Methods for generation of metrology length etalon based on femtosecond stabilized laser

Abstract

Interferometric methods are used for definition of length by distance translation to one of the interferometric arms. The Fabry-Perot interferometer or etalon with very high stable laser produces length etalon sensitive in nanometer scale with linear response to its change. We present in our work improvements of such method by removing the very high stable laser by the femtosecond stabilized optical frequency comb. Train of femtosecond laser pulses produces an optical frequency spectrum separated by equidistant frequency f_rep with frequency offset f_ceo called frequency comb. Stabilized frequency comb generates a very precise frequency rule. Locking the scanning Fabry-Perot interferometer to one of the frequency components could perform length etalon. The Fabry-Perot etalon in simple geometry transmits only the frequencies with integer multiple of f_cav which depends inversely to the mirror spacing. The principle of the first method is based on the direct relation between the repetition frequency of the comb and the etalon length. Fabry-Perot etalon transmits only the optical frequencies of the comb for those f_ceo equal to zero and f_cav is an integer multiple of f_rep. The detected scanning spectrum is characterized by interferogram. The center of the gravity fulfills the condition for direct length transfer between the Fabry-Perot mirror distance length and f_rep of the femtosecond frequency synthesizer. Discrete value of length etalons is obtained. The improvement of the method is based on separation of few optical frequency lines (narrowing the frequency spectrum) from the wide frequency range of the optical frequency synthesizer. It suppresses the presence of the interferogram which has very high demands on the further computer analysis. Widening the scanning Fabry-Perot free spectral range as much as possible enable us to observe each frequency line (line of the comb) only once. The detected scanning spectrum better characterizes the frequency output of the optical frequency synthesizer. This feature enable us to lock the Fabry-Perot intermode distance to one of the detected comb line. Moreover the mirror distance in the Fabry-Perot is arbitrary. We verified the experiment by the fiber based femtosecond stabilized optical frequency synthesizer working at central wavelength 1560 nm and with f_rep 100 MHz. Two He-Ne stabilized lasers for coarse Fabry-Perot mirror spacing length adjustment were separated by different optical polarization. Dielectric multilayer mirror coatings were wavelength dependent. DFB laser diode working on the 1542 nm were added into the set-up. Intermode beat between the frequency comb component and the DFB diode well defines the exact frequency position on which the etalon is locked. Comparison between different Fabry-Perot etalon has been made.