William H. Renninger

All-normal-dispersion femtosecond fiber laser with pulse energy above 20 nJ.

We report a study of the scaling and limits to pulse energy in an all-normal-dispersion femtosecond fiber laser. Theoretical calculations show that operation at large normal cavity dispersion is possible in the presence of large nonlinear phase shifts, owing to strong pulse shaping by spectral filtering of the chirped pulse in the laser. Stable pulses are possible with energies of tens of nanojoules. Experimental results from Yb-doped fiber lasers agree with the trends of numerical simulations. Stable and self-starting pulses are generated with energies above 20 nJ, and these can be dechirped to <200 fs duration. Femtosecond pulses with peak powers near 100 kW are thus available from this simple and practical design.

Properties of normal-dispersion femtosecond fiber lasers

We report a systematic study of all-normal-dispersion mode-locked fiber lasers. Spectral filtering of a chirped pulse in the cavity is a major component of the pulse shaping in these lasers. We identify the nonlinear phase shift accumulated by the pulse, spectral filter bandwidth, and group-velocity dispersion as the key parameters that determine the behavior and properties of these lasers. Trends in the performance as these parameters are varied are summarized. A wide range of pulse shapes and evolutions can occur. Experimental results from Yb-doped all-normal-dispersion fiber lasers agree reasonably well with the results of numerical simulations.

Sub-100 fs pulses at watt-level powers from a dissipative-soliton fiber laser

We report a mode-locked fiber laser that exploits dissipative-soliton pulse shaping along with cladding pumping for high average power. The laser generates 31 nJ chirped pulses at 70 MHz repetition rate, for an average power of 2.2 W. After dechirping outside the laser, 80 fs pulses, with 200 kW peak power, are obtained.

Dissipative solitons in normal-dispersion fiber lasers

Mode-locked fiber lasers in which pulse shaping is based on filtering of a frequency-chirped pulse are analyzed with the cubic-quintic Ginzburg-Landau equation. An exact analytical solution produces a variety of temporal and spectral shapes, which have not been observed in any experimental setting to our knowledge. Experiments agree with the theory over a wide range of parameters. The observed pulses balance gain and loss as well as phase modulations, and thus constitute dissipative temporal solitons. The normal-dispersion fiber laser allows systematic exploration of this class of solitons.

Pulse Shaping and Evolution in Normal-Dispersion Mode-Locked Fiber Lasers

Fiber lasers mode locked with large normal group-velocity dispersion have recently achieved femtosecond pulse durations with energies and peak powers at least an order of magnitude greater than those of prior approaches. Several new mode-locking regimes have been demonstrated, including self-similar pulse propagation in passive and active fibers, dissipative solitons, and a pulse evolution that avoids wave breaking at high peak power but has not been reproduced by theoretical treatment. Here, we illustrate the main features of these new pulse-shaping mechanisms through the results of numerical simulations that agree with experimental results. We describe the features that distinguish each new mode-locking state and explain how the interplay of basic processes in the fiber produces the balance of amplitude and phase evolutions needed for stable high-energy pulses. Dissipative processes such as spectral filtering play a major role in normal-dispersion mode locking. Understanding the different mechanisms allows us to compare and contrast them, as well as to categorize them to some extent.

Giant-chirp oscillators for short-pulse fiber amplifiers

A new regime of pulse parameters in a normal-dispersion fiber laser is identified. Dissipative solitons exist with remarkably large pulse duration and chirp, along with large pulse energy. A low-repetition-rate oscillator that generates pulses with large and linear chirp can replace the standard oscillator, stretcher, pulse-picker, and preamplifier in a chirped-pulse fiber amplifier. The theoretical properties of such a giant-chirp oscillator are presented. A fiber laser designed to operate in the new regime generates ~150 ps pulses at a 3 MHz repetition rate. Amplification of these pulses to 1 μJ energy with pulse duration as short as 670 fs demonstrates the promise of this new approach.

Optical solitons in graded-index multimode fibres

Solitons are non-dispersing localized waves that occur in diverse physical settings, including liquids, optical fibres, plasmas and condensed matter. They attract interest owing to their particle-like nature and are useful for applications such as in telecommunications. A variety of optical solitons have been observed, but versions that involve both spatial and temporal degrees of freedom are rare. Optical fibres designed to support multiple transverse modes offer opportunities to study wave propagation in a setting that is intermediate between single-mode fibre and free-space propagation. Here we report the observation of optical solitons and soliton self-frequency shifting in graded-index multimode fibre. These wave packets can be modelled as multicomponent solitons, or as solitons of the Gross-Pitaevskii equation. Solitons in graded-index fibres should enable increased data rates in low-cost telecommunications systems, are pertinent to space-division multiplexing, and can offer a new route to mode-area scaling for high-power lasers and transmission.

Pulse generation without gain-bandwidth limitation in a laser with self-similar evolution

With existing techniques for mode-locking, the bandwidth of ultrashort pulses from a laser is determined primarily by the spectrum of the gain medium. Lasers with self-similar evolution of the pulse in the gain medium can tolerate strong spectral breathing, which is stabilized by nonlinear attraction to the parabolic self-similar pulse. Here we show that this property can be exploited in a fiber laser to eliminate the gain-bandwidth limitation to the pulse duration. Broad (∼200 nm) spectra are generated through passive nonlinear propagation in a normal-dispersion laser, and these can be dechirped to ∼20-fs duration.

Environmentally stable all-normal-dispersion femtosecond fiber laser

We demonstrate a mode-locked all-normal-dispersion ytterbium-doped fiber laser constructed with polarization-maintaining fibers. Spectral filtering of a chirped pulse in the cavity, along with a semiconductor saturable absorber, produce self-starting femtosecond mode-locked operation with large normal dispersion. Environmentally stable generation of 2 nJ and 300 fs pulses is achieved.

Stabilization of high-energy femtosecond ytterbium fiber lasers by use of a frequency filter

The performance of a femtosecond Yb fiber laser is investigated in the limit of high pulse energy (>10 nJ) and short pulse duration (<100 fs). By reducing the strength of nonlinear polarization rotation and introducing amplitude modulation through an intracavity frequency filter, pulses with unprecedented peak powers can be generated. The laser emits stable 13 nJ pulses that are dechirped to 55 fs duration and are capable of providing 130 kW peak power.