Fizeau interferometry is widely used to measure the surface of optical components and the wavefront aberration of optical systems due to common-path configuration, high accuracy and easy implementation. However, due to the existence of wavefront slope, when the transmission light is not perfectly perpendicular to the reflection flat (RF), the gap between the RF and the test piece will cause the reflected beam to be slightly misaligned from the transmission beam in the transmission test, which is also called beam walk-off error. If the beam walk-off error is less than 1/4 of the pixel spacing, its effects can be ignored. In this work, we will analyze the wavefront slope tolerance of 800mm aperture Fizeau interferometry and suppress it by optimizing the design of the beam expansion system, which is the key component that affects the overall wavefront slope. For the camera with 2048×2048 pixel array, the wavefront slope should be less than 9.77μrad to neglect the influence of beam walk-off error within a cavity length of 5000mm. To obtain the 800mm aperture test light, a Galilean beam expander is designed to expand and collimate the 100mm aperture light into an 800mm aperture test light. Compared with the Keplerian design, the Galilean design can decrease spherical aberration by the combination of positive and negative lenses. Tested by interferometry, the slope of the transmitted wavefront of the Galilean beam expander is 3.83μrad, which ensures the high-precision measurement.
Null interferometric microscope (NIM) is an effective method for detecting the isolated defects on the ICF capsule’s surfaces thanks to its null interference and high-resolution imaging ability. However, the limited depth-of-focus (DOF) caused by the large numerical aperture is the main drawback that prevents measuring defects in the full field of view (FOV) on the curved surface. In this work, a depth-of-focus (DOF) extension method based on numerical propagation is proposed to expand the field of view (FOV) of the NIM. The capability of the proposed DOF extension method is proved by the measurement of a 1-mm diameter ICF capsule. Experiment results indicate that the FOV of NIM is expanded from 140 μm to 320 μm.
In this paper, we demonstrate a spectral beam combining scheme of two ytterbium-doped fiber lasers, running at different wavelengths. An edge filter with high damage threshold (>20 MW/cm2 ) and steep rising edge (<2 nm) is employed as the combining element to overlap the two output beams in the near and far fields. 6.2 kW combined output power is achieved with an efficiency of 97%, which proves the high efficiency of the filter for both the reflection and transmission cases. Despite the broad emission spectrum of the single channel, the beam quality of the output is approximate with the incident emitters in horizontal and vertical directions. In terms of the measurement result conducted with thermal imaging camera, the growth of temperature on the edge filter during the combining process is well within the acceptable range. Compared with the grating based spectral beam combining (SBC) schemes, it permits the efficient combining of broader spectrum and arbitrarily large beams, which shows the potential of the filter-based spectral beam combination system. Scaling by additional and more powerful channels, higher combined output power appears to be feasible.
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