I. Introduction
Ultrafast lasers operating in the spectral region have in recent years undergone rapid developments as driven by a number of potential applications. In particular, lasers based on a fiber format are increasingly desirable as they offer excellent beam quality, compact footprint, broad emission bandwidth and efficient power scaling. Thus far, pulse duration as short as 50 fs has been achieved through extra-cavity techniques [1], [2]. An ultra-broad tuning range of 200 nm was achieved in a nanotube mode-locked ring cavity laser, where a grating mirror is used for wavelength selection [3]. The output power up to 240 watts with picosecond pulse durations has been demonstrated by combining a picosecond fiber oscillator and a three stage thulium-doped amplifier [4]. Despite the impressive advancement, 2 pulsed lasers are still lacking behind in the area of repetition-rate scaling compared to its 1.5 counterpart [5]–[9]. Such multi-gigahertz repetition-rate is essential for novel telecommunication and data processing systems, as well as for sensing and metrology [10]–[14].