Claudecir R. Biazoli

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.

Strain-Temperature Discrimination Using Multimode Interference in Tapered Fiber

Tapering single-mode-multimode-single-mode structures to enhance sensitivity is proposed and experimentally demonstrated. 50-mm-long coreless multimode fiber sections are spliced between single-mode fibers (SMFs) and tapered. They are characterized in strain, and an increase in strain sensitivity is obtained with taper diameter reduction. Sensitivities as high as -23.69pm/με for the 15-μm taper are attained. Temperature sensitivities also depend on taper diameter. A combination of two different diameter tapered SMF MMF-SMF structures, with cross-sensitivity to strain and temperature, is proposed as a sensing system for the simultaneous measurement of strain and temperature with resolutions of ±5.6 με and ±1.6°C, respectively. A good condition number of 3.16 is achieved with this sensing structure.

High sensitivity LPG Mach–Zehnder sensor for real-time fuel conformity analysis

A high sensitivity refractive index sensor based on the combination of mechanically induced long period gratings (LPG) and fiber tapers was developed for real-time fuel quality analysis. The sensor was built in a Mach–Zehnder configuration by employing a pair of in-series gratings. In order to enhance sensor sensitivity, the region between both LPGs was tapered down from 125 to 10 µm. The system was tested by measuring water concentration in ethanol and ethanol concentration in commercial gasoline. The tapered sensor has shown an average sensitivity of 930 nm/RIU, 18 times higher than the non-tapered version. The resolution limit of the system using spectral interrogation was estimated to be 0.06% of ethanol dissolved in gasoline. For the purpose of real-time monitoring, an interrogation system based on white light interferometry (WLI) and virtual instrumentation was employed to evaluate ethanol evaporation in water, avoiding the use of spectral analysis. The WLI system, using phase tracking techniques, enabled us to record the evolution of the ethanol concentration in water with a resolution of 0.005% (v/v).

Selectively coupling core pairs in multicore photonic crystal fibers: optical couplers, filters and polarization splitters for space-division-multiplexed transmission systems

Selective coupling a single pair of cores in a photonic crystal fiber with multiple, initially decoupled, cores is demonstrated through the use of a technique to locally post-process the fiber cross section. Coupling occurs when the hole between the selected core pair is collapsed over a short fiber section, which is accomplished by heating the section while the hole is submitted to an air pressure that is lower than that applied to all other holes in the microstructure. The demonstrated couplers present an estimated insertion loss of ~1 dB and exhibit spectral modulations with a depth of up to 18 dB and a high polarization sensitivity that can be exploited for polarization splitting or filtering in space-division-multiplexed optical interconnection and telecommunication links.

Multimode interference in tapered single mode-multimode-single mode fiber structures for strain sensing applications

Tapering single mode-multimode-single mode structures to enhance sensitivity is proposed and experimentally demonstrated. 50 mm-long coreless MMF sections are spliced between SMFs and tapered. They are characterized in strain and an increase in strain sensitivity is obtained with taper diameter reduction. Sensitivities as high as -23.69 pm/με for the 15 μm taper are attained. A combination of an untapered and tapered SMS is proposed as a sensing system for the simultaneous measurement of strain and temperature.

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.

Tapered GRIN fiber microsensor

The sensitivity of an optical fiber microsensor based on inter-modal interference can be considerably improved by tapering a short extension of the multimode fiber. In the case of Graded Index fibers with a parabolic refractive index profile, a meridional ray exhibits a sinusoidal path. When these fibers are tapered, the period of the propagated beam decrease down-taper and increase up-taper. We take advantage of this modulation -along with the enhanced overlap between the evanescent field and the external medium- to substantially increase the sensitivity of these devices by tuning the sensors maximum sensitivity wavelength. Moreover, the extension of this device is reduced by one order of magnitude, making it more propitious for reduced space applications. Numerical and experimental results demonstrate the success and feasibility of this approach.

Post-processing multicore photonic crystal fibers for locally coupling selected core pairs

Coupling selected cores among otherwise decoupled multiple cores in photonic crystal fibers is demonstrated via the use of differential pressure and heat for local processing. Complex interferometer assemblies for all-fiber devices may be achieved.

Tapered Graded Index Fibers for Sensing Applications

Optical sensors based on modal interference are improved by tapering a small extension of a graded-index fiber. The tapered section modulates the periodic amplitude of the propagated beam, substantially increasing the sensitivity of these devices.

Dispositivos fotônicos baseados em fibras ópticas e fluidos magnéticos

Os fluidos magneticos sao dispersoes de nanoparticulas magneticas em um fluido transportador. Estas dispersoes exibem interessantes propriedades fisicas quando um campo magnetico e aplicado. Neste contexto, as propriedades opticas destes fluidos tem viabilizado a criacao de dispositivos com aplicacao geral. Nos ultimos anos, alguns grupos de pesquisa tem estudado sistemas usando fluidos magneticos para dispositivos fotonicos, que usam propriedades opticas interessantes destes fluidos, por exemplo, como indice de refracao e transmitância sintonizaveis, etc. Um interessante exemplo de aplicacao em fotonica e a producao de um modulador baseado em fibra optica sensibilizada com fluido magnetico. Diversos procedimentos podem ser utilizados para tornar um guia de onda, ou mais especificamente uma fibra optica, sensivel ao meio externo. Entre as principais tecnicas citamos a gravacao de redes de periodo longo, a remocao de parte da casca da fibra por ataque quimico ou a diminuicao de toda a secao transversal da fibra atraves de processo de tapering. Em todos estes casos a luz passa a ser guiada na interface casca/ar sendo que no caso de afinamento a sensibilidade ao meio externo pode ser muito acentuada devido a grande fracao de energia que e guiada fora do guia (no meio externo). Neste trabalho discutimos o caso de um sistema em que uma parte da fibra optica e afinada (tapering) de 2 a 20 micrometros de diâmetro, onde colocamos o fluido magnetico e fazemos o estudo da transmitância da luz na fibra imersa no fluido com um campo magnetico externo aplicado. Com isso podemos modular a luz guiada atraves da fibra optica, pois a atenuacao ou perda da luz guiada nela depende da intensidade do campo magnetico externo aplicado. Portanto, estamos analisando intensidade da luz coletada a partir desta modulacao que muda com a intensidade do campo magnetico, mas faremos tambem o estudo da influencia da concentracao do fluido magnetico usado e do diâmetro do afinamento da fibra optica. Este tipo de dispositivo pode ter vantagens sobre os convencionais, por exemplo, maior sensibilidade, geometria maleavel, acompanhamento remoto, etc.