Doublet/meniscus solution

Doublet/meniscus solution

The Melles Griot optics catalog contains several combinations of doublets and meniscus singlets for high-speed (i.e., large N.A.) focusing systems. One of these, the combination of 01LAO126 followed by a meniscus singlet 01LAM126, has approximately the correct paraxial properties and warrants evaluation.

Click on File >> New. Enter the file name lasrcom2, and select catalog lens.
Since the exact part number is known, click on the part name cell and type 01LAO126, then press ENTER. This part appears in the window as shown. Click OK to close the database and go to the surface data spreadsheet.

Insert the meniscus singlet 01LAM126 behind the doublet. To do this, first set the thickness of surface 3, which was entered with an axial ray height solve automatically by the program, to 1.0 by typing the number into the thickness field. Then click on the row button for surface 4 to select the row, and execute Edit >> Insert Catalog Lens. When the catalog database window appears, enter the part number 01LAM126 and close the spreadsheet.
Delete the extra wavelengths as in the previous example and set the primary wavelength to 0.6328 micrometers. Set the entrance beam radius to 15, and turn on the Autodraw window. Add a solve to surface 5 by clicking on the thickness options button and selecting solves, axial ray height (value = 0). This time, use the Autofocus command to find the best focus by clicking on the SHIFT+F12 toolbar icon. The surface data spreadsheet should now appear as follows:

The Autodraw window appears as shown:

To analyze this new lens system, double-click on the graphics window. The geometric spot size is 0.001790, good enough to meet the specification! Note also the diffraction limit of 0.001527 mm. Whenever the geometric spot size approaches or falls below the diffraction limit, diffraction effects are important. In that case, the geometrical spot diagram becomes a poor model for the physical lens performance.
To check the actual diffraction image, click Calculate >> Point Spread function. In the dialog box, click Plot x and y scans, then dismiss the box by clicking OK. The result is shown in the following figure. The first zero of the pattern appears at about 0.0013, in reasonable agreement with the geometrical analysis.

Before concluding the design task, it is worthwhile to investigate the spacing between the lenses. The lens is being used at a different wavelength than it was designed for (in the Melles Griot optics catalog), so a better spacing solution may exist.

To study the spacing, set up an interactive design window to look at the final solution. The task is to set up the window to show the on-axis ray-intercept curves as surface 3 thickness varies over a range from 0 to 40 mm and as surface 6 thickness (the focus shift from the paraxial image plane) varies from -0.1 to 0 mm. First, set surface 3 thickness to 20 mm (direct), and set th 6 to 0.0, as shown in the following spreadsheet

Click OK to close the spreadsheet and execute Optimize >> Interactive Design. Set the values in the spreadsheet to be the same as those shown. If you have a 486/33 or faster computer with a graphics accelerator, use drag processing. For slower computers, drag processing can be distracting.

Once the data is set up in the interactive design spreadsheet, click on OK to close it. You see two graphic windows, plus a slider window, as follows.

The ray-intercept curves show the displacement of rays from the reference point (here, the optical axis) as a function of the fractional aperture coordinate (FY 1 corresponds to the edge of the exit pupil). Horizontal lines along the axis correspond to a perfect optical system. Tilting corresponds to a focus shift, and curvature to aberrations. Ray intercept curves are widely used in optical design because they provide a great deal of information with minimum computational effort.

4. By dragging the sliders back and forth, you are able to confirm the existence of two solutions, one with a spacing of about 5 mm, and the other with a spacing of about 25 mm. To check the spot diagram corresponding to any lens configuration, double-click on the graphics window containing the spot diagram.

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