The properties of a crossed grating of square pyramids and a crossed grating with hemispherical cavities to eliminate specular reflection from a conducting surface are studied experimentally. Measurements were made in the microwave range of 35 GHz. The best performance is that 99.94% of the power of a TM-polarized incident wave can be scattered into a single spectral order by a pyramidal crossed grating, while for TE polarization the reduction in specular reflection can be as high as 98%. Anti-reflection properties of a crossed grating with hemispherical cavities near normal incidence are also observed. Comparison between the behavior of triangular and pyramidal gratings of the same profile is made. Effects of the profile parameters are investigated. Basically the experimental results agree with the theoretical predictions. This investigation provides a set of experimental data to assist further numerical study. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

A rigorous analysis based upon the extinction theorem is presented to study anomalous resonance effects from single- and multilayer-overcoated, low-efficiency diffraction gratings. Anomalously high diffraction efficiency at resonance results from the coupling of the incident beam into guided waves that can be propagated within the composite structure. Both the traditional characteristic matrix technique and a recursive or R-matrix propagation technique are presented. The R-matrix propagation algorithm was found to be stable numerically, and computational results agree favorably with both experimental and other theoretical work. Numerical results are presented in order to investigate the influence of certain parameters (i.e., groove depth and shape and the number of high- and low-index overlayers) on the diffraction efficiency at resonance. In this analysis, a wavelength of 0.6328 μm and grating period of 0.7 μm were chosen so that only a -1 diffracted order other than the specular is reflected from the gratings. Perfect transfer of the grating relief to the film boundaries does not occur in all instances; it depends on the grating and film characteristics together with the conditions during deposition. Investigated in this work is the effect of nonreplication of the grating profile at film interfaces on anomalous diffraction; a transition from trapezoidal profile at the grating substrate to a rounded relief at the top surface of the multilayer structure is assumed. For the cases studied, it was found that nonreplication has the effect of reducing the strength of the resonance outcoupling. Finally, experimental results on anomalous resonance effects for multilayer-coated gratings are presented. Good agreement with computational results was attained.

This dissertation describes various nonlinear grating coupling phenomena in InSb waveguides. InSb exhibits an opto-thermal, diffusive nonlinearity at 9.6μm. This nonlinearity strongly modifies the growth of a guided wave via the grating coupling of an incident, Gaussian signal. In particular, optical limiting and bistability in the coupled power were demonstrated for a detuned grating coupler. The limits on the detunings were also investigated. The experimental results showed qualitative agreement with a theory based upon a diffusive nonlinearity in the grating coupling process. Also demonstrated and theoretically explained was a new form of "butterfly" bistability in the output coupled signal from a nonlinear waveguide. In this situation, nonlinear interference effects in the substrate modified the proportion of power outcoupled into the cover and the substrate. The effects of a thermal nonlinearity on the response of a distributed feedback grating (DFB) were also studied. The DFB reflection response of an incident guided wave was modified either by varying the power of the incident guided wave or that of a second guided wave. Also, the effects of the DFB reflection were shown to enhance the bistable input grating coupling process by providing an additional feedback. This enhancement resulted in a smaller incident switching power required to obtain bistability in the input coupler.

Fiber Bragg gratings have become an important element in the fields of telecommunications and fiber optical sensing. Their small size, light weight and high tunability have made them ideal for many unique applications. In this paper the fabrication of these devices is investigated. Following a review of current literature and a discussion of the mechanisms underlying the photosensitivity of optical fibers, a new technique for writing fiber Bragg gratings is presented. This technique uses a diode-pumped Nd:YAG laser operating at 266 nm with pulse energies up to 137 ��J per pulse to write high quality gratings in standard optical fiber. This technique allows for the writing of variable wavelength gratings using a standard diffractive optical element (phase mask). The new technique has much lower setup and laser stability requirements than the conventional methods of writing variable wavelength fiber Bragg gratings. Furthermore, it is found to be very tunable and extremely robust, allowing for extended writing times. A preliminary study of the time evolution of the writing process is also presented. It is evident that the process of grating inscription is very complicated and is not always adequately described by current models. In addition. it appears that the process is strongly dependent upon pulse energy and is accelerated by simultaneous heating of the fiber with a C0��� laser and by heating due to the writing laser. A brief study of the thermal stability of different gratings is then given to determine some of the thermal stability properties of the fiber Bragg gratings written at OSU. / Graduation date: 2001

The three-grating optical interferometer is studied to identify restrictions on alignment and improve stability. A description of the way a three-grating interferometer works is given as well as a method for proper set up. The overall power loss through the three gratings is measured and can be used to estimate the amount of atoms that will be detected at the output of the atomic interferometer. Criteria are developed for misalignment affects. Equations are presented for spacing and rotation limitations which can then be applied to the atomic interferometer. A stabilization technique is developed using a servo system. The elimination of low frequency inertial noise is accomplished. / Graduation date: 1997

No description available.

No description available.

M.Ing. / This thesis, devoted to fibre optics, is primarily concerned with the utilization of fibre Bragg gratings and optical couplers to realize optical add-drop multiplexers (OADMs). A comparative study regarding various OADM configurations is undertaken on the basis of manufacturing and the performance in terms of insertion loss, channel isolation, tuning ranges, stability and cost. The heart of most of the OADMs is fibre Bragg gratings. The Runge-Kutta numerical integration method is used to solve the coupled-mode equations in order to simulate the spectral dependence of Bragg gratings numerically. Properties such as the grating strength, the grating length and the grating index profile governing the spectral dependence of Bragg gratings are investigated. In recent years, there has been an increasing interest in the dispersive properties of Bragg gratings. We investigate methods to limit the amount of dispersion induced by fibre Bragg gratings. The tuning of Bragg gratings for dynamic OADMs is also reviewed. High channel isolation Bragg gratings are theoretically and experimentally investigated. DC-apodized gratings were designed and manufactured by using the phase mask method through the use of a preconditioning technique. Bragg gratings with channel isolations of up to 24.61 dB have been realized by using this technique. The spectral dependence of DC-apodized gratings on the amount of preconditioning and the smoothness of the index envelope is simulated and in agreement with the experimental results. An athermal Bragg grating was designed and manufactured, exhibiting an average wavelength-temperature sensitivity of 2.76 pm/oC. An OADM comprising a DC-apodized Kaiser grating and an optical circulator was realized. The device showed an insertion loss of 1.84 dB and a channel isolation of 22.84 dB. The coupling mechanisms for different types of optical couplers are investigated. The distribution of power was established to be either by evanescent field coupling (etched, polished and weakly fused couplers) or due to the beating phenomenon (strongly fused couplers). The beating phenomenon of the HE11 and HE12 modes in the waist of the tapered-fused coupler is modelled and used to simulate different characteristics, such as wavelength, polarization and external refractive index dependence of tapered-fused couplers, in order to realize OADMs.

No description available.

No description available.