Experimental Studies On 8μm ZGP Cascade OPA

The long-wave infrared region range of 8-12μm wavelength is located in the atmospheric window and it is safe for the human eye, the light sources in this wavelength range have important application value in optoelectronic countermeasure, laser medical treatment, laser radar and toxic gas detection, etc, At present, optical parametric oscillation and optical parametric amplification are the main measures to obtain wavelength range of 8-12μm, through nonlinear transformation to convert laser of low wavelength to the long wavelength. ZPG crystal is commonly used in nonlinear frequency conversion because of high effective nonlinear coefficient, good mechanical strength, and high heat conductivity. The wavelength of the pump for ZGP crystal should be more then 2μm limited by the optical properties of ZGP crystal, and it is difficult to get high power laser with the wavelength range of 8-12μm. The purpose of this paper is to explore the power amplification effect of 8μm by cascade OPA method and whether this method has application value.Idle light of 2.8μm is used for pumping signal light of 8μm which are produced by ZGP OPO, this is the cascade OPA technology. According to polarization direction of the pump light, the phase matching of ZGP OPO and OPA can only be type I+type II or type II+type I, the former way is used in this article. By now, cascade OPA technology is not reported in the literature, firstly we need to validate the feasibility of this method. We used the pump light of large pulse energy to validate the feasibility of the method, and use the pump light of high average power to explore the influencing factors of OPA enlargement effect. So we set up two pump sources, the first set is a ZGP OPO pumped by a Ho: YAG laser whose output energy can achieve 30 m J at the frequency of 1 k Hz, the power of 8μm laser is 1.23 W and the power of 2.8μm laser 1.23 W; the second set is another ZGP OPO pumped by a Ho: YAG laser whose output power can achieve 110 W at the frequency of 16 k Hz, the output power of 8μm laser is 3.05 W and the output power of 2.8μm laser is 12.3 W.Optical parametric amplification is a kind of three wave mixing, we obtain the amplification coefficient formula of OPA by solving the three wave mixing coupling equations. the amplification coefficient is related to initial pump light intensity and ZPG needed crystal length. In the OPA experiment which is pumped by large energy, we make the pump and seed light overlap through a beam split and combination device, after OPA,the power of 8μm rise to 1.41 W from 1.23 W, the result demonstrate the cascade OPA method is feasible; In the OPA experiments which pumped by high power, we verify the OPA enlargement effect is concerned with degree of overlap, beam radius and the initial pump intensity. Finally, we optimize optical parametric oscillator and beam split and combination device, cascade OPA method improve average power of 8μm from 3.52 W to 4.7 W, total enlargement factor is 1.34. When the loss of beam split and combination device is taking into consideration, OPA enlargement factor is 1.42, this level reach the demand of the practical application.