James L. Dexter

Fiber-optic prism true time-delay antenna feed

Experimental results for an optical-control technique for implementing a true time-delay function for array antennas are reported. A microwave signal is transmitted on a wavelength-tunable optical carrier through a fiber-optic prism-a set of nominally equal-delay fibers with differing net dispersion-to photodetectors that feed each antenna element. The relative interelement time-delay (beam angle) adjustment is accomplished by tuning the optical carrier wavelength. Measured antenna patterns of a two-element array clearly demonstrate beam steering and true time-delay operation over a two-octave bandwidth of 2-8 GHz.<<ETX>>

Long-Haul Analog Photonics

A paper on long-haul analog photonics is presented using theory and experimental results. Various analog fiber-optic modulation formats are reviewed, including intensity modulation with direct detection, phase modulation with interferometric demodulation, and suppressed-carried techniques. Modulation linearization methods are reviewed. The limitations of and requirements for photodiode detectors are described.

Phase performance of an eight-channel wavelength-division-multiplexed analog-delay line

The design of an eight-channel 25 /spl mu/s wavelength-division-multiplexed (WDM) microwave photonic delay line is described. The performance of the delay line was studied by measuring the temperature dependent relative phase drift between channels and the interchannel crosstalk. A simple method for measuring the maximum phase errors produced by the interchannel crosstalk is described and used to measure the maximum error on a channel in this system. The use of polarization interleaving and dispersion compensation to reduce the effects of crosstalk in the system is examined experimentally.

110 km 256-QAM digital microwave over fiber link

A 110 km digital microwave fiber-optic link has been designed and demonstrated with 256 QAM at 20 GHz. The design approach, involving the management of loss, dispersion, and Brillouin threshold throughout the link is discussed.

Ultraviolet optical isolators utilizing KDP-isomorphs

Abstract The KDP isomorphs have both large Verdet constants and excellent optical properties in the ultraviolet. With these combined features it is possible to construct a compact optical isolator using these materials with rare earth permanent magnets. This paper describes the construction of an optical isolator using the KDP-isomorph ADA, and its operation at 351 and 308 nm. The potential use of the other isomorphs in the ultraviolet will also be discussed.

Two optical-control techniques for phased array: interferometric and dispersive-fiber true time delay

We describe and present experimental results for two optical control techniques for phased array antennas. The first technique is based on interferometric heterodyning of two narrow- linewidth YAG lasers for the generation of required microwave signal and for simultaneous steering of the radiated beam. The constructed system is simple and well-suited for narrowband applications, and it may be built without any active mechanical components. The measured radiated antenna patterns are in close agreement with the predicted ones. The second technique is a novel and elegant method for implementing a true time-delay function for optical control. It relies on using a wavelength-tunable laser to provide the optical carrier for the microwave signal and a fiber-optic prism--a set of equal delay fibers with differing net dispersion. The relative interelement time-delay (beam angle) adjustment is accomplished by tuning the optical carrier wavelength. The experimental results obtained on a compact antenna range clearly demonstrate beam-steering and true time-delay operation over a two-octave bandwidth.

FSO links using high sensitivity gigabit combinational sensors and an adaptive high-throughput error-correction protocol

For terrestrial free space optical (FSO) communication applications, large area photodetectors not only allow for more efficient power coupling but also ease the effects of atmospheric turbulence. When fabricated in array form, these devices have the added capability of operating as both data reception sensors and position sensitive detectors (PSD). Concentric five element impact ionization engineered (I2E) avalanche photodetector (APD) arrays have been developed to operate as combinational sensors at gigabit data rates. While the excellent sensitivity performance of these detectors allows for enhanced link range in calm atmospheric conditions, link availability with a high quality of service for an FSO system is strongly impacted by its ability to dynamically counter-act atmospheric fading with varying temporal scale. In this paper we present initial results using a combinational sensor in tandem with an advanced FSO modem which utilizes low-density parity check coding (LDPC) in an incremental redundancy (IR) hybrid automatic repeat request (HARQ) protocol. System performance enhancements as function of varying atmospheric scintillation will be discussed.

Simultaneous data communication and position sensing with an impact ionization engineered avalanche photodiode array for free space optical communication

Photodetectors in free space optical communication systems perform two functions: reception of data communication signals and position sensing for pointing, tracking, and stabilization. Traditionally, the optical receive path in an FSO system is split into separate paths for data detection and position sensing. The need for separate paths is a consequence of conflicting performance criteria between position sensitive detectors (PSD) and data detectors. Combining the functionality of both detector types requires that the combinational sensor not only have the bandwidth to support high data rate communication but the active area and spatial discrimination to accommodate position sensing. In this paper we present a large area, concentric five element impact ionization engineered avalanche photodiode array rated for bandwidths beyond 1GHz with a measured carrier ionization ratio of less than 0.1 at moderate APD gains. The integration of this array as a combinational sensor in an FSO system is discussed along with the development of a pointing and stabilization algorithm.