In-situ stitching interferometric test system for large plano optics

Transcription

1 Adv. Manuf. (2018) 6: In-situ stitching interferometric test system for large plano optics Xin Wu 1,2 Ying-Jie Yu 2 Ke-Bing Mou 2 Wei-Rong Wang 3 Received: 27 December 2017 / Accepted: 13 April 2018 / Published online: 19 May 2018 Ó Shanghai University and Springer-Verlag GmbH Germany, part of Springer Nature 2018 Abstract In-situ testing is an ideal technology for improving the precision and efficiency of fabrication. We developed an in-situ subaperture stitching interferometric test system for large plano optics in the workshop environment with high precision and satisfactory repeatability. In this paper, we provide a brief account of this system and the principle of insitu subaperture stitching measurement. Several validation tests are presented, which demonstrate that the developed system is capable of realizing in-situ testing. The size of optical flats can be measured is up to 420 mm mm, and repeatability is smaller than 0.03k. The paper also discusses the necessary requirements for a suitable workshop environment for ensuring that the tests are stable and reliable. Keywords In-situ measurement Subaperture stitching Dynamic interferometry Large plano optics 1 Introduction Large aperture optical components (more than 300 mm mm) are employed in many fields, including highenergy laser systems, aerospace, and astronomy [1 3]. The & Ying-Jie Yu yingjieyu@staff.shu.edu.cn College of Mechanical Engineering, Shanghai University of Engineering Science, Shanghai , People s Republic of China Department of Precision Mechanical Engineering, Shanghai University, Shanghai , People s Republic of China Shanghai Machine Tool Works Ltd., Shanghai , People s Republic of China precision, reproducibility, and efficiency of the measurement techniques and systems are increasing in keeping with demands of the optical manufacturing industry. The use of in-situ measurements, while feeding results simultaneously into the machining process, has been effective in improving manufacturing quality. A swing-arm optical coordinate measurement machine, the traditional high-precision in-situ method, was proposed by the University of Arizona [4, 5]. This point position scanning technology could be combined with machine technology and could be adapted for various optical surfaces by changing different probes [6, 7]. Interferometry, a precision noncontact measurement method, can also be used to measure large aspherical surfaces. However, the test region and lateral resolution are limited by the interferometer. There are also deflectometric systems which can be used to measure large plano optical surfaces [8]. Subaperture stitching interferometry based on a multiaperture overlap-scanning technique (MAOST) is an effective approach used to increase the range of measurement and improve lateral resolution [9]. This technique has been widely used to measure planar [10, 11], cylindrical [12, 13], spherical [14], and aspherical [15] surfaces. The interferometry, especially the large aperture interferometry, is rarely used for in-situ measurement in the workshop environment. The earliest general-purpose subaperture stitching workstation product was proposed in 2003 by Fleig et al. [14]. Recently, an on-machine stitching interferometer for spherical surface from Zeeko Ltd. (Coalville, England) was reported [16], which enabled these types of measurements to be conducted in a manufacturing environment. The qualifier on-machine means that an interferometer was mounted on the polishing head for the measurement. To overcome vibration disturbances, a compact simultaneous-phase interferometer from 4D

2 196 X. Wu et al. technology (Tuscon, AZ) was incorporated into the module [17, 18]. In this study, an in-situ stitching interferometric test system (ISITS) is developed to test large plano optics, which has an original framework integrated with the grinding machine and utilizes the one-dimensional movement of its piece carriage. The qualifier in-situ means that the measurement occurs during a fabrication step and in a non-fabricating state, and the workpiece does not need to be taken down from the fabrication machine when measuring. The dynamic interferometer ZYGO DynaFiz TM (Middlefield, CT) is mounted on the removable gantry, which is close to the grinding machine when testing and away from it when manufacturing. This paper provides a brief account of the whole system, and presents the serial tests undertaken on the system. The requirements for a suitable workshop environment to ensure test stability and reliability are also discussed. 2 In-situ stitching interferometric test system The geometry schematic of an ISITS is shown in Fig. 1. The system (2) and the glass grinding machine (1) are integrated and share the piece carriage of the grinding machine (3). The two parts can work independently, and the test piece is placed on the carriage and not removed during processing and testing. The glass grinding machine (1) will stop working while testing is ongoing; during grinding, the system (2) will remain inoperative and farther from machine (1) by gantry guideway (4). A 4 00 dynamic interferometer (7) (ZYGO DynaFiz) is mounted in a downlooking configuration on the test gantry (9), which can be moved in the y direction. It can be tilted and tipped by ± 5 around x and y axes to make sure that there is a clear interferogram in the field of view. The two-directions (x, y) of the subaperture scanning movements for stitching are supplied separately by the grinding machine s x-guideway and the test gantry s y-guideway [19]. The interferometer s data acquisition and analysis software is controlled remotely to interoperate with another independent software package. 2.1 Stitching measurement software The stitching measurement software (SMS) is developed to integrate the entire testing process, and it is installed on the main computer. The main functions of the SMS include precision scanning-motion control, stitching lattice design, dynamic interferometry remote control and data acquisition, and stitching processing completion. The numerical control system of the ISITS is shown in Fig. 2. The SMS, which is installed on the main computer, controls the test gantry and the piece carriage remotely and aligns the interferometer to each subaperture s location. It remotely controls the interferometer s data requirements, and reads 1 glass grinding machine; 2 ISITS; 3 piece carriage with the grinding machine s x-guideway; 4 gantry guideway; 5 grinding head; 6 test piece; 7 dynamic interferometer; 8 fine-adjusting mechanism; 9 test gantry with y-guideway Fig. 1 The geometry schematic of the glass grinding machine and in-situ measurement system

3 In-situ stitching interferometric test system for large plano optics 197 Fig. 4 Automatically designed stitching lattice pattern Fig. 2 Schematic of numerical control system Fig. 3 Flowchart of stitching testing process the subaperture data from the interferometer computer. The subaperture stitching and analysis can also be accomplished using this software. A flowchart of the measurement process using SMS is shown in Fig. 3. The first step in the measurement process is to determine the number and arrangement of all subapertures to cover the entire aperture of the test surface. The operator should enter the size of the test surface into the SMS and select other parameters when creating a new measurement. The software will then automatically suggest the most appropriate lattice of subapertures. The lattice design is based on the stitching algorithm and measurement precision. The default algorithm is MAOST and the overlap ratio between adjacent subapertures is approximately 30%. For example, when testing a plano surface of 300 mm mm in size, the SMS will automatically apply a lattice pattern to cover the whole surface, as shown in Fig. 4. The second step is to select the interferometer s measurement parameters, such as camera resolution, number of averaging, and camera shutter. These selections mainly depend on the testing circumstance and the requirement of stitching precision. Higher camera resolution can provide more information and a large amount of data, but simultaneously increase time and amount of calculation in stitching. Averaging (i.e., making multiple measurements of the same part and averaging the results) is used to eliminate air turbulence. The camera shutter, as a percentage of the camera frame rate, is used to specify exposure time to freeze fringes, which prevents smearing of fringes and loss of contrast. These parameters for all subapertures are uniform and cannot be changed until all subaperture data are completed. After completing the first two steps, the preparation of the testing is then completed. The dynamic interferometer DynaFiz has an industry measuring mode and can measure a series of similar parts after calibrating its retrace error. The calibration is embedded in its measuring process and it can be completed by once measurement in any position of the test surface. We choose the center position of the test surface and take into account the uniformity of the error distribution. The SMS automatically completes the rest of the process until all subaperture data have been acquired. The data can be stitched directly or some appropriate analysis can be performed according to the user s requirements. Finally, the stitching results could be displayed in the software. 2.2 Subaperture stitching Many stitching algorithms have been proposed [20], such as the maximum likelihood (ML) algorithm [21] and the subaperture stitching and localization (SASL) iterative algorithm [22]. QED Technologies Inc. (Rochester, NY) developed the earliest commercial stitching interferometer. This interferometer is capable of providing full-aperture measurement of large-aperture (up to 200 mm or so) convex parts [23]. The stitching method proposed in our system, MAOSTbased algorithm [5], not only reconstructs the low frequencies similar to ML, but also does so more efficiently than SASL. This algorithm requires a certain amount of overlap in adjacent subapertures. All subapertures should cover the entire test surface. The significant advantage of

4 198 X. Wu et al. this system is achieved by simultaneously optimizing alignment terms among multiple overlapping subapertures. The data of the overlap area should be equal when translation is rigid, as illustrated in Eq. (1). U 1 ðx 1 ; y 1 Þ ¼ U 2 ðx 2 ; y 2 Þþax 2 þ by 2 þ c; ð1þ r 2 ¼ min X 2 U i ðx i ; y i Þ U j ðx j ; y j Þþa j x j þ b j y j þ c j ; ð2þ where U 1 ðx 1 ; y 1 Þ and U 2 ðx 2 ; y 2 Þ are data of the overlap area of two adjacent subapertures; and a, b, c are adjustment coefficients. As illustrated in Eq. (2), the deviation of overlap regions of all adjacent subapertures (i and j) can be solved, and all coefficients can be simultaneously obtained using the least squares method. All subapertures can be transferred into a unified coordinate system, which can then be stitched to reconstruct the full-aperture surface. 3 Validation tests The validation testing setup for ISITS (described earlier) is shown in Fig. 5. The test sample, a plano optic, (workpiece in Fig. 2) is placed on the grinding machine s piece carriage. Two-dimensional scanning is necessary for subaperture stitching. The grinding machine supplies the x-direction scanning with a maximum stroke of up to 1300 mm, and the test gantry offers the y-direction scanning with a 500-mm maximum stroke. Currently, this system can accommodate test plano optics up to approximately 1200 mm mm in size. The maximum straightness of both the x- and y- guideways in the range of movement is 3.5 lm, and the maximum positioning accuracy is 5 lm. This means that it can meet the requirements of precision scanning stitching. Thus, the geometrical error of two guideways does not seriously affect stitching precision. 3.1 Preparation of tests Monitoring of vibration Most random errors come from fluctuations in the environmental situation, including vibration, fluctuations in temperature and humidity, and workshop spatiality. The stitching system is placed in a small cabinet with an area of 40 m 2 that provides an independent vibration isolation foundation. This cabinet is located in a large workshop with an area of 450 m 2. Furthermore, the environmental inspection function in the DynaFiz analysis software can be used to monitor current vibration and noise. After several days of monitoring, peak frequency before normal working times was Hz and the peak amplitude was 1 glass grinding machine; 2 ISITS; 3 piece carriage; 4 test sample; 5 interferometer; 6 fine-adjusting mechanism; 7 test gantry with y-guideway; 8 numerical control panel; 9 interferometer computer; 10 reference flat remote adjustment component; 11 main computer Fig. 5 Real in-situ stitching interferometric system a entire in-situ stitching interferometric system, b main stitching test structure and c motion control system

5 In-situ stitching interferometric test system for large plano optics 199 approximately nm. During normal working times, however, the peak frequency was at 0.16 Hz and peak amplitude was nm (as shown in Fig. 6). The peak frequency is the frequency at which the highest peak occurs, and the peak amplitude is the overall height of the highest peak. The conventional phase shifting interferometer cannot provide reliable metrology in such an extremely violent environment. The repeatability measurements of a /100-mm standard reflector with k/20 (k = nm) in different vibration environments are compared in Table 1 to demonstrate the insensitivity of DynaFiz. This disturbance of low frequency with up to 500-nm amplitude is notable for interferometer tests. Thus, to ensure measurement precision and stability, it is important to select the appropriate parameters for subaperture sampling, such as exposure time [24] Monitoring temperature and humidity Two integrated temperature and humidity sensors are installed on the machine: one is beside the sample piece and the second is near the interferometer. Temperature and humidity are recorded every 15 min, and the fluctuations are shown in Fig. 7. Fluctuation occurs during normal working times from 9:00 to 18:00. Generally, the maximum temperature fluctuation is 0.1 C per hour and 0.5 C over 24 h. The maximum humidity fluctuation is 1% per hour and 10% over 24 h. Samples should be placed in the testing cabinet a few hours before testing to equalize the temperature; this usually needs more than 24 h depending on the size and material characteristics of the sample. 3.2 Test results Three sample pieces with different sizes were tested to verify the reliability, stability, and precision that could be reached by the system. These samples were not fabricated by this grinder. The polished samples that were tested using a large aperture interferometer were easy to compare intuitively. According to the discussion in Sect , the camera resolution was set at and the shutter was set to 0.5%, which was approximately 60 ls. Each subaperture measurement took an average of 16 frames. Random noise could be reduced by many times on average, and 16 frames were chosen in this environment according to the experimental results Sample 1 The first test sample is a cuboid K9 optical flat of size 200 mm mm 9 30 mm in which the SMS has divided automatically into 15 subapertures. The whole testing processing took no more than 15 min. During testing, the temperature was based on 22 C with Fig. 6 Environmental conditions a at 7:30 before normal working times (peak frequency = Hz, peak amplitude = nm) and b at 11:40 during normal working times (peak frequency = 0.16 Hz, peak amplitude = nm)

6 200 X. Wu et al. Table 1 Repeatability of a /100-mm standard reflector in different vibration environments (PV: peak to valley; RMS: root mean square) Environmental condition At 7:30 before normal working times At 11:40 during normal working times PV RMS PV RMS Average 0.041k 0.007k 0.045k 0.007k Standard deviation, r 0.003k 0.001k 0.007k 0.001k Fig. 7 Environmental temperature and humidity fluctuations over 24 h fluctuation within 0.1 C, and the humidity was 63% and varied by no more than 1%. Figure 8a shows the test results, and this sample is measured using the full-aperture test result from the Shanghai Institute of Optics and Fine Mechanics, as shown in Fig. 8b. To assess the repeatability of this system, four more stitched data sets were acquired on this flat (see Table 2), and the flat was not removed from the piece carriage Sample 2 The second test piece is also a fused quartz flat that measures 440 mm mm 9 60 mm and is divided into 36 subapertures. The whole testing process took Table 2 Repeatability of example with 200 mm mm surface No. PV RMS Power k 0.099k 0.292k k 0.096k 0.264k k 0.073k 0.182k k 0.071k 0.189k k 0.075k 0.164k Average 0.449k 0.083k 0.218k Standard deviation, r 0.030k 0.014k 0.056k Full-aperture results 0.504k 0.085k 0.242k Fig. 8 Measurement results of example with 200 mm mm surface a the average map of 5 stitched results (PV = 0.478k, RMS = 0.096k, power = 0.264k) displayed in ZYGO MetroPro, b full-aperture phase (PV = 0.504k, RMS = 0.085k, power = 0.242k)

7 In-situ stitching interferometric test system for large plano optics 201 approximately 20 min. The temperature was recorded at 25.6 C; the fluctuation was within 0.1 C, and the humidity was 67% and varied by no more than 1%. One result is shown in Fig. 9a and the full-aperture test result is shown in Fig. 9b. Five results are listed in Table Sample 3 The third test piece is also a larger sample flat that measures 420 mm mm 9 40 mm, and is made of neodymium glass. This sample flat adopted 66 subapertures. This test took nearly 45 min to complete. One stitched fullaperture map is shown in Fig. 10, and three repetitive results are listed in Table 4. This flat was also tested vertically mounted with its surface tilted to 57 against the Zygo interferometer to meet the requirement of full aperture illumination. The result PV is 2.256k, RMS 0.426k, and power k. Because of the different measurement environments, datum, and the different means of mounting the tested samples, this full aperture measurement result cannot be used as an accuracy criterion. Even though these two results have no quantitative comparability, their shapeerror maps have good similarity. Table 3 Repeatability of example with 440 mm mm surface No. PV RMS Power k 0.048k 0.141k k 0.033k 0.057k k 0.072k 0.109k k 0.039k 0.031k k 0.030k 0.024k Average 0.318k 0.044k 0.072k Standard deviation, r 0.029k 0.017k 0.051k Full-aperture results 0.389k 0.044k 0.070k 4 Discussions Two sets of test results of Samples 1 and 2 gotten from the ISITS demonstrate the high precision, reliability, and stability of stitch testing for large optical flats with approximately 36 subapertures in a suitable workshop environment. However, there is the question of how to estimate the stitching results of Sample 3, because Table 4 illustrates the large quantitative difference between the two results. In stitching interferometry, systematic error is introduced primarily by the interferometer s reference wavefront yield. Therefore, determining the reference Fig. 10 Stitched full-aperture map of example with 420 mm mm surface Table 4 Repeatability of example with 420 mm mm surface No. PV RMS Power k 2.500k k k 2.788k k k 2.984k k Average k 2.757k k Standard deviation, r 1.161k 0.243k 1.090k Fig. 9 Measurement results of example with 440 mm mm surface a stitched full-aperture map (PV = 0.287k, RMS = 0.030k, power = 0.024k) displayed in ZYGO MetroPro, b full-aperture phase (PV = 0.389k, RMS = 0.044k, power = 0.070k)

8 202 X. Wu et al. wavefronts from these stitching tests could help reveal the right answer. The simultaneous self-calibrated algorithm suggested by QED technologies [25] uses the real-time calculation of the subaperture data to reconstruct the reference wavefront. We reconstructed the three reference wavefronts shown in Figs. 11a c, which show a slight degradation quantitatively from k/50 to k/40 in PV. The phase map of the used reference flat in DynaFiz interferometer is shown in Fig. 11d, which is approximately k/80 in PV. The similarity of the first three maps demonstrates the stability and reliability of the ISITS for testing large optical plates in the workshop environment, although there are some differences with the reference flat. In other words, the large quantitative difference between the stitched result and full aperture result in Sample 3 is not caused by the increase in the stitching number, but the different mountings of the test components. In our tests all samples were simply placed horizontally on the piece carriage without any holders, and the supporting points are uncertain. Thus, the error from supporting deformation is inevitable, especially for a large thin plate such as Sample 3. Preliminary analysis suggests that the large deviation is due to the change in part fixturing. This will be examined in more detail in future experiments. 5 Conclusions A stitching interferometric test system that was integrated with an ultraprecision glass grinding machine was developed successfully to realize in-situ testing for large plano optics in the workshop environment. This demonstrates that if vibration disturbances in the workshop remain within the levels described in this paper, the conventional phase shifting interferometer cannot even reliably acquire data during normal working hours, in which case a dynamic interferometer is necessary. The first and second test pieces had stitching repeatability of PV 0.03k (standard deviation) and RMS better than 0.017k. This is better than our precision objective of 0.6k for the tested surface no more than this size. However, the stitched result of the third sample was several waves poorer than its full aperture testing result. We believed that two reasons led to this difference. One is an accumulated error from systematic error of each subaperture, especially the second order errors. Another is the supporting deformation caused by the placing pose. Thus, more analyses about these error mitigations should be done. Acknowledgements The authors greatly acknowledge Prof. Ming-Yi Chen from Shanghai University for his enlightening discussions. This work was supported by the National Natural Science Foundation of China (Grant No ) and the National Key Research and Development Project (Grant No. 2016YFF ). References Fig. 11 Reference wavefront phase maps reconstructed by the QED method a sample 1 (PV = k, RMS = k), b sample 2 (PV = k, RMS = k), c sample 3 (PV = k, RMS = k), d reference phase calibrated using a higher precision surface (PV = k, RMS = k) 1. Nostrand MC, Weiland TL, Luthi RL et al (2004) A largeaperture high-energy laser system for optics and optical component testing. Proc SPIE 5273: Jia X, Xing T (2015) Absolute testing of surface based on subaperture stitching interferometry. Proc SPIE 9449: Pant KK, Burada DR, Bichra M et al (2015) Subaperture stitching for measurement of freeform wavefront. Appl Opt 54(34): Anderson DS, Burge JH (1995) Swing arm profilometry of aspherics. Proc SPIE 2356: Su P, Parks RE, Wang Y et al (2012) Swing-arm optical coordinate measuring machine: modal estimation of systematic errors from dual probe shear measurements. Opt Eng 51(4): Dong Z, Cheng H, Ye X et al (2014) Developing on-machine 3D profile measurement for deterministic fabrication of aspheric mirrors. Appl Opt 53(22): Dong Z, Cheng H, Feng Y et al (2015) Calibrating system errors of large scale three-dimensional profile measurement instruments by subaperture stitching method. Appl Opt 54(19): Ehret G, Schulz M, Quabis S (2016) Small angle deflectometer with submillimeter lateral resolution for flatness measurements of optics. Proc AIP 1741: Chen M, Cheng W, Wang C (1991) Multi-aperture overlapscanning technique for large aperture test. Proc SPIE 1553:

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