Abstract
Absolute frequency measurement is of great interest not only for metrology applications but also for high-resolution spectroscopy, optical communications, and physical constant definitions. Frequency standards at optical region based on atomic or molecular absorptions provide high accuracy, but it can’t cover all optical spectrum are limited to some special frequencies. With the progresses of the femtosecond lasers and photonic crystal fibers, the frequency comb from visible to near infrared region is well established. It can accurately measure the absolute frequency of stabilized laser. Mode-locked lasers generate ultrashort optical pulses by establishing a fixed relationship across a broad spectrum of frequency. The pulse duration is about 44 fs at time domain. The pulses train generated by a mode-locked laser has a frequency spectrum that consists of a discrete, regular spaced series of sharps lines, known as a frequency comb at frequency domain. The frequency of the nth comb line is n×frep+δ. Establishing an optical frequency comb by simultaneously stabilizes the repetition rate and offset frequency. We used the 15-cm-long photonic crystal fiber to generate a spectrum that spans from 500 nm to 1550 nm. And obtain the offset frequency by self-reference interferometer. We stabilized the repetition rate and offset frequency by controlling the cavity length and the pump power. We used the optical frequency comb to measure of a10 component of Iodine R(56) 32-0. The accuracy of our femtosecond comb system is 2.3 x 10-12 @ 1 s.