Optical Fiber Basics

Optical Fiber Basics

The previous section examined the phenomenon of total internal reflection and demonstrated how it is a natural consequence of Snell's Law. You should remember that light will be completely reflected at the interface if the angle of incidence is greater than the critical angle for the interface. This critical angle is found by

sin θ_c = n₂/n₁.

This equation is only meaningful when n₁ > n₂, or when light traveling through the denser medium encounters the surface of the rarer medium.

Figure A: Light striking the top edge of the (blue) glass tube at an angle greater than the critical angle is trapped in the tube.

Figure B: Much of the light striking the top edge of the (blue) glass tube at an angle less than the critical angle will escape into the air.

Consider light traveling through a glass tube having an index of refraction of n@1.5, represented by the blue portion of the above figures. When the light strikes an edge of the glass, it encounters the rarer medium of air. Thus a critical angle can be defined for this situation:

sin θ_c = n₂/n₁ = 1.0/1.5 = 0.67
θ_c = 42° (glass-air) Light will be trapped inside the tube if it strikes the wall at an angle greater than 42°, as indicated in Figure A above. The light in Figure B striking the wall at a smaller angle than θ_c is not completely trapped in the pipe. Some is reflected, but much light escapes each time the beam strikes a side of the tube. Notice that the reflected light obeys the law of reflection in both cases.

Are today's optical fibers nothing more than glass tubes?
Continue to the next page to find out!