Investigation Of Single-longitudinai-mode Erbium-doped Fiber Laser With Ring-cavity

As a representative of laser technology of the third generation, fiber laser hasmany advantages such as compact and lightweight structure, low manufacturing costs,high reliability and stability. Therefore, it plays an irreplaceable role in many fieldssuch as Laser medical, optical fiber sensing, spectroscopy analysis, machining.Among the many members of fiber lasers, erbium-doped fiber laser with ring-cavitycan obtain narrower line width and higher power because of its longer cavity length,therefore, it has very broad application prospects in many areas of modern industry.However, with the rapid development of science and technology, people are alreadynot satisfied with the performance indicator of existing fiber laser, many areas putforward higher requirements on the quality (monochrome and direction) of laser beam.Fiber laser of single longitudinal-mode came into being. The exporting laser ofgeneral laser is multi-longitudinal mode or multi-line, fiber laser of singlelongitudinal-mode is required to contain only one longitudinal mode, otherlongitudinal modes is capable of rejecting, which requires researchers propose newstructure or method to achieve single-longitudinal-mode output on the basis ofexisting fiber laser.The article proposes to design a erbium-doped fiber laser with saturated absorberstructure and ring-cavity based on the background of the study above, to optimize theparameters of erbium-doped fiber laser with ring cavity and to study the influence ofsaturated absorber on the output performance of laser through simulation andoptimization of software, and to achieve the output and measurement of singlelongitudinal-mode laser of this structure through experimental way. In addition toexplanation of necessary significance and background of research, we do research onthe following aspects in this paper:1. In this paper, I derive three-level rate equations start from the level structure ofthe erbium ions, and then get the conditions to meet the population inversion. I derivethreshold pump power, slope efficiency and output power equation of erbium-dopedfiber laser with ring cavity using a simplified two-level structure of erbium ions. I analysis the operating characteristics and output power characteristics of the laser ofthis structure, point out that splitting ratio of output coupler and length oferbium-doped fiber are two important parameters to determine the output performance,and both of them have an optimal solution. I also propose three methods to achievesingle longitudinal-mode output of fiber laser, selective analysis the principle of themethod of saturated absorber, and then propose the structure of erbium-doped fiberlaser with saturated absorber and ring-cavity on the basis of this.2. In this paper, the optical design software OptiAmplifier is used to simulateerbium-doped fiber laser with ring-cavity and without saturated absorber, we getoutput laser spectrum of center wavelength of1556nm,3dB bandwidth of1nm, sidemode rejection ratio about75dB and the input and output curve after setting pumppower of130mw, erbium-doped fiber length of2m, coupler splitting ratio of70/30.We get the result that optimal length of erbium-doped optical fiber is2.5m, optimalsplitting ratio of coupler is90/10, thus proving the correctness of the theoreticalderivation. We run the software after stetting the optimization parameters getting thatmaximum output power of the laser increases5.08mw and slope efficiency of2.89%,the effect of optimization is remarkable.3. Performance changes of fiber laser with saturated absorber is also researchedthrough simulation in this paper, we find that at the same settings, output performancewith saturated absorber is worse than that without saturated absorber: the outputthreshold increases from8.65mw to13.95mw, maximum output power decreasesfrom46.366mw to40.439mw, slope efficiency decreases from24.23%to21.74%.We can also find that optimal value of splitting ratio of erbium-doped fiber andcoupler does not change with saturated absorber, they are still2.5m and90/10. Whenthe pump power is200mw, the center wavelength and the3dB bandwidth of theoutput laser do not change, its side mode rejection ratio is about55dB. We find thatthe longer the length is, the higher the threshold power of the laser is, and that thelength of saturated absorber has little effect on maximum output power and slopeefficiency through comparative studies with saturated absorber of2m、4m and5m. Wealso find that the higher the concentration is, the higher the threshold of the laser isand that the change of the concentration has little effect on maximum output powerand slope efficiency these two indicators through comparative studies with saturated absorber of91024m3、1.81025m3and2.71025m3three differentconcentrations.4. We also do experimental study on the designed fiber laser with ring cavity inthis article, based on the conditions of the laboratory, we use a5m long erbium-dopedfiber as the gain fiber, a2m long erbium-doped fiber which is not pumped as saturatedabsorber and a coupler of70/30split ratio to build the laser. When pump powerincreases from0mw to200mw, threshold of the laser is measured as71.78mw, Outputlaser power is2.09mw and slope efficiency is1.63%. When we use opticalspectrometer to measure at200mw pump power, we can get that center wavelength ofoutput laser is1556.14nm,3dB, bandwidth is0.07nm, side mode rejection ratio isabout60dB, we observe the power for more than2hours finding that power is stableand there is no drift of the wavelength basically. Clear and stable singlelongitudinal-mode image is observed by the scanning interferometer of7.5GHz.Experiments show that the principle of the erbium-doped fiber laser with saturatedabsorber and ring cavity designed in this article is correct, the structure designed inthis article is reasonable, the methods using to output and measure single longitudinal-mode laser are viable and operational.