## 2 EDFA's Basic Design and Specifications  
Since the EDFA must be compatible with the CubeSat's form factor, its footprint was designed to be 90 mm \(\times\) 95 mm, and its height was optimized to fit in a case as small as 25 mm (Figure 2) with a total mass of 430 g. The EDFA's interface is made up of power and control. The former requires one line of 3.3 V and one of 5 V, which are common in the CubeSat platform. For the latter, the EDFA's internal microcontroller is equipped with a universal asynchronous receiver-transmitter (UART) port to communicate with the CubeSat's onboard computer (OBC), although a UART-to-RS232 converter was developed for tests with PC. The physical CubeSat's interface is implemented with a 12-pin Molex connector to provide double-pin configuration for redundancy of the five interface lines: 3.3 V, 5 V, GND, UART tx and UART rx. The control of the EDFA allows one to monitor the optical input power, optical output power and case temperature (via an onboard temperature sensor), as well as to set the EDFA's enable/disable and optical output power via driving current of the second pump laser (the first pump laser is set to the maximum current after EDFA  
Figure 1: Three-dimensional model of CubeSOTA showing the EDFA within the lasercom terminal and its impact on the CubeSat’s platforms. Figure 2: The 2-W EDFA’s 3D model (**left**) and flight model (**right**). has been enabled). As for the optical interface, the input and output single-mode fibers both have tight-buffered 900 \(\upmu\)m fiber cables with FC/APC connectors in both ends. A small cavity was designed in one of the faces of the case in order to allow the fibers and the 12-pin cable to go upwards or downwards within the CubeSat structure. The block diagram of the CubeSat's EDFA is shown in Figure 3. The diagram shows the dual-stage configuration which allows one to achieve a better gain and noise figure combination by implementing low-noise pre-amplification in the first stage and high-power amplification in the second stage [9]. The polarization-dependent gain is less than 0.5 dB and typically 0.3 dB, with isolation between input and output bigger than 30 dB. This EDFA was designed to be independent of the laser's polarization state for various reasons. Firstly, polarization-maintaining (PM) fibers are more expensive, especially when required to be radiation-hardened, and this amplifier was conceived to be a low-cost device that facilitates the future adoption of the host lasercom terminal as a commodity when space laser communications are widely used. Additionally, a PM design would introduce supplementary losses, estimated to reach up to an extra 5% in the worst case. Furthermore, polarization-division multiplexing (PDM) is a good candidate for future lasercom systems based on coherent detection, and this technique requires non-PM components. Figure 4 shows the performance of the 2-W EDFA using an optical spectrum analyzer (OSA) for an input-signal's wavelength of 1550 nm. The OSA spectrum shows the 2-W level at 33.26 dBm and the noise due to the amplified spontaneous emission (ASE) at \(-\)18.73 dBm. The EDFA provides an equally approximated saturated gain when the input power is within \(-\)6 dBm and 3 dBm. However, the ASE noise is proportional to the input power, going from \(-\)15.2 dBm/0.1 nm at \(-\)6 dBm to \(-\)21.6 dBm/0.1 nm at 3 dBm. Therefore, the input power should be kept as small as possible within its tolerable range to optimize the optical signal-to-noise ratio (OSNR), which is 51.99 dB when input power is 0 dBm. Figure 4: Spectrum-analyzer measurement of the 2-W EDFA. Figure 3: Block diagram of the 2-W CubeSat’s EDFA. Figure 5 shows the dependency of the output optical power with wavelength for different levels of input power. The maximum output power range is around 1550-1560 nm, and the variation with input power is typically around 0.1 dB within the whole range and smaller than 0.2 dB in the worst case.