技术领域technical field
本发明涉及光学技术领域，尤其涉及一种基于声光低频差移相的双孔外差干涉仪。The invention relates to the field of optical technology, in particular to a dual-aperture heterodyne interferometer based on acousto-optic low-frequency difference phase shifting.
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。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.
目前的干涉仪大多采用偏振移相方式，两束互相垂直的光经过其移相模块，实现四步或多步同时移相，求解被测件的面型。然而，偏振移相方法虽然会抗振，但同时拍摄的多幅移相图要配准解相位，势必会带来匹配误差；此外，采集只能得到有限幅的干涉图，对其相位提取的精度受到制约，并且几幅移相图的移相精度受到移相模块的限制，直接影响到面型反演精度。Most of the current interferometers use the polarization phase shifting method. Two beams of light that are perpendicular to each other pass through the phase shifting module to achieve four or more steps of simultaneous phase shifting to solve the surface shape of the measured object. However, although the polarization phase-shifting method is anti-vibration, if multiple phase-shifting images taken at the same time need to be registered to solve the phase, it will inevitably bring matching errors; The accuracy is restricted, and the phase-shifting accuracy of several phase-shifting maps is limited by the phase-shifting module, which directly affects the accuracy of surface shape inversion.
发明内容Contents of the invention
本发明的目的是提供一种基于声光低频差移相的双孔外差干涉仪，可极大的提高测量精度。The object of the present invention is to provide a dual-aperture heterodyne interferometer based on acousto-optic low-frequency difference phase-shifting, which can greatly improve measurement accuracy.
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:
一种基于声光低频差移相的双孔外差干涉仪，包括：激光器、两块半波片、偏振分光镜PBS、两个声光移频器、两个光纤耦合器、两根保偏光纤、两个分光镜、准直镜、标准镜、待测镜、滤波孔、两个成像镜及两个探测器；其中：A dual-aperture heterodyne interferometer based on acousto-optic low-frequency difference phase shifting, including: a laser, two half-wave plates, a polarization beam splitter PBS, two acousto-optic frequency shifters, two fiber couplers, two polarization-maintaining Optical fiber, two beam splitters, collimating mirror, standard mirror, test mirror, filter hole, two imaging mirrors and two detectors; where:
激光器出射激光经过半波片1后射入PBS，由PBS将入射激光分为两束激光；其中一束激光经过声光移频器1后由光纤耦合器1耦合进入保偏光纤1；另一束激光依次经过半波片2与声光移频器2后由光纤耦合器2耦合进入保偏光纤2；The laser output from the laser passes through the half-wave plate 1 and then enters the PBS, and the PBS divides the incident laser into two beams of laser light; one of the laser beams passes through the acousto-optic frequency shifter 1 and is coupled into the polarization-maintaining fiber 1 by the fiber coupler 1; The laser beam passes through the half-wave plate 2 and the acousto-optic frequency shifter 2 in turn, and then is coupled into the polarization-maintaining fiber 2 by the fiber coupler 2;
通过调整标准镜和待测镜的倾斜，使保偏光纤2出射的激光依次通过分光1、准直镜与标准镜后射入待测镜，经待测镜反射后原路返回，并被分光镜1反射至滤波孔；使保偏光纤1出射的激光均通过依次通过分光1与准直镜后射入标准镜，经标准镜反射后原路返回，并被分光镜1反射至滤波孔；By adjusting the inclination of the standard mirror and the mirror to be tested, the laser light emitted from the polarization-maintaining fiber 2 passes through the beam splitter 1, the collimating mirror and the standard mirror in sequence, and then enters the mirror to be tested. After being reflected by the mirror to be tested, the laser returns to the original path and is split The mirror 1 is reflected to the filter hole; the laser light emitted from the polarization-maintaining fiber 1 passes through the beam splitter 1 and the collimating mirror in turn, and then enters the standard mirror, returns to the original path after being reflected by the standard mirror, and is reflected by the beam splitter 1 to the filter hole;
射入滤波孔的两束激光经过分光镜2后被分为两路；其中一路作为调整监视光路，经过成像镜1射入探测器1中；另一路作为采集干涉图光路经过成像镜2射入探测器2中。The two laser beams injected into the filter hole are divided into two paths after passing through the beam splitter 2; one of them is used as an adjustment monitoring optical path, and is injected into the detector 1 through the imaging mirror 1; Detector 2.
进一步的，所述PBS按照任意功率比将入射激光分为两束激光。Further, the PBS divides the incident laser light into two laser beams according to any power ratio.
进一步的，所述半波片2用于改变激光的偏振方向，使得两束经过声光移频器后的激光偏振方向一致。Further, the half-wave plate 2 is used to change the polarization direction of the laser light, so that the polarization directions of the two laser beams after passing through the acousto-optic frequency shifter are consistent.
进一步的，两个声光移频器的移频量不同，差频后为几赫兹或几十赫兹量级的低差频。Further, the frequency shifting amounts of the two acousto-optic frequency shifters are different, and the difference frequency is a low difference frequency in the order of several hertz or tens of hertz.
进一步的，所述保偏光纤1与保偏光纤2的末端通过固定模块固定，确保两束激光的出射方向一致，并且横向空间有一定错位。Further, the ends of the polarization-maintaining optical fiber 1 and the polarization-maintaining optical fiber 2 are fixed by a fixing module to ensure that the outgoing directions of the two laser beams are consistent and there is a certain displacement in the lateral space.
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，探测器连续采集同一光路干涉图，不存在多幅干涉图样配准的问题，测量精度进一步提高；此外，采用低频差外差干涉与高速相机连续采集，获得的信息量更丰富，更有利于精确解算相位，更有利于克服噪声等因素影响。It can be seen from the above-mentioned technical solution provided by the present invention that the detector continuously collects the interferogram of the same optical path by using the heterodyne interference phase shifting of the acousto-optic frequency shifter, and there is no problem of registration of multiple interference patterns, and the measurement accuracy is further improved; in addition , using low-frequency heterodyne interferometry and high-speed camera continuous acquisition, the amount of information obtained is more abundant, it is more conducive to accurate phase calculation, and it is more conducive to overcoming the influence of noise and other factors.
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 efforts.
图1为本发明实施例提供的一种基于声光低频差移相的双孔外差干涉仪的光路示意图；Figure 1 is a schematic diagram of the optical path of a dual-aperture heterodyne interferometer based on acousto-optic low-frequency difference phase-shifting provided by an embodiment of the present invention;
图2为本发明实施例提供的采集的信号形式示意图。FIG. 2 is a schematic diagram of a collected signal form provided by an embodiment of the present invention.
具体实施方式detailed description
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.
本发明实施例提供一种基于声光低频差移相的双孔外差干涉仪，其光路结构如图1所示，主要包括：激光器、两块半波片、偏振分光镜PBS、两个声光移频器、两个光纤耦合器、两根保偏光纤、两个分光镜、准直镜、标准镜、待测镜、滤波孔、两个成像镜及两个探测器；其中：The embodiment of the present invention provides a double-aperture heterodyne interferometer based on acousto-optic low-frequency difference phase-shifting. Its optical path structure is shown in Fig. Optical frequency shifter, two fiber couplers, two polarization-maintaining fibers, two beam splitters, collimating mirrors, standard mirrors, mirrors to be tested, filter holes, two imaging mirrors and two detectors; where:
激光器出射激光经过半波片1后射入PBS，由PBS将入射激光分为两束激光；其中一束激光经过声光移频器1后由光纤耦合器1耦合进入保偏光纤1；另一束激光依次经过半波片2与声光移频器2后由光纤耦合器2耦合进入保偏光纤2；The laser output from the laser passes through the half-wave plate 1 and then enters the PBS, and the PBS divides the incident laser into two beams of laser light; one of the laser beams passes through the acousto-optic frequency shifter 1 and is coupled into the polarization-maintaining fiber 1 by the fiber coupler 1; The laser beam passes through the half-wave plate 2 and the acousto-optic frequency shifter 2 in turn, and then is coupled into the polarization-maintaining fiber 2 by the fiber coupler 2;
通过调整标准镜和待测镜的倾斜，使保偏光纤2出射的激光依次通过分光1、准直镜与标准镜后射入待测镜，经待测镜反射后原路返回，并被分光镜1反射至滤波孔；使保偏光纤1出射的激光均通过依次通过分光1与准直镜后射入标准镜，经标准镜反射后原路返回，并被分光镜1反射至滤波孔；By adjusting the inclination of the standard mirror and the mirror to be tested, the laser light emitted from the polarization-maintaining fiber 2 passes through the beam splitter 1, the collimating mirror and the standard mirror in sequence, and then enters the mirror to be tested. After being reflected by the mirror to be tested, the laser returns to the original path and is split The mirror 1 is reflected to the filter hole; the laser light emitted from the polarization-maintaining fiber 1 passes through the beam splitter 1 and the collimating mirror in turn, and then enters the standard mirror, returns to the original path after being reflected by the standard mirror, and is reflected by the beam splitter 1 to the filter hole;
射入滤波孔的两束激光经过分光镜2后被分为两路；其中一路作为调整监视光路，经过成像镜1射入探测器1中；另一路作为采集干涉图光路经过成像镜2射入探测器2中。The two laser beams injected into the filter hole are divided into two paths after passing through the beam splitter 2; one of them is used as an adjustment monitoring optical path, and is injected into the detector 1 through the imaging mirror 1; Detector 2.
本发明实施例中，所述PBS按照任意功率比将入射激光分为两束激光。In the embodiment of the present invention, the PBS divides the incident laser light into two laser beams according to any power ratio.
本发明实施例中，所述半波片2用于改变激光的偏振方向，使得两束经过声光移频器后的激光偏振方向一致。In the embodiment of the present invention, the half-wave plate 2 is used to change the polarization direction of the laser, so that the polarization directions of the two laser beams after passing through the acousto-optic frequency shifter are consistent.
本发明实施例中，两个声光移频器的移频量不同，差频后为几赫兹或几十赫兹量级的低差频。In the embodiment of the present invention, the frequency shifting amounts of the two acousto-optic frequency shifters are different, and the difference frequency is a low difference frequency in the order of several hertz or tens of hertz.
本发明实施例中，所述保偏光纤1与保偏光纤2的末端通过固定模块固定，确保两束激光的出射方向一致，并且横向空间有一定错位。In the embodiment of the present invention, the ends of the polarization-maintaining optical fiber 1 and the polarization-maintaining optical fiber 2 are fixed by a fixing module to ensure that the outgoing directions of the two laser beams are consistent and there is a certain displacement in the lateral space.
本发明实施例中，采集干涉图光路中的两束激光经过光纤出射，波面可认为理想波面，各自携带者标准镜和待测镜的面型进行干涉，通过求解干涉图即可得到待测镜的面型。In the embodiment of the present invention, the two beams of laser light in the optical path of the acquisition interferogram are emitted through the optical fiber, and the wave front can be regarded as an ideal wave front. Each of them carries the standard mirror and the surface shape of the mirror to be tested for interference, and the mirror to be tested can be obtained by solving the interferogram face shape.
求解原理如下：The solution principle is as follows:
设移频后两束光的频率分别为ν1和ν2，频差ν1-ν2为赫兹或几十赫兹量级，高速相机采用数百赫兹量级，因此可以准确探测外差的拍频信号。设两束光的光强都为E，则高速相机(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：Suppose the frequencies of the two beams of light after the frequency shift are ν1 and ν2 , and the frequency difference ν1 -ν2 is on the order of hertz or tens of hertz, and the high-speed camera adopts the order of hundreds of hertz, so it can accurately detect the heterodyne beat frequency signal. Assuming that the light intensity of the two beams is E, the interference signal S(t) collected at a point on the high-speed camera (detector) and changing with time t is expressed as:
其中，L为保偏光纤2出射的激光往返待测镜表面时相对于保偏光纤1出射的激光多走的光程，R为待测镜表面粗糙的起伏量，c为光速。高速相机的一点对应待测面上的一个点，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2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，即可获得待测镜表面的起伏量，针对不同频率的傅里叶分析即可分别获得不同的干涉信息。分别计算每相邻点的R值之差就可复原出待测镜的面形。Wherein, L is the optical distance traveled by the laser light emitted from the polarization maintaining fiber 2 to and from the surface of the mirror under test compared with the laser light emitted from the polarization maintaining fiber 1, R is the roughness of the surface of the mirror under test, and c is the speed of light. A point of the high-speed camera corresponds to a point on the surface to be measured. The camera continuously collects a set of area array photos, which is a set of data cubes. The value corresponding to each point is extracted as a cosine period signal, which is S(t) form, as shown in Figure 2. 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. By using Fourier analysis or other data processing methods, the phase of the output signal at each point can be solved to obtain the fluctuation of the surface of the mirror to be tested, and different interference information can be obtained by Fourier analysis for different frequencies. The surface shape of the mirror to be tested can be restored by calculating the difference between the R values of each adjacent point.
本发明实施例的上述方案中，采用声光移频器外差干涉移相，探测器连续采集同一光路干涉图，不存在多幅干涉图样配准的问题，测量精度进一步提高；此外，采用低频差外差干涉与高速相机连续采集，获得的信息量更丰富，更有利于精确解算相位，更有利于克服噪声等因素影响。In the above-mentioned solution of the embodiment of the present invention, the heterodyne interference phase-shifting of the acousto-optic frequency shifter is used, and the detector continuously collects the interferogram of the same optical path, so there is no problem of registration of multiple interference patterns, and the measurement accuracy is further improved; in addition, the low-frequency Heterodyne interferometry and continuous acquisition by high-speed cameras can obtain richer information, which is more conducive to accurate phase calculation, and is more conducive to overcoming the influence of noise and other factors.
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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