Abstract
We propose a series of integrated Bragg grating filters with performance enhancement via the concept of effective medium. The bandstop filters are built in a high-resistivity silicon wafer and operated over the WR-3.4 band (220–330 GHz) with in-plane polarization. The proposed designs use an additional degree of freedom in controlling the effective refractive index so as to fully use the potential of the Bragg grating structures. As a result, the high insertion loss typically observed at the low-frequency bound of the filters due to weak wave confinement can be reduced, while radiation caused by the leaky-wave effect at the high-frequency bound is minimized, allowing for a 40% operation fractional bandwidth. These features are not achievable with conventional waveguide Bragg grating filters. All-silicon prototypes of filter samples are experimentally validated, demonstrating promising performance for a wide range of terahertz applications. The techniques to improve the filter characteristics by controlling the effective medium can be adopted in both microwave and optics domains.
Highlights
Terahertz integrated systems have attracted great attention for their compactness, portability, and low cost compared to conventional quasi-optical systems while promising a wide range of applications from imaging to communications.1,2 In particular, technologies from both electronics and photonics have accelerated the progress in forming terahertz integrated circuits, enabling comprehensive functionalities from signal generation, transmission, and processing to detection with moderately high efficiency.1 To this end, terahertz integrated waveguides with high efficiency, low dispersion, and broad bandwidth are indispensable
These results further show that the proposed filter type can operate in a 40% fractional bandwidth that is much larger than any practical optical band
A class of integrated waveguide Bragg grating filters based on all-silicon effective-medium-clad waveguides has been comprehensively investigated
Summary
Terahertz integrated systems have attracted great attention for their compactness, portability, and low cost compared to conventional quasi-optical systems while promising a wide range of applications from imaging to communications. In particular, technologies from both electronics and photonics have accelerated the progress in forming terahertz integrated circuits, enabling comprehensive functionalities from signal generation, transmission, and processing to detection with moderately high efficiency. To this end, terahertz integrated waveguides with high efficiency, low dispersion, and broad bandwidth are indispensable. The transmission levels at lower frequencies tend to be reduced due to weak wave confinement of guided waves These effects limit the filter operation bandwidth, which is generally not an issue for typical narrow-band operation at optical frequencies, e.g., with a relative bandwidth of around 2.3% at the C-band or 3.8% at the L-band. We present a class of terahertz Bragg grating filters based on our recently proposed effective-medium-clad waveguide.3,4 This waveguide platform allows the modal index to be modified by varying the waveguide core dimensions and/or the cladding configuration. Compared to conventional filters with physical corrugation of the waveguide core, this technique offers an additional level of flexibility to control the filter characteristics, e.g., reducing insertion loss and allowing for broadband operation with a compact footprint These features are very desirable in terahertz applications that typically leverage a vast available bandwidth with limited source power.
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