Unified Optical Scanning Technology

Chapter 4.6.1 - Scanner Devices and Techniques: Scanner-Lens Architecture

4.6.1 Scanner-Lens Architecture

Objective lens complexity (given wavelength, beamwidth, and scan angle) relates primarily to its orientation before or after the scanner. This is introduced and presented in Section 1.5.1, describing the options of postobjective and preobjective scanning.* It is preobjective scanning that imposes the major burdens on lens design and fabrication. A further parameter is the extent of the pupil relief distance, identified in Figure 1.8 and 1.9. Under similar conditions of beamwidth and scan angle Θ, the greater the pupil relief distance, the larger and more costly is the lens assembly. It requires a larger 'clear aperture' for clear propagation of the fully scanned beam through the optical elements.

As detailed in Section 1.5.1, a pyramidal rather than prismatic scanner, also operating preobjective as in Figures 1.9 and 3.6, allows minimization of the pupil relief distance by eliminating the space allocated for the input beam to clear the lens housing. This extra space is evident in Figures 1.8 and 3.7, and Figure 4.2 illustrates the design parameters relating to, for example, Figure 3.7. Keeping in mind that clear passage of a Gaussian beam requires allocating a channel width of almost twice the 1/e2 width of the beam (Equation 2-6), the pyramidal configurations of Figures 1.9 and 3.6 can provide reduced lens size and cost.

A similar reduction in pupil relief distance can be achieved with the use of transmissive deflectors (or facets) instead of the above-described reflective ones. Such is the case with transmissive holographic scanners, exemplified by Figures 4.14 and 4.16. The bow compensation prism of Figure 4.16 inhibits an even closer placement of the F-Θ scan lens to the hologon disk.

In contrast to the demands of wide-angle preobjective scanning on the design of the objective lens, postobjective scanning imposes no such requirement. This operation is represented fundamentally in Figure 1.10, where the objective lens is not only illustrated as a single element, but may, in reality, be reduced to such a simple component. The aberration burden on the lens is minimized with the input beam propagating coaxially with the lens, avoiding all off-axis aberrations. For typical monochromatic operation, the only defect of concern is spherical aberration. Although this could be challenging for focusing from a high numerical aperture (low f-number) to extremely small spots (Equation 2-17), as depicted, for example, in Figures 4.4, even this can be alleviated significantly with the tilted axis alternate of Figures 4.5 and 4.6. These conservation factors are discussed in Section 4.3.5.4.

* Although this presentation relates to active scanning (Section 1.2), it applies equally to passive systems (observing the change in related nomenclature), per Figures 1.1 and 1.2, where the conjugate fixed and moving focal points are oriented at the same sides of both systems.

 

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