Femtosecond laser irradiation of polymethylmethacrylate for refractive index gratings

Polymethylmethacrylate (PMMA) or Perspex is an inexpensive polymer widely used for making the cores of communications grade polymer optical fibres (POFs) and as a substrate for polymer optoelectronic devices and integrated waveguides. Periodic refractive index structures have been written in undoped PMMA using multiple pulses of 40 fs duration from a 1 kHz Ti:sapphire femtosecond laser operating at the fundamental (800 nm). A refractive index change (Δn) of 5 ± 0.5 × 10⁴ was observed before the onset of striations. Optimization of writing conditions for refractive index modification of POP fibres or bulk undoped PMMA will enable structures such as Bragg gratings, long-period gratings, mode couplers, microlens arrays, and zone plates to be written.