Long-period gratings with high insensitivity to external refractive index inscribed by using a CO<sub>2</sub> laser in fibers with parabolic-index cladding

Miroslav Chomát; Jiří Ctyroký; Daniela Berková; Vlastimil Matějec; Ivan Kašík; Jana Proboštová; Filip Todorov; Alexander Jančárek

doi:10.1117/12.723004

16 May 2007 Long-period gratings with high insensitivity to external refractive index inscribed by using a CO₂ laser in fibers with parabolic-index cladding

Miroslav Chomát, Jiří Ctyroký, Daniela Berková, Vlastimil Matějec, Ivan Kašík, Jana Proboštová, Filip Todorov, Alexander Jančárek

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Proceedings Volume 6585, Optical Sensing Technology and Applications; 65850F (2007) https://doi.org/10.1117/12.723004
Event: International Congress on Optics and Optoelectronics, 2007, Prague, Czech Republic

Abstract

Long-period gratings (LPGs) in an optical fiber, which was designed and fabricated for achieving high insensitivity of the inscribed LPGs both to bending and to the external index of refraction, are investigated. The fiber has a double cladding, consisting of a uniform outer cladding and a graded-index inner cladding, where the index of refraction decreases approximately parabolically with radius. This fiber is referred to as the parabolic-index cladding (PIC) fiber. Results obtained in the preparation of preforms and drawing PIC fibers are given. The theoretical analysis, carried out by the mode expansion and beam propagation method, yields field distributions and effective indexes of the fundamental core mode LP₀₁ and several low-order circularly symmetric cladding modes LP_0m for several external indexes of refraction. Experiments are done with an LPG with a period of 214 μm inscribed with a CO₂ laser in the PIC fiber. In experiments, transmission spectra of the LPG immersed in three liquids with refractive indexes in a relatively wide range around the index of silica (water, silica-index-matching liquid, silicone oil) are measured in a temperature range of 25-67.5°C. From experiments it follows, in agreement with the theoretical analysis, that the spectral positions of the investigated attenuation band at &lgr; ≈ 1550 nm (coupling between the LP₀₁ and LP₀₂ mode), determined at temperatures in the above range, are highly insensitive to the external index of refraction.

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Miroslav Chomát, Jiří Ctyroký, Daniela Berková, Vlastimil Matějec, Ivan Kašík, Jana Proboštová, Filip Todorov, and Alexander Jančárek "Long-period gratings with high insensitivity to external refractive index inscribed by using a CO₂ laser in fibers with parabolic-index cladding", Proc. SPIE 6585, Optical Sensing Technology and Applications, 65850F (16 May 2007); https://doi.org/10.1117/12.723004

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KEYWORDS

Optical fibers

Cladding

Refractive index

Photonic integrated circuits

Gas lasers

Refraction

Liquids

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