摘要： Femtosecond laser direct writing (FLDW) of waveguides in dielectric glasses and crystals is well-recognized technique for manufacturing of compact laser sources, optical sensors, labs-on-a-chip and optical chips for quantum computing [1]. For this technology the main obstacle to compaction of optical circuits and an increase in the number of elements on an optical chip is constraint of the refractive index contrast Δn between the core and the cladding of a waveguide that is given by the nature of FLDW. Insufficient RIC also restricts application of the depressed cladding waveguides in mid-IR due to mode leakage [2]. The largest ever-obtained refractive index change is associated with ion migration in a phosphate glass under the thermal regime of laser writing, and it is as high as +0.03 [3]. Recently we found that simultaneous actions of rarefaction and electronic excitation lead to enhanced negative refractive index change in the laser modified spots in silica glass and sapphire [4], wherein the required conditions were produced by a sub-nanosecond burst of femtosecond laser pulses. Here we report on FLDW of a low loss waveguide with enhanced refractive index contrast in 70TeO2-22WO3-8Bi2O3 glass inscribed by bursts of pulses with exponentially decaying amplitudes in each burst and pulse separation of 10 ps. The depressed cladding waveguide composing of 14 parallel tracks with reduced refractive index was inscribed at wavelength of 1030 nm (Fig.1.(a)). The maximum refractive index change Δn produced by train of ordinary pulses in the investigated glass was as high as -0.002 [2]. Mapping of refractive index change in tracks inscribed by the bursts with different burst energies and pulse separation intervals is shown in Fig.1(c) The range of the parameters was found for which the burst produced smooth tracks of negative refractive index change, and its amplitude is increased by factor of 3 in comparison with tracks inscribed by ordinary pulses. The maximum index change Δn was obtained with pulse separation interval of 10 ps. Beside enhanced refractive index change the burst train inscribed tracks with reduced cross section. That is, the track height is less at least by a factor of 3 in comparison with one inscribed by ordinary pulses under the same energy for a burst and an ordinary pulse (Fig1(a),(b)). We consider that the enhanced index change and the reduced track height are inevitably accompanied by strong localization of energy deposition through a reduction in peak pulse intensity that allowed avoiding destructive Kerr self-focusing. We consider that the new technique of FLDW paves the way for better confinement of radiation in a depressed cladding waveguide writing waveguides with well deterministic architectures. The increase in the waveguide non-linearity and extension of operation range to mid-IR is inevitably expected.