| 3?5 ?m mid-infrared laser is located in the important atmospheric transmission window band and covers the absorption peak of many atoms and molecules which make it play an important role in many fields.The optical parametric oscillator(OPO)has many advantages such as compact volume,simple structure and excellent tunability and is an important method for mid-infrared generation.In this paper,a high efficiency,low threshold and multi-wavelength mid-infrared PPLN OPO is realized through the degeneracy of two signal beam.Meanwhile,with intracavity difference frequency mixing technology under wave vector matching condition,the mid-infrared parametric down conversion of various special laser sources is realized.The main content is as follows:Firstly,the research of degeneracy of signal beam theory in multiwavelength PPLN OPO was conducted.The basic model of dual-wavelength optical parametric oscillator is established under the condition of signal optical degeneracy based on the coupled amplitude equations for three waves to mix by Matlab.The effects of pumping temperature and crystal period on parametric gain and signal optical coupling interval in multi-parameter process are calculated and provides theoretical guidance for the process of signal optical degeneracy.Meanwhile,the conversion efficiency and threshold conditions of the dual-wavelength optical parametric oscillator under the condition of signal degeneracy are preliminarily simulatedSecondly,a confirmatory experimental study of dual wavelength PPLN OPO based on signal optical degeneracy was conducted.Dual-wavelength mid-infrared PPLN OPO is pumped by 1018 nm and 1080 nm dual wavelength fiber laser using parallel injection structure.By the tuning of the crystal period,the dual wavelength mid-infrared output is realized under the condition of signal optical degeneracy,and the span of two idlers is 662 nm(3117 nm and 3779 nm).Based on signal beam coupling,two interesting phenomena were observed for the first time.One is that the OPO threshold for 1018 nm and 1080 nm pump beams could be drastically lowered from 15.4 W to 266 m W and from 13.5 W to 1.38 W separately.The other is that the total idler power obtained when the OPO was simultaneously pumped by two fiber lasers was higher than sum of idler power obtained when the OPO was independently pumped by two fiber lasers,revealing obvious parametric gain enhancement.It also indicated that the two independent OPO processes became coherent when two signal beams were coupled.Thirdly,the study of dual wavelength PPLN OPO based on signal optical degeneracy and stimulated Raman scattering was conducted.Tunable dual wavelength mid-infrared OPO pumped by dual wavelength Raman fiber laser is realized for the first time by the tuning of the crystal temperature.The range of signal beam is 1607-1626 nm,and the range of idle beam is 3081 ? 3583 nm.Meanwhile,dual wavelength mid-infrared laser output with low threshold and high efficiency is realized by signal degeneracy.The total output power of dual-wavelength OPO is over 6.8 W when the Raman effect of 1060 nm pumping light is the strongest.The experiment of dual-wavelength mid-infrared OPO based on fiber stimulated Raman effect has been realized under the small signal condition for the first time.When two optical parametric processes cannot reach the OPO threshold simultaneously and the parametric down conversion process works at degenerate points and satisfies the matching condition of wave vector,the conversion efficiency is much higher than the efficiency in DFG process.Fourthly,the study of the parametric down conversion in mid-infrared with special low power laser source was conducted.We present the parametric down conversion of various low power special laser sources based on high intensity signal beam in cavity.The mid-infrared frequency conversion of 1120 nm low-power pulsed fiber laser is realized for the first time.By the tuning of the crystal temperature,both OPO and DFG parametric processes can meet the phase matching conditions.Finally,the 1120 nm pump wave was successfully transformed to 3593 nm mid-infrared wave at every power level and temporal characteristic.Both the slope conversion efficiency exceeded 20% and the pump-to-idler conversion efficiency was around 20% in two parametric processes in the whole experiment,which was the highest result,to the best of our knowledge,for pulsed DFG reports.Meanwhile,the waveform of pulsed pump beam can be preserved in DFG process.The experiment results exhibited great potential in frequency down-conversion of laser sources with lower duty cycle.For the first time,the mid-infrared frequency transformation of the 1064 nm single-frequency fiber laser under the condition of low power is realized,and the output of mid-infrared laser with high efficiency and narrow linewidth is realized.Besides,it is the first time to realize the mid-infrared frequency conversion of a broad spectrum light source under the condition of low power.Single-frequency and broadband low-power parametric down-conversion techniques are demonstrated and the parametric down conversion and broadband matching of spectral modulation source are preliminarily verified. |
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