Design of turn-around-point long-period gratings in a photonic crystal fiber for refractometry of gases

• Published in: Sensors and actuators. B, Chemical Vol. 182; pp. 16 - 24
• Main Author: Kanka, Jiri
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.06.2013
• Subjects: air; Downhill simplex method; engineering; Fiber design; fiber optics; finite element analysis; Finite element method; liquid petroleum gas; Long-period grating; Microstructured optical fiber; Photonic crystal fiber; refractive index; Refractive index sensor; Refractometer; wavelengths; Finite element method; Microstructured optical fiber; Fiber design; Photonic crystal fiber; Downhill simplex method; Refractive index sensor; Refractometer; Long-period grating
• ISSN: 0925-4005; 1873-3077
• DOI: 10.1016/j.snb.2013.02.048

A fiber-optic long-period grating (LPG) operating near the dispersion turning point in its phase matching curve, referred to as a turn-around-point (TAP) LPG, is known to be highly sensitive to external parameters. So far TAP LPGs have been realized in specially designed or post-processed conventional fibers, not yet in PCFs, which allow a great degree of freedom in engineering the fiber's dispersion properties through the control of the PCF structural parameters. We have developed the design optimization technique for TAP PCF-LPGs employing the finite element method for PCF modal analysis in a combination with the Nelder–Mead simplex method for minimizing the objective function based on target-specific PCF properties. Using this tool TAP PCF-LPGs can be designed for specified wavelength ranges and refractive indices of medium in the air holes. We present here the dispersion optimization of PCF-LPG for refractometry of gases. The resonant wavelength of an optimized PCF-LPG is subsequently highly sensitive to the refractive index of gaseous analytes, however, its large shifts could be detected with a substantially reduced resolution because the resonance dip in the TAP LPG transmission spectrum is broad. We have resolved the resolution problem by proposing an interferometric scheme based on twin TAP LPGs.