Wth the femtosecond (fs) direct laser write (DLW) approach FBGs can be produced in all kinds of fibres via localized point-by-point exposure. This method does not require the use of premade masks or altered fiber cores, however, it requires precise tracking and alignment of the fibre core center position during fabrication to achieve the desirable spectroscopic performance.

     The tracking process can get complicated with specialty fibres as more advanced internal structures (such as stress rods in polarization-maintaining (PM) type fibres) can obscure the direct view of the core. However, digital image processing by object edge detection and autofocus algorithms can be used to detect the center position of such fibres (Fig 2 (b)) and therefore can result in high process repeatability, including control of resulting polarization and depolarization effects.

     The goal of our paper is to show how this can be achieved with basic Bright Field Microscopy methods combined with numeric image analysis, reducing the cost and complexity of the, otherwise, challenging process. We will illustrate this for multiple cases and algorithms allowed us to find a 125 µm diameter PM type fibre we could produce a core detection precision of <0.5 µm and show the corresponding FBG's performance.