Investigation Of The Difference Frequency Generation Of Infrared Source Pumped By Dual-wavelength Laser Chirped Pulse Amplifier

The infrared wavelength ranging from 15 to 20 ?m region belongs to the "molecular fingerprint region",in which the explosive molecules show obvious absorption characteristics.This kind of explosive material can be detected remotely by sending this waveband laser source to the scene and using the spectral technology to detect the return signal.There are mainly two methods to generate infrared ultrashort laser pulse,one is directly generated from the laser itself,which is mainly realized by free electron laser,lead salt semiconductor laser,and quantum cascade laser.Although free electron laser can achieve large infrared pulse energy,the cost is too high;the output power of lead-salt semiconductor laser is low,which needs low-temperature cooling;the bandwidth of quantum cascade laser is narrow,and it is difficult to achieve ultrashort pulse operation.The other method is realized by nonlinear optical frequency conversion,which is mainly realized by difference frequency generation,optical parametric amplification,and optical parametric oscillation.Among them,difference frequency generation is the most suitable method to generate the infrared ultrashort pulse.In this thesis,the output of infrared ultrashort pulse is generated by difference frequency generation based on the dualwavelength chirped pulse amplification system.The main innovations are as follows:Firstly,the chirped fiber Bragg grating is utilized to replace the grating stretcher for wavelength selection,the coupling efficiency of the system is increased and finally a compact high-average-power dual-wavelength Yb-doped fiber chirped pulse amplification system with 2.4 W output power is realized.Theoretically,the relationship among the power of signal,ASE and pump in the Yb-doped fiber amplifier are analyzed and calculated,and through which the optimum fiber lengths of the amplification system are determined by theoretical modeling;experimentally,a compact high-average-power dual-wavelength chirped pulse amplification system is designed and built,which delivers two synchronous pulses.The two pulses occupy two different wavelengths(1024 and 1088 nm)respectively.Secondly,a method of fiber end treatment based on a thick-walled quartz tube is proposed.The thick-walled quartz tube is used as the fiber supporting material,which overcomes the problems large-core fiber polishing(such as lacking end-cap,the hardness of metal and ceramic holder does not match that of the quartz).Finally,a high-quality polishing results of the large-core fibers are realized and the realization of high power large-core fiber laser system is guaranteed.Thirdly,the tight-focusing technique is utilized to precisely adjust the focused spot on the crystal for difference frequency generation,the average power of the difference frequency generation infrared source is 2.5 m W at 17.4 ?m.Theoretically,based on the transient nonlinear coupled-wave equation,the evolution of pump,signal and idler pulse with the thickness of Ga Se crystal in the process of difference frequency generation is simulated,and correspondingly,the relationship between the output power and the thickness of the crystal,the product of pump and signal power are simulated;experimentally,a difference frequency generation infrared laser system is designed and constructed,and the wavelength and average output power are measured.