Saturday, December 13, 2008

Tapered Fibers

I don't usually post about my work, partially because it's not very accessible to the lay reader but mostly because I'm the kind of person who leaves work at the workplace. However, I'd like to share some pretty micrographs---photographs taken through a microscope---that Michael D., Brian R. and I took on Friday.

As part of this project, we are learning how to taper an optical fiber, which start out with a diameter of 125 um, down to 5 um or narrower. At this diameter, and at our intended operating wavelength of 1000 or 1550 nm, the evanescent field of the guided optical mode extends beyond the surface of the glass and carries a significant fraction of the optical power. In other words, the fiber still guides light, but much of that light is carried along outside the fiber, in the air or whatever happens to be around the fiber.

Our plan is to take advantage of the properties of this kind of fiber and incorporate one into a passively mode-locked fiber laser. But that's not important for understanding these pics, so I won't elaborate. (Also, I'd rather not disclose our---read, Michael D.'s---clever plans.)

This is an untapered section of the fiber. Note that its diameter is 125 um.

Here you can see the beginning of the taper.

More of the tapered region.

This is the waist, or narrowest part, of the taper.

This is a much higher-magnification image of the waist. Note that the fiber is about 5 um in diameter here. The depth of focus at this magnification is only a few microns, so it's impossible to get a single image in which both the upper surface of glass, closest to the camera, and the edges are in focus. Additionally, becuase the fiber is mounted with a bit of a slope, with the right end of the fiber closer to the camera than the left, we can't get both ends of this image in focus.

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