Femtosecond Fiber Lasers Based On Gold Nanorods/D-shaped Fiber Saturable Absorber

Femtosecond laser is widely used in the fields of medical treatment,industrial processing,national defense and scientific research due to their ultra-short pulse width and ultra-high peak power.Passively mode locking is an effective way to realize femtosecond pulse laser.At present,the commonly used saturable absorber(SA)devices is semiconductor saturable absorber mirrors(SESAMs),but the fabrication of SESAMs is complex and expensive.With the development of nanotechnology,twodimensional materials,such as carbon nanotubes and graphene have attracted extensive attention from researchers.Their common characteristics are high third-order nonlinear coefficient,ultrafast response time,good fiber compatibility and easy preparation.Due to the advantages of higher third-order nonlinear coefficient,ultrafast response time and good fiber compatibility,gold nanomaterials are also regarded as an ideal SA.The saturable absorption characteristics of gold nanomaterials are originate from their surface plasmon resonance(SPR)absorption characteristics.In 2012,our group first realized Q-switched laser at 1.5 ?m by using gold nanomaterials as SA.The gold nanoparticles only have a single transverse SPR absorption peak at 520 nm.Although the aggregated gold nanoparticles can be used to obtained broadband absorption from 500 to 2000 nm,but the aggregation of gold nanoparticles is difficult to control.Compared to gold nanoparticles,gold nanorods(GNRs)have two SPR absorption peaks.One is transverse SPR absorption peaks,the other one is the longitudinal SPR absorption peaks,and can be tuned from visible to infrared wavelength through adjusting the aspect ratio of GNRs,which shows the potential to realize broadband pulse lasers.By using the GNRs as SA,broadband(visible-infrared)pulse lasers were realized by our group.However,high-power and long-term operation fiber laser were limited due to the strong photothermal effect of GNRs.In order to improve the performance of devices based on GNRs,it is of great significance to suppress or reduce the photothermal effect.During master's degree,the author systematically researched the composite of GNRs with evanescent fields and the application of SAs based on this structure.The research results are shows as follows:(1)Through research,the evanescent field effect will generate when light is transmitted in the low refractive index region of the fiber,and the intensity decays exponentially along the radial direction of the fiber,which can be used to increase the damage threshold of the device.The D-shaped fiber has the advantages of high mechanical strength and precise controllability of the evanescent field excitation region,so it is suitable for preparing this type of device.Based on the above ideas,the author proposed to use D-shaped fiber to construct a SA device with evanescent field effects.In the experiment,the cladding of the Hi1060 fiber was partially stripped to make a Dshaped fiber,and the GNRs/D-shaped fiber SA device was prepared by using the evanescent field effect and the GNRs composite.The intensity of the laser radiation received by the GNRs is reduced by 60% compared to the traditional "sandwich" structure.In addition,by adjusting the stripping length of the D-shaped fiber cladding,the effective area of the GNRs can be adjusted,so that the damage threshold can be effectively improved while the saturable absorption performance of the GNRs can be improved.The average aspect ratio of GNRs is 5.3,and the longitudinal SPR absorption peak is located at 900 nm.The absorption characteristics of the composite device were investigate by using theoretical model analysis and measured experimentally.The above results indicate that the device can be used to achieve broadband mode-locked lasers.(2)Further study the application of this type of SA at 2 ?m.The above GNRs/Dshaped fiber SA was placed into a ring cavity thulium-doped fiber laser,and the pump power was increased to 700 mW to achieve a mode-locked laser output with a center wavelength of 1950 nm and a pulse width of 373 fs.The maximum average output power of the pulsed laser was 60 mW.This result is 10 times higher than the maximum average power of the mode-locked laser achieved with a "sandwich" structure GNRs SA.It shows that the damage threshold of GNRs SA has been greatly improved.The long-term operation of the mode-locked lasers for 1.5 hours shows that the SA was not damaged by heat accumulation effect due to the exist evanescent effect..(3)Then,the application of GNRs/D-shaped fiber SA at 1.5 ?m was also investigated.It was placed in a ring cavity erbium-doped fiber laser.After the pump power was increased to 60 mW,the polarization controller was adjusted to achieve a femtosecond laser output with a center wavelength of 1560 nm and a pulse width of 421 fs.It is verified that GNRs/D-shaped fiber SA can be used as a broadband SA to achieve 1.5 ?m femtosecond laser output.