Nonlinear optics in wavelength-size waveguides: how far can the conversion efficiency be pushed?

When their size approach the wavelength of light propagating in them, waveguides exhibit favorable properties such as high nonlinearity and strong confinement. These properties have been used for a variety of nonlinear effects, including supercontinuum generation, second-, third- and one-third- harmonic generation, pulse shaping and nonlinear switching.Although simulations predicted conversion efficiencies well in excess of 30% for most of harmonic generation, experiments showed conversions of a fraction of a percent, at best.This talk will discuss harmonic generation in wavelength-size waveguides and limitations to the practical achievement of theoretical efficiency. In particular, intrinsic surface roughness due to thermal surface waves frozen during fabrication provide a constantly changing detuning from the ideal phase matching conditions, considerably reducing the overall efficiency for waveguides longer than 1mm

Parametric up- and down-conversion in sub-wavelength waveguides: coherent sources in the UV and IR

Parametric up- and down-conversion require phase matching between the generating and generated wavelengths, introducing strict requirements on the refractive index dispersion of materials used for this nonlinear processes. Optical microfibers can exploit the different modal overlap with core and cladding materials to phase match different modes at the required wavelengths. Intermodal phase-matching has been successfully exploited in optical microfibres to generate second- and third-harmonics, both in straight waveguides and related resonators. The simultaneous generation of guided higher harmonics also allowed to exploit other nonlinear effects like four wave mixing to generate light at short wavelengths in the UV part of the spectrum. Parametric down-conversion allows to generate entangled photons, but with efficiencies significantly smaller than their up-conversion processes because of their reliance of spontaneous photons generation from vacuum. The use of more complex architectures, based on multiple coupled waveguides, allows to achieve a quasi-phase matching condition, thus a sizeable improvement on the overall conversion efficiency

Modeling and performance analysis of IP with MPLS/GMPLS over backbone networks

Backbone network is at the core of the computer network infrastructure and we aim to model and analyze the performance of the backbone network under various conditions. In a copper–based or opaque fiber–based backbone network, Integrated Service (IntServ) and Differentiated Service (DiffServ) should be viewed as complementary technologies in the pursuit of end–to–end Quality of Service (QoS). Multi–Protocol Label Switching (MPLS) is a good candidate that facilitates the interoperation of the IntServ and DiffServ network. Meanwhile, with the introduction of Generalized Multi–Protocol Label Switching (GMPLS) as the common control plane between optical crossconnects (OXCs) and electronic switching devices, the integration of Internet Protocol (IP) and Wavelength Division Multiplexing (WDM) can be efficiently achieved. In the case of all–optical IP over WDM network, traffic flows can be directly mapped to optical channels without the intermediate layers such as Asynchronous Transfer Mode (ATM) and Synchronous Optical Networking (SONET) / Synchronous Digital Hierarchy (SDH). Therefore, the bursty nature of IP traffic may have significant effect on the performance of the optical channels. On the other hand, the optical impairments of the optical paths and optical nodes may greatly influence the performance of IP traffic at the network layer. In this thesis, new traffic classification and service mapping among the IntServ, DiffServ and MPLS/GMPLS are proposed to enable the interoperation of these technologies. More importantly, a novel four–queue system is proposed to be located at every output port of the MPLS Label Switch Router (LSR) or GMPLS OXCs, but in different manners respectively. The functionally equivalent electronic queueing system and optical queueing system are illustrated in great details. In this way, the network node in the backbone is able to ensure the QoS of both the IntServ and DiffServ edge customers. The performance analysis of the four–queue system is elaborated and the performance parameters in terms of throughput, loss probability and delay are derived analytically.DOCTOR OF PHILOSOPHY (EEE

Harmonic generation via chi3 intermodal phase matching in microfibers

Intermodally phase matched up- and down-conversion processes based on the third order nonlinearity have been proposed to efficiently generate light in the UV and mid-IR wavelength regions in solid core silica optical fibers and optical microfibers. We study waveguide parameters and practical considerations required for optimum conversion

Modes effective refractive index difference measurement in few-mode optical fiber

We studied the measurement and data analysis of modes effective refractive indexes for few-mode optical fiber and found that measuring refractive index difference from the modes interference pattern was affected by dispersion in the optical fiber. A comprehensive method of accurate measurement of modes effective refractive index differences in few-mode optical fiber was developed. It consists of the measurements of the FBG reflection spectrum and the modes interference spectrum, the simulation of interference with dispersion effect in the interferometer configuration, and the data optimization to match with the measured modes interference. The results show much improvement in the few-mode optical fiber characterization.ASTAR (Agency for Sci., Tech. and Research, S’pore)Published versio

Sensitivity Enhanced Refractive Index Fiber Sensor Based on Long-Range Surface Plasmon Resonance in SiO2-Au-TiO2 Heterostructure

Long-range surface plasmon resonance (LRSPR), generated from a coupled plasmon polariton in a thin metal slab sandwiched by two dielectrics, has attracted more and more attention due to its merits, such as longer propagation and deeper penetration than conventional single-interface surface plasmon resonance. Many useful applications related to light–medium interaction have been demonstrated based on the LRSPR effect, especially in the sensing area. Here, we propose and demonstrate an LRSPR-based refractive index sensor by using a SiO2-Au-TiO2 heterostructure, in which a D-shaped honeycomb-microstructure optical fiber (MOF) is designed as the silica substrate and then deposited with a gold film and thin-layer titanium dioxide (TiO2). By using the full-vector finite-element method (FEM), this heterostructure is numerically investigated and demonstrated to excite LRSPR without a buffer layer, which is usually necessary in previous LRSPR devices. Through comprehensive discussion about the influence of structural parameters on the resonant wavelength, the excitation of the LRSPR in the proposed heterostructure is revealed to be highly related to the effective refractive index of MOF’s fundamental core mode, which is mainly determined by the MOF’s pitch, the thicknesses of the silica web and the planar-layer silica. Moreover, the thin-layer TiO2 plays an important role in significantly enhancing the resonance and the sensitivity to analyte’s refractive index as well, when it is coated on the top of the Au film rather than between the metal and waveguide. Finally, the proposed LRSPR sensor based on SiO2-Au-TiO2 heterostructure shows an ultra-high wavelength sensitivity of 20,100 nm/RIU and the corresponding minimum resolution is as low as 4.98×10−7 RIU. Thus, the proposed LRSPR device offers considerable potential for sensing applications in biomedical and biochemical areas

Efficient phase-matched third harmonic generation from mid-IR to near-IR regions in a double asymmetric plasmonic slot waveguide

Recent years, the research of mid-infrared (mid-IR) photonics has inspired increasingly interest due to their potential applications in a wide variety of areas, including free-space communications, chemical or biological sensors, environmental monitors, thermal imaging, IR countermeasures and medical procedures. On the other hand, third harmonic generation (THG) has been demonstrated to be a versatile tool to realize high speed optical performance monitoring of in-band OSNR and residual dispersion. The mid-IR light sources based third-order frequency conversion opens an entirely new realm of nonlinear interactions. Nevertheless, rare experimental or analytical THG modeling has been published. In this work, we theoretically investigate the possible efficient phase-matched THG in a double symmetric plasmonic slot waveguide (DAPSW) based on a mid-IR light source. Nonlinear organic material DDMEBT with thirdorder susceptibility of χ(3) = 1×10-19 m2/V2 is integrated into the top metallic slot region as the main slot core medium. Silicon (Si) is used to fill the bottom metallic slot region. Silver (Ag) is considered to be the metal medium due to its low Ohmic loss. The needed phase-matching condition (PMC) is satisfied between the zeroth mode at fundamental frequency (FF) and the first mode at third harmonic (TH) by appropriate designing the waveguide geometrical parameters. The associated parameters such as the width and height of the slot, pump-harmonic modal overlap, figureof- merit (FOM), pump power and detuning have been numerically investigated in detail. Finally, the conversion efficiency comes up to 1.69×10-5 with pump power of 1 W and the corresponding waveguide length is 10.8 μm.Published versio

Third harmonic generation from mid-IR to near-IR regions in a phase-matched silicon-silicon-nanocrystal hybrid plasmonic waveguide

The conversion efficiency of third harmonic generation (THG) from mid-IR (3600 nm) to near-IR (1200 nm) regions in a silicon-silicon-nanocrystal hybrid plasmonic waveguide (SSHPW) was calculated. The required modal phase-matching condition (PMC) between the 0-th mode at fundamental wave (FW) and the 2-nd mode at third harmonic (TH) is achieved by carefully designing the waveguide geometry. Benefiting from the hybridized surface plasmon polariton (SPP) nature of the two guided modes, the SSHPW is capable of achieving both high THG nonlinear coefficient I6 and reasonable linear propagation loss, thereby resulting in large figure-of-merits (FOMs) for both FW and TH. According to our simulation, THG conversion efficiency up to 0.823% is achieved at 62.9 ????m SSHPW with pump power of 1 W.Accepted versio