A device for material working by a laser beam wherein an unclad, optically and materially homogeneous quartz rod having enlarged ends tapered to a fiberlike center portion is protectively enclosed in a flexible tube. Preferably, the tube is provided with annular spacer rings of quartz glass to keep the fiber from contacting the tube wall. To provide high power and high efficiency transmission, the tube is filled with a circulating liquid having a refractive index less than that of the fiber, and the spacers are coated with substance having a refractive index which is the same as that of the liquid. The fiber is made by heating a rotating quartz rod and drawing it. The rotation is controllably slowed and stopped to prevent twisting as the rod softens. Alternatively, a quartz rod is etched to the desired form.