We present results of experimental research and comparison of differential mode delay (DMD) maps measured for silica graded index multimode optical fibers (MMFs) with strong and weak diameter variation. Preliminary for two synthesized by MCVD fiber preforms were selected by criterion of expected strong DMD due to great profile dip in the core center. Then two lengths of MFMs were drawn. The first one was manufactured according typical operations with automatic control of technological processes, while the second one was drawn under manual maintenance. Therefore two samples of MMFs of ISO/IEC Cat. OM2 with length about 1 km were manufactured with diameter variation ± 0.3 μm and ± 1.2 μm respectively. At the next stage we performed DMD map measurement of described two MMF 50/125 samples by DMD analyzer lab kit R2D2 according to ratified standards TIA-455-220-A/FOTP-220 and IEC 60793-2-10 to research and analyze influence of fiber diameter variation on mode coupling in the form of additional DMD distortions during laser-excited optical pulse propagation over MMF under a few-mode regime.
We present alternative method for implementation of secure data transmission channel protected over physical layer for the "first/last mile" segment of intra-corporate network by specialized fiber optic link. Proposed solution is based on application of operating in a few-mode laser-based regime multimode optical fibers with special refractive index profile providing optical pulse strong uniquely distortions due to differential mode delay effect that allows to consider those optical fibers as "encryptor". Therefore, transmitted traffic is strongly distorted and it is unable to be processed without corresponding "decryptor" which might be either also another one optical fiber with "inversed" refractive index profile in relation to the "encryptor"-fiber or electronic dispersion compensator device modified for such application. This work is concerned with configurations of proposed "crypto" fiber optic link as well as brief description of math apparatus for design it. Some results of preliminary simulations are also represented.
This work presents results of experimental approbation of earlier on proposed modified fiber optic stress sensor based on a few-mode effects occurring during laser-excited optical signal propagation over silica multimode optical fiber (MMF). Modification is concerned with a passage to quasi-interferometric scheme realized by two multimode Y-couplers with equalized arm lengths improved by fiber Bragg grating (FBG) written on preliminary formed precision macrostructure defects in silica multimode graded-index optical fibers and special offset launching conditions providing laser-based excitation of higher-order modes. The “arms” of quasi-interferometer are two equalized lengths of MMF Cat. OM2 with great central dip of refractive index profile and strong pulse splitting due to high differential mode delay (DMD). We tested FBGs with Bragg wavelength both 1310 nm and 1550 nm written over tapers or up-tapers preliminary formed in short pieces of MMF Cat. OM2+/OM3 and further jointed to the end of one of the arms before output Y-coupler. Researches were focused on comparison analysis of pulse responses under changing of selected excited mode mixing and power diffusion processes due to stress distributed action to sensor fiber depending. Here we considered FBGs not only as particular wavelength reflector during spectral response measurement but also as local periodic microstructure defect which strongly effects on few-mode signal components mixing process also improved by combination with macro-defect like taper or up-taper that should provide response variation. Some results pulse response measurements produced for different scheme configuration and their comparison analysis are represented.
This work presents method for performing precision macro-structure defects “tapers” and “up-tapers” written in conventional silica telecommunication multimode optical fibers by commercially available field fusion splicer with modified software settings and following writing fiber Bragg gratings over or near them. We developed technique for macrodefect geometry parameters estimation via analysis of photo-image performed after defect writing and displayed on fusion splicer screen. Some research results of defect geometry dependence on fusion current and fusion time values re-set in splicer program are represented that provided ability to choose their “the best” combination. Also experimental statistical researches concerned with “taper” and “up-taper” diameter stability as well as their insertion loss values during their writing under fixed corrected splicer program parameters were performed. We developed technique for FBG writing over or near macro-structure defect. Some results of spectral response measurements produced for short-length samples of multimode optical fiber with fiber Bragg gratings written over and near macro-defects prepared by using proposed technique are presented.
The results of experimental study on the main technological aspects relating to a full production cycle of 50/125 μm silica multimode graded-index fibers with the central defect of the refractive index profile realized as a large dip are presented. Preform synthesis conditions for controllable implementation of the mentioned defect via MCVD method are analyzed and optimized. The effect of geometrical irregularities, induced by drawing optical fibers under the manual maintenance of the outer diameter stability, on attenuation has been explored. Applying the Weibull theory, a statistical evaluation of mechanical properties, particularly tensile strength, of the optical fibers drawn at various temperatures has been conducted.
We present modified technique for differential mode delay map measurement. Here according to well-known methods a fast laser pulse is also launched into a tested multimode fiber (MMF) via single mode fiber (SMF), which scans core of MMF under precision offset positions. However unlike known technique formalized in ratified standards, proposed modification differs by addition scanning of the output end of tested MMF by short tail of SMF. Therefore for each radial offset position at the input/output MMF ends, the shape of pulse response of launched optical signal is recorded, that provides to get more informative differential mode delay map. This work presents some results of experimental approbation of proposed modified technique for differential mode delay map measurement.
This work presents results of experimental researches of fiber Bragg gratings (FBG) operating in a few-mode regime.
We tested FBGs written on silica graded-index multimode fibers 50/125 Cat. OM2+/OM3 with Bragg wavelength 1550
nm by using them in a set of developed experimental schemes based on excitation of multimode fibers by corresponding
laser sources. The researches were focused on analysis of both spectral and pulse responses under changing of selected
mode mixing and power diffusion processes due to tension and/or stress local and distributed action to FBG or sensor
fiber. Results of spectral and pulse response measurements at the output of schemes with installed described FBGs are
represented.
This work presents results of experimental approbation of modified fiber optic stress sensor based on a few-mode effects
occurring during laser-excited optical signal propagation over silica multimode optical fiber (MMF). Modification is
concerned with adding of quasi-interferometric scheme realized by two multimode Y-couplers with equalized arm
lengths improved by fiber Bragg grating (FBG) and special offset launching conditions providing laser-based excitation
of higher-order modes. We tested FBGs written on graded-index MMFs 50/125 with Bragg wavelength 1550 nm connected
to different parts of proposed scheme. Researches are focused on comparing analysis of both spectral and pulse
responses under changing of selected mode mixing and power diffusion processes due to stress local and distributed action
to sensor fiber depending on scheme configuration. Here we considered FBGs not only as particular wavelength
reflector during spectral response measurement but also as local periodic microstructure defect strongly effecting few-mode
signal components mixing process that provides pulse response variation. Some results of spectral and pulse response
measurements produced for different scheme configuration and their comparison analysis are represented.
This work is concerned with fiber Bragg grating (FBG) writing technique developed for graded-index multimode optical fibers applied in measurement systems based on a few-mode effects. We present some results of experimental approbation of proposed technique with Bragg wavelength 1310 and 1550 nm on samples of graded-index multimode optical fibers 50/125 of both new-generations Cat. OM2+/OM3 and old Cat. OM2 with preliminary measured refractive index profiles. While the first group fibers of Cat. OM2+/OM3 was characterized by almost ideal smooth graded refractive index profile and some fiber profile samples of this group contains thin central peak, the second fiber group profiles of Cat. OM2 differ by great central core defects representing dip or thick peak. Results of described FBG spectral response measurements under excitation of laser pigtailed by single-mode fiber are represented.
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