Unified Optical Scanning Technology

Chapter - Scanner Devices and Techniques: Phased Array Developments    Phased Array Developments    After the pioneering work identified in Section 4.10.1, more recent development merits review. Figure 4.29c illustrates an array composed of bulk electrooptic crystals of lithium tantalate, which in the reference by Mey require a significant axial length to appreciate 2π phase retardation with reasonable applied voltage. A resonant approach utilizing a compact stack of alternating GaAs and AlAs dielectric layers was reported in 1999 [Keys] having potential for operation in transmission and in reflection. Thirty-period stacks of approximately 5-mm length operating at a selected wave-length are shown to approach full 2π phase delay with some sacrifice in output amplitude stability. Design improvements are discussed.

Work using nematic-phase liquid crystal electrooptic retarders is detailed comprehensively in a 1993 Air Force document [Dor]. An effective description is available in more accessible form [McM]. The types of material are known as E7 and PTTP-33 liquid crystals, having birefringence Δ = (ne - no) 0.2 in the infrared. Thus a cell need be only 5 optical waves thick for a full-wave phase shift in transmission and only 2.5 waves thick in reflection. The thinner the cell, the shorter the time for molecular reorientation. Switching speeds in the millisecond range with high efficiency diffraction-limited steering have been demonstrated at 10.6 μm with CO2 lasers and at 1.06 μm and 0.53 μm with Nd:YAG lasers. Further development tested tandem scanners as a means for adding the contributions of two or more deflectors, each in its optimal operating range. Thin one-dimensional arrays having crossed electrode patterns may be summed, one for azimuth and one for elevation. Or individual deflectors requiring excessive spatial separation may be cascaded [Bei2] using relay optics represented by Figure 1.7 to avoid walk-off of the beam from the second aperture by the action of the first deflector. This is analogous to the familiar woofer-tweeter expansion of acoustic bandwidth. In a major tested system, a course (large angle) deflector was summed with a fine (small angle) one, multiplying the number of steering states of each. A group of discrete large angular positions of the course deflector were filled with additional numbers of small steered positions of the fine unit. Electrical connection was implemented in an elegant leadout arrangement termed 'multiple-state' architecture, which reduced substantially the number of wires to be addressed to 768, whereas 40,000 would have been required for a single fully addressable array having the same number of addressable beam directions.

Another approach to tandem arrays [Tho], called the discrete/offset bias cascade, reduces potential 'noise' (beam artifacts) in the instances of large quantization mismatches when cascading phase-delayed groups. It also reduces significantly the number of control lines that would be required to provide the same resolution from a single phased array. This was accomplished by programming a typical first array of cells of width d arranged in a regular Λ-group fashion (called 'discrete scanning') and directing the resulting wave pattern into a second stage (called 'offset phases') composed of wider cells of width Λ = qd such that they register over the Λ-groups of the first stage. This second stage adds constant phase delays to the wavefront segments from the first array, to form smoothly joined wavefronts. Experiment demonstrated improved overall diffraction efficiency, along with the use of a reduced number of control lines. A similar approach was demonstrated with microlens arrays [Flo] (Section 4.10.2). Also significant in this work is the use of an electrooptic phase retarder other than liquid crystal. The material selected is PLZT (lead lanthanum zirconate titanate), exhibiting a large electrooptic coefficient, broadband optical transmission, very fast switching, and good thermal stability [Hae]. This well-documented ceramic material is familiar in electrooptic modulator and deflector applications.

Mirrored pistonlike phase adjustment has been reviewed [W&M], and later work [Burn] describes both continuous optical phase change and fixed (binary) phase shift. When continuous, the height of the array mirror is decreased by analog electrostatic attraction toward the substrate. In binary operation, a larger fixed voltage pulls the element to the substrate over a fixed distance. The fabrication constraints are stringent, considering its dimensional characteristics, even with application of advanced surface micromachining technology and materials. The first generation device was a linear array of 128 mirrored elements; each 27 μm wide and 110μm long and held 2μm above the substrate by end flexures. Residual stress curved the (intended flat) mirrors to sag depths of approximately 10 nm and 44 nm along the width and length, respectively. Control voltage to 20V provides continuous phase shifts to a full 2π change. Beyond 24.5V, the mirrored element executes 'snap-through" to the substrate, forming the binary condition. Optical efficiency was low, because of poor fill factor of the 19-μm-wide gold reflector in a 30-μm array period and by destructive reflected interference between the gold coating and exposed polysilicon borders. Planned improvements estimate raising maximum efficiencies to 63%.

Problems in broadband operation of phased arrays have been reviewed [McM], and work has been directed toward their solution [Wat3, Sto]. A wavelength-independent phase shift is achieved by polarization modulation of chiral smectic liquid crystals (CSLC), providing action similar to the mechanical rotation of a waveplate. However, grating dispersion remains because of wavelength deviation from nominal 2π phase resets, rendering a variation in efficiency ηd similar to Equation 4-46

where ε is the chromatic error due to mismatch of the nominal 2π phase reset. When in error, not only is energy lost, but sidelobe amplitudes increase and nondiffracted components result in image blurring and interference from sources outside the desired acceptance angle. This dispersion is nulled with application of special achromatic optics [M&Z] to restore the system to single-focus imaging. Additional work using chiral smectic-A ferroelectric liquid crystals (CSFLC) is expressed [Dro] to provide phased arrays with fast switching (1-100 μs) and a low drive voltage requirement (5-10 V).



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