(PHD, 2005)

Abstract:

An analysis is performed of many standard and linearized electrooptic modulators known in the industry. The transfer functions of these modulators are evaluated under a consistent set of performance figures of merit, which are gain and spur-free dynamic range, using a canonical set of optical link parameters. The tolerance of the needed precision of the parameters of the linearization mechanisms of all of these modulators is compared over the entire interesting range of noise bandwidth.

A computer program was written to analyze the frequency dependence of any modulator transfer function under any set of functional inputs. The program is used to illustrate and compare the frequency dependence of the figures of merit of all of the modulators for which a d-c analysis was performed. Further analysis looks at the effect of greater noise-bandwidth and recovering the frequency-dependent degradation of gain and dynamic range through re-phasing techniques. The gain of directional couplers is analyzed in-depth.

Two novel modulator schemes are produced. The first uses reflective wave techniques to retime the electrical and optical waves half way through the modulator. The second uses fabrication geometry and properties of the linearization technique to make a more robust modulator (applicable to three of the modulators analyzed).

A 94 GHz antenna-coupled directional coupler modulator was initially demonstrated using an old modulator chip from Finbar Sheehy. A peculiar bug with the chip was uncovered. And a new modulator experiment was constructed and many aspects of the experimental apparatus were optimized. Though the revised experiment ultimately did not yield modulation side bands, it did couple a 94 GHz microwave signal into the optical waveguide, and many interesting challenges of high frequency electrooptic modulator fabrication were evaluated and improved upon.

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(PHD, 1993)

Abstract:

A new boundary residual mode-matching method is developed to find scattering solutions for an infinitely thin transverse iris (diaphragm) mounted in a waveguide (i.e., junction discontinuities). Differing dielectric constants are allowed on the two sides of the iris, and an optional transverse short placed behind the iris is also treated, allowing solutions useful in understanding planar grids used in microwave/millimeter wave power combining. The new method treats the edge-singularity of the iris (edge-condition) in both the electric and magnetic fields, and at each edge of a multi-edge iris, all simultaneously. This results in a rapidly-converging numerical solution, and oscillation-free transverse (electromagnetic) field plots (not available with methods that ignore singularity extraction). The solution is also free from relative convergence problems. Furthermore, the method when used without singularity extraction results in a formulation that is up to eight times faster than the standard boundary residual method, yet requires only one-fourth the memory, and is simpler to program. Singularity extraction further improves the speed and reduces memory requirements.

In addition, extensions are made to Schwinger’s variational formulas, resulting in formulas that produce highly-accurate answers for certain problems, even with multi-moding.

A new type of problem is introduced, that of a triangular shaped iris, and the complicated inner product integrals are solved analytically in closed rational form. Theory and experiment are compared.

A group of computer programs are developed to apply this new method, and the source code is listed and declared as “public domain.”
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(PHD, 1993)

Abstract:

NOTE: Text or symbols not renderable in plain ASCII are indicated by […]. Abstract is included in .pdf document.

We have demonstrated antenna-coupled electro-optic modulators at frequencies up to 98 GHz. The antenna-coupled design allows the modulator to overcome the velocity-mismatch problem which limits the maximum operating frequency of more conventional designs. Several modulators have been demonstrated, including a prototype narrowband phase modulator (optical wavelength 0.633 […]) at 10 GHz, a narrowband phase modulator (0.633 […]) at 60 GHz, a broadband Mach-Zehnder modulator operated as a phase modulator at 60 GHz, and a broadband Mach-Zehnder amplitude modulator at 94 GHz (optical wavelength 1.3 […]). The performance of the prototype modulator at 10 GHz is not quite as good as that of conventional modulators at this frequency, but is comparable. The performance of the mm-wave modulators cannot be directly compared to conventional modulators, as none exist at these frequencies. However, we have established that the relative performance of the mm-wave modulators is consistent with a simple scaling law.

Linearized […] modulators have been demonstrated recently by other workers. These linearized modulators consist of conventional […] coupler-modulators cascaded with additional couplers. We have considered the general case of cascade-linearized […] modulators, and have shown that it is possible to provide up to four degrees of freedom which can be used to modify the modulator transfer function. We have shown that these degrees of freedom can be identified and separated for design purposes, and have shown how to implement a given design.

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(PHD, 1993)

Abstract:

NOTE: Text or symbols not renderable in plain ASCII are indicated by […]. Abstract is included in .pdf document.

In this thesis, arrays of coupled waveguide […] lasers and some aspects of coupled laser arrays in general are studied. The in-phase coupled supermode is desirable in almost all laser applications, but it is not always obtained in laser arrays. An analytical method is used to investigate the spatial overlap between the distributions of laser mode intensity and the medium gain and cavity loss, and its effect on mode. The result shows that the gain/loss in the inter-element regions of an array determines which supermode to oscillate: for a fixed gain level a high loss favors the out-of-phase supermode while a low loss, the in-phase supermode. Several waveguide […] lasers, including wall-slot coupled ceramic and metal arrays, uncoupled arrays, single slab waveguide lasers, and slab waveguide lasers with unstable resonators were tested for their modal properties and power output. A new structure, the groove-coupled strip waveguide […] laser array, that favors the in-phase coupled supermode was proposed and tested in two, three and five-element arrays. The experimental results showed that this new structure yields robust, pure in-phase coupled mode operation with a reasonable amount of output power.

Also discussed in this thesis are the coupling of Gaussian waves in active media, the experiments in an all-metal, ridge-waveguide […] laser pumped by microwaves, a new type of optical switching based on the unique properties of a coupled twin-element laser, and two-dimensional array of gas discharge lasers.
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(PHD, 1986)

Abstract:

Flexible dielectric waveguides have been demonstrated at 10 GHz and 94 GHz by filling hollow, low dielectric constant polymer tubes with low-loss, high-dielectric constant powders. Flexible guides with losses as low as 0.12 dB/cm were demonstrated at 94 GHz. These guides also exhibited negligible bending loss for radii of curvature greater than 4 cm.

The theory of 3-region cylindrical dielectric waveguide was used to design the powder-filled tube guides, and measured wavelengths for the HE_{11} mode are in agreement with theoretical values. Sets of dispersion curves were calculated numerically from the theory for waveguide parameters typical to our guides.

A powder-filled rectangular groove in the surface of a plastic substrate has also been demonstrated as a dielectric waveguide at 94 GHz. Guide wavelengths measured for these channel guides for various combinations of guide dimensions, powders, and substrate materials agree with values predicted by the approximate theory of Marcatili for the E^{y}_{11} mode. Measured transmission losses were as low as 0.09 dB/cm.

The 94 GHz loss tangents of the powders were calculated by extending Marcatili’s theory to relate channel guide attenuation to material losses. These calculated values of loss tangent increased with powder packing fraction, as predicted by theories of electromagnetic wave propagation in random heterogeneous media. Estimates of the 94 GHz loss tangents of the solid constituent materials were then obtained from these theories using the powder loss tangents.

Powder channel ring resonators had Q’s as high as 2400 at 94 GHz in an 8 cm diameter ring. Directional coupling from adjacent straight channel guides was used to form a transmission filter. Marcatili’s approximate theory of bending loss for channel guide appears to be inadequate for predicting the curvature losses of these resonators.

In a 10 GHz experiment, the coupling between two parallel powder channel waveguides was measured as a function of their separation. The measured coupling was at variance with that predicted by Marcatili’s approximate analysis for parallel channel waveguides.

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(PHD, 1984)

Abstract:

In this work, we develop a new technique for the numerical study of quantum field theory. The procedure, borrowed from non-relativistic quantum mechanics, is that of finding the eigenvalues of a finite Hamiltonian matrix. The matrix is created by evaluating the matrix elements of the Hamiltonian operator on a finite basis of states. The eigenvalues and eigenvectors of the finite dimensional matrix become an accurate approximation to those of the physical system as the finite basis of states is extended to become more complete.

We study a model of scalars coupled to fermions in 0+1 dimensions as a simple field theory to consider in the course of developing the technique. We find in the course of studying this model a change of basis which diagonalizes the Hamiltonian in the large coupling limit. The importance of this transformation is that it can be generalized to higher dimensional field theories involving a trilinear coupling between a Bose and a Fermi field.

Having developed the numerical and analytical techniques, we consider a Fermi field coupled to a Bose field in 1+1 dimensions with the Yukawa coupling λΨΨΨ. We extend the large coupling limit basis of the 0+1 dimensional model to this case using a Bogoliubov transformation on the fermions. Although we do not use this basis in the numerical work due to its complexity, it provides a handle on the behavior of the system in the large coupling limit. In this model we consider the effects of renormalization and the generation of bound states.

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(PHD, 1984)

Abstract:

Scattering of a plane electromagnetic wave from circular conducting and dielectric cylinders is analyzed. Both polarizations of the incident electric field, parallel and normal to the axis of the cylinder, are considered. The study of the rigorous solutions gives an insightful understanding of the scattered field and its dependence on the material of the cylinder, the polarizations of the incident field, and the three dimensionality of the object which usually are not considered in Fourier optics. It is shown that a combination of Fourier optics and ray theory can give good approximations for the scattered field from both conducting and dielectric cylinders. It is shown that the scattered pattern from a conducting cylinder consists of a main lobe and a number of side lobes. The spacing between the side lobes decreases as ka increases, where k is the wave number of the incident field and a the radius of the cylinder. It is found that for a certain conducting cylinder the side lobes terminate in a smaller scattering angle when the incident field is polarized parallel to the axis than when the incident field is polarized normal to the axis of the cylinder. The surface current density in the shadow region is found to be larger for the normal polarization case than for the parallel polarization case. The pattern of the scattered field from a dielectric cylinder has fringes all around the cylinder. The contrast of the fringes for the dielectric cylinder is much larger when the polarization of the incident field is parallel to the axis than that when the polarization is normal to the axis. The backscattered field of the dielectric cylinder has a peak which depends in its position on the refractive-index of the cylinder for the parallel polarization case. The fine structure of the scattering by a dielectric cylinder is studied. It is shown that the dielectric cylinder has resonant frequencies which depend on the radius and refractive-index of the cylinder. It is found that there are significant differences in the shape of the scattered pattern depending on whether the cylinder is at-resonance or at off-resonance. Experimental investigations are carried out to verify the theory derived in this study and it is found that both theory and experiments are in good agreement.

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(PHD, 1983)

Abstract:

This thesis consists of two parts, each representing a different aspect of far-infrared (FIR) physics and technology.

Part I deals with the problems related to the physics and the design of one of the extremely useful coherent sources in the FIR region of the electromagnetic spectrum: the optically-pumped FIR waveguide laser. The effects of small waveguide diameter were studied here particularly because of their importance to the practical realization of a compact coherent FIR laser. Two known theoretical models were used to analyze the performance of CH_{3}OH 118 µm laser and CH_{3}F 496 µm laser; the results from these models were compared with the results of our experimental parametric study on CH_{3}OH 118 µm laser. A simplified model of Lourtioz and Adde agrees reasonably well, in a semiquantitative sense, with our experimental results. The λ^{2}/a^{3} dependence of distributed waveguide loss for FIR radiation turns out to be the major factor that limits the waveguide size.

Part II deals with a problem related to the physics and design of a diplexer for application in the FIR heterodyne radiometry, where signals from the local oscillator and the received signal have to be directed into a detector for frequency mixing and for further signal processing. Since the signal of interest is typically very weak, the diplexer should serve the dual purpose of directing the two beams and filtering out the unwanted frequency components (noise) to enhance the signal to noise ratio, and do so with minimum loss.

The optimum design parameters for a folded Fabry-Perot quasi-optical ring resonator diplexer were derived, and its performance was investigated both theoretically and experimentally. The results were compared with those of the similar diplexers of non-optimum geometry. The advantages and limitations of the optimum diplexer design are analyzed.

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(PHD, 1982)

Abstract:

In this thesis, we analyze high-permittivity dielectric waveguides for use as guiding structures of millimeter waves. Two basic geometries are considered: the circular and rectangular guides.

In Part I, we describe the theory of round fibers surrounded by an infinite cladding. Millimeter wavelengths are comparable to the physical dimensions of the guide. Therefore, a large difference in permittivity between the core and the cladding is required in order to provide for a tight confinement of the fields. We present the results of computations of the propagation characteristics and losses of fibers of very high permittivity. We note that the distribution of the electromagnetic power between the core and the cladding can be deduced from the dispersion curves. Finally, we consider the feasibility of a dielectric fiber made of thallium bromide-iodide (KRS-5) for the long distance transmission of W-band signals (94 GHz). Using our measurements of the dielectric parameters of KRS-5, we find that the losses are several orders of magnitude higher than the losses of conventional metallic waveguides.

In Part II, we analyze rectangular dielectric guides made of high-permittivity materials such as GaAs that would permit the fabrication of active devices directly into the transmission line. We present a new numerical technique base on finite-differences for computing the modes of dielectric guiding structures. This method is simple and efficient in computer storage and computational time. We use it to compute the modes of a rectangular dielectric waveguide and compare the numerical results to those obtained from Marcatili’s closed-form solution. We find that this latter one is a good approximation for the dominant mode of a rectangular guide even when the permittivity of the guide is large compared to the outer medium. For higher order modes, Marcatili’s solution predicts incorrect propagation curves. We have also observed the presence in our numerical solution of “spurious modes” that are thought to be due to the mathematical indefinitiveness of the problem.

In Part III, we present a waveguide technique for the measurement of complex dielectric constants at millimiter wave frequencies: the shorted-waveguide method. Waveguide methods have been extensively used at lower frequencies but this is the first application at 94 GHZ. We use a novel sample preparation technique that allows for an accurate and gap-free positionment of a ductile dielectric material inside a metallic waveguide. We note that the correct choice of sample lengths is critical to the accuracy of the measurement of the loss tangent. Finally, we summarize the results of our measurement of the dielectric constant and loss tangent of thallium bromide-iodide (KRS-5) and thallium bromide-chloride (KRS-6).

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(PHD, 1982)

Abstract:

The conventional chopped laser radiation technique of optoacoustic detection suffers from a sensitivity-limiting synchronous background signal. This thesis reports on theoretical and experimental studies of two novel optoacoustic modulation techniques designed to reduce the background signal and thereby improve the sensitivity. A specialized optoacoustic detector is described, which is capable of being operated in the conventional chopped-laser modulation mode as well as in the new Stark voltage modulation and Stark polarization modulation modes; experimental results are given using all three modes of operation. Stark voltage modulation produces more than two orders of magnitude improvement in sensitivity over conventional modulation under identical conditions. Even greater sensitivities are shown to be attainable. A basic theory of conventional optoacoustic detection is developed and then applied to the new optoacoustic modulation techniques. An introduction to and history of optoacoustic detection are also presented.

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