Three-dimensional computer-generated holography based on the hybrid iterative angular spectrum algorithm.

This paper proposes a novel three-dimensional hologram calculation method based on the angular spectrum approach, with the aim of reducing the noise generated during the hologram reconstruction process. The proposed algorithm divides the spatial domain into multiple layers and employs the angular spectrum method to propagate the image between these layers, thus avoiding the paraxial approximation. To enhance the quality of the hologram, an error iteration algorithm is utilized to alleviate the occurrence of aliasing errors when directly superimposing holograms. Moreover, constraint factors are introduced between different layers within the same region to effectively utilize spatial resources for multi-image reconstruction, thereby mitigating the noise caused by inter-layer crosstalk. The feasibility of the proposed method is demonstrated through numerical simulations and optical experiments, highlighting its potential applicability to a wide range of three-dimensional reconstruction algorithms.

On-site accuracy evaluation of laser tracking attitude measurement system.

In this study, an on-site attitude accuracy evaluation method based on parallel mechanism model and indirect traceability from length to angle is proposed. Firstly, the mathematical model is established. Through orthogonal experimental design, quantitative analysis shows that the ranging accuracy and control layout have a significant impact on the accuracy of the evaluation system. On this basis, the layout of control field is optimized by genetic algorithm. Finally, the practicability of the evaluation method is verified by experiments. The results show that the yaw and pitch accuracy of the method are 0.008°and 0.007°respectively in the range of -25°to 25°within the working distance of 8 m. This method can accurately and effectively evaluate the attitude angle information of the measurement system and is adapted to various on-site environments. The research provides an innovative idea which can be used to ensure the strict requirements of attitude angle measurement in fields such as intelligent manufacturing and in situ processing.

Sub-regional phase error compensation based on a probability distribution function and gamma precoding.

In this paper, a phase error compensation method based on a probability distribution function (PDF) is proposed to improve the accuracy of phase extraction, which is helpful for three-dimensional (3D) reconstruction. First, the relationship between the gamma and the gray values is established to segment the projection regions. Then a new method based on a PDF is designed to represent the variation degree of phase error, which fits the precoded gamma value in the minimum range of the phase error. After that, the error compensation method is applied to the self-built system and packaged independently from the 3D reconstruction system to unwrap phases with high precision. The experimental results show that the proposed method can reduce the standard deviation of the phase error by 46.9% compared without phase error compensation, and decrease the standard deviation of the phase error by 30% compared with the whole precoding. Generally, our method can effectively avoid overcompensation or under-compensation caused by single global gamma precoding correction, and better reduce the phase error and improve the 3D reconstruction accuracy in the fringe projection system.

Ultrasonic Thickness Measurement Method and System Implementation Based on Sampling Reconstruction and Phase Feature Extraction.

The existing ultrasonic thickness measurement systems require high sampling frequencies for echo signal acquisition, leading to complex circuit designs and high costs. Moreover, extracting the characteristics of ultrasonic echo signals for accurate thickness measurement poses significant challenges. To address these issues, this paper proposes a method that utilizes conventional sampling frequencies to acquire high-frequency ultrasonic echo signals, overcoming the limitations of high-frequency data acquisition imposed by the Nyquist-Shannon sampling theorem. By employing an improved sampling reconstruction technique, the multi-cycle sampling signals are reconstructed and rearranged within a single cycle, effectively increasing the equivalent sampling frequency. Additionally, a combination of coarse estimation using fast Fourier transform (FFT) and precise phase extraction using the moving sine fitting algorithm is proposed for accurate thickness measurement, resolving the limitations of common thickness measurement methods such as peak detection, envelope detection, and Hilbert autocorrelation in terms of low measurement accuracy. Experimental results obtained from thickness measurements on 45 steel ultrasonic test blocks within the range of 3 mm to 20 mm indicate a measurement error of ±0.01 mm, while for thicknesses ranging from 1 mm to 50 mm, the measurement error is ±0.05 mm.

Highly uniform parallel scribing inside transparent materials with ultrafast lasers: from 2D to 3D.

Micromachining uniform features inside transparent materials is of great importance. The generation of highly uniform parallel laser beams based on spatial light modulators is a valid way to realize it. A movable magnifying optical feedback approach is proposed. By using a flip mirror and adjusting a movable stage, magnified 3D information such as energy and the position of the split individual parallel laser beams could be obtained and fed back for optimization. Thanks to this setup, active adjustment of holographic algorithm parameters for the energy uniformity and accurate temporal distribution of the parallel laser beams becomes possible. The feasibility and effectiveness of the proposed method are then demonstrated by laser scribing inside silica glass. We pave a way for uniform 3D laser manipulation and subtle microfabrication.

Diffraction characteristics of orthogonal gratings analysis based on a spatial light modulator.

The diffraction characteristics of orthogonal gratings with variable duty cycles and phase modulation depths are analyzed by using a spatial light modulator. The calculation methods of the transmission function, far-field diffraction light field, and diffraction efficiency of orthogonal gratings are deduced in theory. Meanwhile, the influences of the duty cycle and phase modulation depth on the diffraction characteristics of the orthogonal grating are discussed. The simulation and experimental results verify the correctness of the theoretical derivation. This method can be widely used in the fields of an optical vortex array, laser parallel processing, optical computing, optical communication, and optoelectronic hybrid processing.

Beam shaping with high energy utilization and uniformity using gradient orthogonal gratings.

A flattop beam is useful in ultrafast laser processing. A laser beam shaping method for high energy utilization and uniformity is presented using a complex hologram displayed on a spatial light modulator. The hologram consists of a geometric mask, an external blazed grating, and internal gradient orthogonal gratings. The gradient orthogonal gratings can change the incident light energy distribution and obtain flattop beams with high energy utilization. Experimental results show that the presented method can obtain an arbitrary geometric shape with a steep edge and high uniformity. Meanwhile, the bigger the geometric mask size, the higher the energy utilization will be, and it is up to 78.70%.

3D reconstruction of structured light fields based on point cloud adaptive repair for highly reflective surfaces.

In this paper, a novel method, to the best of our knowledge, of structured light fields based on point cloud adaptive repair is proposed to realize 3D reconstruction for highly reflective surfaces. We have designed and built a focused light field camera whose spatial and angular resolution can be flexibly adjusted as required. Then the subaperture image extraction algorithm based on image mosaic is deduced and presented to obtain multidirectional images. After that, the 3D reconstruction of structured light field imaging based on point cloud adaptive repair is presented to accurately reconstruct for highly reflective surfaces. In addition, a method based on smoothness and repair rate is also proposed to objectively evaluate the performance of the 3D reconstruction. Experimental results demonstrate the validity of the proposed method to perform high-quality depth reconstruction for highly reflective surfaces. Generally, our method takes advantage of the multidirectional imaging of the light field camera and can ensure good modulation effect of structured light while avoiding hardware complexity, which makes it application more convenient.

Auto-balancing bridge based wide impedance spectrum measurement with consideration of Op-Amp input impedance.

In an auto-balancing bridge for high impedance measurements, an operational amplifier (Op-Amp) is used to follow the intermediate potential. However, the input impedance of the Op-Amp introduces significant effects in high impedance measurements. This paper proposes a two-step excitation method (TSEM) and an incremental iterative method (IIM). The TSEM determines the magnitude of the Op-Amp input impedance and the initial value of the device under test. The IIM utilizes the TSEM results as initial conditions to quickly bring the bridge to equilibrium. To overcome the distortion issues associated with small amplitude excitation signals generated by the DAC under low resolution conditions, a programmable gain amplifier is designed. Additionally, a half-cycle difference algorithm is developed prior to the three-parameter sine fit to mitigate low-frequency direct-current drift caused by power frequency, thus improving measurement accuracy. Experimental results demonstrate that when the reference impedance is set to 1 MΩ, impedance measurements ranging from 1 kΩ to 100 MΩ can be achieved within the frequency range of 1 to 100 kHz. The precision evaluation reveals a relative standard deviation (RSD) of the modulus better than 0.384% and a standard deviation (SD) of the phase angle better than 3.49 mrad; especially for the impedance under test of 1 MΩ, the RSD is better than 0.006% and the SD is better than 0.1 mrad.