Repetition-rate-multiplication in actively mode-locking fiber laser by using phase modulated fiber loop mirror

We demonstrate a simple scheme to multiply the repetition rate in an actively mode-locking fiber laser by using a phase modulated optical fiber loop mirror (PMOLM). The PMOLM can convert phase modulation to amplitude modulation at double the modulation frequency. In principle, it is easy to double the repetition rate. However, the PMOLM also induces residual chirp which can disable this method in some cases. Numerical simulation shows that the best results can be obtained when the intracavity dispersion is negative and the modulation frequency is an odd harmonic of half of the cavity fundamental frequency. This agrees with the experimental results, in which an 80-GHz 1.74-ps transform limited pulse was obtained. By using rational harmonic modulation which requires detuning the modulation frequency and adjusting the modulation index, the repetition rate can be increased by three or four times.