Biomedical material ablation by femtosecond laser double pulses through small-core hollow fibers

Interest in femtosecond (fs) laser applications in medical fields has been increasing. Many studies which focused on the fs laser ablation processing of biomedical tissues have been reported. The advantages are due to the precise material processing without heat affected zone, and without changing chemical properties of the ablated area. Technical issues of fs laser delivery prevent fs laser processing from endoscopic medical applications. In this paper, a high quality beam delivery using a flexible small-core hollow fiber and the ablation processing of hydroxyapatite (HAp) and collagen with fs laser double pulses through a bend hollow fiber is demonstrated. HAp is a main component of bones and teeth. Two new methods were adopted: one is the use of a hollow fiber with a 320-μm core diameter in order to improve the output beam intensity profile by eliminating the high order modes; the other one is an fs double pulse laser transmission scheme with a tailored ps-order delay time, in which the second pulse comes earlier than the ablation onset. Since the optical damage threshold of the hollow fiber depends on the peak intensity of the incident laser pulse, the double pulse transmission enabled the delivery of higher pulse energies than the single pulse even by keeping the fs laser ablation characteristics. By adopting these two new methods, high energy enough to ablate HAp and collagen was delivered. The delivery of a near Gaussian output beam profile and high ablation rates was demonstrated.