| Ultrashort pulses produced by mode-locked laser have been attracting extensive attention in the fields of industrial processing,scientific research,medical imaging,laser surgery and laser weapons,due to short pulse duration,high peak power and wide spectral range.Ultrafast laser systems with high average power and pulse energy generally consists of mode-locked seed source and laser amplifier.Compared with solid seed source,the fiber seed source using rareearth ion doped fiber as the gain medium have been becoming hot topics in recent years due to their good heat dissipation,compactness,stableness,easy to start and good beam quality.Meanwhile,the solid gain medium was widely used in ultrafast laser amplifiers due to their high peak power and weak nonlinear effect.Therefore,the fiber-solid hybrid amplifier system is a better technical scheme for realizing high power ultrafast laser.Passively mode-locked oscillator play an important role in fiber mode-locked oscillators because of simple structure and short pulse duration.Several main methods of starting mode locking in passively mode-locked fiber oscillator have been proposed,including nonlinear polarization evolution(NPE),nonlinear amplifier loop mirror(NALM)and real saturable absorber.Wavelength tunability has not only enriched the functionalities of mode-locked oscillators themselves but also enabled numerous applications otherwise untouchable by modelocked oscillators with fixed wavelength.In solid gain media,ytterbium-doped crystals have been widely used in ultrafast laser amplification systems in recent year,due to mature manufacturing process,low quantum loss,wide gain spectrum and low-cost laser diode pump source.Compared with most ytterbium-doped crystals,the thermal conductivity of Yb:CALGO is excellent and the emission spectrum of Yb:CALGO is wider.A relatively high emission plateau exists between 990 nm and 1050 nm,and the total fluorescence bandwith is approximately 80 nm,which is very favorable for obtaining short output pulse in laser amplification systems.Therefore,the development potential of Yb:CALGO is great in high power ultrafast laser amplification systems.In this paper,the mainly content is the output characteristics research of mode-locked fiber oscillators based on different mode locking methods,Yb:CALGO continuous wave(CW)laser and Yb:CALGO regenerative amplifier.Firstly,based on the NPE,NALM and SESAM techniques,the stable mode locking pulse train was obtained in a variety of fiber oscillators with different structures,and the influence of intracavity dispersion on oscillator output parameters was investigated.And by utilizing the dispersion effect of grating pairs,wavelength tuning range of 62 nm and 90 nm were realized at 1.5 μm and 1 μm band respectively.Then,in 1μm band,six kinds of typical mode-locked fiber oscillator were crosswise compared and analyzed.Next,based on the Yb:CALGO,the characteristics of CW laser based on single and double crystal ware studied,respectively.Finally,using a wavelength tunable mode-locked fiber oscillator as the seed source,the high repetition rate Yb:CALGO regenerative amplifier was built.The specific research contents of this paper are as follows:1.Based on NPE technology,at 1.5 μm band,firstly,the pulse trains with a center wavelength of 1558 nm,a 3 dB bandwidth of 45 nm,a repetition rate of 174.83 MHz and a pulse duration of 120 fs was obtained in free-space mode locked fiber oscillator.Secondly,by using a polarization controller(PC)instead of spatial waveplates,the all-fiber mode-locked oscillator with 6.5 nm spectral bandwidth was achieved.Finally,the grating pair was introduced into the Er-doped fiber oscillator for the first time,and the continuous wavelength tunable mode-locked operation from 1532 nm to 1594 nm was realized.To the best of our knowledge,under the condition that the oscillator was mode-locked during the wavelength tuning,this is a new record for continuously wavelength tunable mode-locked Er-doped fiber oscillator based on NPE technology.At 1 μm band,the influence of intracavity dispersion on oscillator output parameters was investigated by introducing reflective grating pairs.When the intracavity dispersion was-0.001 ps2,the mode-locked pulse trains with a center wavelength of 1032 nm,a 3 dB bandwidth of 46.5 nm and a pulse duration of 323 fs was achieved.In addition,by utilizing Angle splicing of PM fiber method,The PM-NPE-based mode-locked fiber oscillator with a center wavelength of 1030 nm and a 3 dB bandwidth of 15.1 nm was realized.2.Based on NALM technology,all-fiber "figure-of-8" and "figure-of-9" mode-locked oscillators were achieved by utilizing the transmittivity and reflectivity of fiber loop,respectively.The bandpass filter was used in the "figure-of-8" oscillator to obtain the pulse trains with a center wavelength of 1064.5 nm,a 3 dB bandwidth of 2.6 nm and a repetition rate of 13.2 MHz.Using the dispersion compensation of chirped fiber Bragg Grating(CFBG),the"figure-of-9" oscillator with a center wavelength of 1030 nm,a 3 dB bandwidth of 13.5 nm and a repetition rate of 49.4 MHz was realized.The transmission grating pair was added into the linear arm of "figure-of-9" mode-locked fiber oscillator to better manage intracavity dispersion.In the near zero dispersion region,the pulse trains with a center wavelength of 1034 nm and a 3 dB bandwidth of 24 nm was obtained.By tilting the silver mirror after the grating pair,the center wavelength of free-space "figure-of-9" mode-locked fiber oscillator can be consecutively tuned from 1015 nm to 1105 nm,corresponding to a tuning range of 90 nm.To the best of our knowledge,this is the broadest consecutive tuning range in Yb-doped modelocked fiber oscillator.In addition,the mechanism of filter composed of silver mirror and grating pairs was tentatively analyzed and attributed to the combined action of the spatial dispersion induced by a tiltiing silver mirror and the limited aperture in the system.3.Based on SESAM saturable absorber,a linear cavity mode-locked fiber oscillator composed of CFBG and SESAM was built.When the intracavity dispersion was-0.0468 ps2,the mode-locked pulse trains with a center wavelength of 1030 nm,a 3 dB bandwidth of 12 nm and a repetition rate of 23.74 MHz was obtained.When the cavity structure was adjusted,the repetition rate of SESAM-based fiber oscillator can be increased to 95.36 MHz,but the 3 dB bandwidth of spectrum was only 4.3 nm due to the limitation of intracavity dispersion.4.On the basis of the previous reseach on 1 μm mode-locked fiber oscillator,from the perspective of demand for seed source of ultrafast laser amplification system,six kinds of typical mode-locked Yb-doped fiber oscillator were crosswise compared,including subjectively and qualitatively,objectively and quantitatively.To the best of our knowledge,this is the first time that fiber oscillators based on different mode locking methods were crosswise compared.5.By using Yb:CALGO crystal as the gain medium,after analyzing thermal lensing effect,the CW lasers were realized in single and double crystal short cavities respectively.The CW laser with a maximum output power of 25.9 W was achieved by using single crystal and single end pumping.When the dual-crystal structure was adopted,the maximum output power can reach 64.2 W.The root mean square(RMS)power stability was only 0.1%within one hour,and the beam quality M2 factor was below 1.2.To the best of our knowledge,this is currently the highest CW output power realized by Yb:CALGO crystal.Using the wavelength tunable mode-locked fiber oscillator with a central wavelength of 1050 nm and a spectral bandwidth of 1 6 nm as the seed source,in single crystal Yb:CALGO regeneration amplifier,the femtosecond pulse trains with a repetition rate of 1 MHz,an average power of 13.1 W,a single pulse energy of 13.1μJ and a pulse duration of 185 fs was obtained.When the filter was used to alleviate the gain narrowing effect,the puse duration can be further reduced to 156 fs,but the average output power was reduced to 10.4 W.Keeping the pulse repetition rate at 1 MHz,in dual-crystal Yb:CALGO regeneration amplifier,the femtosecond pulse trains with an average power of 27.5 W,a single pulse energy of 27.5 μJ and a pulse duration of 327 fs can be obtained. |
PDF Full Text Request