<p>The integration of planar on-chip light sources is a bottleneck in the implementation of portable planar chip-scale photonic integrated sensing systems, integrated optical interconnects, and optical signal processing systems on platforms such as Silicon (Si) and Si-CMOS integrated circuits. A III/V on-chip laser source integrated onto Si needs to use standard semiconductor fabrication techniques, operate at low power, and enable efficient coupling to other devices on the Si platform.</p><p>In this thesis, thin film strain compensated InGaAs/GaAs single quantum well (SQW) separate confinement heterostructure (SCH) edge emitting lasers (EELs) have been implemented with patterning on both sides of the thin film laser under either growth or host substrate support, with the devices metal/metal bonded to Si and SiO₂/Si substrates. Gain and index guided lasers in various configurations fabricated using standard semiconductor manufacturing processes were simulated, fabricated, and experimentally characterized. Low threshold current densities in the range of 250 A/cm<super>2</super> were achieved. These are the lowest threshold current densities achieved for thin film single quantum well (SQW) lasers integrated on Si reported to date, and also the lowest reported, for thin film lasers operating in the 980 nm wavelength window.</p><p>These thin film EELs were also integrated with photolithographically patterned polymer (SU-8) waveguides on the same SiO₂/Si substrate. Coupling of the laser and waveguide was compared for the cases where an air gap existed between the thin film laser and the waveguide, and in which one facet of the thin film laser was embedded in the waveguide. The laser to waveguide coupling was improved by embedding the laser facet into the waveguide, and eliminating the air gap between the laser and the waveguide. Although the Fresnel reflectivity of the embedded facet was reduced by embedding the facet in the polymer waveguide, leading to a 27.2% increase in threshold current density for 800 &mum long lasers, the slope efficiency of the L-I curves was higher due to preferential power output from the front (now lower reflectivity) facet. In spite of this reduced mirror reflectivity, threshold current densities of 260 A/cm<super>2</super> were achieved for 1000 &mum long lasers. This passively aligned structure eliminates the need for precise placement and tight tolerances typically found in end-fire coupling configurations on separate substrates.</p> / Dissertation

The phenomenon of polarization interaction in certain nonlinear materials is presented, and the design of an all-optical logic device based on this concept is described. An efficient two-dimensional finite-difference time-domain code for studying third-order nonlinear optical phenomena is discussed, in which both the slowly varying and the rapidly varying components of the electromagnetic fields are considered. The algorithm solves the vector form Maxwell s equations for all field components and uses the nonlinear constitutive relation in matrix form as the equations required to describe the nonlinear system. The stability of the code is discussed and its accuracy demonstrated through the simulation of the self-phase modulation effect observed in Kerr media. Finally, the code is used to simulate polarization mixing in photonic crystal-based line defect and coupled resonator optical waveguides.

Arsenic trisulfide (As₂S₃) waveguide devices on lithium niobate substrates (LiNbO₃) provide a set of compact and versatile means for guiding and manipulating optical modes in infrared integrated optical circuits, including the integrated trace gas detection system. As a member of the chalcogenide glass family, As₂S₃ has many properties superior to other materials, such as high transparency up to 10 [mu]m, large refractive index and high nonlinear coefficient. At the wavelength of 4.8[mu]m, low-loss As₂S₃ waveguides are achieved: The propagation loss is 0.33 dB/cm; the coupling efficiency is estimated to be 81 %; and less than 3 dB loss is measured for a 90-degree bent waveguide of 250 [mu]m bending radius. They offer an ideal solution to the optical interconnection -- the fundamental element of an optical circuit. LiNbO₃ is a birefringent crystal that has long been studied as the substrate material. Titanium diffused waveguides in lithium niobate substrate (Ti: LiNbO₃) have excellent electro-optical properties, based on which, on-chip polarization converters are demonstrated. New benefits can be obtained by integrating As₂S₃ and Ti: LiNbO₃ to form a hybrid waveguide, which benefits from the high index contrast of As₂S₃ and the electro-optical properties of Ti: LiNbO₃ as well as its easy connection with commercial single mode fibers. For hybrid waveguides, the mode coupling is key. A taper coupler is preferred owing to its simplicity in design and fabrication. Although preliminary experiments have shown the feasibility of such integration, the underlying mechanism is not well understood and guidelines for design are lacking. Therefore, a simulation method is first developed and then applied to the taper coupler design. Devices based on taper couplers are then fabricated and characterized. The study reveals that in the presence of mode beating, it is not necessarily the longer taper that is the better coupling. There exists an optimum length for a taper with fixed width variation. A two-stage taper design can largely reduce the total length, e. g. by 64%, while keeping the coupling efficiency above 90%. According to the frequency domain analysis, these practical taper couplers work for a wavelength range instead of a single wavelength.

A rigorous power coupling model for three-core optical waveguides is proposed based on a full-wave vector boundary element method (VBEM). In addition to the influence of the state of the polarization (SOP) of the input light on the coupling behavior of the three-core optical waveguides can be simulated, the polarization dependent loss (PDL) of the three-core optical waveguides can also be investigated by combining the Mueller matrix method into the power coupling model. In this dissertation, the power coupling model is applied to investigate two kinds of power splitters. The first power splitters are constructed of step-index single mode fibers called triangular 3 3 fused tapered couplers. The influence of the SOP of the input light on the coupling behavior of the triangular 3 3 fused tapered couplers and the effect of fabricating parameters of the coupler, fusion degree, and heated length on the PDL of the coupler are investigated in this dissertation. The second kind of power splitters are constructed of photonic crystal fibers (PCFs). And, several fundamental coupling properties of three-core photonic crystal fibers (PCFs) with equilateral triangular cores are investigated numerically included coupling length, bandwidth, and polarization dependent loss (PDL). It is found the three-core PCFs are good candidate to be realized as an ultra-compact power splitter. And, for three-core PCFs that chose a proper coupling point can raise the yield and performance stability of the power splitter. In addition to the coupling behavior of the power splitters, two-dimensional photonic crystals (PCs) are also studied in this dissertation based on finite-difference time-domain (FDTD) method. The phase interference phenomenon due to the multiple plane-wave signals as initial conditions of the FDTD method for computing band structure of two-dimensional PCs is studied in this dissertation. It is found some normal modes supposed to exist could be lost if the phase interference is nearly out of phase at eigenfrequency. To overcome this problem, we proposed a new solving procedure based on FDTD algorithm which can avoid mode loss phenomenon and obtain complete normal modes over interested frequency range.

The communication system using Wavelength-division multiplexing (WDM) allows for better utilization of the spectral bandwidth. Photonic crystals (PhCs) exhibit photonic bandgap (PBG) due to the periodic variation of the dielectric constant and photons with a range of frequencies within the PBG cannot travel through the crystal. By introducing defects into PhCs, it is possible to control the light propagation along certain paths. In this thesis, the characteristics of coupled cavity waveguides (CCWs) and drop filter are discussed. Then we propose a multi-channel WDM system based on CCWs. It can be applied in FTTH to filter the wavelengths of 1310, 1490 and 1550 nm in different CCWs and also can make the bandwidth of output wavelength become narrow to filter more wavelengths. In addition, by modulating the size of the resonator on the PhCs, it can drop the particular wavelength into the waveguide. Finally, we proposed a multi-channel drop filter with FHWM 0.8 nm. This device design is leading the way to achieve CWDM specification with 100% drop efficiency, high quality factor and almost no crosstalk. The operations of such an ultra-compact demultiplexer and drop filter based on PhCs are suitable to be used in WDM optical communication systems.

The work described herein focuses on FT-IR and quantum cascade laser (QCL) based studies towards the development of compact and portable trace gas sensor for benzene, toluene, and xylenes (BTX). FT-IR broadband radiation was used to probe the mid-infrared fingerprint region for quantitatively detecting trace gas levels of BTX. Using direct absorption through a hollow waveguide, parts-per-million (ppm) detection limits for BTX with a response time of 39 seconds was demonstrated. Univariate calibration provided limits of detection (3σ) for benzene, toluene, and meta-xylene at 5, 17, and 11 ppm, respectively. Multivariate calibration using partial least squares regression algorithms were used to simulate real-world conditions with multiple analytes present within a complex sample. A calibration model was built with 110 training set standards enabled by using a customized gas mixing system. Furthermore, a preconcentration/thermal desorption (TD) step was added to the FT-IR HWG trace gas sensor enabling parts-per-billion detection of BTX. A univariate calibration was established in the laboratory with certified gas standards over a dynamic range of 1000 - 100 ppb for benzene, toluene, and the xylenes. The sensor was then taken to an industrial site during a field measurement campaign for the quantitative determination of BTX in field air samples. The laboratory calibration was used to predict unknown concentrations which were in close agreement with industrial hygiene standard techniques, and industrial prototype analyzers, that were simultaneously operated in the field environment. In addition to FT-IR, quantum cascade laser spectroscopy was also investigated due to enhanced spectral density and efforts to precisely overlap emission with analyte absorption. Particular efforts were dedicated on a novel principle for consistent and deliberate QCL emission wavelength selection by varying the QCL cavity length. These studies experimentally confirmed that using this straight-forward post-processing technique, emission wavelength tuning across a range of one hundred wavenumbers range may be achieved. This tuning range was experimentally demonstrated for a QCL emitting across an entire absorption feature of carbon dioxide by tailoring the length of the cavity. Additionally, using an external cavity (EC) - QCL combined with a HWG gas sensor module for the first time enabled the quantitative and simultaneous determination of ethyl chloride, trichloromethane, and dichloromethane within exponential dilution experiments at ppb limits of detection. Multianalyte detection was demonstrated utilizing partial least squares regression for quantitative discrimination of individual constituents within a mixture, yet applying a single broadly tunable QCL light source.

Photonic crystals have allowed unprecedented control of light and have allowed bringing new functionalities on chip. Photonic crystal waveguides (PCWs), which are linear defects in a photonic crystal, have unique features that distinguish these waveguides from other waveguides. The unique features include very large dispersion, existence of slow light, and the possibility of tailoring the dispersion properties for guiding light. In my research, I have overcome some of the challenges in using slow light in PCWs. In this work, I have demonstrated (i) high efficiency coupling of light into slow group velocity modes of a PCW, (ii) large bandwidth high transmission and low dispersion bends in PCWs, (iii) accurate modeling of pulse propagation in PCWs, (iv) high efficiency absorbing boundary conditions for dispersive slow group velocity modes of PCWs. To demonstrate the utility of slow light in designing high efficiency devices, I have demonstrated refractive index sensors using slow light in PCWs. In the end, a few high efficiency devices based on slow light in PCWs are mentioned. The remaining issues in the widespread use of PCW are also discussed in the last chapter.

Silicon based on-chip optical phased arrays are an enabling technology to achieving agile and compact large angle beam steering. In this work, a single layer array is presented, and approaches to multilayer 3D photonic integration for achieving a 2D array are also discussed. Finally, two dimensional optical beam steering is achieved using both thermo-optic and wavelength tuning. Various structures are considered as an alternative to the conventionally used shallow etched surface gratings to achieve narrow beam widths in the far field along with low switching power. The corrugated waveguide interspersed with 2D photonic crystal for crosstalk suppression is presented as a novel structure for coupling to free space that can provide lithographically defined index contrast in a single fabrication step, along with the smallest beam widths presented to date, at 0.25°. In addition, a polysilicon overlay with an oxide etch stop layer on top of a silicon waveguide is also presented as a grating coupler that achieves narrow far field beam widths. With this structure, two dimensional steering of 20° X 15° is demonstrated with a 16 element optical phased array, with a beam width of 1.2° X 0.4° and maximum power consumption of 20mW per channel. / text

A growing need for developing new multi-functional materials operating at microwave frequencies is demanding a better understanding of ferroelectric and ferrimagnetic materials and their combinations. Some of these materials exhibit tunable physical properties, giving an extra degree of freedom in the device design. New multifunctional ferroelectric and ferrimagnetic thin film structures are investigated in this dissertation research, in which dielectric and magnetic properties can separately be tuned over a certain frequency range. The materials of choice, Ba0.5Sr0.5TiO3 (BST) and BaFe12O19 (BaM), both well studied and used in many microwave applications, were prepared using rf magnetron sputtering and pulsed laser ablation. Thin-film bilayers, multilayers and composite thin films were grown on various substrates, and their underlying microstructure and crystallographic properties were analyzed and optimized. After identifying the most successful growth conditions,dielectric and magnetic properties were measured. Unusual features in magnetic hysteresis loops in both sputtered and laser ablated films grown under different conditions were observed. Microcircuits were fabricated using optical lithography and microwave properties and tunability were tested in the range 1-65 GHz.

Bandwidth demands in global telecommunication infrastructures continue to rise and new optical techniques are needed to deal with massive data flows. Generating high bandwidth signals (> 40 GHz) using conventional modulation techniques is hindered by material limitations and fabrication complexities. Similarly, controlling such high bandwidths in both the temporal and spectral domain becomes more problematic using conventional electronic processes. Advances in electro-optic organic materials, fibers/micro-fluidics integration, and nonlinear optics have significant potential for higher bandwidth modulation and temporal/spectral control. The work presented in this dissertation demonstrates the use of various nonlinear optical effects in new photonic device and system designs towards the generation and manipulation of highspeed optical pulses. First, an all fiber-based system utilizing an integrated carbon disulfide-filled liquidcore optical fiber (i-LCOF) and co-propagating pulses of comparable temporal lengths is presented. The slow light effect was observed in 1-meter of i-LCOF, where 18 ps pulses were delayed up to 34 ps through the use of stimulated Raman scattering. Delays greater than a pulse width indicate a potential application as an ultrafast controllable delay line for time division multiplexing in multi-Gb/s telecommunication systems. Similarly, an optically tunable frequency shift was observed using this system. Pulses experienced a full spectral bandwidth shift at low peak pump powers when utilizing the Raman-induced frequency shift and slow light effects. Numerical simulations of the pulse-propagation equations agree well with the observed shifts. Included in our simulations are the contributions of both the Raman cross-frequency shift and slow light effects to the overall frequency shift. These results make the system suitable for numerous applications including low power wavelength converters. Second, a silica/electro-optic (EO) polymer phase modulator with an embedded bowtie antenna is proposed for use as a microwave radiation receiver. The detection of high-frequency electromagnetic fields has been heavily studied for wireless data transfer. Recently there has been growing interest in the field of microwave photonics. We present the design and optimization of a waveguide with an EO polymer core and silica/sol-gel cladding. The effect of electrodes on the insertion losses and poling efficiency are also analyzed, and conditions for low-loss and high poling efficiency are established. Experimental results for a fabricated device with microwave-response between 10 - 14 GHz are presented. Finally, we present the design for a fast optical switch incorporating silicon as the passive waveguide structure and EO polymer as the active material. The design uses a simple directional coupler with coplanar electrodes and promises to have low cross-talk and high switching speed (on the order of nanoseconds). An initial design for a 1x2 switch is fabricated and tested, and future optimization processes are also presented.