Optical metrology is one of the key technologies in today’s manufacturing industry. In this article, we provide an insight into optical measurement technologies for precision positioning and quality assessment in today’s manufacturing industry. First, some optical measurement technologies for precision positioning are explained, mainly focusing on those with a multi-axis positioning system composed of linear slides, often employed in machine tools or measuring instruments. Some optical measurement technologies for the quality assessment of products are then reviewed, focusing on technologies for form measurement of products with a large metric structure, from a telescope mirror to a nanometric structure such as a semiconductor electrode. Furthermore, we also review the state-of-the-art optical technique that has attracted attention in recent years, optical coherence tomography for the non-destructive inspection of the internal structures of a fabricated component, as well as super-resolution techniques for improving the lateral resolution of optical imaging beyond the diffraction limit of light. This review article provides insights into current and future technologies for optical measurement in the manufacturing industry, which are expected to become even more important to meet the industry’s continuing requirements for high-precision and high-efficiency machining.

An insight into optical metrology in manufacturing

The axial resolution of confocal microscopy is not only dependent on optical characteristics but also on the utilized peak extraction algorithms. In previous evaluations of peak extraction algorithms, sample surface height is generally assumed to be zero, and only sampling-noise-induced peak extraction uncertainty was analyzed. Here we propose a sample surface-height-dependent (SHD) evaluation model that takes the combined considerations of sample surface height and noise for comparisons of algorithms' performances. Monte Carlo simulations were first conducted on the centroid algorithm and several nonlinear fitting algorithms such as the parabola fitting algorithm, Gaussian fitting algorithm, and sinc2 fitting algorithm. Subsequently, the evaluation indicators, including mean peak extraction error and mean uncertainty were suggested for the algorithms' performance ranking. Finally, experimental verifications of the SHD model were carried out using a fiber-based chromatic confocal system. From our simulations and experiments, we demonstrate that sample surface height is a critical influencing factor in peak extraction computation in terms of both the accuracy and standard deviations. Compared to the conventional standard uncertainty evaluation model, our SHD model can provide a more comprehensive characterization of peak extraction algorithms' performance and offer a more flexible and consistent reference for algorithm selection.

Influence of sample surface height for evaluation of peak extraction algorithms in confocal microscopy

A self-assembly nanodrug delivery system based on amphiphilic low generations of PAMAM dendrimers-ursolic acid conjugate modified by lactobionic acid for HCC targeting therapy.

Focusing light inside scattering media by optical phase conjugation has been intensively investigated due to its potential applications, such as in deep tissue imaging. However, no existing physical models explain the impact of the various factors on the focusing performance inside a dynamic scattering medium. Here, we establish an angular-spectrum model to trace the field propagation during the entire optical phase conjugation process in the presence of scattering media. By incorporating fast decorrelation components, the model enables us to investigate the competition between the guide star and fast tissue motions for photon tagging. Other factors affecting the focusing performance are also analyzed via the model. As a proof of concept, we experimentally verify our model in the case of focusing light through dynamic scattering media. This angular-spectrum model allows analysis of a series of scattering events in highly scattering media and benefits related applications.

Angular-spectrum modeling of focusing light inside scattering media by optical phase conjugation.

Focused and controllable optical delivery beyond the optical diffusion limit in biological tissue has been desired for long yet considered challenging. Digital optical phase conjugation (DOPC) has been proven promising to tackle this challenge. Its broad applications, however, have been hindered by the system’s complexity and rigorous requirements, such as the optical beam quality, the pixel match between the wavefront sensor and wavefront modulator, as well as the flatness of the modulator’s active region. In this paper, we present a plain yet reliable DOPC setup with an embedded four-phase, non-iterative approach that can rapidly compensate for the wavefront modulator’s surface curvature, together with a non-phase-shifting in-line holography method for optical phase conjugation in the absence of an electro-optic modulator (EOM). In experiment, with the proposed setup the peak-to-background ratio (PBR) of optical focusing through a standard ground glass in experiment can be improved from 460 up to 23,000, while the full width at half maximum (FWHM) of the focal spot can be reduced from 50 down to 10 μm. The focusing efficiency, as measured by the value of PBR, reaches nearly 56.5% of the theoretical value. Such a plain yet efficient implementation, if further engineered, may potentially boost DOPC suitable for broader applications.

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Implementation of digital optical phase conjugation with embedded calibration and phase rectification.

Speed and enhancement are the two most important metrics for anti-scattering light focusing by wavefront shaping (WS), which requires a spatial light modulator with a large number of modulation modes and a fast speed of response. Among the commercial modulators, the digital-micromirror device (DMD) is the sole solution providing millions of modulation modes and a pattern rate higher than 20 kHz. Thus, it has the potential to accelerate the process of anti-scattering light focusing with a high enhancement. Nevertheless, modulating light in a binary mode by the DMD restricts both the speed and enhancement seriously. Here, we propose a multi-pixel encoded DMD-based WS method by combining multiple micromirrors into a single modulation unit to overcome the drawbacks of binary modulation. In addition, to efficiently optimize the wavefront, we adopted separable natural evolution strategies (SNES), which could carry out a global search against a noisy environment. Compared with the state-of-the-art DMD-based WS method, the proposed method increased the speed of optimization and enhancement of focus by a factor of 179 and 16, respectively. In our demonstration, we achieved 10 foci with homogeneous brightness at a high speed and formed W- and S-shape patterns against the scattering medium. The experimental results suggest that the proposed method will pave a new avenue for WS in the applications of biomedical imaging, photon therapy, optogenetics, dynamic holographic display, etc.

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Anti-scattering light focusing by fast wavefront shaping based on multi-pixel encoded digital-micromirror device.

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

The invention belongs to the technical field of microscopicspectral imaging detection, and relates to a split pupil laser differential confocal Raman spectrum test method and device. According to the test method and device, a split pupil laser differential confocal microtechnique and a laser Raman spectrum detection technique are organically combined, precise imaging of three-dimensional geometrical positions is realized through segmentation focal spot differential detection, the optical path structure of a traditional differential confocal microscopic system is simplified, advantages of an original laser differential confocal system and a split pupil confocal system are inherited, and multi-mode switching and processing of split pupil laser differential confocal microscopic detection, laser confocal Raman spectrum detection and laser differential confocal Raman spectrum detection can be realized only through softwareswitching processing. The test method and device provide a new technological approach for detection ofnanoscale microcell three-dimensional geometrical positions and spectrum, can be applied to fields of biomedicine, industrial precision detection and the like, and has the broad application prospect.

Split pupil laser differential confocal Raman spectrum test method and device

A laser reflection-confocal focal-length measurement (LRCFM) is proposed for the high-accuracy measurement of lens focal length. LRCFM uses the peak points of confocal response curves to precisely identify the lens focus and vertex of the lens last surface. LRCFM then accurately measures the distance between the two positions to determine the lens focal length. LRCFM uses conic fitting, which significantly enhances measurement accuracy by inhibiting the influence of environmental disturbance and system noise on the measurement results. The experimental results indicate that LRCFM has a relative expanded uncertainty of less than 0.0015%. Compared with existing measurement methods, LRCFM has high accuracy and a concise structure. Thus, LRCFM is a feasible method for high-accuracy focal-length measurements.

Measuring the lens focal length by laser reflection-confocal technology

A new laser differential confocal ultra-long focal length measurement (LDCFM) method is proposed with the capability to self-calibrate the reference lens (RL) focal length and the axial space between the test lens and RL. Using the property that the focus of laser differential confocal ultra-long focal length measurement system (LDCFS) precisely corresponds to the null point of the differential confocal axial intensity curve, the proposed LDCFM measures the RL focal length fR′ by precisely identifying the positions of the focus and last surface of RL, measures the axial space d0 between RL and test ultra-long focal length lens (UFL) by identifying the last surface of RL and the vertex of UFL last surface, and measures the variation l in focus position of LDCFS with and without test UFL, and then calculates the UFL focal length fT′ by the above measured fR′, d0 and l. In addition, a LDCFS based on the proposed method is developed for a large aperture lens. The experimental results indicate that the relative uncertainty is less than 0.01% for the test UFL, which has an aperture of 610 mm and focal length of 31,000 mm. LDCFM provides a novel approach for the high-precision focal-length measurement of large-aperture UFL.

Rongjun Shao

Papers

Anti-scattering light focusing by fast wavefront shaping based on multi-pixel encoded digital-micromirror device.

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

Split pupil laser differential confocal Raman spectrum test method and device

Measuring the lens focal length by laser reflection-confocal technology

Large-aperture laser differential confocal ultra-long focal length measurement and its system.