To improve the performance of underwater wireless optical communication, a scheme is proposed to increase the bandwidth of underwater optical communication through the multiplexing of vortex beams, and at the same time improve the attenuation length supported by the underwater optical communication by using photon-counting detection. The examination results show that information transmission was achieved in the coastal seawater channel with a slight error rate below the forward error correction threshold, thus verifying the feasibility of the scheme. The transmission power of the communication system is only 3.21 nW after the photon-counting scheme is adopted. Considering only the attenuation of light intensity by the water channel, the supported attenuation length reaches 20.21 in the underwater optical communication system with vortex beam when the transmission power is increased to 1 W. This experiment provides a solution for underwater optical communication that can simultaneously increase the information transfer rate and attenuation length.

The mass-specific backscattering coefficient (bbp*) of suspended particles is mainly affected by their composition and sizes. Studying the variation characteristics of bbp* is of great significance to revealing the types and temporal and spatial distribution of such particles in waters and improving the quantitative accuracy of ocean color remote sensing. In this paper, the Mie theory is applied to calculate the bbp* of various common algae and inorganic mineral particles in seawater with different particle size distributions, relative refractive indexes, and apparent densities by simulation. It is found that the average bbp* of inorganic mineral particles is about twice that of algae particles when their particle size distribution slop ξ is the same. When ξ is 4.0, the bbp* of inorganic mineral particles and algae particles at 532 nm are (9.12±3.18)×10-3 m2·g-1 and (4.09±0.48)×10-3 m2·g-1 respectively. The lower bbp* value of algae particles can be explained by the lower real part of their refractive index. Research results of the measured data on the Yellow and East China Seas show that the spatial variability of bbp* is lower than that of the backscattering coefficient. When the mass concentration of inorganic particles is dominant in the total particles, the average bbp*(532) is 8.46×10-3 m2·g-1, which is 2.3 times that, 3.63×10-3 m2·g-1 to be specific, when organic particles dominate. The measured bbp* decreases in the form of power-law function as the mass concentration proportion of organic particles increases. In view of the simulation results, it is concluded that the variation range of the particle size distribution slope ξ of particles in the study waters is 3.6?4.2. In offshore waters dominated by organic particles, ξ is about 3.9. In the vicinity of the Yangtze River Estuary, the bbp* of the suspended particles varies greatly under the influence of the change in the particle size distribution.

We theoretically investigate the high-order harmonic generation from acceptor-doped semiconductors by numerically solving the one-dimensional time-dependent Schr?dinger equation based on the single electron approximation. The results show that the harmonic efficiency of the second plateau from acceptor-doped semiconductors is about three to four orders of magnitude higher than those from undoped semiconductors. Theoretical analysis shows that doping changes the energy-band structure of the semiconductor, narrows the band gap between the valence band and the first conduction band, and between the first conduction band and the second conduction band. Then it is easier for electrons to tunnel into the higher conduction band, and the electron population of the high conduction band is increased, thus the harmonic efficiency of the second plateau is improved.

Laser speckles create a bottleneck that hinders the development of laser projection display technology. Based on the compound speckle contrast analysis method, the parameters affecting the composite speckle lining ratio in a laser projection system are analyzed. These parameters include the number N of independent speckle samples and the number M of scattered light wave coherent elements within a single imaging-lens resolution element on the projection screen. The occurrence of speckles in the laser projection system can be reduced by changing parameters N and M. These changes can be achieved by adjusting the numerical apertures of the projection-lens and imaging-lens, projection distance, and observation distance. In the simplified projection model, the effects of the random-diffuser spot area and projection-lens numerical aperture on the speckle contrast in laser projection imaging were experimentally analyzed. The results reveal that for a certain size of the projection lens, the contrast of the secondary speckle pattern on the scattering screen can be reduced by increasing the area of the diffuser spot. The occurrence of speckles increases when the numerical aperture is reduced, thereby posing a challenge for the miniaturization of the laser projection system. The findings of this work can provide guidance for the use of a rotating random diffuser in the design of speckle suppression systems for laser projection imaging.

An electron-bombarded active pixel sensor (EBAPS) is a type of vacuum-solid hybrid low light-level device that encapsulates a photocathode, an electronic-sensitive active pixel sensor (APS) chip, and a base, using vacuum technology. Integral sensitivity is a crucial performance parameter of EBAPS, but corresponding test methods in China are lacking. Therefore, based on the working principle of EBAPS and the spectral response test method of a low-light level-image intensifier, the spectral responsivity is characterized by a digital-electron conversion factor and the number of electrons obtained from the output signal in this study. Moreover, a spectral response test system of EBAPS is designed. The spectral response curves of EBAPS at an operating voltage of -800 V are obtained using this system. The EBAPS’ integral sensitivity is 3.2 × 108 (s·lm)-1, and repeatability is 0.65%.

In this paper, the mathematical model of the S-mode of an automatic dependent surveillance-broadcast signal is established, and the realization process and theoretical accuracy of the rising edge and optimal estimation measurement algorithms in the time of arrival (TOA) estimation algorithm are analyzed. First, the theoretical implementation of the single-pulse differential matched filter is deduced and verified, and the improved algorithm of the multipulse differential matched filter for TOA measurement is analyzed based on pulse accumulation. Then, a polynomial fitting algorithm is proposed to reduce the TOA estimation error to solve the time discretization problem caused by hardware sampling in engineering practice. Finally, the TOA estimation performance of each algorithm is analyzed using simulation, and the effectiveness of different algorithms applied to the multilateration system is verified by combining them with the positioning algorithm.

To further study the saturation characteristics of photodetectors irradiated using ultrashort pulse lasers, silicon-based PN-type photodiodes were used as experimental devices to measure the transient response time of picosecond laser irradiated devices with different laser energy densities. The experimental results show that the transient response signal of the device is nonlinearly saturated when the picosecond laser energy density reaches 97 nJ/cm2. Furthermore, we analyze the signal’s full widths at half maximum and signal’s bottom widths of the signal characteristic quantity after the device saturation. The absolute and relative increases are obtained as 154 μs, 157.00% and 157 μs, 47.87%, respectively. Observe that as the laser energy density increases, the response time gradually increases. The time extension of the signal indicates the occurrence of the transient response degeneration of the device. Through the analysis of laser injection carriers, a large number of photogenerated carriers are generated after the laser irradiation, satisfying large injection conditions of semiconductors. This leads to bipolar mobility approaching zero during the initial carrier bipolar transport process. The initial speed of the corresponding signal falling edge attenuates, leading to the degrading response signal of the device.

A high-sensitivity terahertz refractive index sensor controlled by the continuum bound state is designed in this paper. The sensor is composed of all-dielectric grating. By breaking the symmetry of the structure by translation, and the resonance can be transformed into a leakage mode resonance with a high quality factor (Q factor), and the extremely narrow linewidth cavity mode is very sensitive to the change of the refractive index of the surrounding medium. Using the finite element method, the transmission spectra with different asymmetry parameters, grating thickness, grating width, and refractive index of analytes are numerically simulated. The results show that the Q factor of the grating metasurface reaches 12620, the standard sensitivity is 31395 nm/RIU, and the FOM (Figure of merit) is 1000. The structure has potential applications in the field of highly sensitive photon sensors in the terahertz range.

A high-performance polarizing beam splitter is designed based on Al0.9Ga0.1As/Al2O3 subwavelength grating in this paper. The polarizing beam splitter is alternately formed by using Al0.9Ga0.1As and Al2O3, the preparation is simple, and the structure is stable and easy to integrate. The relationship between the reflectivity, duty cycle and period of the grating is analyzed by the rigorous coupled wave analysis method. By selecting the appropriate grating parameters, the grating structure can achieve different diffraction characteristics of transverse electric (TE) and transverse magnetic (TM) polarized light, and the polarization beam splitting function is realized. The simulation and calculation results show that in the wide spectral range of 80 nm (800-880 nm), the reflectivity of the grating polarization beam splitter for TE polarized light is greater than 94%, the transmittance of TM polarized light is greater than 90%, and the polarization extinction ratio is greater than 10 dB, up to 41.37 dB. In addition, the effects of grating parameter variation and incident angle deflection on the performance of the polarizing beam splitter are also investigated. The results show that the polarizing beam splitter has good process tolerance and can tolerate 4° of left and right deflections for vertical incidence, which effectively solves the problems of difficult integration and complex fabrication of traditional polarizing beam splitters, and can be combined with GaAs-based devices. After integration, the control of the incident beam is realized.

From the Fraunhofer far-field diffraction theory and Dammann grating design principle, we used the phase modulation characteristics of the liquid crystal spatial light modulator (LCSLM) to design a 5×5 lattice-structured light pair phase grating. By considering the display of a two-dimensional Dammann grating, we used the simulated annealing algorithm to optimize the phase distribution value within one period of the grating with respect to the expected light energy utilization rate and light intensity non-uniformity. Thus, the beam splitting structure of the Dammann grating, which can form 5×5 lattice, is designed. Next, Matlab was used to simulate the far-field diffraction to obtain the light energy utilization rate and unevenness of light intensity reach 79.94% and 13.28%, respectively. The grating structure was loaded on the LCSLM, and a 5×5 lattice with 27.53% light intensity unevenness and uniform distribution of zero-order diffracted light intensity was obtained. Also, the light energy utilization rate reached 71.06%. Numerical simulation and optical experimental results prove that the LCSLM can clearly reproduce the lattice-structured light in the structured light reproduction optical system. Compared with the traditional structured light, the structured light designed in this paper does not change the distance between points when the diffraction distance changes.

In cognitive radio networks, the number and state of cognitive users and channels vary strongly with time. In complex environments, more factors need to be considered. To evaluate the current performance of various allocation protocols, a performance evaluation system of spectrum allocation protocols (PES-SAP) based on a probability distribution vector is proposed in this study. The system can flexibly evaluate channel allocation protocols. Moreover, an equitable and random allocation protocol is proposed herein to enable maximum secondary users to obtain channels for the available spectrum. The PES-SAP is used to evaluate the comprehensive performance of the equitable and random allocation and traditional random allocation protocols. The simulation results show that the PES-SAP can clearly distinguish these two protocols and some of the equitable and random allocation protocols have better performance within a certain number of values in the random allocation agreement.

In this paper, the pointing error model, which includes the axis and jitter errors, is established based on the unmanned aerial vehicle (UAV) platform and flight environment. Using the atmospheric turbulence model with Gamma-Gamma distribution as the channel transmission model, the bit error rate of UAV laser communication under the influence of turbulence and pointing error is obtained, and the effects of transmitting power, divergence angle, and flight altitude on bit error rate are analyzed. The simulation results showed that when the beam divergence angle is 1 mrad and the pointing error is small, the turbulence had a significant effect on the system’s bit error rate. When the pointing error is more than 0.2 mrad, the system performance is determined by the pointing error. It is even difficult to meet the requirement of the bit error rate of less than 10-6 simply by adjusting a certain parameter. Finally, the influence of system parameters on channel and bit error rate is analyzed, which provides ideas and references for engineering design parameter selection.

Pulse-position modulation (PPM) has the advantages of simplicity and high interference immunity but is affected by the issues concerning low frequency band utilization. To improve the system’s bandwidth utilization and data transmission rate, a hybrid modulated faster-than-Nyquist (PSK-PPM-FTN) rate atmospheric optical communication scheme in combination with phase-shift keying (PSK) and PPM is proposed in this paper. The bit error rate (BER) is derived over Gamma-Gamma-distributed atmospheric channel. In addition, the relationship of BER and transmission distance, laser wavelength, and time acceleration factor τ is analyzed. Monte Carlo simulation results show that when high-order PPM is combined with multi-ary PSK, and additional information can be transmitted at the cost of a slight drop in BER performance in a horizontal communication link with τ≥0.8. When τ=0.8, the amount of information, which is transmitted by a quadrature PSK-PPM4 FTN (QPSK-PPM4-FTN) scheme, is 1.5 times than that of QPSK, and the system bandwidth utilization is 150% higher than that of PPM4.

In view of the secret demand of communication network for unmanned aerial vehicle (UAV) in penetrating reconnaissance mission under the complex battlefield environment, considering the limited energy carried by nodes and the unbalanced of energy consumption during the data transmission, a clustering optimization algorithm for UAV formation based on ultraviolet communication is proposed in this paper. First, on the basis of establishing the wireless ultraviolet communication link model and energy consumption of nodes, the selection of cluster heads and inter-cluster communication process of low energy adaptive clustering hierarchy (LEACH) algorithm is improved. Then, the weight functions of node residual energy and link reliability are introduced to select cluster heads, and the topology optimization between cluster heads is carried out by generating optimally rigid graph. The simulation results show that the algorithm improves the connectivity between clusters and delays the phenomenon of node death in the network. Compared with the LEACH algorithm, the time for the first occurrence of dead nodes and half of dead nodes in the network is delayed by 25.2% and 21.4%, respectively. The connectivity and energy balance of the network are improved.

Aiming at the problem that the unmanned aerial vehicle (UAV) position deployment is highly related to hybrid beamforming, this paper proposes a method based on step-by-step optimization to maximize the total transmission rate of the system. First, the digital beamforming is used to obtain the simplest objective function, and a virtual channel is introduced to solve the optimal position of the UAV deployment. Then, the optimal deployment position of the UAV is substituted into the optimization problem of the offset angle of the phase shifter in the analog beamforming, and the traditional artificial fish swarm algorithm is used to effectively solve the optimal analog beamforming matrix. Finally, the limited resolution of the phase shifter is solved with an improved artificial fish swarm algorithm. The simulation results show that, compared with the existing methods, the method has better performance in location deployment and maximizing the total transmission rate of the system.

This paper proposes a long reach passive optical network based on power-domain non-orthogonal multiple access for the next generation optical access network. Based on channel estimation function, the traditional power-domain non-orthogonal multiple access networks employ successive interference cancellation (SIC) algorithm for signal demodulation and recovery, which require accurate channel estimation and has error propagation problem. To overcome this problem, a signal receiving scheme based on modified convolutional neural network (CNN) is proposed, which exploits a large amount of data to independently fit the channel function of each user, thus to break the dependence chain between users in the SIC demodulation algorithm, and improve transmission performance and fairness of the system. The results show that, compared with the traditional SIC demodulation algorithm in the long reach power-domain non-orthogonal multiple access passive optical network, the receiving scheme based on the modified CNN can improve the transmission performance of far-end users (transmission 60 km) and near-end users (transmission 20 km) by about 0.5 dB and 1.7 dB, respectively, and its fairness index closer to 1.

In the language identification system, the interference of silent segments and the inconsistency of voice decibel range leads to a decline in language identification. Additionally, algorithms using spectrograms for language identification cannot effectively show the information of its low-frequency part, which results in performance failure. To mitigate this, we proposed a language identification method based on joint voice activity detection and dynamic range control. First, we extracted the first dimension coefficient of the Mel-scale frequency cepstral coefficients. Second, we applied median filtering to smooth the feature parameters and perform voice activity detection to remove the silent segment of the voice. Next, we used the dynamic range control to adjust the decibel range of different voices. Finally, we put the log scale spectrogram into the convolutional neural network for classification. The experimental results show that the proposed algorithm improved performance by 7.16 percentage points as compared with the traditional language identification algorithm using spectrogram in the VoxForge public corpus under the ResNeSt network. Additionally, under the same experimental settings, the recognition performance of the log scale spectrogram showed superiority over other mainstream features, which fully validates the effectiveness and superiority of the proposed algorithm and features.

A novel scheme to generate microwave phase-shifting signal with an adjustable frequency-multiplication factor based on polarization division multiplexing dual-parallel Mach-Zehnder modulator is proposed. Without filters, the scheme can generate frequency-doubled, frequency-quadrupled, frequency-sextupled, or frequency-octupled microwave signals by changing the radio frequency drive signals and direct current bias voltages of the modulator. By simultaneously controlling the angle between the polarizer and the principal axis of the modulator, the phase can be continuously adjusted from 0° to 360°. The simulation results show that a 5-GHz radio frequency signal can be converted into full-range linear phase-shifting microwave signals with the frequencies of 10 GHz, 20 GHz, 30 GHz, and 40 GHz. Their power fluctuation is less than 0.5 dB when the phase changes continuously. Moreover, it also shows that the scheme has good frequency tunability. In addition, the operating frequency range of the scheme as well as the influence of modulator extinction ratio, direct current bias voltage drifting, electrical phase shifter amplitude, and phase imbalance on the system performance are studied and analyzed.

An optimization algorithm based on improved salp swarm is proposed to optimize the orthogonal phase-coded waveform of the multiple-input multiple-output (MIMO) radar. The Logistic-Tent chaotic map is introduced into this algorithm to initialize the population, thus increase the probability of obtaining the optimal initial solution position. Then, the leader update equation is improved to expand the range of the optimal solution search. The sine function is applied for weight design, and the followers’ updated positions are flexibly adjusted according to the cost function value to further refine the convergence accuracy. The experimental results show that the auto-correlation sidelobe peak and cross-correlation peak of the phase-coded waveform of the MIMO radar designed based on the improved salp swarm algorithm are reduced by 2.6231 dB and 1.2572 dB, respectively, compared with those of the original algorithm. The orthogonal performance and the convergence accuracy of the algorithm reported here are better than those of the existing results.

The effective identification of fiber optical vibration signals is an important basis for ensuring the operation of the fiber-optical early warning system for oil and gas pipelines. To mitigate the lack of a single classification method in traditional fiber optical vibration signal detection, this paper proposes a fiber optical vibration signal recognition and classification algorithm using AdaBoost ensemble learning. First, we analyzed and studied the characteristics of five fiber optical vibration signals and selected sample entropy, energy distribution, and bandwidth as the three-dimensional feature vectors. Next, this information was sent to the decision tree, support vector machine (SVM), and AdaBoost classification algorithm with the decision tree as the base classifier for training and recognition. Second, the obtained models were optimized and evaluated by cross-validation. Finally, the obtained models were compared. The experimental results show that the AdaBoost ensemble learning algorithm with a decision tree as the base classifier effectively identifies different vibrations and has certain significance for identifying vibration signals from different sources in the fiber-optical warning.

This study demonstrates that the actual thickness of a convex lens is a problem that cannot be ignored, then gradually deduces the object image relationship and focal length of a biconvex symmetric thick lens in the paraxial case in air. The specific differences in focal length and image distance between thick and thin convex lens theories are revealed through examples and drawings, and some of the rules are analyzed. The approximate expression of the focal length of a thin convex lens and the approximate expression of the focal length error of an ideal thin convex lens are derived using approximation, and the latter is almost independent of the focal length. This study also derives that the approximate expressions of image distances of a thin convex lens and ideal thin convex lens on the condition of “the ratio of object distance to focal length” is greater than 2, and the error between them has nothing to do with the focal length, but only with the thickness, refractive index and “the ratio of object distance to focal length” of the lens. At the end of the study, the reason for setting “the ratio of object distance to focal length” greater than 2 is that it is difficult to take approximation when the object distance is within 2 times the focal length, and the reason for dividing the convex lens into the thick convex lens, ideal thin convex lens, and thin convex lens lies in strict distinction.

Microprism reflector films are limited by various factors, such as a narrow range of effective incidence angles (wide angle) and large difference in retroreflection efficiency at different azimuth angles (different directions). These limitations can be resolved by changing the inclination angle of the cube-corner reflector (CCR) element and using the splicing structure. In this study, CCR arrays with different structures are calculated using the ray-tracing software based on the principle of geometrical optics. Herein, the influence of the inclination angle of the CCR element, angle of the incident light, and the splicing structure on the retroreflection efficiency is analyzed using the retroreflection principle of microprism reflective films. The results show that the retroreflective efficiency in the azimuth 0° direction increases by more than 20 percentage points when using a inclination angle CCR array. Furthermore, the maximum deviation of the retroreflective efficiency in different directions reduces by more than 10% after adopting a spliced structure with different inclination angles. The simulation results of the inclination angle of CCR arrays are compared with rightness test results of the reflective film, and the variation trend of the retroreflective efficiency is found to be consistent, confirming the reliability of the simulation model. These research results can be used in the structural design and optimization of microprism reflective films.

Exploiting microwave photonic (MWP) techniques to generate and distribute high-frequency millimeter-wave (mm-wave) signals, termed mm-wave radio over fiber (m-RoF) signals, holds considerable potential for achieving high-density and high-capacity fifth-generation and beyond networks. Herein, we experimentally validate a broadband m-RoF uplink fronthaul transmission system using the MWP downconversion concept, which comprises receiving and processing radio-frequency (RF) signals in the unlicensed V-band at around 60 GHz. The proposed system harnesses the simple cascaded modulator topology, in which an ultrawideband off-the-shelf Mach–Zehnder modulator (MZM) renders a simple-structured remote radio head by directly encoding the broadband 60 GHz uplink RF signal into the optical carrier. The nonlinear transfer function of another MZM at the center unit is explored to achieve subharmonic downconversion using cost-effective low-frequency local oscillator signals. Based on proof-of-concept experiments, mm-wave four quadrature amplitude modulation orthogonal frequency-division multiplexing signals centered at frequencies ranging from 51 GHz to 70 GHz are successfully downconverted into signals at the intermediate frequency (IF) of 1.4 GHz. In the case of 1.2 m mm-wave, free-space, and 5 km m-RoF transmissions, the obtained IF signals with a total bandwidth of 2.4 GHz achieve a bit-to-error ratio performance lower than the 7% hard-decision forward error correction limit of 3.8 × 10-3. A gross bit rate of 10 Gbit/s can be achieved over a total spectrum of up to 10 GHz, which fully covers the globally unlicensed V-band of 57-66 GHz.

In this paper, we present a method for accurately measuring the diopter of both progressive multi-focus and large-range single focal lens. Based on Talbot interferometer methods, we investigated the principle of lens diopter measurement and deduced the formula of optimal diopter measurement. The main parameters of the Talbot interferometer were designed for the range of -8 D to 8 D, and include the grating period, grating angle, and grating spacing. Furthermore, a phase-shifting Talbot interferometer measuring system was constructed and we adopted the calibration method, which uses the target coordinate and grating coordinate systems. Here we applied a five-step phase-shifting algorithm to solve the moiré fringe. The multiple experimental results of multiple diopters and progressive multi-focus lens show that the measurement error is better than 0.1%.

In this paper, we proposed a hybrid coding method that includes both phase order and relative wrapped phase to quickly and accurately obtain three-dimensional information of spatially discontinuous objects. Based on the traditional phase-shifting profilometry, the phase order was modulated into the phase-shifting image using the De Bruijn sequence. The length of the De Bruijn sequence used is the number of periods, where each code value corresponds to a phase period. After calculating the wrapped phase, the code sequence corresponding to each period was matched with the De Bruijn sequence to verify whether the location of the code in the De Bruijn sequence is the phase order. The proposed method need not project additional images to encode the phase order as both the phase order and the wrapped phase required by the unwrapped phase can be obtained from the projected phase-shifting patterns. The experimental results show that when only four images are projected, the three-dimensional morphology of the objects to be measured can be reconstructed, and the standard deviation of the reconstructed plate is 0.319 mm.

In fringe projection profilometry, projector calibration is an important part of system calibration. Accurate projector calibration is key to ensuring the accuracy of three-dimensional shape measurements. The profilometry projector cannot capture the image directly. Thus, obtaining the correspondence between pixels and object points is difficult, which makes projector calibration complex with low precision. To address this problem, we analyzed the factors that influence projector calibration and presented a new projector calibration method based on local homography matrix. First, we conducted experiments to determine the optimum size of the local area. Then, we calculated the local homography matrix of the area where the center of the circle is located and completed the mapping of the projector's coordinates. Finally, we used the camera calibration method to calibrate the projector. Experimental results show that this method reduces the influence of center deviation and phase error on the calibration accuracy of the projector. Based on the idea of local linearization, the subpixel mapping of camera image coordinates to projector image coordinates is established, and the accuracy of projector calibration is improved.

As an optically sensitive unit, the silicon-based integrated photonic resonator plays a vital role in the performance of chip-scale optical gyroscopes. In this paper, the integrated resonator of the photonic integrated gyroscope with the enhanced reciprocity-sensitive dual micro-ring architecture is the research object. The key structural parameters of the dual microring resonator are analyzed through finite-difference time-domain simulation. Constructing a simulation link allowed the study on the internal relationships between quality factor, fineness, coupling coefficient, and transmission loss. The resulting insights formed the basis for structural layout design. The dual microring resonant cavity device is then fabricated through a multiproject wafer tape-out using a passive silicon-based photonic integration process. Test results show that, for a microring radius of 500 μm, the coupling coefficient between waveguide and microring is 0.3, the working wavelength is around 1550 nm, the free spectral range is 0.182 nm, the 3 dB bandwidth is 0.045 nm, the fineness is 4.04, and the quality factor is ~3.4×104. These research results provide a firm foundation for further design optimization of the dual microring resonator.

In this paper, we proposed a finite element model of composite crystals to solve the thermal effect of disk crystal end-pumped by LD. The model was established based on the heat conduction theory, which is related to the working characteristics of continuous LD end-pumped YAG/Yb∶YAG disk. The effects of different factors such as the pump power, composite crystal thickness, and cross section size on the temperature field were analyzed by setting LD pumping parameters. The results showed that when the geometry of the composite crystal is constant, the temperature of the crystal increased with increasing pump power. At 70 W pump power, the pump spot radius was 400 μm via the collimation focusing of the optical coupling system, the thickness of Yb∶YAG crystal was 3 mm, and the thickness bonding of YAG crystal changed from 0 mm to 0.7 mm. Furthermore, the rise in maximum radial and axial temperatures decreased by 39.7 ℃ and 238.3 ℃, respectively. Under the same pumping conditions and with a 0.5 mm thick YAG crystal, the cross section radius of composite crystal increased from 8 mm to 13 mm, the maximum temperature rise along the radial and axial directions decreased by 32.8 ℃ and 171.7 ℃, respectively. Thus, the composite crystals can reduce the thermal effect inside laser crystals, which is significant for realizing high-power outputs of solid-state Yb∶YAG lasers.

The evolution characteristics of Q-switched mode-locked rectangular noise-like pulses were studied in an L-band erbium-doped fiber laser. By varying the pump power and polarization state, continuous-wave and Q-switched mode-locked based on rectangular noise-like pulses with 1600 nm central wavelength were realized in the fiber laser. The fundamental repetition rate and the maximum pulse width of the rectangular noise-like pulse were 1.64 MHz and 17.51 ns, respectively, and the repetition rate of the Q-switched mode-locked rectangular noise-like pulse could be tuned in the range of 8.14-18.18 kHz. The evolution trend of the Q-switched envelope width was opposite to that of the rectangular pulse width. The maximum average energy of the Q-switched envelope and the rectangular pulse inside the Q-switched envelope reached 1115.5 nJ and 24.89 nJ, respectively. The average peak power of the rectangular pulse was maintained at approximately 1 W. These results may aid in understanding the dynamic characteristics of Q-switched mode-locked rectangular noise-like pulses in L-band fiber lasers.

In this paper, the generation and propagation of self-similar pulses in an Er-doped mode-locked fiber laser based on saturable absorber (SA) are numerically investigated. Research results indicate that by using a germanium rod to compensate the cavity dispersion, the intracavity pulse of an erbium-doped fluoride fiber laser based on SA mode-locking can support self-similar pulses evolution when the net dispersion of the cavity is in the range of 0.020-0.048 ps2. Keeping the net dispersion of the cavity with 0.03 ps2, ideal parabolic pulse with a pulse width of 19.7 ps, a peak power of 630 W, and a pulse energy of 12.4 nJ is obtained. To further optimize the output properties of the laser, the effects of small signal gain and gain saturated energy of the gain fiber, as well as the modulation depth and saturated power of the SA on the pulse output properties are studied in detail, which provides reference for the design of self-similar mode-locked Er-doped fluoride fiber lasers.

To improve the laser cutting quality of carbon fiber reinforced plastics, the response surface method was used to establish a mathematical model of each response index with slit entrance width, heat-affected zone width, and kerf taper angleas the response indexes. Thereafter, the influence law of the interaction of each laser parameter on the response indexes was analyzed, and the parameter optimization combination was obtained to minimize each response index using 755 W laser power, 20 mm/s cutting speed, -0.6 mm focus position, and 0.7 MPa air pressure. The predicted values of the response indexes were obtained. Next, the laser cutting test was conducted under the parameter optimization combination, and the differences between the response indexes slit entrance width, heat-affected zone width, and kerf taper angle and predicted values were 12.73 μm, 6.55 μm, and 0.12°, which were within the fluctuation range of the predicted values, proving the effectiveness of the parameter optimization combination. The laser-cut specimens with the parameter optimization combination increased the glued tensile shear strength by 22.86% compared with the specimens before the optimization. The parameter optimization combination improves the laser cutting quality and reduces the impact of laser cutting on the mechanical properties of the material.

Mg element can make the aluminum alloy to obtain better mechanical properties and form a corrosion-resistant spinel film on the surface of the alloy, so that the alloy has better corrosion resistance. Therefore, exploring a method that can quickly and accurately detect the content of magnesium in aluminum alloy quantitatively is of great significance. In this paper, first, the Mg element in 17 aluminum alloy samples is detected and analyzed based on laser-induced breakdown spectroscopy (LIBS) technology. Then, Nd:YAG laser is used as light source, and the partial least squares (PLS) and random forest (RF) models are respectively established, and the prediction performance of the models is analyzed. The experimental results show that for the same test set, the correlation coefficient (Rp2) of the PLS model is 0.6809, and the root mean square error (RMSE) is 1.2042; the Rp2of the RF model is 0.8571 and the RMSEis 1.0918. In order to improve the prediction accuracy of the random forest model, this experiment screened the input variables according to the importance of the variables. When the wavelength point with variable importance greater than 0.11 is selected, Rp2 of the RF model based on variable importance is 0.9461, and the RMSE is 0.9534. Compared with the prediction result of the RF model, Rp2 is increased by 10.38%, RMSEis reduced by 12.68%, and the modeling time is reduced by 91.67%.

In this paper, the time-stretch dispersive Fourier transform is applied to imaging femtosecond laser induced filaments. One-dimensional imaging of the transient evolution of a filament is realized by using spectral-spatial coding method combined with a fast photo-detector and an oscilloscope. The spatial resolution is 60 μm and the refresh rate reaches 54.54 MHz. The proposed method avoids the limitation of refresh rate caused by CCD and provides a new method for studying the dynamics of the filament-matter interaction.

The effect of various beam offsets on the microstructure and the properties of welded joints was studied using optical microscope, scanning electron microscope, and energy dispersive spectrometer, combined with tensile and hardness tests to investigate the challenges caused by large differences in physical properties and poor metallurgical compatibility in the laser welding of dissimilar materials such as copper/stainless steel. The results show that the weld is semi-hourglass shaped under the influence of an evaporation force and Marangoni convection effect. When the beam is biased towards the copper side, the iron-rich phase disperses in the weld due to the liquid phase separation, resulting in an uneven distribution of the weld composition and hardness. In addition, the more copper content in the deposited metal, the lower the joint strength. When the beam is biased towards the steel side, the microstructure of the weld is austenite with a copper-rich phase, and the strength of the welded joint increases. Moreover, the maximum tensile strength can reach 240 MPa, equivalent to the copper base metal. Therefore, the best comprehensive performance of the welding joint can be obtained when the beam offset is -0.4 mm.

We prepared Li+ as a charge compensator using the high-temperature solid-state method and doped it into the silicate matrix of CaMgSiO4∶Sm3+ to improve the luminescence properties of Sm3+ in CaMgSiO4. The experimental results show that the CaMgSiO4∶x Sm3+, y Li+ samples are pure phases, and introducing Sm3+and Li+ do not change the crystal structure. Under the excitation of the near-ultraviolet of 400 nm caused by the 4f-4f transition of Sm3+ ions, the emission peaks of CaMgSiO4∶Sm3+ are 562, 576, 601, and 650 nm respectively. Additionally, from the spectra of co-doped samples, incorporating Li+ significantly enhances the luminescence intensity and emission peak integral area of Sm3+. Moreover, the color coordinates of the samples are in the red region (0.605, 0.394) and the color purity is up to 93.3%. At 150 °C, the peak intensity remains at 73.5% of that at room temperature, indicating that the phosphor has good thermal stability. The phosphor has a potential application prospect in solid lighting.

A dynamic lighting regulation method is proposed based on a rhythm factor and swarm intelligence algorithm to create a healthy, comfortable, and energy-saving visual display terminal(VDT) light operational environment. First, the lighting model is established according to the operational situation and compensation relation of the VDT workspace. The dynamic illuminance regulation model is established by combining the light demand, the compensation relationship between artificial and natural light sources, and rhythm factor. Then, a hybrid algorithm combining the advantages of particle swarm optimization and the firefly algorithm is proposed, and the idea of elite particles is introduced to improve the hybrid algorithm. Furthermore, the illuminance demand for a certain period, compensation amount of the natural light source, and operational area distribution are applied as the input of the illuminance optimization model. Afterward, the improved algorithm is used to solve the light flux of the artificial light source. The improved algorithm is compared with other algorithms. Consequently, its optimization ability and stability are evidently superior to those of other related algorithms. Experiments demonstrate that the optimized VDT operational space light environment based on the dynamic regulation method has a better visual effect and greater energy savings potential.

To achieve the purpose of reducing errors in the measurement of the refraction of the human eye by eccentric photographic optometry and to produce clearer photographs of the human eyes, the imaging system of the device was designed. The working distance of the lens is 1000 mm, the focal length of the system is 46.7 mm, the total length is 51.4554 mm, and the F-number is 2.8. After optimization, the maximum optical system distortion is not greater than 0.3%, and the spherical aberration, field curvature, and distortion are well corrected. The imaging lens has the characteristics of simple, compact, and fine quality, which accords the demand of eccentric photographic optometry equipment.

The design of Offner compensator after paraboloid normalization and renormalization can be regarded as a deterministic problem of optimization in parameter space. The evaluation function distribution in the parameter space is obtained by constructing an example of optimizing the structure of the Offner compensator of the parabolic lens, then the reference map of the design results of the Offner compensator is obtained. A method for selecting the initial structure and evaluating the performance of the compensator is proposed, and the results show that the problem can be solved by numerical method.

Aiming at the border effect caused by ion beam polishing optical components, a suppression method based on polynomial fitting continuation is proposed. The initial surface shape is extended by the polynomial extension method, and the obtained extended surface shape is simulated. The root mean square value of the shape after the simulation is 5.20 nm. The K9 optical element with a diameter of 100 mm is processed by ion beam polishing technology, and the root mean square value of the surface shape after processing is reduced from 19.26 nm to 12.23 nm. Then, 8% of the processed surface shape is selected as the boundary surface shape, and the root mean square value of the boundary surface shape is reduced from 137.23 nm to 56.72 nm. The feasibility of this method is verified through simulation and experiment. This research can provide a new method for optical processing

To speed up the optimization of antenna modeling, this paper proposes a novel deep multi-layer perceptron (DMLP) network based on deep learning network architecture for optimizing ultra-wideband antenna. The DMLP network uses a step-down, connected-layer deep network, and the Adam optimizer automatically updates the learning rate. Dropout technology is used to remove random neurons in the hidden layer, preventing overfitting due to the deep network layers. This paper uses the DMLP network to model the geometric parameters of the ultra-wideband stepped microstrip monopole antenna, extracts features from the eight geometric parameters of the antenna, and predicts the S11 value of the antenna. The experimental results show that compared with traditional multilayer perceptron and radial-basis-function neural networks, the average prediction error of S11 is reduced by 118.32% and 123.76%, respectively, and it has a higher prediction accuracy. In addition, the fitting speed is improved. The feasibility of this network is verified through experiments.

Herein, we present a systematic study on annular plasma photonic crystals (APPCs) with different filling ratios in dielectric barrier discharge using the water electrodes. In this study, the APPCs with dynamically adjustable plasma columns and high spatiotemporal symmetry have been realized, providing possibilities for modulating the microstructures of elements in APPCs. Based on the experimental results, the dispersion relation of different APPCs are studied using the finite element method. Furthermore, the influences of the radius of plasma columns on the positions and sizes of band gaps are analyzed. The results show that band gaps change from unidirectional to omnidirectional with increasing plasma column radius and the widths of omnidirectional band gaps increase considerably. Compared with conventional plasma photonic crystals (PPCs), APPCs can easily produce large omnidirectional band gaps with lower threshold values of the plasma column radius. Additionally, for a given radius, the sizes of band gaps in APPCs are larger than those in conventional PPCs. The novel APPCs with tunable filling ratios proposed here provide more possibilities for engineering the band gaps and offer enlightenment for designing new types of tunable photonic crystals and developing wide band gaps, highly-integrated photonic devices.

The propagation and evolution properties of the finite energy Airy beams generated by exponential attenuation, spatial truncation, frequency truncation, and constant amplitude methods are compared and analyzed using the Huygens-Fresnel diffraction integral. Further, the effects of a transverse scaling factor and aperture truncated width on non-diffracting propagation distance and transverse acceleration are discussed. When the aperture truncated width is constant, the non-diffracting propagation distance and lateral acceleration offset of all types of finite energy Airy beams increase as the transverse scaling factor increases. When the transverse scale factor is constant, the non-diffracting propagation distance of all types of finite energy Airy beams increases as the aperture truncated width increases. When the aperture truncated width and transverse scaling factor are constant, the non-diffracting propagation distance of frequency truncation Airy beam is the shortest, that of the exponential attenuation and spatial truncation Airy beams are longer, and that of the constant amplitude Airy beam is the longest. The results have certain significance for expanding the application of finite energy Airy beams in the long-distance propagation.

Due to the random distribution of micro-structure on the surface, the actual metal has obvious masking effect in light reflection or thermal radiation. The existing bidirectional reflection distribution function model using piecewise geometric attenuation factor is difficult to obtain high characterization accuracy. In this paper, first, the masking and shadowing effect of the inclination difference of adjacent micro-facet on the metal surface of the incident and outgoing light are analyzed. Then, the existing integral masking function is modified based on the geometric relationship between the inclination angle of micro-facet and light, the distribution of micro-facet on the metal surface is characterized by the Cauchy distribution, and a polarization model of spontaneous emission on the metal surface based on the masking function modification is proposed. Finally, the polarization degree of spontaneous emission of different metal materials under heating conditions is obtained through experiments to verify the effect of the modified model. The results show that the spontaneous emission model based on the masking function modification is more consistent with the experimental data than the existing model.

A biological dual parametric sensing structure of D-type photonic crystal fiber based on surface plasmon resonance and directional coupling effects is presented in this paper. First, the surface plasmon resonance effect between the core of photonic crystal fiber and the metal film layer is used to detect the mass concentration of the biological liquid. Then, the directional coupling effect between the core of photonic crystal fiber and the defective core and the temperature-sensitive effect of the temperature-sensitive material are used as the temperature sensing mechanism to measure the ambient temperature. Finally, the photonic crystal fiber sensing structure model is established and numerically analyzed by the finite element software COMSOL Multiphysics. The results show that the mass concentration sensing based on surface plasmon resonance and temperature sensing based on directional coupling are independent of each other. The sensitivity of the sensor can reach 5.44 nm/(mg·mL-1) in the concentration range of 34.6?186.7 mg/mL human serum albumin, and the sensitivity can reach 17.3 nm/℃ in the temperature range of 20?45 ℃.

This study proposes a fiber Bragg grating (FBG) humidity sensor using polyimide (PI) as a humidity-sensitive material. First, to homogenously coat the FBG’s surface, a PI solution is perfused into a PI capillary. Then, a coupling agent is used to strengthen the bond between the humidity-sensitive material and FBG. Finally, three groups of humidity sensors with different thicknesses and lengths are fabricated, and their humidity sensing and temperature cross-sensitivity characteristics are tested. The experimental results show that the thicker the PI film is, the lower the fitting degree is and the higher the humidity sensor’s sensitivity is. The FBG humidity sensor with a film thickness of 0.13 mm shows a sensitivity of 5.5 pm/%RH, a detection range of 30%RH-98%RH, a fitting degree of 0.99, and a nonlinear error of 2%, with good operability, popularization, and applicability.

Mutual interference between LiDARs is a critical problem that affects the safety of driverless cars. In this paper, a vehicle-mounted, light frequency-hopping (LFH) LiDAR system and its related ranging method are proposed. The anti-interference performance of the LFH LiDAR is analyzed under different interference conditions by simulations and experiments. The results show that the LFH LiDAR based on correlation detection can effectively resist the interference from the same type of LFH, pulse, frequency-modulated continuous wave, and continuous-wave LiDAR in different quantity and intensity cases. In the experiment involving different types of interferences, the relative error of the detection range is less than or equal to 1.5%. It indicates that the LFH LiDAR has reliable anti-interference performance.

Terahertz (THz) technology is gaining lots of attention for its unique applications in nondestructive testing, communications, spectral analysis and sensing. However, the traditional THz waveplate has a large size and high loss. As a two-dimensional planar material composed of artificially designed subwavelength periodic meta-atoms, the metasurface can flexibly adjust the amplitude, phase and polarization of electromagnetic waves within the subwavelength thickness, providing a platform for the design of ultra-compact and high-performance THz waveplate. In this paper, the THz polarization waveplate based on natural materials, dielectric gratings and metasurface are reviewed respectively. In particular, the THz polarization waveplate based on metasurface is mainly focused, including different resonant structures, switchable and continuously tuned, etc. Finally, the development of these terahertz polarization waveplate is summarized.

Laser additive manufacturing is widely used in the manufacturing and repairing of petrochemical, aerospace, and marine equipment. However, the residual stress caused by the rapid heating and cooling in the preparation of alloy materials via laser additive manufacturing likely poses a major risk of stress corrosion cracking of the material in harsh environments. This study first reviews the mechanism of residual stress in alloy materials produced via laser additive manufacturing. Second, the main measurement and elimination methods of residual stress in materials are summarized. In addition, the test methods and the mechanism of stress corrosion cracking in alloy materials caused by laser additive manufacturing are summarized. Finally, based on the research status of residual stress and stress corrosion cracking of alloy materials in laser additive manufacturing, the key problems that need to be solved in this field and future development trends are summarized.

Laser polishing technology has the advantages of non-contact processing, no mechanical stress, high polishing accuracy, etc. It is especially suitable for surface processing of brittle and hard materials. This article describes the characteristics and mechanism of laser polishing process, introduces the influence of various parameters in laser polishing process on processing quality, and reviews the research results and current status of laser polishing technology for hard and brittle materials in various countries. The difference and characteristics of laser hot polishing and laser cold polishing, as well as the polishing principle and research progress are emphatically introduced.

Minimally invasive medical treatment has significant advantages compared with general medical treatment. Particularly, minimally invasive surgery can reduce intraoperative blood loss and trauma, improve postoperative recovery, reduce patient pain and doctor fatigue, etc. In recent years, robot-assisted medical system suitable for minimally invasive medical treatment has become one of the hot topics in the world. In addition to easy integration, optical fiber Bragg grating sensor is immunity to electric-magnetic interference, wavelength encoded nature, good linearity, etc. Compared to those traditional electrical sensors, such as piezoresistive sensor, capacitive sensor, and piezoelectric sensor, FBG presents more potentials in the field of minimally invasive medical intelligent robot. In this work, we review the applications of fiber Bragg grating sensor in minimally invasive medical treatment at length and discuss the exiting problems and perspectives.

The fiber Bragg grating sensor reflects the change of the physical quantity measured by the change of the center wavelength, so it is of great significance to improve the demodulation accuracy of the wavelength of the fiber Bragg grating. Most of the traditional demodulation algorithms cannot accurately demodulate overlapping spectra and distorted spectra because of poor anti-noise performance and limited demodulation accuracy, which limits the development of demodulation systems. The high-precision demodulation algorithm can realize accurate demodulation of sensor networks by solving the problems such as poor anti-noise performance and low peak-finding accuracy. The article introduces the sensing principle of fiber grating, explains the main types and production method on fiber grating, and summarizes the high-precision wavelength demodulation algorithms of fiber Bragg grating in recent years. The principle, advantages, and disadvantages of each demodulation algorithm are explained which is divided into two types: single-peak peak-finding algorithm and multi-peak peak-finding algorithm. And the article briefly analyzes and prospects the future of high-precision demodulation algorithms of fiber Bragg grating.

From a global perspective, fossil energy remains the main source of energy supply. For fossil energy development, the petroleum industry has high requirements regarding the detection and monitoring of various gases, which are important for environmental protection and safe production. Tunable diode laser absorption spectroscopy (TDLAS) is a type of trace gas detection technology with advantages of high sensitivity, good selectivity, fast response, and availability for multispecies and multiparameter measurements. It can be widely used for detecting gas volume fraction in the petroleum industry site and monitoring the leakage of hazardous gas. This paper presents, classifies, and summarizes the principle and various detection methods of TDLAS. Furthermore, the application of TDLAS for on-site gas detection in the petroleum industry is reviewed, including detection for flammable and explosive gases (methane, ethane, propane, butane, acetylene, and ethylene), toxic gases (hydrogen sulfide and carbon monoxide), and other gases (oxygen and water). The current applications of the TDLAS standoff detection technology are summarized and future development areas of the TDLAS technology in the petroleum industry are outlined.

The global 5G communication network subscriber base has surpassed 400 million as of May 2021, thanks to the commercial deployment of 5G communication technologies around the world. Research on the next-generation wireless communication technologies has been conducted to address the demand for ultrahigh data rates and ultralow latency for future applications. The large number of absolute bandwidth resources in the terahertz band is the most significant advantage of the terahertz communication, making it suitable for various applications. Herein, the relevant research in the field of terahertz communication is described. First, we introduce the plan and vision of 6G communication network, as well as the current status of the terahertz communication technology on a domestic and worldwide scale. Second, the key terahertz communication technology and potential application scenarios are discussed. Finally, we present a summary of the current research results and an outlook on future research directions to provide new ideas for moving into the “terahertz era.”

Laser particle size analyzer is an instrument based on the principle of static light scattering used to measure particle size. Herein, an ideal optical model of the laser particle size analyzer, which can measure scattered light in the range of 0?π, is proposed and its lower limit of measurement and resolving power are studied. Moreover, the optimal detector density coefficient of the ideal model based on the sampling law is discussed. Experimental results show that when the tolerance limit of scattered light energy distribution loss is 0.1%, the optimal detector density coefficient of the ideal model is 1.0905. When the noise intensity is 0.5%, the lower limit of measurement of the ideal model is 40 nm, and the resolving power for particles with the sizes of 100, 200, and 300 nm is 50.5%, 67.3%, and 75.0%, respectively. Conversely, when the ideal model is used to measure standard polystyrene microspheres, the lower limit of measurement can reach 50 nm, and the resolving power of particles with the sizes of 100, 200, and 300 nm is 50.0%, 55.7%, and 75.0%, respectively.

This study investigated a caries detection method based on the autofluorescence effect to achieve non-destructive detection of early dental caries. The spectrum characteristics of different caries were studied using spectroscopy. A fluorescence spectrometer was used to determine the excitation and fluorescence spectra of different caries samples, and the Savitzky-Golay smoothing method was used to preprocess the spectra. The excitation peak wavelengths of healthy teeth and caries were near 405 nm, and the fluorescence peak of tooth hard tissue was near 480 nm. Carious tissues had fluorescence peaks at 623 nm and 685 nm, corresponding to the peak positions of dental plaque metabolites. The fluorescence spectrum of teeth was significantly different under 405 nm light excitation, and the fluorescence image of healthy teeth was green. With increasing plaque concentration, the fluorescence image of dental caries turned red. The results show that early caries can be detected by analyzing the color difference in fluorescence images.

Soil potassium content affects the quality and yield of crops. In this study, near-infrared (NIR) spectroscopy is combined with a method of selecting feature wavebands for the rapid quantitative detection of soil potassium content. First, the NIR model for the feature waveband optimization of the soil potassium content is established by combining the simulated annealing algorithm and interval partial least squares (SA-iPLS). Then, the optimal feature waveband of SA-iPLS is obtained by adjusting the number of subintervals. Finally, the SA-iPLS model is compared with the partial least squares (PLS), interval PLS (iPLS), and synergy iPLS (SiPLS) models according to the evaluation indicators of the model. The results show that the SA-iPLS model exhibited the best performance on the training set when the number of subintervals is 90, and the prediction root mean square error and correlation coefficient of the test set are 0.0117 and 0.8884, respectively. The prediction root mean square error and correlation coefficient of the full-spectrum PLS model for the test set samples are 0.0140 and 0.8506, respectively. The optimal number of subintervals for the iPLS and SiPLS models are 80 and 70, respectively; the prediction root mean square errors for the test set samples are 0.0155 and 0.0145, respectively; the correlation coefficients are 0.7786 and 0.8420, respectively. Compared to the conventional iPLS and SiPLS models, the SA-iPLS model can retain more useful spectral information and improve the prediction accuracy of the soil potassium content.

In this study, a gas volume fraction-inversion method is proposed to solve the drifting issue of the second harmonic background signal in the wavelength modulation spectroscopy technology. Here, the signal similar to the measured second harmonic signal in the background signal history data is taken as the background signal. First, a laser with a central wavelength of ~5.18 μm is used to build a NO volume fraction measurement system and a background signal library is constructed using the pure N2 background signal measured within 48 h as historical data. Subsequently, the NO is passed into the system, and the absorbed second harmonic signal and the correlation coefficient between it and each background signal in the background signal library is calculated. Finally, the volume fraction of the gas is retrieved after subtracting the background signal from the maximum correlation coefficient. The NO with a volume fraction of 2.5 × 10-6 is monitored for 24 h, and the results show that the method could decrease the average relative error from 6.48% before correction to 3.84%.

Garnet group belongs to nesosilicate minerals with A3B2[SiO4]3 as itsstandard chemical formula. Rarely, few garnets generate two distinct transmission windows due to the isotropic substitution of transition elements, such as Cr and V. This phenomenon leads to color-change as a variation of external light source, which produces purple-green and yellow-red colors under daylight and incandescent light, respectively. As the color-change effect is found in pyrope, spessartine, and pyrope-spessartine, we select the gem grade pyrope-spessartine in this study. Electron probe (EPMA), Fourier transform infrared (FT-IR) spectrometer, ultraviolet-visible (UV-Vis) spectrophotometer, and photoluminescence (PL) spectrometer were conducted on samples to understand the color-change mechanism. EPMA results showed that the samples could be defined as pyrope-spessartine, with few amounts of grossular and almandine. UV-Vis spectra showed that the tailing absorption band around 420 nm (belongs to d-d orbital transition of Mn2+, 6A1g→ 4Eg) overlay with a wide absorption band of 571 nm (belongs to d-d orbital transition of Cr3+,4A2g→4T2g), forming two transmission windows in the blue-green and red regions. The samples excited by daylight and incandescent light produced cyan and orange colors, respectively, due to the alternative transmission between the above two regions leading to the color-change effect. UV-Vis and PL spectra results showed that the isotropic isomorphism mainly occurred on Mn2+, Fe2+, and Cr3+ when substituting in cubic voids and octahedral structures. And the strongest luminescence center of Cr3+ is located near 686 nm, which is also the reason why samples exhibit weak red UV fluorescence at long wavelengths. However, this fluorescence property is inhibited or replaced by Fe2+ for the strong fluorescence quenching effect. The color-change effect in samples was attributed solely to Cr3+.