We have developed a comprehensive 3D steady-state numerical model of high power Yb-doped fiber amplifiers which allows the investigation of thermally induced multimode behavior being the reason for the transversal mode instabilities phenomenon. Numerical simulations show that for the pump powers above 3 kW the refractive index grating inscribed as a result of thermo-optic effect supports an efficient energy transfer from the main mode to high order modes. Coupling of the initial Gaussian beam without any shift or tilt to the investigated amplifier leads to the preferential excitation and further growth of symmetrical high order modes (LP02, LP03). Anti-symmetrical modes are not growing. Increasing the pump power leads to stronger mode coupling which is related to the increase of the amplitude and frequency of the temperature oscillations and corresponding refractive index oscillations over the longitudinal axis of the fiber amplifier.
We have investigated ultrashort parabolic pulse formation via passive nonlinear reshaping in normal dispersive optical fibers at 1550 nm. It was investigated parabolic pulse formation in the transient-state regime and in the steady-state regime. Numerical simulations have been made based on generalized nonlinear Schrödinger equation taking into account high-order dispersion terms and high order nonlinear terms. It was examined the applicability of different commercially available fibers for parabolic pulse formation at 1550 nm. It was found that small amount of positive second-order dispersion and quite sufficient third-order dispersion can restrict strongly the formation of parabolic pulses at 1550 nm. The most suitable fiber for pulse reshaping has been found.
The supercontinuum (SC) generated by pumping in anomalous dispersion is sensitive to the input pulse fluctuations and pump laser’s shot noises and does not possess a single-pulse waveform, so the incident pulse becomes a noise-like train of spikes. A simple method of creating pulsed lasers with either pulse-maintaining ultrabroad SC or specially shaped pulse waveforms can be implemented using all-normal-dispersion microstructured optical fibers (ANDi-MOFs). An ANDi-MOF with a simple topology and dispersion profile maximum at 800 nm was designed using the effective index method. Its properties and suitability were characterized via numerical simulation of femtosecond parabolic pulse formation and generation of an octave-spanning pulse-maintaining SC using a generalized propagation equation. The designed ANDi-MOF is suitable for resolving both problems and allows some detuning of the pulse’s wavelength around 800 nm. However, a better choice for SC generation is pumping at or near the wavelength where the third-order dispersion becomes zero. This configuration benefits from the absence of pulse break-up under large pulse energies, which appears otherwise. The fiber can provide a low-cost method for developing supercontinuum sources and a solution to the problems of parabolic waveform formation to meet the needs of optical signal processing and pulse amplification and compression.
We have designed an all-normal dispersion photonic crystal fiber optimized for pumping at 800 nm with initial pulses which are typical for conventional Ti:Sapphire lasers. Parabolic pulse formation and supercontinuum generation in this fiber is analyzed both in time and frequency domains.
Generation and applications of the optical pulses with a parabolic intensity profile has developed into the area of
dynamic research activity over recent years. Parabolic pulses can propagate remaining their parabolic profile.
Particularly these pulses resist to the deleterious effect of the optical wave breaking. They are of great interest for a
number of applications including the high power pulse generation, and all optical signal processing. Alternative methods
of generating parabolic pulses are of especial interest in the context of non-amplification usage, such as optical
telecommunications. It is found that Gaussian waveforms provide best quasi-parabolic pulses among others and within
shortest distance. There is a range of soliton numbers where the shape of quasi-parabolic pulse is closest to parabolic
one.
We have investigated a nonlinear pulse reshaping towards parabolic pulses in the passive normal dispersive optical
fibers. We have found that pulses with parabolic intensity profile, parabolic spectrum and linear chirp can be obtained
due to the passive nonlinear reshaping at the propagation distance exceeding a few dispersion lengths. These pulses
preserve parabolic profile during subsequent pulse propagation in a fiber. We have examined the influence of initial
pulse parameters and fiber parameters on the resulted pulse shape.
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