Extended Data Fig. 4: Tracking comb timing measurement using an optical timing discriminator.
From: Quantum-limited optical time transfer for future geosynchronous links
(a) Subset of system diagram from Extended Data Fig. 4 showing how the timing of the incoming clock comb is measured with the local tracking comb. (b) System diagram of the optical timing discriminator and subsequent signal processing. The optical timing discriminator generates two measurement channels with the lead and lag positions between the two combs switched between channels. After measuring these channels on balanced photodiodes (BPD), the heterodyne output voltages are demodulated to generate IQ (complex) signals, which are then low pass filtered (LPF). The phase of one signal is used in a phase-locked loop to adjust the demodulation frequency, fDDS, and centre the baseband signals at DC. The magnitudes of the signals are combined to generate an estimate of the power (from their mean squared values) and a timing offset value, δtb, from their normalized difference. BS, beam splitter; ADC, analog to digital converter; NCO, numerically controlled oscillator. (c) Absolute value of the timing discriminator output voltages, |V1| and |V2|, as a function of the time offset between the local tracking comb and incoming clock comb. (d) The error signal generated from the two channels along with the polynomial fit used in the digital processing to generate the timing error value based on the normalized error signal, E.
