Generation of extremely high-angle Bessel beams.

We present a setup to generate tightly focused Bessel beams that is composed of a half-ball lens coupled with a relay lens. The system is simple and compact compared to conventional imaging of axicons based on microscope objectives. We experimentally demonstrate the generation of a Bessel beam with a 42° cone angle at 980 nm in air with a typical beam length of 500µm and a central core radius of about 550 nm. We numerically studied the effects of the misalignment of the different optical elements and the range of tilt and shift that are acceptable to obtain a regular Bessel beam.

Noise in supercontinuum generated using PM and non-PM tellurite glass all-normal dispersion fibers.

Intensity fluctuations in supercontinuum generation are studied in polarization-maintaining (PM) and non-PM all-normal dispersion tellurite photonic crystal fibers. Dispersive Fourier transformation is used to resolve the shot-to-shot spectra generated using 225-fs pump pulses at 1.55 µm, with experimental results well reproduced by vector and scalar numerical simulations. By comparing the relative intensity noise for the PM and non-PM cases, supported by simulations, we demonstrate the advantage of the polarization-maintaining property of the PM fibers in preserving low-noise dynamics. We associate the low-noise in the PM case with the suppression of polarization modulation instability.

Instabilities in a dissipative soliton-similariton laser using a scalar iterative map.

Numerical simulations of a dissipative soliton-similariton laser are shown to reproduce a range of instabilities seen in recent experiments. The model uses a scalar nonlinear Schrödinger equation map, and regions of stability and instability are readily identified as a function of gain and saturable absorber parameters. Studying evolution over multiple round trips reveals spectral instabilities linked with soliton molecule internal motion, soliton explosions, chaos, and intermittence. For the case of soliton molecules, the relative phase variation in the spectrum is shown to be due to differences in nonlinear phase evolution between the molecule components over multiple round trips.

Generation of high conical angle Bessel-Gauss beams with reflective axicons.

We report the generation of Bessel-Gauss beams of high conical angle, up to 35 deg, using reflective off-axis axicons and a magnification optical system. We experimentally characterize the beams with three-dimensional scans. The high precision of fabrication of the axicons in the vicinity of the axicon singularity allows us to generate a beam with intensity distribution close to analytical description.

Universality of the Peregrine Soliton in the Focusing Dynamics of the Cubic Nonlinear Schrödinger Equation.

We report experimental confirmation of the universal emergence of the Peregrine soliton predicted to occur during pulse propagation in the semiclassical limit of the focusing nonlinear Schrödinger equation. Using an optical fiber based system, measurements of temporal focusing of high power pulses reveal both intensity and phase signatures of the Peregrine soliton during the initial nonlinear evolution stage. Experimental and numerical results are in very good agreement, and show that the universal mechanism that yields the Peregrine soliton structure is highly robust and can be observed over a broad range of parameters.

High Aspect Ratio Structuring of Glass with Ultrafast Bessel Beams.

Controlling the formation of high aspect ratio void channels inside glass is important for applications like the high-speed dicing of glass. Here, we investigate void formation using ultrafast Bessel beams in the single shot illumination regime. We characterize the morphology of the damages as a function of pulse energy, pulse duration, and position of the beam inside fused silica, Corning Eagle XG, and Corning Gorilla glass. While a large set of parameters allow for void formation inside fused silica, the operating window is much more restricted for Eagle XG and Gorilla glass. The transient formation of a molten layer around voids enables us interpreting the evolution of the morphology with pulse energy and duration.

Intracavity incoherent supercontinuum dynamics and rogue waves in a broadband dissipative soliton laser.

Understanding dynamical complexity is one of the most important challenges in science. Significant progress has recently been made in optics through the study of dissipative soliton laser systems, where dynamics are governed by a complex balance between nonlinearity, dispersion, and energy exchange. A particularly complex regime of such systems is associated with noise-like pulse multiscale instabilities, where sub-picosecond pulses with random characteristics evolve chaotically underneath a much longer envelope. However, although observed for decades in experiments, the physics of this regime remains poorly understood, especially for highly-nonlinear cavities generating broadband spectra. Here, we address this question directly with a combined numerical and experimental study that reveals the physical origin of instability as nonlinear soliton dynamics and supercontinuum turbulence. Real-time characterisation reveals intracavity extreme events satisfying statistical rogue wave criteria, and both real-time and time-averaged measurements are in quantitative agreement with modelling.

Real-time characterization of spectral instabilities in a mode-locked fibre laser exhibiting soliton-similariton dynamics.

The study of dissipative solitons in mode-locked lasers reveals a rich landscape of interaction dynamics resulting from the interplay of nonlinearity, dispersion and dissipation. Here, we characterize a range of instabilities in a dissipative soliton fibre laser in a regime where both conventional soliton and similariton propagation play significant roles in the intracavity pulse shaping. Specifically, we use the Dispersive Fourier Transform technique to perform real-time spectral measurements of buildup dynamics from noise to the generation of stable single pulses, phase evolution dynamics of bound state "similariton molecules", and several examples of intermittent instability and explosion dynamics. These results show that the instabilities previously seen in other classes of passively mode-locked fibre lasers are also observed in the presence of strong nonlinear attraction of similariton evolution in an optical fibre amplifier.

Machine learning analysis of extreme events in optical fibre modulation instability.

A central research area in nonlinear science is the study of instabilities that drive extreme events. Unfortunately, techniques for measuring such phenomena often provide only partial characterisation. For example, real-time studies of instabilities in nonlinear optics frequently use only spectral data, limiting knowledge of associated temporal properties. Here, we show how machine learning can overcome this restriction to study time-domain properties of optical fibre modulation instability based only on spectral intensity measurements. Specifically, a supervised neural network is trained to correlate the spectral and temporal properties of modulation instability using simulations, and then applied to analyse high dynamic range experimental spectra to yield the probability distribution for the highest temporal peaks in the instability field. We also use unsupervised learning to classify noisy modulation instability spectra into subsets associated with distinct temporal dynamic structures. These results open novel perspectives in all systems exhibiting instability where direct time-domain observations are difficult.

Real-time measurements of spontaneous breathers and rogue wave events in optical fibre modulation instability.

Modulation instability is a fundamental process of nonlinear science, leading to the unstable breakup of a constant amplitude solution of a physical system. There has been particular interest in studying modulation instability in the cubic nonlinear Schrödinger equation, a generic model for a host of nonlinear systems including superfluids, fibre optics, plasmas and Bose-Einstein condensates. Modulation instability is also a significant area of study in the context of understanding the emergence of high amplitude events that satisfy rogue wave statistical criteria. Here, exploiting advances in ultrafast optical metrology, we perform real-time measurements in an optical fibre system of the unstable breakup of a continuous wave field, simultaneously characterizing emergent modulation instability breather pulses and their associated statistics. Our results allow quantitative comparison between experiment, modelling and theory, and are expected to open new perspectives on studies of instability dynamics in physics.

Emergent rogue wave structures and statistics in spontaneous modulation instability.

The nonlinear Schrödinger equation (NLSE) is a seminal equation of nonlinear physics describing wave packet evolution in weakly-nonlinear dispersive media. The NLSE is especially important in understanding how high amplitude "rogue waves" emerge from noise through the process of modulation instability (MI) whereby a perturbation on an initial plane wave can evolve into strongly-localised "breather" or "soliton on finite background (SFB)" structures. Although there has been much study of such structures excited under controlled conditions, there remains the open question of how closely the analytic solutions of the NLSE actually model localised structures emerging in noise-seeded MI. We address this question here using numerical simulations to compare the properties of a large ensemble of emergent peaks in noise-seeded MI with the known analytic solutions of the NLSE. Our results show that both elementary breather and higher-order SFB structures are observed in chaotic MI, with the characteristics of the noise-induced peaks clustering closely around analytic NLSE predictions. A significant conclusion of our work is to suggest that the widely-held view that the Peregrine soliton forms a rogue wave prototype must be revisited. Rather, we confirm earlier suggestions that NLSE rogue waves are most appropriately identified as collisions between elementary SFB solutions.

Experimental generation of parabolic pulses via Raman amplification in optical fiber.

Parabolic pulse generation via Raman amplification is experimentally demonstrated in 5.3 km of non-zero dispersion shifted fiber presenting normal group velocity dispersion at the injected signal pulse wavelength of 1550 nm. The fiber is pumped by a commercially-available continuous wave source at 1455 nm, and the intensity and chirp of the amplifier output are characterized using frequency-resolved optical gating. For 2.4 pJ input pulses of 10 ps duration, the output pulse characteristics are studied as a function of amplifier gain over the range 11-24 dB, allowing the evolution of the input pulse to a parabolic pulse to be clearly seen for amplifier gains exceeding 15 dB. Numerical compression of the output pulses show that near chirp-free pulses can be obtained using only linear chirp compensation.

Differential optical spectroscopy for absorption characterization of scattering media.

Reflectance techniques are commonly used to characterize the optical properties of tissues. However, the precise determination of local chromophore concentrations in turbid media is usually difficult because of the nonlinear dependence of light intensity as a function of scattering and absorption coefficients. A technique is presented to easily determine absorbent compound concentration ratios in a turbid media from three optical reflectance spectra, in the visible range, measured for source-detector distances less than 1cm. The validity of the method is experimentally established, in cases of sets of diluted milk containing absorbent inks, over a relatively wide range of absorption (0.05-0.5 cm(-1)) and reduced scattering (10-20 cm(-1)) coefficients.

Differential optical spectroscopy for absorption characterization of mono & two-layered scattering media.

Diffuse reflectance techniques are commonly used to characterize the optical properties of various biological tissues in the visible and NIR spectral ranges. Nevertheless a sharp determination of local chromophores concentration in turbid media is usually difficult to obtain because of the nonlinear dependence of the reflected light intensity as a function of scattering and absorption coefficients. The proposed technique of this paper allows to determine absorbent compounds concentration ratios in a turbid medium from three reflectance spectra, measured for source-detector distances inferior to 1 cm. This method is validated in the visible range in case of typical biological absorption and reduced scattering coefficients. Two-layered scattering media are also investigated.

Effects of structural irregularities on modulational instability phase matching in photonic crystal fibers.

The effect of structural irregularities in photonic crystal fibers on scalar and vector modulational instability (MI) processes is studied by numerical simulations and experiments. For an anomalous-dispersion regime pump, variations in core ellipticity as small as 0.5% over length scales of the order of several meters are shown to have a negligible effect on scalar MI, yet they completely suppress vector MI. In contrast, for a normal-dispersion regime pump, vector MI is shown to be robust against such fluctuations.