Automatic measurement system for large-aperture-angle non-holonomic spherical stitching with laser differential confocal interference.

In response to the urgent need for highly precise and efficient stitching measurements of large-aperture-angle non-holonomic spherical surfaces, a differential confocal interference automatic stitching measurement system for large-aperture-angle non-holonomic spherical surfaces was developed. The system realizes precise positioning of the confocal position through differential confocal precise focusing technology. Through the stitching model, coordinate transformation and error compensation were performed on subaperture data, and the stitching measurement of the spherical surface shape was realized. The positions and postures of the tested samples were adjusted automatically using an automatic adjustment workbench. The stitching measurement accuracy of this measurement system can attain 0.0013λ, relative error can attain 1.36%, and measurement time for eight subaperture stitching is 6 min. This system achieves automatic and rapid adjustment of large-aperture-angle spherical elements and high-precision, nondestructive, fast, and automatic measurement of surface stitching.

Interferometric measurement of the radius of curvature based on axial displacement from a confocal position and corresponding defocus wavefront.

The radius of curvature (R) is a fundamental parameter of spherical optical surfaces. The measurement range of the widely adopted traditional interferometric method is limited by the length of the precision linear guide rail carrying the measured surface from the cat's eye to the confocal position, and the test result is vulnerable to airflow and vibration in the test environment. An interferometric method is proposed for the radius measurement of spherical surfaces based on a small axial moving distance and the corresponding defocus wavefront to eliminate the dependence on a long guide rail and extend the measuring range. To eliminate the influence of the test environment and calculate the R, a defocus transform algorithm is proposed to instantaneously measure the defocus wavefront from a single interferogram. Numerical simulations theoretically demonstrate that there is no limit to the measurement range of this method because only a short distance of the measured mirror must be moved. A spherical mirror with a radius of curvature of 101.6087 mm is experimentally tested, and the relative measurement error is 0.037%. This method can achieve high accuracy for optical shops and greatly increase the measurement range of the interferometric method without additional equipment.

Laser transverse dual differential confocal radius measurement with high efficiency and high precision.

To meet the need for rapid, high-precision, and non-contact measurement of the radius of curvature (ROC) for large quantities of spherical optics, a radius measurement method based on transverse dual differential confocal (TDDC) detection is proposed in this study. First, a template S0 with a known ROC, R0, is axially scanned on its confocal position to obtain the fitted linear function lTDDC(z) using TDDC. Second, the template S0 is replaced by Sn, which is one of the test sample in large quantities, then the single point TDDC intensity ITDDC(Δzn) is captured without scan, which will be applied to obtain the defocus Δzn according to the linear function lTDDC(z). Finally, the ROC Rn under test is calculated using Δzn and R0. Simulation and experiments show that the measurement accuracy can achieve 8.0 ppm, and the measurement efficiency is 60 times higher than that of the traditional differential confocal scanning measurement. Measurement based on TDDC only requires scanning once and replacing Sn N times to realize the fast, high-precision, non-contact ROC detection of N pieces of spherical optics, which enables the high-efficiency and high-precision measurement of large quantities of spherical optics.

Controlled generation of mode-switchable nanosecond pulsed vector vortex beams from a Q-switched fiber laser.

We reported and demonstrated a ring Q-switched Ytterbium-doped fiber laser that can generate mode-switchable nanosecond pulsed vector vortex beams between two different orders. In the spatial optical path of the fiber laser, several cascaded Q-plates, divided into two Q-plate groups, are applied for intracavity mode conversion between LP01 mode and vector vortex beams. In one Q-plate group, two quarter-wave plates are inserted to achieve the addition and subtraction of the order of Q-plates. By tuning the polarization state in the cavity, mode-switchable vector vortex beams (VVBs), including cylindrical vector beams (CVBs), elliptically polarized cylindrical vector beams (EPCVBs), and vortex beams, of two different orders can be generated on demand. The experimental results show that by using the group of 1st and 3rd orders Q-plates, the 2nd and 4th orders mode-switchable VVBs (vortex beams with topological charges of ±2, ±4, CVBs and EPCVBs of 2nd- and 4th-order) can be obtained from the fiber laser. The slope efficiency, pulse width, and repetition rate are 33.4%, 360 ns, and 241kHz respectively. To the best of our knowledge, this is the first time to realize the direct generation of mode-switchable VVBs on the arbitrary position of the higher-order Poincaré sphere between two different orders from a fiber laser. This work lays the foundation for the flexible generation of arbitrary modes of VVBs with multiple different orders in the laser cavity.

High spatial resolution of topographic imaging and Raman mapping by differential correlation-confocal Raman microscopy.

Confocal Raman microscopy (CRM) has found applications in many fields as a consequence of being able to measure molecular fingerprints and characterize samples without the need to employ labelling methods. However, limited spatial resolution has limited its application when identification of sub-micron features in materials is important. Here, we propose a differential correlation-confocal Raman microscopy (DCCRM) method to address this. This new method is based on the correlation product method of Raman scattering intensities acquired when the confocal Raman pinhole is placed at different (defocused) positions either side of the focal plane of the Raman collection lens. By using this correlation product, a significant enhancement in the spatial resolution of Raman mapping can be obtained. Compared with conventional CRM, these are 23.1% and 33.1% in the lateral and axial directions, respectively. We illustrate these improvements using in situ topographic imaging and Raman mapping of graphene, carbon nanotube, and silicon carbide samples. This work can potentially contribute to a better understanding of complex nanostructures in non-real time spectroscopic imaging fields.

Steep freeform measurement method based on a normal transverse differential confocal.

A normal transverse laser differential confocal freeform measurement (NTDCFM) method was proposed to address the high-precision measurement difficulty of steep freeform surfaces with large variations in inclination, scattering, and reflectance. Using D-shaped diaphragm technology, the freeform surface under test (FSUT) axial variation transformed into a spot transverse movement on the detection focal plane. Meanwhile, a 2D position sensitive detector (PSD) was used to obtain the normal vector of the sampling points so that the measuring sensor's optical axis could track the FSUT normal direction. The focus tracking method extended the sensor measurement range. Theoretical analysis and experimental results showed that the axial resolution of the NTDCFM was better than 0.5 nm, the direction resolution of the normal vector was 0.1°, the maximum surface inclination could be measured up to 90°, the sensor range was 5 mm, and the measurement repeatability of the FSUT was better than 9 nm. It provides an effective new anti-inclination, anti-scattering, and anti-reflectivity method for accurately measuring steep freeform surfaces.

Freeform measurement method based on differential confocal and real-time comparison.

To meet the need for the high-precision contactless measurement of the freeform surface profile during the manufacturing, we propose a high-precision measurement method that combines the laser differential confocal trigger sensor (LDCTS) and the real-time comparison method using reference planes (RCMRP). LDCTS is used to measure the freeform surface under test (FSUT), which enables the high-precision measurement of the surface profile with the large roughness and local inclination. Through the real-time comparisons of the coordinate changes of the reference planes and FSUT, the dominant straightness and rotation errors can be separated based on the error model and thus the spatial motion errors can be significantly reduced along all three axes. Combing these two strategies, we find that the inclination measurement capacity becomes larger than 25° and the repeated measurement accuracy is improved to be better than 10 nm within the horizontal scanning range of 150 mm × 150 mm. Compared with the non-RCMRP method, the repeated measurement accuracy is improved by at least 5 times. We believe the proposed method provides a strategy for the high-precision measurement of freeform surface profile with large local inclination and roughness during different manufacturing periods.

High-precision laser transverse differential confocal radius measurement method.

To meet the current need for high-precision and environment-insensitive measurement of the radius of curvature (ROC), we proposed a transverse differential confocal radius measurement (TDCRM) method based on the optical system of the confocal ROC measurement. Using a D-shaped aperture and the virtual pinhole technology, two signals, analogous to the pre-focus and post-focus signals in the two-detector-based differential confocal radius measurement (DCRM), can be obtained from two segmentations of a single CCD image. The difference of these two signals can be used to precisely determine the cat's-eye and confocal positions, thereby achieving the high-accuracy ROC measurement as DCRM with a relative repeatability of 3.4 ppm. Furthermore, compared to DCRM, no optical alignment is needed after replacing the objective lens, which significantly reduces the time cost of measurements. We believe this novel and high-precision ROC measurement method will widen its application to optical manufacturing and provide an exciting opportunity for mass production of the ROC measurement instrument.

Interferometric measurement of high-order aspheric surface parameter errors based on a virtual-real combination iterative algorithm.

Aspheric surface parameters, including vertex radius of curvature, conic constant, and high-order aspheric coefficients, decide the optical properties of aspheric surfaces. The measurement of aspheric surface parameter errors (SPEs) is a substantial issue for the fabrication of aspheric surfaces. Interferometry is a mature high-accuracy method in aspheric surface figure error measurement, but challenges still exist in the measurement of SPEs for high-order aspheric surfaces or convex aspheric surfaces. We propose an interferometric measurement method for high-order aspheric SPEs based on a virtual-real combination iterative algorithm (VRCIA). We also propose a recommended measurement system including a partial compensation interferometer to obtain the partial compensated wavefront and a laser differential confocal system to obtain the best compensation distance for calculating SPEs through the VRCIA. A high-order convex aspheric surface is measured to demonstrate the feasibility of the method. The relative accuracy of vertex radius of curvature error, conic constant error and fourth-order aspheric coefficient error can reach 0.025%, 0.095% and 3.02%, respectively.

Interferometric microscope with a confocal focusing for inner surface defect detection of ICF capsule.

Inner surface defects of inertial confinement fusion (ICF) capsule are a key factor leading to ignition failure; however, there are still no effective and non-destructive detection methods available. To solve this problem, we propose the first interferometric microscope with confocal focusing (CFIM). CFIM first uses confocal technology to achieve accurate axial positioning of both capsule and the camera, thereby ensuring that the inner surface of the capsule is precisely and clearly imaged at the camera. Then, phase-shifting interferometry based on a short-coherence source and a spherical reference is applied to obtain inner defects result from null inner surface interferograms. In addition, in-situ focusing is realized by the axial adjustment of camera, but not by the capsule, to ensure that the outer defects and the fake inner defects caused by it have the same pixel coordinates, thereby solving the confusion of fake inner defects. The comparative experimental results of the CFIM and the scanning electron microscope (destructive detection) prove the feasibility of the proposed method. With unique precision confocal focusing and in-situ focusing ability, CFIM provides the first approach for non-destructive detection of inner surface defects of ICF capsule to the best of our knowledge.

A general purpose MALDI matrix for the analyses of small organic, peptide and protein molecules.

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) has been widely applied for the analysis of large biomolecules. The emergence of inorganic material substrates and new organic matrices extends the use of MALDI MS for small molecule analyses. However, there are usually preferred matrices for different types of analytes. Here, an organic compound, 4-hydroxy-3-nitrobenzonitrile, was found to be a general purpose matrix for the analyses of small organic, peptide and protein molecules. In particular, 4-hydroxy-3-nitrobenzonitrile has a strong UV absorption property, and it provides a clean background in the low mass range. Its analytical performances as a UV-laser matrix were demonstrated for different types of analytes, including organic drugs, peptides, proteins, mouse brain tissue and bacteria. Compared with commercial matrices, this new matrix has better performances when analyzing small molecules, such as drugs, peptides and lipids, while it has similar performances when analyzing proteins.

Divided-aperture confocal Brillouin microscopy for simultaneous high-precision topographic and mechanical mapping.

Confocal Brillouin microscopy (CBM) is a novel and powerful technique for providing non-contact and direct readout of the micro-mechanical properties of a material, and thus used in a broad range of applications, including biological tissue detection, cell imaging, and material characterization in manufacturing. However, conventional CBMs have not enabled high precision mechanical mapping owing to the limited depth of focus and are subject to system drift during long-term measurements. In this paper, a divided-aperture confocal Brillouin microscopy (DCBM) is proposed to improve the axial focusing capability, stability, and extinction ratio of CBM. We exploit high-sensitivity divided-aperture confocal technology to achieve an unprecedented 100-fold enhancement in the axial focusing sensitivity of the existing CBMs, reaching 5 nm, and to enhance system stability. In addition, the dark-field setup improves the extinction ratio by 20 dB. To the best of our knowledge, our method achieves the first in situ topographic imaging and mechanical mapping of the sample and provides a new approach for Brillouin scattering applications in material characterization.

Differential confocal self-collimation method for high-accuracy measurements of lens decentration.

A differential confocal self-collimation decentration measurement method (DCSDM) is proposed. It uses the differential confocal method to precisely identify the center and vertex positions of the tested lens surface, thereby obtaining the radius of curvature. Then, it uses the self-collimation light-path to detect the position of the reflected light during the rotation of the tested surface, thereby obtaining the center bias. Finally, it calculates the decentration. Theoretical analysis and experiments indicate that DCSDM achieves an accuracy of 0.069". Compared with existing methods, DCSDM significantly reduces the focusing error by using differential techniques, prevents multiple clamping errors by integrating the radius and center bias measurements in one system, and is a feasible method for decentration measurement.

High-precision laser differential confocal measurement method for multi-geometric parameters of inner and outer spherical surfaces of laser fusion capsules.

We propose a well-integrated, high-efficiency, high-precision, and non-destructive differential confocal measurement method for the multi-geometric parameters of the inner and outer spherical surfaces of laser fusion capsules. Based on the laser differential confocal measurement system with high tomography fixed-focus ability and high spatial resolution, the proposed method is used to perform the fixed-focus trigger measurement of the outer vertex, the inner vertex, and the spherical center of the capsule. From the rotation measurement around the Y-axis and the transposition measurement around the Z-axis, the inner and outer diameters, the three-dimensional inner and outer profiles, the shell thickness uniformity, and the shell non-concentricity of the capsule are measured with high precision and no damage. To the best of our knowledge, this is the first method to achieve the high-precision measurement for the multi-geometric parameters of the capsule inner and outer spherical surfaces with the same instrument.

Laser confocal vibration measurement method with high dynamic range.

A new laser confocal vibration measurement method (LCVM) is proposed to meet the requirements of high precision and high dynamic range measurements in micro and nano electromechanical systems. The proposed method uses different measurement modes to ensure that the amplitude solution interval of the out-of-plane is always in the optimal test interval of a confocal curve with the highest sensitivity to axial displacement, and thereby achieving the high-precision extraction of large-scale frequency and the high-precision measurement of large-scale amplitude. Using a 100×, NA=0.9 objective lens with a working distance of 1 mm, the theoretical analysis and preliminary experimental results indicate that the maximum measurable amplitude is 500 µm, the displacement resolution of the amplitude is 4 nm, and the measurable frequency range limited by electrical design is 0-120 MHz. The LCVM provides a novel approach for out-of-plane vibration measurements.

Dual differential confocal method for surface profile measurement with a large sensing measurement range.

To meet the requirements of the large sensing measurement range and high axial depth resolution for profile measurement, a dual differential confocal method (DDCM) is proposed in this paper. The DDCM uses the confocal signal to process separately the signal of two pinholes with axial offset, and it adds the two processed signals to obtain an axial response curve with a large slope and linear response range, thereby achieving a high-precision surface profile measurement with no axial scanning. Preliminary experiments show that the DDCM has a sensing measurement range of 0.54 µm and an axial resolution of 1 nm at the numerical aperture of 0.9. Furthermore, the sensing measurement range of the DDCM is approximately 2.9 times that of the differential confocal microscopy.

Fast and accurate tilt-shift-immune phase-shifting algorithm based on self-adaptive selection of interferogram subblocks and principal component analysis.

To eliminate the effect of tilt-shift error on the accuracy of phase-shifting interferometry (PSI), a fast and accurate tilt-shift-immune phase-shifting algorithm based on the self-adaptive selection of interferogram subblocks and principal component analysis (SSPCA) is proposed. First, each interferogram is divided into several subblocks, and principal component analysis and the least-squares method (LSM) are applied to obtain the phase-shift value of each subblock. Next, according to the correlation coefficients between each phase-shift curve, valid and invalid subblocks can be distinguished. Finally, all phase-shift values of the valid subblocks are used to fit the tilt phase-shift plane, and phase results can be obtained using the LSM. Simulations indicate that the accuracy of SSPCA can reach 0.03 rad both for small (1 rad) and large (${2}\pi $2π rad) tilt amplitudes, and it takes only one-tenth or less of the processing time of iterative algorithms. Experiments proved that SSPCA can be applied even without a precision phase shifter and thus provides a low-cost approach for PSI with both high precision and speed.

Divided-aperture subtraction-differential confocal method with nanoscale axial resolution.

In this paper, a divided-aperture subtraction-differential confocal method (DASDCM) is proposed to meet the requirements of nanoscale noncontact height measurements for precision machining, materials science, and biology. The DASDCM divides the spot on the detection focal plane into two groups of circular detection areas, which are symmetrical to the optical axis and consist of two concentric detection pinholes with different sizes in each group. Then, the DASDCM uses a subtraction of the intensity signals received from the two detection pinholes in each group to suppress the interference of the nonconjugated information on the intensity signal; it also uses the differential subtraction of two obtained circular detection signals to obtain a sensitive axial response curve. Thereby, the DASDCM greatly improves the axial resolution while considering the signal-to-noise ratio and axial dynamic range of the system and can realize surface height measurement without axial scanning by using the linear range of the axial response curve. Theoretical analysis and preliminary experiments show that DASDCM has an axial resolution of 2 nm with a laser wavelength of λ=632.8 nm and a numerical aperture of NA=0.8. It provides an effective technique for nanoscale height detection with high axial resolution.

Three-dimensional super-resolution correlation-differential confocal microscopy with nanometer axial focusing accuracy.

We present a correlation-differential confocal microscopy (CDCM), a novel method that can simultaneously improve the three-dimensional spatial resolution and axial focusing accuracy of confocal microscopy (CM). CDCM divides the CM imaging light path into two paths, where the detectors are before and after the focus with an equal axial offset in opposite directions. Then, the light intensity signals received from the two paths are processed by the correlation product and differential subtraction to improve the CM spatial resolution and axial focusing accuracy, respectively. Theoretical analyses and preliminary experiments indicate that, for the excitation wavelength of λ = 405 nm, numerical aperture of NA = 0.95, and the normalized axial offset of uM = 5.21, the CDCM resolution is improved by more than 20% and more than 30% in the lateral and axial directions, respectively, compared with that of the CM. Also, the axial focusing resolution important for the imaging of sample surface profiles is improved to 1 nm.

Three-dimensional resolution-enhancement divided aperture correlation-differential confocal microscopy with nanometer axial focusing capability.

Divided aperture confocal microscopy (DACM) provides an improved imaging depth, imaging contrast, and working distance at the expense of spatial resolution. Here, we present a new method-divided aperture correlation-differential confocal microscopy (DACDCM) to improve the DACM resolution and the focusing capability, without changing the DACM configuration. DACDCM divides the DACM image spot into two round regions symmetrical about the optical axis. Then the light intensity signals received simultaneously from two round regions by a charge-coupled device (CCD) are processed by correlation manipulation and differential subtraction to improve the DACM spatial resolution and axial focusing capability, respectively. Theoretical analysis and preliminary experiments indicate that, for the excitation wavelength of λ = 632.8 nm, numerical aperture NA = 0.8, and normalized offset vM = 3.2 of the two regions, the DACDCM resolution is improved by 32.5% and 43.1% in the x and z directions, simultaneously, compared with that of the DACM. The axial focusing resolution used for the sample surface profile imaging was also significantly improved to 2 nm.