Remotely manageable phase-locked loop controller for stabilizing optical frequency combs

The optical frequency synthesizers (femtosecond combs) are used mainly in the field of precise measurement of optical frequencies, laser spectroscopy and interferometer distance surveying. They generates thousands narrow-linewidth coherent frequencies at the same time. The spacing of generated components is equaled to the repetition frequency of femtosecond pulses of the laser. The position of the comb spectrum has a frequency offset that is derived from carrier to envelope frequency difference. The repetition frequency and mentioned frequency offset belong to main controlled parameters of this synthesizer. These two parameters are controlled by ultra-stable frequency standards i.e. H-masers, Cs- or Rb- clocks. We present the complete digital signal processing chain for phase-locked loop (PLL) control of the offset frequency. The setup is able to overcome some dropouts causes the femtosecond laser non-stabilities (temperature drifts, ripple noise and electricity spikes). It is designed as two stage control loop, where controlled offset frequency is permanently monitored by a digital signal processing. In case of dropouts of PLL, the frequency-locked loop keeps the controlled frequency in the required limits. The presented work gives the possibility to long-time operation of femtosecond combs which is necessary when the optical frequency stability measurement of ultra-stable lasers is required. The detailed description of the modern solution of the PLL with remote management is presented.