Multiple Solitary Pulses Based On All-normal-dispersion Yb-doped Fiber Laser

Fiber lasers are one of the important research topics in the field of lasers due to its good characteristics,such as high gain,high conversion efficiency,low threshold,simple structure,high stability,low cost and wide application prospects.Multiple solitary pulses laser can be used not only in time division multiplexing and optical comb,but also in distance measurement,laser communications,and optical fiber sensing as well as providing support for studying the complex nonlinearity of the laser system and improving the energy of the output pulse.In this thesis,based on the all-normal-dispersion(ANDi)Yb-doped fiber laser,phenomena and characteristics of multiple solitary pulses from the all-normal-dispersion(ANDi)Yb-doped fiber laser are investigated experimentally,which provides experimental evidence for further understanding the physical mechanism and characteristics of multiple solitary pulses in fiber lasers.The main contents and results are as follows:(1)Using a cascaded long-period fiber grating(CLPG)as a spectral filter and free space components in an all-normal-dispersion Yb-doped fiber laser(total length of the laser cavity is 240 m,and the length of the Yb-doped fiber is 0.18 m),phenomena of multiple solitary pulses are observed,which includes various modes of multiple solitary pulses:(a)double solitary pulses with fixed time interval,which is either 21.1 ns,33.6 ns,or 67.2 ns;(b)multiple solitary pulses with fixed and uniform time interval of 33.6 ns,the number of the pulses could be 3,4,5,6,and up to 7;(c)multiple solitary pulses with non-uniform but fixed time interval of 21.1 ns and 12.5 ns,the number of the pulses is up to 6.In the process of adjusting the polarization of the system,other modes of multiple solitary pulses are also observed,in which the intervals are not fixed but all are associated with 33.6 ns.The second harmonic of quadruple uniform solitary pulses and the third harmonic of triple uniform solitary pulses are observed,all the interval of the uniform solitary pulses is 33.6 ns.Physical mechanism of the formation of the multiple solitary pulses and the intervals between the multiple solitary pulses are analyzed and discussed.(2)The observed phenomena of the multiple solitary pulses in(1)are investigated experimentally in detail.Effects of the cavity parameters,such as cavity length,CLPG filtering,and polarizing on the multiple solitary pulses are performed.Results show that(a)when the cavity length is changed only(all other components in the system and the CLPG are fixed,central wavelength of the CLPG is 1034 nm and 3dB-bandwidth is 7 nm),for example,at 132 m and 350 m randomly,double solitary pulses can also be observed with the time interval being either 21.1 ns or 33.6 ns;when the cavity length is 132 m,the time intervals of the double solitary pulses of 67.2 ns,100.8 ns and 134.4 ns are also observed(integers of 33.6 ns);(b)when the CLPG is changed(central wavelength of the CLPG is 1036 nm and the 3dB-bandwidth is 8 nm),and the cavity length is fixed at 240 nm and all other components keep unchanged,double solitary pulses is also observed,in which the interval is either 21.1 ns or 33.6 ns.Furthermore,double solitary pulses can still be observed even if the CLPG is removed from the system,and the interval is also either 21.1 ns or 33.6 ns;(c)when the cavity length is fixed at 240 m and the CLPG is fixed with central wavelength of 1036 nm and 3dB-bandwidth of 8 nm,only the three waveplates(i.e.,one 1/2 waveplate,and two 1/4 waveplates)are replaced by the all-fiber polarization controller(PC),the double solitary pulses can also be obtained with the interval being either 21.1 ns or 33.6 ns by adjusting the polarization controller.(3)Further comparative studies of the multiple solitary pulses in an all-fiber Yb-doped laser are conducted when the cavity structure keeps the same as that in(1).(a)when the cavity length is 240 m,a CLPG is used as the filter,and the pumping power is kept the same as that in(1),similar phenomena to those in(1)of the multiple solitary pulses are observed,i.e.,double solitary pulses with fixed time interval of 21.1 ns or 33.6 ns;multiple solitary pulses with fixed interval of 33.6 ns;or multiple solitary pulses with non-uniform intervals of 21.1 ns,12.5 ns and 8.6 ns.(b)In order to further confirm the effects of key components in the system on the behavior of the multiple solitary pulses,an all-fiber Yb-doped laser with totally different components from those in(a)is established,in which the cavity length is 91.5 m,the Yb-doped fiber is 0.98 m,and the CLPG is removed from the system.It is found experimentally that stable double solitary pulses can also be obtained with the time interval of either 21.1 ns or 33.6 ns.