Enhanced efficacy in refractive corrections of rabbit corneas with low repetition rate blue femtosecond laser pulses

Abstract

Laser induced refractive index change (LIRIC) technique has been demonstrated as a non-invasive way to alter optical refractive powers of ophthalmic materials including hydrogel-based contact lenses, exercised corneal tissues and even corneas in vivo, via modifying the refractive index through multiphoton absorption process. In our previous work, blue femtosecond laser pulses at 405 nm with a repetition rate of 80 MHz were focused tightly into the stromal region to achieve refractive corrections in live cats via inscribing phase wrapped structures. In order to improve the efficacy of LIRIC, we here demonstrate that blue femtosecond laser pulses at a lower repetition rate range induce larger amounts of phase change with lower laser powers and higher scan speeds owing to a higher pulse energy density deposition. In comparison to a high repetition rate writing at 80 MHz, higher phase change can be attained at 8.3 MHz at the same average power. Furthermore, one wave of phase change measured at 543 nm was attained in rabbit corneas ex vivo for the first time by inscribing single LIRIC layer at 8.3 MHz with a power of 200 mW and a scan speed of 100 mm/s, corresponding to a refractive index change of 0.025 after the layer thickness was estimated to be 20 μm. Accordingly, the optimum laser repetition rate used in femtosecond micromachining corneas can be determined to be around 8.3 MHz, as arbitrary phase structures can be manufactured by wrapping the phase between 0 and 1 wave.