High-power Fiber Lasers Pumped Mid-infrared Optical Parametric Oscillator

Optical parametric oscillator(OPO)based on periodically poled lithium niobate(PPLN)crystal is an effective way to realize mid-infrared(MIR)lasers around 3~5μm,which has a broad range of applications and an irreplaceable roles in many fields,such as environmental monitoring,laser radar,infrared remote sensing,precise spectrum analysis,medical diagnosis and treatment,military infrared countermeasure,and so on.Diode-pumped solid-state lasers are the most common pump sources for most of the traditional PPLN OPOs.However,such laser systems are very large and have significant thermal effect when operating at high power.Thus,the polarization and beam quality of the output laser will be seriously degraded,which is detrimental to the nonlinear frequency conversion process.Moreover,the spectrum of such laser systems is narrow and the wavelength is single,which make it hard to realize the flexible and fast control of the output mid infrared laser.Compared with the traditional solid-state laser pumped OPOs,fiber lasers pumped OPOs have a lot of advantages,such as flexible and compact structure,integration and miniaturization,strong practicability,low cost,stable and reliable work,strong ability to adapt to the environment,high brightness laser output,high efficiency,and so on.Highest on the list of benefits is the ultra-wide stimulated radiation bandwidth characteristics of fiber laser,which makes it possible to achieve widely tunable laser output in the near-infrared wavelength range.Based on the optical parametric oscillation technology,and along with the techniques of temperature tuning,angle tuning,periodically poled tuning,mid-infrared lasers with different spectrum properties can be achieved to meet different application needs.In addition,it is convenient to realize free and flexible switching between the integrated modules when fiber laser is used as the pump source,which plays a crucial role as a scaleplate.At present,the PPLN OPOs pumped by the traditional solid-state laser have achieved tens of watts mid-infrared output,and the research work of fiber laser pumped PPLN OPOs also achieved fruitful results.However,the output power is hard to reach hectowatt scale.In this paper,researches on the mid infrared optical parametric oscillator pumped by high power continuous wave fiber laser were carried out.The power limiting factors were analyzed,which can be guidance to a single high brightness CW mid infrared optical parametric oscillator.Moreover,OPOs pumped by fiber lasers with different spectral and temporal characteristics were also discussed,and multi-beams combination technology were used to achieve high power mid-infrared lasers.All this work established the foundation for high power,excellent beam quality,and high stability MIR solid state laser source.The main contents of this dissertation are listed as follows:Firstly,by using high power continuous wave quasi-single frequency fiber laser as the pump source,a mid-infrared optical parametric oscillator based on the MgO: PPLN nonlinear crystal was built.The highest output power of the system was 34.2W,and the wavelength tuning range of output laser covered 3.2~3.9μm,which represented the highest level in the international public reporting at present for the CW mid-infrared OPO laser.Secondly,the basic model of continuous wave signal singly resonant optical parametric oscillator with external cavity was established.Two major thermal effects among the MgO: PPLN crystal under high output power were analyzed by using the finite element analysis method: the axial temperature distribution and transverse temperature distribution.The results showed that the idler laser is the main reason leading to non-uniform temperature distribution,which results in a slight change of the poling period of the crystal.And it inevitably leads to wavelength shift and decrease of the nonlinear conversion efficiency.The nonuniformity of the transverse temperature distribution of the crystal leads to the distortion of the beam while propagating in the nonlinear crystal,which results in the deterioration of the beam quality of the output laser.Thirdly,the incoherent light has special temporal and spectral characteristics,which can effectively prevent crystals from laser-induced damage caused by self-pulsing effect of the common single mode fiber amplifiers.It is expected to obtain a higher output power for the mid-infrared optical parametric oscillator.For the first time,a continuous wave singly resonant MgO: PPLN optical parametric oscillator pumped by an all-fiberized MOPA structured amplified half-cavity random fiber laser and a full-cavity random fiber laser were reported,respectively.Finally,idler lasers of 2.46 W @ 3752 nm and 4.35 W @ 3271 nm were achieved.The experimental results were of great interest in terms of both investigation of optical parametric processes in the case of continuous spectrum and extension of the range of random fiber laser applications.Next,the amplified spontaneous emission(ASE)fiber sources provide an effective approach to realize ultra-stable,high-power,broad-bandwidth MIR laser sources.ASE fiber source has attracted a great deal of attention for its feature to generate excellent spatial coherence,low temporal coherence,broad spectrum emission,free of longitudinal mode,easy realization of spectrum control and other advantages.Base on this,an ultra-stable,broadband,mid-infrared OPO was established by using a super-fluorescent fiber as pump source.The output power of the MIR laser was up to 11.3W with high beam quality,which exhibited a peak-to-peak stability better than 1.9% and the standard deviation was better than 0.4% RMS in about 1h at the maximum output power.And the idler spectrum was centered on 3325 nm with a FWHM line-width of 72 nm.To the best of our knowledge,this is the highest level in the international public reporting at present of ASE fiber source pumped mid-infrared OPO.Then,according to the variable spectral characteristics of fiber lasers,combined with the operating characteristics of optical parametric oscillator,for the first time,the experimental work about a high power dual-wavelength mid-infrared OPO pumped by a cascade dual-wavelength fiber laser was carried out.A total idler output power of 6.36 W with dual idler-waves at 3125 nm and 3323 nm was obtained under the maximum pump power.It was interesting that only one signal laser was measured in the experiment,which meant a novel method for generating multi-wavelength mid-infrared laser based on intra-cavity difference frequency generation and optical parametric oscillation.Finally,limited by the thermal effects and laser-induced damage properties of the nonlinear crystals,MIR output power of single OPO is hard to get further promoted with excellent beam quality,which seriously cumbers its use in many applications.Multi-beams combination technology is a good choice for achieving high output power with good beam quality.A mid-infrared laser with output power of 12 W at 3.78 μm is obtained by using polarized combination of double beam mid-infrared optical parametric oscillators,and the beam quality of the combined beam achieved 1.2.Based on that,an optical parametric system with MIR output power of 30 W at 3130 nm,3352nm,and 3670 nm was finally obtained by using spectral beam combination of three idler MIR OPOs,and the beam quality was kept within two times of the diffraction limit.Moreover,an hectowatt-level MIR optical parametric system was achieved by multiple space beam combination scheme,which laid the foundation for the realization of high power solid-state laser source.