In recent years the femtosecond optical frequency combs have led to remarkable advances in metrology optical clocks and attosecond generation etc. In general, the photonics crystal fiber is necessary to broaden the laser spectrum for f-2f method implemented in the traditional femtosecond optical frequency comb. The unavoidable instability introduced by the fiber limits the wide use of the frequency comb. However, owing to obviating the photonics crystal fiber and the interferometer replaced by a single PPLN crystal, the locked state of monolithic frequency comb based on Ti: sapphire laser has been significantly extended, typically from about 1 hour to more than 9 hours. Furthermore, the novel design frequency comb has shown its great potential to reach long-term stabilization as long as more advanced technologies for anti-shock and temperature stabilization can be used. We then test the locked state of the comb by recording fceo and frep with universal frequency counters. Besides the long-term locked result, we can also demonstrate that the Allan deviations for the two signals drop as ideal τ−1, respectively. This indicates that the white phase noise is the dominant noise process and then confirms that both fceo and frepcontrolled in experiment are in phase-locked loop. The reproducibility of locked fceo has been tested by standard statistical method, and the distributions of the locked data are uniform at 95% confidence level.
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