Waveguide writing with femtosecond (fs) laser pulses in glasses is compromised by uncontrollable non-linear propagation phenomena for powers exceeding a certain threshold, P-cr which is inversely proportional to the material linear and non-linear refractive indices, n(0) and n(2), respectively. We have studied the behaviour of a commercial glass, Schott SF57, with high values of n(0) and n(2), when it is processed above P-cr with 800 and 1260 nm, 100 A laser pulses to produce waveguide structures. Two types of structures, longitudinal (l-) and transversal (t-) ones depending on the sample translation, were obtained. In both of them, material damage surrounded by guiding regions was generated. We attribute the formation of the guiding regions of increased refractive index to the high local pressure reached in the focal volume where catastrophic damage occurs. This pressure can be high enough to compress the material nearby thus generating regions with a permanent refractive index increase up to 5 x 10(-4). The efficiency of the guiding region is conditioned by the pulse energy. Mode profile analyses reveal double-sided exponential guided modes in the 1-waveguides. As an alternative route to obtain suitable mode shapes and, simultaneously, to increase the coupling efficiency, the use of multiple structures has been investigated. Various 1-structures have been written close to each other generating a region of increased refractive index large enough to effectively support a Lorentzian mode at 633 nm. (c) 2005 Elsevier B.V All rights reserved.