| High-repetition-rate mode-locked fiber lasers have a wide range of applications in high-speed optical sampling,optical arbitrary waveform generation,material micromachining,calibration of astrophysical spectrographs and nonlinear optical imaging.At present,there are two main ways to achieve high-repetition-rate mode-locked pulse:actively mode locking and passively mode locking.Compared with the actively mode-locked method that requires the introduction of a high-frequency signal generator or other additional modulation devices,the passively mode-locked method uses the saturable absorber as a part of the resonant cavity,which is simpler,more compact and more stable,and the obtained pulse width is narrower.Moreover,saturable absorbers mainly include graphene saturable absorbers(GSAs),carbon nanotubes(CNTs),SESAM,etc.Among them,compared with GSAs and CNTs,SESAMs have the advantages of more stability,high damage threshold with the low pump threshold,and more commercially available.So,SESAMs are widely used in mode-locked fiber lasers.However,in high-repetition-rate mode-locked fiber lasers,the repetition rate is related to the length of the resonator cavity.In order to obtain pulsed laser with a high-repetition-rate,the length of the resonant cavity needs to be short enough,that is,the fiber with a high gain coefficient is necessary.Fiber lasers in the 1.5 μm band have the advantage of eye safety and are widely used in laser ranging,optical communication,medical and other fields.The gain fibers for obtaining 1.5 μm laser output are mainly Er-doped silica fibers(EDFs)and Er/Yb co-doped phosphate glass fibers(PGFs).Usually,the gain coefficients of EDFs are low due to the low Er doping concentration in the substrate of SiO2,and it is difficult to realize the pulsed laser output in the ultra-short cavity.Although Er/Yb co-doped PGFs can achieve high Er doping concentration,due to the doping of phosphorus element,the chemical properties of the fibers are unstable,the damage threshold and mechanical strength are low,and they are easy to be broken.Therefore,it is valuable to explore novel gain fibers with high-rare-earth doping concentration,high-chemical stability and mechanical strength.In this paper,some Er:YAG-SiO2 fibers with different core diameters(Er:YAG crystal-derived silica fibers)were prepared and characterized.Based on these fibers,several 1.5 μm stable mode-locked pulse fibers laser with high-repetition-rate over 6 GHz were constructed.At the same time,Er/Yb:YAG-SiO2 fibers with different doping ratios(Er/Yb co-doped YAG crystal-derived silica fibers)were also prepared and studied.And the fibers with the highest gain coefficients were used as the gain medium generating the mode-locked laser pulse with a repetition rate over 8 GHz.The specific contents are as follows:(1)The drawing process of Er:YAG-SiO2 fibers with different core diameters and doping concentrations of 5at.%is introduced.By selecting Er:YAG crystals with different diameters and matching silica tubes,setting different drawing parameters,and adopting a secondary drawing method at～1980 ℃,Er:YAG-SiO2 fibers with the outer diameters of 125 μm were obtained.The core diameters are 12.3 μm,8 μm and 4.5 μm,respectively.Through the element distribution test,it was found that the Er elements were evenly distributed in the fibers’ core.And the gain,absorption and transmission characteristics were measured:the small-signal gain coefficients of the fibers at 1550 nm are 1.46 dB/cm,1.60 dB/cm and 1.74 dB/cm;the absorption coefficients at 976 nm are 1.94 dB/cm,2.71 dB/cm,1.53 dB/cm;the transmission losses measured at 1064 nm wavelength are between 3.60 dB/m and 6.12 dB/m.The experimental results show that Er:YAG-SiO2 fibers which called the special silica fibers with yttrium aluminosilicate as the base material have the high gain characteristics,mechanical and chemical stability,and can be used as the excellent gain mediums for ultra-short resonator lasers.(2)Several Er/Yb:YAG-SiO2 fibers with different Er and Yb elements doping concentrations were prepared by the co-melt in tube method.Selecting the semi-cylindrical shaped Er:YAG and Yb:YAG crystals with the same diameters,then splicing two semi-cylindrical crystals with different doping into the mixed crystals and inserting them into matching silica tubes.The Er/Yb:YAG-SiO2 fibers with same inner diameters and outer diameters of about 14.5 μm and 125 μm were obtained by secondary drawing method at 2000℃ with different drawing parameters.Through the element distribution test of the obtained six co-doped Er/Yb:YAG-SiO2 fibers,it is found that Er and Yb elements are uniformly distributed in the fiber’s core.It shows that the co-melt in tube method is an effective way to realize the preparation of Re:YAG-SiO2 fibers doped with various rare earth elements.Their gain,absorption and transmission losses were measured:the small signal gain coefficients of the co-doped fibers at 1550 nm was between 1.72 dB/cm and 2.31 dB/cm,which was related to the doping concentration of Er and Yb elements;the highest absorption coefficient at 976 nm is 23 dB/cm,which is 8.4 times than Er:YAG-SiO2 fibers;the transmission losses measured at 1064 nm were between 5.3 dB/m and 7.42 dB/m.Compared with Er:YAG-SiO2 fibers,Er/Yb:YAG-SiO2 fibers have higher gain and pump absorption coefficient.They will perform better as the gain mediums for short-cavity lasers.(3)Several 1.5 μm mode-locked pulsed lasers with GHz-repetition-rate based on the homemade Er:YAG-SiO2 fibers were studied.A linear cavity structures was used in the laser,and a SESAM was used as the saturable absorber.The gain fibers lengths of 1.55 cm and 1.45 cm(core diameter of 12.3μm)were used to realize the high fundamental frequency of 6.27 GHz and 6.8 GHz,respectively.And the influence of fiber parameters on the establishment of continuous-wave mode-locked pulses was analyzed.Finally,using a 1.3 cm long gain fiber with a core diameter of 8 μm,a pulsed laser with a repetition rate of 7.45 GHz was further realized.The center wavelength was 1563.72 nm,the pulse width was 9.24 ps,and the signal-to-noise ratio(SNR)was over 55 dB.(3)Two types of Er/Yb:YAG-SiO2 fibers with higher gain coefficients were used as the gain mediums.Based on the 1.16 cm and 1.12 cm long 7.5at.%Yb:YAG+5at.%Er:YAG co-doped fibers,the pulsed lasers with high repetition rate of 8.25 GHz and 8.5 GHz were realized,respectively.Among co-doped fibers,the core crystal with the highest gain coefficient was 5at.%Yb:YAG+5at.%Er:YAG co-doped.With a length of 1.05 cm,a pulsed laser with the repetition rate of 9 GHz was achieved.The center wavelength was 1568.93 nm,the pulse width was 4.97 ps,and the SNR was over 60 dB. |
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