Coupling efficiency into multi-mode fiber can be calculated using the geometric image analysis feature.

In order to use geometrical rays to model multi-mode fiber coupling, the  fiber core diameter has to be at least 10 times larger than the wavelength, such that many transverse modes can be supported. If the fiber is multi-mode in the sense that two or three modes are supported, physical optics must be used. In this article, 'multi-mode' is taken to mean that there are so many modes supported that the fiber can be treated as a light-pipe.
 

The geometric image analysis can generate the irradiance at any surface, from an extended source with specific size and shape at the object surface. In addition, it can filter out rays that have larger incident angle than a definable threshold, at the evaluating surface. Using the attached sample file, we will demonstrate how to use the geometric image analysis feature to calculate multi-mode fiber coupling efficiency.

Open the included sample file from the last page of this article. This file models coupling to a multi mode fiber with 0.1mm core radius and NA of 0.2. For now, we will ignore the Fresnel (reflection) losses from air-glass interfaces, including the fiber.

The size of the fiber core is modeled by specifying 0.1mm radius circular aperture on the images surface. Since in this file the aperture type is “floating”, the size of the circular aperture is controlled by the semi-diameter of the image surface.

Open the Geometrical Image Analysis window under Analysis > Image Analysis > Geometric Image Analysis and open its settings window.

The NA of the receiving fiber is specified in the NA box. Since the we are assuming on-axis point source located at infinity, Field Size (size of the extended source) parameter is zero and the shape of the source, controlled by the “File” option, does not matter. The “Image Size” option determines the area of interest at the evaluation plane; you can also think of it as the size of the detector.  We will limit the number of rays to 10000 to speed up the calculation.

Click OK.