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The paper describes the techniques and instrumentation employed in characterization metrology of devices used in THz communication links, addressing primarily transmitters and receivers, and also applicable to passive elements. THz wireless links are rapidly approaching industrial implementation within specified user-scenarios, such as data centres. Rigorous characterization of emitter and receiver devices utilized in THz wireless links is an essential part of link and network design, and is also a pre-condition for industrial implementation and regulatory acceptance. Deployment will require the ability to manufacture and supply reliable, reproducible emitter and receiver devices in compliance within agreed specifications and international standards. As enabling steps, calibration procedures and standards for these devices must be agreed; suitable instrumentation must be developed; and calibration services must be established providing customer accessibility. Widespread adoption of standards and calibration services must be encouraged.
Measurements and applications at THz frequencies employ two disparate classes of devices and instrumentation platforms: photonics-based, which are predominantly free-space; and electronic, which are generally waveguide-coupled. THz wireless communications utilize both electronic and photonic devices, and both waveguide-coupled interconnects and free-space propagating signals. To date, photonics-based free-space THz systems have strongly dominated applications and uptake; while in recent years THz electronics has also been experiencing rapid growth. Conversely, waveguide-coupled electronic THz systems have enjoyed strong predominance in robust metrology and traceable measurements. However, because wireless signals propagate in free space, the relevant device characterization also has to be performed in free space.
In order to fully characterize emitter and detector devices, a variety of measurements must be performed. It is highly desirable that these measurements be carried out using calibrated instruments and applying traceable techniques. Commercial instrumentation is available for some of the necessary measurements; this is supplemented where needed by laboratory-built equipment. In addition, several types of characterization techniques and instruments have been custom-developed. The paper details the device characterization techniques and their metrological aspects.
Emitters require measurements of power, spectrum, and beam profile. Whereas calibrated power meters are commercially available, bespoke emitter-specific techniques for determining noise and stability for both amplitude and phase had to be developed. Similarly, the centre frequency and linewidth of an emitter can be measured using a traceably calibrated commercial signal analyser. In addition, a laboratory-built lamellar interferometer was employed in order to reveal the broadband spectral profile (e.g. features such as harmonics and side-lobes). Emitter beam profile and divergence was determined using two different approaches: first, and aperture raster-scan, revealing the power profile; and second, electro-optic imaging, revealing field amplitude and phase profiles. Receivers or detectors require measurements of spectral responsivity and spectral NEP (noise-equivalent power), response time, and beam acceptance cone. A calibrated broadband source with known emission spectrum and noise spectrum, such as a black-body, is necessary to determine detector responsivity and NEP. Such source was used together with the lamellar interferometer to obtain spectral response of receivers. The acceptance cone was determined using a well-collimated source and a goniometer-like arrangement where the receiver axis was rotated relative to the beam axis.
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