Quantum dot (QD) has been known as three-dimensional quantum confined structure. Thus, a delta-function type of density with three-dimensional coulomb interaction can have strong dependence on field-driven optical absorption, i.e. Quantum Confine Stark Effect (QCSE), leading to lots of advantages for applications of electroabsorption modulator (EAM). In this work, based on a GaAs substrate, a self-assembly InAs quantum dot (QD) based p-i-n heterostructure is applied for fabricating electroabsorption modulator. The quantum dot electroabsorption modulation is fabricated by wet-etching technique, where the active region is formed by undercut wet-etching technique using selective etching solution (citric acid). In the device characterization, electro luminescence (EL) is first used to examine the optical transition of QD, showing 1280-1320 nm for ground state and 1220-1240 nm for the excite state. Using the photocurrent spectrum measurement, the red shift of 20 nm in photocurrent peaks from 0 V to 7 V is observed. Also, the peaks exhibit a quadratic relation against with bias, confirming QCSE effect of Q.D.. In the optical transmission measurement, 1300 nm light excites on a 300 £gm long device, obtaining 5 dB extinction by voltage swing of 7 V. By comparing with quantum well (QW) structure, the modulation efficient is in the same order of magnitude. However, the active region of QD volume is at least two orders less than QW, indicating strong QCSE can be obtained from QD and QD can have potential for high-efficient modulation. High-speed EO response with -3 dB bandwidth of 3.34 GHz is also obtained, where the main speed limitation is on the electrical isolation on the n-type GaAs substrate. Through optimizing Q.D. structure and also parasitic capacitance, Q.D. EAM can have a great potential for the application of high-speed optical modulation in optoelectronic fields.

A mode locked fiber-ring laser utilizing an optical spot-size converter (OSSC) integrated electroabsorption modulator (EAM) and Erbium-doped fiber amplifiers (EDFA) is demonstrated in this work. By taking advantage of OSSC, the EAMs not only have high-speed performance, but also have high tolerance alignment stability and high power handling capability due to the distributive effect. Thus, by the saturation absorbing and highly nonlinear transmission properties of such EAMs, it can be realized that short optical switching widow operations with high pulse energy excitation driven by EDFA can be used to hybrid mode locked ring laser application. The repetition rate of 10GHz in the mode locked fiber-ring laser is set by a RF synthesizer to driven EAM, creating short time gating widows. EDFA with an optical filter of bandwidth 1.2nm is used for optical gain. By adjusting the operation point in EAM to a bias of 1.72V and a RF-power of Vpp=2.2V, a nearly transform-limited autocorrelated Gauss pulse of 11.72ps (FWHM, the extracted pulsewidth is 8.28 ps) with average power level of 1.2mW is obtained. The time-bandwidth product is 0.478. The optical pulsewidths are mainly limited by the optical filter. Without any feedback circuit, pulse jitter of 993fs measured from a high-speed sampling scope (Agilent 86100A, bandwidth =30GHz) is found from the mode locked pulses, indicating the hybrid mode locked operation can be achieved by the EAM saturation absorbing properties. By measuring the photocurrent of EAM, the highest photocurrent occurs in the conditions of the highest optical transmission and also the shortest optical pulses, suggesting the saturation absorbing of EAM is the main mechanism dominating mode-lock operation. The mode locked operation with repetition rates of 10GHz to 20GHz are also obtained, indicating high-speed operation can be boosted from OSSC integrated EAM to hybrid-type mode locked fiber-ring laser.

Abstract In this thesis, we propose a new geometrical structure of waveguide for the application of traveling-wave electroabsorption modulator (TWEAM). As approaching to high-speed performance in TWEAM, low parasitic capacitance in the waveguide is necessary to get good microwave propagation properties. In this work, a novel processing called two-step undercut-etching the active region (UEAR) is developed to reduce the parasitic capacitance. First of all, Beam Propagation Method (BPM) is used to calculate this 2-D structure optical modes ensuring the guiding capability in such kind of waveguides. Based on an equivalent circuit model, the microwave propagation on different structures of waveguide is then investigated to decide the UEAR waveguide structure. By the selectively etching solution on InP/InGaAsP, the processing by two-step UEAR is developed to reduce the parasitic capacitance in the waveguide core. H3PO4:HCl is used to selectively etch P-InP layer on the top of InGaAsP M.Q.W. (multiple quantum wells, active region). H3PO4:H2O2:H2O is subsequently and selectively remove InGaAsP M.Q.W.s to define the waveguide core. This processing has been successfully developed. The electrical transmission measurement on this kind of TWEAM shows low reflection S11 of < -17.5dB and a low insertion loss S21 of < ¡V2.7dB from D.C. to 40GHz, indicating high microwave performance on such two-step UEAR waveguide can be achieved due to the low parasitic capacitance.

Ultra Wide Band (UWB) is a short-pulse signal which has extremely potential in wireless communication system due to the advantages of high data rate, better immunity to multipath fading, wide bandwidth, and high capability. According to the Federal Communications Commission (F.C.C.), UWB only can be transmitted in short distance of a few to tens of meters due to low power density (-41.3dBm/MHz). However, optical fiber has low loss and cost and wide bandwidth, so it can be achieved in wide area network. In this work, we propose a novel method to generate optical UWB doublet pulse without complicated setup. When electroabsorption modulator (EAM) integrating a taper optical directional coupler (TODC) was applied field, the transmission loss and coupling would change resulting from the absorption coefficient and effective index of active waveguide with the applied field. So, we used a single mode fiber collecting the power after the device, we could get the valley shape transfer function. Using the transfer function of electro-absorption modulator (EAM) integrating TODC we inject a Gaussian pulse into the EAM in the range of valley shape, and it can transform an electrical pulse into optical UWB doublet pulse by acceptable operating point. Therefore, the optical signals could be transmitted in optical fiber so that it can reduce loss of the electro-optic transformation. Experimentally, the full wave at half maximum of doublet pulse, 10dB bandwidth, fractional bandwidth were 75ps, 7.5GHz, 125%, respectively, and power density was less than -41.3dBm/MHz. These were all meeting the F.C.C. standard. In the future, we will use long distant optical fiber to transmitted UWB signal, and compare with different distance. Finally, we will check the UWB signal can be transmitted in optical fiber to achieve wide range signal transmission by bit error rate test.

[pt] No presente trabalho de tese, se faz uma avaliação de moduladores de amplitude baseados no efeito de electro-absorção. As estruturas usadas para a fabricação dos dispositivos foram estruturas com poços quânticos múltiplos de InAlAs/InGaAs e AlGaAs/GaAs. As estruturas de InAlAs/InGaAs foram projetadas para trabalhar na faixa comercial das telecomunicações (1.55 µm). Por isto a importância de aperfeiçoar os parâmetros de desempenho do dispositivo, tais como Stark shift, chirp, razão de contraste, perda por inserção, entre outros. Um estudo sistemático prévio destas estruturas foi realizado por Pires [Pires,1998]. Ele propõe variar a concentração de gálio na liga para produzir uma leve tensão na estrutura e modificar desta forma as propriedades ópticas do material. O estudo de [Pires,1998] propôs uma faixa de valores para variar a concentração de gálio (entre 46 por cento e 52 por cento) onde pode ser encontrada a melhor condição de operação do dispositivo. Cabe a esta tese aprofundar o estudo nesta faixa de valores, e decidir os parâmetros mais adequados para operação. No referente às estruturas de AlGaAs/GaAs, se toma como partida uma proposta teórica de [Batty et al, 1993], e estudada posteriormente por [Tribuzy, 2001], onde se sugere usar finas camadas de dopagem delta nos poços de GaAs para melhorar o deslocamento Stark em 87% para um campo aplicado de 40 kV/cm. O dispositivo foi desenhado e fabricado, obtendo-se um valor de 78 por cento para o mesmo campo aplicado, resultado relevante, pois é a verificação experimental de uma proposta teórica. / [en] In this thesis work, is made an evaluation of modulators of amplitude based in the electrum-absorption effect. The structures used for the devices were multiple quantum wells of InAlAs/InGaAs and AlGaAs/GaAs. The structures of InAlAs/InGaAs are used to work in the commercial band of the telecommunications (1,55 µm). This is the reason it is important to optimize the parameters of performance of the device, such as the Stark shift, chirp, contrast reason, insertion loss, etc. Previously, a systematic study of these structures was made by [Pires, 1998], the gallium concentration was varied to produce a strain in the structure and to modify the optic properties of the material. In the study of [Pires, 1998] considered the Gallium concentration was varied between 46 percent and 52 percent in which range the best condition to operate the device can be found. This is part of the work here presented. In this thesis this range of values was studied in more detail. For the structures of AlGaAs/GaAs, a theoretical proposal of [Batty et al, 1993] was experimentally investigated. It was suggested a nipi structure to use a fine delta doped in the GaAs wells, this delta doped will improve in 87 percent the Stark shift for a field of 40 KV/cm. The device was simulated and manufactured, obtaining a value of 78 percent for the same field applied, this is a excellent result, because this confirm the theoretical prediction.

Electroabsorption is the physical phenomenon by which the absorption of light in a medium can be controlled by applying an electric field. The Quantum–Confined Stark Effect, which makes the absorption band–edge in quantum wells very field–dependent, together with the strong absorption peak provided by excitons, are the physical foundations for the success of electroabsorption modulators based on quantum well structures in telecommunication networks. This thesis describes the design and fabrication of surface–normal electroabsorption modulation devices. The techniques needed to understand the design and fabrication of surface–normal multiple quantum well optical modulators are introduced, as are the various characterisation techniques used during and after the fabrication. Devices for several types of applications have been designed, fabricated, characterised and in some cases integrated into optical systems: – Two–dimensional arrays of 128´128 pixel amplitude modulators grown on GaAs substrates have been fabricated and characterised. Speeds of up to 11700 frames per second were demonstrated, limited by the output electronics of the computer interface. – Large–area modulators grown on GaAs substrates for free–space optical communication were developed, with an active area of 2cm2 and a modulation speed of several megahertz. Contrast ratios up to 5:1 on full modulator areas were measured. Problems limiting the yield and modulation speed of such devices have been studied, and solutions to overcome them have been demonstrated. – Large–area devices grown on InP substrates for free–space optical communication have been developed. Contrast ratios of up to 2:1 for transmissive types have been demonstrated. – Devices consisting of two rows of pixels, grown on GaAs substrates, with an active area of 22mm´5mm, divided into 64 or 128 pixels per row have been developed. These amplitude modulation devices were designed for optical signal processing applications. – One variant of these optical signal processing devices was also characterised as a ternary, binary amplitude and binary phase modulator array. – The use of GaAs multiple quantum well optical modulators in a free–space optical retro–communication system has been studied. An opto–mechanical design for a modulating retro–reflector is described, allowing a large field of view in one direction using reflecting, resonant–cavity modulators for high contrast ratios. / QC 20100802