3.1

Pre-amplifier optical design

The pre-amplifier is a two-stage design as shown in the figure below

Figure 1:

Schematic of the fiber pre-amplifier

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

3.2

Booster optical design

The 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.

Figure 2:

Schematic of the fiber booster amplifier

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.)

3.3

Mechanical design

The 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.

Figure 3

Figure 4:

Booster box with (a) fiber spool and pump and photodiode board and (b) with fiber shield and controller 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.

Figure 5:

Preamp lid

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.

Figure 6:

Assembled preamp and booster unit

Figure 7:

Prototype unit, 160 mm x 120 mm x 50 mm.

3.4

Electronics design

The 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.

Figure 8:

controller electronics board

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.

4.1

Qualification test plan

The additional test that we implemented for the space product qualification are summarized in Table 5.

Table 5:

Test plan

4.2

Radiation test

We 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.

4.3

Temperature 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

4.4

Production and quality assurance testing

Both 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.