12 September 2007 A precise length etalon generator controlled by femtosecond mode-locked laser
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The progress in the field of optical frequency standards is oriented to femtosecond mode-locked lasers stabilized by technique of the optical frequency synthesis. Such a laser produces a supercontinuum light, which is composed of a cluster of coherent frequency components in certain interval of wavelengths. A value of the repetition rate of femtosecond pulses determines (in the frequency domain) spacing of these coherent components. If we control the mode-locked laser by means of i.e. atomic clocks we ensure frequency of these components very stable. With respect to definition of SI unit "one meter" on basis of speed of light the stabilized mode-locked laser can be used for implementation of this definition by non-traditional way. In the work we present our proposal of a system, which converts excellent frequency stability of components generated by the mode-locked laser to a net of discrete absolute lengths represented by a distance of two mirrors of an optical resonator. On basis of theory, the optical resonator with a cavity length has a periodic frequency spectrum Similarly the frequency of i-th comb component could be written as: fi = fceo + i frep, where fceo is the comb offset frequency and frep is the repetition rate. For the simplicity we presume the offset frequency fceo equals to zero. If the supercontinuum beam of the mode-locked laser illuminates the resonator and at the same time the cavity length L is adjusted to length Lp = c / (2 p frep ) then both spectra fit. The symbol 'p' is an integer value. It produces intensity maximum in the output of the cavity, which is detected by a photodetector and locked in the servo-loop. For absolute discrete values of cavity lengths Lp that well satisfy the condition above we obtain precise etalons of length.
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Radek Šmid, Ondřej Čip, Josef Lazar, "A precise length etalon generator controlled by femtosecond mode-locked laser", Proc. SPIE 6673, Time and Frequency Metrology, 66730O (12 September 2007); doi: 10.1117/12.735182; https://doi.org/10.1117/12.735182

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