Optimal design of broadband photonic crystal fibre long-period gratings for evanescent absorption sensing

An index-guiding solid core photonic crystal fibre (PCF) is numerically optimized to achieve through an inscribed long period grating (LPG) broadband coupling between the fundamental mode (LP01) and the first-order symmetric cladding (LP02-like) mode. The vectorial finite element method and the Nelder-Mead simplex method are used for the optimization of the PCF to get for LP01-LP02 mode coupling the phase matching curve with the dispersion turning point located in the center of a selected range of resonant wavelengths. The bandwidth of a LPG close to the dispersion turning point, where the resonance condition is nearly matched for multiple wavelengths, is large. An evanescent power overlap of the LP02 cladding mode with the PCF's air holes is an order of magnitude higher then that of the LP01 mode. We optimize the PCF with only five rings of hexagonally arrayed air holes. By enlarging the air holes in the outmost ring the LP02 mode confinement loss is reduced to a negligible value which allows to lengthen the cladding mode interaction length with an analyte infiltrated into the air holes.

[1]  I. Malitson.  Interspecimen Comparison of the Refractive Index of Fused Silica , 1965 .

[2]  T. Strasser,et al.  Grating resonances in air-silica microstructured optical fibers. , 1999, Optics letters.

[3]  Lou Shuqin,et al.  Mode classification and degeneracy in photonic crystal fibers. , 2003, Optics express.

[4]  W. Jin,et al.  Scaling property and multi-resonance of PCF-based long period gratings. , 2004, Optics express.

[5]  John M. Fini,et al.  Microstructure fibres for optical sensing in gases and liquids , 2004 .

[6]  O. Bang,et al.  Highly sensitive refractometer with a photonic-crystal-fiber long-period grating. , 2007, Optics letters.

[7]  J. Kanka.  Design of photonic crystal fibers with highly nonlinear glasses for four-wave-mixing based telecom applications. , 2008, Optics express.

[8]  David J. Webb,et al.  Long period gratings written into a photonic crystal fibre by a femtosecond laser as directional bend sensors , 2008 .

[9]  Krishna Thyagarajan,et al.  Novel fiber design for broadband long period gratings , 2003 .

[10]  Numerical analysis of refractive index sensitivity of long-period gratings in photonic crystal fiber , 2008 .

[11]  Alan D. Kersey,et al.  Analysis of the response of long period fiber gratings to external index of refraction , 1998 .

[12]  Arijit Roy,et al.  Optimal design of broadband long period grating-based LP01↔LP02 mode converters for dispersion compensation , 2003 .

[13]  John E. Sipe,et al.  Long-period fiber gratings as band-rejection filters , 1995 .

[14]  V. Bhatia.  Applications of long-period gratings to single and multi-parameter sensing. , 1999, Optics express.

[15]  S. James,et al.  Optical fibre long-period grating sensors: characteristics and application , 2003 .

[16]  Ping Miao,et al.  Sensitivity characteristics of long-period fiber gratings , 2005, SPIE/OSA/IEEE Asia Communications and Photonics.

[17]  Kunimasa Saitoh,et al.  Chromatic dispersion control in photonic crystal fibers: application to ultra-flattened dispersion. , 2003, Optics express.

[18]  L Zhang,et al.  Sensitivity characteristics near the dispersion turning points of long-period fiber gratings in B/Ge codoped fiber. , 2001, Optics letters.

[19]  I. Thormählen,et al.  Refractive Index of Water and Its Dependence on Wavelength, Temperature, and Density , 1985 .

[20]  S. Afshar,et al.  Theoretical study of liquid-immersed exposed-core microstructured optical fibers for sensing. , 2008, Optics express.

[21]  D J Richardson,et al.  Inverse design and fabrication tolerances of ultra-flattened dispersion holey fibers. , 2005, Optics express.

[22]  Jeffrey C. Lagarias,et al.  Convergence Properties of the Nelder-Mead Simplex Method in Low Dimensions , 1998, SIAM J. Optim..