Agilent Measurement Journal - Issue 4 - 2008 - (Page 56)

Agilent-funded research: Terahertz connectors and cables Yun Hua Zhang, Ian Robertson and Roger Pollard University of Leeds There is a growing demand for precision measurements above 200 GHz. Advanced devices such as SiGe, GaAs and InP transistors can operate to 500 GHz, and even regular CMOS technology is now reporting potential operating frequencies well beyond 100 GHz as device dimensions scale down to tens of nanometers. At such high frequencies, laboratory measurements are currently conducted with either rectangular waveguides or in some cases, free-space beams. The former are expensive and narrowband; the latter requires setups that are too fragile to be employed in most applications outside the specialist terahertz research lab. The electronics and physics communities require terahertz connectors and cables not only to extend the capabilities of traditional measurement equipment but also to enable a wide range of new medical sensing and imaging systems. This project has compared myriad potential solutions to this terahertz cable requirement. Coaxial cables and connectors are beyond the limits of conventional fabrication. Rectangular waveguides are restricted to bandwidths of less than one octave. Furthermore, metallic guides of any form have high conductor loss in the terahertz region and there is a need for ultra-low surface roughness. As a result, the project is concentrating on investigating the application of dielectric waveguide techniques. Two materials have emerged as strong candidates for this application: PTFE (Teﬂon) and polypropylene have reported loss tangents of approximately 0.006 at 1 THz. PTFE has proven itself extensively in microwave connector applications; polypropylene is very easy to fabricate. Standard circular dielectric waveguide structures — equivalent to conventional optical ﬁbers — have signiﬁcant dispersion and other limitations. This project has demonstrated that the photonic crystal ﬁber (PCF) technique (sometimes referred to as the “holey ﬁber”) from optical applications can be applied to the design of a terahertz dielectric waveguide. Modeling work has been conducted using the full vectorial Effective Index Method (EIM).1 The use of a microstructured guide is intended to achieve endless single-mode behavior (important for ultrabroadband applications in measurement and imaging) with a ﬂattened dispersion characteristic, controllable mode area (for ease of transition design) and low loss. Figure 3 shows the cross section of a conventional hexagonal PCF. To demonstrate the broadband single-mode behavior of PCF in the terahertz band, Figure 4 compares the mode behavior of a step-index ﬁber and a PCF. For the PCF, the single-mode range is from 202 GHz to 2376 GHz; in contrast, the single-mode range of step-index ﬁber is from 200 GHz to 540 GHz. To illustrate the mechanism of single-mode behavior of PCF, it is possible to introduce normalized Veff by making an analogy to normalized V in step-index ﬁber: My relationship with Agilent Roger Pollard My Agilent odyssey started in 1981 as a young faculty member at the University of Leeds looking to take a brief sabbatical — one different from the usual route of going to another university in another country to teach the same subjects to someone else’s students. Some research background in network measurements made the Hewlett-Packard (now Agilent) division in Santa Rosa, California seem the ideal place, so I penned a request to a conference contact. Three months later, a three-line message asked, “Can you start next week?” I spent the next seven months as part of the team that developed the HP 8510A, little realizing the impact this landmark product would have on the microwave industry. I returned to Leeds and settled back into academic life, but the following summer received a surprise call: “So, when can we expect you?” Now, nearly 27 years later, I still spend part of my summers at Agilent in Santa Rosa. I’m grateful to have had the opportunity to collaborate with some of the ﬁnest engineers in the world. Veff 22 a 2 2 eff ncore − nclad l 56 Agilent Measurement Journal

Table of Contents Feed for the Digital Edition of Agilent Measurement Journal - Issue 4 - 2008

Agilent Measurement Journal - Issue 4 - 2008 Delivering Confidence through Compliance with Standards Contents Emerging Innovations Trying Early Device Implementations at the IEEE 1588 PlugFest Achieving Greater Confidence in Measurement Accuracy through Consistency in Calibration Services Overcoming the Challenges of RFID Component Testing 3GPP LTE: Introducing Single- Carrier FDMA Ensuring Reliable Operation and Performance in Converged IP Networks Testing Storage Area Networks and Devices at 8.5-Gb/s Fibre Channel Rates Choosing an Appropriate Calibration Method for Vector Network Analysis Making Traceable EVM Measurements with Digital Oscilloscopes Exploring Terahertz Measurement, Imaging and Spectroscopy: The Electromagnetic Spectrum’s Final Frontier Interpreting Quoted Specifications when Selecting Digitizers

Agilent Measurement Journal - Issue 4 - 2008

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