The disclosed embodiments provide systems and methods for mitigating lensing and scattering as an optical fiber is being inscribed with a grating. The disclosed systems and methods mitigate the lensing phenomenon by surrounding an optical fiber with an index-matching material that is held in a vessel with a sealed phase mask. The sealed phase mask allows it to be in contact with a liquid index-matching material without having the liquid index-matching material seep into the grooves of the sealed phase mask. Thus, for some embodiments, the sealed phase mask may be immersed in a liquid index-matching material without adversely affecting the function of the phase mask.