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How to Create a Double Pass Optical System

This article describes the make double pass tool, which automates the creation of double pass, sequential optical systems.
Akash Arora
3D Geometries

A sequential optical system is modeled in Zemax by individually defining the surfaces with which light interacts. In systems where light interacts with a surface or element more than once, a double pass system, the user must define the second pass instance of any surfaces as additional surfaces. This requires redefining the lens system in reverse, which can be very tedious for complicated systems. Zemax has a make double pass tool that can automate this time consuming and error-prone process. The tool is located under Lens Data Editor > Make Double Pass.

The make double pass tool is very simple to use and only requires the user to specify the surface at which rays are to reflect. The reflect surface is converted to a mirror, all surfaces prior to it are replicated in reverse after the mirror, and pickup solves are applied to all parameters (including lens and extra data values) to link the first and second pass surface data. Note that any nominal surfaces that come after the reflect surface will be deleted when using this tool.

Open the sample file on the last page of this article. The file models an aplanatic laser beam delivery system with a decentered beam incident upon the optical system. Suppose the goal is to simulate a double pass system, whereby the beam reflects from the image plane and travels back through the optical components. This can easily be done with the make double pass tool. Open the tool and select surface 14 as the reflect surface.

Make Double Pass Settings

After pressing OK, notice the layout updates to show the beam coming in through the bottom of the lenses, reflecting off surface 14, and exiting through the top of the lenses. To accomplish this, Zemax duplicated the surfaces in reverse order and applied pickup solves. The solves make it possible to modify, optimize, or tolerance this system while maintaining the same surface definitions in the forward and reverse passes. Replicating the surfaces manually and applying pickup solves would have been a lengthy process.

Make Double Pass Layout

Certain special types of surfaces, such as non-sequential component surfaces, cannot be easily reversed, and are not supported. For a complete description of the feature and its capabilities, see the Tools chapter of the Zemax manual.