Fiber-laser-pumped,mid-infrared Spectral-controllable Optical Parametric Oscillator

3～5 μm mid-infrared laser with special spectra shape is of great application value in many field.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.However,most of reports about OPOs with controllable spectra changed parameters of oscillators to realize the purpose and had complicate structures as well as less flexibility.Fiber lasers have a lot of advantages such as excellent beam quality,high brightness.The most important point is that fiber lasers can output 1 μm laser with controllable spectra.The main propose of this paper is researching spectrum controlling technique of fiber laser pumped mid-infrared PPLN OPO.It combined spectrum controlling technique of fiber lasers together with nonlinear frequency conversion of PPLN crystal to realize spectrum controllable mid-infrared PPLN OPO.The main content is as follows:Firstly,the research of spectral controlling theory of the PPLN OPO was conducted.Based on the coupled amplitude equations for three waves to mix,the gain characteristics of parametric oscillation was derived under the small signal approximation using transfer matrix.The causing reason of broadband phase-matching at certain wavelengths was explained.A 1060&1090 nm dual-wavelength(DW)fiber laser was built using the method of cascade seed injection and applied for pumping a PPLN OPO.The 3.1&3.4 μm DW idler beam with maximum power of 6.57 W was obtained.It was analyzed that the low-power 1090 nm pump beam interacted with intracavity signal beam and high-efficient phase-mismatched intracavity difference frequency generation(DFG)occurred between them.Its conversion efficiency was hundreds of times the theoretical value.This new phenomenon was researched and it was analyed that high-power intracavity signal beam and fiber pump sources were the main factors for causing this phenomenon.The frequency down-conversion of low-power pump source and multiwavelength mid-infared generation can be realized based on the high-efficient phasemismatched intracavity DFG.Secondly,the spectral controllable PPLN OPO based phase-mismatched highefficient intracavity difference frequency generation(DFG)was discussed.A 1060&1080 nm DW fiber laser was built and used to pump a PPLN OPO.DW mid-infrared radiation located at 3.1&3.33 μm and triple-wavelength mid-infrared radiation located at 3.1&3.3&3.33 μm was obtained at different 1080 nm pump power.The maximum triple-wavelength mid-infrared laser power reached 7.38 W.A triplewavelength fiber laser located at 1060&1065&1080 nm was built and used to pump a PPLN OPO.Mid-infrared radiation with four wavelengths was obtained which was located at 3.13&3.17&3.31&3.34 μm.The maximum idler power reached 8.7 W.A tunable DW mid-infrared PPLN OPO was realized which was pumped by a tunable DW fiber laser using cascade seed injection method.The 1060 nm pump beam generated 3.14 μm idler beam as well as 1.6 μm high-power intracavity signal beam.The 1070 nm～1090 nm tunable pump beam was successfully converted to 3227 nm～3413 nm mid-infrared range based on phase-mismatched high-efficient DFG between it and the 1.6 μm signal beam.The tunable DW idler power was over 6 W.This is the first time,to the best of our knowledge,that a tunable DW mid-infrared PPLN OPO was demonstrated which was pumped by a DW fiber laser.A PPLN OPO was pumped by a DW fiber laser which was realized by combined a high-power 1018 nm fiber laser and a low-power tunable fiber laser in parallel using a WDM.The 1018 nm pump beam was used to build parametric oscillation and generate high power intracavity signal beam while the tunable pump beam was used as the detecting beam.The phase-matched point was set at 1080 nm by controlling the PPLN temperature.Finally,the 1065 nm,1070 nm and 1075 nm pump beam was successfully converted into mid-infrared range based on the intracavity DFG between them and signal beam.By comparing the experiment results and theoretically simulated results,it was found that the parametric gain of DFG process at 1065 nm,1070 nm and 1075 nm was enhanced by 40 times,57 times and 91 times separately.Besides,the gain bandwidth of intracavity DFG process was broadened from ～2 nm to about 10 nm.Thirdly,the spectral controllable PPLN OPO was realized based on stimulated Raman scattering.A DW Raman fiber laser located at 1060&1111 nm was built based on fiber stimulated Raman scattering(SRS)effect and used to pump a PPLN OPO.Tunable DW mid-infrared laser around 3.1&3.6 μm was obtained by changing the temperature of PPLN crystal.The total DW idler power under maximum SRS effect ranged from 4 W to 6.4 W.To enlarge the wavelength gap between two idler beams,a high-power 1018 nm fiber laser was connected in parallel with a low-power tunable fiber laser and used to pump a PPLN OPO.The 1018 nm pump beam generated 1495 nm signal beam as well as 1506 nm Raman beam in PPLN crystal.The wavelength of tunable beam was set as 1080 nm so that the phase-matched DFG process occurred between it and the 1506 nm Raman beam.Finally,the DW mid-infrared radiation with large wavelength gap was obtained and located at 3189 nm and 3819 nm.The maximum total idler power reached 8.115 W.Fourthly,the fiber-laser-pumped broadband mid-infrared PPLN OPO was discussed.The experiment results of the amplified spontaneous emission(ASE)laser source pumped PPLN OPO carried out by our research group was analyzed again,and it was found that the pump source just located in the broadband phase-matching area of the PPLN crystal so that the conversion efficiency was relatively high and the generated idler beam was also broadband.A broadband ASE source was built whose central wavelength and bandwidth was 1066 nm and 12 nm separately.It was used to pump a PPLN OPO which was the same as that used in the previous experiment.Finally,under 50 W pump power,the idler power just reached 1.3 W and the conversion efficiency was just 2.41%.Besides,the bandwidth of the idler beam was just 37 nm.It can be concluded that the chief factor affecting the frequency conversion of broadband pump source was the bandwidth of phase-matched area.After then the PPLN OPO was also pumped by a narrow-linewidth fiber laser and the experiment results verified that the spectral intensity distribution was also very important to the output characteristics of broadband-laserpumped PPLN OPO.Then the paper analyzed three methods for how to achieve highefficient frequency conversion when the broadband pump source didn’t locate in the broadband phase-matching area and conclude their limitations.It can be speculated that the spectra shape of idler beam can be changed and its output bandwidth also can be further broadened if the shape of the corresponding gain spectrum can be controlled.Idler beams with different spectra shapes were obtained in the previous ASE pumped OPO experiment.Besides,under the influence of gain spectrum with particular shape,ultrabroadband idler beam was generated and its bandwidth was 16 times as large as that of the pump beam.The results verified the correctness of our speculation and was a new method for broadband mid-infrared generation.