Femtosecond laser writing of symmetrical optical waveguides by astigmatically shaped beams

In this paper we describe a novel approach to the fabrication of optical waveguides by focused low-repetition-rate femtosecond laser pulses. This approach overcomes the main limitation of the technique, i.e. the strong asymmetry of the waveguide profile. By using an astigmatic beam and suitably controlling both beam waist and focal position in tangential and sagittal planes, it is possible to shape the focal volume in such a way as to obtain waveguides with a circular transverse profile and of the desired size. This technique is applied to the fabrication of active waveguides in Er:Yb-doped glass substrates. The waveguides are single-mode at 1.5 mum, exhibit propagation losses of about 0.25 dB/cm and an internal gain of 1.4 dB at 1534 nm.