A surgical technique for removing corneal tissue with scanned
infrared radiation is disclosed which utilizes short mid-
infrared laser pulses to provide a tissue removal
mechanism based on photospallation. Photospallation is a photomechanical
ablation mechanism which results from the absorption of incident
radiation by the corneal tissue. Since photospallation is a mechanical
ablation process, very little heat is generated in the unablated adjacent tissue. The disclosed surgical
system includes a
scanning beam delivery system which allows uniform
irradiation of the treatment region and utilizes low energy outputs to achieve controlled tissue removal. A real-time
servo-controlled dynamic eye tracker, based on a multiple-
detector arrangement, is also disclosed which senses the motion of the eye and provides signals that are proportional to the errors in the lateral alignment of the eye relative to the axis of the
laser beam. Temporal and
frequency discrimination are preferably utilized to distinguish the tracking illumination from the ambient illumination and the surgical
laser beam. A laser parametric generator for surgical applications is disclosed which utilizes short-pulse, mid-
infrared radiation. The mid-infrared radiation may be produced by a pump
laser source, such as a
neodymium-doped laser, which is parametrically down converted in a suitable nonlinear
crystal to the desired mid-infrared range. The short pulses reduce unwanted thermal effects and changes in adjacent tissue to potentially submicron-levels. The parametrically converted radiation source preferably produces pulse durations shorter than 25 ns at or near 3.0 microns but preferably close to the water absorption maximum associated with the tissue. The down-conversion to the desired mid-
infrared wavelength is preferably produced by a nonlinear
crystal such as KTP or its isomorphs. In one embodiment, a non-critically phased-matched
crystal is utilized to shift the
wavelength from a near-infrared
laser source emitting at or around 880 to 900 nm to the desired 2.9-3.0 microns
wavelength range. A
fiber,
fiber bundle or another
waveguide means utilized to separate the pump laser from the optical
parametric oscillation (OPO) cavity is also included as part of the invention.