Ultrashort-pulse laser eye surgery uses fiber technology at 1.6 microns

The strongly localized interactions of ultrashort laser pulses with tissue make femtosecond lasers a powerful tool for eye surgery.1, 2 These lasers are now routinely used in refractive surgery, while several clinical laser systems also offer options for corneal grafting, a surgical procedure that involves replacing all or part of a pathological cornea (the eye’s transparent front section, covering the iris, pupil, and anterior chamber) by donated corneal tissue. Recent studies have also examined the option of performing a surgical procedure in the sclera (the white part of the eye) for the treatment of glaucoma.3 These new forms of laser eye surgery need to take into account the limited tissular transparency. In a pathological cornea, the regular arrangement of the collagen fibrils is perturbed and the tissue’s optical properties become strongly scattering (see Figure 1).4, 5 Sclera is already strongly scattering in its healthy state. While the exact relation between tissular structure and transparency is subject to debate, it is clear that the intensity of the scattering processes strongly decreases with increasing wavelength. It should therefore be beneficial to use longer wavelengths for laser surgery. Earlier work3 and our own results suggest that a spectral region of relative transparency exists between 1.6 and 1.8 m, where the scattering cross-section of edematous cornea is about five times smaller than at wavelengths near 1 m, as typically used. Unfortunately, no laser material is known to directly emit in this wavelength range. Laboratory experiments have been Figure 1. (left) The regular arrangement of the collagen fibrils in the volume of healthy cornea is responsible for its transparency. (right) Most reasons for corneal grafting involve edema (corneal swelling), which perturbs the regular fibril structure and leads to strong optical scattering.