Weibo Wang

Single-frequency, Q-switched Ho:YAG laser at room temperature injection-seeded by two F–P etalons-restricted Tm, Ho:YAG laser

We demonstrated a 1.91 μm pumped, injection-seeded Q-switched Ho:YAG laser operating at room temperature. By inserting two Fabry-Perot etalons into the laser cavity, single-frequency Tm, Ho:YAG seed lasing was achieved at a wavelength of 2090.9 nm, with a typical output power of 60 mW. Single-frequency, nearly transform-limited Q-switched operation of the Ho:YAG laser was achieved by injection seeding. The output energy of the single-frequency Q-switched pulse is 7.6 mJ, with a pulse width of 132 ns and a repetition rate of 100 Hz. We measured the pulse spectrum, half-width of which was 3.5 MHz, by a heterodyne technique.

Long working distance microscope with a low obscuration aspherical Schwarzschild objective

For fine stitching of multiwindow wide-spectrum detectors, an aspherical Schwarzschild objective with an effective working distance of 525 mm has been realized for chromatic aberration-free imaging in 400-900 nm wavelength range and with a numerical aperture of 0.13. A theoretical approach for analytical design of an initial configuration has been modeled using an on-axis Taylor series expansion, and the beam obscuration ratio (OR) has been controlled down to 4%. In comparison against the theoretical spherical-based Schwarzschild objective, 77.5% of the OR has been reduced for improved contrast.

Absolute spherical surface metrology by differencing rotation maps

A simple novel method for the absolute interferometric testing of spherical surfaces is presented. This approach yields an estimate for test surface errors without changing experimental settings, such as cavity length, which may affect the apparent reference errors. The test surface is tested in three orientations: a basic position and two rotation positions. Full-surface absolute maps for each test piece are determined with the proposition that any arbitrary wavefront of a circular cross section may be expressed as a linear combination of polynomial terms and a data-processing technique based on differencing rotation maps of the rotated surface. An optimized numerical reconstruction algorithm employing the least-squares technique to determine the true azimuthal positions of part rotation is used to reconstruct the rotational data. The technique does not require any assumptions about the surfaces under test and additional measurements. The differencing rotation method provides a new approach for rotationally asymmetric removal and azimuthal errors correction in the absolute test. Experimental results are presented to compare the method proposed to the classic two-sphere method.

Stable and robust frequency domain position compensation strategy for Fourier ptychographic microscopy

Fourier ptychographic microscopy is a computational microscopy technique to achieve wide-field and super-resolution complex imaging which has been developed in recent years. The method is based on illuminating the sample by a light source array, and then computationally integrating different images correspondent to each of the sources, in the Fourier domain. Knowledge of the exact relative position of the light sources and the sample is critical for the quality of the final recovered image. In this paper, we present an iterative approach towards correcting the position in the Fourier domain based on Newtons method. Also, an analysis is presented which shows the relation between the position error and the deterioration of the final recovery quality. The effectiveness of the presented method in improving the quality of the final recovered image is demonstrated using simulation and experimental results. Moreover, the method is shown to be more stable and robust to noises in comparison with the state-of-the-art algorithm.

Measuring profile of large hybrid aspherical diffractive infrared elements using confocal profilometer

The discrete profiles of large hybrid aspherical diffractive infrared elements (HADIEs) are measured using a confocal profilometer, and are extracted using a polynomial regression method. In this study, a HADIE with a diameter of 80 mm is measured using a confocal profilometer with maximum horizontal and vertical measurement ranges of 120 mm and 12 mm, respectively. The experimental results indicate that the confocal profilometer can compensate for the fabrication error of the HADIE, which can be reduced from 7.20 µm to 1.05 µm. Furthermore, a profile error of 0.64 µm (measured peak-to-valley (P-V)) is obtained after correcting for the curvature error. The confocal profilometer is a nondestructive tool that can be used to measure the profile of HADIEs, and is particularly suitable in compensating for the fabrication error of fine optical elements.

Error correction for rotationally asymmetric surface deviation testing based on rotational shears.

We present a practical method for absolute testing of rotationally asymmetric surface deviation based on rotation averaging, additional compensation, and azimuthal errors correction. The errors of angular orders kNθ neglected in the traditional multiangle averaging method can be reconstructed and compensated with the help of least-squares fitting of Zernike polynomials by an additional rotation measurement with a suitable selection of rotation angles. The estimation algorithm adopts the least-squares technique to eliminate azimuthal errors caused by rotation inaccuracy. The unknown relative alignment of the measurements also can be estimated through the differences in measurement results at overlapping areas. The method proposed combines the advantages of the single-rotation and multiangle averaging methods and realizes a balance between the efficiency and accuracy of the measurements. Experimental results show that the method proposed can obtain high accuracy even with fewer rotation measurements.

Mirror based microscope with a super-long working distance in wide spectrum imaging

In order for satellite camera installations to make microsize observations at a working distance of hundreds of millimeters, a microscope with a super-long working distance is designed and established for the precise stitching assemblage of multi-window wide spectrum detectors and also for rigorous achromatic aberration. For the case of a numerical aperture of 0.23 the actual working distance is 225 mm and the wavelength ranges from 400 to 900 nm. This system highlights more potential applications for mirror and mirror lens based hybrid microscopy in industrial inspections by challenging the size of traditional optical microscopes.

Interference Confocal Microscope Integrated with Spatial Phase Shifter.

We present an interference confocal microscope (ICM) with a new single-body four-step simultaneous phase-shifter device designed to obtain high immunity to vibration. The proposed ICM combines the respective advantages of simultaneous phase shifting interferometry and bipolar differential confocal microscopy to obtain high axis resolution, large dynamic range, and reduce the sensitivity to vibration and reflectance disturbance seamlessly. A compact single body spatial phase shifter is added to capture four phase-shifted interference signals simultaneously without time delay and construct a stable and space-saving simplified interference confocal microscope system. The test result can be obtained by combining the interference phase response and the bipolar property of differential confocal microscopy without phase unwrapping. Experiments prove that the proposed microscope is capable of providing stable measurements with 1 nm of axial depth resolution for either low- or high-numerical aperture objective lenses.

Reference surface calibration of a Fizeau interferometer through even/odd synthesis.

We propose an even-/odd-synthesis method for the elimination of additional aberration caused by misalignment or environmental vibration during the calibration of a Fizeau interferometer reference surface (RS). The odd and even parts of an RS can be obtained, because surface errors could be divided into rotationally symmetric and nonrotationally symmetric terms. We then propose a least-squares algorithm with a dual-objective optimization function for calibration of the measurement results at the confocal position. Finally, a complete RS can be eventually obtained by synthesizing the odd and even parts of the RS. It has been verified through experiments that the measurement repeatability of the PV value is better than 0.003λ, and the root-mean-square value is better than 0.0003λ.

Calculations of second harmonic generation with radially polarized excitations by elliptical mirror focusing: CALCULATIONS OF SECOND HARMONIC GENERATION

Second harmonic generation (SHG) polarization intensity distribution illuminated with radially polarized beams by lens focusing appears two peaks, when the nonlinear optical coefficients dominate that is relevant to the transverse electric field components. Such two peaks pattern may result in ghosting and the decrement of imaging resolution. In this paper, an elliptical mirror based system is proposed in the case of radially polarized beams illumination for SHG. The calculated predictions and numerical simulations demonstrate that for radially polarized beams, the proportion of transverse field components at the focal plane under the condition of elliptical mirror focusing is 2.6 times smaller than that with lens focusing when its numerical aperture (NA) is 1. Furthermore, the full width at half maximum (FWHM) of the total field intensity profile is approximately 81% of that in a lens focusing system. Due to the enhancement of longitudinal components of incident field, the distribution of SHG polarization presents a single‐peak pattern, in which two peaks can be observed with lens focusing. The SHG polarizations in collagen fiber, KTiOPO4, and LiNbO3 have been numerical simulated and discussed in detail to verify the validity of the proposed method.