## Materials and Methods
### MOCT Laboratory Hardware
#### 2.1.3 Optical Amplifier  
The optical amplifier is responsible for amplifying the 1.5 um seed laser pulses to a high enough peak power in order to be detected by the distant receiver. For MOCT, the optical amplifier is an erbium doped fiber amplifier (EDFA) in a master oscillator power fiber amplifier (MOPFA) configuration. The continuous wave (CW) \(\sim\)980 nm pump laser is coupled with the erbium doped fiber and stores energy via stimulated absorption with the erbium ions. When the seed laser pulse is emitted into the erbium doped fiber that has energy stored in the erbium ions, the seed laser pulse is amplified via stimulated emission. The gain from the EDFA is controlled by how much pump power is coupled into the fiber and how long the erbium doped fiber is charged between seed pulses. In general, higher pump powers and a longer time between pulses are associated with higher amplification (within some physical limitations of the particular amplifier) than lower pump powers and a shorter time between pulses. One characteristic of EDFAs is that they are able to amplify pulses with very little temporal distortions on the original seed pulse. For testing the optical amplifier MOCT hardware in the laboratory environment, the Fiber Laser Amplifier Testbed (FLAT) was constructed and developed. A picture of the FLAT can be seen in Figure 7. The FLAT is the optical amplifier in a spacious configuration, utilizing FC/APC connectors throughout the testbed to rapidly test different amplifier configurations. Additionally, fiber beam splitters were used in conjunction with a PIN diode and power meter to monitor the seed laser and pump laser signals into the optical amplifier. A 976 nm pump laser with a maximum output power of 600 mW CW was selected and used in a forward pump configuration as seen in Figure 8a. This configuration was used in the optical amplifier characterization testing where the power damage threshold is \(\sim\)300 mW CW. A backward pump configuration, as seen in Figure 8b, has a higher gain than forward pumping for the same pump power at the cost of more forward Amplified Spontaneous Emission (ASE). Forward ASE is a band of optical signals near the wavelength of the seed laser, so it can be difficult to reject with filtering, and therefore increases transmitted optical noise. When the  
Figure 6: FLASHE 1.0 current driver module with 1531 nm laser for in-lab experiments. benchtop transceiver testing was conducted, a Wave Division Multiplexer (WDM) with higher power rated connectors was available, allowing this demonstration to implement a backwards pumping configuration. For a space mission, permanent fiber spliced connections and free-space components can be used, which can both greatly increase the optical power damage threshold and enable more pumping configuration freedom. The pump laser currently used on the FLAT has an electrical to optical power efficiency of 35% and consumes \(\sim\)1 W of electrical power. If the same efficiency was on a 4 W pump laser, it would consume 11.4 W of electrical power. Some other components on the FLAT that would not be present on the MOCT space hardware include the U-Bench and the two output detectors. The U-bench launches the optical signal into free-space, sends the signal through free-space optics, and then couples the resulting optical signal back into the fiber. The U-bench was used the attenuate the amplified optical signal by a known amount until it was within the dynamic range of the output detectors. Both a PIN diode and a calibrated power meter were used with a fiber beam splitter so both the temporal characteristics and average power of  
Figure 8: EDFA pumping configurations. Figure 7: The FLAT benchtop optical amplifier setup at the University of Florida in a forward pumping configuration. The FLAT was used to characterize the transmitter hardware for MOCT. the output optical signal could be monitored simultaneously.