Theoretical and experimental studies of real-time interrogation of fiber Bragg grating (FBG) sensors based on chirped pulse compression with increased interrogation resolution and signal-to-noise ratio are presented. Two interrogation systems are proposed in this thesis. In the first interrogation system, a linearly chirped FBG (LCFBG) is employed as the sensing element. By incorporating the LCFBG in an optical interferometer as the sensor encoding system, employing wavelength-to-time mapping and chirped pulse compression technique, the correlation of output microwave waveform with a chirped reference waveform would provide an interrogation result with high speed and high resolution. The proposed system can provide an interrogation resolution as high as 0.25 μ at a speed of 48.6 MHz. The second interrogation system is designed to achieve simultaneous measurement of strain and temperature. In this system, a high-birefringence LCFBG (Hi-Bi LCFBG) is employed as a sensing element.

Theoretical and experimental studies of real-time interrogation of fiber Bragg grating (FBG) sensors based on chirped pulse compression with increased interrogation resolution and signal-to-noise ratio are presented. Two interrogation systems are proposed in this thesis. In the first interrogation system, a linearly chirped FBG (LCFBG) is employed as the sensing element. By incorporating the LCFBG in an optical interferometer as the sensor encoding system, employing wavelength-to-time mapping and chirped pulse compression technique, the correlation of output microwave waveform with a chirped reference waveform would provide an interrogation result with high speed and high resolution. The proposed system can provide an interrogation resolution as high as 0.25 μ at a speed of 48.6 MHz. The second interrogation system is designed to achieve simultaneous measurement of strain and temperature. In this system, a high-birefringence LCFBG (Hi-Bi LCFBG) is employed as a sensing element.

A terahertz photoconductive source structure with nano-grating electrodes is proposed. The resonance modes of the one-dimensional nano-grating and their affect the optical power absorption are studied. In addition, an approach for optimal design of the grating to maximize the photocurrent for different proposed DC biases, is presented. The dependence of the photocurrent on physical parameters of photomixer are analyzed. A fast analysis method for a new terahertz waveguide for photo-mixing is proposed. The wave-guiding mixer structure is a modified parallel plate waveguide (PPWG) in which the top plate is replaced by a periodic array of sub-wavelength nano-slits. The substrate of the PPWG is made of a fast photoconductive material in which laser photomixing/absorption occurs. The characteristic equation of the modified PPWG when used as a THz waveguide is derived analytically, and its guided modes are studied in details over THz range of frequencies. The accuracy of the analytical results are verified by comparison with full-wave numerical simulations. The criteria for choosing the suitable mode for photomixing application are also discussed. Finally, based on dyadic Green’s function representation, a systematic approach is provided for calculating the amplitude of the guided modes that are excited by an arbitrary photocurrent.

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Modeling of wave scattering from grating couplers has become increasingly important due to extensive recent research interest in the problem of plasmonic resonance. Computational algorithms which are specially used to model the problem of scattering from the grating surfaces suffer from several drawbacks such as accuracy, computational efficiency, and generality. To address the challenges of the previous methods, this work presents a novel hybrid Finite Element-Boundary Integral Method (FE-BIM) solution to the problem of scattering from grating surfaces consisting of finite or infinite array of two-dimensional cavities and holes in an infinite metallic walls covered with a stratified dielectric layer. To solve the scattering problem from finite number of cavities or holes engraved in a perfectly conducting screen (PEC), the solution region is divided into interior regions containing the cavities or holes and the region exterior to them. The finite element formulation is applied inside the interior region to derive a linear system of equations associated with nodal field values. Using two-boundary formulation, the surface integral equation employing free-space Green's function is then applied at \emph{only} the opening of the cavities or holes to truncate the computational domain and to connect the matrix subsystem generated from each cavity or hole. The hybrid FE-BIM method is extended to solve the scattering problem from an infinite array of cavities or holes in a PEC screen by deriving the quasi-periodic Green's function. In the scattering problem from an infinite array of cavities, the finite element formulation is first used inside a single cavity in the unit-cell. Next, the surface integral equation employing the quasi-periodic Green's function is applied at the opening of \emph{only} a single cavity as a boundary constraint to truncate the computational domain. Effect of the infinite array of cavities is incorporated into the system of the nodal equations by the quasi-periodic Green's function. Finally, the method based on the hybrid FE-BIM is developed to solve the scattering problem from grating surfaces covered with a stratified dielectric layer. In this method, the surface integral equation employing grounded dielectric slab Green's function is applied at the opening of the cavities or holes inside the dielectric coating to truncate the solution region efficiently. An accurate algorithm is presented to derive the grounded dielectric slab Green's function in spatial domain incorporating the effects of the surface-waves and leaky-waves excited and propagated inside the dielectric slab. Numerical examples of near and far field calculations for finite or infinite array of cavities or holes are presented to validate accuracy, versatility, and efficiency of the algorithm presented in this thesis.

The dual-writing process of holographic technique was applied to fabricate the two-dimensional surface relief gratings on azo-dye doped polymer film in this study. The structure of two-dimensional grooves was investigated under the various polarization state of writing beam, the various angle between two stages of writing, and the adoption of nematic liquid crystals as the interface. The results revealed that the depth of two-dimensional surface relief gratings produced by the writing beams with P-polarization and the interface of nematic liquid crystals in contact with azo-dye polymer film was about 2~3 times of that without nematic liquid crystals as the interface. As the angle between two stages of writing increases, the first order diffraction efficiency and the depth of surface relief structure in the second writing becomes smaller for the writing beam with S polarization; while the first order diffraction efficiency and the depth of surface relief structure becomes larger for the writing beam with P polarization.

Recently, long-period fiber gratings (LPFGs) based on PCFs have been demonstrated by using heating or a mechanically pressure to induce periodic index variations along the fibers. However, LPFGs fabricated by these two methods suffer the structure damage. In this thesis we propose novel optically controllable LPFGs based on the photoresponsive photonic liquid crystal fibers (PLCFs) and no structure damage occurs during the fabrication process. The photoresponsive PLCF was filled with a LC mixture consisting of the nematic LC E7 and the photoresponsive 4MAB. The properties of the photoresponsive PLCF can be modulated by using laser irradiation. In addition, the transmission bands of the photoresponsive PLCF can also be tuned by controlling the 4MAB concentration or operation temperature. An optically controllable LPFG was fabricated based on the photoresponsive PLCF by using blue-laser irradiation through a mask with 700-£gm grating period. The measured resonant wavelength appeared at 1539 nm with the FWHM was 27 nm, and the maximum dip depth was about −15 dB with a 6.5-dB insertion loss. The LPFG was shown to be erasable by using a green laser. In addition, we have also investigated the effects of the number of grating period, 4MAB concentrations, operation temperatures, thermal recovery properties, and irradiation intensity on the LPFGs. Our proposed optically controllable LPFGs possess reversible property and are quite useful to be applied in tunable optical devices.

Silicon-on-Insulator (SOI) micro-ring resonators (MRRs) are versatile elements in high-density integrated optics telecommunication systems. However, small inaccuracies in the fabrication process intensely deteriorate the response of SOI MRRs. By utilizing the racetrack resonator structures with strong coupling abilities, one can improve the fabrication tolerance. For the SOI racetrack resonators, the FSR is usually large. By introducing gratings into SOI racetrack resonators, the mutual mode coupling between the clockwise and counterclockwise modes can be induced and result in the resonance splitting. The grating-assisted SOI racetrack resonators can increase the operation wavelength and open up the possibility to overcome this limitation. In this thesis, we first use the 2-D FDTD method with the effective index method (EIM) to obtain the transmission spectra of the SOI racetrack resonators. The transmission spectra are then fitted by using the time-domain coupled mode theory (CMT) to obtain the quality factor and optical parameters of the SOI racetrack resonators. Next, we demonstrate the characteristics of mode splitting resulted from the mutual mode coupling between the clockwise and counterclockwise modes in the grating-assisted racetrack resonators by utilizing both the CMT and the 2-D FDTD method with the EIM. By tuning the grating configurations, such as the length or the structure of sidewall gratings, one can obtain the desired transmission spectrum of the grating-assisted racetrack resonators. Finally, we numerically investigate the temperature-dependent spectral characterics of the grating-assisted SOI racetrack resonator by taking the thermal-optic responce of the SOI materials into account. The thermal sensitivity of this device is 95.38 pm/¢XC, and the calculted properties can help the further designs based on the grating-assisted SOI racetrack resonators.

Abstract Compared with the other micro acoustic wave devices, the flexural plate-wave (FPW) device is more suitable for being used in liquid-sensing applications due to its higher mass sensitivity, lower phase velocity and lower operation frequency. However, conventional FPW devices usually present a high insertion loss and low fabrication yield. To reduce the insertion loss and enhance the fabrication yield of FPW device, a 1.5 £gm-thick silicon-trench reflective grating structure (RGS), a high electromechanical coupling coefficient ZnO thin-film and a 5 £gm-thick silicon oxide membrane substrate are adopted in this research. The influences of the amount of silicon trench and the distance between inter-digital transducer (IDT) and RGS on the insertion loss and quality factor of FPW device are investigated. The main fabrication technology adopted in the study is bulk micromachining technology and the main fabrication steps include six thin-film deposition and five photolithography processes. Under the optimized conditions of the sputtering deposition processes (200¢J substrate temperature, 200 W radio-frequency power and 75% gas flow ratio), a high C-axis (002) orientation ZnO piezoelectric thin-film with 31.33% electromechanical coupling coefficient can be demonstrated. The peak of XRD intensity of the standard ZnO film occurs at diffraction angle 2£c = 34.422¢X, which matches well with our results (2£c = 34.282¢X). By controlling the thickness of ZnO/Au/Cr/SiO2/Si3N4 sensing membrane less than 6.5 £gm-thick, the fabrication yield of FPW device can be improved and a low operation frequency (6.286 MHz) and high mass sensitivity (-113.63 cm2 / g) can be achieved. In addition, as the implemented FPW device with four silicon trenches RGS and 37.5 £gm distance between IDT and RGS, a low insertion loss (-40.854 dB) and very high quality factor (Q=206) can be obtained. Keywords¡Gflexural plate-wave; silicon-trench reflective grating structure; electromechanical coupling coefficient; ZnO; bulk micromachining technology

In this thesis, a 6-beam combiner using multiplexed holograms in dye-doped polymer is investigated. It is realized by recording six superimposed holographic gratings, which show uniform diffraction efficiency. The coupled wave theory for N superimposed gratings is more generalized and is used to analyze the amplitudes of diffracted waves in three different boundary conditions. Multiple-ring diffracted beam analysis is proposed to determine the dynamic range of a holographic material. The M/# is evaluated by recording a single hologram and counting the number of ring patterns in the diffracted beam. This analysis is extended to assess the equalized grating strength of N superimposed holograms. Six holograms with the equalized grating strength which can be assigned within the dynamic range of our material and show maximum diffraction efficiency are recorded. The phase locking of five beams to one reference beam is performed using PZT controller. The designs of lock-in amplifier, ramp generator and servo using commercial chips are demonstrated. The readout set-up used to split one single beam into six coherent copies is presented. The function of each part of the PZT controller in the readout set-up is discussed in detail. The intensity profile of an N-beam combiner is investigated by varying the phase angle between adjacent input waves. The entire solution which describes the amplitude of a combined beam is derived from generalized coupled wave theory. A simplified experimental set-up without a complicated PZT controller is demonstrated using a planoconvex lens. In order to provide six coherent light sources in future work, the injection locking of a single laser diode to the master laser diode is performed. An expected read-out setup is proposed to carry out both the achievement of six coherent sources and a 6 beam combination.