Optical Frequency Measurement Using Chirped-Mirror-Dispersion-Controlled Mode-Locked Ti:Al2O3Laser

An optical frequency measurement system based on an octave-spanning optical frequency comb generated by a chirped-mirror-dispersion-controlled mode-locked Ti:Al2O3 laser and a photonic-crystal fiber is developed. All of the modes of the octave-spanning optical frequency comb are frequency-stabilized to a microwave frequency standard, where the carrier-envelope offset frequency is phase-locked with self-referencing of the comb. We investigate the methods of controlling carrier-envelope offset frequency in a chirped-mirror-dispersion-controlled mode-locked laser. The rotation of a pair of chirped mirrors is useful for setting the bias of carrier-envelope offset frequency. Although our mode-locked laser has a low pulse-repetition frequency of 150 MHz, a high signal-to-noise ratio in beats results in the direct measurement of beat frequency with a laser to be measured using a frequency counter, and enables us to phase lock carrier-envelope offset frequency merely by using a mixer analogously without the need for a prescaler, with a servo bandwidth at approximately 500 kHz. The uncertainty of our optical frequency measurement system, besides the uncertainty of microwave reference frequency, is 4×10-14, and is limited by the uncertainty of the rf synthesizer used for phase locking and by that of the beat frequency measurement. Frequency measurements of an iodine-stabilized frequency-doubled Nd:YAG laser at 532 nm, an iodine-stabilized He–Ne laser at 633 nm and a rubidium two-photon-absorption stabilized extended-cavity laser diode at 778 nm are conducted. The results contributed to the revision of the practical realization of the metre adopted by the International Conference on Weights and Measures (CIPM) in 2001.