This paper reviews the state‐of‐the‐art of grating fabrication in silica and polymer microstructured optical fibres. It focuses on the difficulties and challenges encountered during photo‐inscription of such gratings and more specifically on the effect of the air hole lattice microstructure in the cladding of the fibre on the transverse coupling of the coherent writing light to the core region of the fibre. Experimental and computational quantities introduced thus far to assess the influence of the photonic crystal lattice on grating writing efficiency are reviewed as well, together with techniques that have been proposed to mitigate this influence. Finally, early proposals to adapt the microstructure in view of possibly enhancing multi‐photon grating fabrication efficiency are discussed.

We have studied transverse propagation of femtosecond pulse duration laser light through the microstructure of hexagonal lattice photonic crystal fibers. Our results provide insight in the role of the microstructure on the amount of optical power that reaches the core of the PCF, which is of particular importance for grating inscription applications. We developed a dedicated approach based on commercial FDTD software and defined a figure of merit, the transverse coupling efficiency, to evaluate the coupling process. We analyzed the propagation of femtosecond laser pulses to the core of a wide range of PCFs and studied the influence of the PCF orientation angle, the air hole pitch and air hole radius on the energy reaching the core. We have found that the transverse coupling efficiency can benefit from a dedicated design of the microstructured cladding and an accurate fiber orientation. We designed a dedicated PCF microstructure that enhances transverse coupling to the core at a wavelength of 800 nm.

We present a design and fabrication approach for 3D printed polymer microstructured optical fiber tapers on standard single-mode glass fibers for efficient and compact mode-field conversion. This paves the way towards complex functionalized fiber tips for various applications, like sensors and beam shaping components, currently limited by the mode-field size and distribution of standard optical fibers. In this paper, we demonstrate the potential of mode-field converting tapers for relaxing the misalignment tolerance in fiber-to-fiber connections and maximizing the coupling efficiency in fiber-to-chip connections. We demonstrate a mode-field diameter expansion ratio of 1.7 and reduction ratio of 3 and show that our microstructured tapers achieve a comparable performance in coupling efficiency as their step-index counterparts, while providing greater robustness.

We studied the influence of the fiber orientation on the growth of fiber Bragg gratings (FBGs) in pure silica microstructured optical fibers (MOFs) during femtosecond UV laser inscription. To do so we simulated the transverse coupling efficiency as a function of the relative angle between the inscribing laser beam and the internal microstructure for hexagonal lattice and highly birefringent MOFs by a finite-difference time-domain (FDTD) method. The orientation was predicted to play a far more important role in the highly birefringent MOF. We confirmed these simulation results with the fabrication of wavelength-multiplexed FBG arrays in pure silica core MOFs under different fiber orientations with 266-nm femtosecond laser pulses and a Talbot interferometer configuration.Index Terms-Femtosecond laser, fiber Bragg gratings (FBGs), microstructured optical fibers (MOFs).