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文中研究的波前测量系统是可以测量可见光、近红外和中红外波前的大口径波前测量系统,主要由前置缩束系统、调光组件、分束镜、离轴抛物面反射镜、波前传感器、测试结果显示模块、电源、机械支撑及调节系统构成,如图1所示。
光学系统采用同轴反射式光路结构,如图2所示,待测波前首先经过由主镜和次镜组成的缩束系统,然后经过调光组件进行光强衰减,再经过分束镜将红外光、近红外光和可见光分开,最后通过离轴抛物面反射镜的准直入射到波前传感器进行波前分析。一部分可见光进入到对点系统,检验光束能否对焦。前置缩束系统、调光组件、分束镜、离轴抛物面反射镜、波前传感器以及对点系统的光路结构设计满足光瞳衔接原理,从而实现对不同波段大口径波前的有效测量。
反射式RC系统是常用的大口径光学前置系统之一[11]。镀膜方式为铝膜加保护膜,工作波长为0.5~5 μm。进行光学设计之前,首先进行RC系统参数的计算。主镜的直径d为450 mm,系统的F数为10,主镜的F数为1.928,主镜和焦点之间的距离s为150 mm,视场角度为0.05°。计算过程如下。
系统的有效焦距和主镜直径的关系为:
$$ f = \left( {f/\# } \right)\left( d \right) $$ (1) 同理,主镜的有效焦距由以下公式计算出:
$$ {f_1} = \left( {{f_1}/\# } \right)\left( d \right) $$ (2) 次镜到焦点的距离B可以表示为:
$$ {{B}} = \frac{{f\left( {{f_1} - l} \right)}}{{{f_1}}} $$ (3) 将B=(l+s)代入公式(3),可得主镜和次镜的间距l:
$$l = \frac{{{f_1}\left( {f - s} \right)}}{{\left( {{f_1} + f} \right)}}$$ (4) 同理可知次镜的焦距长度:
$${f_2} = \frac{{l\left( {l + s} \right)}}{{\left( {2l + s - f} \right)}}$$ (5) 主镜的曲率半径R1、次镜的曲率半径R2和它们焦距的关系为:
$${R_1} = - 2{f_1}$$ (6) $${R_2} = 2{f_2}$$ (7) 主镜的通孔直径表达式为:
$${d_h} = \frac{d}{f} + \frac{{2\left( {FOV} \right){f_1}\left( {{f^2} - {s^2}} \right)}}{{f\left( {f + s} \right)}}$$ (8) 经过上述公式计算,可得表1中系统的近轴参数。
表 1 RC系统近轴参数表
Table 1. Paraxial parameters of RC system
Paraxial parameters f/mm 4 500 l/mm 700 f# 10.0 s/mm 150 B/mm 850 R1/mm −1 762 f1/mm 863 R2/mm −403.4 f2 −201.7 dh/mm 397.9 $${k_1} = - \frac{{2\left( {{f_1} - l} \right)f_1^2}}{{{f^2}l}} - 1$$ (9) $${k_2} = - {\left( {\frac{{{f_1} + s}}{{2l - {f_1} + s}}} \right)^2} + \frac{{2f{{\left( {{f_1} + {f_1} - l} \right)}^3}}}{{l{{\left( {{f_1} - l} \right)}^3}}}$$ (10) RC系统的三次圆锥系数经过计算可得:k1=−1.017 1, k2=−2.344。
根据不同条件下的光强测量要求,使用12个不同衰减强度的中性密度滤光片作为系统的调光组件,利用电动调节装置实现对滤光片的切换,实现衰减倍率1~10 000倍的调节。系统使用两个短波通滤光片作为分束镜,分束镜1截止波长为1. 8 μm,可以实现3~5 μm波段的反射,0.9~1.7 μm和0.5~0.8 μm波段的透射;分束镜2截止波长为900 nm,可以实现0.9~1.7 μm波段的反射,0.5~0.8 μm波段的透射。波前测量系统可以实现测量的波段:0.5~0.8 μm、0.9~1.7 μm和3~5 μm。
为了保证波前质心坐标测量的准确性,需要对波前进行准直后入射到波前传感器,使用离轴抛物面反射镜完成球面波的准直。为满足多通道波前的测量要求,文中的波前测量系统使用三个波前传感器分别对可见光、近红外和中红外波前进行测量。主要考虑到测量波段、有效口径、子孔径数、测量精度和采样频率等因素,选择可见光的夏克哈特曼波前传感器,近红外和中红外的四波剪切波前传感器作为波前探测组件。
三款波前传感器具体参数如表2所示。
表 2 波前传感器参数
Table 2. Parameters of wavefront sensors
Item Visible wavefront sensor Near-infrared wavefront sensor Mid-infrared wavefront sensor Wavelength range/μm 0.5-0.8 0.9-1.7 3-5 Aperture/mm2 3.60×4.50 9.60×7.68 10.88 ×8.16 Number of microlenses 32 × 40 160 × 120 160 × 120 Accuracy 4 nm RMS 15 nm RMS 25 nm RMS Acquisition frequency/Hz 50 120 50 -
根据1.1节计算得到的各光学元件的参数,利用Zemax软件对光学系统进行建模和仿真,仿真结果如图3所示,根据仿真结果不断优化,从而得到光学元件和系统最优设计参数。
系统测量的环形孔径具有中心遮拦,需要Zernike多项式拟合进行像差分析。波长为532 nm时,利用Zemax对波前测量系统不同视场的波前进行分析,随着视场的增加系统像差逐渐增加,0°视场仿真结果PV值为0.009λ,RMS值为0.003λ;0.05°视场仿真结果PV值为0.019λ,RMS值为0.005λ。
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如图4所示,波前测量系统的光机热集成分析结合光学、机械和热学为一体,充分考虑光学系统、机械结构和外部环境温度的影响,利用光学设计、三维建模、有限元分析和接口编程等手段,将温度变化反馈成最终的成像质量,在不断优化的过程中实现波前测量系统的最佳化设计。波前测量系统的工作环境变化时,光学镜片和机械结构的温度会受到环境温度变化的影响,主次镜的面形和主次镜之间的间距会发生变化,影响缩束系统的成像质量,进而影响波前测量系统的测量精度[12-13]。
首先根据系统要求进行光学设计,基于光学系统建立波前测量系统的CAD模型。在结构模型的基础上生成有限元模型,将环境温度的变化作为载荷施加到有限元模型上进行热分析,求得光学镜片的变形和机械结构的刚性位移,结合Zernike多项式对镜片变形拟合分析得到镜片的Zernike系数。将面形改变系数和镜片之间的间距改变带入光学设计软件Zemax中,对光学传递函数和像质进行分析,对光学和结构设计的环节进行优化直至成像质量良好,光机热集成分析是一个首尾呼应的闭环设计过程。
在波前测量系统中,主要考虑温度变化对RC缩束系统的影响,RC缩束系统主要由主镜、次镜、镜片支撑件、底板和支撑脚结构组成。根据材料的不同对模型中的元件进行材料参数的赋值。利用Patran软件进行网格划分,通过约束支腿位移的自由度来定义边界条件,环境温度设置为20 ℃,施加±30 ℃的温度载荷,提交Nastran软件进行计算。利用Patran实现结果关联与分析,热变形云图如图5所示。
温度变化会引起镜片面形变化和镜片间距变化,利用Zernike多项式将镜片热变形数据进行面形拟合,得到各项Zernike系数[14]。文中采用MATLAB软件对接口程序进行编程,可以提高数据的处理效率。将镜片面形变化的Zernike系数和镜片之间间距的变化带入光学设计软件Zemax中,得到系统温度变化后的像差数据,如图6所示。
系统调焦到最佳像面位置,当温度为−10 ℃时,RMS值为0.039λ(RMS, λ=532 nm)。当温度为50 ℃时,RMS值为0.032λ(RMS, λ=532 nm)。如图7所示,系统在此温度区间内的热变形呈现线性规律,初始温度20 ℃,−10 ℃和50 ℃分别代表了系统的最大正负变形量,则在整个温度区间内系统的热变形均满足光学系统的性能指标,因此系统的光机热集成分析符合要求。文中使用的光机热集成设计分析方法对大口径双镜系统光机设计有很好的参考价值。
Design of large aperture multi-spectra channel wavefront measurement system
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摘要: 随着波前测量技术的发展,具有多光谱通道的大口径波前测量系统成为波前测量领域的研究热点。设计了一种大口径多光谱通道波前测量系统,主要由前置RC缩束系统、调光组件、分光组件和波前传感器等组成,有效口径为450 mm,工作波段为0.5~0.8 μm、0.9~1.7 μm和3~5 μm。给出了光学系统的设计参数,描述了系统中光学元件的参数选择,运用Zemax软件完成了光学系统的建模和仿真。进行了机械系统的方案设计,并完成了系统的光机热集成分析。测试了大口径多光谱通道波前测量系统的各项参数,结果表明该波前测量系统的有效口径大于450 mm,在−10~50 ℃工作环境下,对可见光、近红外、中红外波段的波前均可实现高精度实时稳定测量,系统的波前测量稳定性优于0.05λ(RMS,λ=532 nm)。Abstract: With the development of wavefront measurement technology, the large aperture and multi-spectra channel wavefront measurement system had become a research hotspot in the field of wavefront measurement. The large aperture and multi-spectra channel wavefront measurement system was mainly composed of fore RC shrink-beam system, dimming component, beam splitters and wavefront sensors.The system with an effective aperture of 450 mm had working wavelengths of 0.5−0.8 μm, 0.9−1.7 μm and 3−5 μm. The design parameters of the optical system were given, the selection of optical element parameters in the system was described. The modeling and simulation of the optical system were completed by Zemax. The design scheme of the mechanical system was finished, and integrated optical-mechanical-thermal analysis of the system were completed. The parameters of the large aperture and multi-spectra channel wavefront measurement system are tested, the results show that the effective aperture of the wavefront measurement system is larger than 450 mm, at the environment of −10-50 ℃, the system can measure wavefronts in the visible, near-infrared, and mid-infrared bands with high precision and stability, the stability of the wavefront measurement system is better than 0.05λ (RMS, λ=532 nm).
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Key words:
- optical design /
- wavefront measurement system /
- effective aperture /
- stability
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表 1 RC系统近轴参数表
Table 1. Paraxial parameters of RC system
Paraxial parameters f/mm 4 500 l/mm 700 f# 10.0 s/mm 150 B/mm 850 R1/mm −1 762 f1/mm 863 R2/mm −403.4 f2 −201.7 dh/mm 397.9 表 2 波前传感器参数
Table 2. Parameters of wavefront sensors
Item Visible wavefront sensor Near-infrared wavefront sensor Mid-infrared wavefront sensor Wavelength range/μm 0.5-0.8 0.9-1.7 3-5 Aperture/mm2 3.60×4.50 9.60×7.68 10.88 ×8.16 Number of microlenses 32 × 40 160 × 120 160 × 120 Accuracy 4 nm RMS 15 nm RMS 25 nm RMS Acquisition frequency/Hz 50 120 50 -
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