52,071901(2015) 激 与 电字学进展 Laser&Optoelectronics Progress  ̄2015((中国激光》杂志社 Analysis of Frequency Doubling Characteristics of Periodically Poled Crystal Xue Junwen Deng Kaiyong Fang yujie Pei Xuedan Su Binghua B啦 Institute of Technology,Zhuhai,Guangdong 519088,China Abstract Best matching temperature is obtained when the fundamental laser is of normal incidence in periodically poled crysta1.The in—depth analysis of second harmonic nonlinear conversion efficiency uses periodically poled crystal MgO:sPPLT as the research object,and combines the Sellmeier equation and the polar period with temperature thermal expansion relationship.When the fundamental laser tilt iS incident,the equivalent polar period becomes larger,and the best matching temperature is reduced.Acceptable temperature and wavelength bandwidth can be obtained conveniently by using the normalized frequency doubling efficiency curves for the given length of frequency doubling crysta1.The crystal length becomes longer,and the acceptable temperature and wavelength bandwidth become narrower.As long as the Sellmeier equation and polar period with temperature thermal expansion relationship are known,the method used in this study can be extended to other periodically poled crystals.These conclusions are useful for other periodically poled crystals and guiding continuous wave fiber laser external cavity frequency doubling. Key words nonlinear optics;laser;periodically poled crystals;nonlinear frequency coversion;frequency doubling 0CIS codes 】90.4400:190.4360:140.3515:140.3580:1 60.4330 周期极化晶体倍频特性分析 薛竣文 5垲镛 方宇杰 裴雪丹 苏秉华 北京理工大学珠海学院,广东珠海519088 摘要通过深入分析二次谐波转换效率,以周期极化晶体MgO:sPPLT为研究对象,结合其斯涅耳方程和极化周期随 温度的热膨胀关系,得到了基频光正入射时周期极化晶体的最佳匹配温度;当基频光倾斜入射时,极化周期等效变 大,最佳匹配温度降低。利用归一化倍频效率图形,可以方便地得到给定晶体长度的温度和波长接收带宽;同时晶体 通光长度越长,温度和波长接收带宽越窄。只要更换晶体的斯涅耳方程和极化周期随温度的热膨胀关系,所使用的 研究方法可方便地推广到其他周期极化晶体的倍频研究当中。所得到的这些结论对于使用周期极化晶体进行倍频, 尤其是连续光纤激光器腔外倍频有一定的指导意义。 关键词非线性光学;激光;周期极化晶体;非线性频率变换;倍频 E933.43;TN248 文献标识码A 中图分类号doi:10.3788/LOP52.07190l 1 Intro duction Frequency conversion is an effective means to expand the existing laser wavelength scope,and frequency doubling is one important approach.At present,to obtain high efficiency continuous laser ̄equency conversion.intracavity frequency doubling is a general method .However,obtaining high stability,low noise,and single frequency requires a careful design and alignment of the resonant cavity .With the development of laser technology,external cavity requency fdoubling using high power continuous wave(CW) laser by periodically poled nonlinear crystal is eliciting more attention 一 .Hence,optimization is needed for 收稿日期:2015—01—13;收到修改稿日期:2015—03—18;网络出版日期:2015—06—19 基金项目:广东省高等学校优秀青年教师培养计划(Yq2013208)、广东省高等学校优秀青年创新人才培养计划育苗工程 f自然科学)(2013LYM—OlO1)、北京理工大学珠海学院科研发展基金(2013JS02) 作者简介:薛竣文(1978一),男,博士,副教授,主要从事新型激光器及其应用等方面的研究。 E—mail:XUejunwenO01@126.corn 07】901一】 52.071901(2015 ————————————————嫩光与 电子学进展 ————————————————————————————————www.opticsjournal-net —一 the design of frequency doubling characteristics for periodically poled crystals・ Through in—depth analysis of the second harmonic nonlinear conversion efficiency,th s studY uses periodicaIIy poled crystal MgO:sPPLT as the research object and combines the Sellmeier equat on and the pOlar period with temperature thermal expansion relationship.The best matching temperature is obtained when the fundamental laser is in normal incidence into the periodically poled crysta1.When the fundamental laser tilt is incident,equivalent polar period becomes larger,and the best matching temDerature is reduced. Acceptable temperature and wavelength bandwidth can be obta ned cOnveniently via normalized frequency doubling efficiency curves for the given length of frequency doubling crysta1.The crystal length becomes longer,whereasa the acceptable temperature and becomes narrower.These conclusions are useful for other periodically poled wavelength bandwidth crystals and guidance for CW fiber laser external cavity frequency doubling・As long as the Sellmeie equation and polar period with temperature thermal expansion relationship are known,the method used in this study can be extended to other periodically poled crystals・ 2 Second harmonic conversion efficiency The second harmonic conversion efficiency formula for the conditions of the fundamental frequency light is not exhausted and the plane wave approximation is 。 叼_fl r¥  ̄n2c'g0 w/lsln。 一q- j’ 一、) (1) where 77 is second harmonic conversion efficiency,d。 is effective nonlinear coefficient of nonlinear crystal, L is the length of nonlinear crystal,n is the fundamental frequency light refractive index,rt2 is the requency doubling lfight refractive index,c is the speed of light in vacuum,so is vacuum dielectric constant, A is the wavelength of the fundamental frequency light in vacuum,P is fundamental frequency power, A is the average spot area for fundamental frequency light in the nonlinear crystal,Ak is the phase mismatch factor,and sinc(x)is expressed as sin(x)/x. For the chosen nonlinear crystal and certain optical focusing structure,the first term is constant,and does not make any specific analysis.The analysis is focused on the second term.The analysis of second harmonic conversion efficiency is changed into the analysis of the phase mismatch factor. 3 Analysis of phase mism atch factor Nonlinear requency conversion must meet conservation of energy and momentum 。 f一 一 一o, (2) (3) ~ 一 一o, where A。1,A。2,and A。3 refer to the wavelength in vacuum for three light participating in nonlinear effect,n¨rt2,and n,are refractive indexes corresponding to three light,and the index is the function of temperature and wavelength in genera1. The law of momentum conservation in Eq.(3)is actually the phase matching relationship;while for quasi phase matching,the relationship is A 01 一 一 一一A o2 A o3 A-o 1(4) where A is polar period,m is quasi phase matching order,and odds,such as 1,3,5,etc.,are adopted ,n is 1 in general and temperature change is considered.Meanwhile,the quasi phase matching nonlinear effect in the crystal is usually e light.Then,the phase mismatch factor becomes 0719O1—2 52,071901(2015) 2订 撒光与光电子学进展 一 一 n o3, T) A 03 www.opticsJournal net AlT1 1] (5) ,where n(A, )is the refraction index related to temperature and waye1engtheA( )is po1ar perj0d at the temperature of For frequency doublingthe phase mismatch factor becomes ,A =21T 』I ne 2 , ) 2n , ) 一 , (6) e ,where A 2 is wavelength in the vacuum for the frequen refractive indexes for the fundamental ̄equency and respectively. cy doubling light,nT) and ne 2 ,T)are fre quency doubling light at temperature From Eq・(6),one can conclude that the analysis of phase mismatch factor needs the SeIImeiequation of the periodically poled crysta1. er 4 Parameters of periodically poled crystal Given the excellent performance of MgO:sPPLT,this study analyzes its ̄equency doubling characteristics.The Sellmeier equation for MgO:sPPLT is E 。 n 2 一 + + A 一日 +DA: (7) Table 1 shows the parameters of the equation Table 1 Parameters for MgO:sPPLT Equation(7)is valid for the wavelength range of 0.39~4.1 Ixm,and the temperature range of 300℃~ 200℃. Considering the thermal expansion of the crystalpolar period is the function of ternperature : ,Am( )=A(25℃)【l十 ( 一25 cc)+卢( ~25℃) ], where A(25 cC)is polar period at the temperature of 25 oC and 7.97 is adopted7×10一 , f81 is 1.6×10 ,and is When Ak=0 is adopted in Eq.(6)and A 2 :A /2 is considered,polar period for the complete phase matching is )=等【 , ).. ).] 5 Best matching temperature ,(9) When the fundamental ̄equency light is incident into the crystal normallypolar period curves due to thermal expansion(red)and complete phase matching(pink)are drawn in the same graph using Matlab software(Fig・1・P refers to complete phase matching).By enlarging the graph,the intersection abscissa 0719Ol~3 52,071901(2015) 撒兴与光电子学进展 www.opticsjourna1.net of these two curves is 55.17 oC,which is the optimum phase matching temperature for normal incident \ \ \ ~一0。 …1 ~3 P \ \ \ \ \ \ \ \ \ Temper ̄ure/ ̄C Fig.1 Best matching temperature point graphic display However,light will deviate from the normal incident in experiments as shown in Fig.2 l I Fig.2 Light tilt incident into the crystal Equivalent polar period is obtained by refraction law: ( )= 以 ( ) COS西 (10) where refraction angle :arcs  ̄n雨F sin 0],and is incident angle Equivalent polar periods for l。and 3。as the function of temperature are shown in Fig1.Phase .matching temperature can be read as 55.04 oC and 54.0 1℃by enlarging the graphWith increasing tilt .angle results in increased refraction angle.The increase in the equivalent polar period will lead to the drop of phase matching temperature.Based on this characteristic,one can adjust the temperature to the correct direction.The experimental results also proved this point . 6 Relationship between acceptable bandwidth and crystal length The direct relationship between the normalized ̄equency doubling efficiency and temperature and wavelength is shown in Figs.3 and 4 for different lengths of the crysta1. From Fig.3,the best phase matching temperature is 55.17℃,which agrees with the result in Fig1. .From Fig.4,the best phase matching wavelength is 1064 nm. For different lengths of crystal,the results of temperature and wavelength acceptable bandwidths are 葑 10mm 曩 0.8 O.6 _ 20 mm 3Omm 40mm 耄 巷 主 -- 53 L .…一n . ~一 .。.~L.一0~一…52 54 55 56 Temperature/ ̄C 57 J 58 ,一 Wavelength/um Fig.3 Direct relationship between normalized efficiency and temperature Fig.4 Direct relationship between normalized efficiency and wavelength 071901—4 52,07l901(20l5) 激光与光电子学进展 www.opticsjourna1.net shown in Table 2.The crystal is longer,and the acceptable bandwidth of temperature and wavelength is narrower.While longer crystal indicates high frequency doubling eficifency from Eq.(1),the process will need high temperature control precision and narrow line width fundamental laser Table 2 Temperature and wavelength bandwidth 7 C onclusion Based on the second harmonic nonlinear conversion efficiency,using periodically poled crystal MgO: sPPLT as the research object,combined with the Sellmeier equation and the polar period with temperature thermal expansion relationship,the best matching temperature can be obtained when the fundamental laser is in normal incidence to the periodically poled crysta1.When the fundamental laser tilt is incident,equivalent polar period becomes larger,and the best matching temperature is reduced. Temperature and wavelength accept bandwidth can be obtained c0nVeniently by using normalized frequency doubling efficiency curves for given lengths of frequency doubling crysta1.When crystal length becomes longer,acceptable temperature and wavelength bandwidth become narrower.These conclusions are useful for other periodically poled crystals and in guiding cw fiber laser external cavity frequency doubling.As long as the Sellmeier equation and polar period with temperature thermal expansion relationship are known,the method used in this study can be extended to other periodically poled crystals. 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