Ultrafast science has long been dominated by solid-state sources, mainly Ti:sapphire lasers [1
]. Despite their high energy and tunability, these lasers are limited by the power available from high beam quality visible laser pumps. Rare-earth doped fiber lasers offer the potential for integrated air-cooled sources driven by low cost diode pumps. Thanks to advances in understanding of nonlinear pulse evolution, as well as developments in large mode-area fiber technology, fiber lasers are emerging as competitive sources of ultrashort light pulses.
The performance of modelocked fiber lasers is mainly limited by nonlinear phase accumulation. Net-normal dispersion cavities provide the highest pulse energies by enabling chirped-pulse evolutions such as in the self-similar [2
] and the all-normal dispersion (ANDi) lasers [3
]. In particular, the ANDi laser uses spectral filtering of a chirped pulse to achieve dispersion-compensation-free self-consistent pulse shaping. This flexible pulse shaping mechanism supports a wide variety of spectral shapes [4
], and is well-modeled by dissipative solitons of the cubic-quintic Ginzburg Landau equation (CQGLE) [5
A larger effective mode area scales the peak power supported by a given pulse evolution at constant nonlinear phase shift [6
]. So far, the most successful technology for scaling to large single mode cores has been photonic crystal fibers (PCF) and rods [7
]. Oscillators that reach the megawatt peak powers have been reported based on this approach [8
]. Despite their impressive performance, lasers based on PCF have found little practical application; integration into fiber devices is hindered by incompatibility with fusion splicing and sensitivity to bend loss. Large mode-area step-index fibers offer superior integration potential. Amplifiers based on multimode fiber have been demonstrated with close to single-mode beam quality. This is achieved using fundamental-mode excitation and higher-order mode (HOM) suppression, usually through selective bend loss [10
]. There is a brief and isolated report of a modelocked soliton laser based on multimode fiber [11
]. A recent systematic investigation of modelocking under multimode conditions finds that conventional step-index fibers cannot suppress HOMs sufficiently to allow stable modelocking at high single-pulse energies [12
]. Small amounts of energy in HOMs cause multi-pulsing at energies well below those expected from mode-area scaling. There is thus a need for a distributed transverse mode filtering mechanism that is robust enough for an application as sensitive as ultrafast modelocking, yet retains the integration potential of all-glass fibers. Leaky waveguides that selectively couple out HOMs while maintaining low losses on the fundamental mode provide such a mechanism. Examples include leakage channel fibers [13
], as well as Bragg fiber for which a mode-locked oscillator was recently demonstrated [14
]. Another implementation of selective leakage is chirally-coupled core (CCC) fiber, which use a secondary core wound around a large central core to create a distributed and integrated HOM filtering mechanism [15
]. Using no external mode-filtering or mode-matching methods, CCC fiber systems have shown effectively single-mode performance. Although a continuous-wave (CW) laser based on Yb-doped CCC fiber has been demonstrated [16
], an application as sensitive as ultrafast modelocked lasers has yet to be investigated.
In this work, we present a dissipative soliton laser based on Yb-doped large mode-area CCC fiber. This is the first mode-locked laser to use CCC fiber, and demonstrates its ability to scale the pulse energy of ultrafast fiber lasers in a solid glass package with low bend sensitivity and compatibility with fiber fusion technology. The performance matches simulations of dissipative soliton pulse evolution, as well as pulse energies expected from scaling of single-mode ANDi fiber lasers. Beam quality, spectral interference and a high single-pulsing limit demonstrate effectively single-mode operation. The oscillator provides chirped pulses of energies greater than 40 nJ, which dechirp to below 200 fs. With a shorter fiber, pulse durations close to 100 fs are achieved at chirped pulse energies of 22 nJ, limited by the available pump power.