This article describes how to include cleaved fibers in Physical Optics fiber coupling calculations.
This article is also available in Japanese.
Authored By: Mark Nicholson
How to Model Cleaved Fibers
This article is also available in Japanese.
In the article How to Model Coupling Between Single-Mode Fibers we show how to set up a Physical Optics (POP) calculation of the coupling between two fibers with flat end faces:
Summarizing that article, a complex amplitude distribution that represents the source fiber mode is launched, propagated through the optical system, and then the overlap integral of the resulting complex amplitude with the receiver fiber mode is computed.
Often the fiber ends are cleaved, or cut at an angle. This has two consequences. The first is that the fiber mode is no longer rotationally symmetric, but has different widths in x and y. Secondly, the mode is now launched at some angle, and so propagates off-axis through the optical system.
This can be easily included in the calculation. First, the fiber modes of the cleaved fibers can simply be used directly. Zemax can import waveguide mode data from OptiWave and RSoft waveguide software. If the fiber mode for the cleaved fiber is used, that is all that is required. The tilted mode will propagate correctly through the optical system, and its overlap integral with the tilted receiver mode will yield the correct coupling.
The Knowledge Base article How to Get Real Waveguide Mode Data Into Zemax gives example data computed by the OptiWave software for SMF-28 fiber with uncleaved and cleaved fiber ends.
If you do not have direct access to the data for the tilted mode, you can approximate the required data a number of ways:
1. The ZPL keyword ZBFTILT will multiply the data in a .zbf file by a complex phase factor to introduce phase tilt to the beam.
2. A Coordinate Break surface immediately after the surface the POP calculation starts on can provide the required tilt of the launch distribution.
3. The tilt of the receiver fiber can be directly specified in the Fiber Data tab of the POP settings dialog:
Note that a tilt of the receiving fiber mode (#3 above) is not the same as a physical tilt of the receiving fiber defined using a coordinate break.