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Mode Field Diameter Expansion for Single Mode Optical Fibers
Kari S. Rawluk and Allen H. Rose
National Institute of Standards and Technology
May 5, 1997



The purpose of this project was to determine a method by which the Mode Field Diameter (MFD) ofa single mode optical fiber could be consistently increased to 50 um (micrometers). Two methods were tested: Thermally Expanded Core (TEC) fiber and Multi-Mode Fiber (MMF) Graded Index (GRIN) lenses.

The cores of the single mode fibers were expanded using a fusion splicer.This machine was controlled by a C program, which enabled the user to vary the length of the expanded region, the temperatures used, and the speed with which the filament moved along the fiber. The highest MFD consistently achieved using this method was approximately 20 um. The MFD expansion was limited by the length of the heated region along the fiber and fiber warping.

MMF GRIN lenses were constructed using MMF cut to narly a quarter pitch length. The MFD produced when a lens was spliced to the end of a Single Mode Fiber (SMF) is controlled by the fiber parameters. Using a select combination of these two types of fibers, a MFD of approximately 50 um was achieved.



In sensor applications, the MFD (Mode Field Diameter) of the connecting fiber is matched to the MFD of the sensor through a system of lenses. There are two problems with lenses that this project sought to overcome: the attachment process is difficult to mass produce, due to alignment issues, and the conventional lenses are bulky as compared to the fiber optic system. Two procedures for expanding the MF of a Single Mode Fiber (SMF) were explored: Thermally Expanded Core (TEC) fiber and the construction of miniature Multi-Mode Fiber (MMF) Graded Index (GRIN) lenses. Each method must maintain a single mode for polarimetric and interfermetric sensors.



2.1 Theory

2.2 Method: using the FFS-1000 Fusion Splicer

2.3 Results

2.4 Conclusions

Two factors limiting the MFD expansion with the fusion splicer were the deformation of the fiber and the length of the heated region. As shown in Figure 2, the dopants diffuse exponentially with time, and the diffusemore quickly with higher temperatures. However, the strength of the fiber determined the temperatures and the lengths of time used in heating. Higher MFD's could have been achieved if the fiber were able to withstand higher temperatures and longer heating times.

In published papers, researchers were able to expand the MFD of a single mode fiber to 36 um using flames[1]. In figure 9, the flame heats a 5 mm section of the fiber, and the entire length of the heated region is 14 mm. Our experimentation with a flame showed this method to have two advantages over a filament. First, an updraft from the flame supported the fiber, such that it did not bend nor pull apart during high temperature heating. Second, the flame can heat a broader section of the fiber. This provies a more uniform heating, preventing the fiber from developing higher order modes. Future work will include heating the fiber with a flame mounted on a translation stage, so that it can be moved along the fiber to create the dual taper shown in Figure 1.



3.1 Theory

3.2 Method

3.3 Results

3.4 Conclusions





  1. Kihara, Mitsuru;



    Last edited 13Sep2015

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