Femtosecond laser for metrological applications

Present situation of new applications of a femtosecond comb as metrological tool is discussed. Some of last results of worldwide investigation in this field are presented. We have been observing reliable progress in the field of stable lasers and measurement of their frequencies. By now, there is a lot kinds of standards; lasers are controlled in the 4f/f ∼ 10−15 domain using numerous attractive quantum resonators, including Yb+, In+, Sr+, Hg+, and Mg, Ca, Sr, I2, CH4, OsO4, C2HD, and C2H2 [1]. In the past, standards labs had to develop complex chains of phase-related oscillators, organized to phase-coherently connect optical oscillators locked to these narrow optical resonances with frequency standards in the microwave radio frequency domain [2]. Fortunately, later, another way to link optical and radio domains was suggested. It was revolutionary new concept in frequency metrology [3]. The idea is based on use of an equidistant frequency comb generated by a mode-locked laser [4]. But in fact, it became completely realizabled recently when for the first time frequency comb of octave bandwidth was produced by propagation femtosecond radiation through an optical fibre [5,6]. Such comb contains over million components of well-known frequencies. For measuring optical frequencies, this single-laser self-calibrating frequency synthesizer can be ideal, both fo