Minimalist Approach for the Design of Microstructured Optical Fiber Sensors

We report on recent investigations regarding ultra-simplified designs for microstructured optical fiber sensors. This minimalist approach relies on the utilization of capillary-like fibers—namely embedded-core fibers, surface-core fibers, and capillary fibers—as platforms for the realization of sensing measurements. In these fibers, guidance of light is accomplished in an embedded or surface germanium-doped core or in the hollow part of capillaries. External stimuli can alter fiber wall thickness and/or induce birefringence variations, allowing, for the embedded-core and capillary fibers, to operate as pressure or temperature sensors. For the surface-core fiber design, the interaction between the guided mode and external medium allows the realization of refractive index sensing either by using fiber Bragg gratings or surface plasmon resonance phenomenon. Also, we report the realization of directional curvature sensing with surface-core fibers making use of the off-center core position. The attained sensitivities are comparable to the ones obtained with much more sophisticated structures. The results demonstrate that these novel geometries enable a new route toward the simplification of optical fiber sensors

Hollow-core fiber-based speckle displacement sensor

The research enterprise towards achieving high-performance hollow-core photonic crystal fibers has led to impressive advancements in the latest years. Indeed, using this family of fibers becomes nowadays an overarching strategy for building a multitude of optical systems ranging from beam delivery devices to optical sources and sensors. In most applications, an effective single-mode operation is desired and, as such, the fiber microstructure or the light launching setups are typically designed for achieving such a behavior. Alternatively, one can identify the use of large-core multimode hollow-core fibers as a promising avenue for the development of new photonic devices. Thus, in this manuscript, we propose and demonstrate the utilization of a large-core tubular-lattice fiber for accomplishing a speckle-based displacement sensor, which has been built up by inserting and suitably dislocating a single-mode fiber inside the void core of the hollow fiber. The work reported herein encompasses both simulation and experimental studies on the evolution of the multimode intensity distributions within the device as well as the demonstration of a displacement sensor with an estimated resolution of 0.7 {\mu}m. We understand that this investigation identifies a new opportunity for the employment of large-core hollow fibers within the sensing framework hence widening the gamut of applications of this family of fibers

Hollow-core fibers with reduced surface roughness and ultralow loss in the short-wavelength range

While optical fibers display excellent performances in the infrared, visible and ultraviolet ranges remain poorly addressed by them. Obtaining better fibers for the short-wavelength range has been restricted, in all fiber optics, by scattering processes. In hollow-core fibers, the scattering loss arises from the core roughness and represents the limiting factor in reducing their loss regardless of the fiber cladding confinement power. To attain fibers performing at short wavelengths, it is paramount developing means to minimize the height variations on the fiber microstructure boundaries. Here, we report on the reduction of the core surface roughness of hollow-core fibers by modifying their fabrication technique. In the novel process proposed herein, counter directional gas fluxes are applied within the fiber holes during fabrication to attain an increased shear rate on its microstructure. The effect of the process on the surface roughness has been quantified by optical profilometry and the results showed that the root-mean-square surface roughness has been reduced from 0.40 nm to 0.15 nm. The improvement in the fiber core surface quality entailed fibers with ultralow loss in the short-wavelength range. We report on fibers with record loss values as low as 50 dB/km at 290 nm, 9.7 dB/km at 369 nm, 5.0 dB/km at 480 nm, and 1.8 dB/km at 719 nm. The results reveal this new approach as a promising path for the development of hollow-core fibers guiding at short wavelengths with loss that can potentially be orders of magnitude lower than the ones achievable with their silica-core counterparts

All-fiber broadband spectral acousto-optic modulation of a tubular-lattice hollow-core optical fiber

We demonstrate a broadband acousto-optic notch filter based on a tubular-lattice hollow-core fiber for the first time. The guided optical modes are modulated by acoustically induced dynamic long-period gratings along the fiber. The device is fabricated employing a short interaction length (7.7 cm) and low drive voltages (10 V). Modulated spectral bands with 20 nm half-width and maximum depths greater than 60 % are achieved. The resonant notch wavelength is tuned from 743 to 1355 nm (612 nm span) by changing the frequency of the electrical signal. The results indicate a broader tuning range compared to previous studies using standard and hollow-core fibers. It further reveals unique properties for reconfigurable spectral filters and fiber lasers, pointing to the fast switching and highly efficient modulation of all-fiber photonic devices

Optical Sensor Based On Two In-series Birefringent Optical Fibers.

An optical fiber sensor based on the combination of two spliced birefringent optical fiber sections is proposed in this paper. The sensor is built up in a Solc-filter-like configuration and a simple theoretical model based on Jones matrices is employed to predict experimental results. By choosing the suitable birefringent optical fibers (e.g., photonic crystal fibers, birefringent microfibers, elliptical core fibers, PANDA fibers, etc.), the sensor described herein allows for probing of two physical parameters (e.g., refractive index and temperature, hydrostatic pressure and temperature) or sensing the same parameter in two disconnected environments. In order to demonstrate the sensor performance, the system response was evaluated in a temperature-sensing measurement.524915-2

High-temperature sensing using a hollow-core fiber with thick cladding tubes

We report on high-temperature sensing measurements using a tubular-lattice hollow-core photonic crystal fiber displaying a microstructure formed of eight 2.4 µm-thick cladding tubes. The larger thickness of our fiber's cladding tubes compared to other hollow fibers operating in the visible and infrared ranges entails multiple narrow transmission bands in its transmission spectrum (6 bands in the spectral range between 400 nm and 950 nm) and benefits the realization of the temperature sensing measurements. The principle of operation of our device is based on the thermo-optic effect and thermal expansion-induced spectral shifts of the fiber transmission bands due to temperature variations. To study the sensor operation, we monitored the fiber transmission bands' spectral positions from room temperature to 1085 ºC in both ramp-up and ramp-down scenarios. Additionally, we investigated the optimization opportunities by assessing an analytical model describing the fiber transmission characteristics and discussed the alternatives for enhancing the sensor performance. Moreover, our fiber characterization experiments revealed a consistent confinement loss trend aligned with the scaling laws in tubular-lattice hollow-core fibers. We thus understand that the results presented in this manuscript highlight a relevant path for the development of temperature sensors based on microstructured hollow-core optical fibers endowed with thick cladding tubes

Temperature sensing with a liquid-filled hollowcore photonic crystal fiber

We report the realization of temperature sensing measurements using a water-filled hollow-core photonic crystal fiber. The operation of the sensor relies on the thermo-optic effect-mediated spectral shifts of the fiber transmission bands due to temperature variations. The characterization of our device allowed us to estimate a sensitivity of (0.42 ± 0.04) nm/ºC and to identify the studied platform as a valid path for the development of fiber-based temperature sensors

Post-processing of hollow-core photonic crystal fibers: selective hole inflation and tapering

We report on post-processing experiments using hollow-core photonic crystal fibers. We show that, by simultaneously heating and internally pressurizing the fibers, we can taper or modify the sizes of the fiber microstructure features. Particularly, by employing a technique for selectively obstructing the microstructure elements, we could attain tailored modifications of the fiber architecture, namely the inflation of selected cladding tubes, which can be of interest for the development of new devices and sensors

Curvature sensing with a hybrid-lattice hollow-core photonic crystal fiber

We report on curvature sensing measurements using a hybrid Kagomé-tubular hollow-core photonic crystal fiber. The sensing principle is based on bending-mediated resonant couplings between core and airy cladding modes achieved at specific curvature radii and wavelengths. We consider that our investigation identifies a promising use of hollow-core fibers in sensing, thus broadening the application framework of this family of fibers