Objective Chemical reaction energy is converted to optical energy using a high-energy chemical laser. Its laser beam has good collimation and a high-power density when it is exported. The classic laser’s exporting window is normally made of crystalline material, but the crystalline window’s corrupt practice gradually emerges as laser power increases. There is no one type of crystalline window for the middle-infrared band laser that can withstand temperature distortion without exploding due to bulk absorption. Consequently, the free-vortex aerodynamic window (ADW), which seals the optical antrum using aerodynamics, has been commonly used. When the ADW works, ultrasonic airflow can produce an air curtain to seal the optical antrum. Simultaneously, the quality of the output beam would be affected by the gaseous aberration medium formed as the “window.” Thus, conducting a study concerning ADW’s optical quality is necessary for further improving ADW’s performance.

Interferometry, far-field method, shear interferometry, and the Shack-Hartmann (S-H) model method, among others, are available. The reference wavefront interferometry established during ADW’s gauging requires an ideal environment; the far filed process only provides the macroscopic property and cannot quantify the wavefront aberration, putting ADW’s optimization design at a disadvantage; the obtained interferometric fringe shear interference is the result of wavefront difference, in which interpreting both fringes and wavefronts is difficult. The clear aperture of the currently proposed ADW is 280 mm×10 mm, which is a large rectangle. Therefore, the S-H model approach, which uses the Zernike polynomial to rebuild the wavefront, is not appropriate. The unscanned ADW has a large length-width ratio in the paper, resulting in considerable environmental noise. An ADW optical quality detection method that can provide a reference for ADW’s engineering application is required for measuring big length-width ratio ADW’s optical quality in such a complex environment, quantitative analysis wavefront aberration; it should also provide a reference for ADW’s engineering application and have the potential to aid in the future optimization of ADW.

Methods According to the past ADW optical quality measurement and engineering application experience, the preliminary knowledge of ADW’s aberration component is already available. On this basis, an S-H splicing method is investigated in this study, which uses autocollimation S-H to measure wavefront and splicing method to rebuild wavefront; 671-nm optical source is used to verify furthermore. The experiment discusses and analyzes the peak-to-valley (PV) and root-mean-square (RMS) values in restructured wavefront when ADW is not in use and ADW’s working status is stable. This can explain the feasibility of S-H splicing method to measure ADW’s optical quality and its great significance to ADW’s optimization and engineering applications. The method also provides a new perspective to discuss big length-width ratio spot’s optical quality. The autocollimation S-H includes a light source, beam splitter prism, a battery of lenses, beam zoom implements, S-H wavefront sensor, CCD camera, and standard plane mirror; the current designed ADW’s clear aperture is 280 mm×10 mm, the pressure ratio is 100, and the working gas is N₂. The 671-nm light source goes through a battery of lenses, beam splitter prism, and beam zoom to expand a 300-mm diameter annular facula. The facula’s optical axis is parallel to ADW’s optical thoroughfare, and the facula would return the way it came after the incident the standard plane mirror vertically, which is the autocollimation process. ADW imposes restrictions on facula’s size to 280 mm×10 mm; therefore, slit facula returns the way it goes through the microlens array to focus and then image on CCD after shrinking. The autocollimation S-H wavefront sensor was adopted in this study; its microlens’ quantity is 24×24, corresponding to 300-mm diameter annular facula before shrinking, every 10 mm occupy 0.8 subaperture. Therefore, the 280 mm×10 mm rectangle facula focuses on a subspot list after passing through the microlens. To obtain more subspot, avoid the beam zoom implements second mirror block’s influence, the paper bias uses the S-H. The paper used a CCD camera’s collecting frame frequency of 120 Hz, a 2-s working duration of ADW, and subaperture’s quantity of 22. The subspot’s period is coincident and stable after wavefront going through the microlens without aberration. After ADW work, wavefront suggests aberration because of the gas medium’s supersonic flowing; each subspot appears offset with it. The splicing method rebuilds each subwavefront according to the offset between the actual spot center with reference spot center, splices each subwavefront according to the wavefront’s continuity, and rebuilds the whole wavefront.

Results and Discussions According to the measured result of the past ADW and engineering application experience, ADW’s impact on optical quality is considerably reflected in tilt, defocus, and astigmatism aberration. This study uses a 671-nm light source; bias uses a one-dimensional autocollimation S-H wavefront sensor to measure ADW and form a list of spots on CCD (Fig.1). The study aims at this list of spots and proposes the splicing method to rebuild the wavefront; simultaneously, it calculates the tilt aberration. This method covers the shortage that the S-H model method is unsuitable for silt facula (Fig.3). The contrastive paper analyzes the rebuilt wavefront’s long exposure aberration when ADW is not in use, and its pressure is normal, PV value changes from 0.0212λ to 0.1729λ, and RMS value changes from 0.0074λ to 0.0578λ (Fig.6, Table 1). The experiment data contribute to ADW’s further optimization and have great directive significance to ADW’s engineering application.

Conclusions The paper deals with the rebuilt wavefront’s long exposure when ADW is not in use and it has stable pressure. The former y tilt amount is 0.021 μrad, the PV value with a tilt is 0.0297λ, the PV value without tilt is 0.0212λ, and RMS value without tilt is 0.0074λ. The latter y tilt amount is 0.3184 μrad, the PV value with a tilt is 0.2708λ, and RMS value without tilt is 0.0578λ. The gray value of the long exposure image shows the ADW’s current working status. The paper trigger CCD to save images 1 s ahead of ADW launch so that long exposure time is 1 s when ADW is not in use. According to tilt aberration, ADW’s stably working time is 1.3 s, so that long exposure time is 1.3 s when ADW’s pressure reaches the set value and remains stable. When ADW’s pressure reaches the set value and remains stable, the rebuilt wavefront’s long exposure explains that in ADW’s 0--10 cm area, the aberration results are in a relatively large airflow. It is the direction of further optimization. While the whole PV value is controlled in less than half wavelength, it explains that the ADW meets engineering application requirements.