Optimization of Optically and Electrically Modulated Scattering Probes for Field Measurements

In this paper, the problem of optimizing probes used in modulated scattering systems is examined. Instead of the usual empirical or blind numerical optimization, a simple analytical formulation is developed, uniquely based on the impedance states of the modulating devices. This formulation allows obtaining some bounds on system performances and assessing the sensitivity to frequency and component characteristics variations. In particular, an optical modulation component, providing an advantage in terms of low invasiveness, is compared with the more common p-i-n diode-based modulators. The validity of the proposed approach, for high-resolution electrically small probes is demonstrated numerically and experimentally.

I. Introduction

The modulated scattering technique (MST) remains an attractive approach to noninvasive and/or rapid near-field (NF) measurements. This technique is also applicable in arrays of sensors based on radiofrequency identification (RFID) tags [1]–[3]. It consists of a probing technique where no wired RF connection is required between the probe and the receiver. The probe is loaded by a nonlinear component, which can be periodically biased in two distinct states, usually characterized by low (ON state) and high (OFF state) impedance levels. Therefore, the probe scatters a modulated signal, which can be related to the field at the probe location. Modulation is essential to discriminate the probe contribution from the surrounding. Using an optical diode can advantageously reduce the residual perturbation resulting in the modulation wires. Indeed, in this case, very low invasiveness modulation can be achieved using a laser beam [4] or an optical fiber [5]–[8], for instance. This approach has not been widely used due to the shortcomings of commercial low-cost photodiodes such as low impedance contrast and limitation of the modulation frequency to a few kilohertz. Indeed, developing the dedicated photosensitive devices requires substantial technological efforts. For these reasons, electrical modulation with p-i-n diodes using resistive wires has been considered the best solution for a long time. However, the feasibility of optically modulated scattering (OMS) probes using low-cost commercial components has been recently demonstrated [9], [10], renewing the interest for rapid and low-invasive field measurements such as those required by NF microwave imaging techniques for industrial, scientific, and medical applications [11]–[13].