Light with a narrowband spectrum is launched into the FBG, at a wavelength located on one of the two edges of the reflection peak of the FBG, i.e., at a wavelength where the FBG transmits, rather than reflects, light.
An optical device comprising: a first hollow-core photonic-bandgap fiber portion configured to transmit light having a wavelength, wherein the first portion has a first longitudinal axis and is configured to be adjustably twisted about the first longitudinal axis by a torque a