In this activity we propose the characterization and the implementation of optical integrated electromagnetic field sensors suitable for sensing fields in the Fresnel region over a wide frequency range (ELF to UHF band). The optical electromagnetic field sensor proposed in this activity as the first approach is based on an integrated optical LiNbO3 interferometer driven by optical fibre transmission lines, therefore is mainly made by dielectric materials. This device, differently from standard metallic probes, yields minimal coupling with critical radiating structures, and enables their near-field monitoring in industrial, scientific and medical applications. Potential applications include measurements of fields yield by communication systems and devices as critical space and terrestrial RF communication systems, and medical applications as tissue irradiation and electromagnetic field tomography for portable diagnostic instruments.
Figure 1 shows the structure of an interferometric LiNbO3 modulator prototype developed in collaboration with IMM secondary seat of Bologna. This is composed of two y-branch couplers connecting two optical waveguide arms. Several authors improperly refer to this optical structure as Mach-Zehnder interferometer (MZI), which has a different optical scheme with two mirrors M, two beam-splitter and two detection points 0 and 1, as shown in Figure 2. The design of a suitable tapered antenna is derived according to the Wu-King approximation for a resistively loaded antenna, where only the progressive wave has to be present.
The first device made on a LiNbO3 substrate with X-cut, Y-propagating waveguides substrate have the antenna superimposed on the modulator substrate in order to exploit the Pockel effect which results in a linear change of the refractive index due to an external E-field. It was characterized by an electro-optical model shown in Figure 3. RF measurements were performed both in an anechoic chamber with an open-ended wave guide RF radiator, and in laboratory with standard dipole antennas with anti-inductive stands. Measurement in ELF band where performed with capacitive plane cells. The external noise measured in large band in our lab was lower than 300 mV/m, negligible with respect to the test fields used for the probe exposition. Automated measurements were performed connecting the instruments with standard bus for frequency response measurement. The sensor can be designed for a suitable Vp value, in order to avoid phase-wrapping in strong electric near-field field measurements.
The optical electromagnetic field sensor proposed in this activity could also be implemented in future by a polymer and metamaterial based interferometer. TRL can be estimated as 2.
Figure 1. The sensor transverse section
Figure2. Optical schematic of a Mach-Zehnder Interferometer
Figure3. The sensor equivalent circuit model
Contact: Medugno Mario