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马赫-曾德尔干涉装置在工程应用中通常用来测量光学零件或光学系统的像差[13],基于马赫-曾德尔干涉原理的干涉光发射装置具有对远场相干光良好的可控性,其原理如图 1所示。通过装置中分束镜角度的微小偏转可以实现远场相干光场的微调,从而获得理想的远场干涉图样。图中M0, M1, M2和M3是与水平成45°放置的薄透镜,为获得明暗相间的干涉条纹,将分束镜M0顺时针转动θ1。一束高斯光沿z轴方向入射到改装的马赫-曾德尔干涉仪的分束镜M0,经过M0分成两束光l1和l2,继续沿z轴方向传播入射到反射镜M1,经M1反射沿垂直于z轴的y轴方向传播,最后经分束镜M3反射沿z轴方向出射;l1经M0反射,反射光偏转2θ1传播到达反射镜M2,经M2反射继续传播到达分束镜M3,经M3折射后沿与z轴夹角2θ1方向射出,由于M0旋转造成两束光相位差的存在,使得两束光在远场发生干涉。
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如图 1所示,l1和l2分别在分束镜M3的C点和E点处射出,设E点到$\overline {BC} $的距离为a,则由图可得${\rm{tan}}(2{\theta _1}) = \frac{a}{{l + a}} $,从而解出$ a = \frac{{l{\rm{tan}}(2{\theta _1})}}{{1 - {\rm{tan}}(2{\theta _1})}}$, 其中, l是透镜间距。设C点坐标为$ \left( {0, \frac{a}{2}, \frac{a}{2}} \right)$,则有E点坐标为$\left( {0, - \frac{a}{2}, - \frac{a}{2}} \right) $。一束基模高斯光束经分束镜M0分为从C点和E点射出的l1和l2两束光到达距离激光发射装置为L的接收屏上,当扫描时间为t时,设相干高斯光束的重心在接收屏上的坐标为(Δx, Δy),则有:
$ \left\{ \begin{array}{l} \Delta x = {\omega _x}L\left( {t - L/c} \right)\\ \Delta y = {\omega _y}L\left( {t - L/c} \right) \end{array} \right. $
(1) 式中,ωx, ωy分别为探测激光沿x轴正方向和y轴正方向的扫描角速度,c为激光在自由空间中的传播速度。
图 2为相干高斯光束远场扫描示意图。为便于后面的复杂公式推导,引入q参量来简化高斯光束光场的表达形式。假设扫描方向为沿y轴负向,则扫描时间为t时,l1和l2两束光在接收平面上的复振幅分布可以分别表示为[14-15]:
$ \begin{array}{*{20}{c}} {{U_{{l_1}}}\left( {x,y,L} \right) = {U_0}\frac{{{w_0}}}{{w\left( L \right)}}\exp \left[ { - {\rm{i}}\left( {kL + \arctan \frac{L}{g}} \right)} \right] \times }\\ {\exp \left[ { - {\rm{i}}k\frac{{{{\left( {x - \Delta x} \right)}^2} + {{\left( {y - a/2 - \Delta y} \right)}^2}}}{{2q\left( L \right)}}} \right] \times }\\ {\exp \left[ { - {\rm{i}}ky\sin \left( {2{\theta _1}} \right)} \right]} \end{array} $
(2) $ \begin{array}{*{20}{c}} {{U_{{l_2}}}\left( {x,y,L} \right) = {U_0}\frac{{{w_0}}}{{w\left( L \right)}}\exp \left[ { - {\rm{i}}\left( {kL + \arctan \frac{L}{g}} \right)} \right] \times }\\ {\exp \left[ { - {\rm{i}}k\frac{{{{\left( {x - \Delta x} \right)}^2} + {{\left( {y + a/2 - \Delta y} \right)}^2}}}{{2q\left( L \right)}}} \right]} \end{array} $
(3) 式中,
$ \frac{1}{{q\left( L \right)}} = \frac{1}{{R\left( L \right)}} - {\rm{i}}\frac{\lambda }{{{\rm{ \mathsf{ π} }}{w^2}\left( L \right)}} $
(4) $ \left\{ \begin{array}{l} w\left( L \right) = {w_0}\sqrt {1 + {{\left( {\frac{L}{g}} \right)}^2}} \\ R\left( L \right) = L + \frac{{{g^2}}}{L} \end{array} \right. $
(5) 式中,w0为高斯光束的束腰半径,即z=0处的w值,g为高斯光束的焦参量,g=πw02/λ, 波数k=2π/λ。则两束光在距离为L处的接收屏上的相干光场的复振幅分布U为:
$ \begin{array}{*{20}{c}} {U = {U_{{l_1}}} + {U_{{l_2}}} = {U_0}\frac{{{w_0}}}{{w\left( L \right)}}\exp \left[ { - {\rm{i}}\left( {kL + \arctan \frac{L}{g}} \right)} \right] \times }\\ {\left\{ {\exp \left[ { - {\rm{i}}k\frac{{{{\left( {x - \Delta x} \right)}^2} + {{\left( {y - a/2 - \Delta y} \right)}^2}}}{{2q\left( L \right)}}} \right] \times } \right.}\\ {\exp \left[ { - {\rm{i}}ky\sin \left( {2{\theta _1}} \right)} \right] + }\\ {\left. {\exp \left[ { - {\rm{i}}k\frac{{{{\left( {x - \Delta x} \right)}^2} + {{\left( {y + a/2 - \Delta y} \right)}^2}}}{{2q\left( L \right)}}} \right]} \right\}} \end{array} $
(6) 因此,两束干涉高斯光束在接收屏上的相干光场的光强分布为:
$ \begin{array}{*{20}{c}} {I = {U^ * }U = U_0^2\frac{{w_0^2}}{{{w^2}\left( L \right)}} \times }\\ {\left\{ {2 + \exp \left[ {{\rm{i}}k\frac{{a\left( {y - \Delta y} \right)}}{{q\left( L \right)}}} \right]\exp \left[ { - {\rm{i}}ky\sin \left( {2{\theta _1}} \right)} \right] + } \right.}\\ {\left. {\exp \left[ { - {\rm{i}}k\frac{{a\left( {y - \Delta y} \right)}}{{q\left( L \right)}}} \right]\exp \left[ {{\rm{i}}ky\sin \left( {2{\theta _1}} \right)} \right]} \right\} = }\\ {U_0^2\frac{{w_0^2}}{{{w^2}\left( L \right)}}\left\{ {2 + 2\cos \left[ {k\frac{{a\left( {y - \Delta y} \right)}}{{q\left( L \right)}} - } \right.} \right.}\\ {\left. {\left. {ky\sin \left( {2{\theta _1}} \right)} \right]} \right\}} \end{array} $
(7) 式中, U*为U的共轭。
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由(2)式和(3)式两束光在接收屏上的复振幅表达式可知,在马赫-曾德尔干涉装置中,经由分束镜M0产生的两束相干光的光强相同,设为I0,即有Il1=Il2=I0。则两束相干光在远场接收屏上某一点处的相干光强分布还可以表示为[16]:
$ I = 2{I_0} + 2{I_0}\cos \left( {k\Delta S} \right) $
(8) 式中,ΔS为l1和l2传播到距离为L处的接收屏上点P(x, y, L)的光程差。
由(2)式可以得到I0的表达式为:
$ \begin{array}{*{20}{c}} {{I_0} = {U_{{l_1}}}U_{{l_1}}^ * = U_0^2\frac{{w_0^2}}{{{w^2}\left( L \right)}} = }\\ {U_0^2\frac{{{g^2}}}{{{g^2} + {L^2}}} = U_0^2\frac{{{{\rm{ \mathsf{ π} }}^2}w_0^4}}{{{{\rm{ \mathsf{ π} }}^2}w_0^4 + {\lambda ^2}{L^2}}}} \end{array} $
(9) 结合(7)式和(8)式可以得到距离为L处的接收屏上点P(x, y, L)的光程差为:
$ \Delta S = \frac{{a\left( {y - \Delta y} \right)}}{{q\left( L \right)}} - y\sin \left( {2{\theta _1}} \right) $
(10) 两束高斯光束的空间相干性与其方向性密切相关,经过扩束准直的高斯光束可以近似为平行光,所以这里只需考虑分束镜旋转角度对其空间相干性的影响,由物理光学的相关结论可知,两束光若要发生相干,必须满足:
$ \frac{{2{w_0}a}}{{2l}} \le \frac{\lambda }{2} $
(11) 将$a = \frac{{l{\rm{tan}}(2{\theta _1})}}{{1 - {\rm{tan}}(2{\theta _1})}} $带入(11)式中可以得到,若想获得远场干涉图样分束镜旋转角θ1必须满足的条件为:
$ {\theta _1} \le \frac{1}{2}\arctan \left( {\frac{\lambda }{{2{w_0} + \lambda }}} \right) $
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在计算干涉图样远场条纹的表达式时,可以利用三角函数倍角公式对(7)式进行进一步化简,则有:
$ I = 4{I_0}\cos \left\{ {\frac{1}{2}\left[ {k\frac{{a\left( {y - \Delta y} \right)}}{{q\left( L \right)}} - ky\sin \left( {2{\theta _1}} \right)} \right]} \right\} $
(13) 由上式可知,相干光强取到极大值时应有$\frac{1}{2}\left[ {k\frac{{a\left( {y - \Delta y} \right)}}{{q\left( L \right)}} - ky{\rm{sin}}(2{\theta _1})} \right] = m{\rm{ \mathsf{ π} }}(m = 0, \pm 1, \pm 2, \ldots ) $成立,相干光强取到极小值时应有$ \frac{1}{2}\left[ {ka\left( {y - \Delta y} \right)/q\left( L \right) - ky{\rm{sin}}(2{\theta _1})} \right] = (m + 1/2){\rm{ \mathsf{ π} }}(m = 0, \pm 1, \pm 2, \ldots )$成立。远场接收屏上的干涉图样是由一系列平行等距的亮带和暗带组成的,亮带到暗带的逐渐变化即是光强极大值向极小值的渐变过程,通常用条纹间距表示相邻两条亮条纹或暗条纹之间的距离。下面通过计算远场干涉图样上两个极大强度点的位置来推导条纹间距的表达式。由上述讨论可知,接收屏上光强极大值点的纵坐标ym满足:
$ \frac{1}{2}\left[ {k\frac{{a\left( {{y_m} - \Delta y} \right)}}{{q\left( L \right)}} - k{y_m}\sin \left( {2{\theta _1}} \right)} \right] = m{\rm{ \mathsf{ π} }} $
(14) 式中,m=0, ±1, ±2, …时,相干光强取到极大值。
由(14)式可推导出接收屏上光强极大值点纵坐标的表达式为:
$ {y_m} = \frac{{m\lambda + \frac{{a\Delta y}}{{q\left( L \right)}}}}{{\frac{a}{{q\left( L \right)}} - \sin \left( {2{\theta _1}} \right)}} $
(15) 则远场干涉图样的条纹间距e为:
$ e = {y_m} - {y_{m - 1}} = \frac{{\lambda q\left( L \right)}}{{a - q\left( L \right)\sin \left( {2{\theta _1}} \right)}} $
(16) 由条纹间距的表达式可知,远场干涉图样的条纹间距与扫描时间无关,其大小只与马赫-曾德尔发射装置参量与高斯光束参量有关。
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条纹对比度是用来表征干涉场中某一点附近的条纹的清晰程度。在主动相干探测时,必须保证传输到目标处的干涉图样具有满足一定清晰度的条纹,以确保相干探测光经目标反射原路返回到接收端探测器上时能够获得目标时间化的光强变化信息。条纹对比度的定义为:
$ K = \frac{{{I_{\max }} - {I_{\min }}}}{{{I_{\max }} + {I_{\min }}}} $
(17) 式中,Imax为该点附近条纹强度的极大值,Imin为该点附近条纹强度的极小值。
条纹对比度与条纹亮暗差别及背景光强有关,随着传输距离的增大,激光能量会出现衰减,从而使得远场条纹对比度下降。这里从相干光强的表达式出发,考虑影响条纹对比度的因素。由(13)式可以得到距离为L的接收屏上的相干光强的表达式:
$ \begin{array}{*{20}{c}} {I = 4U_0^2\frac{{{{\rm{ \mathsf{ π} }}^2}w_0^4}}{{{{\rm{ \mathsf{ π} }}^2}w_0^4 + {\lambda ^2}{L^2}}} \times }\\ {{{\cos }^2}\left\{ {\frac{1}{2}\left[ {k\frac{{a\left( {y - \Delta y} \right)}}{{q\left( L \right)}} - ky\sin \left( {2{\theta _1}} \right)} \right]} \right\}} \end{array} $
(18) 由上式可知,在考虑某一点附近I的极大值和极小值时,只需考虑$ {\rm{co}}{{\rm{s}}^2}\left\{ {\frac{1}{2}\left[ {k\frac{{a\left( {y - \Delta y} \right)}}{{q\left( L \right)}} - ky{\rm{sin}}(2{\theta _1})} \right]} \right\}$随y轴坐标的变化情况。不妨设:
$ \eta = {\cos ^2}\left\{ {\frac{1}{2}\left[ {k\frac{{a\left( {y - \Delta y} \right)}}{{q\left( L \right)}} - ky\sin \left( {2{\theta _1}} \right)} \right]} \right\} $
(19) 在实际探测时,条纹对比度必须满足一定条件,才能探测到周期细分了的回波信号。因此,在下面仿真分析时,只需考虑任一点处的y轴坐标与光强相对变量η的关系,进而得到对比度与转动角度、传输距离、波长等的关系。
基于M-Z干涉的相干高斯光束远场干涉图样研究
Study of far-field interference pattern for coherent Gaussian beams based on Mach-Zehnder interferometer
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摘要: 为了使远场相干光斑分布及干涉图样的条纹间距和条纹对比度满足激光主动相干探测的要求,对基于马赫-曾德尔干涉原理的相干光发射装置发出的相干高斯光束在远场的扫描相干光场分布特性进行了分析,推导了高斯光束远场扫描相干光场的解析光强分布公式,得到了高斯光束远场扫描干涉图样。利用物理光学相关原理推导了远场的相干光光程差、干涉图样的条纹间距以及条纹对比度的表达式。数值仿真分析了马赫-曾德尔干涉光发射装置的分束镜旋转角度、探测距离和扫描角速度对远场相干光强分布特性的影响,得到了这几个物理量之间的定量关系。结果表明,远场扫描相干光强分布主要受到分束镜旋转角度、探测距离和发射装置扫描角速度等参量影响;干涉光发射装置的分束镜旋转角度同时影响远场接收屏上干涉图样的条纹间距及条纹对比度。该研究结果对实际探测中如何选取分束镜的旋转角、使其在满足条纹间距目标尺寸要求的前提下,适当调节旋转角度,获得尽可能大的条纹对比度是有帮助的。
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关键词:
- 物理光学 /
- 马赫-曾德尔干涉原理 /
- 主动相干探测 /
- 扫描相干高斯光束 /
- 远场干涉图样
Abstract: In order to make the quality of fringe spacing and fringe visibility of interference pattern to meet the strict demands of active coherent laser detecting system of far distance, the distribution characteristics of coherent Gaussian beams based on Mach-Zehnder interferometer were analyzed. Optical intensity distribution of coherent Gaussian beams was deduced. Then interference pattern of scanning coherent Gaussian beams was gotten. The expressions of optical path difference, fringe spacing and fringe visibility based on Mach-Zehnder interferometer were derived by physical optics theory. The influences of rotation angle of beam splitter, detection distance and scanning speed on distribution characteristics of coherent light of far distance were analyzed and simulated numerically. As a result, the quantitative relation of rotation angle of beam splitter, detection distance and scanning speed was found. The results show that the distribution of coherent light of far distance is mainly controlled by rotation angle of beam splitter, detecting distance and scanning speed etc. Rotation angle of beam splitter in the interferometer has the influence on both fringe spacing and fringe visibility. The conclusion is helpful to choose the rotation angle in the experiment. Fringe spacing and fringe visibility should be taken into consideration simultaneously and the rotation angle of beam splitter is adjusted. It's useful to make the fringe visibility as large as possible. -
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