Complex devices fabricated using femtosecond laser direct write
Integrated photonic circuits are designed to manipulate the properties of light for multiple different functions. For example the telecommunications industry uses photonic integrated circuits to efficiently direct light signals at high speeds to provide the internet, and the sensing community is developing compact photonic devices to measure materials. One method to fabricate integrated photonic circuits is femtosecond laser direct-write. This technique induces a refractive index modification inside a glass material localised to the lasers focal point. This enables modifications to be fabricated in three-dimensions, while the strength of the refractive index change can be controlled by the laser’s pulse energy. The femtosecond laser direct-write technique is relatively well established. However, the fabrication of complex devices with large-area index modifications has seen minimal development. In this thesis the challenges associated with complex devices with large-area refractive index modifications, such as controlling the optical path length and managing internal stress, are explored. Two complex devices, the arrayed waveguide grating and gradient index structures that contain large-area refractive index modifications have been chosen to push the limits of fabrication.
Arrayed waveguide gratings are used in telecommunications to multiplex/de-multiplex light into different data streams. These devices contain two large planar index regions. Due to these regions arrayed waveguide gratings have previously only been fabricated using lithographic techniques. In this thesis we fabricate, for the first time, a functional arrayed waveguide grating using the femtosecond laser direct-write technique. The arrayed waveguide grating were fabricated in the visible region at 632:8 nm with a measured free spectral range of 22:2 ± 0:5 nm, and 1:35 ± 0:03 nm resolution. Next three arrayed waveguide gratings were layered vertically to create a novel spectro-interferometer. Finally a 3 port photonic lantern was integrated with an arrayed waveguide grating, for applications such as sensing and astronomy. This was the first time a photonic lantern and an arrayed waveguide grating has been integrated into a single robust chip.
Gradient index structures can be used to alter the irradiance of light beams. While multiple types of beam shapers are already designed to achieve this function, gradient index structures offer the advantage of being compact and simple to use. Unfortunately the fabrication of gradient index volumes is limited to very simple refractive index profiles, due to conventional fabrication methods. Here the fabrication of a customizable refractive index profile, fabricated using the femtosecond laser direct-write method, is demonstrated for the first time. These volumes are shown to re-size Gaussian beams and reshape a Gaussian beam to a flat-top.