Unified Optical Scanning Technology

Chapter 4.6.3 - Scanner Devices and Techniques: Aperture Relaying

4.6.3 Aperture Relaying

This topic, seldom addressed in the context of optical scanning, merits attention. Aperture relaying provides operationally efficient transfer of angular scan from one deflector to another. A prominent example is the formation of two-dimensional angular scan by individual (x) and y deflectors.

Without aperture relay, let the x-deflector execute its required component of scan, which is conveyed over some finite distance to the y-deflector, which provides its component of scan to form the composite function. It is the 'some finite distance' that is of concern. Depending on the distance and the scan angle of the x-deflector, the displacement of the beam on the y-deflector may require an enlargement of its aperture to accept the beam. This could affect significantly its dynamics and cost and, perhaps, its realizability. Even if practical from these viewpoints, the resulting deflection from two disparate centers of scan may burden subsequent optical imaging.

Although aperture relaying requires additional optics, it resolves the above concerns. An aperture relay transfers the dimensions and the axis of angular scan of a first deflector to a second location. This is rendered in Figure 1.7 in its most familiar form. Without deflection, it is recognized as beam expansion or compression. Shown as a beam compressor, it reduces the input beam width D to the output width D'. As a relay, it transfers aperture D at focal distance f1 from lens L1 to a reduced aperture width D' at reduced distance f2 beyond lens L2. The total distance is 2(f1 + f2). In reverse, it provides beam expansion. Note the complementary transfer magnitudes of D and Θ. For f1 = f2, D = D' and Θ = Θ'. Each lens operates telecentrically. As for any substantive angular scan, lens design is to be consistent with acceptable off-axis aberration.

For unity magnification (D = D'), another lens arrangement may be considered (see [Bei2] pp. 129-132), which utilizes a 1 : 1 conjugate image lens and a recollimating lens at D'. This was adapted to reflective optical transfer and became a split confocal resonator, arranged to provide scan amplification, increased angle and resolution through multiple reimaging of a pair of small deflecting mirrors.

 

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