Abstract
Optical frequency standards are important for fundamental research, such as physical constants definitions, frequency metrology, precision optical spectroscopy, and industrial applications, such as optical communication system and optical clock. With the rapid growth of optical communication market, the numerous and important techniques are developed, especially high-density wavelength-division multiplexing (WDM). The WDM system provides multi-channel optical transmissions at single optical fiber. The calibration of channel allocation becomes important. Atomic and molecular absorption lines provide wavelength references that are essential for the calibration of channel allocation. Locking to narrow Doppler-free lines is required for greater frequency stability, accuracy, and reproducibility. We set up the frequency standards for optical communication system based on rubidium 5S-7S two-photon absorptions at 760 nm for optical communication system. The accuracy is 10-11. And we lock the laser frequency on the hyperfine structure of iodine spectroscopy at near infrared. But the frequency standards based on atomic and molecular absorption lines are limited to some specific frequencies. Although the frequency standards based atomic and molecular absorption lines are not difficult to set up, the flexibility is limited. With the progresses of the mode-locked laser and photonic crystal fiber, the frequency comb from visible to near infrared region is realized. The output pulse of the femtosecond laser is launched into a photonic crystal fiber and, as a result, a narrow frequency comb of mode-locked laser pulse is extended into a "comb" of equally spread, very broad light sources that cover almost all the visible spectrum. It is like a “frequency ruler” to measure any unknown frequency at optical region. We set up the femtosecond comb system for the absolute frequency measurement from visible to near infrared and stabilize the repetition frequency and the offset frequency. The frequency jitter of the stabilized repetition frequency is 4 mHz. The frequency fluctuation of the offset frequency is 11 mHz when it is locked. It provides the accuracy of the optical frequency measurements up to the 10-11 level.