Recent advances in the design of electro-optic sensors for minimally destructive microwave field probing.

In this paper we review recent design methodologies for fully dielectric electro-optic sensors that have applications in non-destructive evaluation (NDE) of devices and materials that radiate, guide, or otherwise may be impacted by microwave fields. In many practical NDE situations, fiber-coupled-sensor configurations are preferred due to their advantages over free-space bulk sensors in terms of optical alignment, spatial resolution, and especially, a low degree of field invasiveness. We propose and review five distinct types of fiber-coupled electro-optic sensor probes.

Calibrated 100-dB-dynamic-range electro-optic probe for high-power microwave applications.

Highly stable electro-optic field probe with wide dynamic range is presented. The highly efficient electro-optic modulation mechanism--based on interference and field-induced phase retardations using a new embodiment with a relatively thick sensor crystal mounted on a fiber--is explained. The probe is calibrated up to 3.

A multi-layer electro-optic field probe.

We present a novel design method and sensing scheme for an electro-optic field probe using multi-stratified layers of electro-optic wafers. A serial stack of cascaded layers is found to be capable of enhancing the performance of interferometric electro-optic light modulation that utilizes the slopes of interference fringe patterns and field-induced electro-optic phase retardations within wafers. The absolute sensitivity of the probe is also characterized with a micro-TEM cell that generates electric fields distributions with accurate, calculable strength for use in probe calibration.

An optically-interrogated microwave-Poynting-vector sensor using cadmium manganese telluride.

A single <110> cadmium-manganese-telluride crystal that exhibits both the Pockels and Faraday effects is used to produce a Poynting-vector sensor for signals in the microwave regime. This multi-birefringent crystal can independently measure either electric or magnetic fields through control of the polarization of the optical probe beam. After obtaining all the relevant electric and magnetic field components, a map of the Poynting vector along a 50-Omega microstrip was experimentally determined without the need for any further transformational calculations.

Pulsed-terahertz reflectometry for health monitoring of ceramic thermal barrier coatings.

Terahertz time-domain reflectometry was used to monitor the progress of a thermally grown oxide layer and stress-induced, air-filled voids at the interface of an Yttria-stabilized-zirconia ceramic thermal-barrier coating and a metal surface. The thicknesses of these internal layers, observed in scanning-electron-microscope images to increase with thermal-exposure time, have been resolved - even when changing on the order of only a few micrometers - by distinguishing not only increased delays in the arrival times of terahertz pulses reflected from this multilayer structure, but also changes in the width and shape of the pulses. These unique features can be used to predict the lifetime of thermal-barrier coatings and to indicate or warn of spallation conditions.

Bandwidth enhancement of electro-optic sensing using high-even-order harmonic sidebands.

We demonstrate that the bandwidth of an electro-optic sensing system can be significantly enhanced by the use of even-order harmonic sidebands (order > 2) produced on an optical-carrier probe beam with two cascaded electro-optic modulators. The sensing frequency range may be routinely expanded by at least four times, with respect to the use of fundamental-harmonic sidebands, by enhancing the nonlinearity of high-order-harmonic electro-optic modulation. The creation of harmonic modulation sidebands up to the sixth order of the drive frequency on a laser-diode output is described analytically, as is photonic heterodyne down-conversion of microwave signals using these high-order even harmonics within an electro-optic sensor crystal.

Optimization of sideband modulation in optical-heterodyne-downmixed electro-optic sensing.

Enhancements of the modulation depth in an electro-optic sensing application, through optimization of the modulation sidebands in both an electro-optic modulator and a microcavity electro-optic probe, have been investigated. The principles and methods of optimizing the operational bias points for the devices are described from the point of view of both an electro-optic modulator and probe, and their different optimum characteristics for signal-modulation depth, due to their huge scale gap, are presented. We use an optical-heterodyne receiver employing a fast modulator along with the electro-optic sensing technique to achieve high-frequency microwave electric field mapping.

An optical-fiber-scale electro-optic probe for minimally invasive high-frequency field sensing.

A sub-millimeter-dimension electro-optic probe that provides enhanced scanning accessibility with significantly less intrusiveness than metal-based or even other dielectric probes during electromagnetic characterization of microwave devices is presented. The quantitative and qualitative relative invasiveness of the probe on the operation of an example antenna device-under-test is explored with respect to previously demonstrated fiber and wafer electro-optic sensors. We also demonstrate that the miniaturized probe, with a diameter of 125 microm, can be used to reconstruct the three orthogonal vector components of near-electric fields without the need for different probe crystals or multiple calibration procedures.

Bandwidth enhancement of electro-optic field sensing using photonic down-mixing with harmonic sidebands.

We demonstrate that harmonic sidebands of an electro-optic modulator's driving frequency can be used as the local oscillator in a photonic down-mixing process in order to significantly enhance the bandwidth of near-field, electro-optic, microwave measurements. The creation of second- and third-order-harmonic modulation sidebands on a laser-diode output are described, with heterodyne down-conversion of microwave signals taking place within an electro-optic sensor crystal. The measurement bandwidth of an electro-optic microwave probe can thus be enhanced by as much as a factor of three with respect to the use of conventional, fundamental-harmonic sidebands.