Tunable THz-Wave Radiation Based On PPLN Crystal And Dual-Wavelength Fiber Laser

Terahertz has many unique properties, such as broadband, complementary, transient nature, coherence, etc. It bring a profound impact on applications areas including imaging and sensing and so on, basic research including pharmaceuticals, chemistry, life sciences and others, as well as defense technology field. So, THz science and technology has become more popular research project in recent years. At present, THz has many popular and practical value research such as THz radiation, THz stealth as well as THz waveguide and etc. Among them, how to get tunable THz radiation is key issue in THz science and technology research. Through many experimental study’s verification, the method of obtaining THz radiation by nonlinear optical difference frequency is an ideal method. The advantage is that it can be stable operation at room temperature and can output tunable and coherent THz wave. The principle of generation THz radiation by nonlinear optical difference frequency is that two beams of close wavelengths and good coherence laser pass through nonlinear crystal for difference frequency in condition of phase matching. Most chosen laser is the solid and gas lasers, the fiber laser is unusual, especially, dual-wavelength single-longitudinal-mode erbium-doped fiber laser whose wavelength is near1550nm.This paper focuses on the THz radiation based on PPLN crystal and dual-wavelength fiber lasers, discusses how relevant characteristics of PPLN crystal to affect THz radiation, and analyses tunable THz wave based PPLN crystal. Through analyzing the requirements for dual-wavelength source to nonlinear difference frequency and dual-wavelength fiber laser, the paper receive the conclusion that dual-wavelength single-longitudinal-mode erbium-doped fiber laser is ideal dual-wavelength source. Finally, the paper designs a kind of dual-wavelength single-longitudinal-mode erbium-doped fiber laser based on nonlinear amplifying loop mirror (NALM). Thus, laying the foundation for the future tunable THz radiation by optical fiber composition.Main contents of the paper are as follows:(1) Study the theory of nonlinear difference frequency, establish steady coupled wave equation of nonlinear three-wave interaction, and obtain the condition of generation THz radiation, and describe the method of phase-matching. On the foundation of analyzing the theory of quasi phase matching (QPM), discuss the effect of related features of PPLN crystals to QPM, such as refractive index the coherence length and so on, thus gain the method how to optimize the PPLN crystal structure.(2) Analyze the method of THz wave’s tune, especially temperature tuning, polarization cycle tuning and a combination of both. And analyze the spectrum band of THz wave, so gain the spectrum graph of THz radiation by difference frequency based on PPLN crystals. And gain the polarization cycle allows bandwidth when phase mismatch based on PPLN crystals, finally analyze THz output power.(3) Through analyzing the dual-wavelength source for THz radiation by difference frequency, dual-wavelength erbium-doped fiber laser is an ideal dual-wavelength source. Finally, the paper designs a kind of dual-wavelength single-longitudinal-mode erbium-doped fiber laser based on NALM, thus gain dual-wavelength fiber laser whose wavelength is near1550nm. Through measuring instrument measuring, analyze whether the dual-wavelength meets the requirement for dual-wavelength source of DFG system, laying the foundation for the future tunable THz radiation by optical fiber composition.