We report on the experimental realization of optical frequency comb (OFC) generation in a doubly-resonant cavity second harmonic generation (SHG) system. OFCs continue to attract significant interest, offering a wealth of potential applications beyond frequency metrology. Continuously-driven Kerr microresonators, whose nonlinear response is dominated by the third-order nonlinearity, have proven to be viable alternatives to comb sources based on femtosecond mode-locked lasers. Recently, OFCs have also been directly generated through second-order nonlinear interactions in cw-pumped resonators namely, a singly-resonant cavity SHG system and a nearly-degenerate optical parametric oscillator. Theoretical studies have also predicted OFCs in doubly-resonant cavity SHG systems with a much lower threshold with respect to the singly-resonant configurations. Here we report on the first observations of OFCs in such a doubly-resonant system. The experiment is based on a periodically poled lithium niobate crystal, placed in a traveling-wave optical cavity, pumped by a cw Nd:YAG laser emitting 0.5 W at 1064 nm. The cavity is resonant for frequencies around both the fundamental pump and its second harmonic at 532 nm, and an intracavity adjustable silica window is used to separately set the detunings of the pump and its second harmonic. Stable cavity locking to the pump laser is achieved via the Pound-Drever-Hall offset locking technique, thanks to a counterpropagating orthogonally polarized auxiliary beam. We measured a power threshold for comb formation as low as 5 mW, reduced by more than one order of magnitude with respect to singly-resonant configurations. The locking system permitted to explore frequency detunings up to several cavity linewidths, and to correspondingly observe a large variety of comb regimes, with different teeth spacing and spectral span, as well as the contribution of photothermal effect to the whole dynamics. In this regard, we developed an extended theoretical model that includes thermo-optical nonlinearities.
The last couple of years have witnessed an unprecedented boost in nonlinear nanophotonics, allowed both by constant technological evolution in the fabrication of integrated micro- and nano-structures, and by a deeper understanding of multipolar magnetic and electric response of all-dielectric sub-wavelength nanoparticles. Besides reviewing the recent progress in the hybrid and monolithic semiconductor platforms that are being employed to demonstrate parametric sources on chip, we will focus on high-contrast nonlinear nanophotonic sources in the near-infrared spectral region, where plasmonic devices fall short of broad applicability. To confine photons at the nanoscale by total internal reflection, metal-less devices rely on a high-refractive-index core surrounded by low-index cladding in one or more dimensions, and therefore they typically consist of semiconductor nanostructures that either lie on an oxide substrate or are suspended in air. We will report on our results on χ(2) AlGaAs suspended nanowires  and AlGaAs-on-AlOx nanoantennas , with a perspective view on collective nonlinear effects in 1D and 2D arrays. Finally we will comment on the most sought applications of integrated parametric sources, including optical frequency comb generators and optical parametric oscillators on-chip with sub-milliwatt threshold.
 N. Morais, I. Roland, M. Ravaro, I. Favero, A. Lemaître, S. Wabnitz, M. De Rosa, and G. Leo, SPIE Nanophotonics Australasia 2017, Melbourne 10-13/12/2017.
 V. F. Gili, L. Carletti, A. Locatelli, D. Rocco, M. Finazzi, L. Ghirardini, I. Favero, C. Gomez, A. Lemaître, M. Celebrano, C. De Angelis, and G. Leo, Opt. Express 24, 15965 (2016).
We study the existence and propagation of multidimensional dark non-diffractive and non-dispersive spatiotemporal optical wave-packets in nonlinear Kerr media. We report analytically and confirm numerically the properties of spatiotemporal dark lines, X solitary waves and lump solutions of the (2 + 1)D nonlinear Schrodinger equation (NLSE). Dark lines, X waves and lumps represent holes of light on a continuous wave background. These solitary waves are derived by exploiting the connection between the (2 + 1)D NLSE and a well-known equation of hydrodynamics, namely the (2+1)D Kadomtsev-Petviashvili (KP) equation. This finding opens a novel path for the excitation and control of spatiotemporal optical solitary and rogue waves, of hydrodynamic nature.
We demonstrate optical frequency comb generation in a continuously pumped optical parametric oscillator, in the parametric region around half of the pump frequency. We also model the dynamics of such quadratic combs using a single time-domain mean-field equation, and obtain simulation results that are in good agreement with experimentally observed spectra. Moreover, we numerically investigate the coherence properties of simulated combs, showing the existence of correlated and phase-locked combs. Our work could pave the way for a new class of frequency comb sources, which may enable straightforward access to new spectral regions and stimulate novel applications of frequency combs.
We overview recent advances in the research on spatiotemporal beam shaping in nonlinear multimode optical fibers. An intense light beam coupled to a graded index (GRIN) highly multimode fiber undergoes a series of complex nonlinear processes when its power grows larger. Among them, the lowest threshold effect is the Kerr-induced beam self-cleaning, that redistributes most of the beam energy into a robust bell-shaped beam close to the fundamental mode. At higher powers a series of spectral sidebands is generated, thanks to the phase matching induced by the long period grating due to the periodic self-imaging of the beam and the Kerr effect. Subsequently a broadband and spectrally flat supercontinuum is generated, extending from the visible to the mid-infrared.
We discuss recent advances in the modelling of optical frequency comb generation in quadratic and cubic microresonators.
Different time domain models are presented and compared, and their solutions are analysed by
Optical frequency combs currently represent enabling components in a wide number of fast-growing research fields, from frequency metrology to precision spectroscopy, from synchronization of telecommunication systems to environmental and biomedical spectrometry. As recently demonstrated, quadratic nonlinear media are a promising platform for optical frequency combs generation, through the onset of an internally pumped optical parametric oscillator in cavity enhanced second-harmonic generation systems. We present here a proposal for quadratic frequency comb generation in AlGaAs waveguide resonators. Based on the crystal symmetry properties of the AlGaAs material, quasi-phase matching can be realized in curved geometries (directional quasi-phase matching), thus ensuring efficient optical frequency conversion. We propose a novel design of AlGaAs waveguide resonators with strongly reduced total losses, compatible with long-path, high-quality resonators. By means of a numerical study, we predict efficient frequency comb generation with threshold powers in the microwatt range, paving the way for the full integration of frequency comb synthesizers in photonic circuits.
Various aspects of the nonlinear dynamics of Kerr frequency comb generation in optical microresonators are considered. It is shown that the comb generation process can, for the case of a single continuous wave pump, be given a simple interpretation in terms of modulational instability and that the essential dynamics can be captured using a three wave mode truncation for the pump mode and the dominant sideband pair. This idea is also extended using a four wave model to analyze an alternative dual pump configuration, for which comb generation may occur without a pump intensity threshold in both the normal and the anomalous dispersion regime.
Wavelength tunable synchronous pulse sources are highly desirable for spectroscopy and optical diagnostics. The
common method to generate short pulses in the fiber is the use of nonlinear induced spectral broadening which result in
soliton shaping in anomalous dispersion regime. However, to generate ultra-short pulses, broadband gain mechanism is
also required. In recent years, Raman fiber lasers have retrieved strong interest due to their capability of serving as pump
sources in gain-flattened amplifiers for optical communication systems. The fixed-wavelength Raman lasers have been
widely studied in the last years, but recently, much focus has been on the multi wavelength tunable Raman fiber lasers
which generate output Stokes pulses in a broad wavelength range by so called cascaded stimulated Raman scattering. In
this paper we investigate synchronous 1st and 2nd order pulsed Raman lasers that can achieve frequency spacing of up to
1000cm-1 that is highly desired for CARS microscopy. In particular, analytical and numerical analysis of pulsed stability
derived for Raman lasers by using dispersion managed telecom fibers and pumped by 1530nm fiber lasers. We show the
evolution of the 1st and 2nd order Stokes signals at the output for different pump power and SMF length (determines the
net anomalous dispersion) combinations. We investigated the stability of dispersion managed synchronous Raman laser
up to second order both analytically and numerically. The results show that the stable 2nd order Raman Stokes pulses
with 0.04W to 0.1W peak power and 2ps to 3.5ps pulse width can be achieved in dispersion managed system.
The evolution of short optical pulses towards a self-similar parabolic intensity profile in normally dispersive nonlinear
fiber optic amplifiers can be described with good accuracy by means of a simple analytical model. This approach enables
the evaluation of the optimal input pulse time duration and chirp for decreasing the distance of convergence towards the
asymptotic regime. We show that pulse spectral broadening in dispersive nonlinear fiber amplifiers may be enhanced by
introducing a suitable dispersion tapering. We obtain an analytical dispersion profile which permits to reduce pulse
propagation in a varying dispersion fiber to the case of an equivalent fiber with constant parameters.
The propagation of the solitary waves in the resonant birefringent amplifier with linear losses is considered. The birefringent
optical linear medium contains two-level atoms with the upper state degenerated over projection of angular moment. It
is assumed that population of resonance levels of atoms is inverted. The steady state pulse of polarized radiation that is vectonal
generalization of the known π-pulse was analytically found. Numerical simulations demonstrate the formation dynamics
solitary waves originated in birefringent amplifier.
We present a new multi-parameter family of analytical soliton solutions for nonlinear three-wave resonant interactions.
We show the amplitude, phase-front shapes and general properties of the solitons. The stability of these
novel parametric solitons is simply related to the value of their common group velocity.
We present theoretical and experimental studies of both scalar and polarization or modal pump-divided parametric
amplification in photonic crystal fibers. In the scalar case, we discuss broadband parametric amplification at telecom
wavelengths near 1550 nm. With a pump-divided scattering process, we discuss the possibility of widely tunable
frequency conversion and four-wave mixing gain at visible wavelengths. We confirmed the theory by experiments where
intense, linearly polarized pump pulses at wavelengths ranging from 532 to 625 nm led to the spontaneous generation of
modulation instability sidebands with frequency shifts ranging from 3 up to 63 THz. The observations were in good
agreement the experimental characterization and theoretical modelling ofthe linear and nonlinear properties of the PCF.
We study the interplay between parametric and Raman gain in photonic crystal fibers by taking into account the vector nature of the electric field, the fiber frequency dependent birefringence, the Kerr nonlinear coefficient, the Raman gain profile and chromatic dispersion. In particular, we show that an accurate representation of the frequency dependence of the nonlinear and dispersive properties of a photonic crystal fiber is essential for correctly describing the overall gain profile for a probe signal at large frequency detuning from a continuous wave or pulsed pump. For example, we found that fourth and higher order dispersion have a striking influence on the spectrum of modulation polarization instability gain in both the high and low birefringence regime, in that the vector parametric gain is suppressed above a critical level of linear birefringence. We validated the theory by experimental observations of vector parametric amplification in high birefringence holey fiber with triple defects.