The 9-mm guide is too long for head fixture A3024HFB No2.2, so we sand it down with 15-μm grit paper until it looks right. We glue it in place with five-minute epoxy. We manage to avoid blocking the guide cannula with glue, nor do we see any glue on the taper. The guide cannula and the tapered fiber are shown below, immersed in water, and emitting blue light.
Figure: Head Fixture No2.2. We sanded the 9-mm guide cannula down to a length of roughly 8 mm so that its tip would be adjacent to the base of the taper. The outer diameter of the fiber is 450 μm and of the guide is 700 μm. The misty blue light in the upper foreground is an artifact of the plastic petri dish between the water and our camera.
There are three ways to define the base of the taper. One is the physical base, where we first observe curvature of the outer glass. This is the definition we used when gluing the guide in place. We also have the point along the taper that first emits light when the taper is in air, and when the taper is in water. The taper emits light earlier in water than in air. The brain is mostly water, so we immerse the taper in water for the photograph above. The guide cannula ends 0.3 mm before the emission of light, and the emission of light is 1.0 mm from the tip of the taper, as measured along the fiber axis. Thus the light-emitting length of the taper in water is 1.0 mm.
Which is the best place to have the end of the guide cannula? At the physical taper base, or the water-emission taper base?
UPDATE: [27-JAN-14] Head Fixture No2.5 looks like this in water. The end of the guide cannula is 450 μm from the end of the taper, light-emitting length of the taper in water is 1000 μm.
UPDATE: [31-JAN-14] Head Fixture No2.7 looks like this in air. The end of the guide cannula is 600 μm from the end of the taper, light-emitting length of the taper in air is 700 μm.
