Femtosecond lasers for astrophysics

Passively mode-locked continuous-wave (cw) lasers offer the unique feature of generating a strictly periodic train of absolutely identical femtosecond pulses, their emission spectrum representing a comb of equidistant narrow spectral lines separated by an interval determined by the pulse repetition rate. Thus, a cw femtosecond laser is a source of a regular optical frequency comb (OFC), which can be compared to a cesium frequency standard (standard of second) for the precise measurement of optical frequencies for the development of extremely precise atomic clocks, precise spectroscopy, and metrology. One of the main applications of OFCs based on cw femtosecond lasers is precise spectrometric measurements of Doppler shifts in stellar spectra appearing due to the radial motion of stars with respect to the observer. To provide the high measurement accuracy required for the search for and study of exoplanets, a high-precision calibrator for an astronomical spectrometer is required. Such a calibrator can be based on a femtosecond laser OFC. Laser systems for astronomic investigations, including femtosecond lasers for space studies, are considered. It is assumed that the development of these methods will provide direct measurements of the accelerated expansion of the Universe.