Terence J. Wieting

Infrared absorptances of partially ordered alloys at elevated temperatures

The infrared absorptances of highly polished Ti‐6Al‐4V and 304 stainless steel have been measured between 2 and 20 μm and at temperatures up to 550 °C. Small positive temperature coefficients for both alloys were observed. A generalized Drude model has been developed to interpret the data; the weak temperature dependence of the absorptance has been attributed to the relatively small effect of phonon scattering in these disordered alloys.

Effects of surface condition on the infrared absorptivity of 304 stainless steel

The absorptivity of 304 stainless steel at 10.6 μm has been obtained between room temperature and 1000 °C by laser calorimetry. Highly polished specimens have been compared with as‐received and rough‐polished specimens in order to determine the effects of surface condition on the absorptivity. When the surfaces are well annealed, the calorimetry data show that the absorptivities of the highly polished surface and the rougher surfaces are nearly the same over the entire range of temperature. No influence of grain size on the absorptivity has been observed, despite the substantial growth of the grain boundaries at elevated temperatures. When the surfaces are deformed by cold rolling or rough polishing, the absorptivity at room temperature increases by 22–27% due to increased scattering from nonplanar defects within the surface layer. The absorptivity data on damage‐free surfaces of stainless steel have been found to agree closely with the predictions of generalized Drude theory, applied to disordered alloys...

Piezo-induced sensitivity enhancements in electro-optic field sensors

Responsivity measurements are reported for LiNbO3 electro-optic field sensors operated under applied electric fields at ac frequencies in the vicinity of the acoustic resonance values of the crystals. At these frequencies, piezoelectric effects dominate the sensor output. These resonance effects are well known and commonly considered parasitic. However, we propose their use as a sensitivity-enhancing mechanism for electric-field detection. We have found that our field sensors operated within these resonances responded linearly with the applied field strength and exhibited increases in their intrinsic sensitivities as high as 350 times larger than their normal, electro-optic values. Our modeling of the data suggests that the sensitivity enhancements are produced by the interplay between photoelastic shifts in the refractive indices and the physical vibration modes of the crystals. Aside from narrowband applications, these resonant enhancements can be exploited with fields at frequencies well beyond the nar...

Phase transition in Sr0.75Ba0.25NbO3 near the Curie temperature

Strontium barium niobate has the tungsten-bronze structure with a tetragonal unit cell, and it exhibits negative thermal expansion along the c axis between −120°C and room temperature while having a positive thermal expansion along the a axis for the same temperature range. At higher temperatures, close to the Curie temperature and above, the negative thermal expansion along the c axis changes to positive thermal expansion. The a axis lattice parameter as a function of temperature shows a change in slope at the Curie temperature. These results indicate the presence of a second-order phase transition near the Curie temperature.

The effects of photorefraction on electro-optic field sensors

We have measured the electro-optic response of LiNbO3 and SrxBa1−xNb2O6 crystals to be used for electric-field sensors. Our results indicate that optically induced refractive index variations (photorefractivity) in the crystals affect the temporal stability and sensitivity of the sensors. Spatial distributions of the refractive indices produced from the photorefractivity create incoherence in the polarization of the probing laser passing through the crystal (optical probe). In LiNbO3 crystals, this spatial incoherence was negligible and sensor responsivities close to the theoretical maximum were attained. However, in SrxBa1−xNb2O6 crystals, strong spatial variations of the principal refractive indices resulted in an extremely incoherent polarization of the optical probe and reduced the electro-optic responsivity dramatically. The photorefractive-induced sensitivity loss is modeled using a distribution function (rather than a constant value) to describe the phase of the optical probe. These results emphasi...

Dielectrically induced sensitivity enhancements in electro-optic field sensors

The sensitivity of an electro-optic (EO) field sensor depends inversely on the dielectric constant of the nonlinear crystal. In EO sensors based on lithium niobate the effective value of this dielectric constant is affected by dielectric relaxation effects and is identified with its smaller, high-frequency component. Because of this effect, the EO modulation is significantly enhanced, thus improving the field strength sensitivity.

Spatial and temporal sensitivity variations in photorefractive electro-optic field sensors

Experimental studies of the spatial and temporal fluctuations in photorefractive electro-optic (EO) field sensors have revealed that their maximum intrinsic responsivity is limited by incoherence in the polarization of the probe beam. This incoherence is generated within the irradiated crystal itself. We describe a novel method to measure the incoherence directly while suppressing the temporal fluctuations. Our results indicate that optically induced distributions of birefringence (photorefractivity) generally impair the modulating ability of EO crystals.

The field distribution of a focused Gaussian beam reflected at 45° from a conducting plane and its effects in plasma‐ignition experiments

The electrical field distribution over the focal region of a Gaussian beam, incident at 45° and focused onto a perfectly conducting infinite plane, is calculated numerically by summing the plane waves in an angular spectrum representation. The beam is plane‐polarized with the transverse electric field perpendicular to the plane of incidence. The configuration and parameters were chosen to agree with our experiments on plasma initiation near a metallic conductor by 35‐GHz/8.6‐mm microwave radiation. The results of the calculation show that the field distribution determines the ignition threshold, spatial location, and initial development of the microwave plasma at a metal surface.