Anthony Garzarella

Optimal electro-optic sensor configuration for phase noise limited, remote field sensing applications

Electro-optic (EO) sensors, used for the nonperturbative detection of electric fields, are typically configured to modulate an optical probe beam along a crystal direction in which the EO tensor coefficient is largest. However in fiber optic EO sensors, such configurations can be strongly limited by phase noise and cumbersome compensation optics. Our results demonstrate that a greater signal to noise ratio can be achieved by modulating along a crystal direction of low static birefringence, even when the active EO tensor coefficient is several times smaller.

Anisotropic thermal expansion of strontium barium niobate

Strontium barium niobate is a tungsten-bronze ferroelectric crystal having a tetragonal unit cell. Low-temperature x-ray diffraction studies were performed on a single crystal of Sr0.75Ba0.25Nb2O6 to determine the thermal expansivity along the a- and c-axes. Negative thermal expansion was observed along the c direction while a positive thermal expansion was measured along the a axis. The anisotropic thermal expansion behavior is explained as arising due to the geometry of the crystal structure.

Responsivity optimization and stabilization in electro-optic field sensors

Electro-optic (EO) modulation devices, which utilize an external electric field to modulate a beam of optical radiation, are strongly affected by parasitic effects, which change the polarization state of the optical beam. As a result, very small changes in the birefringence or optical path length within the EO material can result in very large fluctuations of the amplitude and phase of the optical modulation signal. A method of actively analyzing the modulated beam is described and demonstrated, which eliminates these fluctuations and keeps the modulation device stably operating at its peak responsivity. Applications to electric field detection and measurements are discussed.

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...

Failure mechanism of THz GaAs photoconductive antenna

We investigated the failure mechanism of THz GaAs photoconductive antenna using high resolution x-ray diffraction topography. From these studies, it was found that grain boundaries are formed during the high frequency device operation. This results in the segregation of gold at the boundaries causing electromigration of the metal between the gold micro-strips. This disrupts the photocurrents from being produced by femtosecond laser thus preventing terahertz beam generation from the photoconductive antennae leading to device failure.

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.

Optimal crystal geometry and orientation in electric field sensing using electro-optic sensors

For optimal sensitivity in electric field measurements, electro-optic (EO) crystals are typically selected based on their EO coefficients and dielectric constants. However, the conventional figure of merit yields sensitivity predictions regarding EO materials that are inconsistent with experimental data. In this Letter, we demonstrate that depolarization effects, which are often ignored, can dramatically enhance responsivity depending on the shape and orientation of the EO crystal. For optimal sensitivity, these effects are best exploited in longitudinal EO sensors, where they yield an optical modulation depth that increases quadratically with crystal length.