Denis S. Kharenko

20 nJ 200 fs all-fiber highly chirped dissipative soliton oscillator

The dissipative solitons (DS) generated in fiber oscillators with mode-locking mechanism based on nonlinear polarization evolution in a single-mode fiber exhibit stability and energy limits at the cavity lengthening. We demonstrate an alternative approach that enables us to increase the cavity length of the DS oscillator up to 30 m, namely, by the use of a long section of polarization-maintaining (PM) fiber in an all-fiber cavity configuration. We have also identified the next limit of energy scaling related to the onset of Raman conversion of the DS spectrum. The maximum energy of the stable highly chirped DS realized with a 5.5 μm core PM fiber, amounts to ~20 nJ in ~200 fs pulses after a grating compressor. As a next step, energy scaling by means of a fiber core enlargement is discussed.

Multicolour nonlinearly bound chirped dissipative solitons

The dissipative soliton regime is one of the most advanced ways to generate high-energy femtosecond pulses in mode-locked lasers. On the other hand, the stimulated Raman scattering in a fibre laser may convert the excess energy out of the coherent dissipative soliton to a noisy Raman pulse, thus limiting its energy. Here we demonstrate that intracavity feedback provided by re-injection of a Raman pulse into the laser cavity leads to formation of a coherent Raman dissipative soliton. Together, a dissipative soliton and a Raman dissipative soliton (of the first and second orders) form a two (three)-colour stable complex with higher total energy and broader spectrum than those of the dissipative soliton alone. Numerous applications can benefit from this approach, including frequency comb spectroscopy, transmission lines, seeding femtosecond parametric amplifiers, enhancement cavities and multiphoton fluorescence microscopy.

Evolution of dissipative solitons in a fiber laser oscillator in the presence of strong Raman scattering

As recently revealed, chirped dissipative solitons (DSs) generated in a long cavity fiber laser are subject to action of stimulated Raman scattering (SRS). Here we present theoretical and experimental study of the DS formation and evolution in the presence of strong SRS. The results demonstrate that the rising noisy Raman pulse (RP) acts not only as an additional channel of the energy dissipation destroying DS, but on the contrary can support it that results in formation of a complex of the bound DS and RP of comparable energy and duration. In the complex, the DS affords amplification of the RP, whereas the RP stabilizes the DS via temporal-spectral filtering. Stable 25 nJ SRS-driven chirped DS pulses are generated in all-fiber ring laser cavities with lengths of up to 120 m. The DS with duration up to 70 ps can be externally dechirped to <300 fs thus demonstrating the record compression factor.

Highly chirped dissipative solitons as a one-parameter family of stable solutions of the cubic–quintic Ginzburg–Landau equation

In this paper, the stability of the analytical solutions of the cubic–quintic Ginzburg–Landau equation (CQGLE) in the high-chirp approximation has been studied numerically. The existence domain for the stable solution in the CQGLE parameter set has been found. A temporal and spectral shape of the stable solution as dependent of the cavity parameters has been analyzed. Direct comparison of the spectra with numerical calculations has been performed, demonstrating 10−2–10−4 accuracy of the analytical solution for chirp parameter f>10. The stable solutions represent the dissipative soliton family with only one composite parameter. Inside this family, the pulse shape in the time domain evolves from the conventional soliton shape, sech−2, to a rectangular one in the opposite limit with a parabolic shape as an intermediate one. The obtained theoretical results make it possible to classify experimentally observed highly chirped pulses and to optimize experimental schemes with an all-normal-dispersion cavity.

Feedback-controlled Raman dissipative solitons in a fiber laser

Energy of chirped dissipative solitons (DS) generated in fiber lasers may exceed a threshold of stimulated Raman scattering (SRS) leading to formation of a noisy Raman pulse (RP). As we demonstrated recently, a feedback loop providing re-injection of the Raman pulse into the laser cavity can form a Raman dissipative soliton (RDS) with similar characteristics to those of the main dissipative soliton. Here, we present the results of feedback optimization of the generated RDS spectra. First experimental results of coherent combining of DS and RDS are also shown.

Frequency doubling and tripling in a Q-switched fiber laser

The frequency doubling and tripling in a Q-switched all-fiber laser are studied. It is demonstrated that the main limitations on the efficiency of the harmonic generation are related to the random polarization that is nonuniform with respect to axes, the asymmetric pulse shape with a flat trailing edge, and the significant spectral broadening of the multimode radiation of the fiber master oscillator in the fiber amplifier. The methods to increase the efficiency are proposed. For an IR pulse energy of about 0.3 mJ, duration of about 40 ns, and repetition rate of 1 kHz, the second- and third-harmonic pulse energies are greater than 60 and about 10 μJ, respectively.

Frequency doubling of a broadband Raman fiber laser to 655 nm.

655 nm laser radiation with power of >60 mW is generated by frequency doubling of a broadband randomly-polarized 1.31-microm Raman fiber laser (RFL). The red power appears to grow linearly with increasing RFL power up to 7 W at efficiency comparable with that for single-frequency lasers. It has been shown that multiple sum-frequency mixing processes involving different RFL modes provide the main contribution to the output, which is enhanced by 2 times due to the modes stochasticity.

All-fiber highly chirped dissipative soliton oscillator and its scaling

We report on the experimental realization of a highly-chirped dissipative soliton (DS) oscillator with all-fiber cavity consisting of a short single-mode fiber part (for mode locking via nonlinear polarization evolution) and a long PM fiber part (for generation of highly-chirped DS) that enabled to increase cavity length to L ~ 90 m. Stable DS pulses dechirped to ~ 200 fs are generated with maximum energy of ~ 20 nJ. The energy limit is shown to be defined by the onset of Raman conversion of the DS spectrum. The Stokes pulse reaching comparable energy inside the cavity and does not break the soliton stability. Higher DS energy is possible by means of a core enlargement, corresponding experiments are also performed.

50 nJ 250 fs all-fibre Raman-free dissipative soliton oscillator

The regime of highly chirped dissipative solitons is a powerful technique for generating high-energy femtosecond pulses. In this letter we demonstrate a successful scaling of the pulse energy in the all-normal-dispersion all-fibre configuration by increasing the cavity length and the mode-field diameter of the fibre simultaneously. Using a PM fibre of 10 mkm core diameter, it appears possible to lengthen the cavity up to 40 m, thus generating highly chirped pulses with energy above 50 nJ, whereas further growth is limited by the Raman effect. We have also found that the Raman threshold and compressed pulse duration strongly depend on the bandwidth of the intracavity spectral filter. The threshold length is increased by a reduction of the filter bandwidth at the expense of larger compressed pulse duration (250 fs instead of 150 fs). Further energy upscaling by means of core enlargement is shown to be possible, but for that one should waive the all-fiber design, or use custom-made fibre components.

Frequency doubling of a Raman fiber laser

Abstract655 nm laser radiation with power of >60 mW is generated by frequency doubling of a broadband randomly-polarized 1.31-μm phosphosilicate Raman fiber laser (RFL). The red power appears to grow linearly with increasing RFL power up to 7 W at efficiency comparable with that for single-frequency lasers. It has been shown that multiple sum-frequency mixing processes involving different RFL modes provide the main contribution to the output, which is enhanced by 2 times due to the modes stochasticity.