A coaxial laser endoscope with arbitrary spots in endoscopic view for fetal surgery

In this paper, we describe a rigid endoscope that transmits a laser beam coaxially to arbitrary points in the endoscopic view, mainly for treatment of twin-to-twin transfusion syndrome. The endoscope consists of a hotmirror for coaxial transmission of visible light and a Nd:YAG laser beam, and galvanometers for controlling the beam irradiation angle. We evaluated the transmission efficiency of the laser power, the spot size through the endoscope and accuracy in positioning the beam. The maximum laser transmission efficiency was 39% and the spot diameter was 2.2-3.2 mm at a distance of 10-20 mm. The positioning accuracy was mostly within 1.0 mm in the endoscopic view at the distance. The average laser power density on the spot was estimated to be 170-370 W/cm2, and a chicken liver was successfully coagulated by changing the laser beam irradiation angle.