Research and design of high-speed advanced analogue front-ends for fibre-optic transmission systems

In the last decade, we have witnessed the emergence of large, warehouse-scale data centres which have enabled new internet-based software applications such as cloud computing, search engines, social media, e-government etc. Such data centres consist of large collections of servers interconnected using short-reach (reach up to a few hundred meters) optical interconnect. Today, transceivers for these applications achieve up to 100Gb/s by multiplexing 10x 10Gb/s or 4x 25Gb/s channels. In the near future however, data centre operators have expressed a need for optical links which can support 400Gb/s up to 1Tb/s. The crucial challenge is to achieve this in the same footprint (same transceiver module) and with similar power consumption as today’s technology. Straightforward scaling of the currently used space or wavelength division multiplexing may be difficult to achieve: indeed a 1Tb/s transceiver would require integration of 40 VCSELs (vertical cavity surface emitting laser diode, widely used for short‐reach optical interconnect), 40 photodiodes and the electronics operating at 25Gb/s in the same module as today’s 100Gb/s transceiver. Pushing the bit rate on such links beyond today’s commercially available 100Gb/s/fibre will require new generations of VCSELs and their driver and receiver electronics. This work looks into a number of state‐of-the-art technologies and investigates their performance restraints and recommends different set of designs, specifically targeting multilevel modulation formats. Several methods to extend the bandwidth using deep submicron (65nm and 28nm) CMOS technology are explored in this work, while also maintaining a focus upon reducing power consumption and chip area. The techniques used were pre-emphasis in rising and falling edges of the signal and bandwidth extensions by inductive peaking and different local feedback techniques. These techniques have been applied to a transmitter and receiver developed for advanced modulation formats such as PAM-4 (4 level pulse amplitude modulation). Such modulation format can increase the throughput per individual channel, which helps to overcome the challenges mentioned above to realize 400Gb/s to 1Tb/s transceivers.

Voltage properties of optically injected long wavelength VCSELs: Theory and experiment

Future high speed communications networks will transmit data predominantly over optical fibres. As consumer and enterprise computing will remain the domain of electronics, the electro-optical conversion will get pushed further downstream towards the end user. Consequently, efficient tools are needed for this conversion and due to many potential advantages, including low cost and high output powers, long wavelength Vertical Cavity Surface Emitting Lasers (VCSELs) are a viable option. Drawbacks, such as broader linewidths than competing options, can be mitigated through the use of additional techniques such as Optical Injection Locking (OIL) which can require significant expertise and expensive equipment. This thesis addresses these issues by removing some of the experimental barriers to achieving performance increases via remote OIL. Firstly, numerical simulations of the phase and the photon and carrier numbers of an OIL semiconductor laser allowed the classification of the stable locking phase limits into three distinct groups. The frequency detuning of constant phase values (ø) was considered, in particular ø = 0 where the modulation response parameters were shown to be independent of the linewidth enhancement factor, α. A new method to estimate α and the coupling rate in a single experiment was formulated. Secondly, a novel technique to remotely determine the locked state of a VCSEL based on voltage variations of 2mV−30mV during detuned injection has been developed which can identify oscillatory and locked states. 2D & 3D maps of voltage, optical and electrical spectra illustrate corresponding behaviours. Finally, the use of directly modulated VCSELs as light sources for passive optical networks was investigated by successful transmission of data at 10 Gbit/s over 40km of single mode fibre (SMF) using cost effective electronic dispersion compensation to mitigate errors due to wavelength chirp. A widely tuneable MEMS-VCSEL was established as a good candidate for an externally modulated colourless source after a record error free transmission at 10 Gbit/s over 50km of SMF across a 30nm single mode tuning range. The ability to remotely set the emission wavelength using the novel methods developed in this thesis was demonstrated.

Thermal, mechanical and optical modelling of VCSEL packaging

Hybrid OECB (Opto-Electrical Circuit Boards) are expected to make a significant impact in the telecomm switches arena within the next five years, creating optical backplanes with high speed point-to-point optical interconnects. OECB's incorporate short range optical interconnects, and are based on VCSEL (Vertical Cavity Surface Emitting Diode) and PD (Photo Diode) pairs, connected to each other via embedded waveguides in the OECB. The VCSEL device is flip-chip assembled onto an organic substrate with embedded optical waveguides. The performance of the VCSEL device is governed by the thermal, mechanical and optical characteristics of this assembly. During operation, the VCSEL device will heat up and the thermal change together with the CTE mismatch in the materials, will result in potential misalignment between the VCSEL apertures and the waveguide openings in the substrate. Any degree of misalignment will affect the optical performance of the package. This paper will present results from a highly coupled modelling analysis involving thermal, mechanical and optical models. The paper will also present results from an optimisation analysis based on Design of Experiments (DOE).

Polymer waveguide and VCSEL array multi-physics modelling for OECB based optical backplanes [opto-electrical circuit boards]

The deployment of OECBs (opto-electrical circuit boards) is expected to make a significant impact in the telecomm switches arena within the next five years. This will create optical backplanes with high speed point-to-point optical interconnects. The crucial aspect in the manufacturing process of the optical backplane is the successful coupling between VCSEL (vertical cavity surface emitting laser) device and embedded waveguide in the OECB. The results from a thermo-mechanical analysis are being used in a purely optical model, which solves optical energy and attenuation from the VCSEL aperture into, and then through, the waveguide. Results from the modelling are being investigated using DOE analysis to identify packaging parameters that minimise misalignment. This is achieved via a specialist optimisation software package. Results from the thermomechanical and optical models are discussed as are experimental results from the DOE.

VCSEL to waveguide coupling for optical backplanes

Hybrid OECB (Opto-Electrical Circuit Boards) are expected to make a significant impact in the telecomm switches arena within the next five years, creating optical backplanes with high speed point-to-point optical interconnects. The critical aspect in the manufacture of the optical backplane is the successful coupling between VCSEL (Vertical Cavity Surface Emitting Laser) device and embedded waveguide in the OECB. Optical performance will be affected by CTE mismatch in the material properties, and manufacturing tolerances. This paper will discuss results from a multidisciplinary research project involving both experimentation and modelling. Key process parameters are being investigated using Design of Experiments and Finite Element Modelling. Simulations have been undertaken that predict the temperature in the VCSEL during normal operation, and the subsequent misalignment that this imposes. The results from the thermomechanical analysis are being used with optimisation software and the experimental DOE (Design of Experiments) to identify packaging parameters that minimise misalignment. These results are also imported into an optical model which solves optical energy and attenuation from the VCSEL aperture into, and then through, the waveguide. Results from the thermomechanical and optical models will be discussed as will the experimental results from the DOE.

Review of the technology and reliability issues arising as optical interconnects migrate onto the circuit board

Light has the greatest information carrying potential of all the perceivable interconnect mediums; consequently, optical fiber interconnects rapidly replaced copper in telecommunications networks, providing bandwidth capacity far in excess of its predecessors. As a result the modern telecommunications infrastructure has evolved into a global mesh of optical networks with VCSEL’s (Vertical Cavity Surface Emitting Lasers) dominating the short-link markets, predominately due to their low-cost. This cost benefit of VCSELs has allowed optical interconnects to again replace bandwidth limited copper as bottlenecks appear on VSR (Very Short Reach) interconnects between co-located equipment inside the CO (Central-Office). Spurred by the successful deployment in the VSR domain and in response to both intra-board backplane applications and inter-board requirements to extend the bandwidth between IC’s (Integrated Circuits), current research is migrating optical links toward board level USR (Ultra Short Reach) interconnects. Whilst reconfigurable Free Space Optical Interconnect (FSOI) are an option, they are complicated by precise line-of-sight alignment conditions hence benefits exist in developing guided wave technologies, which have been classified into three generations. First and second generation technologies are based upon optical fibers and are both capable of providing a suitable platform for intra-board applications. However, to allow component assembly, an integral requirement for inter-board applications, 3rd generation Opto-Electrical Circuit Boards (OECB’s) containing embedded waveguides are desirable. Currently, the greatest challenge preventing the deployment of OECB’s is achieving the out-of-plane coupling to SMT devices. With the most suitable low-cost platform being to integrate the optics into the OECB manufacturing process, several research avenues are being explored although none to date have demonstrated sufficient coupling performance. Once in place, the OECB assemblies will generate new reliability issues such as assembly configurations, manufacturing tolerances, and hermetic requirements that will also require development before total off-chip photonic interconnection can truly be achieved

Low-Cost Micromechanically Tunable Optical Devices: Strained Resonator Engineering, Technological Implementation and Characterization

The rapid growth of the optical communication branches and the enormous demand for more bandwidth require novel networks such as dense wavelength division multiplexing (DWDM). These networks enable higher bitrate transmission using the existing optical fibers. Micromechanically tunable optical microcavity devices like VCSELs, Fabry-Pérot filters and photodetectors are core components of these novel DWDM systems. Several air-gap based tunable devices were successfully implemented in the last years. Even though these concepts are very promising, two main disadvantages are still remaining. On the one hand, the high fabrication and integration cost and on the other hand the undesired adverse buckling of the suspended membranes. This thesis addresses these two problems and consists of two main parts: • PECVD dielectric material investigation and stress control resulting in membranes shape engineering. • Implementation and characterization of novel tunable optical devices with tailored shapes of the suspended membranes. For this purposes, low-cost PECVD technology is investigated and developed in detail. The macro- and microstress of silicon nitride and silicon dioxide are controlled over a wide range. Furthermore, the effect of stress on the optical and mechanical properties of the suspended membranes and on the microcavities is evaluated. Various membrane shapes (concave, convex and planar) with several radii of curvature are fabricated. Using this resonator shape engineering, microcavity devices such as non tunable and tunable Fabry-Pérot filters, VCSELs and PIN photodetectors are succesfully implemented. The fabricated Fabry-Pérot filters cover a spectral range of over 200nm and show resonance linewidths down to 1.5nm. By varying the stress distribution across the vertical direction within a DBR, the shape and the radius of curvature of the top membrane are explicitely tailored. By adjusting the incoming light beam waist to the curvature, the fundamental resonant mode is supported and the higher order ones are suppressed. For instance, a tunable VCSEL with 26 nm tuning range, 400µW maximal output power, 47nm free spectral range and over 57dB side mode suppresion ratio (SMSR) is demonstrated. Other technologies, such as introducing light emitting organic materials in microcavities are also investigated.

Development of hybrid UV VCSEL with organic active material and dielectric DBR mirrors for medical, sensoric and data storage applications

Lasers play an important role for medical, sensoric and data storage devices. This thesis is focused on design, technology development, fabrication and characterization of hybrid ultraviolet Vertical-Cavity Surface-Emitting Lasers (UV VCSEL) with organic laser-active material and inorganic distributed Bragg reflectors (DBR). Multilayer structures with different layer thicknesses, refractive indices and absorption coefficients of the inorganic materials were studied using theoretical model calculations. During the simulations the structure parameters such as materials and thicknesses have been varied. This procedure was repeated several times during the design optimization process including also the feedback from technology and characterization. Two types of VCSEL devices were investigated. The first is an index coupled structure consisting of bottom and top DBR dielectric mirrors. In the space in between them is the cavity, which includes active region and defines the spectral gain profile. In this configuration the maximum electrical field is concentrated in the cavity and can destroy the chemical structure of the active material. The second type of laser is a so called complex coupled VCSEL. In this structure the active material is placed not only in the cavity but also in parts of the DBR structure. The simulations show that such a distribution of the active material reduces the required pumping power for reaching lasing threshold. High efficiency is achieved by substituting the dielectric material with high refractive index for the periods closer to the cavity. The inorganic materials for the DBR mirrors have been deposited by Plasma- Enhanced Chemical Vapor Deposition (PECVD) and Dual Ion Beam Sputtering (DIBS) machines. Extended optimizations of the technological processes have been performed. All the processes are carried out in a clean room Class 1 and Class 10000. The optical properties and the thicknesses of the layers are measured in-situ by spectroscopic ellipsometry and spectroscopic reflectometry. The surface roughness is analyzed by atomic force microscopy (AFM) and images of the devices are taken with scanning electron microscope (SEM). The silicon dioxide (SiO2) and silicon nitride (Si3N4) layers deposited by the PECVD machine show defects of the material structure and have higher absorption in the ultra violet range compared to ion beam deposition (IBD). This results in low reflectivity of the DBR mirrors and also reduces the optical properties of the VCSEL devices. However PECVD has the advantage that the stress in the layers can be tuned and compensated, in contrast to IBD at the moment. A sputtering machine Ionsys 1000 produced by Roth&Rau company, is used for the deposition of silicon dioxide (SiO2), silicon nitride (Si3N4), aluminum oxide (Al2O3) and zirconium dioxide (ZrO2). The chamber is equipped with main (sputter) and assisted ion sources. The dielectric materials were optimized by introducing additional oxygen and nitrogen into the chamber. DBR mirrors with different material combinations were deposited. The measured optical properties of the fabricated multilayer structures show an excellent agreement with the results of theoretical model calculations. The layers deposited by puttering show high compressive stress. As an active region a novel organic material with spiro-linked molecules is used. Two different materials have been evaporated by utilizing a dye evaporation machine in the clean room of the department Makromolekulare Chemie und Molekulare Materialien (mmCmm). The Spiro-Octopus-1 organic material has a maximum emission at the wavelength λemission = 395 nm and the Spiro-Pphenal has a maximum emission at the wavelength λemission = 418 nm. Both of them have high refractive index and can be combined with low refractive index materials like silicon dioxide (SiO2). The sputtering method shows excellent optical quality of the deposited materials and high reflection of the multilayer structures. The bottom DBR mirrors for all VCSEL devices were deposited by the DIBS machine, whereas the top DBR mirror deposited either by PECVD or by combination of PECVD and DIBS. The fabricated VCSEL structures were optically pumped by nitrogen laser at wavelength λpumping = 337 nm. The emission was measured by spectrometer. A radiation of the VCSEL structure at wavelength 392 nm and 420 nm is observed.

Preparação e caracterização óptica de filmes nanocristalinos de GaAs:H depositados por RF magnetron sputtering

Pós-graduação em Ciência e Tecnologia de Materiais - FC

A new class of bragg stacks and its principle application

This work is focused on the development of high quality nanoporous 1D photonic crystals –so called Bragg stacks – made by spin-coating of approximately 25 nm large SiO2 and TiO2 nanoparticles bearing interparticle voids large enough to infiltrate reactive species. Therefore, the first part of this work describes the synthesis of well-dispersed TiO2 nanoparticles in this size range (the corresponding SiO2 nanoparticles are commercially available). In the second part, a protocol was developed to prepare nanoporous Bragg stacks of up to 12 bilayers with high quality and precision. Tailor-made Bragg stacks were prepared for different applications such as (i) a surface emitting feedback laser with a FWHM of only 6 nm and (ii) an electrochromic device with absorption reversibly switchable by an external electrical bias independently of the Bragg reflection. In the last chapter, the approach to 1D photonic crystals is transferred to 1D phononic crystals. Contrast in the modulus is achieved by spin-coating SiO2 and PMMA as high and low moduli material. This system showed a band gap of fg = 12.6 GHz with a width of Dfg/fg = 4.5 GHz.

Pulse shortening of gain switched single mode semiconductor lasers using a variable delay interferometer

We propose a pulse shaping and shortening technique for pulses generated from gain switched single mode semiconductor lasers, based on a Mach Zehnder interferometer with variable delay. The spectral and temporal characteristics of the pulses obtained with the proposed technique are investigated with numerical simulations. Experiments are performed with a Distributed Feedback laser and a Vertical Cavity Surface Emitting Laser, emitting at 1.5 µm, obtaining pulse duration reduction of 25-30%. The main asset of the proposed technique is that it can be applied to different devices and pulses, taking advantage of the flexibility of the gain switching technique.

High-frequency signal generation using 1550 nm VCSEL subject to two-frequency optical injection

We experimentally investigate high-frequency microwave signal generation using a 1550 nm single-mode VCSEL subject to two-frequency optical injection. We first consider a situation in which the injected signals come from two similar VCSELs. The polarization of the injected light is parallel to that of the injected VCSEL. We obtain that the VCSEL can be locked to one of the injected signals, but the observed microwave signal is originated by beating at the photodetector. In a second situation we consider injected signals that come from two external cavity tunable lasers with a significant increase of the injected power with respect to the VCSEL-by-VCSEL injection case. The polarization of the injected light is orthogonal to that of the free-running slave VCSEL. We show that in this case it is possible to generate a microwave signal inside the VCSEL cavity. © (2013) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Analysis of the effect of transverse modes on free-space optical interconnect performance

Vertical-cavity surface-emitting lasers (VCSELs) and microlenses can be used to implement free space optical interconnects (FSOIs) which do not suffer from the bandwidth limitations inherent in metallic interconnects. A comprehensive link equation describing the effects of both optical and electrical noise is introduced. We have evaluated FSOI performance by examining the following metrics: the space-bandwidth product (SBP), describing the density of channels and aggregate bandwidth that can be achieved, and the carrier-to-noise ratio (CNR), which represents the relative strength of the carrier signal. The mode expansion method (MEM) was used to account for the primary cause of optical noise: laser beam diffraction. While the literature commonly assumes an ideal single-mode laser beam, we consider the experimentally determined multimodal structure of a VCSEL beam in our calculations. It was found that maximum achievable interconnect length and density for a given CNR was significantly reduced when the higher order transverse modes were present in Simulations. However, the Simulations demonstrate that free-space optical interconnects are still a suitable solution for the communications bottleneck, despite the adverse effects introduced by transverse modes.

Lasers - An effective artificial source of radiation for the cultivation of anoxygenic photosynthetic bacteria

The laser diode (LD) is a unique light source that can efficiently produce all radiant energy within the narrow wavelength range used most effectively by a photosynthetic microorganism. We have investigated the use of a single type of LID for the cultivation of the well-studied anoxygenic photosynthetic bacterium, Rhodobacter capsulatus (Rb. capsulatus). An array of vertical-cavity surface-emitting lasers (VCSELs) was driven with a current of 25 mA, and delivered radiation at 860 nm with 0.4 nm linewidth. The emitted light was found to be a suitable source of radiant energy for the cultivation of Rb. capsulatus. The dependence of growth rate on incident irradiance was quantified. Despite the unusual nearly monochromatic light source used in these experiments, no significant changes in the pigment composition and in the distribution of bacteriochlorophyll between LHII and LHI-RC were detected in bacterial cells transferred from incandescent light to laser light. We were also able to show that to achieve a given growth rate in a light-limited culture, the VCSEL required only 30% of the electricity needed by an incandescent bulb, which is of great significance for the potential use of laser-devices in biotechnological applications and photobioreactor construction. (c) 2006 Wiley Periodicals, Inc.

Distance measurement using the change in junction voltage across a laser diode due to the self-mixing effect

Conventional detection scheme for self-mixing sensors uses an integrated photodiode within the laser package to monitor the self mixing signal. This arrangement can be simplified by directly obtaining the self-mixing signals across the laser diode itself and omitting the photodiode. This work reports on a Vertical-Cavity Surface-Emitting Laser (VCSEL) based selfmixing sensor using the laser junction voltage to obtain the selfmixing signal. We show that the same information can be obtained with only minor changes to the extraction circuitry leading to potential cost saving with reductions in component costs and complexity and significant increase in bandwidth favoring high speed modulation. Experiments using both photo current and voltage detection were carried out and the results obtained show good agreement with the theory.