Lowered threshold energy for femtosecond laser induced optical breakdown in a water based eye model by aberration correction with adaptive optics

Abstract

In femtosecond laser ophthalmic surgery tissue dissection is achieved by photodisruption based on laser induced optical breakdown. In order to minimize collateral damage to the eye laser surgery systems should be optimized towards the lowest possible energy threshold for photodisruption. However, optical aberrations of the eye and the laser system distort the irradiance distribution from an ideal profile which causes a rise in breakdown threshold energy even if great care is taken to minimize the aberrations of the system during design and alignment. In this study we used a water chamber with an achromatic focusing lens and a scattering sample as eye model and determined breakdown threshold in single pulse plasma transmission loss measurements. Due to aberrations, the precise lower limit for breakdown threshold irradiance in water is still unknown. Here we show that the threshold energy can be substantially reduced when using adaptive optics to improve the irradiance distribution by spatial beam shaping. We found that for initial aberrations with a root-mean-square wave front error of only one third of the wavelength the threshold energy can still be reduced by a factor of three if the aberrations are corrected to the diffraction limit by adaptive optics. The transmitted pulse energy is reduced by 17% at twice the threshold. Furthermore, the gas bubble motions after breakdown for pulse trains at 5 kilohertz repetition rate show a more transverse direction in the corrected case compared to the more spherical distribution without correction. Our results demonstrate how both applied and transmitted pulse energy could be reduced during ophthalmic surgery when correcting for aberrations. As a consequence, the risk of retinal damage by transmitted energy and the extent of collateral damage to the focal volume could be minimized accordingly when using adaptive optics in fs-laser surgery.

Safety of femtosecond laser‐assisted cataract surgery: assessment of aqueous humour and lens capsule

“…Femtosecond laser‐assisted cataract surgery (FLACS) produces enormous energy on a very narrow space and dissects tissue by photodisruption (Hansen et al. ). It was reported that prostaglandins rise immediately after femtosecond laser treatment (Schultz et al.…”

Fundamentals of Laser‐Based Hydrogel Degradation and Applications in Cell and Tissue Engineering

“…Since hydrogels inherently possess high water content, laser‐induced optical breakdown of hydrogels can be modeled, to some extent, on the optical breakdown of water. Several studies have investigated the optical breakdown of water and simulated biofluids at nano‐, pico‐, and femtosecond pulse durations and the effects of laser properties on shock wave emission, vapor generation, and cavitation . However, since the polymeric component of hydrogels also has a wide range of physical, biochemical, structural, and optical properties, it is important to understand their role in laser‐based degradation.…”

Adaptive optics assisted and optical coherence tomography guided fs-laser system for ophthalmic surgery in the posterior eye

“…If the incident pulse energy exceeds the LIOB threshold energy, laser pulses of the 5-kHz pulse train are scattered and reflected at gas bubbles generated by previous pulses. 23 This resulted in a flickering spot in the camera image visible at the location of the laser focus.…”

“…A wavefront sensor [e.g., Shack-Hartmann sensor (SHS)] measures the system aberrations and the individual eye aberrations, and a wavefront modulator (e.g., deformable mirror) corrects for them. By AO aberration correction, the aberrated point spread function (PSF) of the incident light is reshaped toward the diffraction limit as has been shown for imaging devices such as fundus camera, 20 scanning laser ophthalmoscope, 21 and OCT. 22 In addition, the threshold energy for fs-LIOB in water can be reduced 23 and the cutting precision can be enhanced 24 by aberration correction with AO. Our presented laboratory system for image-guided vitreoretinal fs-laser surgery utilizes AO for spatial beam shaping and integrates OCT for assessing the vitreoretinal transition, the vitreoretinal traction, and the morphology of the fovea after vitreous detachment, and for guiding laser surgery based on this assessment.…”