Unified Optical Scanning Technology

Chapter 4.10.3 - Scanner Devices and Techniques: Summary of Agile Beam Steering

4.10.3 Summary of Agile Beam Steering

This section has concentrated on two dominant approaches to very low-inertia laser beam steering: the phased array and the decentered lens array. Although their operating principles appear to differ, it is shown that they both form diffraction gratings yielding output wavefronts having the characteristic of a blazed grating. The result is high diffraction efficiency in a selected diffraction order in one direction of a symmetric pair of (±) sets. The complementary set is provided by programming the negative action of the first, similar, for example, to mirror deflection within a zero-balanced restoring force, requiring (+) and (-) excitation to steer through the full available field.

Some distinctions between and characteristics of the two array techniques are noteworthy. The dominant phased array method is completely electrooptic, whereas the microlens array requires very small displacements, with concomitant low inertia acceleration/ deceleration. An alternate phased array utilizes minute (≤λ/4) axial displacement of individual micromirrors. However, it encounters difficulty in fabrication to high optical integrity and in achieving an optical fill factor that approaches that of the electrooptic type. Alternate microlens arrays are formed of Fresnel lens-type binary optics. For low f-number lenticules, whose theoretical steered efficiency is high, their minimum feature sizes become miniscule and also, thus far, difficult to fabricate.

Phased arrays utilize complex multielement electrical programming, whereas the lens array systems require controlled and very precise positioning of the lens assembly over variable small distances. These associated operations can impose significant burdens of mass, volume, and cost of the auxiliary facilities required for address and position control. Although this factor is generally not detailed in the literature, a comparative analysis [McD] indicated several related observations regarding the 1995 state of the art. The authors preferred a microlens array over the liquid crystal phased array, avoiding thereby the 'heavy burden' on electronic control of the many individual phase-delay elements. An aplanatic field lens-type (3-lens) array was designed, having a 6-mm path length through a 1-mm aperture of silicon lenslets. Along with x and y translation, z-axis motion was programmed to minimize aberrations. Image quality was within 1.3x diffraction limit. The principal comparison was of this system with respect to a two-galvanometer set, assembled of high-quality commercially available components. Detailed evaluations confirmed that the microlens system steered faster, consumed lower power, and packaged much smaller and lighter. However, no comment appeared on minimizing the scanning mirror sizes and inertia and the mass of the mechanical assembly. Nor was the use of relay optics indicated, to allow the second galvanometer to be as small as the first, to render a dramatic reduction in mass and inertia. Nor was the adaptation of the single articulated mirror that is driven angularly in two dimensions [New, Ball, Ber] expressed. Although some alternatives may exhibit negative trade-offs, these evaluations (including auxiliary control facilities) are very useful in the context of the relative capabilities for meeting system requirements.

 

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