A chromatic aberration pre-compensation scheme with dynamic adjustment ability is proposed to address the problem that chromatic aberration introduced by transmission-based spatial filters in ultrashort high-power laser systems strongly affects the focused intensity. In the pre-compensation scheme, the confocal transfer system comprises of a group of concave lenses and a reflecting system. The proposed scheme can realize dynamic precise compensation for chromatic aberration in the total system. Based on the proposed scheme, an optical path of chromatic aberration compensation is designed and established for the Shen-Guang-II 5 PW (SG-II 5 PW) laser system. The experimental results verify that chromatic aberration in the laser system can be fully corrected by accurately adjusting the pre-compensator, and the peak power density is improved significantly. A proton energy greater than 16 MeV is obtained in the proton acceleration experiment with the SG-II 5 PW laser system after chromatic aberration compensation.

Fast steering mirrors (FSM) are widely used in aeronautical optoelectronic stabilization platform for line-of-sight stabilization. The stability of the FSM is affected by various disturbances especially the vibration in the aviation environment. Traditional anti-disturbance methods, such as proportion integration differentiation controller (PID) and disturbance observer (DOB), have a little effect on suppressing disturbance in FSM. To solve these problems, a fast anti-disturbance strategy based on adaptive robust control (ARC) is proposed. The experimental results show that the steady-state root mean square error of FSM in vibration environment is reduced by about 80% compared with that of the PID control strategy and about 60% compared with that of the DOB control strategy after the introduction of the ARC. It shows that ARC has remarkable effect on improving the anti-interference ability and stability of FSM, and has large engineering application value.

We uses a high-resolution (micrometer scale) photoacoustic microscopic imaging system to continuously monitor early-stage tumor angiogenesis in the ear of mice and its response to anti-angiogenesis therapy. Further, a three-dimensional Hessian-matrix-based vascular extraction algorithm is proposed for the quantitative photoacoustic imaging of tumor vessels to improve the tumor vessel extraction accuracy. Subsequently, the morphological changes in various parameters, such as the diameter, density, and tortuosity, of the tumor vessels are quantitatively analyzed. Furthermore, the potential of high-resolution photoacoustic quantitative imaging to study the pathological mechanisms of tumors and other diseases characterized by vascular changes is demonstrated in this study.

Diffuse correlation spectroscopy (DCS) is a rapidly growing optical technology to noninvasively assess the tissue blood flow index. We develop a multichannel DCS topography system based on a multi-tau photon correlator, which comprises a long coherence length laser, a photomultiplier tube, and a photon correlator. The multi-tau photon correlator structure can obtain the intensity temporal correlation curve with high resolution and large dynamic range. Combining the imaging characteristics of the system, the constrained nonlinear optimization algorithm based on the analytical solution of the correlation diffusion equation is used as the model. The model is applied to match data between actual measurements and model predictions calculated by analytically solving the correlation diffusion equation in semi-infinite geometry. Finally, the dynamic phantom experiments demonstrate that the imaging system can distinguish different flow rates of liquid medium to reconstruct a two-dimensional image of flow rate distribution.

Cancer research has increasingly focused on developing appropriate tumor cell invasion models and developing a quantitative method to monitor tumor cell invasion. In this study, a three-dimensional tumor invasive model with >1 mm thickness is constructed. An ultra-wideband spectral domain optical coherence tomography system is used to detect cell migration and invasion dynamics, and the in vitro invasion process of tumor cells is characterized by the change of cell migration distance and matrix material decomposition. The quantitative detection of tumor cell migration distance based on peak change of the optical coherence tomography scattering interface is combined with three-dimensional images to quantify the matrix surface curvature, thickness, and overall volume change, thereby realizing the characterization of the matrix material decomposition and deformation information during tumor cell invasion. The changes of cell cluster positions caused by tumor cell invasion and morphological changes of matrix materials are matched with hematoxylin-eosin staining sections and laser confocal results, which verifies the feasibility of optical coherence tomography for detecting tumor-cell invasion. Three-dimensional tumor models under different nutrient gradients and different pH microenvironments are utilized. The established optical coherence tomography system accurately quantifies the migration distance of tumor cells and surface curvature and overall volume change of matrix materials at different time and in vitro microenvironments. Compared with hematoxylin-eosin staining and the laser confocal imaging method, the proposed optical coherence tomography-based method enables the continuous monitoring of the invasion process of tumor cells, thereby providing a more comprehensive view of tumor-cell migration and invasion mechanisms.

A method for exciting high-order optical vortex modes in an annular-core fiber is proposed. The method involves controlling the tilt angle and the offset distance between the tapered and lensed single mode fiber (SMF) and the annular-core fiber. Numerical simulation and experimental verification results show that the excitation of the high quality second-order optical vortex mode can be realized by optimizing the tilt angle and the offset distance to approximately 8° and 2 μm, respectively. This tilted offset excitation method based on a tapered and lensed SMF can improve the coupling efficiency because of the focusing effect of the tapered and lensed SMF. Compared with excitation by a standard SMF, an enhancement in the coupling efficiency of approximately 13% is found by using the tapered and lensed SMF. The excited high-order optical vortex modes show considerable potential for high-resolution microscopy, optical micromanipulation, optical sensing, etc.

To improve the signal-to-noise ratio of a Brillouin optical time-domain analysis (BOTDA) system, reduce the cumulative average number, and improve the real-time performance while ensuring measurement accuracy, a BOTDA scattering spectrum image denoising algorithm based on Armijo line search is proposed. The method uses the anisotropy of the partial differential equation from the perspective of energy diffusion to ensure that the noise-reduced image has good edge-holding characteristics and improves the measurement accuracy of the sensing system based on local features. The Armijo retrospective search method is used to adaptively select the steepest descending step size, and 256 cumulative average BOTDA experimental data are denoised. The best noise reduction effect can be achieved in just two iterations, which effectively reduces data acquisition time, thereby improving the real-time performance of the system.

The pseudoscopic-orthoscopic conversion (POC) algorithm, which is applicable to asymmetrical capture and display of three-dimensional (3D) scene information, is utilized to perform the holographic stereogram printing. The matching relationship between pixels on the sampled image and the synthetic parallax image is obtained under different distances between the sampling plane and the holographic plane and different ratios of sampling interval to holographic unit size. The influences of exposure optical system parameters and POC algorithm parameters on the field of view of the stereogram are also analyzed, and the relationship between scene depth and field of view is obtained. The experimental results demonstrate the applicability of POC algorithm to holographic stereogram printing and the validity of the relationship between scene depth and field of view by reconstructing 3D objects at different scene depths. The resolution is reduced because of the reconstructed image ghosting when the depth of the scene is small, and reasons for the ghosting of the holographic stereogram are also explained. The accurate parallax image at the pixel level avoids the influence of data error on the quality of holographic stereogram, and has positive significance in improving the resolution of the reconstructed image.

This paper proposes and experimentally tests a stable single longitudinal mode (SLM) narrow linewidth thulium-doped fiber laser by utilizing a special subring cavity and homemade fiber Bragg grating (FBG). The subring cavity comprising three interconnected optical couplers is employed for suppressing the dense multiple longitudinal and hopping modes. The narrow bandwidth wavelength filter, which is composed of the homemade FBG, achieves stable SLM operation. The experimental result shows that the laser can obtain a stable 1940.6 nm output with an optical signal-to-noise ratio (OSNR) of 60 dB at room temperature. Its frequency noise is measured using a 3×3 optical-fiber coupler with self-homodyne technology. The linewidth, which is calculated from frequency noise, is approximately 8 kHz when measurement time is 0.05 s.

Herein, the atomic emission spectrum of a sodium-argon mixture is obtained after ionization, and its time-resolved characteristics are experimentally studied. When observing the time-resolved evolution of argon spectral intensity at 763.5 nm, two peaks appear. The first peak with a decay time of (33.3±2.3) ns is observed via the fast radiation of particles at argon 2p6 state, which excited through collisional energy transfer from the excited sodium atom [time constant is (15.2±0.8) ns]. Following the recombination of argon ions and electrons, the second peak is observed; this peak''s decay process contains both a fast [(0.24±0.03) μs] and a slow [(3.98±1.03) μs] steps. Using the evolution relationship of electron density with time, the mechanism for decay time, which is impacted by the recombination process, is analyzed. The evolution relationships of electron density and electron temperature with time are obtained. The time-resolved atomic emission spectrum can experimentally explain the unusual phenomenon of the obviously different broadening between the D1 and D2 lines of the sodium spectra; these reasons are the spectral line of argon at 588.9 nm overlaying the sodium D2 line (589.0 nm) after Stark broadening and the self-absorption on two D lines of the sodium. Because the energy level splitting is small for the excited argon atom after recombination, the particle population accumulated on the 2p6 state via cascade relaxation takes a shorter time than that on the sodium 3P state, and the duration of the argon atom emission spectrum is obviously shorter than that of the sodium atom.

With the development of space technology, spatial data transmission rate is becoming a bottleneck associated with its application. Spatial coherent optical communication technology has become a popular research topic in many countries because of its high communication sensitivity, anti-interference ability, and high confidentiality. In this technology, the homodyne coherent optical communication system theoretically exhibits optimal sensitivity and anti-interference ability, but simultaneously requires a complex phase-locked closed-loop system and a local oscillator laser with large linewidth and laser-frequency-tuning bandwidth. In the 1550-nm band, it is difficult for the conventional lasers to simultaneously satisfy the narrow linewidth and high tuning bandwidth requirements. Herein, a narrow linewidth seed source combined with external electro-optic modulation and narrowband grating filtering is used to create a laser source exhibiting a spectral signal-to-noise ratio of approximately 28 dB, a linewidth of approximately 5 kHz, and a laser-frequency-tuning bandwidth of approximately 1.5 MHz.

We experimentally compare the detailed dynamic noise characteristics of a narrow linewidth-swept laser using different tuning mechanisms from three aspects, including the tunable frequency function in the time domain, frequency fluctuation power spectral density in the frequency domain, and its evolution law with time. All the analyses are conducted based on 120° phase difference interferometry and phase real-time reconstruction. The experimental results denote that different frequency-sweeping mechanisms and control parameters exhibit different real-time dynamic noise characteristics. Electro-optic modulation can obtain optimal sweep linearity using an optical phase-locked loop; however, this will result in noise deterioration such as feedback resonance peaks. Simultaneously, the different control parameters applied to the same electro-optic modulator will result in different feedback resonance peak frequencies. Acousto-optic modulation exhibits minimal noise degradation, and piezoelectric modulation results in poor sweep linearity and large tuning noise. This study lays the foundation for the research of the laser-frequency-sweeping mechanism and control technology and the selection of a light source in engineering applications.

Herein, the pulse-duration-dependent clamping intensity in a femtosecond laser filament is systematically investigated. The clamping intensity in the laser filament is directly measured. Results show that the clamping intensity gradually decreases as the pulse duration stretches from 45 fs to 177 fs. The experimental results are in a good agreement with the simulation results obtained by solving the nonlinear Schr?dinger equation. The analysis of pulse-duration-dependent clamping intensity in the laser filament provides scientific basis and new ideas for fully understanding intensity related filament applications.

High-precision space cold atom clocks play an important role in basic physics researches, navigation and positioning systems, and deep space exploration in the future. Herein, a novel microwave cavity is presented, which combines laser cooling and in situ atom detection. In microgravity, 87Rb atoms can be captured and cooled at the center of the microwave cavity, and the cold atom sample can be interrogated by the microwave field of the cavity. The analysis shows that this scheme has considerable advantages over the existing space cold atom clock schemes in reducing the loss of cold atoms, the proportion of dead time, and the range of distributed phase shift in the cavity. The detailed structure and optical design of the microwave cavity are presented herein, and the microwave magnetic field inside the microwave cavity is simulated. The characteristic test is performed in the cavity, and it shows that the design of the microwave cavity meets the requirement of the uncertainty of the space cold atom clock being better than 1×10 -16.

The generation of photoneutrons in ultra-intense short laser-solid interactions is modeled, and the properties of the laser-induced photoneutron sources are studied using the Monte Carlo simulation code Fluka. Further, neutron generation is simulated for different materials and electron temperatures. The results denote that tungsten exhibits the optimal performance and that the neutron yield exhibits different saturation thickness values at different temperatures. The neutron source size can be determined using the electrons'' spread angle and the target thickness. Ratios that are as high as 5 can be achieved between the fluxes in the forward and sideways directions by increasing the target radius. Further, stable energy spectra can be obtained for the photoneutron source when the electron temperature is greater than 4 MeV. The time distribution results denote that the neutron pulse duration is less than 30 ps and that its stretching factor after flight is 100 ps/mm.

An innovative laser amplifier architecture based on cryogenically cooled Yb∶YAG bulk is developed for high-power laser application. The Yb∶YAG bulk with triangular-section channels on the side is cooled using a cryogenic liquid to suppress parasitic oscillation in the medium and improve heat management. This architecture realizes high-efficiency energy storage and extraction of the amplifier, and a laser output with energy of 9.4 J, pulse width of 10 ns, repetition frequency of 5 Hz, diffraction limit far field of 3.3 is obtained, when high energy storage and extraction efficiency of the amplifier are realized. It provides a useful reference for the subsequent design of Yb∶YAG laser amplifier with large energy.

The influences of central wavelength of the seed pulse and gain fiber length of the amplifier on the characteristics of broadband spectra are systematically investigated based on nonlinear ytterbium-doped fiber amplifiers. The dissipative solitons from the nonlinear amplifier are obtained by using the nonlinear polarization rotation mode-locking technique. The best flat wide spectrum with wavelength range of 1040-1600 nm is obtained when the center wavelength of the seed pulse is 1041 nm and the gain fiber length of the amplifier is 8 m. The flatness is about 10 dB, and the wide spectral flatness is less than 1.5 dB from 1040 to 1250 nm.

An all-solid-state ultraviolet 244 nm laser by external cavity double-frequency is designed. Via V-shaped folded-cavity and actively Q-switched technique, the 914 nm and 1047 nm fundamental frequency beams pumped by the double diode arrays are used for intracavity sum frequency generation of a 488 nm high-frequency pulse laser. At the total pumping power of 44 W, the 488 nm laser output power is 527 mW. Using type I phase-matched BBO crystal, the 244 nm deep ultra-violet laser with an average output power of 28 mW is achieved. The repeat rate is 4 kHz with a pulse width of 17.8 ns, and the frequency-doubling conversion efficiency is 5.3%.

Using Janus structures to realize shape transformation is an important method. Here, we present an approach which prepares geometry-switchable Janus micropillars by controlling scanning times of femtosecond laser on a pH-sensitive hydrogel. The applications of the proposed method in encryption, decryption, and display of information are explored. Results show that these micropillars exhibit reversible structural deformation when the pH of the aqueous environment is changed. Because the laser printing technique is highly flexible, the spatial arrangements, pillar heights, and bending directions of micropillars can be readily controlled. Thus, patterns with variable spatial arrangement can be realized.

Femtosecond lasers with different parameters (wavelength, pulse energy, pulse number) are used to process micro-holes of carbon nanotubes film. A theoretical calculation of the single-pulse ablation threshold of carbon nanotubes film for different wavelengths is made by fitting experimental results. The calculation results are 25 mJ·cm -2 at a wavelength of 1030 nm and 39.7 mJ·cm -2 at a wavelength of 515 nm. The influences of laser parameters on material processing results are investigated. The results indicate that the pulse energy of the laser is the primary factor influencing the diameter of the ablation, and a larger area of carbon nanotubes thrown area can be produced by a short wavelength femtosecond laser. Raman spectroscopy is used to assess the incisions of materials cut by femtosecond lasers with different wavelengths, and the results show that a femtosecond laser with a wavelength of 515 nm is more suitable for cutting carbon nanotubes film. The influences of pulse energy and scanning speed on the cutting quality are analyzed. The desired cutting quality is obtained under the optimized process parameters.

A three-dimensional (3D) transient heat transfer and fluid flow model for laser-MIG hybrid welding is developed to investigate keyhole dynamics and temperature and fluid flow fields in weld pools. The effect of the laser-arc tandem relative position on the heat transfer and fluid flow of the weld pool is elucidated. The model considers the effect of the welding torch angle on droplet transfer and the effects of multiple reflections on laser energy distribution. The results show that the downward flow along the keyhole wall forms backward flow and counterclockwise circulation after reflection on the bottom of the weld pool. The backward flow transports heat and momentum to the rear portion and increases weld pool volume. The counterclockwise circulation impinges on the keyhole back wall and reduces the keyhole stability. In laser leading configurations, the droplet and arc pressure impact behind the keyhole and cause two flow patterns, namely the forward and outside flows. The forward flow enhances the impingement of counterclockwise circulation on the keyhole back wall, and the collapse of the keyhole becomes more severe. The outside flow transfers heat to both sides of the weld pool and leads to a wider weld.

The effect of cryogenic laser peening (CLP) on the microstructure of 2024-T351 aluminum alloy and its strengthening mechanism are investigated. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are used to characterize the microstructure evolution of 2024-T351 aluminum alloys. Results show that when compared with room-temperature laser peening (RT-LP), CLP has a more remarkable grain refinement effect and produces high-density dislocation. Additionally, CLP results in more and finer black spherical second phases, which are uniformly distributed throughout the samples and can be analyzed using EDS as the S phase (Al2CuMg). The in-depth microstructures of the samples treated by CLP are shown to have gradient distributions with different morphologies, and the microstructures of the matrix layer of these samples are superior to those obtained from samples subjected to RT-LP. There are two main strengthening mechanisms associated with CLP: one is that the cryogenic environment restrains the dynamic recovery of dislocations,reducing the thermal activation energy needed to promote the inhibitory effect of refined grains and second phases on dislocations; the other mechanism is related to the fact that the plastic deformation and internal stress caused by the volume shrinkage of samples under cryogenic environment can significantly produce structural strengthening effects.

Corrosion properties of CP-Ti and Ti-5%TiN composites produced through selective laser melting (SLM) in an artificial simulated body fluid (Hank''s solution) were investigated by using potentiodynamic polarization curves and electrochemical impedance spectra. The results demonstrate that SLM-produced CP-Ti primarily comprises the acicular α-Ti phase. The addition of TiN particles can bond well with the titanium matrix and refine the α-Ti grains, resulting in more grain boundaries. An SLM-produced Ti-5%TiN sample possesses better corrosion resistance than an SLM-produced CP-Ti sample in Hank''s solution because the tiny TiN particles, which act as micro-cathodes, are uniformly distributed in the titanium matrix. This accelerates the anodic dissolution process of the titanium matrix and allows the Ti-5%TiN composite to first enter the passivation state.

In this study, 3-mm thick low-alloy high-strength steels with different butt gaps were welded by optical fiber laser-metal inert gas (MIG) protection hybrid welding. The forming processes of weld seams with different butt gaps at a constant welding speed were studied and the formed shapes were compared with the morphology of laser-MIG hybrid welding with the same process parameters under zero gap conditions. The results demonstrate that the upper and lower widths of the weld seam and its heat-affected zone are basically same under butt gaps (U-shaped), while the weld-seam width and its heat-affected zone are up-wide and down-narrow under zero-gap (Y-shaped) conditions. The formation mechanism of weld seams is as follows: when laser-MIG hybrid welding is conducted with a butt gap, three different arc plasma shapes exist, i.e. bifurcated arc shapes formed by arc striking/arcing with either side of the test plate and the cross arc shapes formed by arc striking/arcing with both test plates at the same time. The three forms of arc shapes change continuously to form a swing arc pattern. The effect of the swing arc plasma is to preheat and melt the side wall of the workpieces. The laser heat source operates on the molten pool to increase the penetration, stabilize the arc, and eliminate incomplete fusion of the side wall caused by the arc heat source. The arc heat source with a butt gap has a large effective range, and the base metal is heated more evenly. The microstructures of the butt gap welds are more homogeneous than that of the zero-gap welding seams.

TC4+Ni45+CeO2 multi-pass overlapping laser cladding layers with 0, 1%, and 3% CeO2 content are achieved on the surface of Ti811 using synchronous powder-feeding laser cladding. The microstructure and phase compositions of the coating are analyzed via X-ray diffractometry, scanning electron microscopy, and energy dispersive spectrometry. The microhardness and friction and wear properties of the coating are investigated via a microhardness tester, friction and wear test machine, and white-light interferometer. The results demonstrate that the formation phases in the coating include TiC, TiB2, TiB, Ti2Ni, and α-Ti. With the addition of CeO2, the phase compositions of the coating remain unchanged. When the CeO2 content is 0, the internal crystal grains are coarse, the microhardness is 590-640 HV, and the wear mechanisms are primarily abrasive and adhesive wears. When the CeO2 content is 1%, the microstructure of the coating is refined, the orientation of dendrites is weakened, the microhardness is 625-655 HV, and the wear mechanisms are primarily abrasive and adhesive wears. When the CeO2 content is 3%, the reinforcing phase in the coating is changed from dendritic, long strip, and whisker-like to granular, layered, short rod-like, and is uniformly dispersed in the coating. Under this condition, the microhardness is 560-575 HV, and the wear mechanism is primarily abrasive wear.

Ti∶sapphire crystal is a widely-used working material for ultra-short and ultra-intense laser oscillator. Its aperture and surface full-spatial-frequency wavefront errors determine the output energy and beam quality of ultra-short and ultra-intense laser system. However, owing to the extreme difficulty in obtaining good optical homogeneity and high Mohr hardness in the Ti∶sapphire crystal, it remains a great challenge to fabricate a large-aperture Ti∶sapphire with a high-precision transmission wavefront and super-smooth surface. High-precision transmission-wavefront measurements of the Ti∶sapphire crystal are realized through the design of a linear polarization interference detection method to match the crystal axis, which solves the problem that the interference fringes of transmission-wavefront measurements cannot usually be resolved owing to the birefringence caused by the structural characteristics of the Ti∶sapphire crystal. Based on the measurements and analysis on the optical homogeneity of the Ti∶sapphire crystal, a fast polishing convergence process for the transmission wavefront is developed using an uniaxial machine. In order to realize the process for transmission wavefront with high accuracy and low frequency error and the process for super-smooth surface with high frequency error, orthogonal experiments combined with the grey relational analysis method are used to optimize the processing parameters of a computer-controlled small-grinding-head polishing. A small-grinding-head smoothing process using silica sol polishing fluid is developed to reduce the mid-spatial-frequency errors. Experimental results show that the transmission-wavefront errors of the large-aperture Ti∶sapphire crystal at full spatial frequency can be effectively controlled by using multi-way processing technologies. For a large-aperture Ti∶sapphire crystal with a diameter of 120 mm, the peak-valley value of the transmission wavefront can reach 0.283λ (λ=632.8 nm), the power spectral density of intermediate frequency shows that there is no obvious error modulation at special frequency. A super-smooth surface is realized with a high-frequency roughness Rq of approximately 0.262 nm.

A silver (Ag) target was ablated using the XeCl excimer laser at room temperature in vacuum. Films were deposited on the Si(111) substrates parallel to the target surface. The scanning electron microscopy, X-ray diffraction (XRD), and selected area electron diffraction analysis results denote that the deposited films comprise Ag nanoparticles with different sizes. The fixed laser fluence increases the distance from the target to the substrate, decreases the nanoparticle size and depth of the film, and weakens the combinative extent of the nanoparticles. Further, the XRD spectrum intensity of the crystallographic face of (111) decreases, whereas that of the crystallographic face of (200) increases. When the distance from the target to the substrate is fixed, the nanoparticle size and depth of the film increases with the increasing laser fluence, and the combinative extent of the nanoparticles increases. The XRD spectrum intensity of the crystallographic face of (111) increases, whereas that of the crystallographic face of (200) barely decreases. Furthermore, the experimental results are analyzed on the basis of the nucleation and transition of the nanoparticles, mobility of the ablated particles, and surface energy required in film growth along different crystallographic faces.

To analyze the assembly error of the interferometer plate in the Mirau interference-microscope objective, we create a characterization model of interference light intensity of a tilted reference plate. By dividing the test beam and reference beam in the pupil plane, the integral expression of the interference light intensity is obtained when the reference plate is tilted. The influence of the tilted reference plate on the envelope and contrast of interference fringes is quantitatively analyzed by numerical simulation. Then, the tilt error tolerance of the interferometer plate is determined by the results of three-dimensional morphology reconstruction using an eight-step phase-shifting algorithm for broadband light. An experimental verification structure consisting of a Mirau interference-microscope 50× objective is fabricated and a standard step is experimentally measured to verify the accuracy of the simulation model. Results show that the tilt angle of the interferometer plate in a Mirau interference-microscope 50× objective should be constrained to be less than 1.5°.

A method based on spectral estimation and a multispectral technique is proposed to accurately and efficiently measure surface efects of optical elements. Using this method, we estimate and obtain single spectral defect images at different wavelengths and compose a multispectral defect image from different single spectral defect images. The optimized OTSU (Otsu Image Segmentation Algorithm) is used to analyze and compare the images. A defect measurement system of optical elements is developed, and color defect images are obtained upon white-light illumination. The experimental and analytical results show that single spectral or composing multispectral images from different wavelength combinations detect more defects accurately and efficiently. In the composite image, the number of detected defects increases by 1.85 times, and the area of detected defects increases by approximately 6.0 times for different kinds of defects compared to the original white-light image. The composing images from optimized wavelength combinations can be used to detect and obtain different small defects with high efficiency and high quality.

This study proposes a method to measure the backward scattered signals from particles illuminated by Gaussian beams. Micro-glass beads suspended in the fluid are measured, and the measured pulse waveforms are analyzed. Results reveal that the signal profile fits the Gaussian profile; the height of the Gaussian profile is approximately linear to the particle diameter. The standard glass beads with diameter of 19.2 μm are used to acquire signals under different number concentrations in real time, and the dependence of the pulse number on the particle concentration is obtained. Experimental results also confirm that the proposed method can measure both the particle size and concentration, thereby providing a basis for the measurement in the amplifier cavity of a laser system in the future.

In order to conveniently analyze the laser-spot-position detection performance of quadrant detectors (QDs), a new mathematical model of the Gauss spot''s position resolution is established in this paper. First, the principles and approximate mathematical model for QD-based position detection in the Gauss spot model are analyzed. Then, a mathematical model of the relationships among the position resolution, total signal-to-noise ratio, position of spot center, and spot radius are deduced by deriving certain error function and theory properties. Finally, the proposed model''s correctness is verified via numerical simulations and an experimental system. The results show that, over a ±0.45-mm range of spot center position, the proposed model''s estimated error is approximately 36% for a spot radius of 0.74 mm and total signal-to-noise ratio of 66.96 dB. Compared with the original approximate model, the proposed model offers approximately twice the accuracy. It can effectively estimate the resolution of laser-spot-position detection systems, which is of great assistance in engineering applications.

This study proposes a fiber-optic four-channel dual-balanced heterodyne phase detection method based on optical heterodyne detection principle to realize high-sensitivity heterodyne phase detection. A fiber-optic four-channel dual-balanced heterodyne detection optical path is experimentally established, and a quarter-wave plate is used as a phase detection sample to verify the detection performance. Further, the influence of the signal modulation frequency on the phase measurement result in heterodyne detection is analyzed. Results reveal that the phase information cannot be effectively detected by too high or too low modulation frequency due to the limitation of the response bandwidth of the photodetector. Fiber splitting ratio in double-balanced heterodyne interference and the effect of the effective receiving aperture of the coupling lens on phase detection are analyzed based on the optimal signal modulation frequency of 500.5-1550.5 kHz and the actual measured phase root-mean-square of 89.1° with a standard deviation of 0.3°. When the fiber splitting ratio is approximately 1∶1, a more accurate detection result with a higher signal-to-noise ratio is obtained. The actual influence of the effective receiving aperture on the sensitivity of the received signal is verified by varying the emission angle of the coupling lens.

A micro-displacement measurement system is established based on transmissive laser air-wedge interference. The system realizes displacement measurement in one dimension with nanometer resolution. We use an image processing method, applying an extra small displacement and calculating the light intensity difference, to extract the interference patterns buried in stray light and noise, and effectively improve the signal-to-noise ratio of the laser interferometric images. Results show that the relative displacement resolution of the system is better than 10 nm and the absolute displacement measurement uncertainty is better than 5%. The measuring system is compact in structure, easy to install and use, and the measuring resolution reaches the nanometer level. It realizes fast, convenient and stable measurement, and is suitable for temporary demand of high resolution and precision displacement measurements.

A multiplexed optical fiber Mach-Zenhder heterodyne interferometric system which is suitable for on-line displacement measurement is researched. Based on the characteristic that fiber Bragg grating only reflects Bragg wavelength, two independent optical fiber Mach-Zenhder interferometers which possess almost the same optical path are established. One of the optical fiber Mach-Zenhder interferometers is used to perform the measurement task while the other one is used to monitor the environmental disturbances. The system is suitable for on-line measurement after the environmental disturbances are compensated. The acoustic optical modulators embedded in the reference arm of the interferometers shift the frequency of the reference beam, and heterodyne interferometric signals are formed when the reference beam meets the measurement beam, realizing heterodyne interferometric measurement. During the experiments, the standard deviation of 10 repeated measurements for 100 μm displacement is 6 nm.

The distribution characteristics of global surface reflectance and aerosol optical depth are analyzed using a surface-reflectance product of moderate-resolution imaging spectroradiometer and an aerosol-optical-depth product of European centre for medium-range weather forecasts. The effects of surface reflectance and aerosol optical depth on the echo power, detector output signal-to-noise ratio, and relative random error of spaceborne integral path differential absorption lidar systems are analyzed. Results show that with the given system parameters, the single-pulse echo power range is approximately 0.299-321 nW, which requires the detector to have a high dynamic range. The output signal-to-noise ratio of single-pulse echo detector is greater than 13.6 dB, and the output signal-to-noise ratio of detector with an accumulative 148 times (land)/296 times (ocean) pulse is greater than 26 dB. The high values of relative random error appear in the sea near the Sahara Desert and Arabian Peninsula, and the maximum relative random error is 0.22% (0.88×10 -6).

This study builds an intra-pixel response measurement system to overcome the issue arising when the output energy of the point target fluctuates with position, which is caused by the fill factor of the forward-illuminated CMOS image sensor less than 1. According to the interior design structure of the pixel, an intra-pixel response model is established. Two holes with different radii are used to measure the intra-pixel response functions of four pixels in the sensor. The response value of the pixel is recorded when the hole is moved to different positions. The full widths at half maximum of Gauss function of the point target and the intra-pixel response function are calculated. Experimental results show that the spatial distribution characteristics of sensitivity within the pixel can be described by calculating the intra-pixel response function. The measurement and resolution method proposed herein is able to provide a useful reference for the microscopic representation of this type of sensor.

A fiber Bragg grating (FBG) accelerometer based on diaphragm and diamond structure is proposed. The acceleration detection mechanism of the sensor is explained, and its sensitivity and resonance frequency expression are derived. The structural parameters of the sensor are optimized by using ANSYS and MATLAB softwares, and the FBG accelerometer with small structural size and meeting the practical application requirements is obtained. The finite element model is constructed to simulate the vibration characteristics of the sensing system. The sensor is fabricated and tested for dynamic and static characteristics. The experiments show that the sensor has better temperature compensation effect under the temperature of 20-90 ℃, which effectively reduces the influence of temperature on acceleration measurement. The first-order natural frequency of the sensor is about 681.4 Hz. In the range of 0-500 Hz, the sensor''s sensitivity has a good linear relationship with the vibration signal frequency. The combined application of the diaphragm and diamond structure enhances the sensor''s ability to resist lateral interference with a lateral interference of less than 5%.

Herein, the single-target magnetron sputtering method is used to prepare Cu(In,Ga)Se2 thin films at various sputtering times, and rapid quantitative analysis of elemental content ratios of the Cu(In,Ga)Se2 thin films is performed using laser-induced breakdown spectroscopy. Results show that the ratio of the Ga/(In+Ga) spectral line intensities and the forbidden bandwidth of the film vary synchronously. As the sputtering time increases, both parameters initially decrease and subsequently increase. This shows the potential of LIBS in the field of metal thin film analysis; it can play an auxiliary role in the performance analysis of Cu(In,Ga)Se2 thin films and the optimization of preparation parameters.

Quick and nondestructive detection of seed vigor is a popular and difficult task in the seed research field. Based on the relationship between seed respiration and seed vigor, we proposed a rapid non-destructive testing system for seed vigor based on TDLAS technique. The proposed system comprises a distributed feedback laser and its control circuit, a photoelectric conversion and amplification circuit, a data acquisition circuit, an upper computer software, and a seed breathing carbon dioxide (CO2) concentration detection pool based on a multi-reflection pool structure. The detection pool has a volume of 1.5 L, a light path of 16 m, and a laser source band of 2004 nm. Based on Lambert Beer''s law, we use wavelength modulated absorption spectroscopy and second harmonic generation to invert the CO2 concentration generated during seed respiration. Seed vigor is determined according to the concentration of CO2 in seed respiration, and the vigor index obtained from the germination and seedling emergence experiment is compared and validated. The experimental results show that the correlation between the change of CO2 respiratory intensity and seed vigor grade index is greater than 0.9, i.e., the rapid non-destructive testing system for seed vigor based on TDLAS technique can accurately, nondestructively, and efficiently reflect the seed vigor grade. This study provides useful exploration in non-destructive testing and grading of seed vigor using TDLAS technique.

Two Q-switched Nd∶YAG lasers operating at 1064 nm are used and combined in orthogonal beam geometry to perform underwater single and double pulse laser-induced breakdown spectroscopy (SP-LIBS and DP-LIBS) experiments in barium chloride (BaCl2) solution. The influences of key experimental parameters on the spectral characteristics and signal enhancement of underwater SP-LIBS and DP-LIBS are investigated systematically. There is a time window from approximately 500 ns to 900 ns in underwater DP-LIBS, where signal-to-noise ratios are very high. The time window favors the selection of delay time for spectral acquisition and analysis. By properly setting the relative position of two laser foci and using optimized laser energy, a 20-time enhancement of the Ba II 455.4 nm line intensity of DP-LIBS over the maximal intensity of SP-LIBS is obtained. The spectral line of DP-LIBS is narrower and last longer than that of SP-LIBS. In addition, dependence of spectral line intensity of DP-LIBS on the second laser energy is observed, which demonstrates an exponential increase. This is quite different to the linear growth observed in collinear DP-LIBS experiment on the liquid surface, and the difference in growth trend may be related to the gaseous environment in which the second laser induced plasma is produced and confined. Finally, the detection capability of underwater SP-LIBS and DP-LIBS is compared. The results show that the detection sensitivity of DP-LIBS is improved by a factor of 37, and the limit of detection of Ba in bulk water is reduced from 31.35×10 -6 in SP-LIBS to 1.78×10 -6 in DP-LIBS.

To improve the wavelength calibration accuracy of multi-channel grating scanning spectrometers, the principle of linear wavelength scanning of traditional single-channel grating spectrometers is first introduced. Then, the theoretical nonlinear relationship between the spectrometer''s output wavelength and the moving distance of the lead screw is derived based on the common grating axis error. Given the nonlinear relationship, wavelength calibration of a prototype of Fengyun-3 solar irradiance spectrometer is performed. The results demonstrate that the calibration accuracy of the traditional calibration is 0.08 nm,and the calibration accuracy is increased to 0.03 nm by using the wavelength nonlinear formula. This result satisfies the requirements of the instrument''s specifications. Thus, the wavelength nonlinear relationship of a multi-channel grating scanning spectrometer is verified.