Marcos A. R. Franco

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.

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.

Microstructured Optical Fiber for Residual Dispersion Compensation Over

A defected-core microstructured optical fiber (MOF) is numerically investigated for the purpose of residual chromatic dispersion compensation from telecom link. The designed MOF exhibits ultraflattened negative dispersion over S + C + L + U wavelength bands and average dispersion of about -179 ps/nm/km with an absolute dispersion variation of 2.1 ps/nm/km from 1.480 to 1.675 mum (195-nm bandwidth). The designed fiber has eight rings of holes in the cladding resulting in low confinement loss of ~0.002 dB/km at 1.55 mum, and small effective mode area ~6 mum2. To evaluate the feasibility of such proposed MOF, a dimensional sensitivity analysis and an evaluation of coupling efficiency between the designed fiber and a conventional single-mode fiber have been carried out.

S + C + L + U

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.

Wavelength Bands

Singlemode-multimode-singlemode fiber structures (SMS) based on distinct sections of a pure silica multimode fiber (coreless-MMF) with diameters of 125 and 55 μm, were reported for the measurement of curvature and temperature. The sensing concept relies on the multimode interference that occurs in the coreless-MMF section and, in accordance with the length of the MMF section used, two fiber devices were developed: one based on a bandpass filter (self-image effect) and the other on a band-rejection filter. Maximum sensitivities of 64.7 nm·m and 13.08 pm/°C could be attained, for curvature and temperature, respectively, using the band-rejection filter with 55 μm-MMF diameter. A proof of concept was also explored for the simultaneous measurement of curvature and temperature by means of the matrix method.

Multimode interference tapered fiber refractive index sensors

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.

Curvature and Temperature Discrimination Using Multimode Interference Fiber Optic Structures—A Proof of Concept

A microstructured optical fiber with a single design parameter is proposed and demonstrated. In such a structure three thin, long glass webs join in the fiber center, forming its core. By changing the web thickness it is possible to tune the zero-dispersion wavelength from approximately 0.7 to >2.0 microm while keeping a tiny core area and single-mode guidance. Supercontinuum generation is shown in a silica fiber with a web thickness of 850 nm. The small core area and the massive hole area also make the structure very attractive for the sensing and study of fluids.

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

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.

Single-design-parameter microstructured optical fiber for chromatic dispersion tailoring and evanescent field enhancement

Photonic crystal fibers present a new way to guide light The air holes in the fiber work as a cladding, but provide much more flexibility in the design. In this work, the finite element method (FEM) is used to analyze photonic crystal fibers, providing the basic tools for calculating parameters such as mode effective index, mode effective area and dispersion. The results of a design sample are presented in this article, showing the potential of using FEM to analyze this new class of fiber.

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

We propose new microstructured optical fiber (MOF) designs for temperature sensing applications. Such fiber designs consist of a side-polished MOF with the flat side coated with a large thermooptic coefficient external medium. Numerical analysis based on a full-vector finite-element method with a perfectly matched layer was applied to evaluate the power loss caused by the evanescent field going through the external medium. Our calculations demonstrate that it is possible to change the sensor sensitivity by selecting an adequate microstructured fiber geometry. Also, different external media can be used to shift the operational range of temperature sensing.