Electro-optic field-mapping as a diagnostic tool for microwave circuits and antenna arrays

Abstract

The effectiveness of an optical-fiber-mounted electro-optic probe as a scanning electric-fieldmapping tool is demonstrated in diagnostic measurements on microwave and millimeter-wave circuits, antennas, and arrays. A combined electric-field and thermal-imaging capability will also be discussed.The interaction of optical beams and electrical signals in an electro-optic medium provides a fundamental and convenient means for introducing microwave or digital data into an optical transmission channel. This is accomplished through the well-known process of electro-optic modulation. Similarly, the characteristics of an electrical waveform can also be determined using an optical beam when this beam is coincident with, and intensity modulated by, the part of the unknown electrical signal that enters an appropriately oriented electro-optic medium. When a mode-locked laser having a pulse duration much shorter then an electrical feature to be measured is used to interrogate one small part of a repetitive waveform (over an interval At), then the assembly of many "At" constituents from different parts of the waveform into a representation of the original signal is known as electro-optic sampling [1,2]. Dozens of examples of time-domain electro-optic-sampling measurements have appeared in the literature over the past 20 years. They highlight the applications of the technique in high-bandwidth-photodetector characterization [3], discrete-device analysis [4], the extraction of clock and large-signal waveforms from within digital and nonlinear circuits [5], the measurement of pulsed, terahertz signals [6], and other areas. In the diagnostic testing of linear microwave systems or components, another embodiment of electro-optic sampling in which the amplitude and phase of a single frequency are measured at a variety of spatial locations has also been developed [7]. These measurements of spatial field distributions have increased in popularity for the characterization of both active [8] and passive [9] circuits, where the microwave signals are either guided [IO] or radiating [I I]. They can be made using either an electro-optic substrate [I21 or an electro-optic medium externally inserted into a fringing or radiated electric field [13].In this paper, we will discuss the most highly evolved electric-field mapping system that, to our knowledge, has been developed to date. It is a system that: separately distinguishes three orthogonal vector components of the electric field; senses both amplitude and phase; is flexibly positioned due to optical-fiber coupling of its probe; has single-micrometer spatial resolution; has greater than 100-GHz bandwidth; and has a low invasiveness even with its probe positioned in the near field of a device under test (DUT). Furthermore, the system can be modified so that information contained in part of the optical beam is also analyzed to reveal the temperature at the probe location. At the same time the probe is scanned to map out the electric field, it can thus also construct a thermal image. Figure I il...

Improvement of Electrical Near-Field Measurements With an Electro-Optic Test Bench

“…The coaxial probes that we have developed for EMC applications have a spatial resolution of 500 µm for monopole and around 1 mm for dipole probes which do not give enough experimental data with microelectronic components [7][8][9][10]. The optical probes can reach a spatial resolution under 100 µm and thus suitable for measuring microelectronic devices [11][12][13]. As a result, to improve the performance of our current near field bench based on passive detection with coaxial probes, and to analyze these interferences, we are currently developing another near-field test bench using optical technology based on the Pockels effect.…”