基于等效元件和相位补偿法的双倍分辨率波片测量
Wave plates are a key component of optical polarization systems. The study of methods to measure wave plates precisely is becoming increasingly important with continuous improvements in the performance of polarization systems. Currently, most techniques are based on detecting the intensity of light passing through a wave plate. These methods are generally susceptible to fluctuations in the intensity of the light. In contrast, other methods that measure the phase of the light instead typically exhibit greater accuracy and stability. However, relatively few such methods have been described in the relevant literatures for phase measurement. In this study, a high-precision method based on equivalent components and phase compensation is proposed to simultaneously measure the phase retardation and azimuth of arbitrary wave plates.
We insert a rotatable half wave plate in front of a measured wave plate and use a reflector to allow the measurement light to pass through both twice. This effect is equivalent to measuring an equivalent wave plate with a phase retardation twice that of the measured wave plate, and double resolution detection is thus obtained. The proposed system includes a dual-frequency laser source and a phase detector. Simultaneously, the fast-axis azimuth is also determined according to the azimuth of the half wave plate when the maximum or minimum phase difference occurs.
An analysis of error values under the experimental conditions indicated that the measurement uncertainty of the phase retardation was about 3.3', and that of the fast-axis azimuth was better than 5.4''. The results of an experimental comparison show good agreement with the measurement results obtained using other methods. The proposed approach theoretically avoids the influence of fluctuations in the intensity of measurement light that affect conventional methods as well as the typical issue of the influence of the positioning accuracy of birefringent devices azimuth. The proposed optical system adopts a dual-frequency heterodyne interference optical path with common optical path properties to obtain good measurement stability. This system also has a simple structure, requires relatively few components, and can take measurements quickly. In addition, because the measurement beam passes through the position of the measured wave plate twice, birefringent devices with wedge-shaped structures can also be measured.
The study of high-performance methods to measure wave plates has significant practical significance owing to their applications in optical polarization systems. In this study, a high-precision method for simultaneously measuring the phase retardation and azimuth of arbitrary wave plates based on equivalent components and phase compensation is proposed. By inserting a rotatable half wave plate in front of a measured wave plate and using a reflector to allow the measurement light to pass through the half wave plate and the measured wave plate twice, the proposed method is equivalent to measuring an equivalent wave plate with a phase retardation twice that of the measured wave plate, which allows the system to achieve double resolution detection. The system uses a dual-frequency laser source and a phase detection sensor. By rotating the half wave plate to compensate for the phase of the measurement light and adjusting the change in the phase difference of the measurement light relative to the reference light to the maximum or minimum value, the phase retardation of an arbitrary wave plate can be obtained. Simultaneously, the fast-axis azimuth is also determined according to the azimuth of the half wave plate when the maximum or minimum phase difference occurs. This method theoretically avoids the influence of fluctuations of the intensity of the measurement light that affect methods based on light intensity in general, as well as the influence of the positioning accuracy of the azimuth of birefringent devices, which affects many different methods. The optical system owns a dual-frequency heterodyne interference optical path, so it has common optical path property and good measurement stability. The proposed approach also has the advantages of a simple structure, relatively few components, and a quick measurement process. In addition, birefringent devices with wedge-shaped structures can also be measured by the proposed method. As noted above, the results of an error analysis under the experimental conditions showed that the measurement uncertainty of the phase retardation was about 3.3', and that of the fast-axis azimuth was better than 5.4''. The results of an experimental comparison also showed good agreement with the results of measurements obtained using other methods.
1 引言
波片是偏振光学系统中的关键元件之一,已被广泛应用于光通信、传感仪器、医疗健康、航空航天、机械化工和物理学等领域[1-3]。随着生产、科研需求对偏振光学系统性能水平要求的不断提高,探究高性能的波片测量方法具有重要的实际意义。
波片的主要技术指标包括相位延迟量与快轴方位角。目前,大部分波片相位延迟量测量方法是基于光强检测原理工作的,如光强消光法[4]、干涉法[5]、电/磁光调制法[6-7]、光栅法[8]、光谱检测法[9]等。一般而言,这些方法容易受到光强波动的影响。为此,测量系统需要具有较高性能的光源和抗干扰能力较强的稳定光路结构,以获得足够高的精度,但其成本和实现难度难以控制,而且,其中的一些方法还不能同时测量波片的快轴方位角。与光强测量方法相反的是相位测量方法,这类方法通常通过各种原理建立起被测波片的相位延迟量与信号相位之间的关系,从而测得相位延迟量。例如,基于激光反馈谐振的方法利用反射镜和激光谐振腔的反射镜来形成新的外部谐振腔,并在其中插入待测波片,当发生谐振时,测量信号相位和待测波片的相位延迟量之间存在一个简单的函数关系[10],以此进行测量。相位测量方法的精度和稳定性相对强度测量法通常更高,但目前的相位测量方法较少。此外,人们还尝试研究了一些基于其他原理的方法,如频率分割法[11]、椭圆偏振法[12]等,这些都是对波片测量研究的重要补充。
笔者提出了一种基于相位检测原理的波片测量方法,该方法采用等效元件测量和相位补偿思想实现波片相位延迟量的双倍分辨率检测,并可同时确定其快轴方位角。所提方法测量的波片相位延迟量从原理上避免了一般光强测量法所受到的光强波动的影响,以及许多方法所受到的双折射器件方位角定位精度的影响。光路采用双频外差干涉光路,具有共光路性质,稳定性高。测量系统结构简单、元件少,测量快捷。此外,由于测量光束两次通过待测波片的同一位置,因此还可以测量楔形结构的双折射器件。
2 测量原理
测量系统的原理示意图如
设可旋转半波片的快轴与X轴的夹角(即快轴方位角)为ψ,从分束器透射出的光第一次通过半波片后,p、s偏振光的偏振方向各转过2ψ。先假设没有加入待测波片,光束从反射镜反射回来并第二次通过半波片后,p、s偏振光的偏振方向转动的角度与之前的大小相等、方向相反,即恢复为初始状态。此时,如果在半波片和聚焦棱镜之间插入一个相位延迟量为Δ、快轴方位角为θ的待测波片,半波片的作用就相当于使待测波片以光束为轴进行了旋转,且旋转角度是半波片旋转角度的两倍(2ψ)。由于测量光束两次通过了待测波片,相当于光束直接通过了一个相位延迟量为待测波片相位延迟量两倍(2Δ)的波片,即相当于测量了一个波片相位延迟量为2Δ的等效元件,因此测量分辨率提高了一倍。此时,等效元件的快轴方位角为Ω=θ-2ψ。
旋转半波片对测量信号的影响表现在改变其相位上,即对测量信号的相位进行调制。由于参考信号一直保持不变,因此可以将测量信号相对于参考信号的相位差补偿调至为特定值。先考虑p偏振光的频率fp大于s偏振光频率fs的情况。根据偏振理论,某一待测波片的琼斯矩阵为
半波片的琼斯矩阵为
设入射p偏振分量的琼斯向量为
根据
由
因此,由上述分析可得
进而可求得待测波片的相位延迟量为
待测波片的快轴方位角由产生相位差最大值或最小值时所对应的半波片方位角决定,即
再考虑p偏振光的频率fp小于s偏振光频率fs的情况,同样按上述分析可知,相位延迟量与
由
3 误差分析以及实验结果与比对
测量系统实物装置图如
根据实验参数和测量公式可以分析当前实验条件下的测量不确定度。由
表 1. 一个1/4波片的测量结果
Table 1. Measurement result of a quarter wave plate
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表 2. 半波片和1/4波片相位延迟量的测量结果及比对结果
Table 2. Phase retardation measurement results and comparison of half wave plates and quarter wave plates
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1) 相位检测精度导致的误差。实验中采用的相位计的精度为0.1°,根据
2) 待测波片安装误差导致的误差。当待测波片表面不严格垂直于激光束时,光在待测波片中的光程将增加,这会导致测量信号相对于参考信号的相位差变化产生一个额外值。用反射回授方法调整波片与光束表面的垂直度,可以控制其小于2'。根据之前的研究[13],波片厚度和相位延迟量之间的关系为
式中:ne和no分别为材料的非常光和寻常光的折射率;λ为激光在真空中的波长;D为波片厚度;k为波片级数,实验中所用波片的厚度约为1 mm,因此k=14。当波片法线与光束方向的夹角为β时,引起的测量误差为
由
3) 半波片导致的误差。实验所用的半波片是厚度为1 mm、相位延迟精度为λ/300的复合零级波片。半波片可在4个方面引起测量误差。一是其自身的相位延迟量并非理想的180°,二是半波片表面和激光束不严格垂直,这二者的效果可以合并,类似前面的待测波片分析,可知这二者引起的误差约为20''。三是波片倾斜对双折射的影响,同样,该误差约为6''。四是由于采用了复合波片,其本身的轴线对准误差将引起相位延迟量、方位角和附加旋光效应等误差[14],实验所用半波片的轴线对准精度约为0.1°,计算可得产生的误差约为20''。综合上述各误差可知半波片引起的测量误差δ3约为29''。
4) 光源稳定性导致的误差。光源稳定性会导致相位测量产生一定误差。实验所用光源为高精度He-Ne双频激光器,其频率稳定性为2×10-8,光电探测器接收的是双频激光的拍频电压正弦或余弦信号,即信号周期为1/(fp-fs)。相位计测得的是参考、测量两个拍频信号的相位差。光频变化10-8量级引起两个拍频信号的周期发生变化,从而导致二者相位差的变化量远远小于相位计的分辨率,因此其引起的误差可以忽略不计。
5) 偏振片、分束器、探测器等元件性能导致的误差。由于待测波片的相位延迟量是由测量信号相对于参考信号的相位差的极值决定的,也就是测量的是相位计的相位变化,因此偏振片的偏振度和分束镜的残余相位延迟这些元件的自身误差对测量没有影响。
探测器的噪声表现在参考信号和测量信号(余弦电压信号)的稳定性上,从实验中示波器显示的信号波形来看,参考信号和测量信号波形光滑、稳定。同时,所用相位计在将测量信号和参考信号转化为数字信号时采用的是过零比较方式,因此电压信号的波动(即探测器噪声)对测量相位的影响非常小。为评估其影响,笔者进行了系统测量稳定性实验,即让测量系统在非操作的静态下记录相位计数据,结果发现10 min内(测量时间短于该时间)相位值的标准偏差为0.057°。这个相位变化包含了探测器等器件的噪声和响应线性度、光源稳定性等的影响。根据
6) 环境温度波动导致的误差。环境温度波动对波片相位延迟量的影响为
式中:δT为环境温度波动;α为材料的热膨胀系数,实验中所用的波片材料为石英晶体,其α为2.1×10-7 ℃-1。dno/dT=-0.5452×10-5,dne/dT=-0.6509×10-5。
实验在恒温实验室内进行,测量时间短。实验中的环境温度波动为21.9 ℃±0.02 ℃,由
综上所述,在上述实验条件和元件参数下,相位延迟量的测量不确定度为
由式(
实验中测量了2个半波片和2个1/4波片,对每个样品进行了6次重复测量,其中一个1/4波片的实验结果如
4个波片的相位延迟量测量结果如
4 结论
笔者提出了一种基于等效元件思想和相位补偿原理采用相位检测法测量任意波片相位延迟量的方法,在测量光路中的待测波片之前插入可旋转半波片,使测量光两次通过该半波片和待测波片,相当于测量了一个相位延迟量为待测波片两倍的等效波片,实现了双倍分辨率检测,并可同时测量快轴方位角。所测量的相位延迟量从原理上避免了光强波动和双折射器件方位角定位精度的影响。系统具有结构简单、元件少、测量便捷的特点。系统采用的是双频外差干涉光路,具有共光路性质,稳定性高。此外,所提方法还可以测量楔形结构的双折射器件。所提方法十分适合波片等双折射器件的高精度、快速测量。
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Article Outline
陈强华, 邵多, 刘福铭, 关裕, 吕洪波, 司丽娜, 阎红娟, 豆照良. 基于等效元件和相位补偿法的双倍分辨率波片测量[J]. 中国激光, 2024, 51(8): 0804004. Qianghua Chen, Duo Shao, Fuming Liu, Yu Guan, Lü Hongbo, Lina Si, Hongjuan Yan, Zhaoliang Dou. Double Resolution Wave Plate Measurement Based on Equivalent Components and Phase Compensation[J]. Chinese Journal of Lasers, 2024, 51(8): 0804004.