## Materials and Methods
### MOCT Laboratory Hardware
#### 2.1.2 Seed Laser  
The Fast Laser Advanced Switching High-Frequency Emitter (FLASHE), a 1531 nm seed laser and pulse driver, emits stable pulses as short as 100 ps [12]. Its output is the pre-amplified data signal, which goes to the optical amplifier. FLASHE receives two digital signals containing the PPM data string from the modulator, with the second signal inverted and delayed from the first. The pulse duration is set by the time from the first input's falling edge to the second input's rising edge. The digital inputs are sent to two Field Effect Transistors (FETs), which must both be open to send current to the seed laser. The double FET configuration can be seen in the overall circuit design in Figure 4 as Q1 and Q2.  
The circuit was designed to pulse a laser (D1) at 50 MHz with up to 500 mA of current (from II). The threshold current to emit light from the selected laser is 50 mA. The design of FLASHE keeps the current just below the population inversion threshold in between pulses, and jumps above the threshold when emitting light. The negative bias voltage (V1) at the cathode of the laser enables the voltage across the laser to be just below the threshold value. The application of a constant bias reduces the amount of voltage change required to pulse, therefore reducing the rise time. Particular attention was given to decoupling the laser bias to ensure that the bias voltage supply conserved a low impedance at high frequencies. Very low impedance capacitors (C1) have been incorporated to maintain an impedance below 1 Ohm up to 10 GHz.  
The design of FLASHE is novel because it incorporates two slower transistors instead of one fast transistor and removes the need for a radio-frequency amplifier. Both transistors are operated in linear mode (similar to switching power supplies) which enables lower power consumption. However, having two switches increases jitter error [12]. In addition, FLASHE uses gain-switching, which adjusts the laser gain by abruptly providing and removing pump power [12; 13]. Incorporating both gain-switching and two transistors allows FLASHE to generate pulses down to 100 ps (full width half maximum) without requiring radio frequency electronics.  
Capable of emitting pulses in the nanosecond range down to hundreds of picoseconds, the shortest pulse FLASHE has demonstrated is 100 ps as seen in Figure 5[12]. Measured jitter is between 25 ps to 200 ps, which is mainly dependant on the modulation type and the modulator used [12]. The optical  
Figure 4: Seed laser pulse driver circuit.  
peak power range is from \(\sim\)1 to 40 mW. Both the duration and the pulse amplitude can be adjusted to achieve a desired pulse shape.  
There are two versions of the laser driver: FLASHE 1.0, made for in-lab experiments and testing, and FLASHE 2.0, designed as an engineering unit and is currently being fabricated and tested. The power consumption for FLASHE 1.0 has been measured to be 1.3 W. A large portion of that power is dedicated to the current source. In comparison, the FET driver average consumption is estimated to be less than 5 mW. The power consumption of FLASHE 2.0 is expected to be lower due to voltage reductions for its linear current source. A DC/DC current source could further reduce power consumption, below 200 mW for the complete laser driver. The printed circuit board (PCB) for FLASHE 1.0, which can be seen in Figure 6, is approximately 50 mm \(\times\) 80 mm. The 1531 nm laser  
Figure 5: (**A**) 100 ps pulses aligned to the reference clock, 200 ps per division; (**B**) 100 ps pulses self-triggered, 100 ps per division [12].  
has dimensions of 13 mm \(\times\) 48 mm \(\times\) 9.5 mm, which includes the black fiber cover. The Printed Circuit Board (PCB) area utilized for the current source and switches on FLASHE 2.0 is less than 2 cm\({}^{2}\). This reduction in PCB area makes FLASHE 2.0 suitable for a nanosatellite platform.