[[Image:neoscanfig_1_1.png|thumb|right|400px|<i><b>Figure 1.1</b>: EO modulation of an optical signal.</i>]] The [[NeoScan]] real-time field measurement & scanning system provides an entirely new capability for the measurement of high-intensity electric fields. This technology is based on the Pockelâs effect which measures the phase-retardance of an optical beam due to an impinging electric field. This electro-optic effect is observed in non-centrosymmetric crystals when an electric field is directed along certain crystal axes causes a change in the indices of refraction encountered by an incident optical beam. Figure 1.1 shows the basic principle of the electro-optic effect. The electro-optic effect provides a means of modulating the phase or intensity of the optical radiation. In another sense, this effect also makes it possible to detect the presence of an electric field impinging on the crystal. The polarization of an optical beam travelling through a crystal is altered by the electric field in that crystal. The comparison of polarization states allows determination of the amplitude and phase of the existing RF electric field. Since the electro-optic sensing phenomenon relies on small displacements of the atomic crystal structure, the response time of the process is extremely short. This short response time makes it possible to measure high-frequency electric fields up to the terahertz regime.
<table><tr><td>[[Image:neoscanfig_1_2.png|thumb|leftcenter|380px|<i><b>Figure 1.2</b>: A single-axis EO probe. The probe tip is protected by Epoxy toprovide nearly identical performance to a bare probe.</i>]] </td><td>[[Image:neoscanfig_1_3.png|thumb|right|400px|<i><b>Figure 1.3</b>: Illustrates a normal field measurement as oriented with respect to an incident electric field. The normal probe is oriented with the probe normal to direction of propagation.</i>]]</td></tr></table>Â A typical EO probe is composed of an optical fiber affixed with an EO crystal coated with a dielectric reflection layer on its bottom surface as shown in Figure 1.2. These probes have very delicate optical interconnects and extreme care must be taken in handling the probes to prevent excessive shock, bending and out of plane stresses.
Due to its broad measurement bandwidth and high spatial resolution, the EO measurement technique is a promising means to characterize RF systems such as microwave and millimeter-wave integrated circuits, HPM sources and systems, and large-scale active arrays and other radiating structures. Unlike the conventional electrical measurement techniques which require some type of metal structure for the resonant detection of an RF signal, [[NeoScan]]âs unique real-time EO electric field measurement method requires no metal components. As a result, the field perturbation caused by introducing metal within the vicinity of a device under test (DUT) is significantly reduced.
[[Image:neoscanfig_1_3.png|thumb|right|400px|<i><b>Figure 1.3</b>: Illustrates a normal field measurement as oriented with respect to an incident electric field. The normal probe is oriented with the probe normal to direction of propagation.</i>]] Figure 1.3 shows the electric and magnetic fields distribution of a traveling RF wave with a normal probe shown in typical orientation. To detect the maximum electric field in this configuration, the propagation direction of the optical beam of the probe should be parallel to the E-field direction. In general, a normal EO probe is only sensitive to the electric field component parallel to the probe handle, whereas a tangential probe is sensitive to the electric field component perpendicular to the probe handle. Yet, the E-field sensitivity of a tangential probe depends on its crystal orientation sitting on its tip.
A low noise 1550 nm laser diode is used as optical beam source. The optical connections are fiber-based. The beam is delivered to an optical probe. The polarization of the beam is modulated through an electro-optic crystal on the probe tip. The modulated beam is reflected back into the fiber, and back to the mainframe for analysis. An optical analyzer converts the polarization change of the beam into an amplitude change. The amplitude is linearly proportional to the strength of the external electric field at the probe-crystal location. The equation E=αV is used to calculate the electric field, where α is the calibration factor, or the slope between the electric field E (in V/m) and the measured EO signal V (in V/m/uV). For instance, for a calibration factor of 1.082 V/m/uV. a measured EO signal of 1000 V (0.001 V), corresponds to and electric field of 1.082 V/m/uV 1000 V = 1082 V/m.