Abstract
The spherical aberration generated when focusing from air into another medium limits the depth at which ultrafast laser machining can be accurately maintained. We investigate how the depth range may be extended using aberration correction via a liquid crystal spatial light modulator (SLM), in both single point and parallel multi-point fabrication in fused silica. At a moderate numerical aperture (NA = 0.5), high fidelity fabrication with a significant level of parallelisation is demonstrated at the working distance of the objective lens, corresponding to a depth in the glass of 2.4 mm. With a higher numerical aperture (NA = 0.75) objective lens, single point fabrication is demonstrated to a depth of 1 mm utilising the full NA, and deeper with reduced NA, while maintaining high repeatability. We present a complementary theoretical model that enables prediction of the effectiveness of SLM based correction for different aberration magnitudes.
Highlights
Ultrashort pulsed laser fabrication [1] in materials, such as glass or fused silica, is receiving increasing attention due to a range of interesting applications
We investigate how the depth range may be extended using aberration correction via a liquid crystal spatial light modulator (SLM), in both single point and parallel multi-point fabrication in fused silica
It is difficult to confirm the driving mechanism behind the axial extent of the features: (i) the spherical aberration arising from refraction at the sample surface is predicted to generate an intensity distribution axially stretched over 100 μm and (ii) the focal distortion dictates that a higher pulse energy is needed for fabrication, raising the peak power in the fused silica above the critical power for self focussing by the Kerr effect [34]
Summary
Ultrashort pulsed laser fabrication [1] in materials, such as glass or fused silica, is receiving increasing attention due to a range of interesting applications. Implementations have included very high precision machining at high NA [18, 19], incorporation of aberration correction with parallelisation [20], longitudinal waveguide writing [21] and removal of aberrations induced near the sample edge [22]. We explore both theoretically and experimentally the limits for aberration correction using a SLM when machining deep inside fused silica at different numerical apertures
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