the telescope’s optical axis which normally induces aberrations. This ideal scenario is shown in Figure 2a).

Figure 2.

Different pointing scenarios from satellite to OGS using two different wavelengths: a) Optimal divergences and concentric beams, b) Optimal divergences, but angular offset and c) Angular offset with adapted divergences.

A more realistic scenario is shown in Figure 2b). The divergences are not perfectly matched due to residual lens errors and optomechanical offsets which might not be completely compensated or due to a thermal drift.

A worst case scenario is demonstrated in Figure 2c). This scenario must be avioded at all cost, since it is likey that a downlink for all wavelengths at the same time is not even possible. 2c) shows the situation in which the divergences are mismatched, the AOE for the two downlink beams is not matched and thus pointing performance is poor.

2.2

Optical Design

The optical design of the OSIRIS4CubeSat payload is taken as a basis and is created using optics design software Zemax. As only the Tx-wavelengths are changed, the Rx-system is left untouched. A first analysis of the optical properties shows that a redesign of all lenses for the Tx-system is necessary. Therefore, optimisation criteria are adopted to the new wavelengths and with that, the above mentioned criteria have to be adapted as well. The originally used three singlet lens system is split into a system of three achromatic doublets after verifying the feasibility of less achromatic doublets. However, the compensation of chromatic aberrations is not possible without simultaneously inducing spherical aberrations and coma in the telescope.

A cross sectional view of the final optical design model for the Tx-system is shown in Figure 3. The rays indicate the beam paths at 0 ° AOE (blue) and 0.6 ° AOE (magenta) at C-band wavelength. The lens elements are indicated from L1 to L6 beginning with the lens L1, the first element of the achromatic doublet, which collimates the light exiting the optical fibre. Since the elements at the optical path from the fibre tip to L2 are static in their position and aligned to the optical axis, aberrations are lower compared to those occuring in the telescope. It was therefore sufficient to use spherical surfaces for L1 and L2. Since the beam splitter (BS) combines/divides the Tx- and Rx-system, each component in the shared beam path is optimised with respect to the optical properties of both systems. While the FSM is steering the Tx-beam, strong aberrations are induced if the beam path is not on the optical axis. The magenta-coloured rays in Figure 3 show the beam path for an AOE of 0.6 ° which corresponds to an Angle Of Incidence (AOI) of 4.9 ° to a plane perpendicular to the boresight of L3. As not only the AOI on L4 changes but also the position at which the beam is incident on its surface, the induced wavefront errors can be corrected if the shape of the surface was manipulated locally. The induced errors are radially symmetric and correctable with an aspheric shape of one of the surfaces of L3. Different AOI were taken into account for the optimisation process of the lens surfaces in order to assure a reasonable quality not only for the maximum achievable angle, but also for angles in between this range. Aspheric surfaces are more complex and expensive to manufacture and also take more time in the optimisation process. For that reason, all surfaces are spherical in the initial setup of the design process. The surface types for L3-L6 were changed one after another from spherical to aspherical, until a reasonable compromise between quality and cost was found. A market research for stock lenses also showed a potential replacement for L6 which was successfully implemented, as it was also available as an uncoated version. In the final design L6 is an ashperic stock lens; L1, L2 and L5 are custom made spherical lenses; and L4 and L3 are custom made aspherical lenses.

Figure 3.

Cross sectional view of the optical Tx-system. The rays indicate the beam paths at 0 ° AOE (blue) and 0.6 ° AOE (magenta). The optical elements are indicated as follows: Beam Splitter (BS), Lens 1-6 (L1-6), Fine Steering Mirror (FSM).

The selection of the lens material is first done by comparing different flint and crown glasses according to their amount of radiation induced darkening from the ESA RADGLASS catalogue in which data is partly published.⁷

However, after first optimisation it turns out that the radius of curvature of some of the lenses becomes too small for machining a reasonable surface quality, if they were even manufacturable. In particular, the material of L1-L5 has to be changed. Only materials with a high refractive index come into account, so that the radius of curvature can be made larger, while the focal length of the lens is unchanged. For the correction of chromatic aberrations, an achromatic doublet is built by two lens elements per lens. L1, L4 and L5 have a negative focal lengths and are made of a high dispersive material. As the dispersion of standard glass materials is low in the near infrared spectrum, a chalcogenide glass is selected for these lens elements. For the positive focal lengths lens elements L2 and L3, a heavy flint glass is selected as the dispersion is low compared to the chalcogenise glass, but the refractive index is high compared to standard crown glass. In order to minimise back reflections, all lenses are equipped with a custom made anti reflection coating.

Contrary to the complete redesign of the Tx-system including the shared part of the telescope, the optical design of the Rx-system is completely reused. A cross sectional view of the complete optical system is shown in Figure 4. The blue and red rays indicate the beam paths of the Tx- and Rx-system, respectively. Beside the shared beam path, the Rx-system consits of a plano-convex spherical lens (L7) which projects the received beam onto the 4QD with the desired diameter of ~ 600 μm. A prism is used to fold the beam for saving space and a filter (FLT) is placed in the beam path to filter out any stray light of different wavelength so that only the beacon wavelength is transmitted onto the 4QD. As parts of the optics and mechanics could be reused, engineering time and costs were minimised as well as risk of failure during the qualification phase.

Figure 4.

Cross sectional view of the optical Tx- and Rx-system. Rays to indicate the beam paths for the Tx- and Rx-System in blue and red, respectively. Optical elements: Lens 7 (L7), filter (FLT), Beam Splitter (BS), Fine Steering Mirror (FSM), Four Quadrant Diode (4QD).