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 to our knowledge. 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.

Terahertz optical fibers [Invited].

Lying between optical and microwave ranges, the terahertz band in the electromagnetic spectrum is attracting increased attention. Optical fibers are essential for developing the full potential of complex terahertz systems. In this manuscript, we review the optimal materials, the guiding mechanisms, the fabrication methodologies, the characterization methods and the applications of such terahertz waveguides. We examine various optical fiber types including tube fibers, solid core fiber, hollow-core photonic bandgap, anti-resonant fibers, porous-core fibers, metamaterial-based fibers, and their guiding mechanisms. The optimal materials for terahertz applications are discussed. The past and present trends of fabrication methods, including drilling, stacking, extrusion and 3D printing, are elaborated. Fiber characterization methods including different optics for terahertz time-domain spectroscopy (THz-TDS) setups are reviewed and application areas including short-distance data transmission, imaging, sensing, and spectroscopy are discussed.

Sensitivity Analysis of Different Shapes of a Plastic Optical Fiber-Based Immunosensor for Escherichia coli: Simulation and Experimental Results.

Conventional pathogen detection methods require trained personnel, specialized laboratories and can take days to provide a result. Thus, portable biosensors with rapid detection response are vital for the current needs for in-loco quality assays. In this work the authors analyze the characteristics of an immunosensor based on the evanescent field in plastic optical fibers with macro curvature by comparing experimental with simulated results. The work studies different shapes of evanescent-wave based fiber optic sensors, adopting a computational modeling to evaluate the probes with the best sensitivity. The simulation showed that for a U-Shaped sensor, the best results can be achieved with a sensor of 980 µm diameter by 5.0 mm in curvature for refractive index sensing, whereas the meander-shaped sensor with 250 µm in diameter with radius of curvature of 1.5 mm, showed better sensitivity for either bacteria and refractive index (RI) sensing. Then, an immunosensor was developed, firstly to measure refractive index and after that, functionalized to detect Escherichia coli. Based on the results with the simulation, we conducted studies with a real sensor for RI measurements and for Escherichia coli detection aiming to establish the best diameter and curvature radius in order to obtain an optimized sensor. On comparing the experimental results with predictions made from the modelling, good agreements were obtained. The simulations performed allowed the evaluation of new geometric configurations of biosensors that can be easily constructed and that promise improved sensitivity.

Simplifying the design of microstructured optical fibre pressure sensors.

In this paper, we propose a way to simplify the design of microstructured optical fibres with high sensitivity to applied pressure. The use of a capillary fibre with an embedded core allows the exploration of the pressure-induced material birefringence due to the capillary wall displacements and the photoelastic effect. An analytical description of pressure-induced material birefringence is provided, and fibre modal characteristics are explored through numerical simulations. Moreover, a capillary fibre with an embedded core is fabricated and used to probe pressure variations. Even though the embedded-core fibre has a non-optimized structure, measurements showed a pressure sensitivity of (1.04 ± 0.01) nm/bar, which compares well with more complex, specially designed fibre geometries reported in the literature. These results demonstrate that this geometry enables a novel route towards the simplification of microstructured fibre-based pressure sensors.

Integration of bow-tie plasmonic nano-antennas on tapered fibers.

In this article, a new and flexible approach to control the electric field enhancement of bow-tie nano-antennas by integrating them on the lateral of a tapered optical fiber is proposed. The device is driven by a Q-switched laser and the performance of a fabricated nano-antenna in a quartz slide is tested by a Surface Enhanced Raman Scattering (SERS) experiment. A refractive index sensing experiment is also performed and a sensitivity of (240 ± 30) nm/RIU is found in the 1.33-1.35 index range.

Integrated polarizers based on tapered highly birefringent photonic crystal fibers.

This paper proposes and demonstrates the creation of sections with a high polarization dependent loss (PDL) in a commercial highly birefringent (polarization maintaining) photonic crystal fiber (PCF), via tapering with pressure applied to the holes. The tapers had a 1-cm-long uniform section with a 66% scale reduction, in which the original microstructure aspect ratio was kept by the pressure application. The resulting waveguides show polarizing action across the entire tested wavelength range, 1510-1600 nm, with a peak PDL of 35.3 dB/cm (c.f. ~1 dB/cm for a typical commercial polarizing fiber). The resulting structure, as well as its production, is extremely simple, and enable a small section with a high PDL to be obtained in a polarization maintaining PCF, meaning that the polarization axes in the polarizing and polarization maintaining sections are automatically aligned.

Photonic-crystal fiber-based pressure sensor for dual environment monitoring.

In this paper the development of a side-hole photonic-crystal fiber (SH-PCF) pressure sensor for dual environment monitoring is reported. SH-PCF properties (phase and group birefringence, sensitivity to pressure variations) are measured and compared to simulated data. In order to probe two environments, two sections of the SH-PCF with different lengths are spliced and set in a Solc filter-like configuration. This setup allows obtaining the individual responses of the first and second fiber independently, which is useful for a space-multiplexed measurement. As the employed fiber is sensitive to pressure variations, we report the use of this configuration for dual environment pressure sensing.

Detailed analysis of the longitudinal acousto-optical resonances in a fiber Bragg modulator.

The interaction frequencies between longitudinal acoustic waves and fiber Bragg grating are numerically and experimentally assessed. Since the grating modulation depends on the acoustic drive, the combined analysis provides a more efficient operation. In this paper, 3-D finite element and transfer matrix methods allow investigating the electrical, mechanical and optical resonances of an acousto-optical device. The frequency response allows locating the resonances and characterizing their mechanical displacements. Measurements of the grating response under resonant excitation are compared to simulated results. A smaller than <1.5% average difference between simulated-measured resonances indicates that the method is useful for the design and characterization of optical modulators.

Multimode interference tapered fiber refractive index sensors.

Real-time monitoring of the fabrication process of tapering down a multimode-interference-based fiber structure is presented. The device is composed of a pure silica multimode fiber (MMF) with an initial 125 µm diameter spliced between two single-mode fibers. The process allows a thin MMF with adjustable parameters to obtain a high signal transmittance, arising from constructive interference among the guided modes at the output end of the MMF. Tapered structures with waist diameters as low as 55 µm were easily fabricated without the limitation of fragile splices or difficulty in controlling lateral fiber alignments. The sensing device is shown to be sensitive to the external environment, and a maximum sensitivity of 2946 nm/refractive index unit in the refractive index range of 1.42-1.43 was attained.

Ultrahigh-sensitivity temperature fiber sensor based on multimode interference.

The proposed sensing device relies on the self-imaging effect that occurs in a pure silica multimode fiber (coreless MMF) section of a single-mode-multimode-single-mode (SMS)-based fiber structure. The influence of the coreless-MMF diameter on the external refractive index (RI) variation permitted the sensing head with the lowest MMF diameter (i.e., 55 µm) to exhibit the maximum sensitivity (2800 nm/RIU). This approach also implied an ultrahigh sensitivity of this fiber device to temperature variations in the liquid RI of 1.43: a maximum sensitivity of -1880 pm/°C was indeed attained. Therefore, the results produced were over 100-fold those of the typical value of approximately 13 pm/°C achieved in air using a similar device. Numerical analysis of an evanescent wave absorption sensor was performed, in order to extend the range of liquids with a detectable RI to above 1.43. The suggested model is an SMS fiber device where a polymer coating, with an RI as low as 1.3, is deposited over the coreless MMF; numerical results are presented pertaining to several polymer thicknesses in terms of external RI variation.

Efficient and short-range light coupling to index-matched liquid-filled hole in a solid-core photonic crystal fiber.

A photonic crystal fiber (PCF) with a section of one of the holes next to the solid core filled with an index-matched liquid is studied. Liquid filling alters the core geometry, which locally comprises the original silica core, the liquid channel and the silica around it. It is demonstrated that when light reaches the filled section, it periodically and efficiently couples to the liquid, via the excitation of a number of modes of the composite core, with coupling lengths ranging from tens to hundreds of microns. The resulting modal-interference-modulated spectrum shows temperature sensitivity as high as 5.35 nm/°C. The proposed waveguide geometry presents itself as an interesting way to pump and/or to probe liquid media within the fiber, combining advantages usually found separately in liquid-filled hollow-core PCFs (high light-liquid overlap) and in solid-core PCFs (low insertion losses). Therefore, pumping and luminescence guiding with a PCF filled with a Rhodamine solution is also demonstrated.

Analysis and optimization of an all-fiber device based on photonic crystal fiber with integrated electrodes.

We present both numerical and experimental studies of an all-fiber device based on the integration of metallic electrodes into photonic crystal fibers (PCF). The device operation consists on applying electrical current to the electrodes which, by Joule effect, expand and squeeze the PCF microstructure in a preferential direction, altering both phase and group birefringence. We investigate the effect of integrating electrodes into the fiber and the dependence of the device sensitivity on the electrode configuration and composition.

Microstructured-core optical fibre for evanescent sensing applications.

The development of microstructured fibres offers the prospect of improved fibre sensing for low refractive index materials such as liquids and gases. A number of approaches are possible. Here we present a new approach to evanescent field sensing, in which both core and cladding are microstructured. The fibre was fabricated and tested, and simulations and experimental results are shown in the visible region to demonstrate the utility of this approach for sensing.