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Quasi-Optical Directional Coupler for Ultra-Wideband THz Vector Network Analyzers
Keywords: terahertz measurements, vector network analyzer, quasi-optical
At mmW and THz band, on-wafer testing is very critical for on-wafer electronics devices and circuit as well as on-wafer spectroscopy. Nevertheless, current measurement equipment capabilities are limited by contact probe technology and vector network analyzers (VNAs). Recently, we proposed a non-contact method to tackle the issue of using expensive and brittle contact probes. Using proper THz optics and on-chip antennas, we couple the signals from the VNA to the on-wafer device under test (DUT) with minimum insertion loss and unmatched repeatability. However, the limitation of the VNA equipment is still limiting the bandwidth and cutoff frequency of the measurements. State-of-the-art VNAs use external frequency multipliers to up/down convert the VNA signal to the THz band. The problem using VNA extenders is three-fold: 1) The maximum cutoff barely makes it into the THz band, 2) they are bandlimited, and 3) they are extremely expensive due to costs associated with waveguide micromachining and sophisticated semiconductor processes for the electronics. Here, we propose the design of a novel class of VNAs that can employ photonics-based sources and detectors and use THz optical components instead of traditional waveguide structures to route the THz signals. With THz photomixers we can implement cost effective THz sources and receivers that can be efficiently coupled to optics. They feature a relatively simple topology and operate in a very wide bandwidth, typically from less than 100 GHz to more than 3 THz. Since the optical components are frequency independent, by adopting photomixers as THz emitter and receivers, the proposed quasi-optical VNA system has a potential to achieve ultrawide bandwidth measurement capabilities. Nevertheless, isolation between reference and measurement signals is needed to produce a reliable testing bed for THz vector measurements. First, we will talk about the design of quasi-optical coupler consists of two beam splitters, which is the most important part of the quasi-optical VNA. Through a rigorous theoretical analysis and experiments we verified a minimum 60 dB isolation and no more than 3 dB of insertion loss at 330-500 GHz. Then, we will talk about how to use this quasi-optical coupler to construct a one-port quasi-optical VNA. We will also discuss the calibration process and present three calibration standards that eliminate the error terms of the one-port VNA topology. Next, we will show the experimental results we collected from our free-space measurements. We used four different kinds of DUT to test the quasi-optical VNA system at 320- 380 GHz with satisfying agreement between the measured and theoretical values of reflection coefficient in all cases.
Yiran Cui,
Arizona State University
Tempe, AZ

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