Femtosecond laser-induced two-photon polymerization and modeling of multicore optical fibers

Abstract

Abstract Two cutting-edge laser technologies, femtosecond pulsed laser-induced two-photon polymerization to produce ultraprecise 3D micro- and nanoscale structures and multicore fibers for amplification and lasing have been thoroughly investigated in this work. The significant technical development of ultrashort pulsed laser systems has led to novel opportunities for additive manufacturing, including creation of arbitrary and complex 3D structures having features as small as 100 nm through two-photon polymerization processes. Therefore, this approach has attracted significant attention in a large variety of academic and commercial applications ranging from advanced photonics to biomedical applications due to its promising potential as a 3D printing technique to achieve resolution beyond the diffraction limit. In order to stay up-to-date with the latest technology and enjoy the potential benefits of this innovative method, a two-photon polymerization system was completely set up within this project which in fact provided the possibility of using infrared or green laser beams depending on the desired structures. Then, the influence of ultrashort pulsed laser beams on different types of materials including commercial (e.g., femtoBond C, E-Shell 300) and custom-made (e.g., TPGDA, PEGDA, PETIA) photoresists was investigated to determine the entire range of usable laser power, from the polymerization threshold to the damage threshold and subsequently to choose the most suitable resin with the employed fabrication setup. After performing several tests, acrylate-based thermosetting polymer PETIA (Pentaerythritol triacrylate monomer) with 0.2% IRGACURE 819 photoinitiator, showed a considerable potential in reaching the pre-design microstructure. The two-photon polymerization resolution has also been studied by drawing 7 lines with different levels of laser power within this negative photoresist which perfectly demonstrated that by increasing the laser power, the width of the lines increased from approximately 2.67 µm at 20 mW to 9.83 µm at 80 mW at a constant scanning speed of 0.1 mm/s. This implies that the combination of laser power and scanning speed affects directly the width of the lines. After successfully producing simple microstructures such as the small square prisms with different heights from PETIA in a single manufacturing step, the next step was the fabrication of ultraprecise solid microneedles with 49 conical needles arranged in a 7×7 configuration comprising cones with a base diameter of 250 µm, tip diameter of 30 µm, height of 800 µm and pitch of 500 µm. In fact, solid microneedles as a painless novel transdermal drug delivery system suitable for substitution of hypodermic needles are mostly used to create micro-pores in human skin which facilitates the absorption of pharmaceutical formulations such as solutions, creams or gels through the creation of channels allowing more drug molecules to enter it. As seen from the obtained data, the optimal combination of fabrication parameters including laser power, scanning speed, layer height and hatch distance is one of the vital steps required to achieve the desired microstructure. The interaction between a single microneedle and human skin was studied with the finite element method to verify the development of mechanical stress and deformation within both microneedle and skin as well. The results indicated that the minimum force required to reach the ultimate tensile strength of human skin was 10.75 mN for a single MN and 526.75 mN for 49 MNs while the maximum equivalent stress within the PETIA microneedle under the same conditions was 51.8 MPa. Multicore fibers, however, are a revolutionary new approach to improve the threshold limit of nonlinearities and transverse mode instabilities in high-power applications by combining several lower power beams coherently to obtain a single higher power beam. The main applications and opportunities of this cutting edge technology are in telecommunication, optical amplifiers, fiber sensors as well as fiber lasers. However, thermal effects are currently regarded as the main barrier to power scaling of this novel optical fiber lasers. Therefore, the effect of the heat load due to the quantum defect in the amplification process on the performance of both 9 and 16 ytterbium-doped silica cores in 3×3 and 4×4 configurations, featuring the same core diameter ranging from 12 to 19 µm, inner cladding diameter of 420 µm and outer cladding diameter of 600 µm at 1032 nm wavelength, was studied through a combined thermal and optical finite element method simulations. According to the obtained results, fiber guidance properties are strongly affected by mode coupling effect induced by thermally caused refractive index distortion. Furthermore, the core diameter appears to be a critical parameter in ensuring single mode propagation within each core of the fiber where it turns out to be maximum 15 µm or below. In the last step of the electromagnetic modeling, by reducing the value of pitch or center-to-center core distance from 55 to 25 µm, the minimum overlap integral difference consistently decreased for increasing core diameter, to the point of failing to maintain the single-mode criterion. In other words, for core diameters greater than 15 µm, higher order modes become more confined into the cores which causes a negative impact on the optical beam quality.