Wollaston Prism Polarizer

Introduction

A Wollaston prism polarizer consists of two right angle prisms made of a uniaxial crystal such as calcite, which is commonly used for Wollaston prisms. The two pieces of uniaxial crystal are oriented such that the crystal axes are perpendicular to each other. Using the geometry shown in the figure below, horizontally polarized light propagates in the extraordinary index of refraction (ne) in the first section and is refracted into the ordinary index of refraction (no) in the second section. Vertically polarized light undergoes the opposite refraction and the two polarization states are thus separated. This knowledge base article describes a script, wollastonCreator.frs, that creates a Wollaston prism based on user specifications entered into a Basic dialog. It allows for different dimensions in all three directions. The FRED file, wollatsonPrismPolarizers.frd, includes two Wollaston prisms generated using the creator script—one based on a ThorLabs model and another on an Edmund Optics model. The Wollaston creator script and the birefringent material creator script from a previous knowledge base post are included as Embedded Scripts for convenience. The birefringent material creator may come in useful for defining birefringent materials for the Wollaston prism polarizer.

Figure 1. Wollaston prism polarizer with crystal axes labeled as black arrows. Horizontally (x) polarized light is refracted up and vertically (y) polarized light is refracted down (assuming a negative uniaxial crystal with no > ne )

Birefringent Materials in FRED

Before creating elements with birefringent materials, it is useful to understand how they are defined in FRED. A birefringent material is defined by specifying the ordinary and extraordinary indices of refraction at a number of wavelengths and the uniaxial crystal axis vector using “Sampled Birefringent and/or Optically Active Material” as the material type. The crystal axis direction is, by convention, a vector pointing along the direction associated with the extraordinary index ne in global coordinates. When creating a new birefringent material, it is a good idea to include the crystal axis direction in the name or description. When a birefringent material is assigned to a surface, it is put in the local coordinate system of the surface’s direct parent (the target entity). The same is true when a lens, mirror, or prism is created using a birefringent material. For example, if a lens is made out of a birefringent calcite with the crystal axis oriented in the x-direction (1 0 0), the surface material definition is relative to the lens coordinate system. This way if the lens is rotated its properties are maintained. Note that unlike lens, mirror, or prism elements, when Element Primitives are defined using a birefringent material, the material remains in Global Coordinates. There is a utility for modifying, checking, and keeping track of material orientations: “Edit/View GRIN/Birefringent Material Position/Orientation”. This can be accessed in the “Tools” top menu or by right-clicking on an entity in the object tree. As shown in Figure 2, the utility has a drop-down menu with a list of all surface material assignments that include a birefringent or GRIN material. The coordinate system of the selected material is displayed and can be edited.

Figure 2. Edit/View GRIN/Birefringent Material Position/Orientation utility

The Script

Script: wollastonCreator.frs

The script uses a dialog (see Figure 3) to prompt the user for the prism name, description, size, and materials. The first material (Birefringent material 1) will be assigned to the first section of the Wollaston prism and should have its crystal axis oriented along the x-axis (crystal vector [1 0 0]). The second material (Birefringent material 2) should be the same exact material as the first but with the crystal axis oriented along the y-axis (crystal vector [0 1 0]). Both materials must be of the type “Sampled Birefringent and/or Optically Active Material.” An immersion material may also be specified.

Figure 3. Wollaston prism creator dialog

The script first generates a list of all the materials as well as the birefringent materials present in the document. It then gets the user inputs from the dialog and assigns them to variables. The new Custom Element entity is created and then each surface is defined in order.

The Sample Models

FRED File: wollastonPrismPolarizers.frd

The .frd FRED file contains two Wollaston prism models generated using the Wollaston Prism Generator script. They both also contain a rough model of the mount defined using Boolean Element Composites. Multiple polarized and unpolarized sources are defined for analyzing the prisms. In addition to a regular analysis surface, a directional analysis surface is also defined. It can be used to perform an “Intensity on Polar Grid” analysis to determine the angular separation of the orthogonally polarized beams.