Abstract
The measurement of microwave electric-field (E-field) exposure is an ever-evolving subject that has recently led the International Commission on Non-Ionizing Radiation Protection to change its recommendations. With frequencies increasing toward terahertz (THz), stimulated by 5G deployment, the measurement specifications reveal ever more demanding challenges in terms of bandwidth (BW) and miniaturization. We propose a focus on minimally invasive E-field sensors, which are crucial for the in situ and near-field characterization of E-fields both in harsh environments such as plasmas and in the vicinity of emitters. We browse the large varieties of measurement devices, among which the electro-optic (EO) probes stand out for their potential of high BW up to THz, minimal invasiveness, and ability of vector measurements. We describe and compare the three main categories of EO sensors, from bulk systems to nanoprobes. First, we show how bulk-sensors have evolved toward attractive fibered systems that are advantageously employed in plasmas, resonance magnetic imagings chambers or for radiation-pattern imaging up to THz frequencies. Then we describe how the integration of waveguides helps to gain robustness, lateral resolution, and sensitivity. The third part is dedicated to the ultra-miniaturization of components allowing ultimate steps toward electromagnetic invisibility. This review aims at pointing out the recent evolutions over the past 10 years, with a highlight on the specificities of each photonic architecture. It also shows the way to future multi-physics and multi-arrays smart sensing platforms.
Highlights
Exposure to microwave E-fields has been dangerously increasing with the growing wireless telecommunications, boosted by 5G deployment, the Internet of Things, and automotive vehicles
We propose to classify the integrated E-field EO probes into two categories: waveguide-based probes and photonic probes, differentiated by the size of their optical waveguides
The revolution initiated in LN photonic integrated circuits thanks to ion slicing offers the opportunity to reduce the size of LiNbO3 components down to millimeter sizes
Summary
Exposure to microwave E-fields has been dangerously increasing with the growing wireless telecommunications, boosted by 5G deployment, the Internet of Things, and automotive vehicles. The E-field omnipresence and the rise in frequencies induce new health and safety issues, which have pushed the International Commission on Non-Ionizing Radiation Protection to update its exposure guidelines.[1] In this context, E-field measurement is more than ever required, be it for the evaluation of specific absorption rates (SARs), power density,[2,3] or for assessing the emitted radiating fields.[4] In this paper, we focus on the evaluation of E-fields emitted either in biomedical chambers such as magnetic resonance imaging (MRI) or cold plasmas, or in free space due to antennas or electrical devices. Optical nanoantennas have emerged over the past decade as excellent candidates for minimally invasive measurements,[11,12,13] leading to the in vivo study of electro-physiological fields Their submicrometric footprint permits a long-term in vivo evaluation of the electric field impact onto the organism with nanometric spatial resolution.[12] photonic nanoantennas rely on absorption measurements and cannot give direct information about the vectorial nature of fields. The new photonic EO architectures are highlighted and compared with the established EO configurations
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