A protective sheath is provided to prevent mechanical damage to the working channel of the endoscope during insertion of the fiber, insulate the fiber from surrounding cooling fluids, and/or serve as an indicator of overheating that enables early detection of excess heating or burning of tissues or equipment at the treatment site. The protective sheath may include trim markings to facilitate cutting to a desired length, such as the length of the fiber in the scope, may also be arranged to glow when the fiber is pulled too far into the sheath, may be tightly fitted over the fiber to provide a plunger effect, and may include perforations to facilitate sterilization.
