Normal Dispersion Fiber Research Articles

Solid-Core Fiber Spectral Broadening at Its Limits

Broadband optical spectra are employed in ultrafast time-resolved studies, seeding of tunable parametric amplifiers, and selectively addressing transitions in absorption spectroscopy. A strong extension of spectral bandwidth is commonly achieved in nonlinear fiber by means of self-phase modulation. In terms of energy scalability and achievable bandwidth, the method is, however, limited by the damage threshold of the core material. Here, we study spectral broadening in four different single-mode normal dispersive photonic crystal fibers length of 8–10 cm. They are pumped by a thin-disk oscillator emitting 250 fs pulses at peak powers close to the critical power of silica. We demonstrate mode-field diameter-dependent broadening factors from 19 to 51, which are obtained at power spectral densities from 200 to 5 mW/nm. We explain the results by a relation between peak power and broadening factor. This will serve as a fiber selection guideline transferable to lasers emitting at wavelengths different from 1030 nm. In addition, we examine to which extend prechirp leads to energy scalability of spectral broadening in solid-core fiber. By contrast to previous reports, we point out that the applicability of the technique is strongly limited through the requirement of bell-shaped input pulses and the pulse duration dependent material damage threshold.

An efficient way of third-order dispersion compensation for reshaping parabolic pulses through normal dispersion fiber amplifier

Normal dispersion decreasing fibers (NDDFs) employed for parabolic pulse (PP) generation are limited by the presence of inherent third-order dispersion (TOD), which leads to distortion in pulse shape from its parabolic nature. This necessitates a compensation method for TOD for such fibers. Dispersion-compensating fiber (DCF) amplifiers with relatively smaller values of dispersion can also be largely affected by this harmful effect of TOD when used for the same purpose. As a solution, a time-reversal system (TRS) based on a time-conjugating technique is employed with the aim of minimizing the deleterious effects of TOD. A detailed study on the estimation of the critical length where the time lens system needs to be implemented within the fiber link is crucial for reshaping the pulse into parabolic form. An NDDF is designed and optimized to possess a relatively higher value of TOD and a cascaded fiber optic system comprising the first stage of the proposed NDDF, the TRS and the second stage of the NDDF is suggested here to obtain linearly chirped PP at the output end even in the presence of a high value of TOD. The proposed scheme has also proved to be efficient for better pulse reshaping through DCFs possessing a high amount of TOD.

Soliton-like pulse propagation in a normal dispersive liquid-core optical fiber.

Soliton-like pulse propagation, where the pulse intensity profile is conserved, except for its trailing edge, is found in a normal dispersive CS2-core optical fiber by a computer simulation. During propagation, it is found that the chirp of a pulse arising from the delayed nonlinear response compensates for the chirp arising from the second-order dispersion near the pulse peak position and the pulse experiences a negative frequency shift and a negative temporal shift. The effects of the instantaneous nonlinear response on the soliton-like pulse propagation are also evaluated, and the calculated spectrum agreed well with the experimental spectrum.

Near infrared tunable source delivering ultra-short pulses based on an all normal dispersion fiber and a zero dispersion line

A tunable laser source (820–1200 nm) delivering ultra-short pulses in the range of 40–100 fs is investigated. It is based on the filtering of a continuum in the Fourier plane of a zero dispersion line without any phase compensator. The numerical simulations show the origin and the impact of the nonlinear chirp to guarantee ultra-short pulses.

Compression of femtosecond ytterbium fibre laser pulses using nonlinear processes in silica fibre

The temporal compression of 100-fs nanojoule laser pulses using nonlinear processes in silica fibre has been studied by numerical simulation and experimentally. The simulation results demonstrate that, in the case of spectral broadening in a normal dispersion fibre, followed by the temporal compression of the pulse in a compressor with a negative second-order dispersion, up to 90 % of the compressed pulse energy can be concentrated in the main peak, whose duration is a factor of 8 shorter than the initial laser pulse duration. In our experiments, 100-fs ytterbium fibre laser pulses have been compressed to pulses with an ∼13-fs main peak accounting for 87 % of the compressed pulse energy. At an average ytterbium laser output power of 4.2 W, the average power at the compressor output has been 3.1 W.

Internal polarization dynamics of vector dissipative-soliton-resonance pulses in normal dispersion fiber lasers

Internal polarization dynamics of vector dissipative-soliton-resonance (DSR) pulses in a mode-locked fiber laser are investigated. By utilizing a wave plate analyzer configuration to analyze the special structure of a DSR pulse, we find that polarization state is not uniform across a resonant dissipative soliton. Specifically, although the central plane wave of the resonant dissipative soliton acquires nearly a single fixed polarization, the dissipative fronts feature polarization states that are different and spatially varying. This distinct polarization distribution is maintained while the whole soliton extends with increasing gain. Numerical simulation further confirms the experimental observations.

Decomposition of group-velocity-locked-vector-dissipative solitons and formation of the high-order soliton structure by the product of their recombination.

By using a polarization manipulation and projection system, we numerically decomposed the group-velocity-locked-vector-dissipative solitons (GVLVDSs) from a normal dispersion fiber laser and studied the combination of the projections of the phase-modulated components of the GVLVDS through a polarization beam splitter. Pulses with a structure similar to a high-order vector soliton could be obtained, which could be considered as a pseudo-high-order GVLVDS. It is found that, although GVLVDSs are intrinsically different from group-velocity-locked-vector solitons generated in fiber lasers operated in the anomalous dispersion regime, similar characteristics for the generation of pseudo-high-order GVLVDS are obtained. However, pulse chirp plays a significant role on the generation of pseudo-high-order GVLVDS.

Riemann problem for the photon fluid: Self-steepening effects

We consider the Riemann problem of evolution of initial discontinuities for the photon fluid propagating in a normal dispersion fiber with account of self-steepening effects. The dynamics of light field is described by the nonlinear Schroedinger (NLS) equation with self-steepening term appearing due to retardation of the fiber material response to variations of the electromagnetic signal. It is shown that evolution dynamics in this case is much richer than that for the NLS equation. Complete classification of possible wave structures is given for all possible jump conditions at the discontinuity.

Nonlinear Compensation in Optical Communications Systems With Normal Dispersion Fibers Using the Nonlinear Fourier Transform

We investigate the computational cost of the nonlinear Fourier transform (NFT) based on the Zakharov–Shabat scattering problem as a nonlinear compensation technique for quadrature-phase-shift keyed (QPSK) signals with raised cosine frequency characteristic in optical fiber transmission systems with normal dispersion fibers. We show that the primary sources of computational errors that arise from the use of the NFT is the finite eigenvalue resolution of the left and the right reflection spectra. We show that this effect and, consequently, the computational cost of the NFT as a nonlinear mitigation technique in the normal dispersion regime increases exponentially or faster with both the channel power and the number of symbols per data frame even using the most efficient NFT algorithms that are currently known. We find that the computational cost of this approach becomes unacceptably large at data frame lengths and powers that are too small for this approach to be competitive with standard transmission methods. We explain the physical reasons for these limits.

Chaotic one-dimensional domains induced by periodic potentials in normal-dispersion fiber lasers.

We investigate numerically the effects of external time-periodic potentials on time-localized perturbations to the amplitude of electromagnetic waves propagating in normal-dispersion fiber lasers which are described by the complex Ginzburg-Landau equation. Two main effects were found: The formation of domains enclosed by two maxima of the external periodic field and the generation of a chaotic behavior of these domains in the region of relatively high amplitudes and low frequencies of the external fields. Maps and bifurcation diagrams of the largest Lyapunov exponent and moments, such as energy and momentum, are also provided for different values of the amplitude and frequency of such external potentials.

Performance of different normal dispersion fibers to generate triangular optical pulses

To identify the best possible normally dispersive optical fibers (NDF) in view of efficient triangular pulse (TP) generation, the optimization of several passive NDFs are reported in this work. The study shows that TPs, which sustain over a reasonably good fiber length, can be generated at shorter length when higher values of dispersion and nonlinearity are considered. A suitable NDF with high value of dispersion and nonlinearity is chosen and renamed as normally dispersive highly nonlinear fiber (ND-HNLF). The fiber parameters and pulse conditions are optimized here in such a way that the pulse maintains its triangular shape throughout a longer length in transient state. A detail analysis is performed on generation and stability of triangular pulses through the ND-HNLFs in absence and in presence of gain. When compared, the ND-HNLF with gain is found to be preferable due to their lower input power requirement and lesser broadening of the output triangular pulses whereas passive ND-HNLF performs better in terms of formation of TPs at shorter length and sustainability of those pulses over moderately larger fiber length. It is also shown that the suggested fibers, competent enough to generate stable TPs can be potentially applied in the area of optical signal doubling and a new fiber based approach is mentioned as well for saw-tooth pulse formation.

Investigation of all-in-fiber Yb doped femtosecond fiber oscillator for generation of parabolic pulses in normal dispersion fiber amplifier

Abstract In this work femtosecond passively mode-locked environmentally stable Ytterbium fiber oscillator generating pulses with duration of 380 fs is presented. Short pulse duration and smooth spectrum were obtained from the oscillator using chirped fiber Bragg grating with very low anomalous chromatic dispersion (0.15 ps2) and semiconductor saturable absorber mirror. Linearly chirped parabolic pulses were produced after amplification of the oscillator pulses in low concentration ytterbium doped fiber amplifier. Transform limited duration of the generated parabolic pulses was 110 fs.

Limits of coherent supercontinuum generation in normal dispersion fibers

We study the largely unexplored transition between coherent and noise-seeded incoherent continuum generation in all-normal dispersion (ANDi) fibers and show that highly coherent supercontinua with spectral bandwidths of one octave can be generated with long pump pulses of up to 1.5 ps duration, corresponding to soliton orders of up to N=600. In terms of N, this corresponds to an approximately 50 times increase of the coherent regime compared to anomalous dispersion pumping. In the transition region between coherent and incoherent spectral broadening, we observe the manifestation of nonlinear phenomena that we term incoherent cloud formation and incoherent optical wave breaking, which lead to a gradual or instantaneous coherence collapse of supercontimuum (SC) spectral components, respectively. The role played by stimulated Raman scattering and parametric four-wave mixing during SC generation in ANDi fibers is shown to be more extensive than previously recognized: their nonlinear coupling contributes to the suppression of incoherent dynamics at short pump pulse durations, while it is responsible for non-phase-matched parametric amplification of noise observed in the long pulse regime. We further discuss the dependence of SC coherence on fiber design, and present basic experimental verifications for our findings using single-shot detection of SC spectra generated by picosecond pulses. This work outlines both the further potential as well as the limitations of broadband coherent light source development for applications such as metrology, nonlinear imaging, and ultrafast photonics, among others.

Design of All-Normal Dispersion Microstructured Optical Fiber on Silica Platform for Generation of Pulse-Preserving Supercontinuum Under Excitation at 1550 nm

We investigated numerically the possibility of all normal dispersion fiber design for near-infrared supercontinuum generation based on a standard air-silica microstructure. The design procedure includes finding of target dispersion profile and subsequent finding of appropriate geometrical fiber design by inverse dispersion engineering. It was shown that the tailoring of dispersion profile could increase the spectral width of generated supercontinuum while maintaining perfect spectral flatness. Conditions necessary for wide and flat supercontinuum generation as well as restrictions imposed by chosen materials were discussed. As a result of design and optimization procedure, an air-silica design was found providing normal dispersion up to $\text{3}\;\mu {\text{m}}$ . Simulation results with 10 nJ, 100 fs pulses demonstrate supercontinuum generation up to 1.3 octave; whereas pumping with 30 nJ, 100 fs pulses could provide 1.8 octavse supercontinuum.

Soliton interaction and further merging in normal-dispersion fiber lasers modeled with the cubic-quintic CGLE

We study the interaction and merging of optical solitons in normal-dispersion fiber lasers within the framework of the cubic-quintic complex Ginzburg–Landau equation (CGLE). We start finding homogeneous analytic solutions to this equation that are used in numerical simulations to build interacting kinks that lead to the formation of stable solitons. We then study numerically the interaction and merging of these solitons and characterize this process using moments such as the energy and momentum along the fiber. We have found that the merging process occurs only for small enough values of the initial distance (that depends on the phase difference) between solitons, otherwise they repel each other monotonically. The structure of the merging process shows a double peaked energy burst and (depending on the phase difference) a gain and/or loss of momentum. Finally, we propose a fitting model for isolated solitons in order to find a suitable analytical representation for the attractive interaction law before their merging.

Influence of modulation instability on the operation of phase-sensitive optical time domain reflectometers

Modulation instability (MI) in optical fibers adversely affects performance of phase-sensitive optical time domain reflectometers causing reduction of statistical visibility of the coherent reflectograms and corresponding degradation of the phase sensitive signal. This effect limits intensity of the probing pulse and imposes limits on the reflectometer operational range. Intensity limits imposed by the MI development were quantified for different positive dispersion fibers using nearly rectangular ~200 ns pulses at 1.5 µm wavelengths. MI development and reversible character of energy exchange between narrowband probing pulse and wideband optical noise known as Fermi–Pasta–Ulam energy recursion were observed. It is demonstrated both experimentally and numerically that the operational range of coherent reflectometers can be extended by increasing probing pulse energy if negative (normal) dispersion fibers are used, where MI development is suppressed.

Bandwidth-tunable dissipative soliton and noise-like pulse in a normal dispersion fiber laser with a dual-scale saturable absorber

A bandwidth-tunable dissipative soliton (DS) and noise-like pulse are demonstrated in an all-fiber normal dispersion fiber laser by employing a novel spectral filtering scheme. The mode locker is fabricated with a mixture composite that combines the merits of graphene and carbon nanotubes. At a pump power of 39 mW, a DS with knife edges is generated at 1565 nm. One of the steep edges is induced by gain spectral filtering and another is cut by a chirped fiber Bragg grating (CFBG). With higher pump power, the noise-like pulse operation displays an asymmetric spectral profile, confirming the novel combined spectral filtering. When the CFBG is stretched mechanically, the bandwidth of the DS and noise-like pulse can be tuned. The bandwidth-tunable mode-locked fiber laser has numerous potential applications, such as pulse shaping, as an amplifier and in optical nonlinear investigations.

Generation of stretched pulses and dissipative solitons at 2 μm from an all-fiber mode-locked laser using carbon nanotube saturable absorbers.

We demonstrate for the first time, to the best of our knowledge, a thulium-doped, all-fiber, mode-locked laser using a carbon nanotube saturable absorber, operating in the dissipative-soliton regime and the stretched-pulse-soliton regime. The net dispersion of the laser cavity is adjusted by inserting different lengths of normal dispersion fiber, resulting in different mode-locking regimes. These results could serve as a foundation for the optimization of mode-locked fiber-laser cavity design at the 2μm wavelength region.

Group velocity locked vector dissipative solitons in a high repetition rate fiber laser.

Vectorial nature of dissipative solitons (DSs) with high repetition rate is studied for the first time in a normal-dispersion fiber laser. Despite the fact that the formed DSs are strongly chirped and the repetition rate is greater than 100 MHz, polarization locked and polarization rotating group velocity locked vector DSs can be formed under 129.3 MHz fundamental mode-locking and 258.6 MHz harmonic mode-locking of the fiber laser, respectively. The two orthogonally polarized components of these vector DSs possess distinctly different central wavelengths and travel together at the same group velocity in the laser cavity, resulting in a gradual spectral edge and small steps on the optical spectrum, which can be considered as an auxiliary indicator of the group velocity locked vector DSs. Moreover, numerical simulations well confirm the experimental observations and further reveal the impact of the net cavity birefringence on the properties of the formed vector DSs.

Experimental Study on Soliton Rain Patterns in Yb-Doped All-Fiber Standing Wave Cavity Configuration

We demonstrate for the first time, experimental observation of soliton rain (SR) state at relatively low pumping power in Yb-doped fiber laser in all-fiber standing wave cavity configuration. SR is shown to be controllable in terms of speed, density, and direction of drifting. Other SR states, such as stationary SR, cyclic SR, and harmonic SR, were also observed by adjusting polarization controller settings. We carefully identified the portion of the spectra contributing to the SR state and correlate its influence on the speed and direction of rain drift relative to the condensed phase. Observation of SR in our cavity can help in enhancing the understanding of dissipative soliton dynamics in all normal dispersion fiber lasers.