Optimizing the fabrication of diffractive optical elements using a focused ion beam system

In the past, UV lithography has been used extensively for the fabrication of diffractive optical elements (DOEs). The advantage of this technique is that the entire structure can be written at one time, however, the minimum feature size is limited to about 1 μm. Many 1-d and 2-d periodic grating structures may not need such fine details but it is essential for diffractive optics with circular structures. This is because the spacing between features typically decreases towards the edge of the element resulting in the smallest feature falling well below 1 μm. 1-d structures such as sub-wavelength gratings will also have smaller feature sizes throughout the structure. In such cases, advanced techniques such as Focused Ion Beam and Electron-beam Lithography are required for the fabrication of finer structures. In this paper, we present results of DOEs fabricated with a focused ion beam system (Nova Nanolab 600 from FEI) directly on a single mode fibre tip. The ability to write DOEs directly on fibre tip is of great importance in fields such as endoscopy and optical trapping. The DOE itself, transforms the laser beam to a phase and intensity profile that matches the requirement. Because it is located directly on the fibre, no extra alignment is required. In addition, the system becomes more compact, which is especially important for applications in the field of endoscopy. The main goal of the present work was to develop the most accurate method for creating the desired pattern (that is, the DOE structure) into an actually working element. Different exposure strategies for writing test structures directly with the ion beam on the fibre tip have been tested and carefully evaluated. The paper will present in detail the initial fabrication and optical test results for blazed and binary structures of 1-d and circularly symmetric Fresnel axicons on optical fibres