A. Target applications

In addition, at this moment DAS Photonics is developing, under a ESA GSTP activity called Active Optical Cable, a complete set of optical cables to substitute those harness links where the substitution of copper harness by optical cables could improve the most critical parameters for space applications, such as mass, volume, EMI susceptibility, cables handling and earth loops. During first steps of the developments, one of the most important tasks is to identify the most interesting applications to use optical cables.

It is clear that the omnipresent MIL-STD-1553 bus is a communications standard present in practically all onboard equipments, although CAN BUS could be an alternative in the future, but for the short time the 1553 is the most used bus.

It is well known the complexity of the 1553 because of the connectors, backshells, couplers and wires used to communicate the equipments inside the satellite to be controlled. That number of parts used to build the bus affect directly to the mass and cost of it, as indicated by Table 1.

Table 1.

LISA PATHFINDER mass budget for 1553 BUS

Next step to be taken is to determine the mass impact that result of the substitution of the copper 1553 BUS by and active optical bus. Using data collected from GAIA satellite and the description of the complete 1553 BUS, the estimated mass saving for the buses used in this satellite have been calculated in Table 2.

Table 2

GAIA mass saving estimated for 1553 BUS

Other used communications system is the SpaceWire standard (SpW), used for high speed communications from 100 to 400 Mbps. This standard is well know by space community as well as their problems due to the low flexibility, EMI and ground loops because of the characteristics of the copper cable. With a maximum allowed mass of 80 g/m and the protections used, the SpW cable has a handicap in terms of mass and handling. These problems could be solved with optic fibre, being a light transmission medium.

Again, using real data from GAIA satellite for SpW harness it is possible to demonstrate the advantages of using optical cables for SpaceWire communications. These results are shown in Table 3.

Table 3

GAIA mass saving estimated for SpW communications

Other communications protocols will be considered in the initial estimations to detect the mass saving and the impact of the substitution. At the end of the present GSTP activity the final conclusions will present the optimal targets to be substituted and the advantages of that.

B. SIOS Design

Based on the studies performed before about the most common links being used, we concluded that the communications must be divided in two categories, depending on the bit data rate, low speed and medium speed. DAS Photonics developed a suitable solution optimal for each category, increasing the mass saving and decreasing power consumption.

The designs are not static, these could be adapted to any configuration with different connectors or transmitters/receivers distribution, even combining medium and low speed links in the same backshell, being the optical cable of DAS a powerful way to reduce harness mass for intrasatellite communications.

Key devices of the SIOS are the optronic devices and their drivers/amplifiers, they allow the transceivers to have low mass and to reduce the power consumption of the transceivers. In first developments, the maximum power consumption for a full duplex link was around 800mW. For the SIOS included in Alphasat TDP8 the new objective in terms of maximum power consumption was settled as 450mW for a full duplex link. With a more efficient electrical design, this maximum valued has been reduced up to 330mW, with no penalties in terms of bit error rate or reliability.

The components used were selected in order to reduce the power consumption of the optical cables, which is the major handicap of the active optical elements compared with passive copper cable. All optical devices and electronic drivers/amplifiers are COTS, therefore validation campaigns were established to assure the quality and suitability of the COTS for space environment. These COTS have also been submitted to different component test, as a constructional analysis, RGA…, to have more exhaustive data of them to allow their inclusion on the high-rel space environment.

C. Validation campaign

Because of the use of COTS in the optical transceivers, the validation campaigns were focused on both, system and component level.

During the Spanish National Space Program, electrical, mechanical, environmental and radiation test activities were performed, all at system level, to assure the suitability of the parts and the design for space environmental conditions. This campaign was defined using most common levels for space missions in order to assure that the transceivers will accomplish with the target applications detected in previous analysis in a wide range of onboard platforms. The results of this campaign were presented in last ICSO conference.

As part of the qualification activities performed for the acceptance of the transceivers for Alphasat TDP8 experiment, additional tests at system and component level were scheduled in parallel to the General Experiment Board design. This campaign included additional electrical, mechanical and environmental (thermal vacuum and radiation) tests, but in this case, the transceivers submitted were not only electrically tested (from a digital communications point of view), they included also an analogue output to monitor the VCSELs output optical power and the photodiodes input optical power.

Each transceiver comprises four communication links, which were designed with different power budget to check their behavior during the test campaign. This way the impact of the tests on each link can be evaluated based on the power budget. The power budget was designed identical to the power budget to be implemented in the Alphasat experiment.

The campaign had two purposes, to check the suitability of the optical transceivers for the Alphasat experiment, and to check the impact of the tests upon the COTS devices.

The validation campaign comprised the following tests:

At system level:

At component level:

Table 4

Test campaign at system level (transceiver units)

Radiation tests were performed using two units, the nominal and a golden unit. The design of the golden unit was identical to the nominal one. The nominal unit was irradiated while the golden was only taken as reference for results comparison. These golden units were maintained in the same environmental conditions during the testing and annealing.

Fig 3

VCSEL degradation on TID test

Proton test was performed on two fresh units (one medium data rate and one low data rate) and in a unit already submitted to 150Krad TID, with energy and flux levels mentioned before. No errors nor power consumption increment were detected on the fresh transceivers irradiation, the degradation on the power output of the lasers was slight and not sufficient to produce errors on the transmission.

One of the transceivers submitted to 150Krad-Si TID, low data rate transceiver in that case, to see the combined effect of TID plus proton irradiation. This combined test allowed determining the cross section because of the detection of single bit errors in the transmitted patterns.

Table 5

SEE results summary on SIOS transceiver preirradiated in TID test

The errors distribution was not concentred in a single point and after the irradiation no errors were produced again, not being critical for BER values even in low power budget links.

All tests provided very good results, confirming the suitability of the COTS for space environment. For example, for the thermal vacuum case, the maximum deviation in the power output of the lasers (most delicate part in terms of temperature stability) between maximum and minimum temperature was less than 1dB.

Table 6

Maximum deviation on VCSELs output optical power for TVC

For the fiber pull test, the value obtained before fiber/coupling cracks or the losses were higher of the maximum value, was much higher than the 5N of the requirements, in fact, the maximum tensile load before fiber/coupling broken was around 30N.

Fig 4

Maximum tensile load before coupling cracks

The results of the RGA, outgassing, glass seal and thermal conductance were all according to the European Space Agency requirements for flight missions.

No major issues were detected during the validation campaign; however, degradation on the optical devices was found, especially during radiation tests. In any case, the final conclusion for the assessment of the components was that all COTS devices are suitable to be used in the Alphasat experiment and for future missions.