Supercontinuum generation in media with sign-alternated dispersion

When an ultrafast optical pulse with high intensity is propagating through transparent material a supercontinuum can be coherently generated by self-phase modulation, which is essential to many photonic applications in fibers and integrated waveguides. However, the presence of dispersion causes stagnation of spectral broadening past a certain propagation length, requiring an increased input peak power for further broadening. We present a concept to drive supercontinuum generation with significantly lower input power by counteracting spectral stagnation via alternating the sign of group velocity dispersion along the propagation. We demonstrate the effect experimentally in dispersion alternating fiber in excellent agreement with modeling, revealing almost an order of magnitude reduced peak power compared to uniform dispersion. Calculations reveal a similar power reduction also with integrated optical waveguides, simultaneously with a significant increase of flat bandwidth, which is important for on-chip broadband photonics.Comment: Main text and supplementary informatio

Toward integrated tantalum pentoxide optical parametric oscillators

We present a hybrid waveguide-fiber optical parametric oscillator (OPO) exploiting degenerate four-wave mixing in tantalum pentoxide. The OPO, pumped with ultrashort pulses at 1.55 $\mu$m wavelength, generated tunable idler pulses with up to 4.1 pJ energy tunable between 1.63 $\mu$m and 1.68 $\mu$m center wavelength. An upper bound for the total tolerable cavity loss of 32 dB was found, rendering a chip-integrated OPO feasible as a compact and robust light source.Comment: 5 pages and 5 figures, submitted to Optics Letter

Numerical and Experimental Demonstration of Intermodal Dispersive Wave Generation

Abstract Evidence of intermodal dispersive wave generation mediated by intermodal cross‐phase modulation (iXPM) between different transverse modes during supercontinuum generation in silicon nitride waveguides is presented. The formation of a higher‐order soliton in one strong transverse mode leads to phase modulation of a second, weak transverse mode by iXPM. The phase modulation enables not only supercontinuum generation but also dispersive wave generation within the weak mode, that otherwise has insufficient power to facilitate dispersive wave formation. The nonlinear frequency conversion scheme presented here suggests phase‐matching conditions beyond what is currently known, which can be exploited for extending the spectral bandwidth within supercontinuum generation.Intermodal dispersive wave generation mediated by intermodal cross‐phase modulation between different transverse modes during supercontinuum generation in silicon nitride waveguides is demonstrated. The phase modulation enables dispersive wave generation within a weak mode, that otherwise has insufficient power to facilitate dispersive wave formation. This process suggests new phase‐matching conditions for frequency conversion beyond to what is currently known. imag

Bandwidth-limited few-cycle pulses by nonlinear compression in a dispersion-alternating fiber

We demonstrate an improved concept for nearly bandwidth-limited nonlinear pulse compression down to the few-cycle regime in a fiber chain with alternating sign of dispersion. Whereas the normally dispersive fiber segments generate bandwidth via self-phase modulation, the anomalously dispersive fiber segments recompress the broadened spectral bandwidth by an appropriate amount of group velocity dispersion. Nonlinear pulse compression from 80 fs input pulses to nearly bandwidth-limited 25 fs pulses at 1560 nm was achieved, resulting in a pulse compression factor of 3.2. The use of a specific dispersion-compensating fiber eliminated the impact of higher-order dispersion, such that a high spectral coherence was ensured. We show that nonlinear Schrödinger equation simulations were in good agreement with the experimental results and investigated the transfer of input fluctuations to the output. The concept is transferable to longer input pulse durations, resulting in compression factors of 83 for 10 ps input pulses

Mitigating cross-phase modulation artifacts in femtosecond stimulated Raman scattering

In contrast to spontaneous Raman scattering, coherent Raman scattering techniques, such as femtosecond stimulated Raman scattering (FSRS), show advantages for many applications. Besides an enhanced signal strength, FSRS is free of a nonresonant background but is affected by cross-phase modulation (XPM). The resulting artifacts in FSRS become relevant for high pulse intensities (GW/cm2-regime) and ultrashort pulses (<2 ps), both necessary for super-resolution experiments. As the pulse duration is a crucial parameter for XPM as well as for FSRS, we present a setup in which we adjust the pulse duration across the relevant range (0.5–3 ps), in order to investigate the XPM influence on the spectra. Furthermore, we vary the peak intensity, the temporal overlap between the interacting pulses, and the nonlinear refractive index coefficient n2 of the sample, showing that a trade-off between all these quantities enables the measurement of unaffected FSRS spectra

Optical parametric amplification in silicon nitride waveguides for coherent Raman imaging

We present tunable waveguide-based optical parametric amplification by four-wave mixing (FWM) in silicon nitride waveguides, with the potential to be set up as an all-integrated device, for narrowband coherent anti-Stokes Raman scattering (CARS) imaging. Signal and idler pulses are generated via FWM with only 3 nJ pump pulse energy and stimulated by using only 4 mW of a continuous-wave seed source, resulting in a 35 dB enhancement of the idler spectral power density in comparison to spontaneous FWM. By using waveguides with different widths and tuning the wavelength of the signal wave seed, idler wavelengths covering the spectral region from 1.1 µm up to 1.6 µm can be generated. The versatility of the chip-based FWM light source is demonstrated by acquiring CARS images

Investigation on Optical Parametric Oscillators Based on Different Silicon Nitride Waveguide Geometries

Fiber-based optical parametric oscillators (OPOs) are versatile and tunable light sources that are of high interest for applications such as coherent Raman spectroscopy or imaging [1]. However, using integrated waveguides instead of nonlinear fibers as gain medium, waveguide-based OPOs (WOPOs) benefit from higher nonlinearity and compact size of integrated waveguides, which have potential to be used for many lab-on-a-chip applications [2]. To investigate different waveguide geometries to the influence to WOPOs, we study synchronously pumped WOPOs exploiting four-wave mixing (FWM) in silicon nitride (Si3N4) waveguides with varying waveguide widths.</p

Sign-Alternating Dispersion Patterning for Supercontinuum Generation

Supercontinuum generation (SCG), a process that generates a wide and coherent bandwidth of light, has become foundational in emergent optical technologies in a plethora of fields, such as optical coherence tomography (OCT), metrology (e.g., integrated photonic Kerr combs) and precision sensing. As advanced applications of SCG emerge, the requirements on the spectral bandwidth, the coherence, its temporal compressibility, and the quality of the optical spectral content (e.g., a uniformly flat spectral profile) are increasing. Moreover, as these technologies need not be restricted to the lab, the spectral conversion efficiency of SCG must increase from current technology that functions, e.g., in the tens of nJ pulse energy, to sub nJ levels