The use of fiber gratings formed by photosensitivity allows the development of several all-fiber devices, which are simple to implement. The two-mode fiber grating is desirable for many application areas since it is a transmission grating. The devices demonstrated in this work include a digital switch, a tunable wavelength filter, and a tunable polarizer.
These devices based on the mode coupling by two-mode fiber gratings that satisfy the phase-matching condition. Coupled mode equation is derived from the Maxwell equations and interpreted on the Poincare sphere. The mode coupling is characterized for the case of detuning the length of the grating by applying a strain to the grating.
In experiments, gratings are written on elliptical core two mode fibers by the two-photon absorption of blue-green light. The origin of photosensitivity has not well been understood. The present study on the photosensitivity is summarized. The dynamics of grating rowth is studied by numerical calculations and understood by using the Poincare sphere.
The two-mode interferometer is implemented by using the gratings as modal couplers.
The direct temperature sensing with the grating is proposed and demonstrated.
The devices demonstrated in this work can be used only in blue-green region. The fabrications of gratings by side-writing technique is necessary for other wavelengths.
