A topical review of numerical and experimental studies of supercontinuum generation in photonic crystal fiber is presented over the full range of experimentally reported parameters, from the femtosecond to the continuous-wave regime. Results from numerical simulations are used to discuss the temporal and spectral characteristics of the supercontinuum, and to interpret the physics of the underlying spectral broadening processes. Particular attention is given to the case of supercontinuum generation seeded by femtosecond pulses in the anomalous group velocity dispersion regime of photonic crystal fiber, where the processes of soliton fission, stimulated Raman scattering, and dispersive wave generation are reviewed in detail. The corresponding intensity and phase stability properties of the supercontinuum spectra generated under different conditions are also discussed.

Supercontinuum generation, photonic crystal fiber

Sync The Emerging Science Of Spontaneous Order

Building on the scientific understanding and technological infrastructure of single-mode fibers, multimode fibers are being explored as a means of adding new degrees of freedom to optical technologies such as telecommunications, fiber lasers, imaging, and measurement. Here, starting from a baseline of single-mode nonlinear fiber optics, we introduce the growing topic of multimode nonlinear fiber optics. We demonstrate a new numerical solution method for the system of equations that describes nonlinear multimode propagation, the generalized multimode nonlinear Schrodinger equation. This numerical solver is freely available, implemented in MATLAB and includes a number of multimode fiber analysis tools. It features a significant parallel computing speed-up on modern graphical processing units, translating to orders-of-magnitude speed-up over the conventionally-used split-step Fourier method. We demonstrate its use with several examples in graded- and step-index multimode fibers. Finally, we discuss several key open directions and questions, whose answers could have significant scientific and technological impact.

Multimode Nonlinear Fiber Optics: Massively Parallel Numerical Solver, Tutorial, and Outlook

The experimental realization of a Kerr frequency comb represented the convergence of research in materials, physics, and engineering. This symbiotic relationship continues to underpin efforts in comb innovation today. While the initial focus developing cavity-based frequency combs relied on existing microresonator architectures and classic optical materials, in recent years, this trend has been disrupted. This paper reviews the latest achievements in frequency comb generation using resonant cavities, placing them within the broader historical context of the field. After presenting well-established material systems and device designs, the emerging materials and device architectures are examined. Specifically, the unconventional material systems as well as atypical device designs that have enabled tailored dispersion profiles and improved comb performance are compared to the current state of art. The remaining challenges and outlook for the field of cavity-based frequency combs are evaluated.

Emerging material systems for integrated optical Kerr frequency combs

Supercontinuum (SC) generation directly from a random fiber laser (RFL) structure is limited in spectrum span and output power so far. Investigations on wavelength range improvement of SC generated in RFL are analyzed and discussed. The experimental results show that cascaded four wave mixing (FWM) and passive modulation of pump light can explain the appearance of visible components and pulse performance in time domain respectively. To the best of our knowledge, it is the first time a SC covering visible and near-infrared range with 20-dB bandwidth of more than 660 nm is generated directly from a RFL with average output power of 3.4 W, and the spectrum spanning from 600 nm to 1700nm. This work proves that a RFL can be a novel visible to near-infrared SC generation method which has a great potential in various applications.

Random fiber laser directly generates visible to near-infrared supercontinuum.

Continuous-wave pumped optical microresonators have been vastly exploited to generate a frequency comb (FC) utilizing the Kerr nonlinearity. Most of the nonlinear materials used to build photonic platforms exhibit nonlinear losses such as multiphoton absorption, free-carrier absorption, and free-carrier dispersion which can strongly affect their nonlinear performances. In this work, we model the Kerr FC based on a modified Lugiato-Lefever equation (LLE) along with the rate equation and develop analytical formulations to make a quick estimation of the steady state, bistability, self-pulsation, and modulation instability (MI) gain and bandwidth in the presence of nonlinear losses. The analytical model is valid over a broad wavelength range as it includes the effects of all nonlinear losses. Higher-order $(g3)$ characteristic polynomials of intracavity power describing the steady-state homogeneous solution of the modified LLE are discussed in detail. We derive the generalized analytical expressions for the threshold (normalized) pump detuning that initiates the optical bistability when nonlinear losses are present. Free-carrier dispersion-led nonlinear cavity detuning is observed through the reverse Kerr tilt of the resonant peaks. We further deduce the expressions of the threshold pump intensity and the range of possible cavity detuning for the initiation of the MI considering the presence of nonlinear losses. The proposed model will be helpful in explaining several numerical and experimental results which have been previously reported and thereby will be able to provide a better understanding of the comb dynamics.

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Free-carrier-driven Kerr frequency comb in optical microcavities: Steady state, bistability, self-pulsation, and modulation instability

Nonlinear interactions in multimode optical fibers

We demonstrate an efficient ultra-broadband supercontinuum generation in telecom-grade optical fiber by pumping Q-switched subnanosecond laser pulses (0.77 ns) at 1064 nm in the normal dispersion region of the fiber. The fiber supports several spatial modes at this pump wavelength. Multiple sidebands with six Raman stokes spanning over more than 1100 nm (starting well below 600 nm and extending beyond 1700 nm) are generated using very low input pump power (average 57 mW). Theoretical analysis shows that cascaded Raman and cascaded intermodal four-wave mixing processes can account for the generation of multiple sidebands. We also demonstrate that the modal composition of the input pump profile can provide an extra degree of freedom in tailoring the multiple Raman peaks in the output spectrum.

Cascaded Raman and Intermodal Four-Wave Mixing in Conventional Non-Zero Dispersion-Shifted Fiber for Versatile Ultra-Broadband Continuum Generation

The group-velocity-led optical event horizon (OEH) in optical fibers provides a convenient way to efficiently control a weak dispersive pulse by a comparatively strong solitonic pulse. State-of-the-art experiments demonstrating OEH make use of two different light sources, which increases the system complexity and cost. Here, we propose an elegant and cost-effective approach to observe the OEH through intermodal nonlinear interaction between azimuthally symmetric ${\text{LP}}_{01}$ and ${\text{LP}}_{02}$ modes of a specially designed triple-clad multimode fiber through only a single pump. The simultaneous control over dispersion and the walk off of ${\text{LP}}_{01}$ and ${\text{LP}}_{02}$ modes of the designed fiber pave the way for the OEH interaction. The ${\text{LP}}_{02}$ mode possesses anomalous dispersion, while the ${\text{LP}}_{01}$ mode exhibits normal dispersion over the wavelength range of interest. Depending upon the location of the input pump wavelength, the ${\text{LP}}_{02}$ soliton can reflect a weak copropagating ${\text{LP}}_{01}$ dispersive pulse, which possesses the same carrier wavelength as the soliton. The findings of this work might be useful for all-optical switching and also for the optical transistor action, where the increased feasibility is obtained by the fact that the proposed approach does not require separate input wavelengths for the soliton and the dispersive pulse. Moreover, a tunable pump source can be used to observe either a redshift or a blueshift in the dispersive pulse.

Intermodal nonlinear effects mediated optical event horizon in short-length multimode fiber

We demonstrate the formation of Kerr-induced transient long period grating in a standard single-mode fiber (SMF28) using sub-nanosecond laser pulses, which can be used to realize an efficient all-optical mode conversion and temperature sensing. The fiber supports $LP_{01}$ and $LP_{11}$ spatial modes at 1064-nm wavelength. The beating between two guided modes induces refractive index grating exploiting the Kerr effect. The probe beam co-propagating with the pump at the same wavelength but with different polarization exhibits efficient mode conversion efficiency from $LP_{01}$ to $LP_{11}$ mode. Temperature sensitivity is measured owing to the change in mode conversion efficiency due to the shift in resonant peak wavelength with the increase in temperature. The change in temperature is manifested by the change in spectral power of probe beam at the output. The experimental result demonstrates an average temperature sensitivity of 0.15 Watt/°C. The transient nature of the grating with high sensitivity makes our approach unique from the existing results and can open a route for potential applications.

Partha Mondal

Papers

Free-carrier-driven Kerr frequency comb in optical microcavities: Steady state, bistability, self-pulsation, and modulation instability

Nonlinear interactions in multimode optical fibers

Cascaded Raman and Intermodal Four-Wave Mixing in Conventional Non-Zero Dispersion-Shifted Fiber for Versatile Ultra-Broadband Continuum Generation

Intermodal nonlinear effects mediated optical event horizon in short-length multimode fiber

All-Optical Mode Conversion and Temperature Sensing via Transient Grating in Step-Index Fiber