An apparatus is disclosed for advancing an optical fiber in a laser enhanced transluminal angioplasty catheter, and for controlling the introduction of laser energy into the fiber. The device includes a catheter and a fiber advance housing. The optical fiber runs through the catheter and housing, and is optically connected to a source of laser energy. A fiber advance assembly, mounted in the housing, reciprocates to advance and retract the fiber relative to the catheter and housing. An actuator member is provided inside the housing to slide independently of the fiber advance assembly. A switching means, provided in the housing, is closed to enable introduction of laser energy into the optical fiber whenever the fiber advance assembly is disposed forwardly of a select position relative to the actuator member. The switching means is open, thereby preventing introduction of laser energy into the fiber, whenever the fiber advance assembly is disposed rearwardly of the select position. The actuator position is adjustable through a zero adjust slide, connected to the actuator and slidable relative to the housing.