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25 April 2012 Design of turn-around-point long-period gratings in Ge-doped photonic crystal fibers for evanescent sensing
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The numerically optimized Ge-doped photonic crystal fiber long-period gratings operate around the dispersion turning point on the phase matching curve of the coupled modes. This special type of LPG, referred to as a Turn-Around-Point (TAP) LPG, can be employed for evanescent broadband absorption spectroscopy or optical intensity-based refractometry. The numerical optimization of a PCF-LPG utilizes the finite element method for PCF modal analysis and the simplex downhill method to minimize the objective function based on target-specific PCF properties. For gas and aqueous analytes infiltrated into PCF's air holes, the TAP PCF-LPG's periods are shorter than those achievable with a CO2 laser LPG inscription, and therefore the use of a femtosecond laser is supposed. The transmission spectrum of a TAP PCF-LPG is highly sensitive to variations in PCF geometrical parameters. The effects of imprecision in PCF fabrication on the LPG's transmission spectra can be mitigated with a stronger refractive index modulation, which can be achieved easier in a Ge-doped PCF than in a puresilica PCF. Moreover, germanium doping allows to precisely define the grating area for maximizing the coupling coefficient. Potential and limitations of TAP LPGs inscribed with a femtosecond laser into Ge-doped PCFs for evanescent chemical sensing will be evaluated.
© (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Jiri Kanka "Design of turn-around-point long-period gratings in Ge-doped photonic crystal fibers for evanescent sensing", Proc. SPIE 8426, Microstructured and Specialty Optical Fibres, 84260A (25 April 2012);

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