Abstract: In a laser tissue ablation system, a distal end of an optical fiber can extend from an endoscope. The optical fiber can receive therapeutic laser light pulses at first times, receive measurement light pulses at second times, direct the therapeutic laser light pulses and the measurement light pulses toward a target, collect, as collected light pulses, at least some of the measurement light pulses that are reflected from the target, and direct, as return light pulses, at least some of the collected light pulses away from the distal end of the optical fiber. An optical detector can sense at least some of the return light pulses. Processor circuitry can perform a time-of-flight analysis of the sensed return light pulses to determine a spacing between the distal end of the optical fiber and the target and can generate a spacing data signal representing the determined spacing.