Experimental Study Of A Mode-locked Thulium Fiber Laser With A Square Wave Pulse Width Adjustable

As the optical fiber laser has the advantages of good beam quality,good heat dissipation characteristics,compact structure,high conversion efficiency and good stability,the fiber laser has attracted wide attention since the first laser in the 1960 s.On the one hand,since the 1990 s,with the increasing maturity of the laser technology,the thulium doped fiber laser with a wavelength of 2 ?m has also been well developed.The optical fib er laser at 2 ?m has an important application in optical fiber sensing,optical communication,laser radar,laser medical treatment,industrial processing,and it has become one of the research hotspots.On the other hand,the square-wave pulse can effectively suppress the influence of stimulated Brillouin scattering,which make it have high-energy.The fiber laser with square-wave pulse has important application in high-frequency clock signals,high-power laser drilling,signal processing systems,optical fiber sensing and so on.Combined with the forementioned two advantages,to realize square wave pulse output in thulium fiber laser has many advantages.In the thesis,based on nonlinear polarization rotation mode-locking technology,the thulium-doped fiber mode-locked lasers are investigated in experiment.The research content include the following two parts:1.Two types of square wave mode-locked thulium doped fibers laser have been successfully realized,based on the non-linear polarization rotation mode-locking technology.The total length of the resonant cavity is 125 m after adding 100 m single-mode fiber into the cavity.When the pumping power is increased to 14 W,the maximum average output power and the pulse width are 48.5 m W and 21.7 ns respectively.For the cavity lengths of 175 m and 225 m,when the pump power is increased from 6 W to 14 W,the tunable ranges of square wave pulse width are 7.1 ns ? 24.8 ns and 10.3 ns ? 30.6 ns respectively.The experimental results show that the square wave pulse width can be effectively controlled in a certain range by changing the length of cavity and the pump power.2.In further experiments,the cavity length is determined to be 125 m,and the angle of polarization controller is adjusted carefully.Three kinds of different shape pulses are observed,which are h-shaped pulse,ladder pulse and oblique top pulse,respectively.First,the three shape pulses have some characteristics of square wave pulse,such as radio spectrum has modulation characteristics,but the modulation depth of square wave pulse is deeper and the signal-to-noise ratio is higher.Secondly,the pulse widths of the three shape pulses increase linearly with thepump power.At the same pump power,the pulse widths of the three shape pulses are wider than that of the square wave.However,the pulses with the three shapes can not keep the smooth of pulse top.Last,when the pump power is increased to 14 W,the pulse widths of h-shape pulse,ladder pulse and oblique top pulse are 25 ns,58.6 ns and 55.8 ns,respectively.