2?m All-fiber Passively Mode-locked Fiber Laser

Fiber lasers operating at 2 ?m have attracted more and more attention because of their wide applications in biomedical science,ultrafast optics,lidar,materials processing and another field.At present,the mode-locked technology mainly used for 2?m mode-locked fiber lasers is passively mode-locked.The commonly used passively mode-locked devices are mainly various saturable absorptive materials(such as graphene,carbon nanotubes,black phosphorus and so on),although the above saturable absorber is successful to a certain extent,there is a problem that they should be made into "sandwich" type,D type and cone and other device structure,which makes the overall structure of fiber lasers become complex,so the degree of system integration decline,which is not conducive to practical engineering applications.In this paper,a 2?m full fiber-based passively mode-locked fiber laser design based on fiber saturable absorber(FSA)is proposed.In this scheme,the use of FSA not only can achieve a very good passively mode-locked,but also can reflect the advantages of all-fiber laser structure,which greatly expanded the application scope of such lasers.But there is still problem that the output power of this laser is low,in order to further improve the output power of 2 ?m passively mode-locked fiber lasers,a 2 ?m all-fiber passively mode-locked fiber laser based on nonlinear amplifying loop mirror(NALM)was investigated.Through the implementation of the above scheme,a 2 ?m all-fiber passively mode-locked fiber laser with higher output power is obtained.Since this type of fiber laser has the advantages of high output power,simple structure and high integration degree,it has have important value to popularize in practical application.The main contents of this paper are divided into three parts:Firstly,the research status of 2 ?m passively mode-locked fiber lasers at home and abroad were investigated.It is found that there are many saturable absorber materials at present.However,due to the special structure of the materials involved in the literature,the laser structure is complicated,improving the operating costs.Based on this,a technology design scheme that uses FSA to achieve passively mode-locked was presented.The scheme can realize the all-fiber operation of the laser while realizing the passively mode-locked,thus greatly simplifying the structure of the laser and has a good application prospect.Secondly,combining the above design scheme,a 2 ?m all-fiber passively mode-locked fiber laser based on FSA is realized on the basis of the theory of optical fiber saturable absorber mode-locked.by means of reasonable selection of gain fiber,pump source,experimental equipment and saturable absorber,a mode-locked pulse at central wavelength of 2010.11 nm is obtained,the repetition frequency,maximum average output power,power instability,3 dB spectral bandwidth,pulse width and spectral SNR are 9.63 m W,0.33%,0.16 nm,28.2 ns,3.76 MHz and ~37 dB,respectively.Although the desired design goals of all-fiber fiber laser has been achieved,but in view of the needs of practical engineering applications,the key technical indicators that output power still need to be improved.Finally,a 2 ?m all-fiber passive mode-locked fiber laser based on NALM is designed and implemented by introducing NALM passive mode-locking theory in order to solve the problem that the output power can not satisfy the actual requirement in the above scheme.When the pump power is 0.65 W and the angle of polarization controller is slightly rotated,a stable mode-locked pulse at a central wavelength of 1956.7 nm is obtained.The repetition frequency,maximum average output power,power instability,pulse width and spectral SNR are 2.45 MHz,15.48 m W,0.79%,3.8 ns and ~44 dB,respectively.Compared with the fiber-based saturable absorber passive mode-locking program,the power has been greatly improved.