In a paper published in the journal of Optics Express, a study team led by Prof. MAO Qinghe at Anhui Institute of Optics and Fine Mechanics (AIOFM), Hefei Institutes of Physical Science studied feedback mechanism of period-doubling mode-locked (PD-ML) fiber laser frequency comb and the intrinsic correlation property.

Further, they developed a new optical frequency comb technology that enhances the tunable flexibility of repetition frequency based on a designed PD-ML fiber laser.

Fiber-laser-based optical frequency comb (FL-OFC) has attracted more attention because of its simple structure with cost efficient and easy operability. For some application fields, for instance, precision laser spectroscopy and microwave photonics, the comb-teeth spacing is a very important parameter.

The comb-teeth spacing for an FL-OFC is basically determined by the repetition rate of the mode locked pulse trains. However, if a large adjustment range for the comb-teeth spacing is required, one may have to face the difficulty regarding redesign of the cavity length.

Based on the previous work of developed low-noise fiber laser frequency comb, the group found pump power ranges that could be used to control the switching between the fundamental mode locking (FML) state and the Period-doubling mode-locked (PD-ML) state by inserting the OFS-980 fiber into the cavity.

On this, a fiber laser was designed, which could be switched from the FML state to PD-ML state, with almost the same output pulse characteristics between them.

Further, they clarified associated feedback mechanism and intrinsic correlation for such a PD-ML optical frequency comb.

Results showed that the new comb teeth produced by the PD-ML were strongly correlated with the original comb teeth and had a consistent carrier envelope offset frequency. Controlling the pump and cavity length was also suited for phase-locking the PD-ML laser.

With the same f-to-2f heterodyne beat system and locking circuit, phase locking of both PD-ML and FML-based optical combs with two repetition rates of 104.66 and 209.32 MHz, and switching between them, were obtained by changing the pump only.

This switching may present a high potential for the future use in the metrology laboratory and coherent optical pulse synthesis systems.

This work was in part supported by the National Natural Science Foundation of China, the Strategic Priority Research Program of the Chinese Academy of Sciences, and the National Basic Research Program of China.

Schematic diagram of Er-doped fiber optical frequency comb (a) and recorded residual fluctuation of phase-locked repetition rate (b) and its harmonic (c) signals with 1-s gate time. (Image by WU Haoyu)