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近年来,高反射光学元件在激光陀螺[1]、引力波观测[2]、痕量气体检测[3-4]等领域应用越来越广泛。高反射光学元件的反射率直接关系到这些高精密激光系统的性能,如何准确测量高反射元件反射率越来越重要。文献[5]提出通过测量谐振腔相移的衰减相移方法来测量待测元件的反射率,该方法需要确保光源的方向强,相干长度短,这些要求导致在一般情况下测量精度难以保证。文献[6]提出时间衰减法,通过测量光在谐振腔中的衰减时间来确定其腔的损耗,从而得到待测光学元件的反射率。由于该测量方法具有不受光源波动的影响和待测光学元件的反射率越高、测量精度越高等优点,因此被广泛应用于高反射率测量[7-10]。2011年,光学干涉薄膜会议公布了由不同国家、不同机构,通过不同测量方法对同一批次的高反样品的反射率测量结果,发现仅采用时间衰减方法的测量精度高和一致性好,证明了时间衰减法是唯一能够精确测量反射率高于99.99%的方法[11]。目前已经出现许多不同类型的时间衰减技术,如采用脉冲光源[12]或窄谱连续光源[13]的光腔衰荡技术。本文采用半导体激光器作为光源的光反馈光腔衰荡技术[14-15],利用自混合效应(半导体激光器部分出射光经谐振腔反射回半导体激光器,使激光器的输出光谱发生漂移到谐振腔的谐振频率处),提高了激光与谐振腔的耦合效率,从而极大提高了反射率测量精度,而且无需隔离器和频率锁模简化了测量装置。根据菲涅耳公式,光斜入射到介质表面的反射是各向异性的,斜入射时薄膜S、P偏振光的反射率不同,因此准确测量薄膜S、P偏振高反射率显得日趋重要。以前是通过在谐振腔前加偏振片产生S和P线偏振光源或者在谐振腔后加偏振分束器将出射光中的S和P偏振光分开来分别测量谐振腔的偏振损耗[16-17],本文提出无需偏振光学元件,通过一束具有一定偏振比、光强方波调制的偏振光入射到谐振腔中,在方波下降沿记录衰荡腔出射信号,通过双指数拟合同时得到S和P偏振衰荡时间,从而计算得到待测高反射元件的S和P偏振反射率。
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当波长为λ的单色激光入射到如图 1所示的三镜谐振腔时,99%以上光直接由反射镜R3反射出去了,只有相当少经过反射镜R3的透射光,在谐振腔内来回多次反射,记录从出射腔镜R1的透射光。
假设谐振腔腔长(光从R1经R3反射到达R2的长度)为L;腔内介质(空气)的折射率为1;激光束在腔内往返一周的相位延迟总量为δ;g1和g3为高反镜R1和R3的透过系数;r1、r2和r3为高反镜R1、R2和R3的反射系数;入射光电场振幅为A;透射光电场振幅为At;光从谐振腔内传播到周围介质时,反射系数为r;当光从周围介质进入谐振腔内,其相应的系数为r';g3g3+r3r'3=1,r3=r'3,δ=4πL/λ。
当入射光束中既有S偏振分量又有P偏振分量,就需要将光束分解为两个初始相位差为0的S偏振电场分量As和P偏振电场分量Ap,可以得到由腔镜R1第n次透射电场振幅函数为:
$$ {A_\mathit{n}} = {A_s}{g_{3s}}{t_1}{({r_1}r_{3s}^2{r_2}{{\rm{e}}^{i{\varphi _s}}})^n}\mathit{\boldsymbol{j}} + {A_p}{g_{3p}}{t_1}{({r_1}r_{3p}^2{r_2}{{\rm{e}}^{i{\varphi _p}}})^n}\mathit{\boldsymbol{i}} $$ (1) 式中,i,j分别代表P偏振(水平方向)和S偏振方向(竖直方向)的单位矢量。
因此,第n次透射光强为:
$$ {I_n} = {A_n}A_n^*{\rm{ = }}A_s^2{T_{3s}}{T_1}{({R_1}R_{3s}^2{R_2})^n} + A_p^2{T_{3p}}{T_1}{({R_1}R_{3p}^2{R_2})^n} $$ (2) 式中,Ri=ri2=(i=1, 2, 3)为反射镜Ri的反射率;T1=g12;T3s=g3s2;T3p=g3p2。
n为光束在腔内的循环次数:
$$ n = \frac{{tc}}{{2L}} $$ (3) 式中,c为光速;t为光传输时间。据式(2)和式(3)可得:
$$I(t) = {I_{0s}}{{\rm{e}}^{\frac{c}{L}\ln ({R_{3s}}\sqrt {{R_1}{R_2}} )t}} + {I_{0p}}{{\rm{e}}^{\frac{c}{L}\ln ({R_{3p}}\sqrt {{R_1}{R_2}} )t}}$$ (4) 式中,I0s和I0p分别是激光束被关断后经过腔镜R1的S和P偏振光初始透射光强,可以表示为:
$$\left\{ \begin{array}{c} {I_{0s}} = A_s^2{T_{3s}}{T_1}\\ {I_{0p}} = A_p^2{T_{3p}}{T_1} \end{array} \right.$$ (5) 通过式(4)可看出谐振腔的出射信号是随着时间以双指数衰减函数衰减,定义S和P偏振衰荡时间τs和τp分别为从谐振腔出射信号S和P偏振分量的光强衰减到其初始值的1/e倍时所需要的时间,则有:
$$I(t) = {I_s}{{\rm{e}}^{ - \frac{t}{{{\tau _s}}}}} + {I_p}{{\rm{e}}^{ - \frac{t}{{{\tau _p}}}}}$$ (6) 式(6)为当入射光中既有S偏振分量又有P偏振信号的光腔衰荡技术基本公式。通过对谐振腔出射的光强信号以式(6)作为拟合函数进行双指数拟合得到S和P偏振衰荡时间τs和τp后,再根据式(4)和式(6)可以得出S和P偏振光的平均反射率分别为:
$$ \left\{ \begin{array}{c} {R_s} = {R_{3s}}\sqrt {{R_1}{R_2}} {\rm{ = }}{{\rm{e}}^{-\frac{L}{{c{\tau _s}}}}}{\rm{ }}\\ {R_p} = {R_{3p}}\sqrt {{R_1}{R_2}} = {{\rm{e}}^{-\frac{L}{{c{\tau _p}}}}} \end{array} \right. $$ (7) 采用直腔与折叠腔相结合的方法,同理测量直腔的平均反射率$\sqrt {{R_1}{R_2}} $,就可以得到待测光学元件R3的S和P偏振光反射率R3s和R3s。
Simultaneous Measurement of S-and P-Polarization Reflectivity Using Optical Feedback Cavity Ring Down Technique
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摘要: 提出采用光反馈光腔衰荡技术同时测量高反膜的S(垂直于入射平面)和P(在入射平面内)偏振反射率的方法,通过将一束具有一定偏振比、光强方波调制的偏振光入射到衰荡腔中,在调制方波下降沿记录从衰荡腔腔镜透射的光腔衰荡信号,通过双指数拟合得到S、P相应的衰荡时间,从而计算得到高反膜S、P偏振光的反射率。与采用纯S(P)光测量得到的偏振反射率相比,S和P偏振反射率偏差分别仅为1 ppm和15 ppm,证明了同时测量结果的正确性。与传统光腔衰荡测量高反膜偏振反射率方法相比,该测量方法装置简单,操作方便,测量更快速。Abstract: An optical feedback cavity ring down (OF-CRD) technique for simultaneously measuring the S-(perpendicular to the plane of incidence) and P-(in the plane of incidence) polarization reflectivity of highly reflective coatings is developed. Polarized light with a certain ratio of S-and P-polarization power and with square-wave modulated intensity is coupled into a ring-down cavity. The ring-down signal that leaks out from a cavity mirror is recorded at the negative edge of the modulation and fitted to a bi-exponential function to determine simultaneously the ring-down time for S and P polarizations. The reflection coefficients of highly reflective coatings for the S and P polarizations are therefore calculated from the determined ring-down time. Compared to the results obtained with purely S or P polarization, the differences for both S and P polarizations are respectively 1 ppm and 15 ppm, indicating the correctness of the simultaneous measurements. Compared to the conventional CRD approach, this simultaneous measurement method has the advantages of simpler configuration, easier operation, and higher speed.
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