Abstract: A laser system for modification of a sample to form a modified region at a target location within the sample, the target location being disposed below a surface of the sample, the laser system comprising: a laser light source configured to provide laser light; a sample holder for supporting the sample; one or more optical elements configured to direct the laser light from the laser light source into the sample when the sample is supported by the sample holder, wherein the one or more optical elements are configured to focus the laser light into the sample, and wherein the one or more optical elements includes a component configured to correct for spherical aberration caused by mismatch in refractive index at the surface of the sample through which the laser light enters the sample such that the laser light is focused at the target location within the sample, a tilt measurement device configured to measure a tilt angle of the surface of the sample relative to an optical axis of the laser light entering through