An optical feedback control system for the pulsed holmium:YAG surgical laser will be developed to enhance its safety for endoscopic lithotripsy. The system will enable rapid, intrapulse discrimination of, and selective energy delivery to, calculi versus surrounding soft tissue. This technological advance will significantly reduce the risk of laser-induced tissue damage during lithotripsies performed with the holmium laser - a clinical application for which this laser is soon expected to receive FDA approval. The specific aims of the Phase I study are to: 1) record the time- dependent, spectrally-resolved optical emissions induced by pulsed holmium laser irradiation of calculi, urinary tract tissue, urine, blood, etc.; 2) use these emission data to develop an algorithm for detecting, early in a pulse, whether the laser is being discharged on a calculus; and 3) assemble and demonstrate a breadboard opto-electronic control module which will use the developed algorithm to rapidly terminate the laser pulse if the delivery fiber is not targeted on a calculus, thus limiting the exposure risk to healthy tissue. Completion of these Phase I tasks will demonstrate the technical feasibility of the proposed "smart" holmium laser and lay the necessary foundation for subsequent Phase II development efforts.