FIBER-OPTIC TRANSCEIVERS FOR HIGH-SPEED DIGITAL INTERCONNECTS IN SATELLITES

Transcription

1 FIBER-OPTIC TRANSCEIVERS FOR HIGH-SPEED DIGITAL INTERCONNECTS IN SATELLITES Mikko Karppinen, Veli Heikkinen, Antti Tanskanen, Jyrki Ollila, Kari Kautio VTT Technical Research Centre of Finland, Kaitoväylä 1, P.O.Box 1100, FI Oulu, Finland I. INTRODUCTION Current trends in satellite payloads show a rapid increase in data traffic and processing. For instance, the data throughput which has to be processed on board telecom satellites is expanding, and the novel scientific payload instruments, such as high-resolution cameras, fast image compression processors, and synthetic aperture radars, call for high-speed communications on board the satellites. The design of next generation digital telecom payloads is a significant challenge considering the performance provided by current electrical interconnect technology and the overall power dissipation and mass/volume constraints set on the satellite payloads. This motivates to investigate the use of photonic interconnects instead of electrical ones, to save in mass and volume while enabling high-bit-rate performance. A significant advantage of fiber-optic links is lightweight cabling immune to EMI. In the future, optical interconnects might form a generic high-speed medium supporting inter-chip, inter-board and inter-equipment communications in on-board processors and providing almost distance-independent performance as well as high input/output (I/O) and connector densities. Therefore, optical links are studied for more extensive use in space crafts. The space environment sets challenging robustness and reliability requirements not met with the commercial terrestrial photonic components, for instance, due to severe mechanical vibrations and shocks during the launch sequence and due to wide continuous temperature variations as well as ionizing radiation during the long lifetime of the satellite. There are also strong needs to use and develop such high-reliability, high-capacity optical intra-system data links for other harsh environment electronics systems, such as, modern avionics and military vehicle platforms. NASA has been utilizing optical fiber systems in space flight hardware for over thirty years. These early data links operated at 1 Mbps or up to 20 Mbps data rates [1]. During the last decade, the data rates increased to ca 100 Mbps level. Also European Space Agency (ESA) and space industry are pushing the development of digital optical communications on-board satellites [2]. For instance, the Soil Moisture and Ocean Salinity (SMOS) satellite launched in Nov 2009 was the first ESA mission flying that extensively employs on-board fiber-optic communications [3]. The bit-rates reached 125 Mbps. For future payloads, robust multi-gigabit optical interconnection components, which enable dense wide-scale integration, are required. For instance, the European Space Agency promotes the development of the SpaceFibre data link standard to extend the capabilities of the widely used SpaceWire standard for spacecraft onboard communications. The objective is to provide symmetrical, bi-directional, full-duplex, point-to-point communication with 1 10 Gbps data rates over 100 m. ESA is also supporting low or medium data rate link development, for instance, in the form of active optical cables, which could directly replace copper cables in the existing designs [4]. For telecom payloads, ESA is pushing the development of optical interconnects in order to enable very-high-throughput links between the processor ICs and boards [5][6]. VTT has developed optical intra-satellite communication technology since 2004, when ESA commissioned the first SpaceFibre High-Speed Data Link study which included the transceiver component development. The study resulted in successful demonstration of the first generation SpaceFibre fiber-optic transceiver prototypes which operated up to 3 Gbps and passed the environmental tests showing that the SpaceFibre link is a promising candidate for the upcoming high-speed intra-satellite networks [7]. In addition to the SpaceFibre transceivers, the later studies have covered parallel optical links and inter-chip optical interconnects targeting to provide a range of high-bit-rate interconnect solutions for satellite platforms. Among other, the technology was applied in the demonstration of the module-level integration of AD/DA converter and FPGA devices with 25 Gbps multichannel fiber-optic interconnects [8]. In this paper, we describe the transceiver technologies and present some recent developments and test results.

2 II. TECHNOLOGY A. Device Selections The developed optical transceivers technologies are primarily targeted for gigabits per second data rates, in some cases up to 10 Gbps per channel, and at link distances even up to 100 m. The transceiver components are based on the 850-nm vertical cavity surface emitting laser diodes (VCSEL) and the matching GaAs-based PIN photodiodes (PD) which are integrated with the driver and amplifier ICs and and coupled to the 50 µm-core multimode fiber pigtails. These selections enable to meet the high integration density and low power requirements with device technologies already matured in terrestrial use. The first low-bit-rate intra-satellite links by NASA and ESA used 1300 nm LEDs or edge-emitting laser diodes and large core step-index silica fibers. However, recently the VCSEL technology has become more attractive and widely used, partly due to increased data rate requirements. With VCSEL-based technology, the requirements of low-power dissipation can be met, also allowing high integration density and scalability for higher data rates and channel counts. Moreover, the mature 850 nm GaAs-based VCSEL and photodiode technology is adopted for high reliability and have shown good radiation tolerance. Silica multimode fibers with 50 µm core enable VCSEL-based data transmission up to 10 Gbps, and beyond, per fiber in short distances. Today, such kinds of radiation hardened fibers are commercially available from a few suppliers. Thus, the 50 µm multimode fiber was seen as the best choice for the point-to-point high-bit-rate data links. B. Integration and Packaging The applied metal-ceramic hermetic packaging enables lightweight components and high electrical performance, as well as in high reliability and robustness for the harsh spacecraft environments. Ceramic packaging substrates are commonly used in the harsh environment applications with high reliability and stability requirements. We employ low-temperature co-fired ceramic (LTCC) technology as the transceiver module electronics and packaging substrate. Bare die photonic devices and electrical circuits are hybrid integrated into multilayer LTCC carrier substrates. The low conductor resistance and dielectric loss, multilayer structures with fine-line capability and compatibility with hermetic sealing make LTCC a useful technology for high-speed data communications. The thermal management can be improved with thermal vias, and the precision structures manufactured in the LTCC substrate ease the packaging process via the passive optical assembly. In the transceivers the optical coupling between VCSEL/PD and fiber is based on a precision via structure through the multilayer LTCC substrate [9]. Moreover, low outgassing properties of LTCC are advantageous for space use. Dupont 951 LTCC material system is used in the developed transceiver components. Because LTCC substrates are inherently airtight, only the component side of the carrier needs to be sealed, minimizing the size and the weight of the package. Hermetic sealing is made with a Kovar frame and lid mounted onto the LTCC carrier. Then hermetic fiber feed-throughs are made using Kovar ferrules and low temperature glass preforms. When heated to melting temperature of the glass, the preform collapses around the fiber and wets down the inside diameter of the ferrule. The fiber ferrule is soldered to the Kovar frame to complete the feedthrough. In addition, effective fiber strain relief is needed to avoid mechanical stresses to the fiber feedthrough with the stiff glass solder joint. III. TRANSCEIVER COMPONENTS A. SpaceFibre Transceivers Photograph of the Engineering Model prototype of the SpaceFibre transceivers is shown in Fig. 1, both without the lid of the package and as assembled on an evaluation board. The maximum data rate is 6.25 Gbps fullduplex. It is developed to match with the planned ESA SpaceFibre High-Speed Data Link standard [10]. The transceiver integrates 850-nm VCSEL and GaAs PIN photodiode with their driver and amplifier electronics and with 50/125-µm graded-index fiber pigtails. The QFN-type component package has 48-pad layout on the bottom side for mounting on a circuit board. The component has dimensions of 17 x 17 x 5 mm 3 and mass of 3 g without the fiber pigtails. The transceiver has typical power consumption of 210 mw. The specified ambient operating temperature range is from -40 to +85 C at 6.25 Gbps.

3 Fig. 1. SpaceFibre full-duplex transceiver component prototype: Left) Transceiver before sealing with metal lid (and equipped with Radiall LuxCis connector). Right) Transceiver mounted on an evaluation board (and equipped with Diamond AVIM connectors). The transceivers are implemented by assembling all devices on the top side of package carrier (as seen in the Fig. 1, left), and the electrical contact pads are on the bottom side. The TOSA and ROSA are assembled on small LTCC carriers which are vertically mounted on the LTCC-based package carrier. To implement the hermetic sealing of the SpaceFibre transceiver, a Kovar metal frame is first soldered to the metallization on the main LTCC carrier around the devices, TOSA, and ROSA. Then hermetic fiber feed-throughs are made using Kovar ferrules and low temperature glass preforms. In the final assembly stage of the transceiver, the Kovar metal lid is seam-welded to the frame to complete the package. The operability of VTT s SpaceFibre optical links as a part of the Space Wire network has been successfully demonstrated by ESA, Patria Oyj and STAR-Dundee Ltd. Building blocks for the SpaceWire SpaceFibre architecture have been developed in several recent ESA studies recently. In the demonstration, two optical transceivers were interfaced to two STAR Fire SpaceFibre SpaceWire bridges. Proper communication of the SpaceWire data was observed through the 2.5 Gbps Virtual SpaceFibre channel even through 100 m long fibre cable links. B. Parallel Optic Transceiver Another component we have developed is a parallel optic transceiver shown in Fig. 2, left. It is targeted to be used, for instance, in high-bit-rate board-to-board links in high-throughput on-board processors. It is a quad transceiver component operating up to 10 Gbps/channel full-duplex, i.e. it has 4 transmit and 4 receive fibers. The metal-ceramic package integrates 4-channel 850-nm VCSEL and 4-channel photodiode arrays. The driver and receiver electronics are based on 4-channel SiGe ICs. The transceiver is equipped with aerospace 8-fiber ribbon cable (by W. L. Gore & Associates Inc.) pigtail consisting of 50/125-µm fibers. The component dimensions are 17 x 17 x 9 mm 3. The typical power consumption is ca 750 mw, i.e., ca 19 mw/gbps for the full link at 10 Gbps. The structure of the hermetic 4+4-channel transceiver package is shown in Fig. 2, right. The TOSA and ROSA are based on LTCC substrates which are vertically mounted on the LTCC base carrier. The 8-fiber ribbon cable is split into two 4 fiber ribbons outside the package and the 4-fiber ribbons are coupled to optical sub-assemblies through two hermetic feedthroughs. Metal parts support the subassembly substrates and fibers and they also increase the thermal conduction between the TOSA and ROSA and the base LTCC carrier. Similar kind of packaging structure is used also in the SpaceFibre transceivers. Fig. 2. Parallel optical transceiver with 4+4 channels: left) Photo of the component with MTP fiber-ribbon connector. right) Schematic structure of the packaging and fiber-ribbon pigtailing (note, metal lid not shown)

4 IV. ENVIRONMENTAL TESTING The aforementioned SpaceFibre and parallel optic transceiver technologies have been stressed in various reliability and environmental qualification tests. Some results are presented here. The optimized thermal management of the SpaceFibre transceiver enables good signal integrity even at high ambient temperatures (>85 C), which are challenging for typical VCSEL transceivers. This is illustrated by clear eye opening in Fig. 3. Fig. 3. Eye diagrams of the SpaceFibre data link operating at 6.25 Gbps: Left) At room temperature; Right) Stressed eye when the transmitting component is at +90 C ambient and the receiving component is at RT. The typical storage temperature requirement of C with long lifetime causes a significant stress on the hermetic photonic package. However, the hermetic sealing of our packages passed the helium fine leak tests (with rejection limit 10-7 atm cm 3 /s) after been stressed with temperature cycles between C. These tests included up to 500 cycles. And the optical fiber feedthrough has been confirmed even up to 1000 cycles between C. Moreover, the Au thermo-compression flip-chip joints, which are used in the mounting of the VCSEL and PD dies are on the ceramic carriers, have passed temperature cycling tests between C. After 500 cycles no failure or change in the shear strength could be measured. In satellite environment the components are exposed radiation, which can cause a significant total accumulated dose (TID) during the lifetime. The transceivers have passed the specified TID radiation tests with gamma radiation to 100 krad and protons to p/cm 2 at 60 MeV (tested by Alter Technology). Typically some degradation of optical power was seen after the total dose tests, but still the optical power remained within the acceptable margin of the link. In addition, the IC chips used in the SpaceFibre Engineering Model prototype transceiver were tested with heavy ions for possible single-event effects. Only the VCSEL driver showed to be latch up sensitive and did not pass the LET threshold of 70 MeV/mg/cm 2 [9]. This is typical of ICs containing CMOS circuits. Although that VCSEL driver is no longer used in the latest transceiver versions, the test emphasized that either customization of the IC, i.e. radiation hardening, or latch up mitigation by recovery circuitry is required; otherwise the applicability to some missions may be limited. The SpaceFibre transceivers have also passed the mechanical tests with vibrations up to 50 g rms and shocks of g as well as the thermal vacuum tests over the operation temperature range 40 to +85 C (all tested by Alter Technology). V. TOWARDS INTER-CHIP OPTICAL INTERCONNECTS We have also studied potential implementations of inter-chip optical interconnects within ESA TRP study Interchip-Optical Communications and Photonic PCBs for next generation OBP (carried out in collaboration with Atmel, Thales Alenia Space, and Vrije Universiteit Brussel). The future digital telecom payloads call for very-high-throughput interconnections between the processor ICs (ASICs or FPGAs) and boards, while also improving the overall thermal management and reducing power dissipation of the interconnections. In order to best exploit the potential advantages of the optical interconnections, it appears attractive to bring the optical interface very close to the interconnected processor ICs. In other words, it is beneficial to improve the integration density of the IC and optical I/O. This means advancing from the current solution (used in terrestrial applications), where optical link, ASIC and possibly also SerDes circuitry are separate components mounted on PCB, towards the co-packaging or in the long run even monolithic integration of o/e functionalities, ASIC and SerDes. Concerning optical media, the potentially highest integration density may be achieved with integrated waveguides in the PCB, i.e. using the so-called opto-pcb approach. Perhaps such a technology will be gradually adopted into wide use in high-performance electronics and then later on it might be applicable for

5 aerospace applications too. However, for the time being, fiber optics seems the only applicable optical media for space. To achieve higher density signal routing between processors and boards, the fibers can be laminated inside flex circuits. As a potential solution to bring optical I/O very close to the ASIC/FPGA chip, we are proposing to integrate multi-channel fiber-optic transceivers inside the ASIC package, as depicted in Fig. 4. The ASIC (potentially including embedded high-speed serial link blocks) is co-packaged with VCSEL and PD arrays and driver and receiver ICs. This leads to a hybrid-package equipped with fiber-optic interfaces. The package is surfacemountable on the processor PCB and the low data rate signalling and power supplies are implemented through conventional electrical connections. A space-grade ASIC package with optical I/O is illustrated in Fig. 5, where the ceramic package is equipped with four fiber-ribbon connectors; two input and two output connectors, each including 12 fibers. Fig. 4. Schematics of ASIC package with high-bit-rate optical I/O s Fig. 5. Illustration of space-grade ASIC package with embedded fiber-ribbon I/O s: left) Final package; right) Before hermetically sealing with metal lid. VI. CONCLUSIONS High-bit-rate transceiver technologies were developed towards wide use of intra-satellite optical communications, benefiting from the enhanced data throughput, reduced EMI, and savings in mass and volume. The applied ceramics-based hermetic photonic packaging enables high performance and robustness for harsh environments. The components developed include the SpaceFibre transceiver for on-board data communication operating up to 6.25 Gbps full-duplex and the parallel optical transceivers having 4+4 channels each operating up to 10 Gbps/channel with fiber ribbon cables. The parallel transceiver integrates VCSEL and photodiode arrays and a fiber-ribbon pigtail and it is suitable, for instance, for board-to-board interconnect applications. Furthermore, inter-chip optical interconnects were studied and a novel concept for co-packaging of ASIC/processor with integrated optical I/O was presented. ACKNOWLEDGEMENTS The authors would like to thank all partner organizations and companies, including European Space Agency, Thales Alenia Space, Patria, FibrePulse, Alter Technology, Atmel, DAS Photonics, D-Lightsys, W. L. Gore, Vrije Universiteit Brussel, and National Optics Institute, as well as the colleagues therein, for the fruitful collaboration during the studies. ESA is also acknowledged for the financial support. In addition, we thank a number of colleagues for their contribution to the technical work at VTT.

6 REFERENCES [1] Ott, M. N., LaRocca, F., Thomes, W. J., Switzer, R., Chuska, R., and MacMurphy, S., Applications of optical fiber assemblies in harsh environments: The journey past, present, and future, Proc. SPIE 7070, (2008). [2] Karafolas, N., Armengol, J., Mckenzie, I., Introducing photonics in spacecraft engineering: ESA's strategic approach, Proc. of IEEE Aerospace conference, paper #1682, 7-14 March 2009, Big Sky Montana, USA. [3] Kudielka, K., Boehle, P., Tornell, M., and Borges Alejo, A., Design, development, and verification of the fibre-optic harness for SMOS, Elektrotechnik & Informationstechnik 124(6), , (2007). [4] Blasco, J., Navasquillo, O., Sáez, C., Digital and analogue optoelectronic links for intrasatellite Communications, International Conference on Space Optics [5] Vervaeke, M., Debaes, C., Van Erps, J., Karppinen, M., Tanskanen, A., Aalto, T., Harjanne, M., Thienpont, H., Optical interconnects for satellite payloads: overview of the state-of-the-art, Proc. SPIE, 7716 (2010). [6] Baudet, D., Braux, B. O., Prieur, Hughes, R., Wilkinson, M., Latunde-Dada, K., Jahns, J., Lohmann, U., Fey, D., Karafolas, N., Innovative On Board Payload Optical Architecture for High Throughput Satellites, International Conference on Space Optics [7] Heikkinen, V., Alajoki, T., Juntunen, E., Karppinen, M., Kautio, K., Mäkinen, J. T., Ollila, J., Tanskanen, A., Toivonen, J., Casey, R., Scott, S., Pintzka, W., Thériault, S., and McKenzie, I., Fiber-optic transceiver module for high-speed intrasatellite networks, Journal of Lightwave Technology 25(5), (2007). [8] Heikkinen, V., Juntunen, E., Karppinen, M., Kautio, K., Ollila, J., Sitomaniemi, A., Tanskanen, A., Casey, R., Scott, S., Gachon, H., Sotom, M., Venet, N., Toivonen, J., Tuominen, T., and Karafolas, N., Highspeed ADC and DAC modules with fibre optic interconnections for telecom satellites, International Conference on Space Optics [9] Karppinen, M., Kautio, K., Heikkinen, M., Häkkilä, J., Karioja, P., Jouhti, T., Tervonen, A., and Oksanen, M., Passively aligned fiber optic transmitter integrated into LTCC module, Proc. of 51th Electronic Components and Technology Conference Conference (ECTC), (2001). [10]Tuominen, T., Heikkinen, V., Juntunen, E., Karppinen, M., Kautio, K., Ollila, J., Sitomaniemi, A., Tanskanen, A., Pez, M., Venet, N., McKenzie, I., Casey, R., and Lopez, D., SpaceFibre high speed fibre optic data links, Proc. of 3rd Internat. SpaceWire Conference, (2010).