Lukas Chrostowski

Silicon photonic biosensors show great potential for applications in medical diagnostics and healthcare services. Near-infrared transparency and high refractive index of silicon allow us to build compact and efficient circuits leveraging CMOS foundries, which provide low-cost mass production and enable the integration of the optoelectronic components on the same chip. Although silicon photonic biosensors have proven performances close to today's gold standard diagnostics, many applications still require higher multiplexing, as well as more sensitive, reliable and quantitative measurements. This dissertation is based on theoretical and experimental studies of silicon photonic sensing architectures in terms of sensor performance improvement and unit-cost reduction.Specifically, two novel sub-wavelength grating-based (SWG) waveguide configurations are presented to improve the sensitivity. Leveraging the advantage of SWG metamaterials, the substrate-overetch (SOE) and multi-box SWG devices present a largely extended modal size and surface contact area, which gives 10-time enhanced sensitivity compared to the conventional devices. In addition, by employing the Bragg grating as the sensing architecture, the multi-box SWG-based grating configuration achieves a lower detection limit compared to the microring resonator (MRR) counterpart and demonstrates the capability for monitoring small molecule interactions.Replacing the laser with a broadband source can provide a lower-cost solution for the optical system. Therefore, two cost-effective broadband light source-based sensing implementations are proposed and demonstrated with acceptable sensitivities. The first implementation uses cascaded MRRs for index monitoring, where the analyte variation is converted to the photocurrent change as the readout. The second implementation uses a phase-shifted Bragg grating-based symmetrical Mach-Zehnder interferometer, where the analyte variation maps the intensity change at the resonant wavelength. Furthermore, a system-level integration of active silicon photonic sensors using Fan-Out Wafer-Level-Packaging (FOWLP) is proposed in the dissertation, which can reduce the die size down to 1 mm² while simplifying the microfluidic and optical integration. Leveraging the CMOS foundries and the proposed FOWLP technique, the unit cost of each packaged sensing die can be reduced to several dollars.

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Integrating photonics with state-of-the-art nanoelectronics in Silicon (Si) is key to enabling new computing paradigms and sensing applications, as it leverages the well-established complementary metal-oxide-semiconductor (CMOS) foundries used to manufacture the electronics chips at a large-scale with low-cost. Towards this goal, great efforts have been made to integrate all the fundamental photonic building blocks on Si. However, due to a number of challenges, there has been no demonstration of a complete fully-integrated silicon photonic (SiP) chip. This dissertation addresses some of the challenges that hold back the deployment of complete fully-integrated Si chips.Due to Si’s temperature dependency, the performance of ring-based filters, switches, and modulators degrade when the surrounding temperature fluctuates. Second, fabrication imperfections lead to a discrepancy between the designed and measured ring-based filters’, switches’, and modulators’ spectral responses. Third, because of Si’s reciprocal lattice, Si cannot be used to realize optical isolators, which are required to integrate lasers on Si as they block back-reflections from flowing back to the laser and destabilizing its operation.This dissertation addresses the aforementioned challenges as follows. By slightly doping the Si waveguides, defect states are introduced which enable sensing and manipulating light in Si waveguides while absorbing minimal optical power. These doped waveguides are introduced into ring-based filters and switches to correct for fabrication errors and demonstrate the tuning of the largest yet most compact ring-based 16×16 optical switch matrix and 14-ring coupled-resonator optical waveguide (CROW) filter. Second, a new design of a microring modulator (MRM) is demonstrated that allows correcting the spectral features (wavelength, bandwidth (BW) and/or extinction ratio (ER)) of fabricated MRMs and maintain the MRM’s free-spectral range (FSR). Third, a new method for measuring propagation losses in optical waveguides is demonstrated. Finally, a stable quantum well (QW) distributed feedback (DFB) laser without an isolator is demonstrated for the first time. Instead of depositing Si-incompatible magneto-optic (MO) materials, a reflection-cancellation circuit (RCC) is proposed and used to demonstrate laser stability against varying levels of back-reflections in real-time. The same circuit was used to further reduce the linewidth of the DFB laser down to 3 kHz.

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Integrated Bragg gratings (IBGs) developed on the silicon-on-insulator (SOI) platform, owning to their high spectral flexibility, have become key components in photonic integrated circuits. Despite the rapid development of silicon IBG devices, there still lacks a comprehensive design methodology to achieve arbitrary, sophisticated, complex (amplitude and phase) spectral responses on IBGs. In addition, various problems also exist in practical designs and implementations of IBGs. The objectives of this thesis are to address these issues and, thus, to facilitate and improve the spectral tailoring of silicon IBG devices.A comprehensive and sophisticated design methodology of IBGs to achieve arbitrary spectral responses has been developed, and each individual step of the design and implementation process has been elaborated in detail. Furthermore, to address the IBG modeling and apodization issues existing in the design process, we have proposed (1) a highly efficient and reliable IBG modeling method by directly synthesizing the physical structure of the gratings; and (2) a high-performance apodization technique for IBGs based on periodic phase modulation.Multichannel photonic Hilbert transformers (MPHTs) based on complex synthesized IBGs have been designed, fabricated, and experimentally characterized. The realizations of these MPHTs are based on using the comprehensive IBG design methodology developed in this thesis. MPHTs with a total wavelength channel number of up to 9 and a single channel bandwidth of up to 625 GHz have been successfully achieved. The impacts of apodization phase errors (APE) on the spectral responses of apodized silicon IBGs have been characterized. The characterization results show that APE can largely distort the spectral responses of apodized IBGs from the designed ones. Then, to address this issue, a methodology to compensate and thus to eliminate APE of an apodized IBG to correct the distorted response has been developed and experimentally validated. A novel apodization profile amplification technique for IBGs has been proposed. Using this k(z) amplification technique for designing IBGs can bring about significant improvements in the apodization performence for the given fabrication constraints. Therefore, this technique can largely overcome the current apodization limitations of silicon IBGs due to fabrication constraints, thus facilitating their spectral tailoring applications.

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No abstract available.

Microring resonators (MRRs) on silicon photonic platforms allow for low-power, dense, and large-scale manipulation of optical signals on-chip. MRR-based modulators, switches, and filters have become key building blocks in integrated optical circuits for applications in future data communications, high-performance computing, and sensing. This thesis presents solutions for overcoming several challenges towards practical deployment of MRR systems. The performance of MRR is highly susceptible to temperature and fabrication variations, which cause significant shifts in the MRR's spectral responses. In-resonator photoconductive heaters (IRPHs), formed by doping MRRs’ waveguides show high responsivities. As IRPHs do not require additional material depositions, photodetectors, or power taps and use the same contact pads for both sense and tune operations, they can be used to automatically tune and temperature stabilize MRRs without compromising the cost or area of the devices. Automatic tuning and stabilization of one- and two-ring filters are demonstrated.Multi-ring filters offer attractive spectral features such as wide pass-bands, steep roll-offs, and large extinction ratios. Using IRPHS, automatic tuning of a four-ring Vernier ring filter across a record 37.6 nm wavelength and wavelength locking to account for a record 65 degrees temperature variation is demonstrated. A tuning algorithm in which the number of iterations scales linearly with the number of coupled rings in the system is presented. As this method typically does not rely on the output spectral shape of the filter, it is applicable to a wider range of coupled resonator systems. Application of this tuning method is then demonstrated for various multi-ring filters by both simulation and experiment. Crosstalk can be a major source of signal degradation in large-scale MRR systems. Interchannel and intrachannel crosstalk of one- and two-ring MRR filters are experimentally investigated. The power penalties due to interchannel crosstalk are presented as functions of channel spacing and adjacent channel isolation. Intrachannel crosstalk of one-ring, cascaded, and series-coupled add-drop filters are compared and spectral conditions that will ensure low intrachannel crosstalk is presented. MRR filters with extremely small radii of 2.75 um, large free spectral ranges of 34.3 nm, and high thermal tuning efficiencies of 2.78 nm/mW are presented.

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Optical switches are used for signal switching in optical communication networks. Silicon photonics is a low-cost and mature technology to develop high-performance optical switches. This thesis is a theoretical and experimental study on silicon photonic switches, featuring broadband, low-power, high-speed, and low-crosstalk performance.Broadband 3-dB couplers are fundamental building blocks for broadband switches based on Mach-Zehnder interferometer (MZI) structures. A broadband 3-dB coupler, which has a 100 nm operation bandwidth with coupling imbalance being much less than its competitors, i.e., adiabatic couplers and multimode interference couplers, has been theoretically designed and experimentally demonstrated.Switches using thermo-optic phase tuning typically have high power consumption. In this thesis, two methods to improve the tuning efficiency of thermo-optic phase shifters have been investigated and employed: 1) using thermal isolation structures and 2) using folded waveguides structures. Accordingly, thermo-optic switches with state-of-the-art, ultra-low power consumption of down to 50μW/π have been demonstrated.MZI switches using carrier injection phase tuning have high-speed performance but with a large switching crosstalk, due to the imbalanced tuning loss in the MZI structure. A novel carrier injection switch based on a balanced nested Mach-Zehnder interferometer (BNMZI) structure has been theoretically proposed. The BNMZI switch has balanced tuning schemes and therefore can be both high-speed and crosstalk-free. Besides, the switch has three switching states: cross, bar, and blocking.Polarization control is necessary for single-mode switches. A high-performance polarization beamsplitter (PBS), which has a 120 nm operation bandwidth with modal isolations of more than 20 dB, has been designed and demonstrated, and it can be used for polarization control for single-mode switches.Characterizing fabrication variability and performing yield prediction for photonic integrated circuits (PICs) are both challenging for photonics designers. We have developed an accurate and cost-efficient characterization method for fabrication variations, which extracts waveguide dimension variations from the spectral response of a single racetrack resonator. In addition, we have proposed a novel yield prediction method for PICs, which, for the first time in silicon photonics, is able to model the impacts of layout-dependent correlated manufacturing variations and take them into account in circuit simulations.

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This dissertation is a theoretical and experimental study of sub-wavelength grating (SWG) based photonic devices for the silicon-on-insulator (SOI) platform, including high-efficiency sub-wavelength grating couplers (SWGCs), broadband SWGCs, broadband SWG directional couplers, and an SWG polarization splitter-rotator. High-efficiency SWGCs with improved operating bandwidths and sup- pressed back reflections have been demonstrated to couple light into and out of SOI based photonic integrated circuits (PICs). One-dimensional SWGs have been proposed and experimentally demonstrated for the first time to make fully-etched grating couplers, which have performances comparable to the state-of-the-art fully-etched grating couplers, but with better fabrication tolerance, reduced fabrication complexity, and less cost. A theoretical study of the operating bandwidths for grating couplers has been presented and a design methodology has been demonstrated for designing SWGCs with design-intent operating bandwidths. SWGCs with 1-dB bandwidths up to 90 nm have been demonstrated, which have improved the operating bandwidth of fully-etched grating couplers by a factor of 3. Such broadband SWGCs are essential components for applications such as wavelength-division multiplexing (WDM) PICs and bio-sensing. Compact directional couplers, with dimensions about 10 times smaller than its alternatives, i.e., adiabatic couplers and multimode interference couplers have been demonstrated for various power splitting ratios. The operating bandwidths of our directional couplers have been improved by a factor of 2 as compared to conventional directional couplers. The dispersion properties of SWGs have been explored and applied to engineer the wave- length dependancy of conventional directional couplers for broad operating bandwidths, which is the first experimental demonstration of such devices. Polarization splitter and polarization rotators are essential components to address the polarization diversity of PICs. An ultra-compact mode- coupling based polarization splitter-rotator (PSR), which combines functionalities of a polarization splitter and a polarization rotator, with dimensions 15-20 times smaller than its alternative, i.e., mode-evolution based PSRs, has been experimentally demonstrated for the first time, where an asymmetric waveguide system consisting of a strip waveguide and an SWG waveguide were used to improve the fabrication tolerance of such devices. A measured peak polarization conversion efficiency of −0.3 dB with crosstalks below −10 dB over the C-band has been achieved.

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Cell-based screening of cancer treatments is used early in the drug development process to test the efficacy and toxicity of treatment candidates prior to animal and human testing. Current cell-based screening methods offer limited predictive capacity, contributing to the high percentage of drugs that fail during the clinical trial stage (80-95% for cancer treatments). One shortcoming of traditional cell-based screening platforms is their inability to recreate many aspects of the natural environment of tumour cells, which can affect treatment response. One important aspect of the microenvironment that can affect cell behaviour and treatment response is oxygenation. Irregular blood vessel formation can cause tumour oxygenation to be much lower than that of surrounding tissue, and spatial and temporal variations in oxygen can be present. Temporal variations can occur at timescales up to several cycles/hour: changes that are too fast to recreate using standard technologies like well plates due to their long diffusion distances. This thesis presents a novel microfluidic platform to expose cells to both chronic and time-varying oxygen profiles and study their response. Microfluidics technology is combined with 3-D cell culture in tumour spheroids, which can better recreate other aspects of the tumour microenvironment (such as cell-cell and cell-matrix interactions) than traditional 2-D culture. The functionality of the oxygen control device is verified using both finite-element modelling and integrated optical oxygen sensors. Two novel methods for oxygen sensor microfabrication are presented, and the functionality of sensors during long-term experiments is studied. Precise oxygen control is demonstrated using the microfluidic system, with oxygen switching times of 10 minutes. Because 3-D cultures present imaging challenges confounding their data analysis, new, on-chip strategies for improved imaging of 3-D cultures are studied.Finally, this thesis demonstrates the utility of the platform with preliminary biological experiments. Long-term culture of breast tumour cells is demonstrated, and the response of the cells to oxygen changes is analyzed. The microfluidic platform allowed us to observe for the first time that tumour spheroids can reversibly swell and shrink under changing oxygen conditions. Finally, a preliminary evaluation of the suitability of the platform for drug screening experiments is presented.

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This dissertation presents theoretical and experimental results for silicon optical ring resonator filters that meet many of the typical commercial specifications for dense wavelength-division multiplexing (DWDM) filters. First, we theoretically demonstrate a silicon quadruple Vernier racetrack resonator that meets 4-port filter commercial specifications for a clear window of 0.08 nm and a channel spacing of 0.8 nm while being tolerant to typical fabrication variations. Next, we experimentally demonstrate a silicon quadruple Vernier racetrack resonator that meets many 3-port filter commercial specifications for a clear window of 0.048 nm and a channel spacing of 0.8 nm. Then, enhanced resonant tuning range using the Vernier effect is theoretically and experimentally demonstrated using a thermally tunable silicon quadruple Vernier racetrack resonator. Also, we sent 12.5 Gbps data through a thermally tunable silicon quadruple Vernier racetrack resonator and show open eye diagrams in both the drop port and through port of the filter, even within one of the minor through port notches. We then present theoretical and experimental results on a high performance silicon double microring resonator filter using Mach-Zhender interferometer-based coupling that meets numerous 3-port filter commercial specifications for a clear window of 8 GHz and a channel spacing of 200 GHz as well as having an FSR larger than the span of the C-band and low through port passband dispersion. Next, we present a FSR-eliminated silicon Vernier racetrack resonator filter. We demonstrate the performance of this filter both theoretically and experimentally. The FSR of this filter is eliminated by using contra-directional grating couplers (contra-DCs) to suppress all but one of the notches and peaks of the filter's spectra. Lastly, a process calibration procedure is demonstrated that accurately determines the coupling coefficients of fabricated contra-DCs and is used to design a FSR-eliminated silicon Vernier racetrack resonator filter that meets 3-port filter commercial specifications for a clear window of 13 GHz and a channel spacing of 200 GHz. This filter also has low drop port dispersion and low dispersion within the passbands of the through port.

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In the last decade, silicon photonics has become a strategic technology for the development of telecommunications and sensors. Due to its compatibility with well-developed complementary metal oxide semiconductor (CMOS) fabrication processes, silicon on insulator (SOI) wafers can be processed to create thousands of devices per die in a fast and inexpensive way. Being solid state devices with no movable parts, optical gyroscopes have longer life expectancies and shock resistance compared to micro-electro-mechanical gyroscopes. Thus, the implementation of SOI-based gyroscopes is desirable for large-scale, low-cost production. This thesis presents a study of the feasibility of implementing optical gyroscopes in SOI technology. A comprehensive theoretical study has been carried out to develop a device-level optimization and robustness analysis, showing that the most crucial resonator parameter is the propagation loss, followed by length and coupling. For a given propagation loss, there is an optimal resonator size, beyond which the angular speed resolution is severely degraded. On the system level, the impact of signal-to-noise ratio and insertion loss on the resolution are described.Given that the propagation loss is the most important parameter, strategies were proposed to reduce it as much as possible while still using CMOS-compatible processes. The quality factor, Q was chosen as the figure of merit to be maximized during the design iterations. As a result, the largest Q factors reported to date on SOI, using standard CMOS-compatible processes, were achieved. These Q factors are comparable to, or exceed, those of optical resonators intended for gyroscopic applications that are fabricated in materials such as indium phosphide (InP). Innovative approaches to compensate for fabrication variations are proposed, such as thermally-tuneable coupling and reference rings for differential measurements. Complex mechano-opto-electrical measurement setups were designed and implemented to characterize SOI gyroscopes, both at rest and under rotation. As a result, the Microsystem Integration Platform for Silicon-Photonics (Si-P MIP) was created. This characterization platform is now being commercialized by CMC Microsystems for academic and industrial applications. The main practical and theoretical challenges regarding the implementation of optical ring gyroscopes on SOI have been identified. Schemes to address them and suggestions for future work are proposed.

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Silicon-on-insulator (SOI)-based sensors are attractive for sensing applications in environmental safety, oil and gas, medical research, and clinical applications. Since these devices are typically developed using Complementary metal-oxide-semiconductor (CMOS)-compatible multi-project-wafer (MPW) shuttles, they bring the potential for having sensing systems on chips (SSOCs), and for mass fabrication and low cost production. The objective of this thesis is to improve the sensitivity, accuracy, and repeatability of sensors fabricated on the SOI platform. Such sensors have the potential to be the key components of an SSOC.One can increase the sensitivity of a resonator sensor by increasing the interaction between the evanescent field of the guided mode and the analyte. In this thesis, two methods for increasing this interaction in micro-ring resonator-based sensors are investigated: 1) using the transverse electric (TE) guided mode in ultra-thin strip waveguides and 2) using the quasi-transverse magnetic (TM) guided mode in thin strip waveguides. Using analyses and simulations, micro-ring sensors were designed to be fabricated within the constraints of a MPW CMOS-compatible process. Using the TE sensors, the temperature-induced errors were reduced by a factor of three; and the TM sensors exhibited twice the sensitivity of the best SOI micro-ring resonator-based sensors reported to date.Moving towards the actual implementation of an SSOC, a system of sensors was design to correct for unwanted variations in the measurements. This system drew on multivariate techniques to achieve improvements that resulted in measurements that were more repeatable and more accurate in the presence of environmental variations. The capability of this system is investigated by designing a cascade of previously developed micro-ring sensors with various waveguide thicknesses. With this system of sensors, we achieved an R2 value of predictions over 0.996 in the presence of a 2 K temperature drift. This approach significantly improved the repeatability and reliability of the measurements in the presence of undesirable variations and drifts. In another move towards achieving an SSOC, integrating photodetectors in resonator sensors was investigated. To accomplish this, ion-implantation on micro-ring sensors was used. Such integrated photodetector-sensors were designed, fabricated, and tested. Their measured sensitivities were within 90% of the expected values.

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Silicon is the most ubiquitous material in the electronics industry, and is now expected to revolutionize photonics. In just over ten years, silicon photonics has become a key technology for photonic integrated circuits. By taking advantage of silicon-on-insulator (SOI) wafers and the existing complementary metal-oxide semiconductor (CMOS) fabrication infrastructure, silicon photonic chips are now being delivered with low cost and rapidly increasing functionality.This thesis presents the integration of a fundamental optical device - Bragg grating - into SOI waveguides. Various types of waveguides and grating structures have been investigated. All designs are fabricated using CMOS foundry services. We have also explored various applications using the fabricated devices.From the beginning, we focused on strip waveguide uniform gratings, as these are the most simple to design and fabricate. We have studied many design variations, supported by experimental results. In parallel, we have provided insight into practical issues and challenges involved with the design, fabrication, and measurement, such as the lithography effects, thermal sensitivity, and wafer-scale nonuniformity. We then introduce phase-shifted gratings that can achieve very high quality factors and be employed in various applications. We have also demonstrated sampled gratings and the Vernier effect in strip waveguides.To obtain narrow-band gratings, we propose the use of a rib waveguide. We also propose a multi-period grating concept by taking advantage of the multiple sidewalls of the rib waveguide, to increases the design flexibility for custom optical filters. The wafer-scale data shows that rib waveguide gratings have better performance uniformity than strip waveguide gratings, and that the wafer thick- ness variation is critical. Additionally, we have demonstrated very compact Bragg gratings using a spiral rib waveguide.Finally, we demonstrate slot waveguide Bragg gratings and resonators, which has great potential for sensing, modulation, and nonlinear optics. We have also developed a novel biosensor using a slot waveguide phase-shifted grating that has a high sensitivity, a high quality factor, a low limit of detection, and can interrogate specific biomolecular interactions.

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This thesis is a theoretical and experimental study of novel silicon photonic filters, such as traveling-wave resonators (TWRs) and grating-assisted, contra-directional couplers (contra-DCs), for on-chip wavelength-division-multiplexing (WDM) systems and sensing applications. To measure optical losses of photonic components such as Y-branch splitters and waveguide crossings, we have developed a ring-resonator based technique which is accurate, simple, and space-efficient. A number of novel devices have been demonstrated using commercial CMOS-photonics fabrication foundries, with the aim of developing large-scale photonic integrated circuits using the standard process development tools. Two types of wavelength-selective, TWR-based reflective filters have been demonstrated for applications such as remote sensing and tunable lasers. Ultra-compact, high-Q microdisk resonators have been demonstrated, with radii of down to 1.5 µm, free spectral ranges (FSRs) of up to 71 nm, loaded Q's of up to 88,000, and unloaded Q's of over 100,000.Contra-DCs have been studied using coupled-mode theory. An add-drop filter designed using contra-DCs in slab-modulated rib waveguides has been proposed and demonstrated, which shows a flat-top response and a narrow bandwidth of 50--100 GHz, promising for dense-WDM applications. Also, we proposed an out-of-phase grating design to suppress the intra-waveguide reflection in contra-DCs. Using this novel anti-reflection (AR) design, we have demonstrated an add-drop filter with a single-band, flat-top response and a wide channel bandwidth of 6.5 nm, which enables athermal operation in a large temperature span of > 70 K. This AR contra-DC can be used to build an on-chip coarse-WDM system for power-efficient, ultra-high-speed optical interconnects. Furthermore, we have proposed and demonstrated an electrically tunable phase-shifted contra-DC.In order to overcome the challenges facing microring resonators, such as limited FSRs and difficulty in controlling the bus-resonator coupling, we have proposed to integrate contra-DCs with microring resonators for selective bus-resonator coupling. Using this method, we have demonstrated a single dominant resonant mode in a microring resonator that originally has a small FSR of 1.3 nm. This grating-coupled microring resonator is promising for applications that need a huge free spectral range, such as cascaded resonator sensor arrays and ultra-high-bandwidth WDM systems.

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The direct modulation of semiconductor lasers has many applications in data transmission. However, due to the frequency response it has been challenging to use directly modulated lasers for high speed digital transmission at bit-rates above 10 Gbps. With this in mind, designing a laser with a large modulation bandwidth to be used in high data-rate optical links is very important. Transistor lasers (TLs) are a potential candidate for this purpose.Based on these motivations, the main focus of this PhD research is on understanding the physics of the TL and predicting its performance. A detailed model that correctly incorporates the transistor effects on laser dynamics did not exist. The previous models do not differentiate between the bulk carriers and the quantum well (QW) carriers in the rate equations, do not include the effects of the capture and escape lifetimes in the QW, and significantly overestimate the bandwidth.To account for these phenomena, a model has been developed to study the dynamics of the TL. The model is based on the continuity equation in the separate confinement hetero-structure region of the conventional laser and the base region of the TL. It uses the quantum mechanical escape and capture of carriers in the quantum well region and the laser rate equations to model the laser operation. The model has been used to gain insight into the conventional separate confinement hetero-structure lasers, and the results of the model have been compared with the experimental results obtained for 850 nm vertical cavity surface emitting lasers(VCSELs). Analytical expressions have been derived for DC and AC parameters of the TL operating in common-base and common-emitter configurations. It has been shown that the TL operating in the common-emitter configuration has a similar modulation bandwidth as a conventional laser (~ 20 GHz). The common-base configuration, on the other hand, has a very large small-signal modulation bandwidth (> 40 GHz) due to bandwidth equalization in the TL. The large-signal performance of the TL has been studied. Finally, it has been shown that the common-emitter configuration with feedback has improved bandwidth by a factor of 1.5 in high bias currents.

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