In interferometers, defects on the surface of optical elements can generate coherent noise, such as “Newton’s rings,” in interferograms, thereby affecting measurement accuracy. To address this issue, this paper proposes a method for extending the light source using a multimode fiber. By adjusting the parallel beam coupling into the multimode fiber, the coherent noise of the light source can be eliminated. The proposed method is verified in a Twyman-Green interferometer with a 25.4 mm diameter, employing a multimode fiber with a core diameter of 1 mm. Experimental results demonstrate a 36.9% reduction in scattering contrast and a 43% improvement in signal-to-noise ratio (SNR) of the interferometer when the fiber is moved ±1 mm perpendicular to the optical axis, while maintaining a fixed incidence angle of 1° to the optical axis. These findings confirm the effectiveness of the proposed method.
Environmental disturbance caused by mechanical vibration, air turbulence and thermal stratification makes a great impact on the accuracy of absolute flatness measurement, especially for the optical elements with the aperture larger than 600mm. Simplification of the measurement procedure is crucial for ensuring high precision and repeatability measurements. In this paper, the absolute flatness measurement approach combing the shift-rotation method and the second derivative method in polar coordinates is proposed to improve the stability and error tolerance of absolute measurement. The second derivative of the azimuth angle is achieved by rotating the tested flat, while that of the radial direction is achieved by translating high-order Zernike fitting. The absolute surface shape can be calculated simply by integration. The experiment procedure is simplified to two rotations and two translations with two involved mirrors. Furthermore, expanding the wavefront error to second-order can reduce the influence of environmental disturbance and make the measurement more robust. This study may provide technical support on the absolute flatness measurement for large aperture.
The phase-shifting algorithms are essential for a Fizeau interferometer to reconstruct the topography of the optical element’s surface or wavefront. There are differences between different algorithms for reconstruction results, especially for the suppression of noise. To acquire a more accurate Instrument transfer function (ITF) which reflects the axial spatial frequency response of a Fizeau interferometer, the algorithm transfer function which represents the characteristics of the calculation process in spatial frequency was proposed. In this paper, numerical simulations calculated and analyzed several transfer functions of the well-known phase-shifting algorithms. Then, the ITFs of a step plate with a height of 118 nm were measured with different algorithms by experiments and the results were analyzed. The simulations and experimental results indicate that the phase-shifting algorithm has an effect on the ITF measurement but it is not a key factor affecting the ITF measurement.
Optical flats used to realize pulse compression grating in ultra-intense and ultra-fast laser systems are large and have stringent face form requirements. This study employed an interferometer with an optical aperture of Φ800 mm to meet the demand for high-precision surface form measurements of meter-scale optical flats. The measurement accuracy of this interferometer improved using absolute surface form measurement. In addition, the full-aperture reflection surface form measurement of meter-scale optical flats was realized by exploiting sub-aperture stitching technology. The absolute measurement principle, based on the combination of rotational averaging and parity function, was used to calibrate the system error of the interferometer for absolute surface form measurement. The absolute surface form peak-to-valley (PV) repeatability (2σ) of the calibrated interferometer system error was 1.96 nm, and the root mean square (RMS) repeatability (2σ) was 0.44 nm. The calibration results show that the absolute surface-form measurement technique can effectively remove the interferometer system error and improve the surface-form measurement accuracy of the optical flat. The calibrated interferometer was then used to perform a two-dimensional stitching inspection of the meter-scale optical flat. The repeatability (2σ) of the PV value was 21.52 nm, whereas the repeatability (2σ) of the RMS value was 4.43 nm for a 1,680 × 1,100 × 200 mm flat. The measurement results show that the high-precision surface form measurement of meter-scale optical flats was realized.
Optical flats used to realize pulse compression grating in ultra-intense and ultra-fast laser systems are large and have stringent face form requirements. This study employed an interferometer with an optical aperture of Φ800 mm to meet the demand for high-precision surface form measurements of meter-scale optical flats. The measurement accuracy of this interferometer improved using absolute surface form measurement. In addition, the full-aperture reflection surface form measurement of meter-scale optical flats was realized by exploiting sub-aperture stitching technology. The absolute measurement principle, based on the combination of rotational averaging and parity function, was used to calibrate the system error of the interferometer for absolute surface form measurement. The absolute surface form peak-to-valley (PV) repeatability (2σ) of the calibrated interferometer system error was 1.96 nm, and the root mean square (RMS) repeatability (2σ) was 0.44 nm. The calibration results show that the absolute surface-form measurement technique can effectively remove the interferometer system error and improve the surface-form measurement accuracy of the optical flat. The calibrated interferometer was then used to perform a two-dimensional stitching inspection of the meter-scale optical flat. The repeatability (2σ) of the PV value was 21.52 nm, whereas the repeatability (2σ) of the RMS value was 4.43 nm for a 1,680 × 1,100 × 200 mm flat. The measurement results show that the high-precision surface form measurement of meterscale optical flats was realized.
In this paper, we present a wide-spectrum plug-and-play Fizeau interferometric system, which can complete precision interferometric measurement at any wavelength in the range of 600-1600 nm with a maximum measurement aperture of 150 mm. The system can be designed with multiple optical fiber input terminals, different wavelengths share only one set of interferometric system, and no components need to be adjusted when switching the working wavelength. The development of the system is helpful to accurately measure the surface profile error of coated optical elements at a specified wavelength.
A design of high-precision 4-in. transmission spheres that can work in wide-band interferometric measurement is proposed. A motorized precision rotation mount is used to drive one of the lenses in the design to make the transmission sphere work at any wavelength in the wavelength range of 532 to 1550 nm, where the F-number of the transmission sphere remains constant during the movement of the lens. OpticStudio software is utilized to verify the design under F-numbers of 1.0, 2.2, and 3.3, and the results show that a transmission sphere with an F-number of 1.0 to 3.3 can be designed based on our model. The tolerance analysis and the secondary adjustment results show that the peak-to-valley transmission wavefront error of the transmission sphere can be controlled within λ / 20 in actual machining and assembly, which meets the requirement of high-precision interferometric measurement.
We demonstrate that a pair of wedge prisms with opposite angular dispersion can be designed to have zero spherical aberration, zero coma, and zero astigmatism and be simultaneously achromatic in convergent rays, and the system can easily reach the diffraction limit and be utilized in precise three-dimensional imaging, special optical testing, etc., for widening the imaging range. Aberration equations of a pair of thin wedge prisms in paraxial optics are first derived, from which a special solution is obtained by making a few presuppositions. Then three initial structures are built to verify our inference. The near-zero Zernike standard coefficients and tiny rms radii of the image spots given by OpticStudio prove that the derived equations hold. After that the refractive indices of two prisms are both tested, where the testing results show that prisms with high-refractive indices are more capable of compensating monochromatic aberrations. With the aid of Monte-Carlo analysis method, tolerance analysis is carried on with the worst cases of the three optimized systems. The results show that a pair of wedge prisms with the opposite angular dispersion can effectively work within the diffraction limit even if the optical manufacturing and assembly errors are large. In the end, the system design limitations are given, and we suggest that the working F-number of the eyepiece should preferably be larger than 8.0.
The giant laser device used in inertial confined fusion (ICF) experimental research is the largest optical engineering ever built by humans. It requires thousands of large-diameter optical components, especially for optical components with diagonal dimensions close to or exceeding 1 meter, which leads to the manufacturing is extremely difficult. Wavefront characteristics are one of the key parameters of meter-size optical components. Since different degrees of wavefront error are introduced during the material preparation, manufacturing and coating processes, it requires precise measurement and precise control throughout the manufacturing process. In this paper, the research work on key problems such as measurement accuracy, measurement aperture and measurement efficiency in the wavefront error detection of meter-size optical components is carried out and summarized.
The deformation of the φ600mm reference mirror under gravity cannot be ignored when measuring optical components with a φ600mm interferometer. Therefore, it is necessary to choose a reasonable support and rotation mechanism to adjust the reference mirror of the interferometer in order to meet the accuracy requirements. In this paper, two mechanical structures for supporting and rotating the φ600mm reference mirror are analyzed. One is using the roller slings to provide tension, and the conveyor belt drives the reference mirror to rotate by friction; while the other one is using glue to connect the silica gel and the edge of the reference mirror where silica gel is fixed on the metal frame. The rotation of the reference mirror can be realized by manually rotating the frame. In order to analyze the influence of the two mechanisms on the mirror under working conditions, a three-dimensional model of the φ600mm reference mirror with its supporting system was established using the finite element analysis software ABAQUS, and the contact conditions and boundary conditions were reasonably applied to the reference mirror. Reference mirror is analyzed in both statical and dynamical states. The changes of internal stress and surface shape are compared when the reference mirror is supported and rotated on both mechanisms. The advantages of different mechanisms are elaborated, and the parameters of mechanisms are optimized theoretically. Analysis results can provide a useful guide for the support of the reference mirror and the adjustment of the φ600mm large aperture interferometer.
Satellite laser range system measures the distance between the satellite and the surface of the earth by figuring out the transit time of laser pulse. The beam is refracted when it goes through the atmosphere. The atmosphere refraction effect causes laser propagation delay and path bending, which is one of the key factors to restrict the accuracy of laser ranging. In order to improve the accuracy of atmospheric refraction delay correction, it is necessary to strengthen the study of atmospheric group refractivity models and atmospheric refraction delay correction method. According to the datum of Xuzhou upper air meteorological station, which are the monthly values of upper limit layers for 30 years (1981-2010) in China, three atmospheric group refractivity models were analyzed and compared. The atmospheric refraction delays to LiDAR were calculated by ray tracing method. The differences among the group refractivity models as a function of month and angle of direction were given, which lay the foundation for the practical application and precision evaluation of LiDAR.
The wavefront of coated optics is one of critical performances. Due to the interference between the coating layers, the measurement results will be totally different if the measurement wavelength is different from the working wavelength. However, all of the commercial interferometers have single measurement wavelength, which can’t treat the optical coatings working at various wavelengths. A wavelength-switchable interferometer (WSI) capable of detecting wavefront information in a wide wavelength range of 488-1064 nm is proposed in this paper. The principle of design and performance of the system are given in detail. Some typical measurement applications, such as reflection plate and optical filters will also be presented.
A common way to test high-quality aspherical lenses is to use a measurement system based on a set of null corrector and
a laser interferometer. The null corrector can either be a combination of spherical lenses or be a computer generated
hologram (CGH), which compensates the aspheric wave-front being tested. However, the null optics can’t be repeatedly
used once the shape of tested optics changes. Alternative active null correctors have been proposed based on dynamic
phase modulator devices. A typical dynamic phase modulator is liquid crystal spatial light modulator (LCSLM), which
can spatially change the refractive index of the liquid crystal and thus modify the phase of the input wave-front. Even
though the measurement method based on LCSLM and laser interferometer has been proposed and demonstrated for
optical testing several years ago, it still can’t be used in the high quality measurement process due to its limited accuracy.
In this paper, we systematically study the factors such as LCSLM structure parameters, encoding error and laser
interferometer performance, which significantly affect the measurement accuracy. Some solutions will be proposed in
order to improve the measurement accuracy based on LCSLM and laser interferometer.
The multi-object broadband imaging echellette (MOBIE) is the seeing-limited, visible-wavelength imaging multi-object spectrograph (MOS) planned for first-light use on the thirty meter telescope (TMT). The current MOBIE optical design provides two color channels, spanning the 310nm–550nm and 550nm-1000nm passbands. The involved large optics includes an atmospheric dispersion corrector (ADC) prism (1.4m in diameter), a collimator (1.7mx1.0m), a dichroic(680 mm x500 mm x 30 mm), a red folding mirror and two corrector lenses(570mm in diameter) for different channels. In the past two years, Shanghai Institute of Optics and Fine Mechanics (SIOM) has been included in the preliminary study of folding mirror sub-system in MOBIE, especially the study on the large optics manufacture techniques. The research progress of these large optics will be reviewed in this paper. The influence of optical quality of the large optics on the MOBIE is analyzed in order to define the specifications of the large optics. The manufacture methods are designed for different large optics. In order to testify the effectiveness of the manufacture methods, some samples have been processed and the final performance including wavefront error and spectral properties are tested. Finally, the future work including remaining problems and possible solutions are introduced.
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