Shaped Core Multimode Fibers for Improved Power Delivery
DOWNLOAD PDF

Authors: John H. Shannon, Richard J. Timmerman, Anyil Rhee and Todd H. Ewing

Recently, a new type of multimode optical fiber has been developed at Polymicro Technologies which has a modified geometry for the optical core region. Instead of the normal round cross-sectional core shape, square and rectangular core cross-sections have been demonstrated, with other shapes possible. These shapes alter the modal structure of the propagated light, improving the optical performance in varying ways for different system designs.

One potential benefit would be to homogenize the output of the fiber, potentially converting a non-uniform light source into a top-hat uniform output distribution. Also, high angle skew modes are cut off. There are potential applications for this are in many areas, including laser cutting and marking.

In addition, these fiber designs can be tailored to match the shape of an input light source or an output photodetector, increasing system efficiency. This especially could be applied to laser diode stripe arrays, potentially removing the need for complex beam shaping bundle arrays.

Shaped Fiber Design
The basic configuration for this type of fiber is shown in Figure 1 below. The fiber consists of a custom pure silica core, in this case shaped into a square, surrounded by a relatively thin region of doped silica optical cladding. Around this is a pure silica outer clad and a protective polymeric buffer to retain the fiber’s strength. Depending on the target operational wavelength, the core silica material can be varied, with either ultra low -OH silica for the visible/ near IR, or high -OH for UV/visible operation. The Numerical Aperture (NA) of these fibers is in the range of .16+.02.

Figure 1. Shaped Square Core Fiber Schematic

Demonstrated Designs

At Polymicro, several different configurations of shaped core designs have been fabricated for customers with varying applications. Fiber core sizes have varied from 20µm up to beyond 1000µm with square geometries as well as rectangular cores with aspect ratios up to 3:1. Examples of these designs are shown below. Figure 2 shows the uniform output from a 600µm square core fiber.

Figure 2. Illuminated Square Core Fiber

Figure 3 shows the profile and output of two similar rectangular core fibers, one 20 by 60µm next to a 100 by 300µm fiber. Once again the near field power profile is very uniform.

Figure 3. Illuminated Rectangular Core Fibers

Optical Test Results

The optical performance of shaped core fibers is comparable to standard multimode fibers with similar size and NA. In this section, the fibers shown in figures 2 and 3 have been subjected to spectral attenuation testing and NA measurement. The results are shown in Figures 4 and 5 below.

Figure 4. 600µm Square Core Fiber Spectral Attenuation

Figure 5. 100µm x 300µm Rectangular Core Fiber Spectral Attenuation

Conclusions:

Special multimode optical fibers with shaped cores have been fabricated and demonstrated at Polymicro Technologies. These fibers have potential for improving output modal properties, as well as more efficiently matching specialized input sources and output detectors. The fiber core -OH content can be optimized for either UV/VIS or VIS/NIR operation. While square and rectangular cores have been demonstrated, many other configurations and shapes potentially could be built to meet customer requirements.