Tripling the 99% mode conversion bandwidth of gratings using a single phase shift

Achieving large bandwidth at strong mode conversion strengths is a central problem in few-mode optical waveguides. Externally-written gratings for broadband operation have missed multiple points as yet: (a) use of a single phase shift in a standard grating, (b) requisite control of the critical phase shift value at resonance wavelength, and (c) deterministic method for placement of the phase shift(s) accommodating the change in waveguide properties during grating writing. From semi-analytical considerations, we show that our unique design of a single-phase-shifted grating increases the bandwidth at 99 \% conversion strength by more than 2.8 times, for which we also elucidate crucial parameter tolerances. The phase-shift placement is determinable on-the-go from the conversion spectra, thereby obviating any estimation of modified waveguide properties during grating-writing. This offers the longed on-demand in-fiber solution for dispersion control in fiber laser cavities and communication systems.