On-chip two-octave supercontinuum generation by enhancing self-steepening of optical pulses

On-chip mid-IR octave-tunable Raman soliton laser

Photonic chip-based continuously tunable lasers are widely recognized as an indispensable component for photonic integrated circuits (PICs). Specifically, mid-infrared (mid-IR) laser sources are of paramount importance in applications such as photonic sensing and spectroscopy. In this article, we theoretically investigate the propagation dynamics of mid-IR Raman soliton in Ge₂₈Sb₁₂Se₆₀ chalcogenide glass waveguide. By carefully engineer the waveguide dispersion and nonlinear interaction, we propose a suspended chalcogenide glass waveguide device that allows an octave-tuning, from 1.96 µm to 3.98 µm, Raman soliton source. The threshold pump energy is in the low pico-Joule range. Our result provides a solution to continuously tunable on-chip mid-IR ultrafast laser sources.

Breaking fundamental noise limitations to supercontinuum generation

Supercontinuum generation in the anomalous group-velocity dispersion regime is widely considered to be inherently unstable against input pulse fluctuations. This constraint has compelled a coherent supercontinuum to be triggered by femtosecond pulses. In this work, conditions for breaking this fundamental limitation are analytically derived and realized in a silicon waveguide by exploiting the Kerr nonlinearity dispersion. On this basis, coherent supercontinuum generation with picosecond pulses and anomalous group-velocity dispersion is numerically demonstrated, which crosses a long-standing frontier in nonlinear optics.

Highly coherent and multi-octave mid-infrared supercontinuum generations in a reverse-strip AlGaAs waveguide with three zero-dispersion wavelengths

In this paper, a reverse-strip AlGaAs waveguide with three zero-dispersion wavelengths (ZDWs) is designed. The corresponding three ZDWs are located at 3.74, 6.56, and 8.89 µm. The nonlinearity coefficient of the proposed reverse-strip AlGaAs waveguide is calculated as 2.09W^-1m^-1 at wavelength 4.9 µm. The effects of pump pulse parameters, waveguide length, and noise coefficient on the nonlinear dynamics of supercontinuum (SC) generation are investigated. When the hyperbolic secant pump pulse with a wavelength of 4.9 µm, peak power of 900 W, and duration of 100 fs is launched into the proposed waveguide and propagated after a 3 mm length, highly coherent and multi-octave mid-infrared (MIR) SC spanning from 2.2 to 14.5 µm (more than 2.7 octaves, at -40dB level) is generated. Finally, a possible fabrication process of the reverse-strip AlGaAs waveguide is introduced. Our research results have important applications in MIR photonics, MIR spectroscopy, optical precision measurement, and more.

Extreme polarization-dependent supercontinuum generation in an uncladded silicon nitride waveguide

We experimentally demonstrate the generation of a short-wave infrared supercontinuum in an uncladded silicon nitride (Si₃N₄) waveguide with extreme polarization sensitivity at the pumping wavelength of 2.1 µm. The air-clad waveguide is specifically designed to yield anomalous dispersion regime for transverse electric (TE) mode excitation and all-normal-dispersion (ANDi) at near-infrared wavelengths for the transverse magnetic (TM) mode. Dispersion engineering of the polarization modes allows for switching via simple adjustment of the input polarization state from an octave-spanning soliton fission-driven supercontinuum with fine spectral structure to a flat and smooth ANDi supercontinuum dominated by a self-phase modulation mechanism (SPM). Such a polarization sensitive supercontinuum source offers versatile applications such as broadband on-chip sensing to pulse compression and few-cycle pulse generation. Our experimental results are in very good agreement with numerical simulations.

Measuring self-steepening with the photon-conserving nonlinear Schrödinger equation

We propose an original, simple, and direct method to measure self-steepening (SS) in nonlinear waveguides. Our proposal is based on results derived from the recently introduced photon-conserving nonlinear Schrödinger equation (NLSE) and relies on the time shift experienced by soliton-like pulses due to SS upon propagation. In particular, a direct measurement of this time shift allows for a precise estimation of the SS parameter. Furthermore, we show that such an approach cannot be tackled by resorting to the NLSE. The proposed method is validated through numerical simulations, in excellent agreement with the analytical model, and results are presented for relevant spectral regions in the near infrared, the telecommunication band, and the mid infrared, and for realistic parameters of available laser sources and waveguides. Finally, we demonstrate the robustness of the proposed scheme against deviations expected in real-life experimental conditions, such as pulse shape, pulse peak power, pulsewidth, and/or higher-order linear and nonlinear dispersion.

Improved broadband dispersion engineering in coupled silicon nitride waveguides with a partially etched gap

In order to meet the targeted dispersion requirements of applications, hybridized modes of coupled waveguides, whose dispersion characteristics can be significantly altered around their mode cross-point, have been recently investigated. The applications have been applied to microresonators based on concentric waveguides with a thin silicon nitride layer. However, it is still challenging to achieve a low and flattened anomalous dispersion profile just by optimizing the gap width between the waveguides. We propose to investigate the dispersion characteristics of coupled silicon nitride waveguides with a partially etched gap. It is shown that low and broadband anomalous dispersion can be achieved based on relatively thin 515 nm thick silicon nitride layer waveguides. The mechanism of a partially etched gap on dispersion engineering can be attributed to the tuning of the coupling strength between the waveguides. Therefore, when combined with other design parameters, it offers an additional "tuning knob" of advanced dispersion engineering when designing such coupled-waveguide devices for nonlinear photonic applications.

Towards On-Chip Self-Referenced Frequency-Comb Sources Based on Semiconductor Mode-Locked Lasers

Miniaturization of frequency-comb sources could open a host of potential applications in spectroscopy, biomedical monitoring, astronomy, microwave signal generation, and distribution of precise time or frequency across networks. This review article places emphasis on an architecture with a semiconductor mode-locked laser at the heart of the system and subsequent supercontinuum generation and carrier-envelope offset detection and stabilization in nonlinear integrated optics.

Extremely high dispersions in heterogeneously coupled waveguides

We present a heterogeneously coupled Si/SiO₂/SiN waveguide structure that can achieve extremely high dispersions (> | ± 10⁷| ps · nm^-1km^-1). A strong mode coupling between the Si and SiN waveguides introduces a normal dispersion to symmetric mode and an anomalous dispersion to anti-symmetric mode, and the large group velocity difference between the two waveguides results in such high dispersions. Geometric parameters of the structure control the peak dispersions and the central wavelength of the mode coupling, and these engineering capabilities are studied numerically. Analytical representations on the heterogeneously coupled waveguides are also introduced and these equations explain the effects of geometric parameters. This extremely dispersive waveguide scheme can be constructed with other material combinations as well and should be of interest in ultrafast signal processing and spectroscopic applications.

Compact broadband low-loss taper for coupling to a silicon nitride photonic wire

We demonstrate an ultra-compact waveguide taper on a silicon nitride platform. The proposed taper provides a coupling efficiency of 95% at a length of 19.5 μm in comparison to the standard linear taper of length 50 μm, which connects a 10 μm wide waveguide to a 1 μm wide photonic wire. The taper has a spectral response >75% spanning over 800 nm and resilience to fabrication variations; ±200  nm change in taper and end waveguide width varies transmission by <5%. We experimentally demonstrate taper insertion loss of <0.1  dB/transition for a taper as short as 19.5 μm, and reduce the footprint of the photonic device by 50.8% compared to the standard adiabatic taper. To the best of our knowledge, the proposed taper is the shortest waveguide taper ever reported in silicon nitride.

Mid-IR supercontinuum generated in low-dispersion Ge-on-Si waveguides pumped by sub-ps pulses

Ge-on-Si is an attractive material platform for mid-IR broadband sources on a chip because of its wide transparency window, high Kerr nonlinearity and CMOS compatibility. We present a low-loss Ge-on-Si waveguide with flat and low dispersion from 3 to 11 µm, which enables a coherent supercontinuum from 2 to 12 µm, generated using a sub-ps pulsed pump. We show that 700-fs pump pulses with a low peak power of 400 W are needed to generate such a wide supercontinuum, and the waveguide length is around 5.35 mm.

Reverse pillar chalcogenide glass waveguides with ultraflat and low dispersion profile over an ultrawide bandwidth

Two types of reverse pillar integrated chalcogenide glass (As₂S₃) waveguides are proposed in this study. These geometries exhibit an ultraflat and low dispersion profile with four zero dispersion wavelengths. Its low dispersion is approximately ±10  ps/nm/km over a 2240 nm bandwidth (for L waveguide) and ±13  ps/nm/km over a 2030 nm bandwidth (for F waveguide). Waveguide dispersion engineering is achieved by tuning the structural parameters of the waveguide, which has less sensitivity to the variations of structural parameters compared with silicon pillar waveguides. Moreover, the nonlinear coefficient and phase-matching condition in four-wave mixing (FWM) showed a great potential for broadband FWM processes in the near- and middle-infrared regions.

On-chip visible-to-infrared supercontinuum generation with more than 495 THz spectral bandwidth

We report ultra-broadband supercontinuum generation in high-confinement Si3N4 integrated optical waveguides. The spectrum extends through the visible (from 470 nm) to the infrared spectral range (2130 nm) comprising a spectral bandwidth wider than 495 THz, which is the widest supercontinuum spectrum generated on a chip.

Silicon-on-sapphire pillar waveguides for Mid-IR supercontinuum generation

We propose pillar integrated silicon waveguides to exploit the entire transparent window of silicon. These geometries posses a broad and flat dispersion (from 2 to 6 μm) with four zero dispersion wavelengths. We calculate supercontinuum generation spanning over two octaves (2 to >8 μm) with long wavelengths interacting weakly with the lossy substrate. These structures have higher mode confinement in the silicon - away from the substrate, which makes them substrate independent and are promising for exploring new nonlinear phenomena and highly sensitive molecular sensing over the entire silicon's transparency range.

Supercontinuum generation in strip/slot hybrid waveguide with flat and low dispersion

A strip/slot hybrid waveguide with double horizontal silicon nanocrystals slots is proposed to achieve flat and low dispersion with four zero dispersion wavelengths. By tuning structural parameters of the waveguide, dispersion tailoring is fully characterized. The flat dispersion varying between -13 and 14  ps/(nm·km) is obtained over an 845 nm bandwidth. A broadband supercontinuum spectrum, spanning from 1.15 to 3.65 μm in the -15  dB level, was generated in this waveguide pumped by a femtosecond pulse at 1.86 μm. Results indicate that the waveguide has great potential in near- and mid-infrared nonlinear applications.

Visible-to-near-infrared octave spanning supercontinuum generation in a silicon nitride waveguide

The generation of an octave spanning supercontinuum covering 488-978 nm (at -30  dB) is demonstrated for the first time on-chip. This result is achieved by dispersion engineering a 1-cm-long Si3N4 waveguide and pumping it with an 100-fs Ti:Sapphire laser emitting at 795 nm. This work offers a bright broadband source for biophotonic applications and frequency metrology.

Simulation of octave spanning mid-infrared supercontinuum generation in dispersion-varying planar waveguides

A dispersion-varying tapered planar waveguide is designed to generate supercontinuum efficiently in the mid-infrared region. The rib waveguide of lead-silicate glass on silica is 1.8 cm long, consisting of a segment with longitudinally increasing etch depth. The mechanism involves nonlinear soliton dynamics. The dispersion profile is shifted along the propagation distance, leading to continuous modification of the phase-matching condition for dispersive wave (DW) emission and enhancement of energy transfer efficiency between solitons and DWs. With low input pulse energy of 45 pJ, simulation demonstrates the generation of both broadband and flat near-octave spectrum spanning 1.3-2.5 μm at the -20  dB level.

Spatial effects in supercontinuum generation in waveguides

An economic, space- and time-resolved method to model ultra-short, intense pulse propagation in waveguides is described. Simulations of supercontinuum generation on a chip demonstrate the utility of the approach. Comparisons with the generalized nonlinear Schrödinger equation elucidate spatial effects, which influence pulse dynamics and the generation of new spectral components.

Supercontinuum generation in an on-chip silica waveguide

Supercontinuum generation is demonstrated in an on-chip silica spiral waveguide by launching 180 fs pulses from an optical parametric oscillator at the center wavelength of 1330 nm. With a coupled pulse energy of 2.17 nJ, the broadest spectrum in the fundamental TM mode extends from 936 to 1888 nm (162 THz) at -50  dB from peak. There is a good agreement between the measured spectrum and a simulation using a generalized nonlinear Schrödinger equation.

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

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.

Octave-spanning supercontinuum generation of vortices in an As2S3 ring photonic crystal fiber

We propose As(2)S(3) ring photonic crystal fiber (PCF) for supercontinuum generation of optical vortex modes. Due to the large material index contrast between As(2)S(3) and air holes in the designed ring PCF, there is a two-orders-of-magnitude improvement of the difference between the effective refractive indices of different vortex modes compared with regular ring fiber. The design freedom of PCFs enables a low dispersion (<60 ps/nm/km variation in total) over a 522 nm optical bandwidth. Moreover, the vortex mode has a large nonlinear coefficient of 11.7/W/m at 1550 nm with a small confinement loss of <0.03 dB/m up to 2000 nm. An octave-spanning supercontinuum spectrum of the vortex mode is generated from 1196 to 2418 nm at -20 dB by launching a 120 fs pulse with a 60 W peak power at 1710 nm into a 1 cm long As(2)S(3) ring PCF.

Silicon waveguide with four zero-dispersion wavelengths and its application in on-chip octave-spanning supercontinuum generation

We propose a novel silicon waveguide that exhibits four zero-dispersion wavelengths for the first time, to the best of our knowledge, with a flattened dispersion over a 670-nm bandwidth. This holds a great potential for exploration of new nonlinear effects and achievement of ultra-broadband signal processing on a silicon chip. As an example, we show that an octave-spanning supercontinuum assisted by dispersive wave generation can be obtained in silicon, over a wavelength range from 1217 to 2451 nm, almost from bandgap wavelength to half-bandgap wavelength. Input pulse is greatly compressed to 10 fs.