a The characterization setup. A frequency comb (FC), which is located in a different laboratory from the OFS, is used for an out-of-loop measurement. The carrier offset frequency and repetition rate of the FC are locked to the 10 MHz reference with a 100 m RF cable. The output of the FC beats with the synthesizer output after transmission through a 300 m single mode fiber (SMF). The beat note from the BPD is then filtered by a BPF and measured by a frequency counter. b The measurement results with six different M values. For all cases, f2 = 6.16 MHz. c Allan deviation for the six measured synthesizer frequencies across the C-band, showing a frequency instability of 10−12 at 1 s. d Fine frequency tuning to generate the MIT logo pattern (the averaging time of the frequency counter is 200 ms).