Faezeh Fesharaki

Low-Loss and Low-Dispersion Transmission Line Over DC-to-THz Spectrum

Transmission lines or waveguides are the most fundamental building blocks of all electronic and photonic circuits and systems. Efforts have been made to incrementally evolve and improve existing transmission line structures to meet the increasingly stringent demands for signal transmission bandwidth and performance. However, a potentially revolutionary scheme or disruptive concept is required in support of future technological needs and bridging the gap between electronics and photonics. In this paper, we report on a fully integrated transmission line with simple structure that overcomes the long-standing bottleneck problems of high attenuation, strong dispersion, and low mode confinement in the guided-wave signal transmission from dc to terahertz (THz). This so-called mode-selective transmission line (MSTL) supports super-broadband and/or ultrafast pulse signal propagation, making it a disruptive solution for building future high-performance analog and digital integrated electronics and photonics. To demonstrate this scheme, an MSTL on fused silica substrate is designed, fabricated, and experimentally measured from near-dc to 0.5 THz, showing less than 0.35 dB/mm attenuation and low dispersion characteristics over the entire frequency range.

Band-Pass Non-TEM Mode Traveling-Wave Electro-Optical Polymer Modulator for Millimeter-Wave and Terahertz Application

High-frequency electro-optical modulator is critical for enabling signal processing and distribution in the next generation cloud-computing, tele-medicine, and telecommunications. In this paper, substrate integrated waveguide (SIW) is exploited as an alternative fundamental transmission line structure in support of electrical signal for the design and development of millimeter-wave and terahertz (THz) traveling-wave polymeric electro-optic (EO) modulator. Optical and full-wave electromagnetic analyses are carried out and structure optimization is made on the basis of such analyses in order to obtain millimeter-wave transmission characteristics and optical response. Compared to its conventional TEM-mode transmission lines, this bandpass non-TEM mode SIW-based EO modulator presents numerous advantages, namely compact structure, low transmission loss, low driving power, simple packaging and flat optical response over a wide frequency range.

Plasmon-enhanced LT-GaAs/AlAs heterostructure photoconductive antennas for sub-bandgap terahertz generation.

Photocurrent generation in low-temperature-grown GaAs (LT-GaAs) has been significantly improved by growing a thin AlAs isolation layer between the LT-GaAs layer and semi-insulating (SI)-GaAs substrate. The AlAs layer allows greater arsenic incorporation into the LT-GaAs layer, prevents current diffusion into the GaAs substrate, and provides optical back-reflection that enhances below bandgap terahertz generation. Our plasmon-enhanced LT-GaAs/AlAs photoconductive antennas provide 4.5 THz bandwidth and 75 dB signal-to-noise ratio (SNR) under 50 mW of 1570 nm excitation, whereas the structure without the AlAs layer gives 3 THz bandwidth, 65 dB SNR for the same conditions.

S-Parameter Deembedding Algorithm and Its Application to Substrate Integrated Waveguide Lumped Circuit Model Extraction

In this paper, a unified method is introduced and formulated for deriving the equivalent circuit model of a guided wave structure as well as its characteristic impedance for any mode. This method is developed through the combination of a generalized multimode calibration and a full-wave simulation, and it is applicable to the fundamental mode as well as higher order modes. The method is verified for both TEM and non-TEM propagating modes and is applied to extract the characteristic impedance and circuit model of non-TEM guided-wave structures. This scheme allows for the effective use of a commercial electromagnetic field simulator in removing inherent numerical simulation noises or errors. Extracting circuit parameters through this technique for establishing equivalent circuit models can overcome the shortcomings encountered in the modeling of guided-wave eigenvalue problems as well as in the design of multimode structures.

Optical properties of epitaxial Ca x Ba 1-x Nb 2 O 6 thin film based rib-waveguide structure on (001) MgO for electro-optic applications.

In this work, optical properties of epitaxial CaxBa1-xNb2O6, CBN (x = 0.28) thin film based waveguides are studied at 1550 nm optical communications wavelength. CBN thin films are deposited epitaxially on MgO substrates using Pulsed Laser Deposition and characterized by prism coupling to extract the refractive index and propagation loss. It is shown that the 2 µm-thick epitaxial CBN thin films have a refractive index close to the bulk form and the CBN planar waveguides have a propagation loss of 4.3 ± 0.5 dB/cm. 1 cm-long rib waveguide structures were fabricated using a high density plasma etching. Their propagation losses were measured by the cutback method at 8.4 ± 0.6 dB/cm.

Theoretical Analysis and Experimental Evaluation of SiO 2 -CBN-MgO Rib Waveguide Structure

This letter reports the design, simulation, optimization, and optical characterization of an electro-optical SiO2-calcium barium niobate (CBN)-MgO rib waveguide structure. To achieve the single mode and low loss operation, the proposed rib waveguide structure is simulated and optimized with respect to CBN thin film thickness, etching depth, and rib width. Propagating optical mode profile of the waveguide is simulated using a beam propagation method solver. To verify the design and simulation, CBN thin film is coated on MgO substrate using a pulsed laser deposition technique, and SiO2-CBN-MgO is fabricated with various CBN thicknesses and rib widths. A smooth and nearly vertical etching of rib side wall enables the fabrication of high quality rib waveguides. The waveguides are characterized and measured wave profiles are compared with simulated results. It is found that the guided modes are well confined within the rib and extended through the core. An excellent agreement between simulations and measurements is observed, thereby validating the design method.

Guided-Wave Properties of Mode-Selective Transmission Line

The so-called mode-selective transmission line or simply “MSTL” is studied theoretically and experimentally. This low-loss and low-dispersion transmission line operates with a frequency-dependent mode-switching behavior. This self-adaptive mode-selective guided-wave structure begins with the propagation of electromagnetic waves over the lower frequency range in the form of a quasi-TEM fundamental mode similar to the microstrip line case, then followed by a fundamental quasi-TE10 mode with reference to rectangular waveguide over the higher frequency region. To gain insight into the physical mechanism and fundamental features of this mode-selective transmission line, a detailed semi-analytical hybrid-mode analysis is developed through the application of a method of lines. This method allows accurate and effective modeling of MSTL guided-wave properties. Propagation characteristics of this proposed mode-agile structure in terms of dispersion, modal, and loss properties are examined, which leads to the establishment of some basic MSTL design guidelines. Numerical results confirm the expected mode conversion and low-loss behavior through the observation of field evolutions along the structure. For experimental verification, a set of MSTL prototypes are fabricated on two different substrates through dissimilar fabrication processes. Measurements are carried out from dc-to-500 GHz using a vector network analyzer. Excellent agreement between theoretical and experimental results is observed. It is confirmed that the low-dispersion and low-loss behavior of MSTL makes it an outstanding integrated waveguide in support of high-performance super-broadband signal transmission and/or ultra-fast pulse propagation in a fully-integrated platform.

Physical evidence of mode conversion along mode-selective transmission line

This work investigates mode conversion along a longitudinally uniform mode-selective transmission line (MSTL) and demonstrates physical evidence of the mode conversion. Mode conversion of the fundamental mode is observed through examining the field distributions of the MSTL. Characteristic mode conversion frequency is defined based on the distribution properties of the longitudinal magnetic field component and the intrinsic physical implications. The accuracy and effectiveness of this definition are verified numerically and experimentally. The result indicates that a quasi-TEM fundamental mode and a quasi-TE10 fundamental mode dominate in the MSTL below and above this frequency, respectively. This mode conversion could have specific applications in the design of microwave, millimeter-wave, and THz wideband and multi-band components and systems.

High-integrity terabit-per-second signal interconnects with mode-selective transmission line

An alternative global interconnects solution with minimal signal distortion and propagation loss is explored and reported in this work. This interconnect scheme is made possible thanks to the use of a mode-selective transmission line (MSTL), which supports a fundamental TEM mode in lower frequency range covering DC while it is automatically reconfigured to support the fundamental TE10 mode at higher frequency. An MSTL is designed, fabricated, and characterized providing low-attenuation and dispersion-free picosecond pulse propagation with an excellent output signal-to-noise ratio. This technology allows for a jitter-free signal transmission with data-rates greater than 200 Gb/s through just one printed circuit board.

Mode-selective transmission line for DC-to-THz super-broadband operation

A class of transformative transmission lines and waveguides called mode-selective transmission line (MSTL), is devised and demonstrated in this work. MSTL, in a fully integrated form, exhibits a disparate modal characteristic of traditional planar microwave transmission lines and non-planar waveguides. Whereas at low frequency, MSTL operates under the TEM regime, the operating mode is gradually converted to low-loss TE10 mode for operation as frequency moves up such as millimeter-waves and THz. An experimental MSTL prototype on fused silica substrate is designed, fabricated, and measured from DC to 500 GHz, showing unprecedented low-attenuation and low-dispersion characteristics over the entire frequency range.