Soliton Fission Process Research Articles

Optical heating-induced spectral tuning of supercontinuum generation in liquid core fibers using multiwall carbon nanotubes.

In this work, we demonstrate the optical heating modulation of soliton-based supercontinuum generation through the employment of multi-walled carbon nanotubes (MW-CNTs) acting as fast and efficient heat generators. By utilizing highly dispersion-sensitive liquid-core fibers in combination with MW-CNTs coated to the outer wall of the fiber, spectral tuning of dispersive waves with response times below one second via exploiting the strong thermo-optic response of the core liquid was achieved. Local illumination of the MW-CNTs coated fiber at selected points allowed modulation of the waveguide dispersion, thus controlling the soliton fission process. Experimentally, a spectral shift of the two dispersive waves towards the region of anomalous dispersion was observed at increasing temperatures. The presented tuning concept shows great potential in the context of nonlinear photonics, as complex and dynamically reconfigurable dispersion profiles can be generated by using structured light fields. This allows investigating nonlinear frequency conversion processes under unconventional conditions, and realizing nonlinear light sources that are reconfigurable quickly.

Tailoring the waveguide dispersion of nonlinear fibers for supercontinuum generation with superior intrapulse coherence

Spectral coherence is an important prerequisite for many applications of femtosecond supercontinua, including precision frequency metrology, attosecond science, and high-field physics. These applications often critically depend on measurement of the carrier-envelope phase via f − 2 f interferometry. As mode-locked laser sources cannot directly provide octave-spanning spectra, one typically resorts to spectral broadening in highly nonlinear optical fibers to generate the necessary spectral coverage. This process comes with a caveat, as coherence can be severely degraded in the broadening process. This degradation can be mitigated by suitable choice of a fiber dispersion profile. Here we numerically investigate prototypical fiber designs and analyze their susceptibility to coherence degradation. It is found that the generally favored soliton fission process provides the best broadening efficiency at the expense of rather scarce coherence conditions. Sufficient pulse energy provided, all-normally dispersive fiber designs fare far better in terms of spectral coherence. Finally, fiber designs with flattened normal dispersion provide the best compromise between efficiency and resulting spectral coherence. Our study indicates that the spectral width of a supercontinuum may be deceiving. Even when compressible into a short pulse, this does not automatically qualify for sufficient spectral coherence to conduct carrier-envelope-phase-sensitive experiments. The numerical simulations in this study provide a guideline for coherent spectral broadening. In turn, these guidelines may help to improve carrier-envelope phase stabilization and, in particular, to enable stabilization of laser oscillators that have previously proven difficult to stabilize.

Slow-Nonlinearity Assisted Supercontinuum Generation in a CS2-Core Photonic Crystal Fiber

In this paper, we theoretically investigate the supercontinuum generations (SCGs) in a carbon disulfide (CS 2 )-core photonic crystal fiber (PCF). We show that the intrinsic slow nonlinearity of CS 2 plays a significant role to control the soliton fission process. The initiation of the soliton fission process can be distinctly delayed. More importantly, the transition between the smooth soliton fission and the sub-solitons' chaotic-like interference is sufficiently extended so that the optical spectrum can keep broadening continuously while still maintaining a smooth spectral profile. When pumping a designed CS 2 -core PCF at wavelength 1.55 μm in the anomalous dispersion region, we obtain temperature-controllable and highly coherent SCs spanning over one octave at the -30-dB spectral intensity. The unique feature of large slow nonlinearity and the controllable dispersion and nonlinearity of CS 2 -core PCF confirm that as a versatile platform for highly coherent and octave-spanning SCGs.

Versatile supercontinuum generation in parabolic multimode optical fibers.

We demonstrate that the pump's spatial input profile can provide additional degrees of freedom in tailoring at will the nonlinear dynamics and the ensuing spectral content of supercontinuum generation in highly multimoded optical fibers. Experiments and simulations carried out at 1550 nm indicate that the modal composition of the input beam can substantially alter the soliton fission process as well as the resulting Raman and dispersive wave generation that eventually lead to supercontinuum in such a multimode environment. Given the multitude of conceivable initial conditions, our results suggest that it is possible to pre-engineer the supercontinuum spectral content in a versatile manner.

Modelling of Supercontinuum Generation in the Germanium-on-Silicon Waveguided Platform

The influence of high-order susceptibilities on the supercontinuum generation process in germanium-on-silicon waveguides has been investigated in detail in the midinfrared. The frequency dispersion of both the three-photon absorption coefficient and the real part of the fifth-order susceptibility has been investigated in order to develop a self-consistent modeling based upon a generalized nonlinear Schrodinger equation. Numerical results demonstrate that germanium-on-silicon waveguides can be used to induce supercontinuum generation over 1.34-μm-wide bandwidth with 200-fs input pulses pumped at a wavelength of 4 μm with peak pump powers of 89 W and waveguide lengths of 4.6 mm. The soliton fission process has also been observed as it is the main mechanism behind the supercontinuum generation.

Dressed dynamics of two time-reversed shapes of Airy pulses in a relaxing nonlinear medium

We investigate the dressed dynamics of two kinds of asymmetric Airy pulses with time-reversed shapes in a relaxing nonlinear medium. Airy tails act as low intensity “dressing” pulses, refuel the Airy main lobe, and modify propagation dynamics of Airy pulses. It is demonstrated that a soliton sheds from the main lobe of Airy pulses and the spectrum maximum intensity is periodically manifested due to the effect of self-phase modulation. The intrinsically asymmetric nature of Airy pulses is revealed through the nonlinear generation of a Raman soliton self-frequency shift and the generation of static solitons driven by the soliton fission processes and the interactions with dressing Airy tails. The resulting Raman-induced frequency shift can be controlled by using time-reversed Airy pulses and varying truncation coefficients. Numerical results show that the frequency shift can be enhanced with a larger input peak power as well.

Improved intrapulse raman scattering control via asymmetric airy pulses.

We experimentally demonstrate the possibility of tuning the frequency of a laser pulse via the use of an Airy pulse-seeded soliton self-frequency shift. The intrinsically asymmetric nature of Airy pulses, typically featured by either leading or trailing oscillatory tails (relatively to the main lobe), is revealed through the nonlinear generation of both a primary and a secondary Raman soliton self-frequency shift, a phenomenon which is driven by the soliton fission processes. The resulting frequency shift can be carefully controlled by using time-reversed Airy pulses or, alternatively, by applying an offset to the cubic phase modulation used to generate the pulses. When compared with the use of conventional chirped Gaussian pulses, our technique brings about unique advantages in terms of both efficient frequency tuning and feasibility, along with the generation and control of multicolor Raman solitons with enhanced tunability. Our theoretical analysis agrees well with our experimental observations.

Ultrafast multi-wavelength switch based on dynamics of spectrally-shifted solitons in a dual‑core photonic crystal fiber

Nonlinear propagation of ultrafast near infrared pulses in anomalous dispersion region of dual-core photonic crystal fiber was studied. Polarization tunable soliton-based nonlinear switching at multiple non-excitation wavelengths was demonstrated experimentally for fiber excitation by 100 fs pulses at 1650 nm. The highest-contrast switching was obtained with the fiber length of just 14 mm, which is significantly shorter compared to the conventional non-solitonic in-fiber switching based on nonlinear optical loop mirror. Advanced numerical simulations show good agreement with the experimental results, suggesting that the underlying dual-core soliton fission process supports nonlinear optical switching and simultaneous pulse compression to few-cycle durations at the level of 20 fs.

Soliton-based ultrafast multi-wavelength nonlinear switching in dual-core photonic crystal fibre

Systematic experimental study of ultrafast multi-wavelength all-optical switching performance in a dual-core photonic crystal fibre is presented. The focus is on nonlinearly induced switching between the two output ports at non-excitation wavelengths, which are generated during nonlinear propagation of femtosecond pulses in the anomalous dispersion region of a dual-core photonic crystal fibre made of multicomponent glass. Spatial and spectral characteristics of the fibre output radiation were measured separately for both fibre cores under various polarization and intensity conditions upon selective, individual excitation of each fibre core. Polarization-controlled nonlinear switching performance at multiple non-excitation wavelengths was demonstrated in the long-wavelength optical communication bands and beyond. Depending on the input pulse polarization, narrowband switching operation at 1560 nm and 1730 nm takes place with double core extinction ratio contrasts of 9 dB and 14.5 dB, respectively. Moreover, our approach allows switching with simultaneous wavelength shift from 1650 to 1775 nm with extinction ratio contrast larger than 18 dB. In addition, non-reciprocal behaviour of the soliton fission process under different fibre core excitations was observed and its effect on the multi-wavelength nonlinear switching performance was explained, taking into account the slight dual-core structure asymmetry. The obtained results represent ultrafast all-optical switching with an extended dimension of wavelength shift, controllable with both the input radiation intensity and the polarization by simple propagation along a 14 mm long fibre.

Coherence Properties of Supercontinuum Generated in Dispersion-Tailored Lead-Silicate Microstructured Fiber Taper

This article details the numerically studied coherence properties of the supercontinuum generated in a lead-silicate microstructured fiber taper, with an increasing core radius along the propagation distance that tailors the dispersion property. Simulations are conducted by adding quantum noise into the input pulse at 1.55 μm, and the complex degree of first-order coherence function and the overall spectral coherence degree are both calculated. Although the spectral broadening is comparable, the coherence degree is shown to vary with different pumping conditions. It decreases with higher peak power and longer duration due to the significant competition between the soliton-fission process and the noise-seeded modulation instability. By controlling the input pulse parameters, it is possible to generate a perfectly coherent supercontinuum with a flat broadened spectrum extending from ~1 μm to ~5μm in this fiber taper.

Supercontinuum generation for coherent anti-Stokes Raman scattering microscopy with photonic crystal fibers

Photonic crystal fiber (PCF) designs with two zero-dispersion wavelengths (ZDWs) are experimentally investigated in order to suggest a novel PCF for coherent anti-Stokes Raman scattering (CARS) microscopy. From our investigation, we select the optimum PCF design and demonstrate a tailored spectrum with power concentrated around the relevant wavelengths for lipid imaging (648 nm and 1027 nm). This new PCF is characterized by varying the fiber length, the average power, and the pulse width of the fs pump pulses. It was found that the selected PCF design gave a significantly improved spectral distribution compared to an existing PCF for CARS microscopy. Furthermore, the PCF is designed in a twofold symmetric structure allowing for polarization maintaining propagation. Finally, the pulse propagation is investigated numerically showing good agreement with the measured spectrum. From the numerical analysis, the nonlinear effects responsible for the spectral broadening are explained to be soliton fission processes, dispersive waves, and stimulated Raman scattering.

Cascaded interactions between Raman induced solitons and dispersive waves in photonic crystal fibers at the advanced stage of supercontinuum generation

The complex mechanism of multiple interactions between solitary and dispersive waves at the advanced stage of supercontinuum generation in photonic crystal fiber is studied in experiment and numerical simulations. Injection of high power negatively chirped pulses near zero dispersion frequency results in an effective soliton fission process with multiple interactions between red shifted Raman solitons and dispersive waves. These interactions may result in relative acceleration of solitons with further collisions between them of quasi-elastic or quasi-plastic kinds. In the spectral domain these processes result in enhancement of certain wavelength regions within the spectrum or development of a new significant band at the long wavelength side of the spectrum.

Role of dispersion profile in controlling emission of dispersive waves by solitons in supercontinuum generation

We study the supercontinuum process in optical fibers numerically for a variety of dispersion profiles to investigate how a specific dispersion profile controls the emission of dispersive waves. We conclude that the number of zero-dispersion points in the dispersion profile of a fiber is an excellent predictor of the dispersive-wave peaks when it is pumped with femtosecond pulses in the anomalous dispersion regime. Our study reveals that two or more such peaks can form on the same side of the input wavelength in specially designed and practically achievable dispersion profiles. We show that dispersive waves are emitted even in the case of normal dispersion where soliton fission does not occur. We suggest that a phenomenon related to soliton spectral tunneling is responsible for this radiation. Distinct dispersive peaks may also appear when an optical pulse, launched in the normal dispersion region, later begins to propagate in the anomalous dispersion regime because of its spectral broadening. Several dispersion profiles are numerically employed to show how the soliton fission process creates non-solitonic radiation even under normal dispersion pumping. A time-domain picture clearly shows this radiation when the conventional phase matching condition is satisfied. We also propose a realistic photonic crystal fiber with a dispersion profile that supports dispersive-wave generation in the normal-dispersion region. Our study should prove useful for experiments designed to control the generation of blue light by launching femtosecond pulses into optical fibers.

Enhanced degree of temporal coherence through temporal and spatial phase coupling within a focused supercontinuum

In the diffraction of a supercontinuum source, a redistribution of amplitude and phase at the focal region is incurred by the coupling between the supercontinuum and the spatial phase caused by the lens diffraction, making it extremely difficult to predict the focal behaviour. We show that the coupling between the temporal phase of a SC source and the spatial phase from the diffraction by a low numerical aperture (NA) lens causes dramatic alterations in the spectra and the temporal coherence near the focal region, and that this effect is maximized in points of singularity. Furthermore, we show that such an enhancement in temporal coherence can be controlled by the pulse evolution through the photonic crystal fiber, in which nonlinear and disperive effects such as the soliton fission process provides the key phase evolution necessary for dramatically changing the coherence time of the focused electromagnetic wave.

Polarization-controlled dispersive wave redirection in dual-core photonic crystal fiber

The complex study of the polarization-controlled supercontinuum generation in a dual-core square lattice photonic crystal fiber made of multicomponent glass was accomplished. The fiber was excited by 100-fs pulses at a 1250-nm wavelength in the anomalous dispersion region and the registered spectra exhibited soliton fission and Raman-induced self-frequency shift processes. The study also involved a detailed analysis of the infrared-to-visible light conversion exhibiting a dispersive wave origin. The special dual-core properties were investigated by separate spectral analysis of each of the two cores and the near-field profile registration. The emphasis was on the visible part of the spectrum where the input energy and polarization direction dependences were studied. The increase in the input energy allowed for the tuning of the wavelength of the visible spectral features, a further rotation of the polarization direction had an effect on the spectral dependence of the light distribution between the cores. The dual-core fiber exhibited a significant coupling performance and the spectral dependence of the visible light distribution is in good agreement with the simulated coupling length spectral characteristics. Single-core excitation in the linear regime revealed the possibility of coupling 50% of the energy to the other core and the same polarization-controlled redirection possibilities as that at the nonlinear experiments. Dual-core excitation of the fiber enhances the light redirection effect with the application potential for the polarization-controlled directional coupler accompanied by nonlinear frequency conversion.

Supercontinuum generation and soliton timing jitter in SF6 soft glass photonic crystal fibers

We use broadband frequency-resolved optical gating (FROG) and cross-correlation FROG (XFROG) techniques to study the details of the supercontinuum generated in extruded soft glass SF6 photonic crystal fibers pumped with envelope-modulated 100 fs pulses at telecom wavelengths. Strong temporal jitter of solitons is observed with highly non-Gaussian statistics, which is related to the statistics of the pump pulse envelope shape fluctuations. The ripples present on the input pulse seed the modulation instability at high pump powers, affecting soliton fission. Numerical modeling confirms strong sensitivity of the soliton fission process to the presence of ripples on the pump pulse envelope.

Chirp-controlled soliton fission in tapered optical fibers

Manipulation of the input pulse chirp during supercontinuum generation in tapered fibers provides precise control of the soliton fission process, and can be applied to compensate for deleterious effects due to untapered pigtails.

Generation of a broadband continuum with high spectral coherence in tapered single-mode optical fibers

By means of a delayed pulsed method, we carry out an experimental study of the mutual spectral coherence of supercontinuum trains generated through a tapered fiber. We observe a strong dependence of the spectral coherence on the input wavelength. Analysis of the interferograms shows that this is related to the robustness of different order soliton fission processes. A broadband continuum with 20dB wavelength from 500nm~1300nm with high coherence (mean visibility g(12)~0.7) is obtained.