The Photonic Lantern

Photonic lanterns are made by adiabatically merging several single-mode cores into one multimode core. They provide low-loss interfaces between single-mode and multimode systems where the precise optical mapping between cores and individual modes is unimportant.Comment: 45 pages; article unchanged, accepted for publication in Advances in Optics and Photonic

Advanced materials, process, and designs for silicon photonic integration

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2009.Includes bibliographical references (p. 229-235).The copper (Cu) interconnect has become the bottleneck for bandwidth scaling due to its increasing RC time constant with the decreasing gate line width. Currently, silicon based optical interconnect is widely pursued as the most promising technology to replace Cu in microprocessor chips. Silicon optical interconnect is based on integrated silicon nanophotonic technologies. It can leverage the large scale and low cost of CMOS technology and deliver higher bandwidth with no EMI and low heat dissipation. Passive photonic component, such as waveguides, couplers, filters, splitters, are the backbone of integrated photonic circuit. This thesis is dedicated to the development of low loss, high performance, high index contrast optical waveguides and couplers via materials, processes engineering, development, and device designs. We primarily focus on SOI single crystalline silicon (c-Si or SOI), PECVD amorphous silicon (a-Si:H, or simplified as a-Si), and PECVD silicon nitride (SiNxHy) based single mode channel waveguides.We have previously identified that sidewall roughness scattering is the dominant loss mechanism for the TE mode in high index contrast single mode channel waveguides. In this thesis, we provide a comprehensive understanding of the roughness scattering and its positive correlations with (1) sidewall optical intensity; (2) sidewall RMS roughness; and (3) sidewall index contrast. Novel processes and designs, such as hard mask and chemical oxidation, are developed based on the above understanding. In single mode, 500 x 200 nm2 c-Si channel waveguides, we have achieved world-record 2.7 dB/cm and 0.7 dB/cm transmission loss coefficients for the TE mode and the TM mode, respectively.For deposited waveguides, bulk absorption loss is also important for both TE and TM modes.For PECVD a-Si, we adapt hydrogen passivation to reduce dangling bond density.(cont.) We also use a thin silicon nitride as the over cladding layer to help preserve H passivation and to reduce sidewall index contrast, acting as the graded index layer for a-Si waveguide core. We have accomplished the lowest reported loss coefficients in directly etched, single mode, 700 x 100 nm2 a-Si channel waveguides of 2.7 dB/cm for the TE mode, comparable to c-Si waveguide with similar dimensions. For the first time, damascene process has also been demonstrated as a promising process for a-Si waveguide fabrication. We have achieved a record-low loss of 2.5 dB/cm in 600 x 100 cm2 a-Si channel waveguides. Chemical-mechanical polishing (CMP) is the most critical step.For PECVD SiNxHy, we have previously identified that the absorption loss is due to the resonant absorption caused by N-H vibration. In this thesis, three different low temperature approaches have been developed and optimized to reduce NH concentration in as-deposited SiNxHY via (1) deposition chemistry; (2) post-deposition Ultraviolet light (UV) treatment; and (3) post-deposition, in-situ N2/Ar plasma treatment. All three processes are compatible with CMOS back-end processes, such as a-Si process. While changing deposition chemistry is the simplest method to obtain low NH containing SiNxHy, it comes with high SiH concentration and may have undesirable properties. Experimentally, for UV treatment, the highest H removal percentage is 60%; for plasma treatment, - 90%. UV treatment shows strong compositional dependence. The underlying mechanism of such dependence is identified and confirmed by Monte-Carlo modeling. Low loss and spectrally broadband optical couplers are indispensable optical components in an integrated photonic circuit. A high performance coupler should be capable of overcoming the mode-size mismatch, mode-shape mismatch, mode-position mismatch, and polarization mismatch, bridging different optical devices with minimal coupling loss. In this thesis, we have demonstrated a fiber-to-waveguide coupler based on asymmetric graded index taper and monolithically integrated cylindrical lens.(cont.) It is capable of transforming single mode light between single mode fiber and waveguides with minimal coupling loss of 0.45 dB between 1520 nm and 1630 nm. We have also demonstrated a vertical waveguide-to-waveguide coupler that is based on complementary inverse tapers. This design is tolerant of large refractive index mismatch between the two waveguides and also of any fabrication variation that would affect the effective indices of the two waveguides. We have achieved a minimal coupling loss of 0.25 dB per coupler and excellent broadband behavior is also demonstrated. Slot waveguides are a newly developed class of waveguides with unique optical properties. Slot waveguides can achieve exceptional high optical field in nanometer sized low index regions. In this thesis, we have demonstrated low loss transmission of 6 dB/cm for the fundamental slot mode in horizontal slot waveguides at 1550 nm. The horizontal slot configuration removes the constraints of thin slot definition by lithography and allows an arbitrarily thin slot to be fabricated via deposition or oxidation. Because the resulting interface is much smoother than the etched interface, the transmission loss in horizontal slot waveguides is much lower than in vertical slot waveguides. We also demonstrated that multiple slot configurations result in higher optical confinement compared to single slot configurations with the same slot thickness. The low loss and high optical confinement in the low index slot region realized in horizontal slot waveguides promises many useful applications, such as Er-doped silicon-based light emitters. For integration of slot waveguides with conventional channel waveguides, we have designed and simulated mode couplers and polarization rotators for slot-slot, slot-channel waveguide mode transformations.Athermal operation is important for realizing stable passive, WDM optical network on silicon. Athermal design of silicon waveguide systems uses advanced polymer cladding of large negative TO coefficient to provide compensation for the large positive TO coefficient in silicon. The reduced thermo-optic (TO) effect is experimentally demonstrated by reducing TO coefficient from 85 pm/K to 11 pm/K using polymer films.by Rong Sun.Ph.D

Subwavelength Surface Plasmons Based on Novel Structures and Metamaterials

With the rapid development of nanofabrication technology and powerful computational tools over the last decade, nanophotonics has enjoyed tremendous innovation and found wide applications in ultrahigh-speed data transmission, sensitive optical detection, manipulation of ultra-small objects, and visualization of nanoscale patterns. Surface plasmon-based photonics (or plasmonics) merges electronics and photonics at the nanoscale, creating the ability to combine the superior technical advantages of photonics and electronics on the same chip. Plasmonics focuses on the innovation of photonic devices by exploiting the optical property of metals. In particular, the oscillation of free electrons, when properly driven by electromagnetic waves, would form plasmon-polaritons in the vicinity of a metal surface and potentially result in extreme light confinement, which may beat the diffraction limit faced by conventional photonic devices and enable greatly enhanced light-matter interactions at the deep subwavelength scale. The objective of this dissertation is to develop subwavelength or deep subwavelength plasmonic waveguides and explore their integration on conventional dielectric platforms for multiple applications. Three novel structures (or mechanisms) are employed to develop and integrate nanoplasmonic waveguides; each consists of one part of the dissertation. The first part of this dissertation covers the design, fabrication, and demonstration of two-dimensional and three-dimensional metal-insulator-metal plasmonic couplers for mode transformation between photonic and nanoplasmonic domains on the silicon-on-insulator platform. In particular, deep subwavelength plasmonic modes under 100-nm are achieved via end-fire coupling and adiabatic mode transformation at telecom wavelengths. The second part studies metallic gratings as spoof plasmonic waveguides hosting deep subwavelength surface propagation modes. Metallic gratings under different dielectric coatings are numerically investigated for terahertz and gigahertz regions. The third part proposes, explores, and experimentally demonstrates the metametal for super surface wave excitation based on multilayered metal-insulator stacks, where the dispersion of the supported surface modes can be engineered by insulator dopant films in a given metal. The final part discusses the potential applications of active plasmonics for optical sensing, modulation and photovoltaics

High Efficiency Silicon Photonic Interconnects

Silicon photonic has provided an opportunity to enhance future processor speed by replacing copper interconnects with an on chip optical network. Although photonics are supposed to be efficient in terms of power consumption, speed, and bandwidth, the existing silicon photonic technologies involve problems limiting their efficiency. Examples of limitations to efficiency are transmission loss, coupling loss, modulation speed limited by electro-optical effect, large amount of energy required for thermal control of devices, and the bandwidth limit of existing optical routers. The objective of this dissertation is to investigate novel materials and methods to enhance the efficiency of silicon photonic devices. The first part of this dissertation covers the background, theory and design of on chip optical interconnects, specifically silicon photonic interconnects. The second part describes the work done to build a 300mm silicon photonic library, including its process flow, comprised of basic elements like electro-optical modulators, germanium detectors, Wavelength Division Multiplexing (WDM) interconnects, and a high efficiency grating coupler. The third part shows the works done to increase the efficiency of silicon photonic modulators, unitizing the χ(3) nonlinear effect of silicon nanocrystals to make DC Kerr effect electro-optical modulator, combining silicon with lithium niobate to make χ(2) electro-optical modulators on silicon, and increasing the efficiency of thermal control by incorporating micro-oven structures in electro-optical modulators. The fourth part introduces work done on dynamic optical interconnects including a broadband optical router, single photon level adiabatic wavelength conversion, and optical signal delay. The final part summarizes the work and talks about future development

SEMICONDUCTOR AND GLASS MICRO-RESONATORS

In this thesis we have demonstrated the cascading of two photonic AND logic gates by using two symmetric semiconductor GaAs microring resonators. In addition, we have developed a new, low-cost method for fabricating glass microring resonators. In the first part of this work, we discuss the properties of microring resonators and describe the fabrication of semiconductor microring resonators by the research team of which I was a member. In the experiments on cascaded logic gates we launched one probe and pump beam into different input waveguides, respectively. The first ring works as a AND logic gate for probe and pump beams. The output beam from the first ring goes to the second ring. The second ring also work as a AND logic gate using the second pump to switch the beam coming from the first ring. We successfully demonstrated cascading two photonic logic gates by using two symmetric semiconductor GaAs microrings. In the second part of this work, we extended our prior work on the fabrication of semiconductor microrings in a clean room to a purely mechanical method of glass microring fabrication. Many laboratories, including ours, lack the expensive facilities needed for the lithographic fabrication of microrings. And, a low-cost, high yield method of fabrication may have significant application in the development of disposable microring sensors. We have built up a complete mass=production capability based on glass capillary pulling and micro-polishing to fabricate glass microrings, because there were no available off-the-shelf systems available from industry at affordable prices. This method of producing highly polished glass micro-resonators has many advantages, such as fast fabrication (≦6 weeks), high yield (≧50%) (percentage of devices w/o cracks on edge), low cost (no need to use costly facilities in a clean room), mass production (800~1200 devices per batch). The surface quality of glass resonators should be excellent because capillaries were made at high temperature ≧1000℃ and devices were polished by suspension slurry of 70 nm colloidal silica. Further measurements that are beyond our current capability are needed for final verification. If some fabrication steps could be optimized in the future, we estimate that the fabrication time could be within 2 weeks, the yield rate would be higher than 90 %, and the number of devices per batch could be more than 1,200. This innovative method opens a new path for microresonator fabrication at low cost and in fast mass production. In sensor applications where low cost and mass production could be important, our work is an important first step to making microring sensors inexpensive, if further work in characterizing them can be done. Glass microresonators can play a key role, for example, in gas sensors, chemical sensors, liquid sensors, biological sensors, and vibration sensors. Two appendices in this thesis list the most significant sub-systems of the whole system we designed and built for producing glass microring resonators. Designs and engineering drawings are also listed in Appendices

Femtosecond Transient Bragg Gratings

Fiber Bragg gratings (FBGs) have found numerous applications in fiber lasers, sensors, telecommunication, and many other fields. Traditionally, they are fabricated using UV laser sources and a phase mask or other interferometric techniques. In the past two decades, FBGs have been fabricated with femtosecond lasers in either the point-by-point method or by using a phase mask, in a similar configuration as with UV laser sources. In the following, we briefly review the advantages of femtosecond fabrication of fiber Bragg gratings. We then focus on transient FBGs; these are FBGs that exist for a short duration only, for the purpose of all-optical, in-fiber switching and modulation and the possible mechanism to implement them with a high-power femtosecond laser. The theory behind transient grating switching is outlined, and we discuss related experimental results achieved by our group on both permanent grating inscription and the generation of transient (dynamic) fiber Braggs gratings

Sensing using Specialty Optical Fibers

Fiber optic based sensing is a growing field with many applications in civil and aerospace engineering, oil and gas industries, and particularly in harsh environments where electronics are not able to function. Optical fibers can be easily integrated into structures, are immune to electromagnetic interference, can be interrogated from remote distances, and can be multiplexed for distributed measurements. Because of these properties, specialty fiber designs and devices are being explored for sensing temperature, strain, pressure, curvature, refractive index, and more. Here we show a detailed analysis of a multicore fiber (MCF) for sensing, including its design and optimization in simulation, as well as experimental operation when used as sensor. The multicore fiber sensor\u27s performance as a function of temperature, strain, bending, and acoustic waves are all explored. The MCF sensors are shown to be able to withstand temperatures up to 1000°C, making them suitable to be harsh environment sensors. Additionally, a simple method for increasing the sensitivity of the MCF to longitudinal force is shown to multiple the sensitivity of the MCF sensor by a factor of seven. Also, a configuration for decoupling force and temperature will be presented. Finally, a developing all-fiber device, a photonic lantern, will be shown in conjunction with the MCF in order to increase sensitivity, add directional sensitivity, and lower the cost of the sensor interrogation for bending measurements. In addition to the multicore fiber, an analysis of anti-resonant hollow core fiber (ARHCF) is also presented. The fibers\u27 design-dependent propagation losses are explored, as well as their higher order mode content. Also, a potential application of an ARHCF for an in-fiber Raman air sensor is introduced, and the design optimization in simulation is shown

Towards Compact and High Speed Silicon Modulators

Los moduladores son elementos claves para la transmisión de la señal y el procesamiento de la información. Las técnicas de fabricación avanzadas "complementary metal-oxide semiconductor" (CMOS) permiten reducir drásticamente las dimensiones de estos dispositivos de interés para la implementación a gran escala en un chip de silicito a bajo coste. El trabajo realizado en esta tesis se centra en el diseño, la fabricación y la caracterización de estructuras de onda lenta con el objetivo de realizar moduladores compactos y eficientes integrados en un chip de silicio. El trabajo se divide en cuatro capítulos y un capítulo de conclusión y perspectivas. El capítulo uno introduce los fundamentos de física del estado sólido y de los mecanismos básicos de propagación guiada de la luz por reflexión total interna. El capítulo dos presenta los parámetros importantes de los moduladroes electro-ópticos así como un trabajo de recopilación de todos los mecanismos físicos que pueden ser empleados para modular la luz en silicio. Además, se presenta el estado del arte de los moduladores basados en silicio. El capítulo tres presenta el diseño , fabricación y caracterización de un modulador electro-óptico en silicio compacto y eficiente basado en el efecto de onda lenta en una estructura periódica unidimensional integrada, cuya geometría, similar a la de una red de Bragg, permite reducir la velocidad de grupo de un paquetes de ondas. Dicho efecto, se emplea para incrementar la interacción luz-materia y por lo tanto la eficiencia del modulador electro-óptico. El capítulo cuatro demuestra experimentalmente que dicha guía unidimensional periódica puede ser mejorada a fin de conseguir que el efecto de baja velocidad de grupo suceda en un rango mayor de longitudes de onda para posibles aplicaciones como la multiplexación por división de longitudinal de onda. En el capítulo cinco, se proporcionan conclusiones y perspectivas sobre el trabajo realizado.Brimont ., ACJ. (2011). Towards Compact and High Speed Silicon Modulators [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/14345Palanci