Frequency control of a chirped-mirror-dispersion-controlled mode-locked Ti:Al₂O₃ laser for comparison between microwave and optical frequencies

The use of an optical frequency comb generated by an ultrafast mode-locked laser has been realized as a promising method of the direct comparison between microwave and optical frequencies. We are currently investigating frequency control of a chirped-mirror-dispersion-controlled mode-locked Ti:Al₂O₃ laser. We stabilized the pulse repetition rate f_rep to a rf synthesizer locked to a cesium (Cs) clock to the Allan deviation of 4 × 10^-12 in 1 s. We found that the position of the crystal, rotation of the chirped mirrors, and change of the pump-laser power can be used in controlling the carrier-envelope offset frequency f_CEO. We extended the span of the comb to over one octave, i.e., from 530 nm to 1190 nm, at-20 dB using a photonic-crystal fiber made at the University of Bath. We are currently trying to measure the frequency of an iodine-stabilized Nd:YAG laser using a floating ruler of a f:2f frequency interval chain. We detected the self-referencing beat between the fundamental and second-harmonic frequencies of the comb, which will enable further precise comparison between microwave and optical frequencies through the control of the f_CEO.