Experimental And Applicational Studies Of Mode-locked Fiber Lasers Based On Saturable Absorber

Ultrafast pulses have multiple important applications in fields of economy,military,daily life,and so on.Mode-locked fiber lasers display several advantages,such as wide operating wavelength range,high gain coefficient,stable performance,simplity and compactness in cavity design.So mode-locked fiber lasers have been attracted a great deal of research interest.Nanophase materials have been utilized to fabricate saturable absorber(SA)in recent years.They exhibit significant advantages on integrated method and cost saving over conventional mode-locked technique.In our dissertation,we have experimentally investigated the performances of mode-locked fiber lasers in two aspects(cavity design and SA).We have obtained mode-locked fiber laser emitting various solitons and bisoliton fiber laser emitting bisoliton with multiple discrete equilibrium distances.The contents are as follows:1.In the linear cavity,by deploying the vestile fature of FBG in band with and wavelength as spectral filter,we obtain switchable fiber laser at different wavelengths.With the help of the polarization controller(PC),the pulses are emitted at different wavelengths.The fiber Bragg gratings(FBGs)are employed as end mirrors of the linear cavity,bisoliton fiber laser are realized with multiple discrete equilibrium distances.In the bisoliton fiber laser,multiple types of bound soliton are generated.Bisolitons have multiple discrete equilibrium distances with the quantized separations,which are confirmed by the theoretical analysis and the experimental observations.The interaction between bisolitons is also shown with numerical simulations.2.In the ring cavity,the ultrafast pulse laser is obtained by employing dispersion management technigue.Stretch pluse and dissipative soliton can be emitted separately.The operation direction of the laser in the cavity is controlled by the attenuator.The dispersion in the clockwise direction of the cavity is normal,so the dissipative soliton can be output.The dispersion in the counter-clockwise direction of the cavity is nearly zero,and the stretch pluse can be output.Attributing to the cavity asymmetry between two directions,two kinds of mode-locked pulses cannot be formed simultaneously.Although the two solitons have the similar central wavelength,the pulse width and spectrum bandwidth are different.3.By combining linear cavity and ring cavity together,the ultrafast pulse laser emitting conventional and dissipative soliton is realized.The FBGs are used as mirrors,and the linear cavity is embedded in the ring cavity.There are two cavities in one system:abnormal dispersion for the linear cavity and normal dispersion for the ring cavity.Via employing two FBGs as end mirror,the linear-cavity fiber laser delivers conventional soliton with the central wavelength of 1550 nm.Dissipative soliton with central wavelength of 1594 nm is output by the ring-cavity fiber laser.Two kinds of solitons have completely different spectrum,pulse duration and pulse energe.When two kinds of pulses pass through each other,they keep their soliton properties unchanged during their interaction.The results of numerical simulation agree well with the experimental observations.4.New-type fiber lasers are constructed by exploiting saturable absorption properties of nanophase materials as the mode locking device.The carbon nanotubes interact with evanescent field overflowed from the D-shaped fiber,and then ultrafast pulse at wavelengthes of 1529 nm,1531 nm,1532 nm and 1556 nm are produced by alternating the state of PC.The fiber laser can output stable ultrafast pulse under the maximum pump power of 550 mW.The saturable absorption propertie of molybdenum telluride(MoTe2)is employed in fiber laser.The SA is constructed by transferring the ultra-thin MoTe2 film onto the pinhole of fiber head.The S A made of ultra-thin MoTe2 film has excellent modulation depth,which enable the fiber laser self-starting mode-locked operation.5.We apply TIWDM(Tap-Isolator-WDM)device into mode-locked fiber lasers and effectively reduce the number of device in the fiber laser.Then we replace the WDM terminal pigtail fiber with Erbium-doped fiber(EDF),ultrafast pulse can be output,so we can reduce the fiber solder in the fiber laser and make fiber laser more compact,thus further miniaturization of the mode-locked fiber laser.