Photonic devices with MQW active material and waveguide gratings : modelling and characterisation (original) (raw)
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Quantum well intermixed waveguide grating
2011
A waveguide grating have been designed suitable for Coarse Wavelength Division Multiplexing applications in which the refractive index is perturbed by the spatial tailoring of the band gap with fluorine ion implanted quantum well intermixing of the In0.95Ga0.05As0.10P0.90/InP multi quantum well structure. The gratings have been modeled using coupled mode theory and diffusion equations and Schrödinger wave equations are used to model quantum well energy while interdiffusion. A four channel waveguide grating from 1,550 to 1,610 nm at a span of 20 nm have been simulated with a channel bandwidth of 13 nm and a cross talk of −5 to −10 dB.
Journal of Lightwave Technology, 2017
An equivalent surface current method is used to derive an analytic expression that approximates the coupling coefficient, reflectivity bandwidth and group velocity for coupling between identical and non-identical TE modes in an asymmetric three-layer waveguide with a sinusoidal grating at one waveguide interface. The analytic expression for the coupling coefficient agrees with expressions derived by two other methods for contra-directional coupling between identical modes. The results from the analytical expressions are compared to results from a numerically accurate Floquet-Bloch solution. The analytical expressions, which do not depend on which interface contains the grating, provides almost identical results obtained by the accurate solution for shallow grating depths and small index changes at the grating interface, a case typical of single-mode distributed feedback lasers. However, the accurate numerical solution, unlike the analytic solution, shows that in waveguides with only a large index step at the grating interface, increasing the grating depth can result in decreasing the coupling coefficient, a case typical of some distributed Bragg reflector lasers. In highly confined silicon photonic waveguides (large index steps at both interfaces), the analytic expression gives accurate results even for deep gratings. The derivation of the analytical expression for the coupling coefficient in this paper using the equivalent surface current method extends the application of the previous analytic formulas to non-identical mode coupling where the forward and backward modes are not identical, which has application to gratings in multi-mode broadened waveguide lasers and amplifiers.
Experimental analysis and modeling of buried waveguides fabricated by quantum-well intermixing
IEEE Journal of Quantum Electronics, 1999
The fabrication of buried waveguides in InP-based quantum-well (QW) material through the use of implantationenhanced QW band edge blue-shifting is reported. First, the lateral selectivity of implantation-induced QW intermixing is investigated using a specially designed implantation mask and photoluminescence. The refractive index change of the intermixed material is measured near the band edge. In combination, the lateral resolution and the index difference have allowed for the fabrication of narrow buried waveguides in an InP-based laser structure. Detailed modeling of the mode excitation and beam propagation of these devices is performed, and results are compared with experimental near-field profiles. We demonstrate that the waveguides are single-mode for both TE and TM polarizations at wavelengths near the band edge. The potential applications of this waveguide structure include post-growth fabrication of buried waveguide lasers and other integrated components.
Multiple quantum well structures as optical waveguides
1986
This thesis is concerned with the design, fabrication and characterisation of semiconductor optical waveguides in which the high index guiding layer is a multiple quantum well structure (MQWS), consisting of alternate layers of high and low band gap semiconductors with the electrons and holes in the MQWS being confined to the low band gap material. This confinement in two dimensions alters greatly the electronic and optical properties of the MQWS in comparison to the bulk properties of the constituent layers. The basic concepts involved in MQV waveguides are introduced using an elementary quantum mechanical analysis of quantum wells together with a brief description of the properties of dielectric waveguides. A more detailed treatment of the electronic and optical properties of MQVS and a review of published experimental work is used to show that the fundamental absorption edge is much more abrupt than that in the corresponding bulk material with strong excitonic characteristics being evident even at room temperature. In addition, the absorption edge is seen to be anisotropic with the fundamental energy gap being larger for light polarised perpendicular to the MQW layers. This anisotropic absorption edge, together with the layered dielectric nature of MQVS, makes them biréfringent with a smaller refractive index for light polarised perpendicular to the MQV layers. The quantum confinement of carriers in MQVS also enhances their electro absorption and electro-optic properties through the quantum confined Stark effect. Standard techniques used in the design, fabrication and analysis of bulk semiconductor waveguides are developed for application to MQV waveguides. These include analytical and numerical techniques for the design of dielectric waveguides; dry etching and metallisation processes for the fabrication of devices; and a laser/optics system to analyse the waveguide devices. To verify these techniques they are first applied to the well-understood case of n/n^ GaAs waveguides and are used to successfully fabricate and analyse single-mode, passive, rib waveguides at X=1.15pra. The electro-optic coefficient is also measured in an active, planar n/n" waveguide and found to be close to that reported by other workers. ÏÏ The design techniques are then applied to MQV8 waveguides resulting in the design of a MQV double heterostructure (MQW-DH), p-in diode which was predicted to produce the required quantum properties (strong, room temperature, excitonic behaviour), waveguide properties (single-mode propagation up to the fundamental absorption edge) and electronic properties (a high reverse bias breakdown voltage and uniform applied electric field). Most of the theoretical work and all the experimental work included is devoted to MQVS in the (Al,Ga)As, III-V semiconductor alloy system. Accordingly, the methods available for growing MQVS in this system are reviewed with Molecular Beam Epitaxy (MBE) being found the most likely method to satisfactorally reproduce the desired structure. MQV-DH were grown at two establishments and are initially studied by photoluminescence and scanning electron microscopy before their planar optical waveguide characteristics are checked using the laser system. Only one sample is found to satisfy all the design requirements, and then only partially. Detailed analysis of the properties of MQV waveguides is therefore limited to this structure. Passive MQV-DH waveguides are demonstrated to exhibit an anisotropic absorption edge as predicted, and it is shown that the design and fabrication techniques developed can be successfully used to obtain single, double and multi-mode strip loaded waveguides. Single-mode waveguides are also used to fabricate passive directional couplers with coupling lengths in good agreement with theoretically predicted values. A semi-empirical model is put forward to describe the band edge electro-absorption of MQVS. Although simple, the model is in qualitative and approximate quantitative agreement with published results. To allow a comparison to be made, the electro-absorption of bulk GaAs is also modelled. A realistic electro-absorption figure of merit is the ratio of the change in absorption for a given applied field to the initial absorption for no applied field. It is found that the maximum figure of merit obtainable in MQVS is 5-6 times larger than that in bulk GaAs. In MQVS this maximum is obtained at an optimum applied field of approximately 30% of the available avalanche breakdown field whereas in bulk GaAs the figure of merit steadily increases with applied field to reach a maximum at the avalanche breakdown field. Electro-absorption is experimentally investigated in both planar and stripe MQV-DH waveguides. Although no quantum confined Stark effect is observed there is a strong shift in band edge with applied electric voltage. This is measured to produce a figure of merit of 35 for a voltage of-4 volts compared with a published figure of merit of 4.6 at-24 volts for GaAs-DH. The very large electro-absorption observed precludes any measurement of the electro-optic effect in MQV-DH close to the band edge. A strong electro-optic effect is observed at X=1.15pm but further work is required before this can be confidently stated to imply an enhanced electro-optic coefficient. In conclusion recommendations are made concerning the application of the observed properties of MQV-DH p-in diodes to integrated optics. These properties, together with the reported low passive waveguide loss of MQV laser diodes, make MQV-DH an ideal medium for the monolithic integration of optical circuits.
Computational environment for the study of optical waveguides
IEEE Transactions on Education, 1999
A notebook in the software Mathematica is developed here for the analysis of planar multilayer dielectric waveguides with the objective of using it as a didactic computational tool, with a possible inclusion in the electrical engineering package library of this same software. The scattering and guiding phenomena in a given structure are analyzed in the notebook, through the use of the programming facilities of the Mathematica software. The user may thus specify the physical and geometrical parameters to be analyzed or make a choice from a model's library that includes periodic structures such as Bragg reflectors and multiple quantum well (MQW) structures.
Analysis of integrated optical waveguides
An overview of the analysis of integrated optical waveguides is presented. Starting from the Maxwell's equations, a formulation of the problem for general 3-D structures will be introduced. Then, for longitudinally invariant structures, problem for waveguides with 2-D cross section is presented for vectorial, semivectorial, and scalar formulations. Simpler 1-D case for planar structure will then be discussed in more detail. A novel scheme developed for the analysis of planar structures is given based on the variational method. Its application using also effective index method for reduction of dimensionality to analyze channels and directional couplers made by diffused Z-cut Y-propagating LiNbO 3 crystal are demonstrated.
Modeling, Design and Applications of Optical Amplifiers and Long Period Gratings
The scope of this thesis is modeling, design and application of optical amplifiers and long period gratings (LPGs). Typically, we concentrate on erbium doped fiber amplifiers, erbium doped lithium niobate waveguide optical amplifiers (EDWAs), and semiconductor optical amplifiers (SOAs). These three devices besides being employed as optical amplifiers also promise their feasibility to be used as optical signal processors. Modeling and simulation of these amplifiers is thus an important tool in the understanding and designing of these amplifiers. This can further aid in the development on structural modifications which can help in implementing new technologies and look for novel applications of the same structure. The salient features of the underlying work can be summarized as below: The thesis is structured as follows: After a brief introduction to the state of art in Chapter 1, Chapter 2 provides a review of the concepts used in subsequent chapters. Confinement and guidance of electromagnetic waves through waveguides and fibers is presented. Specifically, we discuss the variational method for channel waveguides and analysis for optical fibers taking into account the core, cladding and ambient refractive index regions. The basic characteristics of long period gratings are presented along with the coupled mode theory for determining the coupling between the fundament core mode and co-propagating cladding mode. Finally, the general principle of optical amplification is presented and some common optical amplifiers are discussed. In Chapter 3 power coupled equations for modeling of EDFA’s are discussed. Recently, researchers have proposed EDF's with LPG written in them for better gain flattening. These structures are analyzed using modified coupled mode analysis. We have evolved a methodology to incorporate ASE taking into consideration the random phase of spontaneous emission. Our results show that LPG written in EDF itself, not only brings about gain flattening, but also suppresses the ASE. Chapter 4 looks into the analysis of erbium doped titanium in-diffused lithium niobate waveguide optical amplifiers. The gain coupled differential equations involve integrals and depend explicitly on the modal fields, making it time consuming to solve. In this chapter we approximate the Hermite-Gaussian modal field obtained from the variational analysis by suitably chosen approximations. These approximations reduce the integrals to analytical forms. This results in a computationally efficient solution. In Chapters 5-7, we focus on semiconductor optical amplifiers. Chapter 5 discusses the basic semiconductor physics controlling the behavior of an SOA. The chapter also discusses the widely accepted Connelly model, to model an SOA in steady state. Chapter 6 investigates the possibility of using ATLAS, a Silvaco’s physics based simulator tool, to model the behavior of SOA. The results obtained by ATLAS are compared with the experimental data, validating the use of ATLAS to simulate SOAs. In Chapter 7 we further investigate using ATLAS, the effect of modification in SOA design on gain saturation and alpha factor. Specifically, we look into the effects of doping the active layer of the SOA, changing the depth of the active layer and changing the width of ridge. Our results show that it is possible to engineer the gain saturation and alpha factor of an SOA. During our study of the transmission spectra of LPG written in it, we observed that at a certain wavelength greater than the resonance wavelength, the transmitted core power varies significantly from 0 (no transmission) to 1 (full transmission) as the ambient index is varied. This motivated us to investigate the possibility of intensity based refractive index sensor using LPG. Hence, in Chapter 8, we look into the application of LPGs as refractive index sensors. We propose a design recipe to tailor a refractive index sensor with maximum sensitivity in the desired refractive index range. Finally we present an outlook on future research.