Otto Schwelb | Concordia University (Canada) (original) (raw)

Papers by Otto Schwelb

Optical resonators have micrometer size dimensions and come mostly in two flavors, namely circula... more Optical resonators have micrometer size dimensions and come mostly in two flavors, namely circular and racetrack shaped microrings (MR), and microdisks (MD), although microsphere (MS) and photonic crystal microring (PCMR) resonators are also expected to gain prominence. Highly advanced fabrication techniques in recent years resulted in the reduction of propagation losses and in a remarkable increase of resonator Q factor and finesse. Newly developed microresonators are therefore ideally suited for applications in highly selective communication filters, delay lines, distributed and localized sensing, industrial measurements, microlaser mirrors and high-resolution spectroscopy. Since the optical signal recirculates and spends a relatively long time trapped in a high Q cavity, microresonators enhance light-light and light-particle interactions and are for this reason most promising to exploit nonlinear effects.

Numerically simulated performance is presented in a new type of bandstop and bandpass filters, bu... more Numerically simulated performance is presented in a new type of bandstop and bandpass filters, built with discontinuity-assisted ring resonator (DARR)-loaded couplers. The operating characteristic of the bandpass filter is similar to that of a long grating with a centered cavity or a photonic crystal with a defect. The filters are tuned by varying the refractive index or the length of the waveguide forming the ring or the cavity. Selectivity and tuning range are controlled by the coupling coefficient of the couplers, by the order of resonance in the rings, and by the reflection coefficient of the discontinuities. A design procedure and the effect of loss are also presented.

Microring and microdisk resonators are used in photonic circuits to perform filter functions, ser... more Microring and microdisk resonators are used in photonic circuits to perform filter functions, serve as bandlimited mirrors in lasers, compensate for dispersion and group delay distortion, are used in multiplexers in telecommunication systems, and a number of other applications such as medical instrumentation, biosensing, interferometry, to name a few. This paper outlines some challenges and solutions in this relatively new and prolific discipline.

An overview of microring resonator architectures, fabricated using both fiber and integrated tech... more An overview of microring resonator architectures, fabricated using both fiber and integrated technologies, whose function is to reflect nearly the entire incident signal within a specified band of frequencies is presented. The signal components that lie outside this band, called the stopband, are either transmitted through the device to exit through another port, or dissipated. The stopband reflectivity is ideally close to unity, while outside the stopband the reflectivity is very small; the device performs the function of a band limited optical mirror. A primary application for such a mirror is in semiconductor laser devices, where its frequency selective properties are exploited to eliminate unwanted resonances of an active medium with a broad gain characteristics. Another application involves the mirror group delay characteristics in reflection, which can, in some architectures, be shaped to compensate for the group delay distortion of an incident signal, or control the Q factor of an oscillator. Comparisons of the reflection coefficient provided by various architectures in terms of resonator loss and as a function of the coupling strength to the input waveguide are included.

Ring-resonator-based optical mirrors are analyzed and their characteristics numerically evaluated... more Ring-resonator-based optical mirrors are analyzed and their characteristics numerically evaluated. The mirrors are optimized to provide high reflection in a relatively narrow band. A design procedure that relates the required coupling to the bandwidth (BW) and reflection coefficient is presented. The effect of waveguide loss on the transmission and reflection performance is computed. Group delay and dispersion under the influence of loss and circuit-parameter variations are also treated.

Results of numerical experiments on several interferometric networks built with 2 2 2 couplers ar... more Results of numerical experiments on several interferometric networks built with 2 2 2 couplers are presented. The networks investigated include resonant and nonresonant lattices, generalized Mach-Zehnder (MZ) configurations and transversally coupled spectral filters. Several novel interferometric configurations and novel properties of known configurations are discussed. The numerical method is based on the concept of a universal coupler and on the generalized analysis of interferometric networks described in an earlier paper [5]. It has been found that relatively simple configurations can provide the very high spectral selectivity needed in some communication applications and the flexibility desired in various sensor applications. Group delay and loss characteristics are also given.

Performance characteristics and the design for optimum crosstalk suppression and extended bandwid... more Performance characteristics and the design for optimum crosstalk suppression and extended bandwidth are described for lossy microring add/drop multiplexers. Restrictions constraining the single-ring architecture are contrasted with the increased flexibility of the series-coupled double-ring design. Expressions to compute the bandwidth of the lossy filter maintaining a minimum crosstalk are provided.

Coupled resonator optical waveguides (CROW) comprised of only a few resonators including one defe... more Coupled resonator optical waveguides (CROW) comprised of only a few resonators including one defective element are investigated analytically and numerically. The defect is introduced by perturbing weakly the resonator at the center of the chain, creating a 'cavity' in what is an analogue of a Fabry-Pérot resonator or a defect-assisted photonic crystal structure. The device is found to have a very narrow resonance within the original stopband of the unperturbed CROW, and an accompanying extremely large group velocity excursion, that extends over both the subluminal and the superluminal ranges. The properties of the resonance and the magnitude of the group velocity dispersion are found to be conveniently controllable by coupling strength and by the number of resonators used. The perturbation can be caused thermo-optically or electro-optically. The complex effects of waveguide loss are extensively discussed.

In Part I of this paper, the author presented a method to simplify the analysis and design of int... more In Part I of this paper, the author presented a method to simplify the analysis and design of interferometric microwave and optoelectronic networks such as filters, sensors, ring resonators, etc., comprised of 2 2 2 couplers, waveguides, reflectors, and mismatched interfaces. The simplification was achieved by introducing a generalized single-mode lumped-element 2 2 2 coupler with arbitrary coupling paths. In Part II, the author numerically examines a number of interferometric devices utilizing the analysis described in Part I. These devices include feedbackassisted couplers, multiple-ring resonators, unit transmittance circuits, add/drop filters, grating-assisted Mach-Zehnder (M-Z) interferometers, etc. Results are presented on the characteristics of the output and circulating wave in the resonant rings as a function of frequency, coupling coefficient, loss, and other component parameters. Results on group-delay characteristics are also given.

A method is introduced to simplify the analysis and design of microwave and optoelectronic networ... more A method is introduced to simplify the analysis and design of microwave and optoelectronic networks such as spectral filters, interferometric sensors, etc., comprised of 2 2 2 couplers, waveguides, reflectors, and mismatched interfaces. The key element which makes it possible to reduce topological complexity and rearrange a network into a chain of cascaded four-ports, is a generalized, single-mode lumped-element 2 2 2 coupler with arbitrary coupling paths. As a result, one can now enumerate and evaluate all possible feedback-assisted and resonant configurations. The emphasis is on providing a computationally efficient method of analysis applicable to a wide variety of networks, rather than on obtaining the simplest and most transparent analytical expressions for a particular configuration.

Coherent, passive, linear optical signal processing networks, built with 2×2 couplers, gratings, ... more Coherent, passive, linear optical signal processing networks, built with 2×2 couplers, gratings, mirrors and waveguides are discussed. The signal processing function is accomplished by the interference of coherent signals. Through the artifice of a generalized coupler complex network topologies are reduced to cascaded four-port and feedback-assisted cascaded four-port circuits, which are then treated by standard network analysis techniques. A wide range of configurations can thus be evaluated through a single numerical code. Several applications relevant to telecommunication, measurement and sensing are enumerated.

Relations between coupling strength and resonator mode numbers that ensure invariant resonance sp... more Relations between coupling strength and resonator mode numbers that ensure invariant resonance splitting in stand-alone (i.e., isolated from external circuitry) multiring resonators of diverse architectures are investigated analytically and numerically. The loss-insensitive splitting determines the bandwidth of the associated circuitry.

Transmission of optical signals intensity-modulated by an RF carrier is investigated. Linear and ... more Transmission of optical signals intensity-modulated by an RF carrier is investigated. Linear and nonlinear effects are found to be independent as long as the nonlinear part of the fiber phase-constant is less than one third of the dispersive part. Soliton-like effects are discovered at higher power levels.

Microring and microdisk resonators are used in photonic circuits to perform filter functions, ser... more Microring and microdisk resonators are used in photonic circuits to perform filter functions, serve as bandlimited mirrors in lasers, compensate for dispersion and group delay distortion, are used in multiplexers in telecommunication systems, and a number of other applications such as medical instrumentation, biosensing, interferometry, to name a few. This paper outlines some challenges and solutions in this relatively new and prolific discipline.

Optical resonators have micrometer size dimensions and come mostly in two flavors, namely circula... more Optical resonators have micrometer size dimensions and come mostly in two flavors, namely circular and racetrack shaped microrings (MR), and microdisks (MD), although microsphere (MS) and photonic crystal microring (PCMR) resonators are also expected to gain prominence. Highly advanced fabrication techniques in recent years resulted in the reduction of propagation losses and in a remarkable increase of resonator Q factor and finesse. Newly developed microresonators are therefore ideally suited for applications in highly selective communication filters, delay lines, distributed and localized sensing, industrial measurements, microlaser mirrors and high-resolution spectroscopy. Since the optical signal recirculates and spends a relatively long time trapped in a high Q cavity, microresonators enhance light-light and light-particle interactions and are for this reason most promising to exploit nonlinear effects.

Numerically simulated performance is presented in a new type of bandstop and bandpass filters, bu... more Numerically simulated performance is presented in a new type of bandstop and bandpass filters, built with discontinuity-assisted ring resonator (DARR)-loaded couplers. The operating characteristic of the bandpass filter is similar to that of a long grating with a centered cavity or a photonic crystal with a defect. The filters are tuned by varying the refractive index or the length of the waveguide forming the ring or the cavity. Selectivity and tuning range are controlled by the coupling coefficient of the couplers, by the order of resonance in the rings, and by the reflection coefficient of the discontinuities. A design procedure and the effect of loss are also presented.

Microring and microdisk resonators are used in photonic circuits to perform filter functions, ser... more Microring and microdisk resonators are used in photonic circuits to perform filter functions, serve as bandlimited mirrors in lasers, compensate for dispersion and group delay distortion, are used in multiplexers in telecommunication systems, and a number of other applications such as medical instrumentation, biosensing, interferometry, to name a few. This paper outlines some challenges and solutions in this relatively new and prolific discipline.

An overview of microring resonator architectures, fabricated using both fiber and integrated tech... more An overview of microring resonator architectures, fabricated using both fiber and integrated technologies, whose function is to reflect nearly the entire incident signal within a specified band of frequencies is presented. The signal components that lie outside this band, called the stopband, are either transmitted through the device to exit through another port, or dissipated. The stopband reflectivity is ideally close to unity, while outside the stopband the reflectivity is very small; the device performs the function of a band limited optical mirror. A primary application for such a mirror is in semiconductor laser devices, where its frequency selective properties are exploited to eliminate unwanted resonances of an active medium with a broad gain characteristics. Another application involves the mirror group delay characteristics in reflection, which can, in some architectures, be shaped to compensate for the group delay distortion of an incident signal, or control the Q factor of an oscillator. Comparisons of the reflection coefficient provided by various architectures in terms of resonator loss and as a function of the coupling strength to the input waveguide are included.

Ring-resonator-based optical mirrors are analyzed and their characteristics numerically evaluated... more Ring-resonator-based optical mirrors are analyzed and their characteristics numerically evaluated. The mirrors are optimized to provide high reflection in a relatively narrow band. A design procedure that relates the required coupling to the bandwidth (BW) and reflection coefficient is presented. The effect of waveguide loss on the transmission and reflection performance is computed. Group delay and dispersion under the influence of loss and circuit-parameter variations are also treated.

Results of numerical experiments on several interferometric networks built with 2 2 2 couplers ar... more Results of numerical experiments on several interferometric networks built with 2 2 2 couplers are presented. The networks investigated include resonant and nonresonant lattices, generalized Mach-Zehnder (MZ) configurations and transversally coupled spectral filters. Several novel interferometric configurations and novel properties of known configurations are discussed. The numerical method is based on the concept of a universal coupler and on the generalized analysis of interferometric networks described in an earlier paper [5]. It has been found that relatively simple configurations can provide the very high spectral selectivity needed in some communication applications and the flexibility desired in various sensor applications. Group delay and loss characteristics are also given.

Performance characteristics and the design for optimum crosstalk suppression and extended bandwid... more Performance characteristics and the design for optimum crosstalk suppression and extended bandwidth are described for lossy microring add/drop multiplexers. Restrictions constraining the single-ring architecture are contrasted with the increased flexibility of the series-coupled double-ring design. Expressions to compute the bandwidth of the lossy filter maintaining a minimum crosstalk are provided.

Coupled resonator optical waveguides (CROW) comprised of only a few resonators including one defe... more Coupled resonator optical waveguides (CROW) comprised of only a few resonators including one defective element are investigated analytically and numerically. The defect is introduced by perturbing weakly the resonator at the center of the chain, creating a 'cavity' in what is an analogue of a Fabry-Pérot resonator or a defect-assisted photonic crystal structure. The device is found to have a very narrow resonance within the original stopband of the unperturbed CROW, and an accompanying extremely large group velocity excursion, that extends over both the subluminal and the superluminal ranges. The properties of the resonance and the magnitude of the group velocity dispersion are found to be conveniently controllable by coupling strength and by the number of resonators used. The perturbation can be caused thermo-optically or electro-optically. The complex effects of waveguide loss are extensively discussed.

In Part I of this paper, the author presented a method to simplify the analysis and design of int... more In Part I of this paper, the author presented a method to simplify the analysis and design of interferometric microwave and optoelectronic networks such as filters, sensors, ring resonators, etc., comprised of 2 2 2 couplers, waveguides, reflectors, and mismatched interfaces. The simplification was achieved by introducing a generalized single-mode lumped-element 2 2 2 coupler with arbitrary coupling paths. In Part II, the author numerically examines a number of interferometric devices utilizing the analysis described in Part I. These devices include feedbackassisted couplers, multiple-ring resonators, unit transmittance circuits, add/drop filters, grating-assisted Mach-Zehnder (M-Z) interferometers, etc. Results are presented on the characteristics of the output and circulating wave in the resonant rings as a function of frequency, coupling coefficient, loss, and other component parameters. Results on group-delay characteristics are also given.

A method is introduced to simplify the analysis and design of microwave and optoelectronic networ... more A method is introduced to simplify the analysis and design of microwave and optoelectronic networks such as spectral filters, interferometric sensors, etc., comprised of 2 2 2 couplers, waveguides, reflectors, and mismatched interfaces. The key element which makes it possible to reduce topological complexity and rearrange a network into a chain of cascaded four-ports, is a generalized, single-mode lumped-element 2 2 2 coupler with arbitrary coupling paths. As a result, one can now enumerate and evaluate all possible feedback-assisted and resonant configurations. The emphasis is on providing a computationally efficient method of analysis applicable to a wide variety of networks, rather than on obtaining the simplest and most transparent analytical expressions for a particular configuration.

Coherent, passive, linear optical signal processing networks, built with 2×2 couplers, gratings, ... more Coherent, passive, linear optical signal processing networks, built with 2×2 couplers, gratings, mirrors and waveguides are discussed. The signal processing function is accomplished by the interference of coherent signals. Through the artifice of a generalized coupler complex network topologies are reduced to cascaded four-port and feedback-assisted cascaded four-port circuits, which are then treated by standard network analysis techniques. A wide range of configurations can thus be evaluated through a single numerical code. Several applications relevant to telecommunication, measurement and sensing are enumerated.

Relations between coupling strength and resonator mode numbers that ensure invariant resonance sp... more Relations between coupling strength and resonator mode numbers that ensure invariant resonance splitting in stand-alone (i.e., isolated from external circuitry) multiring resonators of diverse architectures are investigated analytically and numerically. The loss-insensitive splitting determines the bandwidth of the associated circuitry.

Transmission of optical signals intensity-modulated by an RF carrier is investigated. Linear and ... more Transmission of optical signals intensity-modulated by an RF carrier is investigated. Linear and nonlinear effects are found to be independent as long as the nonlinear part of the fiber phase-constant is less than one third of the dispersive part. Soliton-like effects are discovered at higher power levels.

Microring and microdisk resonators are used in photonic circuits to perform filter functions, ser... more Microring and microdisk resonators are used in photonic circuits to perform filter functions, serve as bandlimited mirrors in lasers, compensate for dispersion and group delay distortion, are used in multiplexers in telecommunication systems, and a number of other applications such as medical instrumentation, biosensing, interferometry, to name a few. This paper outlines some challenges and solutions in this relatively new and prolific discipline.