At present, the high-precision adjustment of infrared refracting lenses at home and abroad mainly adopts the method of precise centering. According to the centering instrument to measure the trajectory of the image point of the optical part rotating with the rotary axis, the center deviation of the optical part is calculated and adjusted. However, commonly used infrared refraction materials mainly include germanium, silicon, zinc selenide, and chalcogenide glass. Due to the characteristics of the material preparation process, it is difficult to guarantee the uniformity of the refractive index. For example, the uniformity of the refractive index of the germanium material is about 2 ×10⁻⁴, the refractive index uniformity of chalcogenide glass at 3.39 μm is about 1×10⁻⁴. Compared with the 10⁻⁶ level of refractive index uniformity of visible refractive materials, the high deviation of the refractive index uniformity of infrared materials is equivalent to introducing additional irregular aberrations in the imaging system, which will lead to abnormal lens wavefront errors and image quality This is the main difficulty in the adjustment of high-performance infrared refracting lenses, and the core of precision centering adjustment is the control of the optical axis consistency and spacing of optical components, which is powerless to correct the irregular aberrations of the system.
According to the report of Memes Consulting, recently, the scientific research team of Beijing Institute of Space Mechatronics published an article on the theme of "Infrared Refractive Lens Assembly and Adjustment Technology for High Performance" in the journal "Infrared and Laser Engineering". The first author and corresponding author of this article is Huang Yang, a senior engineer, who is mainly engaged in the research work of optical assembly and detection.
Aiming at the problem that the uneven refractive index of infrared optical materials leads to abnormal aberrations in the system wavefront, which leads to a serious decline in the image quality of the lens, this study proposes a system wavefront compensation method that combines iterative adjustment of the position of the optical components and surface modification. , to realize the adjustment of high-performance infrared refraction lens.
Analysis of Influence of Infrared Material Refractive Index Uniformity Deviation
The optical materials used for infrared refraction lenses mainly include germanium, silicon, zinc selenide and chalcogenide glass, etc., which are mostly prepared by crystal growth. The materials are affected by fluctuations in temperature or pressure during the growth process, which will cause The residual stress level of lattice growth in different regions is different, which causes a certain deviation in the refractive index of each region of the material.
Optically, a four-step interferometry is used to measure the uniformity of the refractive index of optical materials, as shown in Figure 1, by testing the error distribution of the reflected wavefront, transmitted wavefront, and cavity wavefront of the test optical path on the front and rear surfaces of the planar sample of the material to be tested .
Figure 1 (a) S1 reflection wavefront on the front surface of the sample; (b) S2 reflection wavefront on the back surface of the sample; (c) transmission wavefront of the sample; (d) cavity wavefront of the test optical path
Using a 3.39 μm infrared interferometer to measure the uniformity of the refractive index of the Φ200 mm infrared refractive material samples provided by some domestic material manufacturers, the results are shown in Figure 2. The deviation of the uniformity of the refractive index of the samples silicon and zinc selenide is small and the distribution is relatively Uniform, but the uniformity of refractive index of samples germanium and zinc sulfide has large deviation and irregular distribution.
Figure 2 Refractive index uniformity distribution of (a) sample silicon, (b) sample zinc selenide, (c) sample germanium, (d) sample zinc sulfide
Lens setup for high performance
Correction of primary aberrations
Position iterative adjustment to correct the wavefront principle: According to the theory of computer-aided adjustment technology, the functional relationship between the comprehensive aberration of the ideal optical system and the position and structure parameters of each optical component can be expressed by an approximate linear equation.
According to the residual aberration of the measured optical system, the primary aberration of the optical system can be reduced through the position compensation adjustment of the optical parts, so that the adjusted optical system index is as close as possible to the theoretical system. However, due to the high deviation of the uniformity of the refractive index of infrared optical materials, after the lens is precisely centered according to the optical design value, a large number of irregular aberrations remain in the wavefront of the system, and there is a nonlinear relationship with the adjustment amount, so it can only be adjusted by the adjustment amount ΔX The residual primary aberrations are converged in an iterative manner.
On-line adjustment detection: In order to realize the optimal treatment of the residual primary aberration of the infrared refraction lens, it is necessary to unify the lens adjustment and detection. Aberration online iteratively adjusts the position of optical components.
For this reason, based on the precise centering of the infrared refracting lens, a 45° turning mirror is placed on the precision turntable of the centering instrument, and the lens supporting tooling is used to convert the vertical optical axis of the lens to the horizontal direction, as shown in Figure 3. Install a plane reflector with an adjustable angle above the entrance pupil of the lens, through the screwing in and out of the working condition switching device, the lens detection and the arbitrary switching of the working conditions of the adjustment can be realized, and combined with the infrared interferometer, the infrared refracting lens can be adjusted The process has the function of system wavefront detection.
Figure 3 Model diagram of online assembly and testing platform
The key to online iterative adjustment is that the optical parts can be adjusted in real time in tilt, translation and spacing in the lens barrel. The axial combined lifting of the three support points on the end face of the optical part, combined with the radial coordinated expansion and contraction of the top wire on the side wall of the lens barrel, realizes the five-dimensional freedom adjustment of the optical part in the lens barrel.
Figure 4 Schematic diagram of position iterative adjustment
Compensation for middle and high order aberrations
The principle of wavefront compensation by surface shape modification: the outgoing wavefront of an ideal imaging optical system should be a perfect spherical wave. When the system has aberrations, the outgoing wavefront of the system will be deformed, and there is wave aberration between the actual wavefront and the ideal wavefront, as shown in Figure 5 shown.
Figure 5 Schematic diagram of the influence of aberration on the wavefront of the system
The remaining mid-to-high order aberrations in the optical system usually have little to do with the positional misalignment of the optical parts, but are mainly related to the surface shape of the optical parts, the processing accuracy of the parameters, and the material properties. According to the compensation principle of the wavefront, a compensation wavefront for anti-residual wave aberration can be introduced into the system by deforming the surface shape of the optical parts, and the outgoing wavefront can be corrected into a perfect spherical wave, as shown in Figure 6.
Figure 6 Schematic diagram of system wavefront compensation
The distribution of wave aberration of optical system and surface aberration of optical components can be characterized by Zernike polynomial in polar coordinate form. In order to reduce the difficulty of surface shape modification of the deformable mirror and improve the matching degree of the compensation wavefront, the primary aberration of the wavefront of the system is corrected by adjusting the position of the optical parts, and only the remaining middle and high-order aberrations are compensated for the wavefront by surface shape modification. Generally speaking, the surface shape of optical components used at full aperture at the pupil of a refractive lens has the most balanced impact on the wave aberration of the full field of view system. In view of the difficulty of shape repair, the anti-residual wave aberration is equivalent to the additional deformed surface shape on the surface of the optical component at the pupil, and the surface shape repair is performed strictly according to the aberration distribution of the surface shape.
Anti-residual wave aberration calculation: Refractive lenses usually have a large field of view. Due to the high deviation of the uniformity of the refractive index of the material, the residual wave aberration of the system in different fields of view will have a certain relative irregularity due to the different incident light paths. , it is impossible to obtain a wavefront that can perfectly compensate for the entire field of view.
Figure 7 Schematic diagram of the relationship between wave aberration and incident angle
Experimental verification
In order to verify the wavefront compensation effect of the system, this technology is used to install and adjust a mid-wave infrared refracting lens with a full field of view of 13°. The optical structure of the lens is shown in Figure 8, which consists of a window and six lenses. The front surface of the window is the pupil of the system, the design value of the MTF (@25 lp/mm) of the full field of view of the lens is 0.70, and some parameters of the optical system are shown in Table 1.
Figure 8 Optical structure diagram of infrared lens
Table 1 Some parameters of infrared lens optical system
The lens assembly adopts a precise centering method. In order to facilitate online iterative adjustment of optical parts, a clamping device installation position is reserved on the side wall of the lens barrel. An online assembly and adjustment detection platform was constructed based on the infrared centering instrument and infrared interferometer. The adjustment accuracy of the clamping device is ±5 μm, and its structure and connection with the lens barrel are shown in Figure 9.
Figure 9 (a) The structural form of the clamping device; (b) The connection method between the clamping device and the lens barrel
After the lens is assembled and adjusted in strict accordance with the optical design tolerance, the plane reflector is rotated into the optical path, and the system wavefront and MTF (@25 lp/mm) of the three fields of view of the lens are tested online using a 3.39 μm infrared interferometer, and the aberration distribution 36 Zernike coefficients were used for fitting, and the test results are shown in Table 2. Due to the high deviation of the uniformity of the refractive index of the infrared material, the wavefronts of each field of view system are distributed in a clover shape, and large aberrations of each order remain. The average MTF (@25 lp/mm) of the three fields of view is only 0.31 , much lower than the design value.
Table 2 Image quality test results after precise centering of the lens
Based on the sensitivity matrix of mirrors 1 to 6, the wavefront primary aberrations of the three field of view systems are corrected. It is calculated by the iterative adjustment and compensation method of the position of the optical parts that through the combined adjustment of the axial movement of the mirror 2 and the radial translation of the mirror 4, the primary aberration of the wavefront of the system can be corrected quickly and optimally. On the installation and testing platform, according to the actual wavefront measured by the system, the axial movement of mirror 2 and the radial translation of mirror 4 are iteratively adjusted in real time by using the clamping device and the top screw on the side wall of the lens barrel until the primary aberrations reach the minimum value. After adjustment, the wavefront and MTF (@25 lp/mm) test results of the lens system are shown in Table 3. The primary aberrations of the system wavefront in the three fields of view are basically corrected, and the residual middle and high-order aberrations are basically unchanged. MTF (@25 lp/mm) increased to 0.46.
Table 3 Image quality test results after lens iterative adjustment
For the remaining middle and high order aberrations in the three fields of view of the lens, the anti-residual wave aberration is calculated according to the method of surface shape repair to compensate the wavefront, and the surface shape is equivalently repaired on the front surface of the window at the pupil of the system. The test results of the transmitted wavefront of the modified window are shown in Table 4.
Table 4 Test results of the transmitted wavefront of the repaired window
Install the repaired window into the lens barrel according to the wavefront compensation matching orientation. The system wavefront and MTF (@25 lp/mm) test results of the three fields of view of the lens are shown in Table 5. The aberrations of each order of each field of view are effectively compensated. The average MTF (@25 lp/mm) increased to 0.67, basically close to the design value. After the wavefront compensation of the system is completed, radially inject glue on each optical part from the glue injection hole on the side wall of the lens barrel. Tuning is complete.
Table 5 Image quality test results after lens surface modification
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