Semiconductor Electroabosortion Modualtor (EAM) has become an important element in optical fiber communications because of its capability to integrate with other semiconductor devices, high-speed and low driving voltage. However, high optical insertion loss and low tolerance in optical power coupling are main general problems to be solved in order to get high electro-optical (EO) efficiency. Monolithically integrating EAM with optical spot-size converter (SSC) can lead to high-efficiency single-mode fiber coupling, but the price is on the complex fabrication methods. In this paper, based on previous work, the selective undercut etching active region (UEAR) and the whole wet-etching techniques are employed to fabricate the integration of laterally tapered SSC and EAM. Also, by applying the ion-implantation in SSC region, the reliable transfer efficiency and also high-speed performance are obtained based on the high resistance and low parasitic capacitance in SSC. The active region containing 10 strain compensated multiple-quantum-wells (MQWs) sandwiched by n-InP (bottom) and p-InP (top) for the electroabsorption region of EAM and also the top region of lateral tapered SSC. The converted waveguide in SSC consists of alternating InGaAsP and InP layers. An HBr-base etching solution is first used to define the top p-cladding with the widths of from 6um to 8um. An H2O2-base solution is then utilized to selectively undercut-etch the MQWs from InP material. The active waveguide p-cladding in EAM is set as 8um. After defining EAM and SSC, the converted waveguide is fabricated by aligning the top SSC and then wet-etched. By using an e-beam evaporator, Ti/Pt/Au and Ni/AuGe/Ni/Au are deposited as p- and n-type metallization, respectively. PMGI is spun serving as the passivation, planarization and bridging. The microwave coplanar waveguide (CPW) line is finally defined by depositing Ti/Au for microwave load- and feed- lines and connecting EAM. The length of SSC is 350um. The Spot-Size Converter monolithically integrated with Electroabsortion Modulator using whole wet-etching technique is demonstrated. ¡V12.5dB of fiber-to-fiber insertion loss and 10dB (TE) 10dB(TM) extinction ration in 1V(1570nm excitation) is obtained in this device. Using Fabry-Perot method, the average optical transfer loss in SSC is extracted to be 2dB, quite consistent with simulation results. By applying ion-implantation on SSC, the broadband EO performance 45GHz of ¡V3dB bandwidth is achieved for 100um long device due to the low capacitance and the high resistivity in SSC.

We present the fabrication of InGaAsP/InP buried heterostructure spot-size converter lasers. In the lateral conversion, we use photolithography to make tapered ridge waveguides. In the vertical conversion, we use a pair of step-index passive waveguides, namely guard waveguides (GWs), in the two sides of the step-index active waveguide region to increase optical-field profile. In order to decrease leakage current, we use a p-n-p current blocking structure by MOCVD regrowth. From numerical simulations, the far-field divergence is 21x21. The step-index GW structure shows an internal efficiency of 63%. However, the BH lasers did not lase from our fabrication processes. From the I-V characteristics, a large leakage current has bypassed through the blocking structure. The reason may relate to the high background doping concentration of our MOCVD growth.

Abstract Optical spot-size converter (SSC) is an essential element in the opto-electronic integrated circuit because of its direct coupling to single-mode optical fiber, low-cost from the misalignment issue in package. By the tapered waveguide structure, SSC offers a capability to transfer an elliptical optical mode of optical waveguide to single-mode fiber matched mode, allowing the independence of device design from the coupling issue. However, the conversion efficiency of SSC is strongly reliant on tapered waveguide structure, material index, and also excitation wavelengths, restricting design and fabrication of SSC. In order to enhance the reliability on SSC fabrication, a voltage-controllable SSC integrated with electroabsorption modulator (EAM) is designed and fabricated. The tapered waveguide processing is based on the selective undercut wet-etching on InGaAsP-material system. The active waveguide width of EAM is 3.5£gm, where the tapered waveguide widths is ranged from 1.7£gm to 3.5£gm. The transferred optical waveguide (passive waveguide) is 8£gm wide. By adjusting the voltage of tapered waveguide, coupling efficiency is enhanced by 2dB from 1560nm to 1570nm. A reliable SSC-integrated EAM with -12dB of insertion loss, extinction ratio of 22dB is demonstrated. By extracting the index change from Fabry-Perot optical mode of waveguide, index variation from reverse bias is in the order of 10-3, consistent with the calculation based on Kramer-Kronig model.

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.

Abstract We use InGaAlAs and InGaAsP as materials of 1.55mm multi-quantum-well spot-size converter ridge waveguide lasers. On lateral conversion, we fabricate a taper ridge waveguide. On vertical conversion, we add guard layers on each side of active layer. For InGaAlAs ridge waveguide lasers, simulation results show a far field 16o ¡Ñ 27o¡]lateral ¡Ñ vertical¡^at guard layer width S = 0.1 mm with 300-150-50 mm narrow-tapered waveguide structure. Due to large Zn background contamination in the MOCVD growth chamber, we did not fabricate the InGaAlAs lasers successfully. For the InGaAsP ridge waveguide lasers, we measure a far field 18o ¡Ñ 28o and a threshold current 23 mA for the 200-250-50 mm narrow-tapered waveguide structure; a far field 20o ¡Ñ 26o and a threshold current 22 mA for the 200-250-50 mm wide-tapered waveguide structure.

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.

High efficient optical spot size converter (SSC) is one of the most important building blocks for dense optical interconnection network and high-speed optical fiber communications due to efficient optical power transfer between different optical modules. Using tapered optical direction coupler (TODC) as SSC can reduce the dimension with high efficiency because of resonant condition and tapered structure. However, the strong dependence of operation on wavelength leads to narrow band operation, reducing the usage of wavelength division multiplexer (WDM) technique for upgrading optical data capacity through optical spectrum. In this work, based on multi- resonant points of TODC, a broadband SSC integrated with optical electroabsorption modulator (EAM) is proposed, designed, and fabricated. By tapering quantum well of the top active waveguide (AW), the integration with bottom passive waveguide (PW) can form a TODC. With the tapered structure, the gradually varied effective index forms a resonant point along wave propagation, inducing strong coupling, collecting optical power after resonant point, and thus leading to high efficient coupling. With multi-section of resonant conditions and also tapered structure, broadband operation can be realized. Through 3 section of tapered AW of TODC, the calculated coupling efficiency from bottom PW with larger than 70% is found for the regime of 1530nm~1550nm wavelength. The EAM-integrated SSC is also fabricated. With counter direction of optical coupling, the measured photocurrent in EAM shows a broadband of flap coupling from 1570nm~1585nm is observed, suggesting the multi-section TODC can bring out broadband operation.