Preliminary Theoretical Study Of Photonic Crystal Laser Mode-control Characteristics

The laser mode is usually divided into two categories:the longitudinal mode and transverse mode. The spectral characteristics of the lasers, such as the coherence length and line width, are determined by the longitudinal mode. On the other hand, the beam divergence angle, beam diameter and energy distribution depend mainly on the transverse mode. So that, the controlling of the mode properties has been an important issue in the research of lasers. Photonic crystals are new type of functional materials with artificial structure. In recent years, they have been extensively studied both theoretically and experimentally. Due to its periodic structure, photonic crystals have the characteristic of photonic band gap, determining that they have unusual ability to control and manipulate light. Thus, photonic crystals are used for the control of the laser mode properties has broad application prospects.In this thesis, a preliminary study on the application of two photonic crystals structure on controlling the mode properties has been done. The main contents include:the theory and numerical simulation research of one-dimensional photonic crystal structure in terahertz free-electron lasers (THz FELs) vertical model control and two-dimensional photonic crystal structure in photonic crystal fiber laser (PCFL) transverse model control.On one-dimensional photonic crystal section, the method to control the reflectivity, transmissivity of the longitudinal mode is given by the study of the relationship between one-dimensional photonic crystal reflection spectrum and the lattice constant, dielectric material, the defect configuration. In addition, the feasibility of photonic crystal mirrors for the THz FELs high reflectance mirror is verified and the method to adjust the mirrors with dynamically tunable reflectivity to be used as efficient, broadband out-couplers for THz FELs is also obtained in this paper.On two-dimensional photonic crystal section, through the research of the relationship between two-dimensional photonic crystal structure parameters and the number of transverse modes, the mode field distribution characteristics in PCF, the feasibility of two-dimensional photonic crystal structure in the transverse model control is verified. Through flexible design, we can adjust the PCF structure parameters, to control the PCF mode properties in order to meet the actual needs of work. Furthermore, the modal cutoff of the square lattice, triangle lattice large mode area PCF and the thermal effect in PCF is calculated and analyzed. Thermal effect is even more important in the low numerical aperture PCF, expressed as the contraction of mode area and the excitation of higher order mode.