We propose for the first time a new method to electronically control the cavity length of mode-locked fibre lasers, allowingdifferent lasing conditions and performance in a single laser configuration. A special cavity design was developed thatcombines a multi-kilometre fibre resonator boosting pulse energy to hundreds nJ with an all-PM fibre resonator providingnearly 200-m long environmentally-stable cavity round trip. Switching between the cavities is achieved automatically byadjusting modulation frequency of an optical switcher (used for active mode locking) to match the cavity round-trip timeof one of the cavities. We discuss details of our method, its possibilities and limitations, in particular related to stability ofpulse parameters.
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