Measurement Of Mode Loss And Mode Competition In Er-doped Fiber Laser

Erbium-doped fiber lasers operating at 1.55 μm band have some advantages,such as low threshold,compact structure and tunability.They have important applications in optical fiber communication technology,optical fiber sensors,optical information processing,and so on.Mode competition is one of the important parts in lasers before reaching stability.The key problems of mode competition in lasers are studied in this dissertation,the main research contents are as following:1.The operating characteristics of erbium-doped fiber laser is studied by using rate equation theory,the analytical expressions of the output power,threshold power and slope efficiency of the laser are derived.The effects of pump power and erbium-doped fiber length on its output characteristics.It’s demonstrated that the output power is proportional to pump power,the shorter the length of erbium-doped fiber is,the lower the slope efficiency is,and the less the loss of cavity is.2.The cavity ringdown spectroscopy characteristic of fiber Bragg grating FP cavity is studied theoretically,the analytical expressions of the light field through fiber Bragg grating FP cavity is derived,the effect of the reflectivity of fiber Bragg grating on cavity ringdown characteristics is discussed.Numerical results demonstrate that the reflectivity of the fiber Bragg grating decreases,the output loss of fiber Bragg grating FP cavity increases,and the ringdown time of the cavity decreases.3.The output characteristics of single and cascaded fiber Bragg gratings are studied experimentally,the relationship between the Bragg wavelength of fiber Bragg gratings and its operating temperature is explored.The mode losses in erbium-doped fiber laser are measured by using cavity ringdown spectrum technique,the measurement of the mode losses is divided into three stages: 1)determine the mode(presented by wavelength);2)measure the ring-down time and determine the corresponding mode loss at the wavelength above;3)determine the mode loss profile of the fiber Bragg grating FP cavity.Synchronously changing the operation temperature of the two fiber Bragg grating,repeating stages 1)and 2)some times to obtain relative data,then,using fitting method,the mode loss profile of the fiber Bragg grating FP cavity is determined.Experimental results demonstrate that one of the modes in the laser oscillates,single wavelength output,in the case that the two fiber Bragg gratings constructing FP cavity operates at the same temperature,which is one of the many modes in the fiber Bragg grating FP cavity.The oscillating mode in the cavity can be scanned by adjusting the operating temperature of the two fiber Bragg gratings,the loss of fiber Bragg grating FP cavity at 1549.7 nm wavelength(mode)is the smallest.4.The mode competition effect in erbium-doped fiber laser is experimentally studied,the cavity of the laser is constructed by a piece of 25 cm length erbium-doped fiber connected two fiber Bragg grating,the mode competition effect in observed.It’s demonstrated that the longitudinal mode losses in erbium-doped fiber laser are different.The mode loss at 1549.63 nm is greater than that at 1550.08 nm,the mode loss at 1549.56 nm is greater than that at 1550.02 nm,the mode loss at 1549.86 nm is greater than that at 1549.44 nm,the mode loss at 1549.89 nm is greater than that at 1549.43 nm.The threshold power at 1549.63 nm is higher than that at 1550.08 nm,the threshold power at 1549.56 nm is higher than that at 1550.02 nm,the threshold power at 1549.86 nm is higher than that at 1549.44 nm and the threshold power at 1549.89 nm is higher than that at 1549.43 nm mode.