Keith D. Masterson

An optically linked three-loop antenna system for determining the radiation characteristics of an electrically small source

Experimental results on an antenna system for determining the radiation characteristics of an electrically small source are presented. Three orthogonal loop antennas, each terminated at diametrically opposite points with identical loads, encircle the source and characterize its equivalent electric and magnetic dipole moments. The total radiated power can be determined from this near-field measurement of the device under test. The test system operates from 3 kHz to over 100 MHz with up to 90 dB of dynamic range. Test results are presented for a system constructed from three 1-m-diameter loops.<<ETX>>

Accurate bias point control for an electrically isolated Mach-Zehnder interferometric modulator via an analog optical-fiber link

We describe a new method to keep an electrically isolated Mach-Zehnder electro-optic modulator biased in quadrature despite changes in temperature and optical coupling efficiency. The modulator is part of a system to measure electromagnetic waves in an outdoor environment. All data and control signals between the modulator and electronics control module are carried by analog optical-fiber links, and all of the bias signals and control procedures are implemented with an onboard computer and digital signal-processing unit. Our method compares the average DC optical power in the complementary outputs of the modulator and adjusts the bias point by controlling the optical power to small photocells in the sensor head. A second control loop balances responses from a small dither signal applied to the modulator, consequently balancing the optical losses in each of the complementary outputs by way of a variable optical attenuator. This control system allows us to maintain the bias to within ±2 degrees of the quadrature point. The response time of the control loop is about 10 ms.

Photonic Electric-Field Probe For Frequencies Up To 2 GHz

A photonic electric field probe using the Pockels effect in bulk LiNbO3 is used to measure electromagnetic fields from 10 to 100 V/m. The observed frequency response is flat up to 1.6 GHz and extends beyond 2 GHz. Over the majority of the frequency range, field strengths down to about 6 V/m would be detectable above the noise floor when using a 3 kHz detection bandwidth. Present experimental results indicate a linear dynamic range of 30 dB for the probe. Increasing the optical carrier power and lowering the noise floor are expected to improve the dynamic range to above 50 dB.

Electromagnetic shielding characteristics of optical-fiber feedthroughs

A number of commercially available optical-fiber connector styles ST, SC, and FC were tested to determine the extent to which their use in bulkhead adapter feedthroughs would compromise the shielding of electromagnetic interference for electronic enclosures. Connectors and adapter barrels made from metal, ceramic, and polymer materials were included in the tests. The tests were carried out using a nested reverberation cell and covered a frequency range from 1 to 16 GHz. Although we rely on data acquired by making shielding effectiveness (SE) measurements, we report the results in terms of a transmission cross-section which is the ratio of the power coupled into an enclosure through an aperture to the power per unit area incident on the aperture. The amount of coupling through the feedthroughs we tested varied over 70 dB. The shielding of an all-metal FC connector system was nearly equal to that of a blank reference plate. In some cases, a feedthrough coupled more energy into the enclosure than was coupled through the empty hole required to mount the adapter barrel. Unlike SE, the transmission cross sections do not depend on the volume, shape, or construction materials of the enclosure and can be used to estimate the degradation in the SE for enclosures other than our reference cell. The uncertainties in the reported transmission cross sections are calculated to be /spl plusmn/3 dB and are small compared to the differences between the measured cross sections.

Broadband, Isotropic, Photonic Electric-Field Meter For Measurements From 10 kHz To Above 1 GHz

An isotropic, photonic electric-field meter (PEFM-15) having 15 cm resistively tapered dipole elements and Pockels effect electro-optic modulators is used to measure electric fields of 10 to 100 V/m from 10 kHz to beyond 1 GHz. The probes frequency response is flat within ±3 dB from 30 kHz to 100 MHz except for a region between 1 and 10 MHz where acoustic resonances occur in the LiNbO3 modulator crystals. For a 3 kHz detection bandwidth, the noise equivalent field is approximately 7 V/m, thereby giving a calculated linear dynamic range of 68 dB in field power density. The probes isotropic response is flat within ±2 dB, and the response of each dipole closely follows the curve predicted by theory. An optical-beam switch that connects the individual dipoles to a laser source and optical receiver is also described.

Thin-film design for enhanced stability of optically pumped vertical-cavity surface-emitting lasers (VCSELs)

We have employed a novel approach to enhance the optical pump-coupling stability of vertical-cavity surface-emitting lasers (VCSELs). These structures are composed of thin-film semiconductor multilayers and are manufactured entirely by thin-film deposition. In the past, pump light was coupled into VCSEL cavities through the short wavelength interference notches in the reflectance spectrum. However, the steep slope and narrow width of the notches inherently makes pump-coupling sensitive to device temperature. We have employed traditional thin-film optimization to the multilayer etalon to create a low ripple, wideband pump region of low reflectance while maintaining cavity-mode field overlap at the quantum wells. We demonstrate a factor of 2.5 times better stability across a 35 nm spectral range.

An integrating sphere spectrometer for high-temperature materials characterization

Abstract A state-of-the-art, high-temperature integrating sphere spectrometer has been fabricated and characterized. The liquid-cooled sphere is combined with a furnace designed for a maximum specimen temperature of 1200°C. The spectrometer is capable of directly measuring the diffuse and hemispherical-directional reflectance and transmittance over the wavelength range of 0.35–2.5 μm. The specular properties and absorptance are calculated from the directly measured quantities. Chopped white light from a high pressure xenon arc-lamp is introduced into the sphere, alternately reflected from the sphere wall and reflected from or transmitted through the specimen, spectrally resolved by a monochromator, and focused onto a two-color Si/PbS detector. Data are sampled at wavelengths corresponding to equal energy bands of the terrestrial solar distribution and are graphically displayed. Data acquisition and hardware module commands are computer controlled. The sphere and light collecting optics can be rotated about the horizontal optical axis of the monochromator so that the specimen port is at the top, side, or bottom of the sphere. By appropriately orienting the sphere, the optical properties can be measured for a variety of materials such as solids, molten salts, powders, or a falling curtain of particles, all of which are relevant to high-temperature solar applications. Absolute reflectance measurements of a standard white reference material agreed to within 2% of the NBS reference curve for the material over most of the spectral range of interest. After calibration the accuracy of solar weighted optical property measurements is better than 1%.

Transmission cross sections of optical-fiber feedthroughs for suppression of electromagnetic interference

Various optical-fiber, bulkhead adapter feedthroughs were tested over a range from 1 GHz to 16 GHz to determine how much they degraded the shielding effectiveness of electronic enclosures. We report transmission cross sections for the feedthroughs which range from 10/sup -10/ m/sup 2/ to 10/sup -2/ m/sup 2/. These can be used to calculate the shielding effectiveness for various enclosures of interest.

Standard antennas designed with electro-optic modulators and optical fiber linkage

We describe the design of standard reference antennas that use an electrooptic transducer together with optical-fiber linkage to preserve the amplitude and phase information of the received signal. They will be used over a range from 10 MHz to 2 GHz at our open area test site in order to reduce measurement uncertainties attributable to the ambient electromagnetic spectrum. The transducer consists of an optical-fiber directional coupler with unbalanced legs and LiNbO3 phase modulators. The output signal is processed to servocontrol the wavelength of the laser and maintain the optimal optical bias point in changing environments. The complementary rf signals are subtracted to provide common mode noise rejection. The output signal is modeled to determine the design and operating parameters required for good repeatability and accuracy. The results show that spurious reflections in the modulator legs need to be < -50 dB in order to obtain the desired stability.