Optimal design of single resonant and ultrabroadband long-period fiber grating filters

We propose a spectral flat-top, single resonant, and ultrabroadband-more than 180 nm in a −20-dB bandwidth-long-period fiber grating (LPG) filter. The ultrabroadband LPG is based on a thin cladding layer LPG synthesized by the Lagrange multiplier optimization (LMO) algorithm. As the bandwidth and resonant spectra cover a very wide band, both material dispersion and waveguide dispersion were included in the calculations of the LMO method. To the best of our knowledge, the bandwidth of the designed flat-top LPG filter in the −20-dB coupling is the broadest currently existing in the literature. Such designed LPG devices can be very useful for a variety of applications in broadband optical communication systems.