Mode Coupling in Space-division Multiplexed Systems

Even though fiber-optic communication systems have been engineered to nearly approach the Shannon capacity limit, they still cannot meet the exponentially-growing bandwidth demand of the Internet. Space-division multiplexing (SDM) has attracted considerable attention in recent years due to its potential to address this capacity crunch. In SDM, the transmission channels support more than one spatial mode, each of which can provide the same capacity as a single-mode fiber. To make SDM practical, crosstalk among modes must be effectively managed. This dissertation presents three techniques for crosstalk management for SDM. In some cases such as intra-datacenter interconnects, even though mode crosstalk cannot be completely avoided, crosstalk among mode groups can be suppressed in properly-designed few-mode fibers to support mode group-multiplexed transmission. However, in most cases, mode coupling is unavoidable. In free-space optical (FSO) communication, mode coupling due to turbulence manifests as wavefront distortions. Since there is almost no modal dispersion in FSO, we demonstrate the use of few-mode pre-amplified receivers to mitigate the effect of turbulence without using adaptive optics. In fiber-optic communication, multi-mode fibers or long-haul few-mode fibers not only suffer from mode crosstalk but also large modal dispersion, which can only be compensated electronically using multiple-input-multiple-output (MIMO) digital signal processing (DSP). In this case, we take the counterintuitive approach of introducing strong mode coupling to reduce modal group delay and DSP complexity

VCSEL Techniques for Wavelength-Multiplexed Optical Interconnects

The majority of global data communication is taking place within data centers where data is stored and processed and where the largest part of the power used for global networking is consumed. With the rapidly increasing use of Internet-based applications and services, data centers are equipped with a larger number of servers and switches requiring higher bandwidth connectivity. Optical interconnects (OIs) are used to provide the connectivity needed. Short-reach OIs are dominated by 850 nm GaAs-based vertical-cavity surface-emitting lasers (VCSELs) due to their low fabrication cost, low power consumption, high modulation speed, and circular output beam. With the need for even higher bandwidth connectivity, large efforts have been invested in the development of VCSEL-based OIs offering higher aggregate capacity. Until now, higher capacity has been achieved mostly through an increase of the lane rate by higher speed VCSELs and higher order modulation formats. Furthermore, spatial division multiplexing (SDM), using parallel fibers or multicore fibers, has proven effective for increasing the aggregate capacity. With these techniques, it is expected that the OI capacity will saturate at the 1 Tbit/s level.Capacity beyond the limits of current technologies is expected by also exploring the wavelength dimension, referred to as wavelength division multiplexing (WDM). This calls for the development of high-speed VCSELs at multiple wavelengths. To also enable the very small footprint transceivers and high bandwidth density needed as transceivers move closer to the switch AISC, the multiple wavelength VCSELs should be in a monolithic array. This requires a VCSEL technology where the wavelength of individual VCSELs can be precisely set in a post-growth fabrication process. As an integration platform for multiplexing and fiber coupling we envision a photonic circuit on Si with Si3N4 waveguides and grating couplers for VCSEL integration. With such waveguides being single mode and the grating couplers being polarization sensitive, the VCSELs in the array should be single transverse and polarization mode, in addition to having a high modulation bandwidth.In this thesis, an intra-cavity phase tuning technique, based on an Ar ion-beam etching process with sub-nm precision, is demonstrated for setting the resonance wavelength of VCSEL resonators with <2 nm precision in the wavelength range 1040-1070 nm. Single transverse and polarization mode VCSELs with a record output power of 6 mW are also demonstrated. Suppression of higher order transverse modes and the orthogonal polarization state is achieved by etching a shallow mode filter in the surface of the VCSEL

VCSEL and Integration Techniques for Wavelength-Multiplexed Optical Interconnects

GaAs-based vertical-cavity surface-emitting lasers (VCSELs) are dominating short-reach optical interconnects (OIs) due to their high modulation speed, low power consumption, circular output beam and low fabrication cost. Such OIs provide the high bandwidth connectivity needed for interconnecting servers and switches in data centers. With the rapidly increasing use of Internet-based applications and services, higher bandwidth connectivity and higher aggregate capacity VCSEL-based OIs are needed. Until now, this has been achieved mostly through an increase of the lane rate by higher speed VCSELs and higher order modulation formats. Furthermore, spatial-division-multiplexing has proven effective for increasing the aggregate capacity. Much higher capacity can be achieved by multiple wavelengths per fiber, known as wavelength-divisionmultiplexing (WDM). Moreover, smaller footprint and higher bandwidth density WDM transceivers can be built using monolithic multi-wavelength VCSEL arrays with densely spaced VCSELs. This requires a VCSEL technology where the wavelength of individual VCSELs can be precisely set in a post-epitaxial growth fabrication process and a photonic integrated circuit (PIC) for multiplexing and fiber coupling. Flip-chip integration over grating couplers (GCs) is considered for interfacing VCSELs with waveguides on the PIC. In this thesis, an intra-cavity phase tuning technique is demonstrated for setting the resonance wavelength of VCSELs in a monolithic array with an accuracy in spacing of <1 nm. Uniform performance over the array is achieved by spectral matching and balancing of mirror reflectances, optical confinement factor and optical gain. Single transverse and polarization mode VCSELs, as required for flip-chip integration over GCs, with a record output power of 6 mW are also demonstrated.Finally, an investigation of angled flip-chip integration of a VCSEL over a GC on a silicon photonic integrated circuit (Si-PIC) is presented. Dependencies of coupling efficiency and optical feedback on flip-chip angle and size of the VCSEL die are studied using numerical FDTD simulations. Moreover, flip-chip integration of a VCSEL over a GC on a Si-PIC is experimentally demonstrated. The insertion loss from the VCSEL at the input GC to a singlemode fiber, multimode fiber or flip-chip integrated photodetector over the output GC was measured and quantified. The latter forms an on-PIC optical link

Multilevel Modulation and Transmission in VCSEL-based Short-range Fiber Optic Links

As the demand for ever higher throughput short-range optical links is growing, research and industry associations have shown increased interest in multilevel modulation formats, such as the four leveled pulse amplitude modulation, referred to as 4-PAM. As on-off keying (OOK) persists to be the choice for low latency applications, for example high performance computing, datacenter operators see 4-PAM as the next format to succeed current OOK-based optical interconnects. Throughput can be increased in many ways: parallel links can be deployed, multicore fibers can be used or more efficient modulation formats with digital signal processing is an alternative. Therefore, to improve link data rates, the introduction of new modulation formats and pre-emphasis are primarily considered in this thesis. In a bandwidth-limited link, turning towards spectrally efficient formats is one of the methods to\ua0 overcome the bandwidth requirements of OOK. Such are the considerations when opting for 3-PAM or 4-PAM schemes. Both require lower bandwidth than OOK and are potential candidates in such scenarios. 4-PAM provides double spectral efficiency and double data rate at the same symbol rate as on-off keying, but, as with any technology transition, new challenges emerge, such as a higher SNR requirement, a lower tolerance to VCSEL nonlinearities and skewing of the signal in the time domain. 3-PAM could potentially be an in-between solution, as it requires 33% less bandwidth than OOK and is less sensitive to VCSEL dynamics which could impair the transmission. A study is presented where 3-PAM has outperformed both OOK and 4-PAM in the same link. Detailed investigation of legacy 25G class VCSELs has shown that devices with moderate damping are suitable for the transition to 4-PAM. The pre-emphasis of signals is a powerful tool to increase link bandwidth at the cost of modulation amplitude. This has been investigated in this thesis for on-offkeying and has shown 9% and 27% increase in bit rate for error-free operation with two pre-emphasis approaches. Similarly, pre-emphasis of a 4-PAM electrical signals has enabled 71.8 Gbps transmission back-to-back with lightweight forward error correction and 94 Gbps net data rate was achieved with the same pre-emphasis and post-processing using an offline least-mean-square equalizer

Genetic algorithm for optimizing Bragg and hybrid metal-dielectric reflectors

Highly efficient reflectors are in demand in the rapidly developing optoelectronics. At the moment, distributed Bragg reflectors made of semiconductor materials are mainly used in this capacity. A lot of time and financial resources are spent on their production. Reducing the thickness of the reflector while maintaining its reflectivity would make these devices more affordable and extend their lifetime by reducing thermal noise. With the help of genetic optimization algorithms, the structures of multilayer semiconductor and combined metal-semiconductor reflectors were obtained, having a smaller thickness and equal optical characteristics than those of classical analogues. In particular, a 29% reduction in the thickness of the silicon/silica Bragg reflector was achieved without compromising performance.The work has been supported by the Russian Science Foundation 21-12-00304. This work is financially supported by the Government of the Russian Federation (The federal academic leadership program Priority 2030)

High performance photodetectors for multimode optical data links

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2006.Includes bibliographical references (p. 233-240).The majority of photodetectors presented in the literature, or available commercially, have dimensions on the order of 50 Ym or smaller, suitable for glass multimode or single mode fibre applications. The recent successful commercialisation of very large core diameter plastic optical fibre in systems based around 650 nm emitters, as well as the recent emergence of new polymer materials enabling relatively low loss at the more standard 780 nm and 850 nm wavelengths, has exposed the need for integrated photodetectors with dimensions well above 100 /m and capable of bitrates from 250 Mb/s for low-cost consumer applications to multiple Gb/s for high performance short reach interconnects. This size-performance regime has been largely ignored until now. This work examines interdigitated detector structures in multiple material systems by measurement and simulation. An optoelectronic frequency response measurement system was designed and implemented for this work, allowing measurement up to 8 GHz using 850 nm or 1550 nm sources. The full expression for frequency response of diffusion current under different illumination scenarios was derived, a topic normally omitted in the discussion of photodetectors, and applied to the analysis of device measurements.(cont.) Silicon detectors of various geometries were fabricated, with measured bandwidths at 5 V reverse bias up to 2 GHz for 200 ym diameter devices and 4 GHz for 50 and 100 ym diameter devices. The latter is the highest bandwidth reported for a silicon detector fabricated in a CMOS-compatible process and biased at a practically accessible voltage. Device performance was confirmed by simulation, and a novel structure is proposed featuring a buried junction on SOI determined by simulation to have twice as high a responsivity-bandwidth product as the best reported devices fabricated on high resistivity SOI. The silicon device structure was modified for epitaxial germanium wafers, and devices were fabricated. The germanium devices were simulated to determine the appropriate technology scaling direction and maximum device dimensions for desired performance specifications.by Wojciech Piotr Giziewicz.Ph.D

Optische Kurzstreckenverbindungen auf der Basis polymeroptischer Komponenten

Passive, optical components for short range data transmission are basic components for optical networks. As low cost material for optical waveguides plastic will be chosen. To enable high speed communication between CPUs in multiprocessor computers a component for multichannel datatransfer will be realised. For highly accurate positioning of the thin Plastic Optical Fibre (POF) in a two dimensional array established technologies are used. The achieved results and the integration in a demonstrator are described in details. To realise POF based optical networks starcouplers based on plastic are needed. Two ways for the realisation are examined: One is based on the lithography technique known from semiconductors and a second on etch technique. After electroplating a tool is used to transfer the structures with hot embossing. With an optical glue, fixing of a top foil and passive coupling of the POF the component is finished.Passive optische Bauteile für die Übermittlung von Daten über kurze Strecken stellen Basiskomponenten für optische Netzwerke dar. Um die Kosten gering zu halten werden in dieser Arbeit Lichtleiter auf Kunststoff-Basis verwendet. Zur schnellen Kommunikation zwischen Prozessoren in Multiprozessor Computern wird ein Bauteil für die vielkanalige Datenübermittlung realisiert. Um eine hochpräzise Positionierung der dünnen Plastik Optischen Faser (POF) in einem zweidimensionalen Raster zu ermöglichen, werden etablierte Technologien verwendet. Die erzielten Ergebnisse und die Integration in einen Demonstrator werden ausführlich dokumentiert. Zur Realisierung von optischen Netzwerken auf POF-Basis werden in Kunststoff realisierte Sternkoppler benötigt. Zwei Kopplerarten werden realisiert: Eine wird mit Hilfe der Lithographietechnik und eine zweite mit Ätztechniken erreicht. Nach galvanischer Abformung eines Stempels wird durch Heissprägen die Struktur in Plastik übertragen. Nach Verkleben und Aufbringen eines Deckels, sowie der passiven Ankopplung der POF, ist das Bauteil realisiert