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How to Get Real Waveguide Mode Data Into Zemax

Zemax now supports the input of mode field data from Optiwave's OptiBPM and OptiFiber codes. This makes it easy to improve the accuracy of fiber and waveguide coupler designs using Physical Optics in Zemax, by providing more accurate input data. This article gives several examples.

 
 
Mark Nicholson
01/20/2007
Fiber Coupling

Introduction

ZDC thanks Steve Dods of OptiWave Corporation for supplying the SMF-28 fiber simulation data used in this article.

In the article How to Model Coupling Between Single-Mode Fibers SMF-28 single mode fiber is modeled using data from the manufacturer's datasheet. The only data provided on the optical radiation produced at 1.31 is the mode field diameter, which is stated to be 9.2 ± 0.4 µm.

As a result, the fiber mode of both launch and receiver fibers was entered as a Gaussian (TEM0,0) mode of waist 4.6µ. The resulting fiber coupling calculation agrees well with experimental measurement.

However, Zemax can now import directly the modal fields computed by OptiWave Corporation's OptiBPM and OptiFiber codes. These codes are specialized integrated-optics codes which produce field distributions for optical fiber or waveguides integrated on a substrate, including channel waveguides, rib or ridge waveguides, buried waveguides or waveguides from a diffused process. This allows fields computed by these specialized codes to be propagated through bulk optical systems in Zemax. OptiBPM and OptiFiber can both read the .zbf files produced by Zemax, so that the results of a physical optics propagation through an optical system can be used as the input to one of their waveguide models.

In this article we will compare the OptiBPM simulation of SMF-28 fiber to the manufacturer's datasheet.


Importing the OptiBPM field distribution

OptiBPM and OptiFiber share a file format *.f3d which is proprietary to Optiwave Corporation. Zemax can read this data via Tools...Miscellaneous...Convert File Format.

The file conversion utility

The details are described in the manual, but note the following points:
  • ZBF files have a number of pixels in x and y which must be a power of 2: the .f3d files do not. Zemax will therefore zero-pad the .f3d data to make it equal the nearest larger power of 2
  • .f3d files do not contain the wavelength of the data, which must therefore be entered separately
  • The .f3d E-field phase data is referenced to a local plane. Zemax will fit a Gaussian beam to the E-field data to estimate the pilot beam properties required for subsequent propagation in Zemax, and store this data in the ZBF file.

The zip file at the end of this article contains two OptiBPM simulations of SMF-28. One is measured with no tilt,and the other contains a 2 degree tilt that represents a cleaved fiber end. The untilted beam can be converted and then read into Zemax like so:

Inputing this beam and analyzing it with the POPD operand shows the following:

  • effective width in x: 4.492 microns
  • M2 in x: 1.022
  • effective width in y: 4.479 microns
  • M2 in y: 1.021

The modal field as computed by OptiBPM is therefore very slightly asymmetric, and very slightly non-Gaussian (the M2 parameter = 1 for any pure TEMx,y mode). The simulation is certainly within the specification of a modal field diameter of 9.2 ± 0.4 µm.

The real use of this capability is when the input file is not a good Gaussian, of course. The second file contains a tilt, and analysis with POPD shows:

  • effective width in x: 6.64 microns
  • M2 in x: 1.3
  • effective width in y: 6.59 microns
  • M2 in y: 1.3