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Can I use a fiber to focus light?

Contrary to what some folks believe, fiber optic glass strands have no ability to focus light. Many times designers and engineers believe that using fiber with a small numerical aperture (N.A) will collimate (or focus) light.

While it’s true that the light output cone will emit at full angle commensurate with N.A., a small N.A. will also restrict light collection. If the application measures output power in the center area, on axis with the fiber and within the cone of the small NA fiber, the light power projected from a small NA fiber will have the same value as light power projected from a larger NA fiber (but smaller NA fiber almost always costs more!).

Where it counts

If the application uses a wide angle source, and the designer includes focusing optics (even simple focusing optics) at the launch end of the fiber, the amount of power delivered by a .66NA fiber (above) could be (as much as) a factor of 3 greater than the same light focused and delivered by the .22 fiber at the same point. The reason for the difference is the focusing optic…it collects higher angle light emitted by the higher NA fiber and focuses the light (redirects it) within the field of interest.

In the world of fiber optic lighting, more efficient fibers can be less efficient.

Silica fiber is more efficient (lower attenuating loss) than leaded glass fiber. The statement is particularly true in communication applications wh ere the source is wavelength specific laser energy (very narrow launch angle power).

However, silica fiber is less efficient (collecting light energy) than leaded glass fiber when the source emits high angle, broad band (White light), particularly over shorter runs. Almost all non-telecom applications use this type of source. Once again, the main reason is NA. Most leaded glass fibers have a NA between .44 and .66 (Some go as high as .87) Most Silica fibers have NA between .22 and .37 (Some go as high as .48). Silica fibers will not collect as much light from these sources as the leaded glass types.

From a design and cost perspective, unless the fiber run is very long or the wavelength of interest is outside the visible spectrum, it’s quite possible the design would benefit more from using leaded glass rather than silica!

Conclusions

As you work through design parameters, keep the light source output characteristic in mind, along with required light characteristics to optimize your fiber selection.

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