Broadband And Multiple-wavelength Quasi-phase Matching Structure Design

With the development of information technology and the requirement of society,the laser technology is becoming more and more important.Especially at the age of high-speed information optical devices have began to pay a key role.There is high demand of the laser light source,not only for high intensity output,but also for a wide spectral as far as possible.The nonlinear optical frequency conversion technique can broaden the laser light source.A variety of wavelengths obtained by frequency conversion can be applied to many different fields.One of the most common used technique in frequency conversion is quasi-phase matching(QPM).In recent years,the applied electric field polarization technique is mature,which makes the quasi-phase matching technique become more flexible and diverse.The structure design of frequency conversion based on quasi-phase matching technique has attracted much interest.From the periodic structure which realizes a single wavelength frequency conversion output initially,to subsequent periodic sequence structure,quasi-periodic structure,chiral structure and to non-periodic structure,structures have become more and more complex.And the dimension experiences one-dimensional,two-dimensional,even three-dimensional structure.Nonlinear processes become more complex and spectral output more diverse to meet all kinds of requirements.In order to achieve the QPM structure and provide good guidance for experimental fabrication,in this paper we study simultaneous multiple-wavelength output and wide band spectrum of frequency conversion.Sum frequency generation and second harmonic generation of the QPM crystals are studied.The principle of structure design is presented and influence factors are analyzed.The main contents include the following:First of all,based on nonlinear quasi-phase matching principle,several methods to design nonlinear quasi-phase matching structure are introduced.Secondly,we propose a method by introducing disorder into periodic structure for achieving broadband QPM frequency.The influence of the center length and randomness of the non-periodic structure on the broadband spectrum output is analyzed quantitatively.The effect of the total length of the crystal on conversionefficiency and bandwidth is also studied.By the proposed structure,high efficiency second harmonic output with broad bandwidth is obtained.That provides a good guidance for the research of second harmonic generation and the production of QPM device with high efficiency.Finally,from the perspective of intensity,an aperiodic structure is designed to achieve multi-wavelength output.By using Matlab simulation,the crystal structure is obtained and six simultaneous sum-frequency generation processes are realized successfully.By the introduction of weighing factor the relative optical intensities of six processes can be flexibly adjusted.