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Browsing CAP - Indexed Articles in Journals by Author "5872"
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    Femtosecond laser direct written off-axis fiber Bragg gratings for sensing applications
    ( 2020) Carlos Duarte Viveiros ; Vítor Oliveira Amorim ; João Miguel Maia ; Susana Oliveira Silva ; Orlando Frazão ; Pedro Jorge ; Fernandes,LA ; Paulo Vicente Marques ; 3565 ; 4061 ; 4287 ; 4678 ; 5872 ; 6407 ; 6596
    First order off-axis fiber Bragg gratings (FBGs) were fabricated in a standard single mode fiber (SMF-28e) through femtosecond laser direct writing. A minimum offset distance between the grating and core center of 2.5 µm was found to create a multimode section, which supports two separate fiber modes (LP0,1 and LP1,1), each split into two degenerate polarization modes. The resulting structure breaks the cylindrical symmetry of the fiber, introducing birefringence (˜10-4) resulting in a polarization dependent Bragg wavelength for each mode. Based on the modal and birefringence behavior, three off-axis FBGs were fabricated with 3.0, 4.5 and 6.0 µm offsets from the core center, and then characterized in strain, temperature, and curvature. The tested off-axis FBGs exhibited a similar strain sensitivity of ~1.14 pm/µ? and a temperature sensitivity of ~12 pm/C. The curvature and orientation angle were simultaneously monitored by analyzing the intensity fluctuation and the wavelength shift of the LP1,1 Bragg resonance. A maximum curvature sensitivity of 0.53 dB/m-1 was obtained for the off-axis FBG with a 3.0 µm offset. © 2020 Elsevier Ltd
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    Femtosecond laser direct written off-axis fiber Bragg gratings for sensing applications
    ( 2020) Carlos Duarte Viveiros ; Vítor Oliveira Amorim ; João Miguel Maia ; Susana Oliveira Silva ; Orlando Frazão ; Pedro Jorge ; Fernandes,LA ; Paulo Vicente Marques ; 3565 ; 4061 ; 4287 ; 4678 ; 5872 ; 6407 ; 6596
    First order off-axis fiber Bragg gratings (FBGs) were fabricated in a standard single mode fiber (SMF-28e) through femtosecond laser direct writing. A minimum offset distance between the grating and core center of 2.5 µm was found to create a multimode section, which supports two separate fiber modes (LP0,1 and LP1,1), each split into two degenerate polarization modes. The resulting structure breaks the cylindrical symmetry of the fiber, introducing birefringence (˜10-4) resulting in a polarization dependent Bragg wavelength for each mode. Based on the modal and birefringence behavior, three off-axis FBGs were fabricated with 3.0, 4.5 and 6.0 µm offsets from the core center, and then characterized in strain, temperature, and curvature. The tested off-axis FBGs exhibited a similar strain sensitivity of ~1.14 pm/µ? and a temperature sensitivity of ~12 pm/C. The curvature and orientation angle were simultaneously monitored by analyzing the intensity fluctuation and the wavelength shift of the LP1,1 Bragg resonance. A maximum curvature sensitivity of 0.53 dB/m-1 was obtained for the off-axis FBG with a 3.0 µm offset. © 2020 Elsevier Ltd
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    Femtosecond laser micromachining of an optofluidics-based monolithic whispering-gallery mode resonator coupled to a suspended waveguide
    ( 2021) João Miguel Maia ; Amorim,VA ; Carlos Duarte Viveiros ; Paulo Vicente Marques ; 4287 ; 5872 ; 6596
    A monolithic lab-on-a-chip fabricated by femtosecond laser micromachining capable of label-free biosensing is reported. The device is entirely made of fused silica, and consists of a microdisk resonator integrated inside a microfluidic channel. Whispering gallery modes are excited by the evanescent field of a circular suspended waveguide, also incorporated within the channel. Thermal annealing is performed to decrease the surface roughness of the microstructures to a nanometric scale, thereby reducing intrinsic losses and maximizing the Q-factor. Further, thermally-induced morphing is used to position, with submicrometric precision, the suspended waveguide tangent to the microresonator to enhance the spatial overlap between the evanescent field of both optical modes. With this fabrication method and geometry, the alignment between the waveguide and the resonator is robust and guaranteed at all instances. A maximum sensitivity of 121.5 nm/RIU was obtained at a refractive index of 1.363, whereas near the refractive index range of water-based solutions the sensitivity is 40 nm/RIU. A high Q-factor of 10(5) is kept throughout the entire measurement range.
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    Femtosecond laser micromachining of suspended silica-core liquid-cladding waveguides inside a microfluidic channel
    ( 2022) João Miguel Maia ; Carlos Duarte Viveiros ; Amorim,VA ; Paulo Vicente Marques ; 4287 ; 5872 ; 6596
    This work addresses the fabrication of straight silica-core liquid-cladding suspended waveguides inside a microfluidic channel through fs-laser micromachining. These structures enable the reconfiguration of the waveguide's mode profile and enhance the evanescent interaction between light and analyte. Further, their geometry resembles a tapered optical fiber with the added advantage of being monolithically integrated within a microfluidic platform. The fabrication process includes an additional post-processing thermal treatment responsible for smoothening the waveguide surface and reshaping it into a circular cross-section. Suspended waveguides with a minimum core diameter of 3.8 µm were fabricated. Their insertion losses can be tuned and are mainly affected by mode mismatch between the coupling and suspended waveguides. The transmission spectrum was studied and it was numerically confirmed that it consists of interference between the guided LP01 mode and uncoupled light and of modal interference between the LP01 and LP02 modes. © 2022 Elsevier Ltd
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    Inscription of surface waveguides in glass by femtosecond laser writing for enhanced evanescent wave overlap
    ( 2020) Vítor Oliveira Amorim ; João Miguel Maia ; Carlos Duarte Viveiros ; Paulo Vicente Marques ; 4287 ; 6596 ; 5872 ; 6407
    Near-surface optical waveguides were fabricated in alkaline earth boro-aluminosilicate glass (Eagle2000), by femtosecond laser direct writing, using two distinct approaches. First, the capability of directly inscribing optical waveguides close to the surface was tested, and then, compared to the adoption of post writing wet etching to bring to the surface waveguides inscribed at greater depths. Laser ablation was found to limit the minimum surface to core center distance to 6.5 mu m in the first method, with anisotropic wet etching limiting the latter to 3 mu m without any surface deformation; smaller separations can be achieved at the cost of the planar surface topography. Furthermore, the waveguide's cross-section was seen to vary for laser inscription nearing the surface, observations that were also corroborated by its distinct guiding characteristics when compared to the adoption of post writing wet etching. The spectral analysis (in the 500-1700 nm range) also evidenced an increase in insertion loss for longer wavelengths and smaller surface to core center separations, caused, most likely, by coupling loss due to the interaction between the propagating mode and the surface. Different lengths of waveguide exposed to the surface were also tested, revealing that scattering loss due to surface roughness is not an issue at the centimeter scale.
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    Intensity-modulated refractometer based on mode-mismatch in surface waveguides inscribed by femtosecond laser direct writing
    ( 2021) Vítor Oliveira Amorim ; Carlos Duarte Viveiros ; João Miguel Maia ; Paulo Vicente Marques ; 6407 ; 6596 ; 4287 ; 5872
    Optical waveguides were fabricated at the surface of Eagle2000 glass substrates, using femtosecond laser direct writing and wet etching, and their potential as intensity-modulated refractometers was assessed. Through the analysis of their broadband spectral response to different refractive index oils, we observed that mode mismatch is present when the guided mode reaches the surface of the substrate and interacts with the external medium, thus enabling the use of such optical waveguides in refractive index sensing. Refractive indices equal to or greater than that of the substrate also induced a coupling mechanism that was shown not to be suitable in these devices. The device's wavelength of operation was found to be tunable by controlling the distance between the surface and the center of the optical waveguide. However, the sensitivity was seen to diminish by increasing the latter, being nonexistent for distances greater than 5.5 mu m. In this study, the maximum sensitivity values were found for a surface to core center distance between 1 and 2 mu m, in the biological range, and 2.5 to 3 mu m, for a refractive index nearing that of the substrate. Accordingly, maximum sensitivities of approximate to 25 dB/RIU and approximate to 1200 dB/RIU were found between 1.300 < n(D)(25)degrees(C) < 1.400 and 1.490 < n(D)(25)degrees(C) < 1.500, respectively.
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    Loss Mechanisms of Optical Waveguides Inscribed in Fused Silica by Femtosecond Laser Direct Writing
    ( 2019) Vítor Oliveira Amorim ; João Miguel Maia ; Carlos Duarte Viveiros ; Paulo Vicente Marques ; 4287 ; 5872 ; 6407 ; 6596
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    Mach-Zehnder interferometer-based evanescent refractometer inscribed at the surface of Eagle2000 by femtosecond laser writing
    ( 2021) Vítor Oliveira Amorim ; João Miguel Maia ; Carlos Duarte Viveiros ; Paulo Vicente Marques ; 4287 ; 5872 ; 6407 ; 6596
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    Magnetic field sensors in fused silica fabricated by femtosecond laser micromachining
    ( 2020) João Miguel Maia ; Vítor Oliveira Amorim ; Carlos Duarte Viveiros ; Paulo Vicente Marques ; 4287 ; 5872 ; 6407 ; 6596
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    Mass Producible Low-Loss Broadband Optical Waveguides in Eagle2000 by Femtosecond Laser Writing
    ( 2019) Vítor Oliveira Amorim ; Carlos Duarte Viveiros ; João Miguel Maia ; Paulo Vicente Marques ; 4287 ; 5872 ; 6407 ; 6596
    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 (& x003C; 0.05 dB $\cdot$ cm $<^>{-1} {\cdot }\,\,\mu \text{m}<^>{4}$ ) 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.
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    Spectral Tuning of Long Period Fiber Gratings Fabricated by Femtosecond Laser Micromachining through Thermal Annealing
    ( 2019) Carlos Duarte Viveiros ; José Almeida ; Luís Carlos Coelho ; Helena Soares Vasconcelos ; Vítor Oliveira Amorim ; João Miguel Maia ; Pedro Jorge ; 3565 ; 5256 ; 5799 ; 5872 ; 6407 ; 6596 ; 7222
    A femtosecond laser direct writing system was developed to explore the fabrication of long-period fiber gratings (LPFGs) in SMF28 fibers. The LPFGs, showing the mode LP1,6 at 1500 nm, were exposed to high-temperature annealing up to 950 °C. Modifications in the refractive index (RI) modulation are observed through a blue-shift in the LPFG attenuation bands and above 850 °C, the mode LP1,7 appear at 1600 nm. The wavelength sensitivity to external RI from 1.300 to 1.452 was estimated for both modes before and after annealing. Greater sensitivity was found for the higher order mode in the entire range reaching 2400 nm/RIU around 1.440.
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    Temperature Stability and Spectral Tuning of Long Period Fiber Gratings Fabricated by Femtosecond Laser Direct Writing
    ( 2020) Carlos Duarte Viveiros ; José Almeida ; Luís Carlos Coelho ; Helena Soares Vasconcelos ; João Miguel Maia ; Vítor Oliveira Amorim ; Pedro Jorge ; Paulo Vicente Marques ; 3565 ; 4287 ; 5256 ; 5799 ; 5872 ; 6407 ; 6596 ; 7222
    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 (LP1,6 and LP1,12, respectively). For the grating periods of 182.7 and 192.5 µm, it was verified that the LP1,12 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.
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    Turn Around Point Long Period Fiber Gratings With Coupling to Asymmetric Cladding Modes Fabricated by a Femtosecond Laser and Coated With Titanium Dioxide
    ( 2021) Carlos Duarte Viveiros ; José Almeida ; Luís Carlos Coelho ; Vasconcelos,H ; João Miguel Maia ; Amorim,VA ; Pedro Jorge ; Paulo Vicente Marques ; 3565 ; 4287 ; 5256 ; 5799 ; 5872 ; 6596
    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 (TiO2) 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 mu m, the LP1,12 mode exhibits a TAP at 1442.7 nm in air, and for a period of 192.5 mu m, 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 TiO2 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 similar to 8051.4 nm/RIU was measured for the 192.5 mu m LPFG coated with a 30 nm thick TiO2 film.