Dispersion tailoring in both integrated photonics and fiber-optic based devices

Tesis por compendio[EN] This Thesis focuses on the study, implementation and characterization of chromatic dispersion tailoring employing both optical fiber and photonic integrated waveguides. Chromatic dispersion causes that the different spectral components of an optical pulse travel at different velocities. This effect can be separated into two different fundamental contributions, material dispersion and waveguide dispersion. Chromatic dispersion can be tailored through the design of the structural parameters of the device in order to obtain specific characteristics in the resulting dispersion profile such as low values of dispersion and/or zero dispersion at a desired wavelength, for example. This approach is very useful in dispersion-dependent applications. In this PhD, we investigate chromatic dispersion tailoring in two different transmission mediums, photonic integrated waveguides and optical fiber. In the first case, two different geometries of Silicon-on-Insulator (SOI) integrated waveguides, strip and slot, are considered. By varying structural parameters such as the cross-section, aspect ratio or fill factor, different chromatic dispersion profiles are obtained. In addition, the influence of the slot location is evaluated. This study is carried out using simulation software in order to obtain the effective refractive index profile as a function of wavelength, which is later differentiated to obtain the final dispersion values. Besides, chromatic dispersion in both waveguide geometries is experimentally measured using an interferometer technique. In the second case, the chromatic dispersion present in a tapered fiber is studied. A tapered fiber consists of a narrow waist located between two transition regions and it allows the modification of the conventional propagation conditions due to the interference between the modes propagating through the waist. This interference between modes creates a transmission pattern which depends on the waist length and the effective refractive indexes of the modes travelling through the waist. By applying stress to the tapered fiber its interference pattern can be modified. Chromatic dispersion profile of tapered fibers is obtained, tailored and compared with the dispersion profile of conventional single-mode fibers.[ES] Esta Tesis se centra en el estudio, implementación y caracterización del control de la dispersión cromática empleando tanto fibra óptica como guías integradas fotónicas. La dispersión cromática provoca que las distintas componentes espectrales asociadas con el pulso óptico viajen a diferentes velocidades. Este efecto puede ser dividido en sus dos contribuciones fundamentales, la dispersión del material y la dispersión de la guía. La dispersión cromática puede ser controlada a través del diseño de los parámetros estructurales del dispositivo para poder obtener así determinadas características en el perfil de dispersión resultante como por ejemplo bajos valores o localización de la longitud de onda de dispersión cero en una longitud de onda deseada. Este método es muy útil en aplicaciones dependientes de la dispersión. En esta Tesis, investigamos el control de la dispersión cromática en dos medios de transmisión diferentes, las guías fotónicas integradas y la fibra óptica. En el primer caso, se consideran dos geometrías diferentes de guías integradas en silicio, las guías convencionales y las guías ranuradas. Mediante la modificación de los parámetros estructurales como la sección transversal de la guía, su relación de aspecto o el factor de llenado, se obtienen diferentes perfiles de dispersión cromática. Además, se evalúa la influencia de la situación de la ranura. Mediante software de simulación, se obtiene el perfil de índice de refracción efectivo en función de la longitud de onda, que posteriormente se deriva y se obtiene el valor de la dispersión. Asimismo, se mide experimentalmente la dispersión en ambas geometrías utilizando una técnica interferométrica. En el segundo caso, se analiza la dispersión cromática que presenta una fibra de tipo taper. Esta geometría consiste en una cintura estrecha situada entre dos regiones de transición y permite la modificación de las condiciones de propagación convencionales debido a la interferencia entre los modos que se propagan por la cintura, que crea un patrón de transmisión dependiente de la longitud de la cintura y de los índices efectivos de los modos. Aplicando tensión sobre la fibra, su patrón de interferencia puede ser modificado. La dispersión cromática de las fibras taper se obtiene, se modifica y se compara con el perfil de dispersión de una fibra convencional.[CA] La tesi a exposar se centra en l'estudi, implementació i caracterització del control de la dispersió cromàtica empleant la fibra òptica i les guies integrades fotòniques. La dispersió cromàtica provoca que els distints components espectrals associats amb la pols òptica viatgen a diferents velocitats. Aquest pot dividir-se en les dos contribucions fonamentals corresponents: la dispersió del material i la dispersió de la guia. La dispersió cromàtica pot controlar-se a través del disseny dels paràmetres estructurals del dispositiu per poder obtindre aixi determinades característiques en el perfil de dispersió resultant, com per exemple, baixos valors o localizació de la longitud d'ona de dispersió zero a una longitud d'ona desitjada. No obstant això, aquest mètode és molt útil en aplicacions depenents de la dispersió. A més a més, investiguem el control de dispersió cromàtica en dos mitjans de transmissió diferents, les guies fotòniques integrades i la fibra òptica. D'una banda, es consideren dos geometries diferents de guies integrades en silici, les guies convencionals i les ranurades. Mitjançant la modificació dels paràmetres estructurals com la secció transversal de la guia, la relació d'apecte o el factor d'ompliment, obtenim diferents perfils de dispersió cromàtica. Fins i tot, s'avalua la influència de la situació de la ranura. Mitjançant el programari de simulació, obtenim el perfil d'índex de refracció efectiu en funció de la longitud d'ona, que posteriorment es derivarà i s'obrindrà el valor de la dispersió. Tanmateix, es mesura experimentalment la dispersió en les dos geometries utilitzant una tècnica interferomètrica. D'altra banda, analitzam la dispersió cromàtica que presenta una fibra de tipus taper. Aquesta consisteix en una cintura estreta situada entre dos regions de transició que, ens permet la modificació de les condicions de propagació convencional com a causa d'una interferència entre els modes que es propaguen per la cintura i els índex efectius dels modes. Si apliquem tensió sobre la fibra, el seu patró d'interferència podria ser modificat. La dispersió d'una fibra cromàtica de les fibres taper s'obté, es modific i es compara amb el perfil de dispersió d'una fibra convencional.Mas Gómez, SM. (2015). Dispersion tailoring in both integrated photonics and fiber-optic based devices [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/54113TESISCompendi

Micro-combs: a novel generation of optical sources

The quest towards the integration of ultra-fast, high-precision optical clocks is reflected in the large number of high-impact papers on the topic published in the last few years. This interest has been catalysed by the impact that high-precision optical frequency combs (OFCs) have had on metrology and spectroscopy in the last decade [1–5]. OFCs are often referred to as optical rulers: their spectra consist of a precise sequence of discrete and equally-spaced spectral lines that represent precise marks in frequency. Their importance was recognised worldwide with the 2005 Nobel Prize being awarded to T.W. Hänsch and J. Hall for their breakthrough in OFC science [5]. They demonstrated that a coherent OFC source with a large spectrum – covering at least one octave – can be stabilised with a self-referenced approach, where the frequency and the phase do not vary and are completely determined by the source physical parameters. These fully stabilised OFCs solved the challenge of directly measuring optical frequencies and are now exploited as the most accurate time references available, ready to replace the current standard for time. Very recent advancements in the fabrication technology of optical micro-cavities [6] are contributing to the development of OFC sources. These efforts may open up the way to realise ultra-fast and stable optical clocks and pulsed sources with extremely high repetition-rates, in the form of compact and integrated devices. Indeed, the fabrication of high-quality factor (high-Q) micro-resonators, capable of dramatically amplifying the optical field, can be considered a photonics breakthrough that has boosted not only the scientific investigation of OFC sources [7–13] but also of optical sensors and compact light modulators [6,14]

Discrete Wave Propagation In Quadratically Nonlinear Media

Discrete models are used in describing various microscopic phenomena in many branches of science, ranging from biology through chemistry to physics. Arrays of evanescently coupled, equally spaced, identical waveguides are prime examples of optical structures in which discrete dynamics can be easily observed and investigated. As a result of discretization, these structures exhibit unique diffraction properties with no analogy in continuous systems. Recently nonlinear discrete optics has attracted a growing interest, triggered by the observation of discrete solitons in AlGaAs waveguide arrays reported by Eisenberg et al. in 1998. So far, the following experiments involved systems with third order nonlinearities. In this work, an experimental investigation of discrete nonlinear wave propagation in a second order nonlinear medium is presented. This system deserves particular attention because the nonlinear process involves two or three components at different frequencies mutually locked by a quadratic nonlinearity, and new degrees of freedom enter the dynamical process. In the first part of dissertation, observation of the discrete Talbot effect is reported. In contrast to continuous systems, where Talbot self-imaging effect occurs irrespective of the pattern period, in discrete configurations this process is only possible for a specific set of periodicities. The major part of the dissertation is devoted to the investigation of soliton formation in lithium niobate waveguide arrays with a tunable cascaded quadratic nonlinearity. Soliton species with different topology (unstaggered all channels in-phase, and staggered neighboring channels with a pi relative phase difference) are identified in the same array. The stability of the discrete solitons and plane waves (modulational instability) are experimentally investigated. In the last part of the dissertation, a phase-insensitive, ultrafast, all-optical spatial switching and frequency conversion device based on quadratic waveguide array is demonstrated. Spatial routing and wavelength conversion of milliwatt signals is achieved without pulse distortions

Fiber-based phase-sensitive optical amplifiers and their applications

Optical parametric amplifiers rely on second-order susceptibility (three-wave mixing) or third-order susceptibility (four-wave mixing) in a nonlinear process where the energy of incoming photons is not changed (elastic scattering). In the latter case, two pump photons are converted to a signal and to an idler photon. Under certain conditions, related to the phase evolution of the waves involved, this conversion can be very effi-cient, resulting in large amplification of an input signal. As the nonlinear process can be very fast, all-optical applications aside from pure amplification are also possible. If the amplifier is implemented in an optical input-phase-sensitive manner, it is possible to amplify a signal wave without excess noise, i.e., with a noise figure of 0 dB. In this paper, we will provide the fundamental concepts and theory of such amplifiers, with a focus on their implementation in highly nonlinear optical fibers relying on four-wave mixing. We will discuss the distinctions between phase-insensitive and phase-sensitive operation and include several experimental results to illustrate their capability. Different applications of parametric amplifiers are also discussed, including their use in optical communication links

Nonlinear Applications using Silicon Nanophotonic Wires

This thesis is concerned with an emerging set of nonlinear-optical applications using silicon nanophotonic "wires" fabricated on a silicon-on-insulator photonic chip. These deeply scaled silicon nanophotonic wires are capable of confining the telecom and mid-infrared (mid-IR) light tightly into an optical-modal area ~ 0.1 μm2. The tight optical confinement leads to many advantageous physical properties including enhanced effective nonlinearity, flexible control of waveguide dispersion, and short free-carrier lifetime. All these advantages make silicon nanophotonic wires an ideal platform for a variety of nonlinear applications. The first part of my thesis study is focused on nonlinear applications in the telecom bands. In Chapter 3, I study the frequency dependence of optical nonlinearity in silicon nanophotonic wires, and its influence on the propagation of ultra-short optical pulses in such wires. I show that silicon nanophotonic wires possess a remarkably large characteristic time associated with the self-steepening effect and optical-shock formation. In Chapter 4, I present an experimental demonstration of an ultrafast cross-phase-modulation-based wavelength-conversion (XPM-WC) technique for telecom RZ-OOK data. I also investigate the effect of pump-probe detuning on the efficacy of this XPM-WC technique. In Chapter 5, I show a (primarily) numerical study of a method for dispersion-engineering of silicon nanophotonic wires using a conformal thin-silicon-nitride dielectric film deposited around the silicon wire core. My simulation results show that this approach may be used to achieve the dispersion characteristics required for broadband phase-matched four-wave-mixing processes, while simultaneously maintaining strong modal confinement within the silicon core for high effective nonlinearity. The second part of my thesis is devoted to investigations of nonlinear applications in mid-IR spectral region, in which nonlinear optical loss due to parasitic two-photon absorption can be significantly reduced and therefore a large nonlinear figure of merit can be achieved in order to facilitate efficient nonlinear processes. In Chapter 6, I present an experimental demonstration of a mid-IR-silicon-nanophotonic-wire optical parametric amplifier with 25.4 dB on-chip gain. This gain achieved with only a 4-mm-long silicon nanophotonic wire is sufficient enough to overcome all the insertion loss, resulting in 13 dB net off-chip amplification. In addition, I show, on the same waveguide, efficient generation of 4 orders of cascaded FWM products enabled by the large on-chip gain. In Chapter 7, I report a comprehensive study of the propagation characteristics of a picosecond pulse through a 4-mm-long silicon nanophotonic wire with normal dispersion with excitation wavelengths crossing the mid-infrared two-photon absorption edge at λ = 2200 nm. Significant reduction in nonlinear loss due to two-photon absorption is demonstrated as the excitation wavelengths approach 2200 nm. Self-phase modulation at high input power is also observed. Analysis of experimental data and comparison with numerical simulations illustrates that the two-photon absorption coefficient obtained from nanophotonic wire measurements is in reasonable agreement with prior measurements of bulk silicon crystals, and that bulk silicon values of the nonlinear refractive index can be confidently incorporated in the modeling of pulse propagation in deeply-scaled waveguide structures. In Chapter 8, I investigate a higher-order phase matching technique utilizing the 4th-order dispersion term for realizing a broadband or discrete band parametric process in silicon nanophotonic wires. I demonstrate experimentally, on a silicon nanophotonic wire designed to exhibit a desired 2nd-order and 4th-order dispersion, broadband/discrete-band modulation instability and 50 dB Raman assisted parametric gain

High-efficiency dissipative Kerr solitons in microresonators

The microresonator comb (microcomb) is a laser source that generates equally spaced coherent lines in the spectral domain. Having a chip-scale size and the potential of being low cost, it has attracted attention in multiple applications. Demonstrations have included high-speed optical communications, light detection and ranging, calibrating spectrographs for exoplanet detection and, optical clocks. These experiments typically rely on the generation of a dissipative Kerr soliton (DKS) --- a temporal waveform that circulates the microresonator without changing shape. However, these DKS states have thus far been limited in certain technical aspects, such as energy efficiency, which are essential for realizing commercial microcomb solutions.This thesis studies the dynamics of DKSs in microresonators aiming at developing a reliable and high-performing microcomb source. The investigation will cover DKSs found both in the normal and anomalous dispersion regime of silicon nitride microresonators. The performance of microcombs in terms of line power is numerically explored in single-cavity arrangements for telecommunication purposes. DKSs generated in linearly coupled microcavities are investigated, revealing exotic dynamics and improved performance in terms of power efficiency and DKS initiation. These studies facilitate reliable energy-efficient microcombs, bringing the technology a step closer to commercial use

Versatile short-wave and mid-infrared sources based on wideband parametric conversion

The mid-infrared part of the optical spectrum is of high interest in a wide range of applications such as environmental gas monitoring, contaminant detection in the chemical, food or pharmaceutical industry, medical diagnosis, or defense and security. Relevant molecules can readily be identified through their mid-infrared absorption spectra, as the latter contains the fundamental resonances of a number of pollutant and toxic gases. Consequently, spectroscopic apparatus, light detection and ranging systems or free-space communication links all benefit from the progress accomplished by mid-infrared technologies over the last years. However some shortcomings in the light emitters capabilities are still to be addressed. In this research work, we aim at designing a mid-infrared laser as versatile as possible and based on nonlinear wavelength conversion. The conversion relies on third-order parametric effects in waveguides such as optical fibers made of various types of glass, or integrated semiconductor chips. The objective is to leverage mature communication-band components to generate and shape the seed optical signals, then mixed in the abovementioned waveguides to down-convert them towards midinfrared. The wavelength conversion is performed in two stages, and the first stage consists of a parametric source emitting in the short-wave infrared range. This thesis mostly focuses on the design and realization of this stage. As such, it is closely linked to the field of nonlinear fiber optics, where the use of silica is preponderant. We build on the research performed over the last years on parametric amplifiers, initially used for the re-amplification of communication signals, and we combine it with technologies dedicated to short-wave infrared fiber lasers. As such, we are able to build wavelength tunable and modulation-capable short-wave infrared sources, significantly more powerful and versatile than previous broadband parametric converter designs. The end of the dissertation is then dedicated to the solutions that are then envisioned to realize the second conversion stage, towards mid-infrared. Very promising numerical and experimental results indicate a successful outcome to the project, confirming the validity of the laser concept

A route to high peak power and energy scaling in the mid-IR chirped-pulse oscillator-amplifier laser systems

The paper introduces a new route towards the ultrafast high laser peak power and energy scaling in a hybrid mid-IR chirped pulse oscillator-amplifier (CPO-CPA) system, without sacrificing neither the pulse duration nor energy. The method is based on using a CPO as a seed source allowing the beneficial implementation of a dissipative soliton (DS) energy scaling approach, coupled with a universal CPA technique. The key is avoiding a destructive nonlinearity in the final stages of an amplifier and compressor elements by using a chirped high-fidelity pulse from CPO. Our main intention is to realize this approach in a Cr2+:ZnS-based CPO as a source of energy-scalable DSs with well-controllable phase characteristics for a single-pass Cr2+:ZnS amplifier. A qualitative comparison of experimental and theoretical results provides a road map for the development and energy scaling of the hybrid CPO-CPA laser systems, without compromising pulse duration. The suggested technique opens up a route towards extremely intense ultra-short pulses and frequency combs from the multi-pass CPO-CPA laser systems that are particularly interesting for real-life applications in the mid-IR spectral range from 1 to 20 um.Comment: 16 pages, 14 figure