CN109990976A - 以非均匀延迟间隔测量通过液晶可变延迟器的路径延时 - Google Patents

以非均匀延迟间隔测量通过液晶可变延迟器的路径延时 Download PDF

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CN109990976A
CN109990976A CN201811473087.9A CN201811473087A CN109990976A CN 109990976 A CN109990976 A CN 109990976A CN 201811473087 A CN201811473087 A CN 201811473087A CN 109990976 A CN109990976 A CN 109990976A
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A·黑格伊
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Abstract

对液晶可变延迟器施加电压,所述电压在一时间段内单调地改变延迟且改变所述液晶可变延迟器的所述延迟相对于时间的一阶导数。测量在所述时间段期间穿过所述液晶可变延迟器的光的干涉图。

Description

以非均匀延迟间隔测量通过液晶可变延迟器的路径延时
本公开涉及以非均匀延迟间隔测量通过液晶可变延迟器的路径延时。在一个实施例中,对液晶可变延迟器施加电压,所述电压在一时间段内单调地改变延迟且改变所述液晶可变延迟器的所述延迟相对于时间的一阶导数。测量在所述时间段期间穿过所述液晶可变延迟器的光的干涉图。
在另一实施例中,对液晶可变延迟器施加电压,所述电压在一时间段内单调地改变所述液晶可变延迟器的延迟。在所述时间段期间以非均匀延迟间隔对穿过所述液晶可变延迟器的光的干涉图进行采样。
鉴于以下详细讨论和附图,可以理解各种实施例的这些和其它特征和方面。
以下讨论参考以下附图,其中相同的附图标记可用于识别多个图中的类似/相同的组件。图式未必按比例。
图1是根据实例实施例测得的通过液晶可变延迟器的归一化干涉图的标绘图;
图2是根据实例实施例的液晶可变延迟器的延迟响应曲线的标绘图;
图3和4是示出根据实例实施例的各个延迟响应曲线可如何影响经由偏振干涉仪测得的干涉图的标绘图;
图5是根据另一实例实施例的液晶可变延迟器的延迟响应曲线的标绘图;
图6是根据实例实施例的液晶可变延迟器的电压波形和延迟响应的标绘图;
图7是根据实例实施例的设备的图式;
图8和9是根据实例实施例的方法的流程图;且
图10是示出根据实例实施例的液晶可变延迟器的不同干涉图采样间隔的标绘图。
本公开涉及对利用干涉计的光学装置的控制。干涉仪配置成在穿过所述干涉仪的光的分量之间引入光学路径延时。一种类型干涉仪是偏振干涉仪,其使用放置在两个偏振器之间的可变光学延迟器。可变光学延迟器在入射偏振方向上的第一光线与在正交偏振中的第二光线(例如,普通光线和非常光线)之间产生可选的路径延时。此路径延时在第一光线与第二光线之间引起与波长相关的相移,使得离开偏振干涉仪的光产生干涉图,所述干涉图经由例如焦平面阵列等光学传感器检测。此类干涉计可用于例如超光谱成像等高光谱带宽光学应用。超光谱成像是指用于获取可包含图像的超光谱数据集或数据立方体的方法和装置,在所述图像中在每个像素处提供密集采样、精细分辨的光谱信息。
偏振干涉仪可使用一个或多个液晶(LC)单元作为可变光学延迟器。此装置在本文中被称作液晶可变延迟器(LCVR)。一般来说,液晶(LC)材料是具有一些晶体特性(例如,内部结构的定向,例如指示LC分子的局部平均对准的LC指向矢)的液体,所述晶体特性可通过施加例如电场或磁场等外部刺激而可选择地更改。LC指向矢的定向的改变会更改LC材料的光学特性,例如改变LC双折射的光轴。
LCVR在行进通过液晶的光的两个正交偏振之间产生可变光学路径延时(也被称作可变延迟)。LCVR内的一个或多个液晶单元充当电调谐双折射元件。通过变化跨液晶单元的电极的电压,单元分子改变其定向,且有可能在一段时间内可控制地改变光学路径延时。
为了形成具有LCVR的偏振干涉仪,将LCVR放置在第一偏振器与第二偏振器之间,具有标称并行或垂直的偏振轴。LCVR的慢轴(具有可变光学路径延时的偏振轴)定向成相对于第一偏振器的偏振方向的标称45度。入射光被第一偏振器偏振到入射偏振方向。由于LCVR的慢轴相对于此入射偏振方向为45度,所以可以根据平行于LCVR的慢轴偏振的一部分光和垂直于此轴偏振的一部分光来描述偏振入射光。
当光穿过LCVR时,它获得第一偏振和第二偏振之间的波长相关的相对相移,从而导致偏振态的波长相关的变化。与第一偏振器并行或垂直定向的第二偏振器或分析器干涉平行于LCVR的慢轴偏振的所述部分光,所述部分光垂直偏振,从而在LCVR的输出处将与波长相关的偏振态改变为可通过光检测器(例如,焦平面阵列)感测的与波长相关的强度图案。通过在变化LCVR的延迟的同时感测此强度图案,有可能测量入射光的干涉图,其可用以确定入射光的光谱特性。这些光谱特性可用于超光谱成像。
超光谱成像仪扫描干涉仪的延迟,而空间分辨光学检测器记录瞬时干涉图强度。在图像中的每个点处同时记录干涉图,且相对于延迟对干涉图进行傅里叶(Fourier)变换以获得超光谱数据立方体。存在可通过适当考虑可变光学延迟器的延迟与时间轨迹(或类似地,干涉图采样间隔的变化)改善的数个性能因素(例如,灵敏度、光谱分辨率)。例如,线性延迟与时间轨迹(或相应的均匀干涉图采样间隔)对于所有应用可能不是最佳的。
据干涉测量法领域已知,通过干涉仪提供的光谱率分辨有效地等于(取决于其如何定义)干涉仪的最大光学路径延时或最大延迟Γmax的倒数,如下文等式(1a)和(1b)中所示出。等式(1a)定义了波数方面的分辨率Δk,且等式(1b)定义了波长方面的分辨率Δλ。因此,对于给定的最大路径延时,对于较短波长的光谱分辨率箱将比对于较长波长更密集地间隔开(在波长方面)。
Δk=1/Γmax (1a)
Δλ=λ2max (1b)
这种非均匀光谱分辨率对宽带光谱有负面影响,在灵敏度受信号本身散粒噪声限制的情况下测量所述宽带光谱。如果宽带光谱在不同波长下的每单位波长具有近似相等的强度,那么处于较短波长的干涉图信号将比处于较长波长的每个光谱分辨率箱弱得多。然而,来自所有波长的散粒噪声将在重构光谱上均匀分布,因此在较短波长下信噪比将显着降低。此现象有效地限制了利用干涉测量法的任何散粒噪声限制光谱测量系统的可检测光谱的动态范围,所述干涉测量法例如本发明的实施例的超光谱成像仪。
存在限制较短波长的灵敏度的额外因素。检测器响应度在较短波长处降低;这是每单位能量较低的量子效率和较少的光子(因此产生较少的光电子)的综合结果。在较短波长下材料吸收倾向于增大。随着波长变短,可变光学延迟器的通光孔径上的路径延时不均匀性导致更大的相位不确定性,且因此在所记录的干涉图中存在更多的退相干和更少的干涉图条纹对比度。
在图1中,堆叠的标绘图示出了根据实例实施例的通过偏振干涉仪测量的不同波长的窄带源的归一化干涉图。窄带源在底部标绘图中从400nm开始,并以10nm的增量增加到1,000nm。横轴是时间,而不是延迟,但是延迟与时间几乎线性相关。这些标绘图的重要特征是随着波长从最上面的标绘图到底部的减小,信号量值向零路径延时周围的区骤降。零路径延时点大致对应于区100,其中所有波长的相位重合。
通过减小最大光学路径延时来增加分辨率箱的大小,从而可以部分地弥补差的短波长灵敏度。虽然这在光谱分辨率最佳的较短波长下是可以容忍的,但是在已经具有相对差的光谱分辨率的较长波长下这是不可容忍的。或者,可以通过增加总积分时间、在相同积分时间的情况下采用更多样本或增加每个样本的积分时间来弥补,但这可能并非始终可行。
如果过快扫描可变光学延迟器的延迟,那么更难控制且因此样本之间的路径延时间隔将不均匀,在一些实施例中在傅里叶变换之前需要进行插值。然而,此插值还将在较短波长下引起更多信号损耗。并且,采样速率越接近干涉图的奈奎斯特(Nyquist)频率,其条纹对比度越小,因为样本代表每个采样间隔上的信号的时间积分。如果在最短波长下每个条纹存在N个样本,那么此对应于使用sinc滤波器对信号进行滤波,其中第一个零点处于每个条纹仅有一个样本的频率下。因此,信号将通过sinc(π/N)衰减。在奈奎斯特频率下,这是2/π≈64%,但如果存在额外相位偏移,那么干涉图将进一步衰减,这是因为仅可测得干涉图的实部。
考虑到在存在较长波长的光的情况下测量较短波长的干涉图的固有限制,期望以有可能牺牲较长波长的信号为代价优先对较短波长进行采样。这可通过增加接近零路径延时的采样密度和/或增加接近零路径延时的样本(例如,接近图1中的区100)的积分时间来完成,其中有助于较短波长的信号大部分位于所述接近零路径延迟的样本中。为了保持相同的总采集时间,将花费更少的时间在测量处于较长路径延时的干涉图的部分,其中存在较长波长的信号,但不存在较短波长的信号。
所期望的延迟与时间轨迹的一般形式(在本文中也被称为“延迟响应曲线”)将在较小延迟下具有较缓斜率且在较大延迟下具有较陡斜率,且存在多种方式来对其进行数学建模(此处注意“较小”和“较大”是指延迟的量值或绝对值)。定义延迟响应的一种方式是随着分段线性曲线,将恒定延迟速度(延迟的时间导数)分配到延迟扫描的不同间隔。这在图2中示出,其中延迟曲线200包含三个线性区部200a-c。每个区部200a-c具有相比于先前区部单调递增的延迟值和更大的延迟斜率(其对应于延迟相对于时间的一阶导数,或延迟速度)。在此图中,LCVR的零延迟点202将有可能存在在第一区段200a的中间某处。
定义延迟响应曲线的另一方式是施加恒定延迟加速度(延迟速度的时间导数)。这将在每个波长下对相位赋予线性啁啾。现假设相对时间可变x∈{0…1},其中相对延迟速度线性增大到等式(2)中所示出的公式可用以产生延迟响应曲线,所述延迟响应曲线从x=0处的0延迟增大到x=1处的Γmax,恒定相对加速度对应注意可将任何恒定延迟值添加到等式(2)的右侧且其将仍产生具有期望恒定加速度特征的延迟响应曲线;这将是有用的,例如以形成延迟响应曲线,所述延迟响应曲线开始于略微负路径延时且通过零路径延时点增大到较大正路径延时。
Γ(x)=Γmax(2x+(a-1)x2)/(a+1) (2)
在图3和4中,标绘图示出445nm(蓝色)LED的干涉图如何随从具有a=1的线性延迟响应曲线切换成具有a=6的啁啾曲线而改变。数据包含用以特征化延迟响应曲线的参考光源的干涉图。标绘图300、301是线性延迟响应曲线的测量结果且标绘图400、401是啁啾曲线的测量结果。标绘图300、400是蓝色LED的空间平均干涉图且标绘图301、401是参考源的干涉图。啁啾曲线相对于如根据干涉图标准差所测量的线性曲线将测得的蓝色LED的信号增大≈31%。
图5中示出对于正常500和啁啾502采集测得的延迟与帧数(例如,干涉图样本数)。基于测量参考激光的相位延时获得延迟。使用等式(2),相位速度以恒定加速度从开始到结束增加因数a=6。使用Takeda方法从参考激光干涉图(例如,干涉图301、401)获得相位延时,但还可通过LCVR的一个或多个单元的电容测量更直接地测得所述相位延迟。
瞬时延迟速度还可用瞬时延迟按比例调整以得到指数延迟与时间曲线。此曲线将从以下观点以未加偏置的方式处理每个波长而得到:当给定波长达到N次振荡时,波长的相位速度应与波长无关。给定时间点处给定波长的相位速度是当延迟等于N个波长时,每个波长的相位速度可以是相同值,因此Γ=Nλ。如果此相位速度值参数化为2π/ntexp,其中n是所述波长下每个条纹的帧数且texp是每个帧的时间,那么这得到以下等式(3)和(4),其中是帧数。
可通过使用最大延迟Γmax应在最后一帧M处达到的事实来求解积分常数。总之,这是最佳(从一个角度)延迟轨迹,用于向上扫描到M帧中的延迟Γmax,其中在其振荡N次之后对每个波长每条纹采样n次。在其它实施例中,延迟相对于时间的一阶导数与延迟加在某一时间间隔内相对于时间的恒定偏移成比例:
任何合适的延迟轨迹也可逆向行进。然而,可能必需的是开始从LCVR电极通常保持在较高电压量值的方向到电极通常保持在较低电压量值的方向的延迟扫描,且取决于LC单元类型(例如,平面或竖直对准),这可确定延迟速度的符号。一般想法是以相对于较高延迟的较低延迟对LCVR延迟进行最大控制,如之前所提到,这是存在来自短波长的信号的情况,且短波长需要更大的延迟控制以适当地进行测量。
在图6中,使用200帧的LCVR或每个样本标绘了针对两个后续干涉图采集的电压波形600(左标度)和参考源602的相位延时(右标度)。左标度在跨LCVR单元的AC电压的量值乘以液晶介电各向异性(其可通过修改AC电压的频率而改变)的符号方面进行绘制。正介电各向异性下的最高电压对应于低延迟点。应注意,为方便起见,曲线602的最小延迟点已移位到~0拉德,但LCVR的零路径延时点604出现在~15拉德处。随着延迟和延迟速度继续增大,将电压缓慢降低到弗雷德里克兹(Freedericksz)电压606周围且维持在敏感点附近。为了进一步加速延迟速度,介电各向异性的符号在点608处改变且电压量值快速增大直到采集结束为止。接着,在正介电各向异性频率下电压量值快速升高610以在下一次采集之前快速重置单元状态。
液晶单元的响应时间随施加的电压平方而变化,因此如果较高的电压量值对应于具有正单轴双折射的平面或同构单元的较低延迟,那么单元响应更快,且因此更容易控制在较低延迟。替代地,具有负单轴双折射(和负介电各向异性)的单元将产生相同的结果。
如图6中所示出,从较高电压向较低电压扫描的另一个原因在于避免从下方向高于弗雷德里克兹转变阈值电压606扫描液晶单元,所述阈值电压其高于液晶分子响应于施加的场而旋转的电压。此电压可能对外部条件敏感,从而使所述点周围的延迟行为难以控制。阈值行为通常在单元电压在正方向上超过阈值电压时而非在单元衰减时出现。理想地,单元电压始终保持在此阈值电压之上,以便保持LC分子朝向施加的场的轻微倾斜,且因此明显更快地响应施加的场。
最后,期望快速重复延迟轨迹,以用于后续图像采集。如果延迟轨迹的开始对应于高压量值,那么单元状态可比可能的更快地恢复到其初始配置,如果所述初始状态对应于低压量值。可能仍期望在采集期间将单元有源地驱动到高延迟状态中,以便实现啁啾采集所需的高延迟速度。在此情况下,使用双频LC将是有益的。通过改变LC单元的驱动频率,介电各向异性将改变符号,且单元电压可再次增大以将单元驱动到其高延迟状态中。即使可通过介电各向异性的符号的改变而将单元有源地驱动到其高延迟或“关断”状态,来重置单元,但此过渡没有有源地驱动到“接通”状态那么快,这是由于高于过渡频率的负介电各向异性的量值大体上小于低于过渡频率的正介电各向异性的量值。因此,仍优选地利用驱动到“接通”状态作为图像采集之间的快速重置。
在图7中,框图说明根据实例实施例的执行图像处理的设备700。设备700包含装置控制器702,其可包含一个或多个处理器,例如中央处理单元、子处理器、图形处理单元、数字信号处理器等。控制器702耦合到存储器704,所述存储器704包含将在下文更详细地描述的功能模块。存储器704可包含易失性存储器和非易失性存储器的组合,且可存储如本领域中所已知的指令和数据。
设备700包含具有外部光接口708的光学区部706,所述外部光接口708从设备700外侧接收光。外部光接口708可包含适合于使光709从设备700外侧传递到内部光学组件的窗口、透镜、滤波器、孔口等。在此实例中,外部光接口708示出为耦合到外部透镜710。
偏振干涉仪712位于设备700的光学区部706。偏振干涉仪712例如经由电信号线耦合到控制器702。控制器702将信号施加到偏振干涉仪712,以引起作为干涉仪712的一部分的LCVR 712a中时变光学路径延时或延迟。此时变光学路径延时引起光709的不同偏振之间的转换,从而导致形成随光学路径延时而变的干涉图的输出光711。通过还耦合到控制器702的图像传感器714(例如,传感器像素阵列、焦平面阵列)检测干涉图。图像传感器714可基于干涉图形成静态图像和/或视频帧。
延迟控制器718指导装置控制器702对LCVR 712a施加控制信号以实现时变延迟轨迹。图像处理器720(其可完全或部分实施于设备700和/或计算机724中)使用此延迟轨迹以及在图像传感器714处检测到的干涉图作为时变路径延时的量度。可以通过计算作为LCVR712a的对应位置处的路径延时的函数的变换来处理每个检测到的干涉图,且与作为位置的函数的处理后的干涉图一起产生超光谱数据立方体。超光谱数据立方体可呈现为静态图像和视频中的一个或两个。
为了增加干涉图的短波长分量的测量灵敏度,控制器702将电压施加到LCVR 712a的电极712aa。电压在一时间段内单调地改变延迟且改变LCVR 712a的延迟相对于时间的一阶导数。例如经由成像仪传感器714测量在所述时间段期间穿过LCVR 712a的光的干涉图。图像处理器720可在所述时间段内变换测得的为延迟的函数的干涉图以光谱地分辨光709。
在图8中,流程图示出根据实例实施例的方法。方法涉及对液晶可变延迟器施加800电压。电压在一时间段内单调地改变延迟且改变液晶可变延迟器的延迟相对于时间的一阶导数。测量801在所述时间段期间穿过所述液晶可变延迟器的光的干涉图。方法可任选地涉及在所述时间段内变换802测得的为延迟的函数的干涉图以光谱地分辨光。
在图9中,流程图示出根据实例实施例的方法。方法涉及对液晶可变延迟器施加900电压,所述电压在一时间段内单调地改变液晶可变延迟器的延迟。在所述时间段期间以非均匀延迟间隔测量901穿过液晶可变延迟器的光的路径延时。
在图10中,标绘图1000示出根据实例实施例的可如何以对应于不同延迟范围的不同间隔对通过LCVR测得的干涉图进行采样。测量干涉图涉及在第一采样时间1006内对以第一延迟1004为中心的第一延迟范围1002内的光进行采样。此范围1002是较低延迟范围,意指其比第二延迟范围1010和第三延迟范围1018更接近零路径延时,所述第二延迟范围1010和所述第三延迟范围1018是在竖直方向上具有增加的路径延时的更高延迟范围。在第一采样时间1006中获取的比在第二采样时间1008中获取的更大数目的样本用以对在第二延迟范围1010内的光进行采样,所述第二延迟范围1010以第二延迟1012为中心,所述第二延迟1012的量值比第一延迟1004的量值更大(更远离零路径延时)。第一时间段1006和第二时间段1008之间的样本数目的此差值导致在包含第一时间段1006和第二时间段1008的时间段期间以非均匀延迟间隔对穿过液晶可变延迟器的光的干涉图进行采样。
第一延迟范围1002可具有相同或不同于第二延迟范围1010的大小。应注意,这可应用于如果示出的线性延迟曲线1014或应用于具有增加的延迟和其一阶导数的延迟曲线,例如类似于如图2中所示出的曲线200的分段线性曲线或类似于如图5中所示出的曲线502的平滑曲线。在给定采样时间中获取的不同数目的样本可应用于多于两个延迟范围,如由在采样时间1020中采样的第三延迟范围1018所指示。

Claims (10)

1.一种方法,其包括:
对液晶可变延迟器施加电压,所述电压在一时间段内单调地改变延迟且改变所述液晶可变延迟器的所述延迟相对于时间的一阶导数;以及
测量在所述时间段期间穿过所述液晶可变延迟器的光的干涉图。
2.根据权利要求1所述的方法,其中所述干涉图包括短波长分量和长波长分量,所述延迟的所述单调改变和所述一阶导数的所述改变增加所述干涉图的所述短波长分量的所述测量的灵敏度。
3.根据权利要求1所述的方法,其中测量所述干涉图包括在第一采样时间内对以第一延迟为中心的第一延迟范围内的所述光进行采样,所述第一采样时间大于用以对第二延迟范围中的所述光进行采样的第二采样时间,所述第二延迟范围的大小与所述第一延迟范围相同且以第二延迟为中心,所述第二延迟的量值大于所述第一延迟。
4.根据权利要求1所述的方法,其中所述延迟的所述单调改变和所述一阶导数的所述改变使得通过较低延迟范围的过渡时间长于通过更高延迟范围。
5.根据权利要求1所述的方法,其中所述延迟相对于时间的所述一阶导数与所述延迟加在某一时间间隔内相对于时间的恒定偏移成比例。
6.根据权利要求1所述的方法,其中所述一阶导数根据分段常值函数而变化。
7.一种方法,其包括:
对液晶可变延迟器施加电压,所述电压在一时间段内单调地改变所述液晶可变延迟器的延迟;以及
在所述时间段期间以非均匀的延迟间隔对穿过所述液晶可变延迟器的光的干涉图进行采样。
8.根据权利要求7所述的方法,其中以基本均匀的时间间隔对所述干涉图进行采样,同时改变所述延迟相对于时间的一阶导数。
9.一种设备,其包括:
偏振干涉仪,其包括液晶可变延迟器;以及
控制器,其耦合到所述液晶可变延迟器,所述控制器可用于:
对所述液晶可变延迟器施加电压,所述电压在一时间段内单调地改变延迟且改变所述液晶可变延迟器的所述延迟相对于时间的一阶导数;以及
测量在所述时间段期间穿过所述液晶可变延迟器的光的干涉图。
10.根据权利要求9所述的设备,其中所述干涉图包括短波长分量和长波长分量,所述延迟的所述单调改变和所述一阶导数的所述改变增加所述干涉图的所述短波长分量的所述测量的灵敏度。
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