Abstract
Femtosecond laser waveguide writing is a powerful technique to create photonic devices and is already widely applied in glasses and crystals [1]. In polymers however, relatively little work has been reported so far. Material specific effects during femtosecond laser irradiation such as chain scission or crosslinking and an overall low damage threshold demand for alternative writing schemes to overcome drawbacks such as reported high propagation losses of 3 dB/cm [2]. In [3] we already demonstrated that a cascaded-focus during the writing process increases the quality of single-track waveguides. Here, we propose an alternative multitrack writing scheme for polymers, employing the same cascaded-focus modifications. Utilizing PMMA, we demonstrate that parallel tracks with a well-defined gap l in the middle are suitable to confine a waveguide in between. The refractive index increase is induced by material compression and stress-related effects which are caused by a quickly expanding plasma core. We find that adding further modification tracks with spacing ∆y on each side increases the densification and therefore the refractive index at the center up to ∆n = 2 · 10−3. A number of 4 modifications on each side, as depicted in Fig. 1(a), proves to be optimal in our case [4]. A higher refractive index contrast is beneficial to minimize bend losses occurring in a curved waveguide. To investigate these bend losses, s-curved waveguides were fabricated. Each track follows an individual path with varying radii of curvature at constant angle δ as shown in Fig. 1(b). The reference radius R is defined by the center of the waveguide core.
© 2017 IEEE
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