A novel orthogonal grating has been proposed and demonstrated. The grating is inscribed over inner cladding of a multi-cladding fiber containing two orthogonally cascaded “cladding” fiber Bragg gratings. Femtosecond laser side-illumination technique is utilized to ensure the two grating inscriptions are precisely positioned and compact in size. With such configuration, two pairs of well-defined cladding resonances have been achieved in reflection, which are originated from the internal cladding (two weakened resonances at shorter wavelength side) and from the fiber core (two stronger resonances at longer wavelength side), respectively. Most importantly, the “cladding” resonances are strongly directional sensitive to the small-vibration.
A fiber Bragg grating (FBG)-based gas refractometer is proposed and demonstrated experimentally. The configuration consists of a short section of S-type taper followed with a FBG. The S-taper is capable to couple the core mode to cladding modes into the downstream SMF, and the low-order cladding modes can be reflected back to the fiber core via the FBG, in which the recoupling efficiency is highly dependent on surrounding refractive index (RI) of liquid and gas. Experimental results show that some recoupled cladding modes show high sensitivities to surround RI. This power-referenced RI measurement and wavelength-referenced temperature measurement have been achieved via selective cladding modes monitoring.