A polarization independent electrooptic add/drop tunable filter fabricated on a LiNbO3 substrate with a narrow -3 dB bandwidth (FWHM) of 1.56 nm operating in the 1.55 μm wavelength regime has been developed to meet the demands of fast tuning speed and increased channel capacity for dense wavelength division multiplexed (WDM) networks. The operation of the filter is based on passive polarization beam splitters and strain-induced phase-matched TE↔TM polarization mode converters. Extinction ratios as high as 20 dB for polarization beam splitters were achieved using zero-gap two-mode interference directional couplers with an opening angle of 0.55°. A tunable TE↔TM polarization mode converter with 98.2 % conversion efficiency was obtained using a strain-induced refractive index grating consisting of 765 parallel strips 10.5 μm wide in a strained SiO2 surface film with a spatial period of 21 μm. Thermal and electrooptic tuning of the polarization mode converters were examined. A polarization independent electrooptic add/drop tunable filter in which the fabrication parameters of the splitter and the mode converter were optimized was produced. Fiber-to-fiber insertion loss less than 6.3 dB was measured on a 62 mm long filter device. The spectral characteristics reveal a -3 dB bandwidth of 1.56 nm and nearest sidelobes about 12 dB below the center peak. A thermal tuning rate of -0.903 nm/°C was realized. Electrooptic tuning was also demonstrated. A tuning range of 14.08 nm with applied DC voltages from -80 V to +80 V was achieved indicating an electrooptic tuning rate of 0.086 nm/V. The filter performance for both TE and TM modes was examined and polarization independence of the spectral characteristics was confirmed.

A new tuning scheme for linearly tunable high-Q filters is proposed. The tuning method is based on using the phase information for both frequency and Q factor tuning. There is no need to find out the relationship between a filter's passband magnitude and Q. A gm-C biquadratic filter is designed to demonstrate the proposed tuning circuitry. The project includes a phase locked loop (PLL) based frequency tuning loop, reference clock generator, and differential difference amplifier (DDA) for dealing with frequency and Q factor tuning loop and linearly tunable second order gm-C bandpass filter. Simulation results for a 10 MHz prototype filter using AMI 0.5μm process is presented. The chip testing results show that the automatic frequency tuning error is 2.5% for the 10 MHz case.

A new tuning scheme for linearly tunable high-Q filters is proposed. The tuning method is based on using the phase information for both frequency and Q factor tuning. There is no need to find out the relationship between a filter's passband magnitude and Q. A gm-C biquadratic filter is designed to demonstrate the proposed tuning circuitry. The project includes a phase locked loop (PLL) based frequency tuning loop, reference clock generator, and differential difference amplifier (DDA) for dealing with frequency and Q factor tuning loop and linearly tunable second order gm-C bandpass filter. Simulation results for a 10 MHz prototype filter using AMI 0.5μm process is presented. The chip testing results show that the automatic frequency tuning error is 2.5% for the 10 MHz case.

A polarization-independent electro-optically tunable add/drop filter utilizing non-polarizing novel relaxed beam splitters has been developed in LiNbO3 at the 1.55μm wavelength regime. The operation of this filter is based on passive directional coupler type beam splitters and strain-induced phase-matched TE↔TM polarization mode converters on an asymmetric Mach-Zehnder interferometer waveguide configuration. Fabrication parameters for channel waveguides, relaxed beam splitters and polarization mode converters were optimized individually then integrated to produce the final device. Single mode channel waveguides for both TE and TM polarizations were realized by the diffusion of 7μm wide Ti strips into LiNbO3 substrate. Relaxed beam splitters were produced using Ti diffused waveguides in a directional coupler configuration with 3.5mm long coupling region, 0.6º bending angle, and separation gap of 11μm and 13μm between waveguides. Tunable TE↔TM polarization mode converters with 99.8%

A polarization-independent electro-optically tunable add/drop filter utilizing non-polarizing novel relaxed beam splitters has been developed in LiNbO3 at the 1.55μm wavelength regime. The operation of this filter is based on passive directional coupler type beam splitters and strain-induced phase-matched TE↔TM polarization mode converters on an asymmetric Mach-Zehnder interferometer waveguide configuration. Fabrication parameters for channel waveguides, relaxed beam splitters and polarization mode converters were optimized individually then integrated to produce the final device. Single mode channel waveguides for both TE and TM polarizations were realized by the diffusion of 7μm wide Ti strips into LiNbO3 substrate. Relaxed beam splitters were produced using Ti diffused waveguides in a directional coupler configuration with 3.5mm long coupling region, 0.6º bending angle, and separation gap of 11μm and 13μm between waveguides. Tunable TE↔TM polarization mode converters with 99.8%

This thesis presents the theory and design of fixed and Radio Frequency (RF) Microelectromechanical Systems (MEMS) -based tunable microwave filters for RF and microwave applications. The methodology for the design of coupled resonator filters is explained in detail and is used to design an end-coupled microstrip filter at 1.5 GHz with inductive loading using a stepped microstrip discontinuity to lower the resonance frequency of the half-wavelength microstrip resonator. The fabricated endcoupled filter shows center frequencies of 1.36 GHz and 1.03 GHz in the unloaded and loaded state respectively, with insertion losses between 1.2-1.5 dB and return loss better than 10 dB in both states. The filter response shows spurious passbands at approximately twice the filter center frequencies. To overcome this problem and improve the upper rejection skirt of the filter, microstrip resonators with tapped input/output coupling and mixed inter-resonator coupling are used to suppress the spurious passband by introducing a transmission zero at spurious resonance frequency. Measurement results for the fabricated tapped-resonator filters show an improvement of the upper rejection skirt due to spurious suppression to a level of -40 dB, with insertion loss of 1.2-1.5 dB for the same center frequencies. The concepts developed from fabrication and measurement of fixed-tuned microstrip filters are used to design an inductively-loaded RF MEMS tunable filter with adjustable spurious suppression implemented using packaged metal-contact switches. The two-pole 5% filter has a tuning range of 17% from 1.06 GHz to 1.23 GHz with an insertion loss of 1.56-2.28 dB and return loss better than 13 dB over the tuning range. The inductive loading mechanism is used to tune the open-ended quarter wavelength stub such that a tunable transmission zero supresses the spurious resonance as the filter center frequency is tuned. The spurious passband response in both states is suppressed below -20 dB. The unloaded quality factor (Q) of the filter varies from 127 to 75 as the filter is tuned. The equivalent circuit model for the series metalcontact packaged RF MEMS switch used in the tunable filter is derived from full-wave electromagnetic simulations and used to predict the effect of MEMS switch parasitics on the overall performance of the tunable filter.

Today's microwave circuits, whether for communication, radar, or testing systems, need compact tunable microwave filters. Since different microwave circuit applications have radically different size, power, insertion loss, rejection, vibration, and thermal requirements, new filter technologies with different balances between these requirements are always desirable. Negative Refractive Index (NRI) transmission media was discovered 10 years ago with the unique property of negative phase propagation. A literature review was conducted to identify potential NRI methods for filters and other devices, but no NRI tunable filters were found. To address this gap, a family of tunable NRI bandpass filters was simulated and constructed successfully using end-coupled zeroth order resonators. Tuning was accomplished by controlling the negative phase length of the NRI sections with varactors. The resulting L-band filters exhibited a 25-40 percent tunable range, no higher order resonances, and required only one fourth the length of a coupled-line filter constructed from traditional 180 degree microstrip resonators.

RF/microwave tunable filters are widely employed in radar systems, measurement instruments, and communication systems. By using tunable filters, the frequency bandwidth is utilized effectively and the system cost and complexity is reduced. In the literature, various tuning techniques have been developed to construct tunable filters. Mechanical tuning, magnetic tuning, and electrical tuning are the most common. In terms of quality factor, power handling capability, and linearity, mechanical tuning is superior to the other two tuning techniques. Unfortunately, due to their bulky size, heavy weight, and low tuning speed, mechanically tunable filters have limited applications. MicroElectroMechanical Systems (MEMS) technology has the potential to produce highly miniaturized tunable filters; however, most of the MEMS tunable filters reported so far have a relatively low quality factor. The objective of the research described in this thesis is to investigate the feasibility of using MEMS technology to develop tunable filters with a high quality factor. The integration of MEMS tuning elements with a wide range of filter configurations is explored, from micromachined filters to traditional dielectric resonator filters, from planar filters to cavity filters. Both hybrid integration and monolithic integration approaches are carried out. To achieve tunability, MEMS tuning elements are embedded within RF and microwave filters. Tuning is accomplished by disturbing the electromagnetic fields of resonators with nearby MEMS tuning elements, which in turn change the resonant frequency of the resonators. First, the proposed tuning concept is experimentally demonstrated by integrating a surface micromachined planar filter with MEMS thermal actuators as the tuning elements. Then, a novel micromachined ridge waveguide filter embedded with similar MEMS tuning elements is proposed and constructed by using the EFAB^{TM} micromachining technique. A power handling analysis is performed for the newly devised 3D micromachined filter, and potential failure mechanisms such as air breakdown are identified. For the first time, a tunable dielectric resonator bandpass filter, incorporating vertical long-throw MEMS thermal actuators as tuning elements, is developed to achieve a wide tuning range, high quality factor, and large power handling capability. Several prototype tunable filter units are fabricated and tested. The experimental results reveal that the tunable filters maintain a relatively high quality factor value over a wide tuning range. In addition to the hybrid integration approach, a monolithic integration approach is investigated. A novel surface micromachining process is developed to allow monolithic integration of MEMS tuning elements into micromachined filters. Due to a stress mismatch, MEMS actuators fabricated by this process obtain a vertical deflection of several hundred microns, resulting in a wide tuning range. Various latching mechanisms are created, based on the micromachining processes that are used to fabricate the MEMS tuning elements. These out-of-plane latching mechanisms with multi-stable states have the potential to be adopted not only for tunable filter applications but also for switches and phase shifters.

RF/microwave tunable filters are widely employed in radar systems, measurement instruments, and communication systems. By using tunable filters, the frequency bandwidth is utilized effectively and the system cost and complexity is reduced. In the literature, various tuning techniques have been developed to construct tunable filters. Mechanical tuning, magnetic tuning, and electrical tuning are the most common. In terms of quality factor, power handling capability, and linearity, mechanical tuning is superior to the other two tuning techniques. Unfortunately, due to their bulky size, heavy weight, and low tuning speed, mechanically tunable filters have limited applications. MicroElectroMechanical Systems (MEMS) technology has the potential to produce highly miniaturized tunable filters; however, most of the MEMS tunable filters reported so far have a relatively low quality factor. The objective of the research described in this thesis is to investigate the feasibility of using MEMS technology to develop tunable filters with a high quality factor. The integration of MEMS tuning elements with a wide range of filter configurations is explored, from micromachined filters to traditional dielectric resonator filters, from planar filters to cavity filters. Both hybrid integration and monolithic integration approaches are carried out. To achieve tunability, MEMS tuning elements are embedded within RF and microwave filters. Tuning is accomplished by disturbing the electromagnetic fields of resonators with nearby MEMS tuning elements, which in turn change the resonant frequency of the resonators. First, the proposed tuning concept is experimentally demonstrated by integrating a surface micromachined planar filter with MEMS thermal actuators as the tuning elements. Then, a novel micromachined ridge waveguide filter embedded with similar MEMS tuning elements is proposed and constructed by using the EFAB^{TM} micromachining technique. A power handling analysis is performed for the newly devised 3D micromachined filter, and potential failure mechanisms such as air breakdown are identified. For the first time, a tunable dielectric resonator bandpass filter, incorporating vertical long-throw MEMS thermal actuators as tuning elements, is developed to achieve a wide tuning range, high quality factor, and large power handling capability. Several prototype tunable filter units are fabricated and tested. The experimental results reveal that the tunable filters maintain a relatively high quality factor value over a wide tuning range. In addition to the hybrid integration approach, a monolithic integration approach is investigated. A novel surface micromachining process is developed to allow monolithic integration of MEMS tuning elements into micromachined filters. Due to a stress mismatch, MEMS actuators fabricated by this process obtain a vertical deflection of several hundred microns, resulting in a wide tuning range. Various latching mechanisms are created, based on the micromachining processes that are used to fabricate the MEMS tuning elements. These out-of-plane latching mechanisms with multi-stable states have the potential to be adopted not only for tunable filter applications but also for switches and phase shifters.

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