Design of an ultra-broadband optical filter based on a local micro-structured long period fiber grating near PMTP.

This paper presents a local micro-structured long period fiber grating (LMS-LPFG) ultra-broadband optical filter based on the wide bandwidth near the phase-matching turning point (PMTP). The structure of LMS-LPFG is obtained by dividing a LPFG into two parts of equal length and reducing the cladding radius of the second LPFG. At this time, the LMS-LPFG can be regarded as a cascade of two equal-length LPFGs with different resonance wavelengths. The cladding mode and grating period are determined to make the first LPFG work in the double-peak resonance state, and the second LPFG operates near PMTP. It is found that the transmission spectra of the two LPFGs can be superimposed to form a wide loss bandwidth. Then the cladding radii of the second LPFG and grating structure parameters are designed based on coupled-mode theory. First, the grating period corresponding to the operating wavelength is determined from the phase-matching curve of LMS-LPFG. Then, the radius of the second LPFG with a designated grating period is selected to make LPFG 2 work in PMTP by reducing its cladding radius. In addition, the grating lengths of the two LPFGs are determined by maximizing the loss of the LMS-LPFG's transmission spectrum. Finally, the two LPFGs are cascaded into a LMS-LPFG, and the optical transmission spectrum of the LMS-LPFG is calculated by the transfer matrix method. Simulation results show that the bandwidth of the transmission spectrum can reach 380 nm. In addition, the flexibility of design for the operating wave band is discussed and confirmed, and can meet different actual requirements of optical communication.