2.1

Optical bench function

The optical bench combines the functionality of all building blocks. It forms the physical and functional connection of all optical and opto-mechatronic components. To accommodate this function, it provides beam conditioning, splitting and shaping directing the optical beams towards the targeted components. The key requirements and functions are:

2.2

Telescope function

The telescope expands and decreases the beam size from the entrance pupil to the optical bench. As sufficient area is needed to collect sufficient amount of light, it determines the antenna gain for both transmit and receive beam. The optical quality of the telescope determines partly the transmission of the optical system, since transmission can be degraded by wave front errors. This is especially relevant, when the optical beam is coupled into a single mode fiber. Since a telescope consists of multiple optical elements, it requires quite some design effort to keep it low SWaP.

2.3

Coarse pointing function

Coarse alignment of the optical terminal is done via the coarse pointing assembly (CPA), which steers and aligns the optical beam for both the receive and transmit beam. This is used to provide the basic angular motion of the laser beam needed for acquisition mode and to anticipate on the relative motion between the optical terminals. To realize this the key requirements and functions are:

2.4

FSM function

Complementary to the coarse steering function of the CPA, the fast, fine steering mirror (FSM+) provides the fine resolution steering, which is required for both the received and transmitted laser beam. The received laser beam needs to be pointed onto a photodetector, or into a single mode fiber to demodulate the data from the optical beam. Since the divergence of the optical beam is small due to high antenna gains for optical communications systems, it is required to steer the transmit beam with micrometer accuracy. Pointing errors will lead to reduced optical power at the receiving end of the optical communication channel.

The fine pointing functionality of the FSM is also used to mitigate high frequency disturbances in the common optical path. These disturbances generally originate from various vibration sources on the satellite platform. And for ground based terminals, atmospheric turbulence need to be corrected [10]. The key requirements and functions of the FSM are:

2.5

Controller function

The control system ties the electronic part of all subsystems together. It is responsible to drive the CPA, the FSM, the transmit laser and the modulation mechanism to realize tracking and communication in all operational modes. It collects the several sensor inputs, such as the orientation of the satellite from the inertial measurement unit IMU, the positions of the CPA and FSM and the pointing sensor. It drives the CPA and the FSM based on the sensor inputs and their setpoints. The setpoint trajectory is generated using knowledge of the position and orientation of the satellite and the orbital information of the neighboring satellite or position of the ground terminal. As the FSM and CPA work realize pointing together, the controller must be able to schedule tasks and combine position feedback control.

Figure 2

TESLA-C optical bench

3.1

Tesla status

TNO successfully designed, integrated, tested and qualified for flight the optical bench for the Optel-μ terminal in the TESLA-C Project (primed by TAS-CH), for direct to ground laser downlinks from LEO satellites. The emphasis was a compact and robust terminal that mainly addresses the needs of the emerging market of small satellites to increase data download capabilities with higher security, at comparable on-board resource constraints. In this project, TNO achieved the following results:

An IOD of the Optel-μ terminal is planned shortly with the aim of demonstrating the functionality of the system, and in doing so also demonstrating the performance of the TESLA-C optical bench.

TNO have been keen to build upon this achievement, spinning off the developed technology towards other applications such as:

This paper reports on the status of these developments at TNO.

Figure 3

Preliminary design of a LEO communication terminal with 70 mm entrance aperture front side (a) and back side (b).

3.2

LEOCAT optical communications head for intersatellite links

For the mentioned massive LEO constellations, TNO is developing a compact LEO communication optical head for intersatellite links (LEOCAT). An CAD drawing of the LEOCAT optical head is depicted in Figure 3. The optical head has a 70 mm entrance aperture and a telescope field of view (FOV) of ±0.25°, which is a good compromise between directivity of the optical beam and the achievable pointing precision [11], [12]. In order to be compact, the optical design has been optimized, so the global outside dimensions of the terminal fits in a volume of 190x190x250mm. The optical head contains the following functionalities:

One of the prominent risks for this optical head is the challengingly low required wave front error (WFE) of less than 30 nm RMS, while remaining a low recurrent cost price. To mitigate this risk, TNO manufactured a prototype of the telescope, shown in Figure 4. Focus of this risk mitigation comprises the manufacturing of the mirrors, but also the effort required to assemble and align the optical system, since it also has a major impact on recurrent price.

Figure 4

Picture of prototype telescope.

The telescope mirrors and the structure are all made from the same type of aluminum, to reduce thermal stresses due to material differences. For this sake, the telescope mirrors where not NiP (Nickel-Phosphor) plated. Instead, TNO has the unique possibility to achieve excellent optical performance, using only diamond turning, so no form of polishing is applied after the diamond turning. The achieved results are shown in Figure 5. The measured WFE is 24 nm RMS, which is well within the targeted WFE of <30 nm RMS. Also the surface roughness of the mirrors is between 2 and 4 nm RMS, which is beneficial for improving the transmissivity of the system and to reduce scattering of the Tx beam into the Rx channel.

Figure 5

RMS WFE of prototype telescope.

With the obtained results of the prototype telescope, TNO concluded, that it should be feasible to manufacture these telescopes, with high optical quality, for a low recurring cost price. An engineering model of the optical head, with the full functionality as described above is planned to be realized before the end of 2019.

3.3

CubeCat architecture

Small satellites, also known as CubeSats could provide an excellent opportunity for cost-effective satellite communications services. Due to the advantages of optical satellite communications, their small size is suited very well suited towards the low SWaP requirements, with still a high data rate. Hence, it provides a good alternative for large, but cost effective LEO constellations, or for smaller experimental satellite missions or for surveillance.

As it is predicted that satellite services need to increase in the coming future, e.g. by SpaceX or Morgan Stanley, more and more companies are available that can make small CubeSats. In order to use this opportunity, TNO is developing CubeCat, a communication terminal suited towards a CubeSat in collaboration with industrial partners. As shown in Figure 6, the design is already in an advances stage; the preliminary design phase (PDR) has already been closed, and at the end of this year there should be hardware on the table.

Figure 6

(a) CAD render of the optical bench (b) cad rendering of the optical bench in an cubecat structure

Table 1

Specifications of the CubeCat terminal for CubeSats

The entire CubeSat optical terminal fits in a 1U standard CubeSat volume (1dm³), including all parts, such as the control electronics. The downlink data rate is 1 Gbit/s at 1545 with an on and off keying (OOK) modulation scheme. For the uplink channel, a data rate of 100 kbit/s is foreseen at 1590 nm, with an OOK modulated beam. The uplink beam is also used as beacon for the CubeSat for coarse orientation of the CubeSat and to correct the optical beam with an FSM, for frequencies below 10 Hz.

At the end of 2018 the integration phase of the CubeCat should be finished. As it is important that the technology gain space heritage, it is planned to launch the CubeCat during 2020, for in orbit demonstration. In orbit tests of the CubeCat will not only promote confidence for this specific type of technology, but will also demonstrate the subsystems, such as the onboard detector technology and control system.

3.4

Course Pointing Assembly status

Course pointing assemblies (CPAs) are very application specific components of optical communication terminals, as the optimal CPA-architecture is strongly determined by the required field of regard (FOR). TNO is developing a mirror based and a Risley prism based CPA as shown in Figure 7 (a) and (b) respectively.

Figure 7

Two coarse pointing assembly (CPA) architectures TNO is currently developing. (a) A mirror-based configuration targeted for LEO-LEO use-cases. (b) A CPA architecture based on prisms following the Risley-principle which is targeted for LEO-Ground use cases.

LEO constellations have their satellites typically on orbits with the same altitude. For inter-satellite communications between two LEO satellites, the CPA is mainly pointing at satellites with the same altitude. For this reason, a relatively large azimuth angle is needed to be able to point in all direction, but only a small elevation angle to anticipate on the exact deviation from this orbital planes. The targeted azimuth range is ±180° and the elevation range is from -5° up to around +20°. This architecture is shown in Figure 7 (a).

For LEO-Ground use cases, the azimuth and elevation angles should be equal, to target a conical FOR. By using the Risley architecture with two counter rotating prisms, this can be achieved, while using the advantages of high compactness and low mechanical complexity. Under an ESA Artes program, TNO is also developing an CPA based on this Risley principle. The Risley architecture as shown in Figure 7 (b), allows for a conical FOR with a range of ±60°. The main advantage of the Risley architecture is the compactness and low mechanical complexity. For bi-directional communications over single channels low complexity singlet prisms suffice. To utilize the Risley architecture in combination with wavelength division multiplexing (WDM), i.e. using multiple wavelengths, the dispersion of the prisms need to be compensated in order to ensure that all communication channels are pointed towards the same direction. To achieve this, TNO is investigating the use of diffraction gratings etched on one side of the prisms. This architecture is also referred to as Grism’s.

The heart of the both CPA’s is a motorization axis, consisting of a bearing, motor and encoder, which can be strong cost drivers of the overall CPA-system. TNO is developing a cost-effective solution for the motorization axis, based on an integrated motor and encoder concept. This motorization solution is applicable for both CPA-architectures and has the potential of low-recurring cost for small series.