A three-dimensional polarimetric ptychography (3D-PP) technique is proposed to reconstruct the residual stress distribution in optical elements in three dimensions by combining polarization imaging and a 3D ptychographic iterative engine (3D-PIE). 3D-PP mainly uses two orthogonal linear polarizers placed along the light direction to capture the photoelastic information within an optical element. Based on phase shift theory, a three-step phase shift method is proposed to record three dark field images, allowing for calculating the isometric line of residual stress within the optical element. This method enables the layer-by-layer measurement of residual stress within thick samples and assesses the distribution of residual stress along the detection axis, enhancing the precision of stress measurements.

A laser Fizeau interferometer measures surface form by comparing the wavefronts reflected from a reference surface and the surface under test. The measurement accuracy is limited by the unknown absolute form error of the reference flat. Absolute testing offers a strategy to break through the limitation. In this work, we propose a three-flat absolute testing method using minimum norm least squares solutions (MNLS). After acquiring three measurement sets from pairwise combinations among the flats, one flat is rotated by 90° or 180° for the fourth measurement. The simulation demonstrates the effectiveness of the proposed MNLS method and its robustness against rotational angle errors. Our absolute testing method is experimentally validated using a home-built and a commercial Fizeau interferometers. Correcting the absolute forms of the home-built Fizeau interferometer’s reference flat improves its measurement accuracy. The experimental outcomes of the MNLS absolute testing technique are compared with two alternative absolute testing methods. The MNLS method’s advantage is demonstrated.

In this study, a fusion splicing system that facilitates the splicing of a monolithic linear array fiber end cap is constructed. The fusion splicing thermal field of the end cap irradiated by a CO2 laser is theoretically simulated using the finite element method. The experimental fusion splicing temperature agrees well with the simulation results. The splicing strength, misalignment, and high-power withstanding capacity of the fusion-spliced monolithic linear 3-fiber end cap are investigated. The use of the fabricated device in a dual-grating spectral beam combination (SBC) is also demonstrated with three-channel laser beams, achieving a combined beam with near-diffraction-limited beam quality M2=1.101. It shows that the development of the monolithic linear array fiber end cap is meaningful for achieving a compact high-brightness SBC system.

In order to boost the energy of the femtosecond regenerative amplifier (RA), we adopt the chirped pulse amplification technique to stretch the seed pulses through the Martinez stretcher and inject them into our specially designed dual-crystal regenerative cavity based on the Yb:CaGdAlO4 (Yb:CGA) crystals. To avoid damaging the component coating, we meticulously regulate the size of the cavity mode on the surface of each component within the cavity to ensure that the energy density remains below the damage threshold. The final output of 11.3 mJ pulse energy was obtained at a 1 kHz repetition rate with a power stability of 0.35% over 1 hour, which is the highest energy we know of for the regenerative output of Yb:CGA crystals. Additionally, leveraging the wide gain spectrum of the Yb:CGA crystal, we achieve a spectrum full width at half maximum (FWHM) of 9 nm along with a compressed pulse duration of 198 fs. The combination of the dual-crystal setup, superior thermal properties of the Yb:CGA crystals, quasi-continuous wave pumping approach, and the thermal-insensitive design of the regenerative cavity effectively minimize the thermal impact on the crystals. The output beam exhibits near-diffraction-limited performance, with an M2 value of only 1.05 × 1.06.

Mid-infrared frequency-comb spectroscopy enables measurement of molecules at megahertz spectral resolution, sub-hertz frequency accuracy, and microsecond acquisition speed. However, the widespread adoption of this technique has been hindered by the complexity and alignment sensitivity of mid-infrared frequency-comb sources. Leveraging the underexplored mid-infrared window of silica fibers presents a promising approach to address these challenges. In this study, we present the first, to the best of our knowledge, experimental demonstration and quantitative numerical description of mid-infrared frequency-comb generation in silica fibers. Our all-silica-fiber frequency comb spans over two octaves (0.8 μm to 3.4 μm) with a power output of 100 mW in the mid-infrared region. The amplified quantum noise is suppressed using four-cycle (25 fs) driving pulses, with the carrier-envelope offset frequency exhibiting a signal-to-noise ratio of 40 dB and a free-running bandwidth of 90 kHz. Our developed model provides quantitative guidelines for mid-infrared frequency-comb generation in silica fibers, enabling all-fiber frequency-comb spectroscopy in diverse fields such as organic synthesis, pharmacokinetics processes, and environmental monitoring.

A low sampling resolution scheme for coherent Doppler wind LiDAR (CDWL) is proposed. The CDWL offers advantages in precision and detection resolution but suffers from the requirement of high-speed data acquisition (DAQ) with high sampling resolution, such as 12- or 14-bit, which leads to an increase of the computational complexity and the system cost. The use of a DAQ system with lower sampling resolution can provide a solution to mitigate this problem. The feasibility of the proposed scheme is validated by simulations and experiments. The detection performance can be greatly affected by the quantization interval selected during sampling. It is shown that the optimal quantization interval exists and only depends on the carrier-to-noise ratio (CNR), and the optimal quantization intervals of several sampling resolutions are given at different CNRs. With the given optimal quantization configuration, the low sampling resolution data can be used for reliable wind field measurements. For long-distance detection with a CNR lower than −13dB, the CNR deterioration of 1-bit, 2-bit, 3-bit, and 4-bit signals can be as low as 2, 0.5, 0.2, and 0.1 dB.

The microlens array (MLA) is used in the flexible pupil shaping module of lithography illumination system for energy balance. However, the coherence of the light source can lead to interference patterns. Most of the decoherence methods are restricted for the pulse laser and the scanning exposure application. To solve this issue, a specific random phase plate (RPP) is designed to eliminate the interference patterns caused by the coherence of the light source. The model of partially coherent light propagation in the MLAs homogeneous system is established based on the mutual intensity theory, and the genetic algorithm is introduced to design a high-performance RPP. The corresponding measurement device is constructed, and the intensity of the image plane light field is measured. Experiment results indicate that the intensity integral non-uniformity is 32.14% in the x-direction and 66.04% in the y-direction without the RPP, and the intensity integral non-uniformity is 22.95% in the x-direction and 19.18% in the y-direction with the RPP, which verifies the feasibility of the proposed method.