Theoretical And Experimental Study On Multiwavelength Erbium-Doped Fiber Laser And Broadband Source

Multiwavelength erbium-doped fiber lasers aroused extensive attention in the passed two decades, due to their potential applications in optical fiber communication system, fiber sensors, optical detection, microwave photonics, optical spectrum analysis and so on. However, the erbium-doped fiber is a homogeneous broadening medium at room temperature, which leads to fierce mode competition and makes it different to obtain stable lasing. Various techniques have been proposed to deal with the critical issue. The intensity-dependent loss is an effective technology to suppress the mode competition and obtain the multiwavelength lasing at room temperature.The intensity-dependent loss in the laser cavity can be obtained by power-asymmetric nonlinear optical loop mirror, which based on the self-phase modulation, or the struction of polarization controller-single mode fiber-polarization controller-polarizer, which based on the nonlinear polarization rotation. As one of effective methods to obtain multiwavelength fiber laser, intensity-dependent loss has apparent advantages such as tunable operation of the wavelength number and spectral region, more output wavelengths, high output power flatness and high stability. Despite of some experiment research on this type laser, few theoretical investigations have been made on the analysis of intensity-dependent loss assisted the multiwavelength erbium-doped fiber laser, and the reason of the tunable operation of the wavelength number and spectral region could not been explained up to present. Therefore, in this dissertation, both the theoretical and experimental works are made to study the behavior of this type of lasers.Firstly, a new type multiwavelength erbium-doped fiber laser has been proposed and demonstrated. The intensity-dependent loss induced by the nonlinear polarization rotation in a power-symmetric nonlinear optical loop mirror suppresses the mode competition and ensures stable multiwavelength operation at room temperature. The polarization state and its evolution conditions for stable multiwavelength operation in the ring laser cavity are discussed. The number and spectral region of output wavelength can be controlled by adjusting the work states of power-symmetric nonlinear optical loop mirror.Secondly, a model is presented to describe the intensity-dependent loss induced by a symmetric nonlinear optical loop mirror assisted multiwavelength erbium-doped fiber laser. Both the power-dependent inhomogeneous loss and the power-independent homogeneous loss are analyzed to study the effect of intensity-dependent loss for the stable multiwavelength lasing. Based on the numerical model, the reason of the tunable operation of the wavelength number and spectral region has been investigated. The influences of the intracavity loss, pump power, and the work state of the nonlinear optical loop mirror, which is determined by the input polarization, the angle of quarter wave plate and the length of the NOLM, on the multiwavelength output characteristics have been analyzed. These findings are useful for the design and optimization of the multiwavelength system.Thirdly, a pump power cotrolled multiwavelength fiber laser with adjustable channels at fixed wavelength is proposed and experimently demonstrated. As the pump power increases from 53 mW to 410 mW, 1~7 lasing line(s) at fixed wavelength around 1571 nm are obtained. A theoretical model describing the nonlinear optical loop mirror and the erbium-doped fiber is established to investigate the effect of the pump power on the lasing characteristics. Numerical results confirm the experiment measurements and indicate that it is the intensity-dependent loss of lasing beams generated in the nonlinear optical loop mirror that leads to such phenomena. A multiwavelength Erbium-doped fiber laser with linear cavity configuration is proposed and demonstrated. A power-symmetric nonlinear optical loop mirror is utilized to induce an intensity- dependent transmission in the linear cavity, which suppresses the mode competition in the Erbium-doped fiber and ensures stable multiwavelength operation at room temperature. Both the wavelength number and the output power of the laser can be flexibly controlled by adjusting a quarter wave plate and/or a polarization controller.At last, a new type of flat broadband erbium-doped fiber amplified spontaneous emission source is proposed and demonstrated. The intensity-dependent loss induced by a symmetric nonlinear optical loop mirror is used to flatten the spectrum and broaden the bandwidth. The 3 dB bandwidth over 50 nm has been obtained with 15 m EDF and 100 mW pump power based on the single-pass forward pump configuration. This spectrum flattening and broadening method is quite effective over a large range both in pump power and EDF length.