Abstract The high-speed performance of the lump-type electroabsorption modulator (EAM) is mainly limited by RC-effect. By taking advantage of the distributive effects, the traveling-wave structure can overcome the RC-lump effect. However, in order to enhance the limitation imposed by the conventional slow-waveguide type of traveling-wave structure, the speed of the device is still mainly restricted by the distributed capacitance of the waveguide. In this work, a novel type of traveling-wave-electroabsorption-modulator based on the undercut-etching the active region is successfully fabricated and measured. The methods of the processing adopted here is to lower the capacitance by chemical-wet-etching and two-time subsequent undercut etching on active region to further decrease the parasitic capacitance between P-type and N-type cladding layer. Also, the optical scattering loss may be reduced due the smooth sidewall of the waveguide from the wet etching. The whole processing shown in this thesis includes the lift-off technique by lithography, the metalization for n-, p- contacts (by thermal evaporator) and CPW microwave transmission (by e-beam evaporator), and PMGI-planarization. ¡V15dB optical transmission, ¡V6dB electrical transmission loss and >20GHz 3dB bandwidth of electrical-to-optical response at 50£[termination is measured on this kind of devices. It exhibits a high potential on the application of high-speed optical-fiber link in the future.

The work of this thesis includes designs, molecular beam epitaxy (MBE) growths and optical study of electro-absorption modulator (EAM) structures. Three EAM structures are designed near 1.5 um : symmetric, asymmetric multiple quantum wells (MQWs) of TE polarization, and polarization insensitive MQWs. For symmetric and asymmetric MQWs simulation of TE polarization, their red-shift are 31 nm and 50 nm, respectively, as the electric field decrease from -40 kV/cm to -120 kV/cm. For polarization insensitive MQWs, we use the strained quantum-well concept to achieve same transition energy and absorption. After growth by MBE system, the samples were fabricated in mesa type by photolithography and wet etching. For symmetric and asymmetric quantum wells of TE polarization¡Gthe red-shift are 16 nm and 49 nm, respectively, as the bias decrease form 0-1 volt to 0-6 volt. Because of small ¡µn near subband transition energy, these two samples exhibit small chirp parameter. However, the photoluminescence (PL) and photocurrent spectra of these two ones were not near 1.5 um and obvious absorption edge. The possible reason is that the molecular beam flux have changed during growth. For polarization insensitive MQWs, the PL spectra shows 1494 nm, which only 25.6 nm differ from our design. Also, the photocurrent spectra of TE and TM polarization nearly exhibit same transition energy and have small chirp parameter.

The thesis includes two aspects. The first part includes designs and optical study of electro-absorption modulator structures. Three structures are designed near 1.5

Electro¡VAbsorption Modulator has become a very important element in optical fiber communication due to its capability of integrating with other semiconductor devices. In order to get high-speed performance, the small size of waveguides is necessary. But it also brings to high coupling loss, resulting in low optical fiber link. In general, the waveguide mode is elliptical shape with sizes of 1¡Ñ2£gm to 1¡Ñ3£gm, which will definitely lead to high mode mismatch as adapted to conventional single¡Vmode optical fibers of 8£gm circular mode and cause 7~10 dB insertion loss[21]. Typically, micro lens, tapered fibers or taper optical waveguides are used to confine optical fiber mode to waveguide in order to reduce the insertion loss. In the thesis, we have developed a novel structure of tapered optical spot-size mode converter monolithically integrated with traveling-wave electro-absorption modulator (TWEAM) by using whole wet-etching processing. The optical waveguides are fabricated by wet-etching and subsequent selective undercut etching. By adjusting the wet-etching time, the waveguide core for TWEAM and the tapered spot-size mode converter can thus be engineered. The selective undercut wet etching not only can reduce the optical scattering loss, but also decrease the parasitic capacitance, leading to high optical and microwave transmission of TWEAM. Based on the model described in literature [4-8] and also Beam Propagation Method (BPM), the optical index of epi-layers is used to calculate the three¡Vdimension modal of optical mode and coupling efficiency. The microwave equivalent circuit is used to calculate and design device structure. In this thesis, the Spot¡VSize Converter monolithically with Traveling¡VWave Electro¡VAbsorption Modulator device is successfully fabricated and demonstrated. TWEAM integrated spot-size optical mode converter is measured and compared with single TWEAM (without converter) with optical wavelength of from 1550nm to 1570nm. The average optical insertion loss of about 4dB is found. The maximum extinction-ratio is about 21dB with modulation efficiency of 21dB/V, E-O response about 12GHz of ¡V3dB bandwidth at 50£[ termination is demonstrated.

Decades of efforts have pushed the replacement of electrical interconnects by optical links to the interconnects between computers, racks and circuit boards. It may be expected that optical solutions will further be used for inter-chip and intra-chip interconnects with potential benefits in bandwidth, capacity, delay, power consumption and crosstalk. Silicon integration is emerging to be the best candidate nowadays due to not only the dominant status of silicon in microelectronics but also the great advantages brought to the photonic integrated circuits (PICs). Regarding the recent breakthroughs concerning active devices on silicon substrate, the question left is no longer the feasibility of the optical interconnects based on silicon but the competitiveness of the silicon device compared with other alternatives. This thesis focuses on the study of two key components for the optical interconnects, both especially designed and fabricated for silicon platform. One is a high speed electroabsorption modulator (EAM), realized by transferring an InP-based segmented design to the hybrid silicon evanescent platform. The purpose here is to increase the speed of the silicon PICs to over 50  Gb/s or more. The other one is a high performance grating coupler, with the purpose to improve the optical interface between the silicon PICs and the outside fiber-based communication system. An general approach based on the transmission line analysis has been developed to evaluate the modulation response of an EAM with a lumped, traveling-wave, segmented or capacitively-loaded configuration. A genetic algorithm is used to optimize its configuration. This method has been applied to the design of the EAMs on hybrid silicon evanescent platform. Based on the comparison of various electrode design, segmented configuration is adopted for the target of a bandwidth over 40 GHz with as low as possible voltage and high extinction ratio. In addition to the common periodic analysis, the grating coupler is analyzed by the antenna theory assisted with an improved volume-current method, where the directionality of a grating coupler can be obtained analytically. In order to improve the performance of the grating coupler, a direct way is to address its shortcoming by e.g. increasing the coupling efficiency. For this reason, a nonuniform grating coupler with apodized grooves has been developed with a coupling efficiency of 64%, nearly a double of a standard one. Another way is to add more functionalities to the grating coupler. To do this, a polarization beam splitter (PBS) based on a bidirectional grating coupler has been proposed and experimentally demonstrated. An extinction ratio of around -20 dB, as well as a maximum coupling efficiency of over 50% for both polarizations, is achieved by such a PBS with a Bragg reflector underneath. / QC 20100906

Les formats de modulation avancés, codant l’information sur la phase, la polarisation ou plusieurs niveaux d’amplitude de la lumière reçoivent aujourd’hui un intérêt croissant. En effet, ceux-ci permettent d’atteindre une meilleure efficacité spectrale et par conséquent des débits plus élevés. Ces caractéristiques sont actuellement très recherchées dans les télécommunications pour répondre à la demande constante d’augmentation de capacité des transmissions optiques fibrées. L’essentiel du travail effectué porte sur la génération de tels signaux dans des sources photoniques monolithiques sur InP faisant appel à un concept nouveau de commutation de phases optiques préfixées avec des modulateurs électro-absorbants. Une comparaison de notre technologie intégrée avec la technologie actuelle de génération de formats de modulation avancés démontre des possibilités nouvelles de réduction de taille, de diminution de consommation énergétique et d’évolution en vitesse de modulation jusqu’à 56 GBauds. Suite à la validation, par simulations, d’architectures de transmetteurs spécifiques pour la génération de formats de modulation avancés, nous réalisons en salle blanche les circuits photoniques intégrés d’étude. Les caractérisations statiques confirment le fonctionnement de toutes les fonctions intégrées des circuits et soulignent l’efficacité de la filière technologique. Pour une première démonstration de fonctionnalité nous choisissons un transmetteur BPSK capable de générer une modulation de phase à 12,4 GB. Ce résultat démontre la plus petite source intégrée BPSK à l’heure actuelle. Un autre circuit capable de générer des formats de modulation plus complexes est aussi caractérisé / Advanced modulation formats, encoding data on the phase, polarization or multi-level intensity of the light are currently a hot topic in the telecommunication domain. By using them, high spectral efficiency and therefore higher bit rate signals could be generated. Those characteristics are really attractive for the telecommunication systems manufacturers in order to answer to the constant need of increased bandwidth in fiber optic communications. The study of advanced modulation formats generation in Photonic Integrated Circuits (PICs) based on a new concept of preset phases switching by Electro-Absorption Modulators is the main task of the current work. Compared to the actual technology used for generate advanced modulations, our choice could allow a strong reduction of the dimensions and of the energy consumption of the transmitter as well as bit rate up to 56 GB. After validating specific transmitters’ architectures by simulations, we fabricated the studied photonic integrated circuits in clean room. Through static characterizations, we verify that all integrated functions of the transmitters are working and we show the efficiency of our technological choices. Using the available equipments at the lab, we prove the validity of our concept of EAM based phase switching by using a BPSK transmitter. A 12.4 GB BPSK modulation is obtained as well as a wide open eye diagram. This result demonstrates the smallest BPSK integrated photonic source at this time. Another photonic circuit able to generate more complex modulation formats is also measured

According to the 2018 Ethernet Roadmap projections, the requirements for high speed links keep increasing every year, always keeping an eye on the energy per bit consumption of the communication system. The Ethernet requirements are estimated to reach 1Tbps by 2022-2025. Optical links are one of the most concrete solutions to satisfy bandwidth requirements at low energy consumption. An optical link is a communication system that consists of a single end-to-end optical circuit. In contrast with vertical-cavity surface-emitting laser (VCSEL) technology, which is based on a direct laser modulation, silicon photonics technology (SPT) is based on indirect modulation. In order to perform the modulation, electro-optical modulators are needed in the optical link system for electrically modulating the optical power.This master thesis, developed at imec, will present the modeling of two different technologies of optical modulators: Silicon Ring Modulator and the Franz-Keldysh ElectroAbsorption Modulator. The work was initiated, since there were no available models of these devices in the actual framework for link analysis. First a preliminary study of the physical principles of the two devices was performed, in order to build the Matlab models. Then, these models were fitted with measurements, in order to adjust them to real-life behavior. After having obtained two working models of the two modulators, an alreadyexisting framework was used, to compare the energy consumption per bit in the optical link. However, the results obtained with the simulation didn’t highlight a technology to be preferred to the other. / Enligt prognoserna från Ethernet Roadmap 2018 fortsätter kraven på höghastighetslänkar att öka varje år och håller alltid ett öga på kommunikationssystemets energi konsumtion per bit. Ethernet-kraven beräknas nå 1Tbps 2022-2025.Optiska länkar är en av de mest konkreta lösningarna för att tillgodose bandbreddskrav vid låg energiförbrukning. En optisk länk är ett kommunikationssystem som utgör en punkt till punkt förbunden optisk krets. I motsats till teknik som utnyttjar den vertikala kavitetsytemitterande lasern (VCSEL), som bygger på en direkt lasermodulering, baseras kisel fotonikteknik (SPT) på indirekt modulering. För att utföra en modulering behövs elektrooptiska modulatorer i det optiska länksystemet för att elektriskt modulera den optiska effekten.Denna masteruppsats, utvecklad vid imec, kommer att presentera modelleringen av två olika teknologier av optiska modulatorer: Silicon Ring Modulatorn och Franz-Keldysh Electro Absorption Modulator. Arbetet, genomfördes eftersom det inte fanns några tillgängliga modeller av dessa enheter i själva ramverket för länkanalys. För det första genomfördes en preliminär studie av de två enheternas fysikaliska principer för att bygga Matlab-modellerna. Sedan jämfördes dessa modeller med mätningar för att anpassa dem till verkligt beteende. Efter att ha fått två arbetsmodeller av de två modulatorerna användes ett redan existerande ramverk för att jämföra energikonsumtionen per bit i den optiska länken. De resultat som erhölls med simuleringen indikerade emellertid inte någon teknik som skulle föredras framför den andra.