Rare events of extremely high optical intensity are experimentally recorded at the output of a mode-locked fiber laser that operates in a strongly dissipative regime of chaotic multiple-pulse generation. The probability distribution of these intensity fluctuations, which highly depend on the cavity parameters, features a long-tailed distribution. Recorded intensity fluctuations result from the ceaseless relative motion and nonlinear interaction of pulses within a temporally localized multisoliton phase.

The pioneering paper 'Optical rogue waves' by Solli et al (2007 Nature 450 1054) started the new subfield in optics. This work launched a great deal of activity on this novel subject. As a result, the initial concept has expanded and has been enriched by new ideas. Various approaches have been suggested since then. A fresh look at the older results and new discoveries has been undertaken, stimulated by the concept of 'optical rogue waves'. Presently, there may not by a unique view on how this new scientific term should be used and developed. There is nothing surprising when the opinion of the experts diverge in any new field of research. After all, rogue waves may appear for a multiplicity of reasons and not necessarily only in optical fibers and not only in the process of supercontinuum generation. We know by now that rogue waves may be generated by lasers, appear in wide aperture cavities, in plasmas and in a variety of other optical systems. Theorists, in turn, have suggested many other situations when rogue waves may be observed. The strict definition of a rogue wave is still an open question. For example, it has been suggested that it is defined as 'an optical pulse whose amplitude or intensity is much higher than that of the surrounding pulses'. This definition (as suggested by a peer reviewer) is clear at the intuitive level and can be easily extended to the case of spatial beams although additional clarifications are still needed. An extended definition has been presented earlier by N Akhmediev and E Pelinovsky (2010 Eur. Phys. J. Spec. Top. 185 1-4). Discussions along these lines are always useful and all new approaches stimulate research and encourage discoveries of new phenomena. Despite the potentially existing disagreements, the scientific terms 'optical rogue waves' and 'extreme events' do exist. Therefore coordination of our efforts in either unifying the concept or in introducing alternative definitions must be continued. From this point of view, a number of the scientists who work in this area of research have come together to present their research in a single review article that will greatly benefit all interested parties of this research direction. Whether the authors of this 'roadmap' have similar views or different from the original concept, the potential reader of the review will enrich their knowledge by encountering most of the existing views on the subject. Previously, a special issue on optical rogue waves (2013 J. Opt. 15 060201) was successful in achieving this goal but over two years have passed and more material has been published in this quickly emerging subject. Thus, it is time for a roadmap that may stimulate and encourage further research.

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Phase coherently linking optical to radio frequencies with femtosecond mode-locked laser frequency combs enabled counting the cycles of light and is the basis of optical clocks, absolute frequency synthesis, tests of fundamental physics, and improved spectroscopy. Using an optical microresonator frequency comb to establish a coherent link between optical and microwave frequencies will extend optical frequency synthesis and measurements to areas requiring compact form factor, on chip integration and comb line spacing in the microwave regime, including coherent telecommunications, astrophysical spectrometer calibration or microwave photonics. Here we demonstrate a microwave to optical link with a microresonator. Using a temporal dissipative single soliton state in an ultra-high Q crystalline microresonator that is broadened in highly nonlinear fiber an optical frequency comb is generated that is self-referenced, allowing to phase coherently link a 190 THz optical carrier directly to a 14 GHz microwave frequency. Our work demonstrates precision optical frequency measurements can be realized with compact high Q microresonators.

We report on high-energy ultrashort pulse generation from an all-normal-dispersion large-mode-area fiber laser by exploiting an efficient combination of nonlinear polarization evolution (NPE) and a semiconductor-based saturable absorber mode-locking mechanism. The watt-level laser directly emits chirped pulses with a duration of 1 ps and 163 nJ of pulse energy. These can be compressed to 77 fs, generating megawatt-level peak power. Intracavity dynamics are discussed by numerical simulation, and the intracavity pulse evolution reveals that NPE plays a key role in pulse shaping. © 2010 Optical Society of America OCIS codes: 060.2310, 060.2320 High-performance laser sources delivering ultrashort pulses with energies at the hundred nanojoule level or beyond will open up new directions for ultrafast scientific and industrial applications, ranging from highprecision laser structuring of various micro-/nanotargets to nonlinear optics. Rare-earth-doped fiber lasers present unique properties due to diffractionless propagation and the absence of thermo-optical problems. Because of the light confinement, fiber lasers offer a remarkable opportunity to develop highly stable laser sources, making them ideal candidates for real-world applications. The development of mode-locked fiber lasers operating in the normal dispersion regime to achieve higher pulse energies has been demonstrated [1,2]. Such lasers support dissipative solitons [3] and could produce sub-100 fs pulses with energies as high as 30 nJ using standard single-mode fibers [4]. More recently, exceptional performances in terms of pulse energy and peak power have been demonstrated in mode-locked fiber lasers using large-mode-area (LMA) photonic crystal fibers [5][6][7][8][9]. However, these lasers operate at a low accumulated nonlinear phase and produce relatively long pulses >300 fs. The generation of ultrashort pulses in a dissipativesoliton laser strongly relies on the amount of nonlinearity accumulated along the cavity, which should be enough to ensure sufficient spectral broadening. However, generation of ultrashort pulses with broad bandwidths in an allnormal-dispersion laser needs a strong pulse-shaping mechanism to prevent excessive temporal expansion. One solution is the use of a narrow spectral filter to achieve self-consistent evolution inside the laser cavity [4]. Indeed, spectral filtering of a chirped pulse produces a strong amplitude modulation in the time domain, which increases with spectral breathing. Recently, the generation of sub-150 fs pulses with 24 nm spectral width has been demonstrated in an Yb-doped LMA fiber laser using a saturable absorber mirror (SAM) [10].In this Letter we report the generation of sub-80 fs pulses from a passively mode-locked all-normaldispersion laser featuring an LMA photonic crystal fiber. By exploiting the combined action of an SAM and nonlinear polarization evolution (NPE) for pulse shaping, the laser directly generates 1 ps chirped pulses with pulse energies above 160 nJ at watt-level average power. These pulses...

Following the first experimental observation of a new mechanism leading to optical rogue wave (RW) formation briefly reported in Lecaplain et al (2012 Phys. Rev. Lett. 108 233901), we provide an extensive study of the experimental conditions under which these RWs can be detected. RWs originate from the nonlinear interactions of bunched chaotic pulses that propagate in a fiber laser cavity, and manifest as rare events of high optical intensity. The crucial influence of the electrical detection bandwidth is illustrated. We also clarify the observation of RWs with respect to other pulsating regimes, such as Q-switching instability, that also lead to L-shaped probability distribution functions.

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Performance scaling of passively mode-locked ultrashort-pulse fiber oscillators in terms of average power, peak power, and pulse energy is demonstrated. A very-large-mode-area fiber laser in an all-positive group-velocity-dispersion ring cavity configuration with intracavity spectral filter, mode-locked by nonlinear polarization evolution, emits 66 W of average power at 76 MHz repetition rate, corresponding to 0.9 μJ pulse energy. The pulses are dechirped to 91 fs outside the cavity with an average power of 60 W remaining after the compressor. The generated pulse peak power is as high as 7 MW.