Photonic structures fabricated in polymer materials using femtosecond laser irradiation

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Abstract

Sub-surface modification using a frequency doubled Ti: Sapphire femtosecond (fs) laser at 1kHz repetition rate, producing 100-fs pulse duration at 400nm, is studied in order to fabricate optical components within non-photosensitised polymethyl methacrylate (PMMA). This thesis explores the feasibility of producing three-dimensional optical devices in bulk polymers and polymer optical fibre (POF) using fs laser direct-writing techniques. For effective and optimal structuring, the laser writing parameters and focusing conditions, such as focusing depth, translation speed, and accumulated fluence are investigated by means of photo-modification thresholds; structural changes in dimensions and morphologies; and the magnitude of the refractive index modulation. The highest refractive index change is 3.2x10^(-3) achieved by using a dry (non-immersion) 0.45-NA objective for a single laser scan. Variations in damage threshold with focusing depths are attributed to a combination of material absorption or surface scattering of light due to contamination or surface imperfections, as well as oxygen diffusion and spherical aberration. Distortion of the laser-induced feature size and shape due to spherical aberrations is controlled and compensated by adjusting the laser power near the damage threshold. Permanent refractive index structures with cross-sectional dimensions of 2μm by 0.9μm and 3μm by 1.4μm are demonstrated at depths of 300μm and 500μm below the surface, resulting in the axial/ lateral ratio of 2.2 and 2.1, respectively. A novel phenomenon relevant to effects of translation speed on the fs laser modification is observed for the first time. As translation speeds reduce from 1.2 to 0.6mm/s, the optical damage threshold power decreases by 6μW, whilst other writing conditions remain constant. However, the damage threshold increases by 74μW with decreasing speeds from 0.6 to 0.35mm/s. This significant increase in threshold power enables inscription of refractive index gratings <5μm below the surface, because irradiation on the surface or near the surface initiates ablation rather than refractive index changes, and this forms a limit for writing useful structures. Compensating for this limit by using appropriate writing parameters highlights the potential of fabricating three-dimensional integrated optical circuits in thin (100μm) polymer substrates. Finally, highly localised fabrication of long period gratings into step-index single mode polymer fibres is demonstrated by removing distortion effects due to the curved surface. The distortion is compensated by sandwiching the fibre with two flat PMMA sheets, between which index-matching oil (n=1.5) is injected. This arrangement enables precise laser micro-structuring with flat interfaces and continuous inner material. The first demonstration of a 250-μm-period fibre grating, resulting in attenuation bands in the visible spectral region at 613, 633, 728, 816, 853, 877 and 900nm, is presented.

Keyword(s)

direct-writing techniques; femtosecond (fs) laser irradiation; long period fibre gratings; photonic structures; polymer optical fibre (POF); polymethyl methacrylate (PMMA); refractive index modification

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