Middle Infrared Mode-locked Cr:ZnSe Laser Pumped By Thulium Doped Fiber Laser

Since first reported by the Lawrence Livermore National Laboratory?LLNL?in1996,transition metal doped II-VI lasers have exhibited excellent mid-infrared laser characteristics.Among them,Cr²⁺-doped II-VI semiconductor crystals feature favorable characteristics,especially Cr:ZnSe/S.Their characteristics include a four-level structure,an absence of excited state absorption and close to 100%quantum efficiency of fluorescence at room temperature.Their broad absorption bands cover a variety of fiber laser sources.At the same time,the ultra-wide emission bands can support the generation of few-cycle mid-infrared ultra-short pulses and large wavelength tuning range.Cr:ZnSe/S lasers have advantages of excellent room temperature performance,high electro-optical conversion efficiency,large wavelength tuning range,and low cost.These advantages can satisfy many demands,such as new emerging materials processing,medical treatment,scientific research and environmental detection.Especially,ultrashort pulse Cr:ZnSe/S lasers can be used in mid-infrared optical frequency combs,synchronous pumped OPOs and synchronously pumped Fe:ZnSe/S lasers.Limited by high-efficiency pump sources,high-quality crystals and mid-infrared coatings,there is a big gap between the domestic research level and the international advanced level.In view of this,this research project focuses on exploring the ways to generate ultrashort pulse Cr:ZnSe laser.We started from the pump source and built a high power 1908 nm thulium-doped fiber oscillator.Then,the wavelength tunability of the Cr:ZnSe crystal was studied by a diffraction grating.After that,a SESAM was used in the Cr:ZnSe laser to achieve mode-locking.The performance of this crystal about the generation of ultrashort pulses was investigated.An interferometric autocorrelator based on two-photon absorption was built to measure the pulse width of the mid-infrared mode locked pulses.Finally,a Cr:ZnSe laser synchronously pumped by a short-wavelength 1935 nm picosecond thulium-doped fiber laser was built.The research contents of this thesis include:1.Tunable and self-Q-switched Cr:ZnSe laser pumped by a 1908 nm thulium doped fiber laser.Firstly,a thulium doped fiber laser was built as the pump source of the Cr:ZnSe laser.The wavelength tunability of the single crystal Cr:ZnSe laser was studied by a diffraction grating.Self-Q-switched phenomenon was also observed in the Cr:ZnSe laser.The concrete content is as follows:1)A 1908 nm thulium doped fiber oscillator was built.A pair of 1908 nm FBGs was separately spliced to each end of a thulium-doped fiber to complete a laser cavity.The maximum output power of 30 W was obtained.To facilitate pumping the Cr:ZnSe laser,the output port of the 1908 nm thulium doped fiber oscillator was spliced with a pigtailed collimated isolator.2)A Littrow Cr:ZnSe laser was built with a mid-infrared diffraction grating and a continuous tuning range of 432 nm was achieved between 2284 nm and 2716 nm.With a 22%OC,an output power of more than 500 mW was achieved in the range of2350 nm to 2510 nm under a pump power of 5 W.3)Using an X-type cavity,we achieved a self-Q-switched operation.Under a pump power of 4.91 W,a maximum output power of 273 mW was achieved,corresponding to a slope efficiency of 4.9%.The maximum repetition rate was 1.28MHz and the corresponding pulse width was 160 ns.2.SESAM mode-locked Cr:ZnSe laser pumped by a 1908 nm thulium doped fiber laser1)In order to measure the pulse width of the mid-infrared ultrashort pulses at 2.4?m,a collinear interferometric autocorrelator based on two-photon absorption was built.2)A SESAM with a modulation depth of 0.6%was used to initiate mode locking and bound-states mode-locking was realized.In our experiment,two different sapphire plates with thickness of 3 mm and 5 mm were used to compensate the dispersion.Bound states with various pulse separations at different dispersion regimes were obtained.With a 4.5%OC,the maximum output power of 403 mW was achieved.3.A Cr:ZnSe laser synchronously pumped by a 1935 nm picosecond thulium doped fiber laserThe Cr:ZnSe laser was synchronously pumped by a thulium doped fiber laser.The fiber laser had an average power of 4.5 W,a center wavelength of 1935 nm,a repetition rate of 104.5 MHz and a pulse width of 112 ps.The fiber laser was a MOPA structure.A 10/130 single-mode double-clad polarization-maintaining thulium doped fiber was used in the main amplification stage.A 4.5 W linear polarization output could be obtained under a pump power of 14 W.A 104.5 MHz synchronous mode-locked output was achieved by matching the cavity length of the Cr:ZnSe laser with the pump laser.By fine-tuning the Cr:ZnSe laser,a 209 MHz harmonic mode-locked operation was achieved.An output power of 243 mW was obtained with a 4.5%OC under a pump power of 4.5 W,corresponding to a slope efficiency of6.8%.A chopper wheel was inserted into the Cr:ZnSe laser cavity and its self-starting performance was investigated.