Abstract
The influence of laser parameters on silica based waveguide inscription is investigated by using femtosecond laser pulses at 1030 nm (near-IR) and at 343 nm (UV). Negative phase contrast microscopy technique is used to measure the refractive index contrast for different photo-inscribed waveguides and shows the effects of both laser wavelength and scanning speed. In particular, UV photons have a higher efficiency in the waveguide production process as also confirmed by the lower optical losses at 1550 nm in these waveguides. These measurements are combined with micro-Raman and photoluminescence techniques, highlighting that laser exposure induces both structural modification of the silica and point defects generation. The contribution of induced defects to the total refractive index change is singled out by applying two different thermal treatments on the waveguide. The first, up to 500 °C, is able to remove the most of the induced non-bridging-oxygen-hole-centers (NBOHCs) while the second up to 750 °C erases almost all absorbing induced defects, highlighting the strong contribution of additional defects, not yet identified.
Highlights
The development of high intensity femtosecond pulsed lasers has given a strong boost to the production of efficient and innovative devices in the field of photonics [1]
In the present work we have studied the laser wavelength effects on the waveguide inscription, using femtosecond laser pulses at 1030 nm and 343 nm
The waveguides inscribed with the near-IR laser reach the Type II before to attain ∆n comparable with those of waveguides inscribed at 343 nm
Summary
The development of high intensity femtosecond (fs) pulsed lasers has given a strong boost to the production of efficient and innovative devices in the field of photonics [1]. Combining the high collimation with the improvements of the chirped pulse amplification technique [2], ultrafast laser manufacturing takes advantage of the very high peak intensity (∼10 TW/cm2) to induce permanent structural modifications in transparent materials, such as silica glasses. These changes lead to a refractive index variation along laser’s path that can be controlled with a high spatial resolution, adjusting the laser fluence at μm and sub-μm scales [3]. For Type II, the strong temperature increase of the focal point drives the inscription to thermo-mechanical expansion of the material with a negative refractive index change, leading in certain conditions to void formations
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