LAUSR

Direct femtosecond laser inscription has become an advanced and widespread maskless technology for waveguides writing inside optical materials. One of the advantages of the technology consists in combination of both high efficiency of integral optics and perfect thermal and spectral characteristics of bulk optical elements. In the paper we proposed, for what is believed to be the first time, femtosecond inscription of induced structures with modified refractive index (reduced down to −6 × 10−3) within significantly broad depth range (more than 550 μm) and waveguide with mode field diameter 200 μm, 30 mm length and numerical aperture 0.04. Proposed result broadens field of application of femtosecond writing technology into large mode diameter waveguides formation and hybrid integral-bulk laser schemes that create an opportunity to develop novel optical elements. Laser induced tracks with controlled refractive index were written with aspherical lens at all the depths within range between 550 and 1100 μm under the surface of the sample. Continuous writing process at significantly different depths was based on dynamic adjustment of femtosecond pulses energy to compensate negative effect of spherical aberration. In the investigation we examined dependences of writing energy range and geometry of induced refractive index tracks on writing depth. Experiments were carried out in Nd:phosphate glass in thermal cumulative regime (repetition rate of femtosecond pulses was 2 MHz) and compared with numerical simulation. It was shown, that dependence of working range on focusing depth is nonlinear, nonmonotonic and significantly differs for positive and negative shift from optimal correction depth of the lens.