Optical Low-coherence Reflectometry Technique Research Articles

Versatile Characterization of Specialty Fibers Using the Phase-Sensitive Optical Low-Coherence Reflectometry Technique

Emergence of new fibers families induces considerable requirements in terms of characterization and metrology (group delay, chromatic dispersion, birefringence, bending losses, etc.). The optical low-coherence reflectometry (OLCR) technique is demonstrated as a versatile method for the characterization of most types of optical fiber. A synthesis of multiple analysis concerning different families of specialty fibers including rare-earth-doped fibers, few-mode fibers, and microstructured fibers will be presented. OLCR allows measuring precisely the group velocity dispersion value for both polarization modes and birefringence. It is also possible to measure small refractive-index variations in a pumped Erbium-doped fiber. Unique dispersive properties of higher order modes fiber offer novel solutions for dispersion compensation or nonlinear effects management. OLCR can allow each LP mode characterization without the requirement for mode converters. A new method, called ldquotime-wavelength reflection mapping,rdquo based on the OLCR interferogram processing is applied to the determination of chromatic dispersion of each guided LP mode whatever their group index. Finally, different characterization results concerning photonics crystal fibers with guiding based on the conventional total internal reflection principle (high-index guiding) or photonic bandgap effect (low-index guiding) will be presented.

Optical low-coherence reflectometry for complete chromatic dispersion characterization of few-mode fibers

A phase-sensitive optical low-coherence reflectometry (OLCR) technique is demonstrated to simultaneously measure the absolute chromatic dispersion values of each guided LP mode of a few-mode fiber. We show that the OLCR technique requires only short samples of fiber (<1 m) and has no need for high-ratio mode converters to reach an accurate wavelength-dependent group delay evolution of every mode. As an example we present for the first time to our knowledge a direct and complete analysis of few-mode fibers with high, low, positive, and negative modal dispersion values, leading to chromatic dispersion parameters in good agreement with theoretical predictions.

DS-OCDMA encoder/decoder performance analysis using optical low-coherence reflectometry

Direct-sequence optical code-division multiple-access (DS-OCDMA) encoder/decoder based on sampled fiber Bragg gratings (S-FBGs) is characterized using phase-sensitive optical low-coherence reflectometry (OLCR). The OLCR technique allows localized measurements of FBG wavelength and physical length inside one S-FBG. This paper shows how the discrepancies between specifications and measurements of the different FBGs have some impact on spectral and temporal pulse responses of the OCDMA encoder/decoder. The FBG physical lengths lower than the specified ones are shown to affect the mean optical power reflected by the OCDMA encoder/decoder. The FBG wavelengths that are detuned from each other induce some modulations of S-FBG reflectivity resulting in encoder/decoder sensitivity to laser wavelength drift of the OCDMA system. Finally, highlighted by this OLCR study, some solutions to overcome limitations in performance with the S-FBG technology are suggested

Characterization of residual strains in an epoxy block using an embedded FBG sensor and the OLCR technique

Cylindrical specimens made of epoxy and centrally located glass fibre are used to characterize residual stresses due to curing and post-curing of the epoxy. The fibre contains a Bragg grating which is used as a strain sensor along the fibre axis. A novel optical low-coherence reflectometry technique combined with a layer-peeling algorithm allows the direct reconstruction of the optical period of the grating and provides the strain distribution due to polymerisation of the epoxy along the fibre without any assumption. For the purpose of analysis, the polymerisation process is considered as an equivalent thermo-elastic problem. Using the experimental data as an input to a finite element model of the fibre-epoxy specimen, a non-homogeneous strain field is predicted in the whole specimen. Particularly, the experimental data are in good agreement with finite element simulations and a theoretical model [8] along the embedded fibre.

Phase-sensitive optical low-coherence reflectometry technique applied to the characterization of photonic crystal fiber properties

Localized measurements of group-velocity dispersion and birefringence of photonic crystal fibers are achieved with a phase-sensitive optical low-coherence reflectometry technique. This technique is efficient for fiber samples no longer than 1 m. Theoretical simulations are in good agreement with experimental results. As a result, the stress-induced birefringence proves to be at most 1 order of magnitude below the geometrical-shape birefringence.