An apparatus for treating a biological tissue of a patient in situ includes an optical fiber for guiding a coherent waveform of a selected wavelength to a fiber tip. The wavelength is selected to have a low absorption in water. The fiber tip is contained within a guide member having a discharge bore to define an unobstructed light pathway. The guide member is adapted to be slidable along a tissue surface in atraumatic sliding engagement with the discharge bore opposing the surface. The guide member is carried on an elongated flexible member.