Ultrafast laser inscription of efficient volume Bragg gratings deep in fused silica using active wavefront shaping

Abstract

The maximum depth that photonic structures such as volume Bragg gratings (VBGs) can be precisely fabricated inside dielectric materials using ultrafast laser inscription (ULI) is limited by the aberration imparted on the laser beam by the air-substrate interface as it is focused into the substrate. Here, we use a computer-controlled spatial light modulator (SLM) to shape the wavefront of the ULI laser before it is focused into the substrate, such that the impact of this aberration on the manufacture of VBGs is minimized. We show that this technique allows us to inscribe efficient VBGs at depths in fused silica that would otherwise result in low efficiency VBGs. We find that an optimized “reference” grating fabricated at a mean depth of 200 µm without wavefront shaping exhibited a maximum relative first-order diffraction efficiency of 48%, whereas a grating fabricated at a mean depth of 900 µm using identical parameters exhibited an efficiency of 6.2% – both measured with 633 nm light polarized perpendicularly to the grating lines. Using the SLM to control the wavefront of the ULI laser beam, we were able to pre-compensate for the effect of the substrate surface aberration and fabricate gratings at a mean depth of 900 µm that increased the first-order relative diffraction efficiency to ∼42%. A further plasma study provided significant evidence to the effectiveness of Zernike polynomials for spherical aberration correction. Combing both plasma imaging and laser writing approaches, a set of polynomials for aberration correction at a range of depths was produced with scope for arbitrary depth correction.