Minimally invasive optical sensors for microwave-electric-field exposure measurements

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. © The Authors. Published by SPIE under a Creative Commons Attribution 4.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.

Limitations of using electro-optical probes for the measurement of the electromagnetic field emitted by the new generations of wireless communication devices

“…However, even with the recent progress of the EO field sensing technology, their sensitivity remains insufficient for sensing the EMF originated from state-of-the-art communication devices. The sensitivity of the EO probes was documented to be as low as 33mV/m at a RBW of 10 Hz [15] for continuous wave. Even with this improved sensitivity, the RBW value is insufficient for enabling fast SWT at the SA, required to capture the emitted signal time dynamics.…”

“…The sensitivity of the probes is 40 mVRMS/m Hz¹/² for the longitudinal probe and 200 mVRMS/m Hz¹/² for the transversal probe. Other EO probes with increased sensitivity were documented [15] but their spatial resolution is much higher than the selected probes. (1)…”

“…They have several advantages like compact size, strong adaptability to harsh environments, and immunity to electromagnetic interference [13] but are also limited by the bandwidth to be measured (mainly for low frequencies) [14] and their sensitivity. Recent progress has been made on EMF measurements based on EO probes [13][14][15], and their improved characteristics sustain their use for in-situ measurements of EMF originated from communication devices. Developments have been made for measuring soldier exposure in operational environments in the 1-500 MHz frequency range [16].…”