Periodical corrugated structure for forming sampled fiber Bragg grating and long-period fiber grating with tunable coupling strength

A novel corrugated structure on an optical fiber is proposed and demonstrated to produce tunable long-period index modulation based on photoelastic effect. The corrugated structure is a periodic variation made by chemical etching on the cladding radius of an optical fiber. By imposing this corrugated structure upon a built-in fiber Bragg grating (FBG), a superstructure grating with tunable reflectance is formed. In addition, couplings between the fundamental core mode and cladding modes take place under such a corrugated structure when the phase-matching condition is satisfied. Thus, the device can also act effectively as a long-period fiber grating (LPFG) with tunable coupling strength. We also develop the coupled-mode theory based on Fourier series expansion to describe such a corrugated sampled Bragg grating. The phase-matching condition for various sampling orders can be derived, and the calculated spectra are compared with those based on the fundamental matrix method. Optical measurements demonstrate some unique characteristics of these devices, and good qualitative agreements between simulation and measurement verify the viewpoint that long periodic index modulation is indeed induced by such a corrugated structure through the photoelastic effect when a tensile force is applied.