Paulo V. S. Marques

Optical microbubble resonators (OMBRs) – understood as localised thin wall bulges induced in silica microcapillaries – are gaining an ever-growing interest in microfluidic sensing applications due to their capability to sustain whispering gallery modes (WGMs) and confine the fluidic sample within their own hollow-core microcavity. Currently, most applications use an external tapered optical fibre for coupling light to the resonator. This arrangement is known to be fragile and prone to vibrations. In this work an alternative approach, based on coupling OMBR with a femtosecond (fs) laser written optical waveguides, integrated at the surface of fused silica substrate, is proposed. In this configuration, a stable and robust final structure is accomplished by gluing the two ends of the microcapillary, on which the OMBR is made, to the substrate. The OMBR quality factors, measured at the excitation wavelength of 1540 nm, show values close to 104 in the case of a water-filled cavity, with a maximum coupling efficiency up to 6.5%. Finally, the operation of the integrated optical devices as refractometres is demonstrated by delivering different solutions with successively increasing concentrations of NaCl inside the OMBR. An average sensitivity of 45 nm/RIU is obtained, yielding a resolution of 4.4×10 ^-5 RIU, creating the potential for this platform to be applied in chemical/biochemical sensing.

A detailed study of turn around point (TAP) long period fiber gratings (LPFGs) with coupling to the asymmetric cladding modes of a standard single-mode fiber (SMF-28e), fabricated by femtosecond (fs) laser direct writing was realized. The entire fabrication process, including the coating with different titanium dioxide (TiO ₂ ) film thicknesses of LPFGs and the corrections needed to achieve coated devices operating precisely in the TAP condition with coupling to the asymmetric cladding modes, was addressed. The significant fabrication details are also given, such as inscription periods, shape and localization of the refractive index modifications across the core. The fabrication process described allows the optimization of the LPFGs sensitivity in regards to the surrounding refractive index (SRI). Optimization of the writing parameters to obtain gratings working at the TAP for two different media surrounding the fiber (water and air) was achieved. It was demonstrated that for a grating period of 191.8 μm, the LP _1,12 mode exhibits a TAP at 1442.7 nm in air, and for a period of 192.5 μm, the same mode exhibits a TAP at 1448.6 nm in water. The LPFGs operating at the TAP in air and water were coated with 10, 20, and 30 nm thin TiO ₂ film thicknesses and the spectral behavior characterized. The wavelength sensitivity to the surrounding refractive index (SRI) was assessed in the range between 1.3700 to 1.4120, and a maximum sensitivity of ~8051.4 nm/RIU was measured for the 192.5 μm LPFG coated with a 30 nm thick TiO ₂ film.

Low loss optical waveguides are the key component for the fabrication of more complex integrated optics devices. In most works related to femtosecond laser written waveguides, the values presented give results at a single wavelength or in a narrow wavelength band; but some applications in optical sensing, for example, would benefit from waveguides having good propagation properties in a larger wavelength range. This paper presents results that allow one to gain insight into the major loss mechanisms present in laser written waveguides in two different types of glasses (fused silica and Eagle 2000 glass) and the dependence of those on the fabrication parameters. Finally, an example of application of broadband operating waveguides is given.

Long period fiber gratings (LPFGs) were fabricated in a standard single mode fiber (SMF-28e) through femtosecond (fs) laser direct writing. LPFGs with longer and shorter periods were fabricated, which allows coupling from the fundamental core mode to lower and higher order asymmetric cladding modes (LP_1,6 and LP_1,12, respectively). For the grating periods of 182.7 and 192.5 µm, it was verified that the LP_1,12 mode exhibits a TAP at approximately 1380 and 1448 nm in air and water, respectively. Characterization of the LPFGs subjected to high-temperature thermal treatment was accomplished. Fine-tuning of the resonance band’s position and thermal stability up to 600 °C was shown. The temperature sensitivity was characterized for the gratings with different periods and for different temperature ranges. A maximum sensitivity of −180.73, and 179.29 pm/°C was obtained for the two resonances of the 182.7 µm TAP LPFG, in the range between 250 and 600 °C.

A Fabry-Pérot interferometer was fabricated inside a fused silica substrate through femtosecond laser micromachining. The influence of the waveguide’s writing parameters on the measured signal’s quality was studied for an interferometer with a 27-μm wide cavity. Optimal signal-to-noise ratio and fringe visibility were obtained for waveguides written at 75 nJ and 50 μm/s. The same device was characterized with different refractive index liquids, and a maximum sensitivity of 1181.4±23.6 nm/RIU was obtained in the index range of 1.2962 to 1.3828 (at 1550 nm) for the spectral order 𝑚 = 46.

Optical waveguides were fabricated in alkaline earth boro-aluminosilicate glass, by femtosecond laser direct writing, with varying pulse energy and scan velocity. A spectral characterization, from 500 nm to 1700 nm, was made in order to determine their losses and understand its dependence on the processing parameters. Three major loss mechanisms were identified. At longer wavelengths, loss is mainly due to weak coupling. On the other hand, the behavior at shorter wavelengths is governed by propagation loss due to Rayleigh scattering, which was shown to be practically eliminated (<0.05 dB7cm-µm4) at higher scan velocities. Bulk absorption was also found to have an influence in the propagation losses at higher wavelengths. The combination of intermediate pulse energies (between 125-250 nJ) and high scan velocities (above 6 cm/s) allowed the fabrication of optical waveguides offering low losses across the entire range of wavelengths tested, facilitating applications that require larger wavelength working bands. Furthermore, since optimal fabrication conditions are achieved at higher scanning velocities, mass production with reduced fabrication times can be achieved.

A highly sensitive D-shaped optical fiber refractive index sensor based on surface plasmon resonance is designed and analyzed by using numerical simulations based on the finite element method. The flat surface of the fiber is coated with a gold layer that works as the plasmon active metal, followed by a titanium oxide (TiO2) layer, which is employed to enhance the performance of the sensor. The results demonstrate that the proposed sensor’s properties highly depend on the metal and dielectric coating’s thickness, enabling the tuning of the resonance wavelength. By supposing the system noise to be 0.1 nm, the theoretical maximum sensitivity was found to be 30000 nm/RIU, with a resolution of 3.33×10-6 RIU and a figure of merit (FOM) of 312.46 RIU-1, for an analyte with a refractive index of 1.41. The sensor’s sensitivity and FOM is improved upon the state of the art, possibly opening new windows of study in the fields of biological and chemical sensing.