Alessio Stefani

High Sensitivity Polymer Optical Fiber-Bragg-Grating-Based Accelerometer

We report on the fabrication and characterization of the first accelerometer based on a polymer optical fiber Bragg grating (FBG) for operation at both 850 and 1550 nm. The devices have a flat frequency response over a 1-kHz bandwidth and a resonance frequency of about 3 kHz. The response is linear up to at least 15 g and sensitivities as high as 19 pm/g (shift in resonance wavelength per unit acceleration) have been demonstrated. Given that 15 g corresponds to a strain of less than 0.02% and that polymer fibers have an elastic limit of more than 1%, the polymer FBG accelerometer can measure very strong accelerations. We compare with corresponding silica FBG accelerometers and demonstrate that using polymer FBGs improves the sensitivity by more than a factor of four and increases the figure of merit, defined as the sensitivity times the resonance frequency squared.

Tunable Polymer Fiber Bragg Grating (FBG) Inscription: Fabrication of Dual-FBG Temperature Compensated Polymer Optical Fiber Strain Sensors

We demonstrate stable wavelength tunable inscription of polymer optical fiber Bragg gratings (FBGs). By straining the fiber during FBG inscription, we linearly tune the center wavelength over 7 nm with less than 1% strain. Above 1% strain, the tuning curve saturates and we show a maximum tuning of 12 nm with 2.25% strain. We use this inscription method to fabricate a dual-FBG strain sensor in a poly (methyl methacrylate) single-mode microstructured polymer optical fiber and demonstrate temperature compensated strain sensing around 850 nm.

Narrow Bandwidth 850-nm Fiber Bragg Gratings in Few-Mode Polymer Optical Fibers

We report on the inscription and characterization of narrow bandwidth fiber Bragg gratings (FBGs) with 850-nm resonance wavelength in polymer optical fibers (POFs). We use two fibers: an in-house fabricated microstructured POF (mPOF) with relative hole size of 0.5 and a commercial step-index POF, which supports six modes at 850 nm. The gratings have been written with the phase-mask technique and a 325-nm HeCd laser. The mPOF grating has a full-width at half-maximum (FWHM) bandwidth of 0.29 nm and the step-index POF has a bandwidth of 0.17 nm. For both fibers, the static tensile strain sensitivity is measured to be 0.71 pm/με at 850 nm and 1.3 pm/με at 1550 nm.

Dynamic Characterization of Polymer Optical Fibers

With the increasing interest in fiber sensors based on polymer optical fibers, it becomes fundamental to determine the real applicability and reliability of this type of sensor. The viscoelastic nature of polymers gives rise to questions about the mechanical behavior of the fibers. In particular, concerns on the response in the nonstatic regime find foundation in the viscoelasticity theory. We investigate the effects of such behavior via analysis of the mechanical properties under dynamic excitations. It is shown that for low strain (0.28%), the Youngs modulus is constant for frequencies up to the limit set by our measurement system. A more detailed analysis shows that viscoelastic effects are present and that they increase with both applied strain and frequency. However, the possibility of developing sensors that measure small dynamic deformations is not compromised. A stress-relaxation experiment for larger deformations (2.8%) is also reported and a relaxation time around 5 s is measured, defining a viscosity of 20

Direct Writing of Fiber Bragg Grating in Microstructured Polymer Optical Fiber

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Polymeric THz 2D Photonic Crystal Filters Fabricated by Fiber Drawing

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Five-ring hollow-core photonic bandgap fiber with 1.8 dB/km loss

We report point-by-point laser direct writing of a 1520-nm fiber Bragg grating in a microstructured polymer optical fiber (mPOF). The mPOF is specially designed such that the microstructure does not obstruct the writing beam when properly aligned. A fourth-order grating is inscribed in the mPOF with only a 2.5-s writing time.

Fiber drawn 2D polymeric photonic crystal THz filters

In this paper, we report on a new form of polymeric 2D photonic crystal filters for THz frequencies fabricated using a standard fiber drawing technique. The band stop filters were modeled and designed using the generalized multipole technique. The frequency and angle-dependent transmission characteristics of the photonic crystal structures were characterized in a THz time-domain spectrometer.

Fiber design and realization of point-by-point written fiber Bragg gratings in polymer optical fibers

A 19-cell hollow-core photonic bandgap fiber reaching 1.8±0.5 dB/km loss is reported. Despite expanded corner-holes in the first ring adjacent to the core, and only five cladding rings, the minimum loss is close to the previous record of 1.2 dB/km.

Optical fibre Bragg grating recorded in TOPAS cyclic olefin copolymer

In this paper, we report on different polymeric 2D photonic crystal filters for THz frequencies which are fabricated by a standard fiber drawing technique. The bandstop filters were simulated and designed by the generalized multipole technique (GMT). The frequency and angle dependent transmission characteristics of the photonic crystal structures were characterized in a pulsed terahertz (THz) time domain spectrometer.