Electroabsorption modulators (EAM) have attract a lot of interests in high-speed optical communication due to low chirp, high-efficiency operation and the capability to be integrated with other semiconductor devices. Enhancing the operation by overcoming the trade-off between RC-limitation and high-speed performance, traveling-wave types of EAMs (TWEAM) have been documented to be a good candidate. In this thesis, a novel type of TWEAMs, namely undercut-etching-the-active-region type (UEAR), have been characterized and analyzed. There are two topics in this thesis, namely (1) one is to compare the performance of the UEAR and conventional ridge-waveguide (RW) types of TWEAM. The regime of D.C. to 40GHz small-signal modulation and 10 Gbits/sec large-signal data transmission are used to characterize and compare the performance of TWEAMs. In comparison with conventional ridge-waveguide (RW) TWEAM, 3dB lower optical-insertion-loss, at least 6dB higher in RF-link (D.C. to 40GHz) and faster electro-optical response (3dB bandwidth of 25GHz at 50£[-termination for UEAW and 15GHz for RW) are obtained in UEAW-TWEAM. Error-free 10Gbits/sec operation with high sensitivity of ¡V36.2 dBm and low swing voltage of 0.6V have been achieved in UEAW-TWEAM, a 3.2dB enhancement over RW-TWEAM, indicating the trade-off in designing electroabsorption modulators can be greatly released by novel type structure (UEAW). (2) The other topic is the application of TWEAM to all-optical wavelength converters. The generating photocurrent by optical absorption is the effect accompanying with the electrical-to-optical modulation in the EAM. Using the properties of cross-absorption and generating photocurrent, high-speed all-optical modulation can be potentially implemented in the application of EAMs. Long -waveguide high-speed TWEAMs can thus have high-extinction ratio performance in all-optical conversion. The conversion efficiency of -26dB and high extinction ration of >20dB are obtained in this experiment, showing the potential in the application of all-optical conversion.

Abtract Three different microstrip transmission-line layouts between the pads and an optical modulator waveguide have been designed and compared for their circuit response. In the first design, two 50£[ input and output microstrip transmission lines are connected to the center of the 24.58£[ semiconductor waveguide. In the second design, two 78£[ input and output microstrip transmission lines are connected to the center of waveguide. The third design represents a traveling-wave EA modulator. After optimizing the lengths of the transmission lines through our circuit simulation, we find that the traveling-wave optical modulator (the third layout) have the best circuit response of bandwidth ~22.4GHz and voltage reflection coefficient ~40%. The traveling-wave optical modulator not only overcomes the RC constant limit but also makes use of long waveguide to satisfied the high-speed requirement. At the same time, this circuit increases the extinction ratio and optical saturation power. In the fabrication process, we use polyimide to planarize both sides of the ridge waveguide and evaporate the top electrode on the polyimide. In addition, we use a special wet etching to form sloped edges for rasing ground pads up to the same level of the signal pad. This will allow us to use coplanar microwave probes to measure the devices.

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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.

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.

Multiple-quantum-well (M.Q.W.) and quantum-confined-stark-effect (Q.C.S.E.) have been widely used in designing and fabricating electroabsorption modulators. In this paper, material InAlAs/InGaAlAs near 1500nm transition is used to be our target for designing and fabricating EAM due to its high band-offset ratio (electron to hole) and the strong exciton effect. A calculation model for quantum well absorption has been developed to design EAM active region. Asymmetrically inserting a thin-spiked potential barrier into wide Q.W. structure, the Q.W. can have high efficiency of Q.C.S.E. without lowing the electron-hole wave function overlap integral, causing high electroabsorption coefficient and optical modulation. Tuning material composition (~-0.4% tensile strain ) is also used for polarization independence characteristics. Traveling-wave EAM based on InAlAs / InGaAlAs material system is also fabricated and measured. Polarization independence 2~5 dB operation, low voltage swing of 1V for 15 dB extinction ratio, high-speed electrical-to-optical response with ¡V3dB bandwidth of >20GHz at 50£[ termination have been achieved showing high potential in broad band fiber optical communication.

Semiconductor optical amplifiers (SOA) and electroabsorption modulators (EAM) have been become vital elements to obtain high-output-power and high-speed optical signal in the optical fiber communications. In this paper, we propose a novel type cascaded integrated SOAs and EAMs, which are monolithically integrated in the same chip without any regrowth. In the active region, high electron bandgap offset material, InGaAsP/InAlGaAs, is used in order to get high optical gain and also high modulation. Using cascaded SOAs and EAMs, high impedance of microwave stripe lines are the bridges connecting the small EAM elements, bring up higher impedance and thus enhancing the microwave transmission. The optical waveguide is made by selectively undercut etching InGaAsP/InAlGaAs material in order to reduce the optical scattering loss and also the microwave loss due to the low parasitic capacitance. The processing is described by the following steps: (1) ion implantation to get electrical isolation; (2) wet etching to form the optical waveguide ridge; (3) e-gun evaporation to get n- and p- metalization ; (4) spinning PMGI as planarization; (5)Final thick metalizations as for microwave transmission line. The final integrated cascaded SOAs and EAMs has been successfully fabricated and measured. In comparison with single EAM, higher than 10GHz of ¡V3dB electrical transmission has been found, indicating the cascaded integration structure has better impedance matching and also higher electrical transmission. The measured optical gain is higher than 5dB with 11dB/V modulation efficiency at excitation wavelength of 1568nm.