| Abstract [eng] |
Nowadays ultrafast laser micromachining of transparent dielectrics, such as fused silica, is becoming one of the most promising and intriguing areas of photonics. One of the key factors owing to this, is the unique property of this type of radiation to induce refractive index changes, when focused inside the material, opening possibility to integrate various three dimensional photonic devices directly into the bulk of transparent materials. One of the simplest yet most versatile of such devices when it comes to enabling light manipulation inside a piece of transparent medium, necessary in such areas as optical communication, optofluidics, sensing and astrophotnics, is a femtosecond laser written waveguide formed by exploiting type I material modification, i.e. smooth refractive index increase. This work investigates the possibility of writing waveguides inside fused silica by femtosecond direct writing technique using Yb:KGW solid state laser generating 320 fs pulses. It is shown that, by using slit beam shaping method, optimising laser pulse energy and focusing depth it is possible to suppress spherical abberations affecting the laser beam and form type I modifications with a symmetrical cross-section as well as single mode waveguides and splitters supporting 632.8 nm Gaussian mode propagation. By using the cut-back technique and Fabry – Perot interferometric method it is estimated that at a wavelength of 632.8 nm these waveguides exhibit attenuation coefficient as low as 0.79 dB/cm, similar to typical values obtained by using titanium sapphire laser systems, generating < 100 fs pulses. These results suggest that, contrary to the widely accepted belief in the femtosecond micromachining community, under certain laser beam focusing conditions, by using laser pulses longer than 200 fs, there might be a possibility to induce type I modifications inside fused silica without the undesirable formation of nanogratings, responsible for induced birefringence. |
