## 1 Introduction
### MOCT Architecture  
The Miniature Optical Communication Transceiver is a lasercom system capable of both transmitting and receiving optical pulses. This system includes a modulator, transmit channel, and receive channel. The transmit channel is composed of a seed laser and optical amplifier and the receive channel is composed of an Avalanche Photodetector (APD) and a Time-to-Digital Converter (TDC). The MOCT design scope does not include the transmit optics to collimate the outgoing pulse, the receive optics to capture an optical pulse, nor the precision pointing system for the transmitted beam. These non-trival elements would need to be integrated with MOCT in order to utilize MOCT on a nanosatellite. A block diagram of the MOCT architecture can be seen in Figure 2.  
When the spacecraft wants to transmit data over an optical link to a distant receiver, the spacecraft sends the modulator the data string to be transmitted. The modulator is a novel software defined pulse modulator (SDPM) implemented on a Microsemi SmartFusion2 [8] field programmable gate array (FPGA) and utilizes a Microsemi Chip Scale Atomic Clock (CSAC) [9] as the reference oscillator. The modulator takes the data string and encodes it using pulse position modulation (PPM) by varying the time delay between generated electrical pulses. Two 2.5 GHz Serializer/Deserializer (SERDES) hardware modules onboard the FPGA, offset from each other by a calibrated Delay Locked Loop (DLL), form the physical layer of the modulator. The modulator is able to set \(\tau\) and \(T_{g}\) with a resolution of 400 ps, and control the pulse width with 1 ps increments. The rising and falling edges of the modulated pulses have a random time error (jitter) of less than 30 ps. As opposed the modulators commonly used in communication that rely on fast slot clocks with high power consumption to make slots, the MOCT modulator utilizes timing delays. The electrical pulses from the modulator are sent to the pulse driver, which converts the electrical pulses into optical pulses. The pulse driver is composed of a seed laser and a custom designed pulse diver circuit board called the Fast Laser Advanced Switching High-Frequency Emitter (FLASHE). The seed laser is a 1531 nm laser capable of delivering up to 40 mW of power. FLASHE utilizes Gallium-Nitride (GaN) field-effect transistors (FETs) for radiation resilience. The circuit configuration of the FETs on FLASHE enable the seed laser to produce optical pulses as short as \(\sim\)100 ps. Nominally, the seed pulses on MOCT will have a pulse width of \(\sim\)1 ns and peak power of \(\sim\)5 mW. The seed pulses are coupled into an optical amplifier capable of amplifying the pulse to peak powers of \(\sim\)1 kW without significant distortion the pulse shape. The optical amplifier is a single stage, erbium doped fiber amplifier (EDFA) in a master oscillator power fiber amplifier (MOPFA) configuration. The optical amplifier has a single stage with a counter-propagating (backwards) \(\sim\)980 nm pump laser configuration. The pump power can be adjusted to provide only enough amplification to close an optical link. The erbium doped fiber type used is Liekki ER80-8/125 [10], which has a high doping concentration (\(\sim\)80 dB/m) that enables short lengths of fiber that have  
Figure 2: MOCT architecture. high gain and reduce nonlinear effects like Stimulated Raman Scattering (SRS), Stimulated Brillouin Scattering (SBS), and Four-Wave Mixing (FWM) [10]. Additionally, this fiber has a large single mode core for \(\sim\)1.5 \(\upmu\)m light that also makes coupling the pump light easier. After the optical amplifier, the amplified pulse is sent to the distant receiver through the transmit optics. When the spacecraft captures an optical pulse from a distant transmitter in the receive optics, the optical pulse is detected by the APD. The APD converts the captured optical pulse into an electrical pulse. A transimpedance amplifier (TIA) at the exit of the APD helps this detector achieve a 1 GHz bandwidth.