Femtosecond laser writing in transparent materials

Abstract

Optical waveguides (type-I), with perfect mode matching to the standard single-mode fibre and with an overall insertion loss of ~1 dB, have been demonstrated in high index bismuth borate glass by femtosecond laser direct writing. Broadening of the transmitted light source by as much as 500 nm was demonstrated through a waveguide with a length of 1.8 cm. Finally, passive waveguide components such as Y-splitters and directional couplers have also been fabricated using this writing technique.

Self-assembled, sub-wavelength periodic structures (type-II) are induced in fused silica by a tightly focused, linearly polarized, femtosecond laser beam. Two different types of periodic structures, the main one with period (Lambda.E) in the direction of the laser beam polarization and the second with period (Lambda.k) in the direction of the light propagation, are identified from the cross-sectional images of the modified regions using scanning electron microscopy. The period Lambda.E is proportional to the wavelength of the writing laser and the period Lambda.k in the head of the modified region remains approximately the wavelength of light in fused silica.

A new phenomenon in ultrafast laser processing of transparent optical materials, in particular silica glass, manifested as a change in material modification by reversing the writing direction, is observed. The effect resembles writing with a quill pen and is interpreted in terms of new physical effect - anisotropic trapping of electron plasma by a tilted front of the ultrashort laser pulse. Different types of modifications are induced in fused silica by controlling the pulse front tilt.

Birefringent modification is demonstrated in the chalcogenide glass by femtosecond laser direct writing. The optical axis of the birefringent region is not determined by the laser polarization direction. It is observed that the information on the direction of writing can be recorded and be rewritable in the chalcogenide glass.

Finally, a unique non-reciprocal photosensitivity is identified for the lithium niobate crystal for ultrafast laser direct writing. Therefore, in a non-centrosymmetric medium, modification of the material can be different when light propagates in opposite directions.

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