Ultrafast Optical Properties Of Photonic Devices With Large Modulation Depth

Ultrafast fiber laser has attracted the attention of many researchers because of its low thermal effect,high pulse energy and excellent spatial-temporal resolution.So far,the ultrashort pulse technology based on fiber laser not only plays an important role in material processing,femtosecond time spectroscopy,nanoscale imaging and other fields,but also greatly promotes the innovation and development of related disciplines.At present,a variety of methods have been proposed to realize ultrashort pulse output based on saturable absorber,nonlinear polarization rotation,octagonal cavity and so on.Among them,saturable absorber(SA)is considered to be an economic and effective method to generate ultrashort pulse from picosecond to femtosecond.In recent years,many SA have shown excellent nonlinear optical properties in ultrafast photonics.However,the preparation of novel SA with high performance and further improvement of the performance of fiber lasers still need to be studied.This paper focuses on the research of high stability and narrow pulse fiber laser.By means of material design and improved preparation method,a new type of SA with high damage threshold and large modulation depth is prepared to improve the stability of fiber laser.At the same time,on the basis of the saturable effect,combined with the nonlinear polarization rotation technology,the more precise polarization state adjustment can be realized,and the ultrashort pulse with high power can be generated while ensuring the narrow pulse width.The specific research contents are as follows:(1)Mode locked fiber laser based on WTe2 S A.WTe2 S A was fabricated by combining magnetron sputtering with tapered fiber.The modulation depth of WTe2 SA is 37.95%,and the corresponding nonlinear absorption coefficient is-3.78x10-5 cm/W.Then,the mode-locked fiber laser is realized by WTe2 SA.The mode-locked threshold is 244 mW.When the pump power is 630 mW,the stable mode-locked pulse with the shortest pulse width of 164 fs and signal-to-noise ratio of 75 dB is obtained.(2)Fiber laser based on SnSSe.SnSSe thin films were prepared by chemical vapor deposition(CVD)and characterized.SnSSe SA was prepared by transferring the film to the center of the fiber ferrule.The modulation depth of the SA was as high as 57.5%.A Q-switched laser with a central wavelength of 1550 nm is built.The shortest pulse duration and the maximum output power of the Q-switched sequence are 547.8 ns and 11.14 mW,respectively.Then,we continue to optimize the intracavity dispersion and nonlinearity,and realize the mode-locked fiber laser based on SnSSe SA.The output ultrashort pulse has the shortest pulse width of 158.6 fs,which shows the potential of SnSSe in the generation of ultrashort pulse.(3)Mode locked fiber laser based on CoOF.CoOF nanosheets were prepared by rapid thermal annealing.CoOF SA with modulation depth of 56.79%was synthesized by transferring the nanosheets to the waist cone region of tapered fiber.The central wavelength of the mode-locked fiber laser based on CoOF SA is 1562 nm,and the output pulse has a maximum average output power of 20 mW and a minimum pulse width of 156 fs.The experimental results show the feasibility of CoOF in ultrafast photonics.