## Materials and Methods
### Bentchtop Transceiver Testing
#### 2.3.2 BER Measurement  
Bit Error Ratio (BER) is the ratio between the number of bit errors to the total number of transmitted bits. BER is often used as a metric to determine how well data can be transmitted across a communication link. An experiment was conducted in order to determine the BER for the benchtop MOCT hardware. This measurement was conducted for the LEO to Earth link requirements since the current benchtop hardware is anticipated to be able produce high enough power to close the link, as determined from the results of the amplifier verification test results. The parameters used can be found in Table 4. The guard time was selected to be 51.2 ns due to the modulator time setting resolution and the need to have at least 50 ns to store energy in the fiber, which was determined from the results of the optical amplifier characterization testing and simulation results. The FLAT was used to test the transceiver performance at optical powers expected on orbit. As seen in Figure 12, a U-bench was used between the exit of the optical transmitter and the receiver to attenuate amplified pulses and emulate the link losses that would be present for a transmitted optical signal. The U-bench allows for free-space attenuation to be inserted without adding uncertainty from reconnecting fibers that some in-fiber attenuation can be sensitive to. A calibrated PIN diode on a pickoff is used before the EDFA to monitor the seed laser signal. After the U-bench, the attenuated pulse passes through a 99:1 fiber beam splitter. A calibrated PIN diode on the 99% portion of the beam splitter is used to monitor the attenuated pulse. The 1% portion of the beam is coupled with an 18.75 mm focal length, 3.6 mm diameter collimator and directs the beam to the APD, which is the receiver for MOCT. A 100 mm focal length, 25.4 mm diameter bi-convex lens placed in front of the APD. Adjustments were made to the lens until a maximum in the APD signal amplitude for a fixed optical pulse was achieved. The output signal of the APD is an analog differential pair. On a satellite, the MOCT receiver would use both the positive and negative portions of the differential pair to time the rising and falling edge of the received pulse. Timing both edges of the received pulse and utilizing knowledge of the pulse shape reduces timing uncertainty associated with optical power fluctuations. For this laboratory jitter characterization test, the positive output was sent into the comparator and the negative output was monitored by an oscilloscope. Prior to transmitting the communication string, the FPGA modulator is set to pulse continuously at a repetition rate of 15.625 MHz, which is equivalent to the same symbol being repeated for the selected communication parameters. The pump power and attenuation were used in conjunction to \begin{table}
\begin{tabular}{c c c c c} \hline
**Pulse Width** & **Num. Slots, \(M\)** & **Guard Time** & **Slot Width** & **Raw Data Rate** \\ \hline
2.5 ns & 32 & 51.2 ns & 400 ps & 78.1 Mbps \\ \hline \end{tabular}
\end{table} 
Table 4: Optical communication parameter set that was investigated. The guard time was selected to close the LEO to Earth optical link. The corresponding raw data rate was calculated from Equation (2). achieve the desired optical power on the receiver by monitoring the PIN on the output. The oscilloscope was used with its fast frame acquisition on the APD signal to determine a starting threshold value for the comparator. A sampling of the fast frame acquisition of the negative signal from the APD differential pair can be seen in Figure 14.  
For any given optical power, the reverse bias on the APD and the PGA amplification (from \(-12\) to \(+52\) dB) were tailored to the received optical signal prior to data collection. Due to the APD-integrated transimpedance amplifier (TIA) saturation limits, the reverse bias voltage was limited at high optical powers.