Stable 6.8 GHz actively modelocked pulses were generated from composite cavity Er-doped fibre ring lasers. The composite cavity fibre lasers operate as virtually short cavity lasers, and thus modelocking at a lower harmonic number can be possible. The cavity mode spacing of the laser is over 10 times larger than usual ring cavity mode spacing with the same cavity length.
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N. Onodera (1997) studied this question.