技术领域technical field
本发明涉及光学成像技术领域，尤其涉及一种全视场低频外差点衍射干涉仪。The invention relates to the technical field of optical imaging, in particular to a full-field low-frequency heterodyne diffraction interferometer.
背景技术Background technique
以深紫外光刻机投影曝光系统为代表的高端光学设备，对光学元件的加工、光学系统的集成提出了极大挑战。干涉仪作为高精度光学元件加工和光学系统集成不可或缺的核心检测设备，检测精度要求不断提高。The high-end optical equipment represented by the projection exposure system of the deep ultraviolet lithography machine poses great challenges to the processing of optical components and the integration of optical systems. Interferometer is an indispensable core detection equipment for high-precision optical component processing and optical system integration, and the detection accuracy requirements are constantly improving.
传统光学加工中采用的光学面形检测方法包括哈特曼传感器法、刀口法和轮廓法等。这些方法分别存在着非数字化需主观判读或接触损伤待测件等不同的缺点，且很难达到较高的测量精度，是简单测量方法。The optical surface shape detection methods used in traditional optical processing include Hartmann sensor method, knife edge method and contour method. These methods have different disadvantages such as non-digital subjective interpretation or contact damage to the test piece, and it is difficult to achieve high measurement accuracy. They are simple measurement methods.
干涉检测法早在百年前就已经被使用，属于非接触式测量，且具有大量程、高灵敏度、高精度等特点，在高精度检测时被广泛应用，其原理是一束光照射标准的参考平面作为参考光，另一束光照射被测面返回带有面形信息作为测量光，两束光干涉时由于光斑不同位置相位不同产生光程差从而产生弯曲的干涉条纹，即可判断待测面的面形起伏。直到1974年Bruning等人提出移相干涉技术，把通讯理论中同步相位探测技术引入到光学干涉术中，使得干涉检测球面面形的精度大大提高。其基本原理是经过四步或多步移动待测元件，以改变测试波和参考波之间的位相差，光强也随之改变，从而得到一系列的方程。最后，通过求解方程组得到待测元件(或系统)的位相值。移相干涉技术已经相当成熟，在光学检测领域具有不可替代的地位。The interferometric detection method has been used as early as a hundred years ago. It is a non-contact measurement and has the characteristics of large range, high sensitivity, and high precision. It is widely used in high-precision detection. The plane is used as the reference light, and the other beam of light illuminates the surface to be measured and returns the surface shape information as the measurement light. When the two beams of light interfere with each other, the optical path difference is generated due to the different positions and phases of the light spots, resulting in curved interference fringes, which can be used to judge the measured surface. The shape of the face is undulating. Until 1974, Bruning et al. proposed the phase-shifting interferometry technology, which introduced the synchronous phase detection technology in the communication theory into the optical interferometry, which greatly improved the accuracy of the interference detection spherical surface. The basic principle is to move the component under test in four or more steps to change the phase difference between the test wave and the reference wave, and the light intensity will also change accordingly, so as to obtain a series of equations. Finally, the phase value of the component under test (or system) is obtained by solving the equations. Phase-shifting interferometry technology is quite mature and has an irreplaceable position in the field of optical detection.
高精度干涉检测方法的发展按干涉仪结构与特点可以分为：泰曼-格林型干涉仪、斐索型干涉仪和点衍射干涉仪等。The development of high-precision interferometric detection methods can be divided into three types according to the structure and characteristics of interferometers: Tieman-Green interferometer, Fizeau interferometer and point diffraction interferometer.
传统球面干涉检测法都是利用一个具有较高面形精度的参考球面获得所需参考波面，进而与含有待测面形信息的检测波面进行比较，由此得到待测面形数据。因而标准镜上参考面的面形精度直接限制了传统干涉系统所能实现的检测精度，而点衍射干涉仪(Point Diffraction Interferometer，简称PDI)的出现和发展很好地解决了该问题。点衍射球面干涉检测技术的基本思想是利用点衍射原理来获取理想的球面波，并将衍射波前的一部分作为参考波前，另一部分作为检测波前，进而可实现球面面形的高精度检测。利用点衍射原理获得理想球面波前，避免了传统干涉检测系统中由于标准镜面形误差对于系统检测精度的限制，因而可以达到衍射极限性能的分辨率，并使得检测精度具有较好的再现性。按照点衍射波前的获取方式不同，可将点衍射球面干涉检测法分为光纤点衍射干涉检测法和针孔点衍射干涉检测法。The traditional spherical interferometric detection method uses a reference sphere with high surface accuracy to obtain the required reference wave surface, and then compares it with the detection wave surface containing the information of the surface shape to be measured, thereby obtaining the surface shape data to be measured. Therefore, the surface shape accuracy of the reference surface on the standard mirror directly limits the detection accuracy of the traditional interferometric system, and the emergence and development of the Point Diffraction Interferometer (PDI) has solved this problem well. The basic idea of point diffraction spherical interference detection technology is to use the principle of point diffraction to obtain an ideal spherical wave, and use part of the diffracted wavefront as a reference wavefront, and the other part as a detection wavefront, thereby realizing high-precision detection of spherical surface shape . The principle of point diffraction is used to obtain an ideal spherical wavefront, which avoids the limitation of the detection accuracy of the system due to the standard mirror shape error in the traditional interference detection system, so that the resolution of the diffraction limit performance can be achieved, and the detection accuracy has good reproducibility. According to the different acquisition methods of the point diffraction wavefront, the point diffraction spherical interferometry method can be divided into the fiber point diffraction interferometry method and the pinhole point diffraction interferometry method.
典型的光纤点衍射干涉系统如图1所示，其利用光纤产生点衍射光，由短相干光源发出的光分为两束后分别经角锥镜反射后合束耦合进入光纤，一束光作为测量光另一束作为参考光，参考光的角锥镜由压电陶瓷驱动进行机械移相。从光纤另一端出射的光锥一部分照向被测镜，返回后聚焦在光纤端面反射，与出射光锥的另一部分重合干涉，形成干涉图，由于光源相干长度短，通过前端的光程匹配，测量光自身无法与从被测镜返回的光干涉，就避免的串扰。干涉图通过机械移相后分别记录，从而可以解算出被测镜的面型。然而，由于采用高精度的机械移动作为移相方法，因此精度仍不够高，成本高，研制难度大，尤其对大口径面形的测量，精密机械驱动精度降低，测量精度也随之降低；另外，采用光纤出射光锥的一部分作为测量光，所以可测量的镜面数值孔径范围受到较大限制。A typical optical fiber point diffraction interference system is shown in Figure 1. It uses optical fiber to generate point diffracted light. The light emitted by the short coherent light source is divided into two beams, which are respectively reflected by the corner mirror and coupled into the optical fiber. One beam is used as The other beam of measuring light is used as the reference light, and the corner mirror of the reference light is driven by piezoelectric ceramics for mechanical phase shifting. A part of the light cone exiting from the other end of the optical fiber illuminates the mirror under test, and after returning, it is focused on the end face of the optical fiber and reflected, and overlaps and interferes with the other part of the exiting light cone to form an interference pattern. Due to the short coherent length of the light source, the optical path through the front end is matched. The measurement light itself cannot interfere with the light returning from the mirror under test, so crosstalk is avoided. The interferograms are recorded separately after mechanical phase shifting, so that the surface shape of the mirror under test can be calculated. However, due to the use of high-precision mechanical movement as a phase-shifting method, the accuracy is still not high enough, the cost is high, and the development is difficult. Especially for the measurement of large-diameter surface shapes, the accuracy of precision mechanical driving is reduced, and the measurement accuracy is also reduced; in addition , using a part of the optical fiber exit light cone as the measurement light, so the range of measurable mirror numerical aperture is relatively limited.
典型的针孔点衍射干涉系统如图2所示，其利用微小针孔作为点衍射孔产生高精度参考光，也是取用出射光锥的一部分作为测量光照射被测镜面，在小孔处镀半透半反膜，聚焦的光回到小孔后经反射与光锥另一部分重合干涉，使用压电陶瓷推动被测镜面进行机械移相测量。然而，压电驱动的机械移相不适用大口径的被测目标镜，压电陶瓷难精确驱动质量很大的物体；另外，被测镜面会聚的光斑如果较大，则返回到小孔处容易受到小孔边缘衍射的干扰，改变波前相位分布，降低测量精度，且微小的衍射小孔制作工艺复杂，成本高。A typical pinhole point diffraction interference system is shown in Figure 2. It uses tiny pinholes as point diffraction holes to generate high-precision reference light. The semi-transparent and semi-reflective film, the focused light returns to the small hole and then is reflected and interferes with the other part of the light cone. The piezoelectric ceramic is used to push the measured mirror to perform mechanical phase shift measurement. However, the mechanical phase shift driven by piezoelectricity is not suitable for large-diameter target mirrors, and it is difficult for piezoelectric ceramics to accurately drive large objects; in addition, if the converged light spot of the measured mirror is large, it is easy to return to the small hole. Interference by diffraction at the edge of the small hole will change the wavefront phase distribution and reduce the measurement accuracy, and the manufacturing process of the tiny diffraction small hole is complicated and the cost is high.
发明内容Contents of the invention
本发明的目的是提供一种全视场低频外差点衍射干涉仪，具有较高的测量精度，且抗干扰性能较好；同时，其研制难度与成本较低，尤其对于大口径面形的测量，相比机械驱动同样的优势更明显。The purpose of the present invention is to provide a full-field low-frequency heterodyne diffraction interferometer, which has high measurement accuracy and good anti-interference performance; at the same time, its development difficulty and cost are relatively low, especially for the measurement of large-diameter surfaces , Compared with the same advantage of mechanical drive, it is more obvious.
本发明的目的是通过以下技术方案实现的：The purpose of the present invention is achieved through the following technical solutions:
一种全视场低频外差点衍射干涉仪，包括：激光器、第一与第二半波片、偏振分光镜、第一与第二声光移频器、第一与第二光纤耦合镜、第一与第二单模保偏光纤、分光平片、成像镜头、面阵探测器；其中：A full-field low-frequency heterodyne diffraction interferometer, comprising: a laser, a first and a second half-wave plate, a polarization beam splitter, a first and a second acousto-optic frequency shifter, a first and a second fiber coupling mirror, a first One and the second single-mode polarization-maintaining optical fiber, a spectroscopic flat film, an imaging lens, and an area array detector; wherein:
所述激光器出射的激光经过第一半波片与偏振分光镜分为偏振方向相互垂直的两束光；The laser light emitted by the laser is divided into two beams of light whose polarization directions are perpendicular to each other through the first half-wave plate and the polarizing beam splitter;
其中一束光依次通过第二半波片、第一声光移频器、第一光纤耦合镜及第一单模保偏光纤进入分光平片，并作为参考光射向成像镜头；One beam of light sequentially passes through the second half-wave plate, the first acousto-optic frequency shifter, the first fiber-coupled mirror and the first single-mode polarization-maintaining fiber into the beam-splitting flat plate, and is sent to the imaging lens as a reference light;
另一束光次通过第二声光移频器、第二光纤耦合镜及第二单模保偏光纤进入分光平片，并作为测量光照向被测镜；射入被测镜的测量光被被测镜发射至分光平片，再经分光平片反射垂直发散向另一方向与参考光重合；Another beam of light passes through the second acousto-optic frequency shifter, the second fiber coupling mirror and the second single-mode polarization-maintaining optical fiber to enter the spectroscopic flat film, and is directed to the mirror under test as the measurement light; the measurement light incident on the mirror under test is The measured mirror is emitted to the spectroscopic flat plate, and then vertically diverges to the other direction after being reflected by the spectroscopic flat plate to coincide with the reference light;
重合后的两束光产生干涉，经成像镜头后在面阵探测器上获得干涉图。The overlapped two beams of light produce interference, and the interferogram is obtained on the area array detector after passing through the imaging lens.
进一步的，还包括：分光片、监视镜头与监视相机；Further, it also includes: spectroscopic film, surveillance lens and surveillance camera;
所述分光片设置在分光平片与成像镜头之间，用于分束出一部分重合后的光束，再经监视镜头射入监视相机。The beam-splitter is arranged between the beam-splitter and the imaging lens, and is used to split a part of the overlapped light beams, and then enter the monitoring camera through the monitoring lens.
进一步的，所述第一与第二单模保偏光纤的出头均为角度为i的端面，且与所述分光平面胶合在一起；Further, the outlets of the first and second single-mode polarization-maintaining optical fibers are both end faces with an angle i, and are glued together with the splitting plane;
设角度为i的端面射出的光束方向与分光平面成45°，则：Assuming that the direction of the beam emitted from the end face with angle i is 45° to the beam splitting plane, then:
其中，n表示分光平面的折射率。Among them, n represents the refractive index of the beam splitting plane.
进一步的，面阵探测器上一点采集的随时间t变化的干涉信号S(t)表示为：Further, the interference signal S(t) collected at a point on the area array detector and changing with time t is expressed as:
其中，E表示两束光的光强，ν1与ν2分别表示经过第一与第二声光移频器调频后的光束频率，R为被测镜粗糙的起伏量，c为光速，L为测量光往被测镜表面时相对于参考光多走的光程。Among them, E represents the light intensity of the two beams of light, ν1 and ν2 represent the beam frequencies after the frequency modulation of the first and second acousto-optic frequency shifters respectively, R is the rough fluctuation of the measured mirror, c is the speed of light, L It is used to measure the optical distance traveled by the light to the surface of the mirror under test relative to the reference light.
一种全视场低频外差点衍射干涉仪，包括：激光器、半波片、偏振分光镜、第一与第二声光移频器、第一与第二光纤耦合镜、第一与第二单模保偏光纤、分光平片、成像镜头、面阵探测器；其中：A full-field low-frequency heterodyne diffraction interferometer, including: a laser, a half-wave plate, a polarizing beam splitter, a first and a second acousto-optic frequency shifter, a first and a second fiber coupling mirror, a first and a second single Mode polarization-maintaining fiber, spectroscopic flat film, imaging lens, area array detector; of which:
所述激光器出射的激光经过波片与偏振分光镜分为偏振方向相互垂直的两束光；The laser light emitted by the laser is divided into two beams of light whose polarization directions are perpendicular to each other through a wave plate and a polarizing beam splitter;
其中一束光依次通过第一声光移频器、第一光纤耦合镜及第一单模保偏光纤进入分光平片，并作为参考光射向成像镜头；One beam of light sequentially passes through the first acousto-optic frequency shifter, the first fiber coupling mirror and the first single-mode polarization-maintaining fiber into the spectroscopic flat film, and is sent to the imaging lens as a reference light;
另一束光次通过第二声光移频器、第二光纤耦合镜及第二单模保偏光纤进入分光平片，并作为测量光照向被测镜；射入被测镜的测量光被被测镜发射至分光平片，再经分光平片反射垂直发散向另一方向与参考光重合；Another beam of light passes through the second acousto-optic frequency shifter, the second fiber coupling mirror and the second single-mode polarization-maintaining optical fiber to enter the spectroscopic flat film, and is directed to the mirror under test as the measurement light; the measurement light incident on the mirror under test is The measured mirror is emitted to the spectroscopic flat plate, and then vertically diverges to the other direction after being reflected by the spectroscopic flat plate to coincide with the reference light;
重合后的两束光产生干涉，经成像镜头后在面阵探测器上获得干涉图。The overlapped two beams of light produce interference, and the interferogram is obtained on the area array detector after passing through the imaging lens.
进一步的，还包括：分光片、监视镜头与监视相机；Further, it also includes: spectroscopic film, surveillance lens and surveillance camera;
所述分光片设置在分光平片与成像镜头之间，用于分束出一部分重合后的光束，再经监视镜头射入监视相机。The beam-splitter is arranged between the beam-splitter and the imaging lens, and is used to split a part of the overlapped light beams, and then enter the monitoring camera through the monitoring lens.
进一步的，所述第一与第二单模保偏光纤的出头互相垂直且分开一定距离放置并与所述分光平片贴近；Further, the outlets of the first and second single-mode polarization-maintaining optical fibers are perpendicular to each other and placed at a certain distance apart, and are close to the light-splitting flat plate;
所述的分光平片为超薄分光平片，其厚度小于1mm。The spectroscopic plain plate is an ultra-thin spectroscopic plain plate, the thickness of which is less than 1mm.
进一步的，面阵探测器上一点采集的随时间t变化的干涉信号S(t)表示为：Further, the interference signal S(t) collected at a point on the area array detector and changing with time t is expressed as:
其中，E表示两束光的光强，ν1与ν2分别表示经过第一与第二声光移频器调频后的光束频率，R为被测镜粗糙的起伏量，c为光速，L为测量光往被测镜表面时相对于参考光多走的光程。Among them, E represents the light intensity of the two beams of light, ν1 and ν2 represent the beam frequencies after the frequency modulation of the first and second acousto-optic frequency shifters respectively, R is the rough fluctuation of the measured mirror, c is the speed of light, L It is used to measure the optical distance traveled by the light to the surface of the mirror under test relative to the reference light.
由上述本发明提供的技术方案可以看出，采用声光移频器外差干涉移相，有效避免干涉仪存在运动件，测量精度进一步提高，抗干扰性好，且研制难度与成本可以降低，相比机械驱动同样的优势更明显。另外，采用低频差外差干涉与面阵探测器进行全视场连续采集，获得的信息量更丰富，更有利于精确解算相位；对大口径、长焦距曲面反射镜的面型测量，由于测量光程长，特别容易受到震动、气流等因素干扰，本发明采用的全视场低频外差干涉仪方案具有抑制震动、气流等因素干扰的能力，结合本发明设计的光路，特别适合于大口径、长焦距曲面反射镜的面型的动态测量。同时，采用两根光纤分别产生测量光与参考光，而不是用出射光的一部分，结合了光纤点衍射光容易获得，灵活调整的优点，相比典型的光纤点衍射干涉仪系统，又可以提高测量数值孔径范围。此外，采用研磨角度的光纤头与分光平片相胶合的方式实现点衍射测量光与参考光的合束，合束由半透半反平片的外侧面实现，避免了会聚光斑的波前受到衍射孔边缘的干扰，且容易实现，成本低廉。It can be seen from the above-mentioned technical solution provided by the present invention that the heterodyne interferometric phase-shifting of the acousto-optic frequency shifter can effectively avoid the existence of moving parts in the interferometer, the measurement accuracy is further improved, the anti-interference performance is good, and the difficulty and cost of development can be reduced. Compared with mechanical drive, the same advantages are more obvious. In addition, the use of low-frequency difference heterodyne interferometry and area array detectors for continuous acquisition of the entire field of view can obtain more information, which is more conducive to accurate phase calculation; for the surface shape measurement of large-aperture and long-focal-length curved mirrors, due to The measurement optical path is long, and it is particularly susceptible to interference from factors such as vibration and airflow. The full-field low-frequency heterodyne interferometer scheme adopted in the present invention has the ability to suppress interference from factors such as vibration and airflow. Combined with the optical path designed in the present invention, it is especially suitable for large Dynamic measurement of the surface shape of the caliber and long focal length curved mirrors. At the same time, two optical fibers are used to generate measurement light and reference light respectively, instead of using part of the outgoing light, which combines the advantages of easy acquisition of fiber point diffraction light and flexible adjustment. Compared with typical fiber point diffraction interferometer systems, it can also improve Measure the numerical aperture range. In addition, the combination of the point diffraction measurement light and the reference light is realized by gluing the optical fiber head with a grinding angle and the spectroscopic flat plate. The interference at the edge of the diffraction hole is easy to implement and low in cost.
附图说明Description of drawings
为了更清楚地说明本发明实施例的技术方案，下面将对实施例描述中所需要使用的附图作简单地介绍，显而易见地，下面描述中的附图仅仅是本发明的一些实施例，对于本领域的普通技术人员来讲，在不付出创造性劳动的前提下，还可以根据这些附图获得其他附图。In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the accompanying drawings that need to be used in the description of the embodiments. Obviously, the accompanying drawings in the following description are only some embodiments of the present invention. For Those of ordinary skill in the art can also obtain other drawings based on these drawings on the premise of not paying creative work.
图1为本发明背景技术提供的典型的光纤点衍射干涉系统示意图；Fig. 1 is the typical optical fiber point diffraction interference system schematic diagram that background technology of the present invention provides;
图2为本发明背景技术提供的典型的针孔点衍射干涉系统示意图；2 is a schematic diagram of a typical pinhole point diffraction interference system provided by the background technology of the present invention;
图3为本发明实施例提供的一种全视场低频外差点衍射干涉仪的结构示意图；3 is a schematic structural diagram of a full-field low-frequency heterodyne diffraction interferometer provided by an embodiment of the present invention;
图4为本发明实施例提供的面阵探测器采集的信号形式示意图；Fig. 4 is a schematic diagram of the signal form collected by the area array detector provided by the embodiment of the present invention;
图5为本发明实施例提供的另一种全视场低频外差点衍射干涉仪的结构示意图。FIG. 5 is a schematic structural diagram of another full-field low-frequency heterodyne diffraction interferometer provided by an embodiment of the present invention.
具体实施方式Detailed ways
下面结合本发明实施例中的附图，对本发明实施例中的技术方案进行清楚、完整地描述，显然，所描述的实施例仅仅是本发明一部分实施例，而不是全部的实施例。基于本发明的实施例，本领域普通技术人员在没有做出创造性劳动前提下所获得的所有其他实施例，都属于本发明的保护范围。The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only some of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by persons of ordinary skill in the art without making creative efforts belong to the protection scope of the present invention.
图3为本发明实施例提供的一种全视场低频外差点衍射干涉仪的结构示意图。如图3所示，其主要包括：FIG. 3 is a schematic structural diagram of a full-field low-frequency heterodyne diffraction interferometer provided by an embodiment of the present invention. As shown in Figure 3, it mainly includes:
一种全视场低频外差点衍射干涉仪，其特征在于，包括：激光器、第一与第二半波片、偏振分光镜、第一与第二声光移频器、第一与第二光纤耦合镜、第一与第二单模保偏光纤、分光平片、成像镜头、面阵探测器；其中：A full-field low-frequency heterodyne diffraction interferometer is characterized in that it includes: lasers, first and second half-wave plates, polarization beam splitters, first and second acousto-optic frequency shifters, first and second optical fibers Coupling mirror, first and second single-mode polarization-maintaining optical fibers, spectroscopic flat film, imaging lens, area array detector; wherein:
所述激光器出射的激光经过第一半波片与偏振分光镜分为偏振方向相互垂直的两束光；同时，通过旋转第一半波片可以任意调整两束光的分光比；The laser light emitted by the laser is divided into two beams of light whose polarization directions are perpendicular to each other through the first half-wave plate and the polarizing beam splitter; at the same time, the beam splitting ratio of the two beams can be adjusted arbitrarily by rotating the first half-wave plate;
其中一束光依次通过第二半波片、第一声光移频器、第一光纤耦合镜及第一单模保偏光纤进入分光平片，并作为参考光射向成像镜头；One beam of light sequentially passes through the second half-wave plate, the first acousto-optic frequency shifter, the first fiber-coupled mirror and the first single-mode polarization-maintaining fiber into the beam-splitting flat plate, and is sent to the imaging lens as a reference light;
另一束光次通过第二声光移频器、第二光纤耦合镜及第二单模保偏光纤进入分光平片，并作为测量光照向被测镜；射入被测镜的测量光被被测镜发射至分光平片，再经分光平片反射垂直发散向另一方向与参考光重合；Another beam of light passes through the second acousto-optic frequency shifter, the second fiber coupling mirror and the second single-mode polarization-maintaining optical fiber to enter the spectroscopic flat film, and is directed to the mirror under test as the measurement light; the measurement light incident on the mirror under test is The measured mirror is emitted to the spectroscopic flat plate, and then vertically diverges to the other direction after being reflected by the spectroscopic flat plate to coincide with the reference light;
重合后的两束光产生干涉，经成像镜头后在面阵探测器上获得干涉图。The overlapped two beams of light produce interference, and the interferogram is obtained on the area array detector after passing through the imaging lens.
进一步的，还包括：分光片、监视镜头与监视相机；Further, it also includes: spectroscopic film, surveillance lens and surveillance camera;
所述分光片设置在分光平片与成像镜头之间，用于分束出一部分重合后的光束，再经监视镜头射入监视相机；用来辅助光路与干涉图的调整。The beam splitter is arranged between the beam splitter and the imaging lens, and is used to split a part of the overlapped light beams, and then enter the monitoring camera through the monitoring lens; it is used to assist the adjustment of the optical path and the interference pattern.
进一步的，所述第一与第二单模保偏光纤的出头均为角度为i的端面(如图1中的第一与第二角度光纤头)，且与所述分光平面胶合在一起；Further, the heads of the first and second single-mode polarization-maintaining fibers are both end faces with an angle i (such as the first and second angle fiber heads in Figure 1), and are glued together with the splitting plane;
设角度为i的端面射出的光束方向与分光平面成45°，则：Assuming that the direction of the beam emitted from the end face with angle i is 45° to the beam splitting plane, then:
其中，n表示分光平面的折射率。Among them, n represents the refractive index of the beam splitting plane.
本发明实施例中，第一与第二声光移频器可以改变激光频率，两个声光移频器的移频量不同，差频可以为几赫兹或几十赫兹量级的低差频，阵探测器相机采用数十或数百赫兹量级采样频率，因此可以准确探测外差的拍频信号。面阵探测器上一点采集的随时间t变化的干涉信号S(t)表示为：In the embodiment of the present invention, the first and second acousto-optic frequency shifters can change the laser frequency, the frequency shifts of the two acousto-optic frequency shifters are different, and the difference frequency can be a low difference frequency on the order of several hertz or tens of hertz , the array detector camera adopts a sampling frequency of tens or hundreds of hertz, so it can accurately detect heterodyne beat frequency signals. The interference signal S(t) changing with time t collected at a point on the area array detector is expressed as:
其中，E表示两束光的光强，ν1与ν2分别表示经过第一与第二声光移频器调频后的光束频率，R为被测镜粗糙的起伏量，c为光速，L为测量光往被测镜表面时相对于参考光多走的光程。Among them, E represents the light intensity of the two beams of light, ν1 and ν2 represent the beam frequencies after the frequency modulation of the first and second acousto-optic frequency shifters respectively, R is the rough fluctuation of the measured mirror, c is the speed of light, L It is used to measure the optical distance traveled by the light to the surface of the mirror under test relative to the reference light.
由于拍频的存在，干涉条纹会以ν1-ν2的频率扫描起来，面阵探测器的一点对应待测面上的一个点，探测器相机连续采集一组面阵照片，即为一组数据立方，对应相同每一点的值抽取出来为一余弦周期信号，即为S(t)的形式，如图4所示。由信号形式可以看出，不同点由于粗糙起伏的R值不同，造成相机上对应点探测的信号相位不同。利用傅里叶分析或其它数据处理方法可解算每点出信号的相位，合成起来进行去噪、相位解缠解、面型复原等计算后即可得到待测表面的起伏量，即实现了全视场外差对面型的测量。Due to the existence of the beat frequency, the interference fringes will be scanned at the frequency of ν1 -ν2. One point of the area detector corresponds to a point on the surface to be measured. The detector camera continuously collects a group of area array photos, which is a set of In the data cube, the value corresponding to each point is extracted as a cosine periodic signal, which is in the form of S(t), as shown in Figure 4. It can be seen from the signal form that different points have different R values due to rough fluctuations, resulting in different phases of signals detected by corresponding points on the camera. Use Fourier analysis or other data processing methods to solve the phase of the signal at each point, and combine them for denoising, phase unwrapping, surface restoration, etc. After calculation, the undulation of the surface to be measured can be obtained, which is realized. Measurement of surface type by heterodyning across the entire field of view.
针对现有技术几乎都采用机械驱动进行移相，达到高精度难度大，抗扰性差的确定，本发明上述方案采用声光移频器外差干涉移相，有效避免干涉仪存在运动件，测量精度进一步提高，抗干扰性好，且研制难度与成本可以降低。相比机械驱动同样的优势更明显。In view of the fact that almost all existing technologies use mechanical drive for phase shifting, it is difficult to achieve high precision, and the determination of poor immunity, the above scheme of the present invention uses acousto-optic frequency shifter heterodyne interference phase shifting, effectively avoiding the existence of moving parts in the interferometer, measuring The precision is further improved, the anti-interference performance is good, and the development difficulty and cost can be reduced. Compared with mechanical drive, the same advantages are more obvious.
此外，采用低频差外差干涉与面阵探测器进行全视场连续采集，获得的信息量更丰富，更有利于精确解算相位。对大口径、长焦距曲面反射镜的面型测量，由于测量光程长，特别容易受到震动、气流等因素干扰，本发明采用的全视场低频外差干涉仪方案具有抑制震动、气流等因素干扰的能力，结合本发明设计的光路，特别适合于大口径、长焦距曲面反射镜的面型的动态测量。In addition, the use of low-frequency difference heterodyne interferometry and area array detectors for continuous acquisition of the entire field of view can obtain more information, which is more conducive to accurate phase calculation. For the surface shape measurement of large-diameter and long-focus curved mirrors, due to the long measurement optical path, it is particularly susceptible to interference from factors such as vibration and airflow. The ability of interference, combined with the optical path designed by the present invention, is especially suitable for the dynamic measurement of the surface shape of the curved mirror with large aperture and long focal length.
同时，采用两根光纤分别产生测量光与参考光，而不是用出射光的一部分，结合了光纤点衍射光容易获得，灵活调整的优点，相比典型的光纤点衍射干涉仪系统，又可以提高测量数值孔径范围。At the same time, two optical fibers are used to generate measurement light and reference light respectively, instead of using part of the outgoing light, which combines the advantages of easy acquisition of fiber point diffraction light and flexible adjustment. Compared with typical fiber point diffraction interferometer systems, it can also improve Measure the numerical aperture range.
另外，采用研磨角度的光纤头与分光平片相胶合的方式实现点衍射测量光与参考光的合束，合束由半透半反平片的外侧面实现，避免了会聚光斑的波前受到衍射孔边缘的干扰。且容易实现，成本低廉。In addition, the combination of the point diffraction measurement light and the reference light is achieved by gluing the optical fiber head with a grinding angle and the spectroscopic flat plate. Interference at the edge of the diffraction hole. And it is easy to implement and low in cost.
本发明实施例还提供一种全视场低频外差点衍射干涉仪，如图5所示，其主要包括：激光器、半波片、偏振分光镜、第一与第二声光移频器、第一与第二光纤耦合镜、第一与第二单模保偏光纤、分光平片、成像镜头、面阵探测器；其中：The embodiment of the present invention also provides a full-field low-frequency heterodyne diffraction interferometer, as shown in FIG. One and the second fiber optic coupling mirror, the first and the second single-mode polarization-maintaining fiber, a spectroscopic flat film, an imaging lens, and an area array detector; wherein:
所述激光器出射的激光经过波片与偏振分光镜分为偏振方向相互垂直的两束光；The laser light emitted by the laser is divided into two beams of light whose polarization directions are perpendicular to each other through a wave plate and a polarizing beam splitter;
其中一束光依次通过第一声光移频器、第一光纤耦合镜及第一单模保偏光纤进入分光平片，并作为参考光射向成像镜头；One beam of light sequentially passes through the first acousto-optic frequency shifter, the first fiber coupling mirror and the first single-mode polarization-maintaining fiber into the spectroscopic flat film, and is sent to the imaging lens as a reference light;
另一束光次通过第二声光移频器、第二光纤耦合镜及第二单模保偏光纤进入分光平片，并作为测量光照向被测镜；射入被测镜的测量光被被测镜发射至分光平片，再经分光平片反射垂直发散向另一方向与参考光重合；Another beam of light passes through the second acousto-optic frequency shifter, the second fiber coupling mirror and the second single-mode polarization-maintaining optical fiber to enter the spectroscopic flat film, and is directed to the mirror under test as the measurement light; the measurement light incident on the mirror under test is The measured mirror is emitted to the spectroscopic flat plate, and then vertically diverges to the other direction after being reflected by the spectroscopic flat plate to coincide with the reference light;
重合后的两束光产生干涉，经成像镜头后在面阵探测器上获得干涉图。The overlapped two beams of light produce interference, and the interferogram is obtained on the area array detector after passing through the imaging lens.
进一步的，其还包括：分光片、监视镜头与监视相机；Further, it also includes: spectroscopic film, surveillance lens and surveillance camera;
所述分光片设置在分光平片与成像镜头之间，用于分束出一部分重合后的光束，再经监视镜头射入监视相机。The beam-splitter is arranged between the beam-splitter and the imaging lens, and is used to split a part of the overlapped light beams, and then enter the monitoring camera through the monitoring lens.
本发明实施例中，所述第一与第二单模保偏光纤的出头互相垂直且分开一定距离放置并与所述分光平片贴近(如图5所示的第一光纤头与第二光纤头)；所述的分光平片为超薄分光平片，其厚度小于1mm。基于这种结构可使测量光经超薄分光平片反射的光可以避开参考光光纤端面而不产生杂光串扰。In the embodiment of the present invention, the heads of the first and second single-mode polarization-maintaining fibers are perpendicular to each other and placed at a certain distance apart, and are placed close to the beam splitter (the first fiber head and the second fiber as shown in Figure 5 head); the spectroscopic plain film is an ultra-thin spectroscopic plain film, and its thickness is less than 1mm. Based on this structure, the light reflected by the ultra-thin spectroscopic flat sheet of the measuring light can avoid the end face of the reference optical fiber without generating stray light crosstalk.
另外，面阵探测器上一点采集的随时间t变化的干涉信号S(t)表示为：In addition, the interference signal S(t) changing with time t collected at a point on the area array detector is expressed as:
其中，E表示两束光的光强，ν1与ν2分别表示经过第一与第二声光移频器调频后的光束频率，R为被测镜粗糙的起伏量，c为光速，L为测量光往被测镜表面时相对于参考光多走的光程。Among them, E represents the light intensity of the two beams of light, ν1 and ν2 represent the beam frequencies after the frequency modulation of the first and second acousto-optic frequency shifters respectively, R is the rough fluctuation of the measured mirror, c is the speed of light, L It is used to measure the optical distance traveled by the light to the surface of the mirror under test relative to the reference light.
本发明实施例提供的全视场低频外差点衍射干涉仪与前述实施例的全视场低频外差点衍射干涉仪原理基本一致，具体的可参见前文的描述，此处不再赘述。The principle of the full-field low-frequency heterodyne diffraction interferometer provided by the embodiment of the present invention is basically the same as that of the full-field low-frequency heterodyne diffraction interferometer in the foregoing embodiments. For details, please refer to the foregoing description, and details will not be repeated here.
以上所述，仅为本发明较佳的具体实施方式，但本发明的保护范围并不局限于此，任何熟悉本技术领域的技术人员在本发明披露的技术范围内，可轻易想到的变化或替换，都应涵盖在本发明的保护范围之内。因此，本发明的保护范围应该以权利要求书的保护范围为准。The above is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any person familiar with the technical field can easily conceive of changes or changes within the technical scope disclosed in the present invention. Replacement should be covered within the protection scope of the present invention. Therefore, the protection scope of the present invention should be determined by the protection scope of the claims.
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