Enhancing Optical Nonlinear Processes Performance of Silicon Nitride Waveguides

Silicon nitride has recently attracted significant interest for nonlinear integrated optics. In addition to its excellent linear properties, silicon nitride offers a bandgap larger than silicon, and a wide transparency window spanning from the visible all the way to the middle infrared (mid-IR). As such, research efforts have been focused on leveraging these properties in order to achieve efficient nonlinear wave-mixing of the third and second order. In this paper we will show how mid-IR light can be efficiently generated in large cross section silicon nitride waveguides through careful dispersion engineering and how second order nonlinearities can be significantly enhanced through an all-optical poling process.

Experimental and numerical investigation of two-dimensional photonic crystals for application in integrated optics

David Leuenberger

Two-dimensional (2D) photonic crystals (PhCs) at near-infrared wavelengths are promising candidates for novel integrated optics applications. The main focus being on 'all PhC' monolithically integrated optical circuits. Because of well established fabrication technologies particular emphasis is on 'quasi' 2D PhCs where a 2D lattice of air holes is etched through a planar step-index waveguide providing the optical confinement in the third dimension. This approach has been successfully demonstrated both in GaAs and InP-based systems. We have studied vertical low-index-contrast structures both in GaAs and InP. These structures suffer inherently from radiation losses that are strongly affected by the air hole depth and shape. A semi-analytical 'ε"-model' enables out-of-plane scattering to be described by an effective imaginary dielectric constant in the air-holes which can be used in 2D finite difference time domain (FDTD) calculations. The model, once validated by 3D FDTD, allows us to deduce from optical simple slab transmission measurements the structural parameters (e.g. size, depth and form of the hole). The experimental transmission spectra are obtained with the so-called 'internal light source technique' (ILS), which allows for quantitative normalised transmission measurements, thus eliminating the uncertainties due to the in-out coupling arising in alternative characterisation techniques, i.e. end-fire. The ILS technique proved to be a versatile method without stringent limits for assessing the fabrication parameters (air-filling factor and loss) but also proved to be suitable for testing building blocks. In this thesis different building blocks were fabricated in the GaAs system and analysed using diverse modeling techniques (e.g. plane wave expansion, FDTD and coupled mode theory). The first building block was used to study contra-directional coupling of the fundamental mode with higher-order modes in channel waveguides. It is an extension of the work on waveguides with symmetric periodic corrugation to the case of multi periodicity. We showed that by breaking the translational symmetry in these structures referred to as hybrid waveguides, the number of mini-stopbands is increased and not, as one could have expected, decreased. A second building block analysed to guide light around the corner, are cavity resonant bends (CRBs). As our analysis showd that CRBs work best with non-degenerate cavity modes, different low-symmetry order cavities were studied with differ coupling coefficients between cavity and waveguides. Unfortunately, the resulting peak transmission and overall performance was below our expectations. An important building block in integrated optics are directional couplers. They have a potential for applications such as 3dB-intensity splitting or channel add-drop filtering. To reduce the coupling length, transmission measurements of a series of coupler structures of different lengths were taken. In the case of two W3 waveguides separated by a single row, the coupling length proved to be very dependent on the hole diameter in the single row. For this case the experimentally determined coupling length, which has been validated by both PWE and FDTD simulations, was of the order of 350 periods which corresponds to an absolute length of about 80 μm, a value that is small for conventional optics, but rather large for the domain of PhC optics where due to the unusually high propagation losses the integrated circuit has to be kept small. This value could, however, be reduced by a factor of at least three when reducing the ratio between the barrier hole and the nominal hole radius to 0.5. Finally we studied the waveguides needed to connect the different building blocks. The standard solution are channel waveguides, line-defects obtained by omitting a number of rows of holes. An alternative approach are coupled cavity waveguides (CCWs). They not only offer the potential for waveguiding but may also be used for non-linear optics by exploiting the special dispersion properties of CCWs (e.g. low group-velocity). A tight-binding model has been set-up that describes the interaction between the neighbouring cavities and that allows one to deduce the dispersion properties of the chain from the single cavity field distribution.

EPFL2004

Mid-infrared continuous-wave parametric amplification in chalcogenide microstructured fibers

Camille Sophie Brès, Davide Grassani, Svyatoslav Kharitonov, Sida Xing

The persistent growth of interest in the middle infrared (MIR) is stimulating the development of sources and components. Novel waveguides and fibers for the efficient use of nonlinear effects in the MIR are being intensively studied. Highly nonlinear silica fibers have enabled record performances of highly versatile parametric processes in the telecommunication band. However, no waveguiding platforms (to our knowledge) have yet solved the trade-off among high nonlinearity, low propagation losses and dispersion in the MIR. As single waveguide designs have not yet hit this particular optimal point, only pulsed–pumped demonstrations have been carried out, hindering any application requiring narrow linewidth, continuous-wave (c.w.) operation, or signal modulation. Here, we show MIR c.w. parametric amplification in a Ge10As22Se68 tapered fiber. Leveraging state-of-the-art fabrication techniques, we use a photonic crystal fiber (PCF) geometry combining high nonlinearity and low dispersion, while maintaining single mode and low losses in the short-wave IR and MIR. We experimentally demonstrate 5 dB signal amplification and 3 dB idler conversion efficiency using only 125 mW of pump in the 2 μm wavelength range. Our result is not only the first c.w. parametric amplification measured at 2 μm in any waveguide, but it also establishes GeAsSe PCF tapers as the most promising all-fibered, high-efficiency parametric converter for advanced applications in the MIR. © 2017 Optical Society of America

2017

Ultralow-power chip-based soliton microcombs for photonic integration

Botao Du, Bahareh Ghadiani, Hairun Guo, Maxim Karpov, Tobias Kippenberg, Junqiu Liu, Martin Hubert Peter Pfeiffer, Arslan Sajid Raja, Michail Zervas

The generation of dissipative Kerr solitons in optical microresonators has provided a route to compact frequency combs of high repetition rate, which have already been employed for optical frequency synthesizers, ultrafast ranging, coherent telecommunication, and dual-comb spectroscopy. Silicon nitride (Si3N4) microresonators are promising for photonic integrated soliton microcombs. Yet to date, soliton formation in Si3N4 microresonators at electronically detectable repetition rates, typically less than 100 GHz, is hindered by the requirement of external power amplifiers, due to the low quality (Q) factors, as well as by thermal effects that necessitate the use of frequency agile lasers to aces the soliton state. These requirements complicate future photonic integration, heterogeneous or hybrid, of soliton microcomb devices based on Si3N4 microresonators with other active or passive components. Here, using the photonic Damascene reflow process, we demonstrate ultralow-power single-soliton formation in high- Q (Q(0) > 15 x 10(6))Si3N4 microresonators with 9.8 mW input power (6.2 mW in the waveguide) for devices of electronically detectable, 99-GHz repetition rate. We show that solitons can be accessed via simple, slow laser piezo tuning, in many resonances in the same sample. These power levels are compatible with current silicon-photonics-based lasers for full photonic integration of soliton microcombs, at repetition rates suitable for applications such as ultrafast ranging and coherent communication. Our results show the technological readiness of Si3N4 optical waveguides for future all-on-chip soliton microcomb devices. (C) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

OPTICAL SOC AMER2018

Experimental and numerical investigation of two-dimensional photonic crystals for application in integrated optics

David Leuenberger

EPFL2004