Evgeniy V. Podivilov

Approaching the microjoule frontier with femtosecond laser oscillators: theory and comparison with experiment

A detailed numerical analysis of heavily chirped pulses in the positive-dispersion regime (PDR) is presented on the basis of the distributed cubic–quintic generalized complex nonlinear Ginzburg–Landau equation. It is demonstrated that there are three main types of pulse spectra: truncated parabolic-top, Π- and M-shaped profiles. The strong chirp broadens the pulse spectrum up to 100 nm for a Ti:Sa oscillator, which provides compressibility of the picosecond pulse down to sub-30 fs. Since the picosecond pulse has a peak power lower than the self-focusing power inside a Ti:Sa crystal, the microjoule energies become directly available from a femtosecond oscillator. The influence of the third- and fourth-order dispersions on the pulse spectrum and stability is analysed. It is demonstrated that the dynamic gain saturation plays an important role in pulse stabilization. The common action of dynamic gain saturation, self-amplified modulation (SAM) and saturation of the SAM provides pulse stabilization inside the limited range of the positive group-delay dispersions (GDDs). Since the stabilizing action of the SAM cannot be essentially enhanced for a pure Kerr-lens mode-locking regime, a semiconductor saturable absorber is required for pulse energies of >0.7 μJ inside an oscillator. The basic results of the numerical analysis are in an excellent agreement with experimental data obtained from oscillators with repetition rates ranging from 50 to 2 MHz.

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.

Heavily-chirped solitary pulses in the normal dispersion region: New solutions of the cubic-quintic complex Ginzburg-Landau equation

A new type of the heavily-chirped solitary pulse solutions of the nonlinear cubic-quintic complex Ginzburg-Landau equation has been found. The methodology developed provides for a systematic way to find the approximate but highly accurate analytical solutions of this equation with the generalized nonlinearities within the normal dispersion region. It is demonstrated that these solitary pulses have the extra-broadened parabolic-top or fingerlike spectra and allow compressing with more than a hundredfold growth of the pulse peak power. The obtained solutions explain the energy scalable regimes in the fiber and solid-state oscillators operating within the normal dispersion region and promise to achieve microjoules femtosecond pulses at MHz repetition rates.

High-order random Raman lasing in a PM fiber with ultimate efficiency and narrow bandwidth.

Random Raman lasers attract now a great deal of attention as they operate in non-active turbid or transparent scattering media. In the last case, single mode fibers with feedback via Rayleigh backscattering generate a high-quality unidirectional laser beam. However, such fiber lasers have rather poor spectral and polarization properties, worsening with increasing power and Stokes order. Here we demonstrate a linearly-polarized cascaded random Raman lasing in a polarization-maintaining fiber. The quantum efficiency of converting the pump (1.05 μm) into the output radiation is almost independent of the Stokes order, amounting to 79%, 83%, and 77% for the 1st (1.11 μm), 2nd (1.17 μm) and 3rd (1.23 μm) order, respectively, at the polarization extinction ratio >22 dB for all orders. The laser bandwidth grows with increasing order, but it is almost independent of power in the 1–10 W range, amounting to ~1, ~2 and ~3 nm for orders 1–3, respectively. So, the random Raman laser exhibits no degradation of output characteristics with increasing Stokes order. A theory adequately describing the unique laser features has been developed. Thus, a full picture of the cascaded random Raman lasing in fibers is shown.

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.

Relative intensity noise in cascaded-Raman fiber lasers

Pump-to-Stokes and Stokes-to-Stokes relative intensity noise (RIN) transfer is discovered now for cascaded Raman fiber lasers (RFLs) with multimode laser pumping. We report on the first experimental study of this effect observed with two-stage phosphosilicate RFL and also propose its analytical explanation. It is shown that the peaks of radio-frequency (RF) spectrum associated with longitudinal mode beating in the pump laser cavity almost uniformly transfer to the first and second Stokes RF-spectra, thus increasing RIN of the RFL in megahertz-frequency domain.

Self-scanned single-frequency operation of a fiber laser driven by a self-induced phase grating

The selector-free single-frequency operation of an Yb-doped fiber laser with scanning in the range of ?20?nm is demonstrated. The frequency and intensity evolution is shown to be driven by a self-induced phase grating in the active fiber defined by gain saturation in a standing-wave. A theory has been developed that describes well the main features of the experiment and provides possibilities for optimization of laser parameters. Perspectives for utilizing the self-scanned laser in fundamental studies and practical applications are discussed.

Broad-range self-sweeping of a narrow-line self-pulsing Yb-doped fiber laser

The effect of broad-range (16 nm) self-sweeping of a narrow-line (less than 1 pm) Yb-doped fiber laser has been demonstrated experimentally. It is found that the effect arises from the self-sustained relaxation oscillations. As a result, the sweeping rate increases as square root of the laser power and decreases with increasing cavity length. Based on these results we propose a model describing dynamics of the laser frequency. The model takes into account the effects of gain saturation at the laser transition and spatial hole burning in the self-pulsing regime.

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.