[1] EVENSON K M, WELLS J S, PETERSEN F R, et al.Accurate frequencies of molecular transitions used in laser stabilization:the 3.39 μm transition in CH4 and the 9.33 μm and 10.18 μm transitions in CO2 [J].Appl Phys Lett, 1973, 22(4):192-195.

[2] SCHNATZ H, LIPPHARDT B, HELMCKE J, et al.First phase-coherent frequency measurement of visible radiation [J].Phys Rev Lett, 1996, 76(1):18-21.

[3] TERRIEN J.News from the International Bureau of Weights and Measures [J].Metrologia, 1968, 4(1):41-45.

[4] UDEM TH, REICHERT J, HOLZWARTH R, et al.Absolute optical frequency measurement of the cesium D1 line with a mode-locked laser [J].Phys Rev Lett, 1999, 82(18):3568-3571.

[5] SHELTON ROBERT K, MA Long-sheng, KAPTEYN HENRY C, et al.Phase-coherent optical pulse synthesis from separate femtosecond lasers [J].Science, 2001, 293(5533):1286-1289.

[6] JONES DAVID J, DIDDAMS SCOTT A, RANKA JINENDRA K, et al.Carrier-envelope phase control of femtosecond mode-locked lasers and direct optical frequency synthesis [J].Science, 2000, 288(5466):635-639.

[7] MORGNER U, ELL R, METZLER G, et al.Nonlinear optics with phase-controlled pulses in the sub-two-cycle regime [J].Phy Rev Lett, 2001, 86(24):5462-5465.

[8] DIDDAMS SCOTT A, JONES DAVID J, YE JUN, et al.Direct link between microwave and optical frequencies with a 300 THz femtosecond laser comb [J].Phys Rev Lett, 2000, 84(22):5102-5105.

[9] FORTIER T M, JONES DAVID J, CUNDIFF S T, et al.Phase stabilization of an octave-spanning Ti:sapphire laser [J].Opt Lett, 2003, 28(22):2198-2200.

[10] ZIMMERMANN MARCUS, GOHLE CHRISTOPH, HOLZWARTH RONALD, et al.Optical clockwork with an offset-free difference-frequency comb:accuracy of sum- and difference-frequency generation [J].Opt Lett, 2004, 29(3):310-312.

[11] DIDDAMS S A, UDEM TH, BERGQUIST J C, et al.An optical clock based on a single trapped 199Hg+ ion [J].Science, 2001, 293(5531):825-828.

[12] DIDDAMS S A, BERGQUIST J C, JEFFERTS S R, et al.Standards of time and frequency at the outset of the 21st century [J].Science, 2004, 306(5700):1318-1324.

[13] MARGOLIS H S, BARWOOD G P, HUANG G, et al.Hertz-level measurement of the optical clock frequency in a single 88Sr+ ion [J].Science, 2004, 306(5700):1355-1358.

[14] MA Long-sheng, ROBERTSSON LENNART, PICARD SUSANNE, et al.First international comparison of femtosecond laser combs at the International Bureau of Weights and Measures [J].Opt Lett, 2004, 29(6):641-643.

[15] MA Long-sheng, BI Zhi-yi, BARTELS ALBRECHT, et al.Optical frequency synthesis and comparison with uncertainty at the 10-19 level[J].Science, 2004, 303(5665):1843-1845.