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1.INTRODUCTIONSatellite-to-satellite laser communication1 will enable multi gigabit data transfers in space that will surpass present microwave systems. With these high-speed links, new satellite constellations will become global telecommunications networks. LEO satellite-to-satellite links may extend over distances of more than 5000 km, necessitating an optical-power budget of ~70 dB to compensate for diffraction-limited optical-beam divergence. This can be done by boosting the transmitted laser diode signal to around 1 W, and by amplifying the received signal by 40 dB. Such performances are achieved by terrestrial fiber amplifiers, operating in the telecom 1550-nm wavelength window, which are produced in large volume and at relatively low cost. However, they are not space qualified. Space technology is usually more expensive, on account of the more rigorous tests and controlled-environment manufacturing environment and because, until recently, the number of satellites is traditionally small (often one), thus preventing economies of scale. This is changing with the proposed new constellations for which hundreds, and even more than 1000 satellites, will be launched, each often comprising four laser communications terminals (LCT). Low-cost LCT are now required as satellite constellation providers seek more affordable solutions for their very large constellations. Based on MPBC’s extensive production experience with fiber amplifiers and its history of space system design and manufacturing, the Company has proposed and begun development on a new line of lower-cost space-borne LCT terminals. We designed a package to hold both the transmission optical booster unit and receiver optical pre-amplification unit, because all links need both a powerful transmitter and a sensitive receiver. These units are compact and stackable. This allows weight savings and provides superior radiation shielding. This design also helps the control costs in assembly and testing. To improve reliability, all active components are redundant. They are built using of commercial off-the-shelf (COTS) components whenever possible. The fiber optic components used in large volume for our terrestrial amplifiers are already qualified to telecommunications standards. However, additional tests are required for operation in a space environment. We have subjected the components, both active and passive, to Gamma and proton radiation testing, to total ionization doses of up to 100 krad, which can be accumulated over 15 years is space. We are also performing temperature vacuum cycling over extended ranges of temperatures, i.e., -25 to 70 °C, to understand the behavior of the COTS components. Finally, they must be tested to more severe vibration and shock tests. We can thus select COTS that will work in our space applications. Accordingly, the amplifier is designed with power in reserve to compensate any anticipated degradation over the lifetime (5 to 15 years) of operation. 2.OBJECTIVESThe large constellations, with their hundreds of identical satellites, are a game changer for the production of space hardware. The new onboard systems must still be reliable and able to survive years in the space environment, but they must also be produced in large volume over periods of months or years. This becomes very similar to the production of high-quality terrestrial systems, which achieve lower cost because of volume. In order to produce large volumes of satellite subsystems, one must not only consider the design for space, but also the lowering the cost coming from the manufacturing processes and the supply chain. Therefore, to address the 10 Gbits LCT application, the baseline design was based on terrestrial amplifiers that are produced in volume but with these additional space related requirements.
The design and testing will be detailed in the following sections. 3.UNIT DESIGN3.1Pre-amplifier optical designThe pre-amplifier is a two-stage design as shown in the figure below The first stage is a low-noise high-gain amplifier optimized for amplification of very weak signals (in the range of -40 dBm). The second-stage booster the signal to 1 dBm. The laser diodes are 400 mW single-mode 980-nm pump lasers that are normally limited to operate at half their power rating. In this schematic, the pump power is shared between the two stages, but each stage could have its own separate lower-power pumps. The preamp has an input signal monitor to detect the presence of an incoming signal, and, if yes, to indicate to the control electronics to “turn on” the preamp. It also has an output power monitor, so the gain may be adjusted. Depending on customer requirements, this schematic can be modified. An example would be the elimination of the input power monitor if the satellite can supply to the preamp a receiving signal command. Table 1:Parts list for pre-amplifier
Table 2:Pre-amplifier operation specifications
3.2Booster optical designThe booster is a two-stage design as shown in Figure 2. The first stage is a single-mode amplifier to amplify the modulated signal (typically a few milliwatts) from the source laser. The pump lasers are 400-mW single-mode 980-nm laser diodes that are operated at half their power rating. The second-stage booster amplifies the signal to ~30 dBm. It is pumped by 10-W multimode pump laser diodes at 940 nm, use at a maximum of half their power rating. The booster has an input signal monitor to determine if a signal is coming in and turn on the amplifier. It also has an output power monitor, so the gain can be adjusted. Depending on customer requirements, this schematic can be modified. An example would be the elimination of the input power monitor if the satellite can supply to the booster a receiving signal command. Table 3:Parts list for booster amplifier
Table 4:Booster operation specifications
*(It should be appreciated that, when used for transmission of digital optical signals, the specified noise figure (NF) above is not particularly meaningful, since the shot noise comprising the noise figure is also attenuated by the same link-loss value as the signal level. This is generally true for all optical power amplifiers.) 3.3Mechanical designThe single unit box is the assembly of two halves made of aluminum. One of the halves contains the booster and the other the preamp. They are both joined together to form the unit, as illustrated in Figure 3. As illustrated in Figure 4 (a), the larger side of the booster box is used as the optical tray. All the passive optical components are mounted in that space. There are two angled protected fiber outputs, one for the input signal and the other for the output signal. It also includes a spool for the DCF gain fiber. The pumps are mounted on the smaller side. They are connected via an electronics board that also includes the photodiodes, but they are directly thermally interfaced to the side. Once the unit is assembled, this small side actually becomes the bottom of the unit and is the heatsink surface, thus enabling he high-power pumps to be directly and passively cooled. On top of the fiber spool, an aluminum shield is put in place. The shield helps control the radiation exposure for the most sensitive component, which is the gain fiber. We have simulated the exposure of the gain fibers and with a proper choice of the shield and enclosure thickness, we can limit the total radiation dose to under 20 krads for the operational lifetime of the system. The shield also serves as support and heat sink to the controller electronics board. The design concept is also applied to the preamp lid portion of the unit. The large side serves as the optical tray and the small side is for mounting the pump and photodiode board. The pumps are again directly thermally coupled to the side, but the heat dissipation is much smaller than for the high-power pump. The heat is dissipated through the lid and conducted through the booster box to the heat sink. The two halves are optically and electronically independent, so they can both be assembled and tested separately. When they are assembled, a back panel is added to complete the enclosure and provide the back support for the electrical connector, as shown in Fig 6. The fiber outputs are in the front of the unit, the heat sink surface is at the bottom. The unit dimensions are 160 mm x 120 mm x 50 mm. Additional features can be added to the unit so that it can be mounted to a frame in the satellite. Depending on the shielding required, the weight of the unit is typically between 800 g and 1000 g. 3.4Electronics designThe electronics design includes two electronics boards for each half of the unit. Each half includes a pump and photodiode board 133 mm x 33 mm. It can hold up to 4 high power multimode pump laser diodes, 2 single-mode 980 nm pump laser diodes, and 4 photodiodes. The board details depend on the selected pumps. This board provides wiring to the different opto-electronic component pins but does not have any other function. The control is effected through the primary larger board, illustrated in Figure 8. This board support laser driver for each redundant pump diodes, 2 redundant CPUs for control and a 25 Pin MILDTL-83513 connector. The connector can be changed depending on customer requirement. Its dimensions are 148 mm x 80 mm. For cost reduction and production purposes, both the preamp and the booster use the same electronic boards. The only difference is that the preamp boards are not fully populated as it requires fewer elements. The serial interface can be LVTTL, RS 232 or RS-485 (preferred) depending on customer requirements. 4.QUALIFICATION TESTINGFor the space environment and reliability, testing is essential. Though we use COTS components that have been fully tested for terrestrial applications, extra tests have to performed, both for qualification and quality assurance. We summarize these in this section 4.1Qualification test planThe additional test that we implemented for the space product qualification are summarized in Table 5. Table 5:Test plan
4.2Radiation testWe have performed radiation testing on gain fiber, passive fiber component and the optoelectronics components. The results are qualitatively summarized in the following table. Table 6:Gamma radiation testing
We also performed high energy proton exposure and have not measured any performance degradation of the optical and optoelectronic parts. The problem is with electronics and especially isolated single events, perturbing the electronic data. This is mitigated by the redundancy of the electronics and robust firmware coding. 4.3Temperature in vacuum test (TVAC)We have performed thermal vacuum testing on relevant passive fiber component and the optoelectronics components. The results are qualitatively summarized in the following table. Table 7:Thermal vacuum testing
4.4Production and quality assurance testingBoth amplifiers are tested during assembly. They are functional at room temperature before the unit assembly. However, once the two halves are integrated, an additional TVAC hi-low test is performed and the unit is characterized over the temperature range. This will root out assembly problems and provide calibration data for the system. 5.CONCLUSIONWe have design a 1550 nm LCT amplifier unit that integrates both the preamp and the booster amplifier into a single enclosure weighing less than 1 kg. COTS components are used after being qualified in radiation and TVAC testing. When properly selected, they show little degradation in performance at the extreme of the temperature range and the system can accommodate the reduce efficiency with its power margin. The unit has also been optimized for volume manufacturing. REFERENCESH. Kaushal and G. Kaddoum,
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