An assembly for detecting a substance on an outer surface of a shaft. According to one implementation the assembly includes an elongate shaft having an outer surface and a radially emitting fiber spirally disposed inside the elongate shaft. A plurality of through holes are located between the radially emitting fiber and the outer surface with the through holes being longitudinally spaced-apart from one another. A laser under the control of a controller is optically coupled with the radially emitting fiber. The controller is configured to cause the laser to deliver pulsed light with a pulsed width into the radially emitting fiber. The longitudinal distance between adjacent through holes are such that the pulse width is shorter than a time for the light to travel between the adjacent through holes.