Distance estimation between a fiber end and a tissue using numerical aperture modulation

A tissue treatment system to treat target tissue comprising:

• a pulsed treatment laser, arranged to treat the target tissue by delivering energy via an optical fiber to the target tissue, the optical fiber having a proximal end and a distal end;
• and
• a tissue distance estimator, the tissue distance estimator comprising:
• a modulator positioned at the proximal end of the optical fiber and arranged to modulate a numerical aperture of a light beam transmitted through the optical fiber and directed at the target tissue via the distal end of the optical fiber;
• a sensor positioned at the proximal end of the optical fiber and arranged to measure intensity values of reflections of the light beam reflected from the target tissue and transmitted backward through the optical fiber;
• and
• a processing unit arranged to receive the intensity measurements from the sensor and estimate, by comparing intensity values of the reflections associated with two or more numerical aperture values, a distance between the target tissue and the distal end of the optical fiber;
• and;
• wherein the modulator and the sensor are arranged to respectively carry out the modulation and the measurement between pulses of the treatment laser.