Control of repetition frequency of a femtosecond laser by an external cavity working as the length sensor

New lasers, special digital signal processing electronics, algorithms and new materials for optics enabled development of new absolute distance measurement methods. The phenomenon of the mode-lock of the femtosecond pulse laser increased a number of potential applications with distance surveying where a stable generator of very short and periodically repeated coherent pulses can be used. The aim of the work is a presentation of our unique control algorithm for measuring the length of unknown distance with direct traceability to a time standard. The principle is based on a passive optical cavity with mirrors keeping distance to be measured. Time spacing of short femtosecond pulses generated by femtosecond laser is optically phase locked to the cavity free spectral range. A value of the repetition frequency of the laser determines the measured distance in that case.