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

Abstract Group IV photonics hold great potential for nonlinear applications in the near- and mid-infrared (IR) wavelength ranges, exhibiting strong nonlinearities in bulk materials, high index contrast, CMOS compatibility, and cost-effectiveness. In this paper, we review our recent numerical work on various types of silicon and germanium waveguides for octave-spanning ultrafast nonlinear applications. We discuss the material properties of silicon, silicon nitride, silicon nano-crystals, silica, germanium, and chalcogenide glasses including arsenic sulfide and arsenic selenide to use them for waveguide core, cladding and slot layer. The waveguides are analyzed and improved for four spectrum ranges from visible, near-IR to mid-IR, with material dispersion given by Sellmeier equations and wavelength-dependent nonlinear Kerr index taken into account. Broadband dispersion engineering is emphasized as a critical approach to achieving on-chip octave-spanning nonlinear functions. These include octave-wide supercontinuum generation, ultrashort pulse compression to sub-cycle level, and mode-locked Kerr frequency comb generation based on few-cycle cavity solitons, which are potentially useful for next-generation optical communications, signal processing, imaging and sensing applications.

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Nonlinear Group IV photonics based on silicon and germanium: from near-infrared to mid-infrared

We show that octave-spanning Kerr frequency combs with improved spectral flatness of comb lines can be generated in dispersion-flattened microring resonators. The resonator is formed by a strip/slot hybrid waveguide, exhibiting a flat and low anomalous dispersion between two zero-dispersion wavelengths that are separated by one octave from near-infrared to mid-infrared. Such flattened dispersion profiles allow for the generation of mode-locked frequency combs, using relatively low pump power to obtain two-cycle cavity solitons on a chip, associated with the octave-spanning comb bandwidth. The wavelength dependence of the optical loss and of the coupling coefficient and thus wavelength dependent Q-factor are also considered.

Generation of two-cycle pulses and octave-spanning frequency combs in a dispersion-flattened micro-resonator

A hybrid four-hole AsSe2-As2S5 microstructured optical fiber (MOF) is fabricated by the rod-in-tube drawing technique. The core is made of AsSe2 glass and the cladding is made of As2S5 glass. The material refractive indices of the core and the cladding are ∼2.832 and ∼2.219 at the wavelength of ∼2.8 μm, respectively. The calculated zero-dispersion wavelength of the AsSe2-As2S5 MOF is 2.759 μm. Soliton self-frequency shift with a soliton central wavelength from 2.986 to 3.419 μm is observed when the AsSe2-As2S5 MOF is pumped by an optical parametric oscillator at the pump wavelength of ∼2.8 μm. At the same time a blue-shift dispersive wave is emitted by the soliton.

Soliton self-frequency shift and dispersive wave in a hybrid four-hole AsSe2-As2S5 microstructured optical fiber

We numerically investigate the influence of high-order dispersion (HOD) on temporal and spectral characteristics of microresonator-based optical frequency combs. Theoretical analysis based on the moment method associated with numerical simulations are utilized to study the comb evolution dynamics, showing that temporal shifts of steady-state intracavity solitons are induced by high-odd-order dispersion rather than high-even-order dispersion. The role of HOD on comb spectral envelopes is also elucidated through analyzing the intracavity dispersive wave generations. We further demonstrate that the spectral envelope of a broadband optical frequency comb can be engineered by using a cavity dispersion profile with multiple zero dispersion wavelengths.

Broadband Kerr frequency combs and intracavity soliton dynamics influenced by high-order cavity dispersion

We propose an approach for an efficient generation of optical Cherenkov radiation (OCR) in the near-infrared by tailoring the waveguide dispersion for a zero group-velocity mismatching between the radiation and the pump soliton. Based on an As2S3 slot waveguide with subwavelength dimensions, dispersion profiles with four zero dispersion wavelengths are found to produce a phase-matching nonlinear process leading to a broadband resonant radiation. The broadband OCR investigated in the chalcogenide waveguide may find applications in on-chip wavelength conversion and near-infrared pulse generation.

Optical Cherenkov radiation in an As_2S_3 slot waveguide with four zero-dispersion wavelengths

The invention discloses a multi-mode/single-mode optical fiber connector based on a double-clad optical fiber, which comprises a single-mode optical fiber, the double-clad optical fiber with a certain length and a multi-mode optical fiber. The double-clad optical fiber is provided with an inner clad, an outer clad and a fiber core. The refractive index of the fiber core of the double-clad optical fiber is the same with that of the outer clad and is a little higher than that of the inner clad; the diameter of the fiber core of the double-clad optical fiber is the same with that of the fiber core of the single-mode optical fiber; the diameter of the inner clad is the same with that of the fiber core of the multi-mode optical fiber; and the single-mode optical fiber, the double-clad optical fiber with a certain length and the multi-mode optical fiber are sequentially welded end to end. According to the connector, the special structure of the double-clad optical fiber is adopted, the double-clad optical fiber plays the role of a coupler, so that mode conversion and energy coupling between the multi-mode optical fiber and the single-mode optical fiber are realized. According to the invention, the energy coupling efficiency can be improved, a manufacturing process is simple and the reliability is high.

Multi-mode/single-mode optical fiber connector based on double-clad optical fiber

The invention provides a switchable multi-wavelength erbium-doped optical fiber laser based on an optical fiber Mach-Zehnder interferometer, which comprises a 980nm pump light source, a wavelength division multiplexer, a segment length erbium-doped optical fiber, a first double-clad fiber, a first single-mode fiber, a second double-clad fiber, a second single-mode optical fiber, a first polarization controller, a polarization correlated optical isolator, a second polarization controller and a 9:1 coupler. The 980nm pump light source is connected with one input end of the wavelength division multiplexer and used for providing persistent energy for the laser. The wavelength division multiplexer forms an optical fiber loop together with the segment length erbium-doped optical fiber, the first double-clad fiber, the first single-mode fiber, the second double-clad fiber, the second single-mode optical fiber, the first polarization controller, the polarization correlated optical isolator, the second polarization controller and the 9:1 coupler in sequence. The switchable multi-wavelength erbium-doped optical fiber laser has a compact structure, can be integrated with the existing optical fiber system easily and can realize the switch conversion of different wavelengths and the number conversion of different output wavelengths by adjusting the polarization controller.

Switchable multi-wavelength erbium-doped optical fiber laser based on optical fiber Mach-Zehnder interferometer

Summary form only given. Optical Cherenkov radiation (OCR) is referred to as dispersive wave (DW) generation or non-solitonic radiation originating from soliton propagation perturbed by high order dispersion (HOD). The OCR becomes of particular importance for octave-spanning spectral broadening and blue-shifted supercontinuum generation (SCG) [1]. When the OCR generation lies in the anomalous GVD region and tends to be a soliton state, the phase-matching (PM) condition between linear DWs and the soliton could become a soliton PM condition: β s (ω s ) = β s (ω r ), provided that v g (ω s ) = v g (ω r ). β s (ω k ) ≡ β(ω k ) + (ω-ω k )/v g (ω k ) + q k reflects the nondispersive nature of the soliton at the center frequency ω k and v g (ω k ) is the group velocity. k = r, s represents the radiation or soliton wave. A sharp spectral switching of the soliton due to the soliton spectral tunneling (SST) effect in the photonic crystal fibers (PCFs) can occur across a normal group velocity dispersion (GVD) region, which is sandwiched in the anomalous GVD regions [2]. This approach shows a new efficient wavelength conversion scheme, and we explore the cascaded OCR to enlarge the span of the wavelength conversion.We show the flexible dispersion profiles with three controlled ZDWs, λ1 <; λ2 <; λ3, as shown in Fig. 1(a), by tuning the pitch Δ and the relative hole size D = d/Δ in PCFs. λ2 and λ3 shift to longer wavelengths as the increase of D. Three anomalous GVD regions separated normal GVD are formed. Fig. 1(b) shows the predicted PM wavelength versus soliton wavelength when the input pulse has a full width at half-maximum (FWHM) of 50 fs and the soliton number N = 1. The soliton PM condition is satisfied at a so-called critical point (CP), where two radiation bands are degenerate. Two different CPs are chosen to make soliton spectral tunneling in two-segment PCFs.

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Engineering wavelength conversion span in cascaded broadband Cherenkov radiation

We propose As 2 S 3 slot waveguides, which have four zero-dispersion wavelengths and exhibit a flattened and low dispersion over a 1800-nm bandwidth from near-IR to mid-IR wavelength range. The dispersive wave generation
is investigated based on this kind of dispersive profile by launching femtosecond pulses into As 2 S 3 waveguides. Detailed simulations under realistic conditions show that considerable amount of dispersive wave emission occur
in As 2 S 3 waveguide and therefore can be desirable for optical communication and on-chip signal processing, such as frequency metrology, optical coherence tomography and microscopy.

Jungao Hu

Papers

Optical Cherenkov radiation in an As_2S_3 slot waveguide with four zero-dispersion wavelengths

Multi-mode/single-mode optical fiber connector based on double-clad optical fiber

Switchable multi-wavelength erbium-doped optical fiber laser based on optical fiber Mach-Zehnder interferometer

Engineering wavelength conversion span in cascaded broadband Cherenkov radiation

Dispersive wave generation in As2S3 slot waveguide with four zero-dispersion wavelengths