Christopher S. McDermitt

Design and Performance of Frequency Selective Surface With Integrated Photodiodes for Photonic Calibration of Phased Array Antennas

The design, fabrication, and integration of a frequency selective surface (FSS) with integrated photodiodes to allow for photonic calibration of phased array antennas is presented. The design includes embedding electrically short dipole antennas in each unit cell of the FSS, with a zero-biased photodiode placed across the gap of the diode. Fibers from an optical distribution network are passed through the honeycomb core of the frequency selective surface and pigtailed to the photodiodes. The RF performance of the frequency selective surface with integrated optics is investigated via simulations and measurements, and the results show that the structure maintains RF-transparency.

RF Characterization of Zero-Biased Photodiodes

The characteristics of zero-biased InGaAs p-i-n photodiodes were studied for use as optical transducers in an in situ phased array radar calibrator. Radio frequency (RF) operation is especially important for the intended application in which the zero-biased photodiodes drive patch antennas embedded in a radome. The photodiodes act as sources for real-time calibration of the phase and amplitude of each array element and will operate with only optical input from a modulated optical fiber link. Unfortunately, little is known of the RF performance of photodiodes operated without voltage bias. In this paper, both the dc and RF performance of a particular photodiode were studied to determine the optimum operating conditions and to understand the type and severity of any limitations.

The Performance of a Fiber-Optic Link Using Unbiased Balanced Photodiodes for Antenna Array Calibration

A quadrature-biased dual-output Mach-Zehnder modulator is used to drive unbiased balanced photodiodes for use in antenna array applications. The RF gain is measured as a function of frequency and photocurrent. Balanced photodiode two-tone measurements at 6 GHz are compared with those of a single photodiode to show 26-dB suppression of the second-order distortion. This results in a second-order limited spur-free dynamic-range improvement of 12 dB. The phase error introduced in unbiased photodiodes is compared to that of the biased case to demonstrate that it would have minimal effect on the performance of the antenna array system.

The performance of unbiased balanced photodiodes for optical calibration of radio-frequency antenna arrays

The accurate calibration of radio-frequency (RF) antenna arrays is critical to their performance. Real-time, in situ calibration is particularly desirable and recent photonic implementations [1],[2] have stirred interest for onboard naval applications. These photonic architectures are primarily suited for large shipboard arrays [3] but may also be pertinent to the calibration of the smaller arrays found in avionic platforms. The concept is to feed each receive-mode antenna element with an optically-encoded test signal that is demodulated with unbiased photodiodes placed near every element [1],[2]. This technique provides the distinct advantage of having no metallic components near the aperture but the unbiased photodiodes will exhibit reduced performance as compared to reversed-biased photodiodes of the same type. Most notably, an unbiased photodiode will demonstrate decreased optical-to-electrical conversion efficiency and increased RF distortion.

Free-Space Analog Optical Links: Systems, Performance and Statistical Properties

We review properties of free-space optical links for analog transmission including RF performance for various photoreceiver configurations and experimental results at 1 GHz over a 32 km link.

RF frequency shifting via optically switched dual-channel PZT fiber stretchers

We demonstrate a new technique for RF serrodyne frequency shifting with ultrahigh sideband suppression. This is accomplished using two parallel optically switched piezo fiber stretchers driven with two out-of-phase serrodyne modulations. We achieved sideband suppression of nearly 50 dB, an improvement of almost 30 dB compared to the standard fiber stretching approach. The frequency shifter is easily integrated into photonic and RF systems, is functional in certain broad-band applications, and allows for tunable translations.