An apparatus for forming a lesion in cardiac tissue 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 for a direction of energy discharge to define an unobstructed light pathway from the fiber tip through a discharge end of the discharge bore. A fluid pathway is in fluid flow communication with the guide member to admit a bio-compatible flushing fluid to the discharge bore.