In this paper, we rigorously deduce the coupled-mode equations of a long-period fiber grating and fiber Bragg grating in their cascaded structure (CLBG), based on coupled-mode theory. Next, through the difference iterative method, the total transfer matrix of CLBG is obtained. Mistakes in previous literature involving coupled-mode equations and the transfer matrix of CLBG are corrected, including the neglect of coupling behaviors and the application of incorrect methods of solving coupled-mode equations. Then, both reflection and transmission spectrum characteristics are simulated numerically. The simulation results indicate that the position and intensity of the transmission and reflection peaks coincide with existing experimental results, and the transmission spectrum is highly consistent with the reflection spectrum. However, a large deviation exists in the previous literature: the transmittance exceeds one in the simulation results. Finally, the temperature and surrounding refractive index characteristics of the reflection spectrum are theoretically simulated, and further verified by experiments. The experimental results coincide well with our simulation results, indicating the correctness of our proposed method. The method provided in the paper could provide effective and reliable theoretical guidance for CLBG characteristic analysis, structure optimization, and sensing applications.
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.
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