This tutorial review paper deals with various methods for solving a basic problem of wideband phased arrays, i.e. beam squinting, using optical technologies. The problem of beam squinting in phased arrays is analyzed and the concept of true-time delay is introduced. The advantages of realizing variable delay lines by optical rather than by microwave means are reviewed, together with principles of operation. Among the techniques described are switched-path length delay lines, fiber stretchers, tunable lasers with highly dispersive fiber, and coherent techniques incorporating dispersive delay. Recent experimental results are discussed in the light of practical system requirements. >

Optically generated true-time delay in phased-array antennas

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. >

Fiber-optic prism true time-delay antenna feed

This paper reviews the topic of true time delay (TTD) and its importance and implications in wideband imaging phased arrays. The paper describes an early analog version of TTD using the “Rotman lens,” discusses various architectures, and talks about photonic implementations. Also emphasized are the differences in design between narrowband arrays which are calibrated at individual frequencies and wideband imaging arrays where TTD is necessary.

True Time Delay in Phased Arrays

A true time-delay beamformer based on a fiber-optic dispersive prism is developed and characterized. The beamformer is used to control an ultrawideband time-steered array antenna, which is a significant improvement over inherently narrowband phased-array antennas. The time-steered transmitter antenna consists of eight broadband spiral elements in a sparsely-populated array. In transmit mode the bandwidth is microwave-component limited to 2-18 GHz. The transmitter shows an unprecedented performance with >100/spl deg/ azimuth steering and no observed squint over the full frequency range. We also extend the beamformer functionality and demonstrate, we believe for the first time, fully-independent dual-beam dual-frequency ultrawideband antenna transmitter operation. Furthermore, the beamformer is shown to be capable of controlling the transmitter under pulsed operation with microwave pulse-widths as short as 75 ps. In the phase-steered receive mode, the antenna is component-limited to two elements and a frequency range of 6-16 GHz. However, we can still demonstrate squint-free receiver steering over >70/spl deg/ azimuth over the full available frequency range. >

True time-delay fiber-optic control of an ultrawideband array transmitter/receiver with multibeam capability

Microwave photonics is a rapidly-growing interdisciplinary field linking microwave and optical technologies. This paper intends to trace the evolution of microwave photonics over the last three decades emphasizing some major research areas and application fields. Currently, microwave photonics is a vivid research area with several practical applications. In this paper, we attempt to project potential future trends as well.

Microwave Photonics—A Historical Perspective

The authors have developed a low-loss integrated 6-bit photonic time-delay unit to be used in the demonstration of a photonic beamformer for eight subarrays of a 3-6 GHz phased array radar. The current architecture for the electronic true-time-delay beamformer, and the proposed architecture for the equivalent photonic true-time-delay beamformer are presented. The 6-bit photonic time-delay unit design, showing the four 4*4 crossbar switch arrays realized using two 3-in LiNbO/sub 3/ substrates, is outlined. The time-delay unit fabrication is discussed. The results of the insertion loss and crosstalk measurements for each of sixty-four paths through the photonic time-delay unit are presented. >

W. Minford

Papers

Integrated 6-bit photonic true-time-delay unit for lightweight 3-6 GHz radar beamformer