The electroabsorption modulators (EAM) play an important role in the optoelectronic integrated circuits. InGaAs/InAlAs is an excellent material system for fabricating 1.55-£gm EA modulators. Natural high band-offset ( ) ratio structure and the strong exciton effect make this kind of material a good candidate for high saturation power operation and high speed. In order to design good quantum-well (QW) for electroabsorption, the Vegard¡¦s law is used to obtain the parameters of In1-x-yGaxAlyAs material by interpolating the relevant binary semiconductor material. The bowling factor is included in finding the right bandgap. Using the Resonant Scattering Method, the QW energy levels and electron-hole overlap integrals can be obtained to calculate the optical electroabsorption effects. In this thesis, the TWEAM based on In1-x-yGaxAlyAs material is also fabricated. In order to get low parasitic capacitance for high-speed operation, a processing called undercut-etching the active region is developed in this work. A selective etching solution (citric acid : H2O2) is used to etch InGaAs layers from InAlAs and the undercut-etching structure InGaAlAs EAM has been successfully fabricated. The processing includes 1)optical waveguide formation with wet etching; 2)n-contact evaporation and contact annealing; 3)mesa etch, PMGI passivation/bridging/planarization; 4)final metallization, cleaving line formation, wafer lapping and device cleaving.

In this paper, a distributed Electroabsorption Modulator (EAM) monolithically integrated with Semiconductor Optical Amplifier (SOA) is analyzed. Using the distributed effects on the optical modulation and amplification, several advantages have been found in this work, such as high-speed modulation, microwave performance, low nose properties, and low chirp, which is quite fitted to the requirements of optical fiber communications. EAMs have been widely used due to high-speed, high extinction ratio, the compactness, and the capability of integration. However, due to the highly loaded capacitance in the waveguide, EAMs generally suffer from high microwave reflection and thus low modulation efficiency during high-speed modulation. By the distributed structure, SOA-integrated EAMs can not only enhance the impedance match by adopting distributed high impedance transmission line (HITL), but also offer optical gain. By the optical processing scheme of re-amplification and re-modulation, it also has been found that the extra amplified spontaneous emission (ASE) noise coming from SOA can be reduced to get lower noise figure (NF). Appling the saturation on SOA, the positive frequency chirp of EAM can be compensated to give overall low chirp. By the distributed structure, chirp compensation has been realized by this characterization. In this work, the distributed EAM-SOA scheme and the traditional single section EAM-SOA scheme are used for comparison, higher speed and lower NF are observed in distributed scheme. Due to impedance matching improvement in distributed scheme, a -3dB bandwidth of higher than 40GHz and 40Gbit/s data transmission is achieved, while a 15GHz of -3dB bandwidth is obtained in single device. Also, in 10Gbit/s data transmission, a 3dB lower of power penalty occurs in distributed scheme, while the lower NF is the mainly dominating mechanism.

This thesis proposes a novel structure to realize the monolithic integra-tion of optical spot-size converter (SSC) and high-speed electroabsorption modulator (EAM). The SSC is based on a scheme of coupled asymmetric waveguide fabricated by tapered undercut waveguide. Using a selectively undercut-etching-active-region (UEAR), the laterally tapered undercut ac-tive waveguide (LTUAWG) can be processed from a wide tapered ridge waveguide using in situ control to avoid submicron photolithography as well as complex processing, such as selective area growth, selective area etching and re-growth. By monolithically integrating EAM and SSC, the EAM waveguide width can be beneficial from scaling down the waveguide size for enhancing the EAM bandwidth, while the optical coupling loss from single mode fiber can still be kept low. In this finished SSC-integrated EAM, a 1-dB misalignment tolerance of ¡Ó2.9£gm (horizontal) and ¡Ó2.2 £gm (vertical) is obtained from SSC side, which is better than the results, ¡Ó1.9£gm (horizontal) and ¡Ó1.6£gm (vertical), from EAM side. The measured far-field angles for SSC and EAM are 6.0 (horizontal) ∗ 9.3 (vertical) and 11 (horizontal) ∗ 20 (vertical) respectively. As low as mode transfer loss of -1.6 dB is obtained in such SSC. All the simulation results are quite fitted with the experiment results, realizing the function of SSC by LTUAWG. The fabricated EAM waveguide width is 2.5 £gm, leading to over 40 GHz of -3-dB electrical-to-optical (EO) response. The high efficient SSC integrated with high-speed EAM suggests that the LTUAWG technique can have potential for applications in high-speed optoelectronic fields.

<p>The ever advance of CMOS digital circuit process leads tothe trend of digitizing an analog signal and performing digitalsignal processing as early as possible in a signal processingsystem, which in turn leads to an increasing requirement onanalog- to-digital converter (ADC). A wireless transceiver is asuch kind of signal processing system. Conventionaltransceivers manipulate (filter, amplify and mix) the signalmostly in analog domain. Since analog filters are difficult todesign onchip, the system integration level is low. Moderntransceivers shift many of these tasks to digital domain, wherethe filtering and channel selection can be realized moreaccurately and more compactly. However the price for the highintegration level is the critical requirement on the ADC,because the simplified analog part sends not only the weaksignal but also the unwanted strong neighboring channel to theADC. In order to digitize the needed signal in the presence ofstrong disturbances, a high dynamic-range and high-speed ADC isneeded.</p><p>Sigma Delta ADCs are promising candidates for A/D conversionin modern wireless transceivers. They are naturally suitablefor high-resolution narrow-band A/D conversions. With thedevelopment of processing and design techniques, sigma deltaADCs are expanding their applications to moderate-band area,such as wireless communication baseband processing. Currentlymobile communication systems are migrating from 2G to 3G. In 2Gsystems the baseband width is in the order of hundred kHz,while in 3G systems the baseband width is in the order of MHz.To face the challenge of designing a high resolution sigmadelta ADC with large bandwidth, a multi-bit internal quantizeris often used. In this thesis special design considerations onmulti-bit sigma delta modulators are discussed. The biggestdrawback of multi-bit sigma delta modulators isthe need of anextra circuit to attenuate or compensate the internal multi-bitDAC non-linearity. This thesis provides a comprehensiveanalysis of the solution which combines a multi-bit quantizerwith a 1-bit DAC in a sigma delta modulator. The theoreticalanalysis result is verified by measurement results. Anothertopic addressed in the thesis is how to reduce the multi-bitquantizer complexity. It is shown that by using a semiuniformquantizer, the quantizer can reduce its complexity by one-bityet still maintain the same modulator dynamic range. Theperformance of the semi-uniform quantizer is also verified bymeasurement results.</p>

We investigate the properties of integrated dielectric filters for the purposes of on-chip routing of photons. We started with the use of high quality factor tunable photonic crystal nanobeam cavities and moving on to examine a new class of reflection based reverse designed filters that maintain the footprint of a waveguide while allowing for arbitrary amplitude and phase response. / Engineering and Applied Sciences

Zugl.: Erlangen, Nürnberg, Univ., Diss., 2007

In this research an approach to design and implement all-optical delta-sigma modulator (ODSM) has been expanded. The two main blocks of this modulator are “leaky integrator” and “bi-stable switch” designed and implemented by using active element like semiconductor optical amplifier (SOA) and other passive elements like optical filter, isolator and coupler. All experiments are done on optical table and proper results achieved. Thus the new bi-stable switch is designed and implemented by using “inverted bistable switch” and “non-inverted bi-stable switch”. This switch is made by five ring lasers. Right wavelengths have chosen for each ring laser to achieve a novel characteristic called “Proteresis”. All control parameters of this switch was investigated The major impact of this research will be in the area communication system, which need high resolution and fast modulation speed with less noise in their systems.

Electro-optic (EO) polymers are an attractive alternative to inorganic nonlinear materials. EO polymers with a Pockel's coefficient, r33, greater than 320 pm/V have been recently demonstrated. In addition to their high EO activity, EO polymers have the additional benefit that their dielectric constants at optical and millimeter wave frequencies are closely matched which allow for bandwidths which are limited only by the resistive losses of traveling wave electrodes. The amorphous nature of the host polymer makes heterogeneous integration of the materials on any substrate possible. The devices which will have the most immediate impact based on these recent materials developments are EO waveguide modulators. Performance benchmarks of less than 6 dB insertion loss, sub-volt Vpi and greater than 100 GHz bandwidth have been achieved separately however, the challenge of achieving all of these benchmarks in a single device has not yet been met.The aim of this dissertation is to optimize passive materials to achieve efficient in device poling of EO polymers, optimize the chromophore loading of the active polymers and to optimize waveguide modulators for device performance within a particular system, analog RF photonic links. These optimizations were done by defining figures of merit for the materials and modulators. This research strategy has led to significant improvements in poling efficiency as well as modulators with record low insertion losses which maintain a low half-wave voltage; on the order of 1 - 2 Volts. Using this optimization strategy and state of the art EO polymers, devices which meet or surpass the benchmark performance values in all categories are expected in the near future.

No description available.

The sigma-delta modulator is a popular signal amplitude quantization error (or noise) shaper used in oversampling analogue-to-digital and digital-to-analogue converter systems. The shaping of the noise frequency spectrum is performed by feeding back the quantization errors through a time delay element filter and feedback loop in the circuit, and by the addition of a possible stochastic dither signal at the quantizer. The aim in audio systems is to limit audible noise and distortions in the reconverted analogue signal. The formulation of the sigma-delta modulator as a discrete dynamical system provides a useful framework for the mathematical analysis of such a complex nonlinear system, as well as a unifying basis from which to consider other systems, from pseudorandom number generators to stochastic resonance processes, that yield equivalent formulations. The study of chaos and other complementary aspects of internal dynamical behaviour in previous research has left important issues unresolved. Advancement of this study is naturally facilitated by the dynamical systems approach. In this thesis, the general order feedback/feedforward sigma-delta modulator with multi-bit quantizer (no overload) and general input, is modelled and studied mathematically as a dynamical system. This study employs pertinent topological methods and relationships, which follow centrally from the symmetry of the circle map interpretation of the error state space dynamcis. The main approach taken is to reduce the nonlinear system into local or special case linear systems. Systems of sufficient structure are shown to often possess structured random, or random-like behaviour. An adaptation of Devaney's definition of chaos is applied to the model, and an extensive investigation of the conditions under which the associated chaos conditions hold or do not hold is carried out. This seeks, in part, to address the unresolved research issues. Chaos is shown to hold if all zeros of the noise transfer function lie outside the unit circle of radius two, provided the input is either periodic or persistently random (mod delta). When the filter satisfies a certain continuity condition, the conditions for chaos are extended, and more clear cut classifications emerge. Other specific chaos classifications are established. A study of the statistical properties of the error in dithered quantizers and sigma-delta modulators is pursued using the same state space model. A general treatment of the steady state error probability distribution is introduced, and results for predicting uniform steady state errors under various conditions are found. The uniformity results are applied to RPDF dithered systems to give conditions for a steady state error variance of delta squared over six. Numerical simulations support predictions of the analysis for the first-order case with constant input. An analysis of conditions on the model to obtain bounded internal stability or instability is conducted. The overall investigation of this thesis provides a theoretical approach upon which to orient future work, and initial steps of inquiry that can be advanced more extensively in the future.