Ultra-precision spherical optical elements are widely used in optical systems such as inertial confinement fusion devices, extreme ultraviolet lithography objectives, high-end mobile phone/vehicle imaging lens modules, and other optical systems due to their excellent processing properties. The performance of the optical elements and the overall optical system is determined by various parameters, such as curvature of radius, thickness, refractive index, focal length, and surface shape. The high-precision measurement of the above parameters is an important means to ensure the performance of optical elements and systems.
However, it is difficult for the existing methods to realize the high-precision common reference comprehensive measurement of multiple parameters of spherical optical elements. The common bottleneck problems are as follows.
1) It is difficult to break through the bottleneck of high resolution and precise focusing of the measured surface. In fact, one of the key reasons why the accuracy of existing optical measurement instruments is difficult to be significantly improved is that their axial resolution and focusing capacity are difficult to be significantly improved due to the limitation of the optical diffraction limit, which restricts the improvement of the surface focusing accuracy of components and the measurement accuracy of parameters.
2) It is difficult to break through the bottleneck of anti-surface scattering and anti-environmental disturbance measurement. The measurement method based on interference technology is difficult to measure samples with surface scattering characteristics, and the measurement accuracy is seriously affected by air flow disturbance, ground vibration, and other environmental factors.
3) It is difficult to break through the bottleneck of multi-parameter common reference comprehensive measurement. The existing multi-parameter measurement principles of spherical elements are different, and the measurement results of different instruments are difficult to be traced uniformly, so it is difficult to realize the high-precision common reference comprehensive measurement of multiple parameters.
To sum up, it is difficult for existing measurement methods to achieve the high-precision measurement of multiple parameters of spherical optical elements due to the difficulty in breaking through the technical bottlenecks of high-resolution and accurate focusing, anti-surface rough scattering, and anti-environmental disturbance of the measured surface, and it is difficult for existing measurement instruments to achieve the common reference and high-efficiency comprehensive measurement due to different measurement principles. Furthermore, it greatly restricts the improvement of processing precision and efficiency of high-end spherical optical elements.
Based on the proposed method, the laser differential confocal-interference high-precision measurement instrument (Figs. 7 and 8) is invented and developed. The instrument adopts a highly stable He-Ne laser to realize multi-parameter comprehensive measurement on the same instrument, and then the multi-parameter results can be uniformly traced to the wavelength of the light source, thus effectively shortening the traceability chain and realizing common reference measurement.
We propose the principle of the laser differential confocal-interference high-precision measurement method (Fig. 1). The differential confocal measurement optical path and Fizeau interference optical path are organically fused into the same measurement optical path. Differential confocal detection is used to achieve high-precision, anti-scattering, and anti-disturbance focusing (Fig. 2), and thereby high-precision common reference measurement of multiple parameters [Figs. 3(c)-(h)] is achieved. Fizeau phase-shifting interference technology is used to achieve the high-precision measurement of the surface shape of the spherical/plane element [Fig. 3(b) ].
The laser differential confocal-interference high-precision measurement method has broken through the common technical bottlenecks faced by the current spherical optical element parameter measurement, such as high-precision focusing, anti-surface rough scattering and environmental disturbance, and multi-parameter common reference comprehensive measurement. For the first time, the high-precision common reference measurement of multiple parameters, such as curvature radius/ultra-large curvature radius, focal length/ultra-long focal length, thickness, refractive index, and surface shape has been realized on the same laser differential confocal-interference high-precision measurement instrument, which provides an important technical means for the ultra-precision processing and testing of high-end spherical optical components.
