Sub-surface modification by a femtosecond laser, producing pulses of 100fs duration at 400nm wavelength for fabrication of optical components is investigated, within sub-millimetre thick sheets of undoped polymethyl methacrylate (PMMA). For effective structuring, the influence of laser writ-ing parameters and focusing conditions were studied to optimise the pulse energy, pulses spacing and focusing depth in the material. Permanent sub-surface refractive index structures with a mini-mum feature size (~3μm) at a depth of ~500μm demonstrated the feasibility of writing flexible 3-D microstructures within a thin PMMA substrate. This investigation of focusing depth location in PMMA bulk material enabled photonic structures at the desired spatial resolution to be written within the core of single mode polymer optical fibre under controlled laser writing conditions. A long period fibre grating (LPG) consisting of a 2.8μm wide refractive index structure with a perio-dicity of 250μm and containing a series of attenuation bands in visible range, demonstrated a photonic structure written into a single mode polymer optical fibre.