The existence of atmospheric turbulence and platform micro-vibration will cause the random channel attenuation of the satellite-to-ground laser communication links， which leads to the unstable communication quality. To further analyzes the performance of satellite-to-ground laser communications with the influence of atmospheric turbulence and platform micro-vibration， a random attenuation model of joint satellite-to-ground laser link is established. Based on binary phase shift keying modulation and heterodyne coherent reception， the closed form expression of system average bit error rate under the model is given. The influence of different satellite-to-ground laser communication system parameters on the bit error rate performance is analyzed by simulation. The research results can provide a theoretical reference for the design of practical satellite-to-ground laser communication system.

Aerosol optical depth （AOD） is a key factor for characterizing aerosol content and atmospheric pollution. In this study， level 2 aerosol profile products of the CALIPSO satellite were used to analyze the temporal and spatial variation characteristics and trends of AOD in specific regions worldwide from 2009 to 2018. Results show that AOD has a certain temporal and spatial difference in different areas. On the spatial scale， high AOD value centers are mainly distributed in India， Saudi Arabia， and other regions； moreover， on the temporal scale， noticeable seasonal differences exist. Specific regions such as India， Saudi Arabia， and northern China show peak AOD during the MAM and DJF periods， while Brazil peak during the JJA period. Moreover， significant AOD trend differences exist in each typical region， with the strongest upward trend in India.

The inversion method of temperature and pressure from the upper and middle troposphere to the lower stratosphere is studied and simulated. An absorption line is selected， which is not sensitive to temperature but sensitive to pressure， and the relationship among the absorption cross section， pressure and the absorption coefficient is used to determine the pressure by using the iterative method. An absorption line is selected near the weak absorption peak and the relationship among the absorption coefficient， pressure and temperature is used according to the retrieved pressure value to determine the temperature by using the iterative method. In order to reduce the influence of other absorption and scattering factors on the inversion results， the simulation process adopts the method of differential wavelength. In the oxygen absorption band， the appropriate absorption line is selected， and the profile of the differential absorption coefficient at the tangent altitude of each laser track is obtained by retrieving the simulated data of the laser occultation differential transmittance from the Abel integral inverse transformation， and then the pressure and temperature at each tangent altitude are retrieved using the differential absorption coefficient. The simulation results show that the inversion error of pressure is primarily affected by the inversion error of the differential absorption coefficient， which increases with altitude decreasing， and the maximum error is approximately 6%； the inversion error of temperature is affected at the same time by the inversion error of pressure and the differential absorption coefficient； the two influences are partially offset， and the maximum error is 1.5 K near an altitude of 5 km. Through analysis of the error model， some change trends and influence factors in the inversion error are explained. Under the condition of eliminating the inversion error of the differential absorption coefficient， the pressure and temperature are solved once in a cycle， the maximum inversion error of pressure is approximately 0.3%， and the temperature is approximately 1 K. The comparison between this inversion error and the inversion error of the differential absorption coefficient highlights the importance of reducing the latter.

Aiming at the problem of limited speed measurement of moving targets in a large dynamic range， a four-light coherent mixing detection method is proposed. By extracting the Doppler frequency and the difference frequency between frequency shift and Doppler frequency， the speed measurement with a larger dynamic range can be achieved. In the process of four-optical coherent mixing detection， there are many output signals and the problem of difficult signal discrimination. In order to solve the above problems， a four-light coherent mixing detection structure with spatial polarization splitting structure is proposed and designed. Polarization splitting technology is used to achieve complete spatial separation of four output signals， avoiding mutual influence between signals and reducing the difficulty of signal processing. In theory， the structure is analyzed in principle， and the optical simulation software is used to simulate and analyze it to verify the feasibility of the theory and structure. At the same time， the direction of the target movement can be judged according to the output signals of different detectors.

A high sensitivity sensor based on anapole mode is designed， which consists of a metal reflector， a SiO2 thin film and a slotted silicon disk. The anapole mode in silicon nanoarray is used to excite the metal reflector layer to generate plasma resonance， and sharp resonance peaks can be generated in the periodic nanostructures through mode matching and coupling between adjacent structures. The structure is simulated and analyzed by finite difference time domain method， and the influence of different parameters on the sensitivity of the sensor is studied. The simulation results show that the nanostructures can significantly enhance the local electric field and reduce the energy absorption rate. The spin-orbital angular momentum conversion around nanostructures can be realized by using circularly polarized light， which is beneficial to the sensing of biomolecules such as neurotransmitters.

Composition segregation can easily occur during the non-equilibrium solidification of SiGe alloys. The non-uniform composition distribution caused by the fiber fabrication process leads to high fiber loss. In this paper， the thermal processing of the SiGe core silica cladding fiber prepared by the assembly of semi-cylindrical Si and Ge rods is conducted using a CO2 laser， and the effects of different thermal processing conditions on the fiber core composition distribution are studied. The experimental results show that continuous Si-rich SiGe alloys can form at edge of laser scanning area along the laser scanning direction， and the closer to the edge， the more uniform the composition distribution of the fiber core is. This study provides a method to fabricate and optimize a SiGe core fiber to achieve uniform composition distribution of the fiber core.

To address the path selection for quantum communication between any two users around the world， according to the structural characteristics of the dandelion spherical quantum satellite network， a quantum routing scheme with two-end entanglement exchange is proposed. In this scheme， entanglement is established from the source node and the destination node to the intermediate node， and the Bell measurement results are transmitted through the wireless channel， so as to complete the transmission process of quantum state information from the source node to the destination node. Simulation results show that： with the increase of the number of switching satellites， the two-end entanglement exchange routing scheme can effectively reduce the transmission time of quantum information； with the increase of the number of switching satellite nodes， the quantum channel establishment rate of the two-end entanglement exchange routing scheme is significantly superior to that of the hop by hop entanglement exchange scheme， and its routing transmission cost is less than that of the hop by hop entanglement exchange scheme. It can be seen that the quantum routing scheme of the two-end entanglement exchange has the advantages of short information transmission time， high quantum channel establishment rate， and small wireless transmission overhead.

A new photon scheme for linear frequency modulation （LFM） signal generation and dechirping is proposed. In the proposed scheme， at the radar transmitter， a double-parallel Mach-Zehnder modulator is used to generate quadruple frequency suppression carrier double-sideband signal. The two sidebands are separated after the fiber grating and modulated by a cyclic phase， and the octave frequency LFM signal is generated by photoelectric conversion through a photodetector. At the receiving end， the echo signal is phase-modulated， the carrier wave and the first-order sideband containing the echo information are filtered through a filter， and the beat frequency is realized to obtain a single-frequency signal. Thus， the dechirp processing of the echo signal is completed. In the analysis and verification by Optisystem， the frequency of the radio frequency signal was set to 10 GHz， the amplitude of the parabolic signal was 1， and the width of the parabolic signal was 10 ns. An LFM signal with a center frequency of 80 GHz， bandwidth of 32 GHz and time bandwidth products of 320 was generated. The tunability and compression performance were verified， and LFM signals with time bandwidth products of 480， 640， and 3200 were generated. At the receiving end， echo signals with delays of 1.0 μs and 0.9 μs were dechirped at the receiver， corresponding to detection distances of 150 m and 135 m， respectively. The scheme is compact and has good reconfigurability.

In this study， the performance of a three-hop hybrid radio frequency/free space optical/radio frequency （RF/FSO/RF） airborne communication link based on decode-and-forward （DF） relaying is investigated. The connection link between the airborne subnode and the backbone node is the RF link， whereas that between the airborne backbone node and the backbone node is the FSO link. The same modulation method is assumed in the three hops. The RF channel is characterized using the Nakagami-m fading model， and the FSO channel follows the Exponentiated Weibull atmospheric turbulence distribution model. Further， closed expressions are obtained for the end-to-end outage probability （OP） and the average bit error rate （BER）. Subsequently， the effects of turbulence intensities， RF channel fading coefficients， and modulation modes on the OP and average BER are analyzed. The obtained results indicate that in the three-hop DF relaying mode， the optimal OP and BER are dependent on the hop with the worst channel quality. Furthermore， binary phase shift keying （BPSK） modulation outperforms binary frequency shift keying （BFSK） modulation and high-order PSK with respect to the BER. The deduced closed expression is conducive for quantitatively analyzing the link performance and provides a reference for system design.

In this paper， the upper limit of sampling rate of chaotic signals which are generated by an optoelectronic oscillator and used as the entropy source of physical random number is studied. The effects of different sampling rates on the random characteristics of the chaotic entropy source are analyzed. The maximum sampling rate for random sampling is obtained. The random characteristics of the signal are characterized by the autocorrelation function and power spectra. The maximum sampling rate is numerically studied by the run test and normality test. Simulation results show that， for the random sampling of chaotic signals， there exists an upper limit of sampling rate. On this basis， the maximum random sampling rate and the maximum normal sampling rate are obtained by calculations. The changes of these two maximum sampling rates with the feedback intensity of the optoelectronic oscillation system are numerically studied.

A rotating laser-scanning measurement system utilizes a photoelectric-scanning timing method and combines the principle of space-time conversion to provide angle-rendezvous positioning. The accuracy of the time-domain information in the photoelectric signal is an important factor， which affects the measurement accuracy. To address the problem of delay in the synchronization signal in the measurement system， we propose a delay-evaluation method that uses forward- and reverse-measurement targets on the transmitter base-station turntable according to the characteristics of the angle-measurement error. By quantitatively analyzing the delay in the synchronization signal， we first establish a delay model for the synchronization signal. Using a programmable logic device， we then analyze the influence of the delay on the accuracy of the angle measurement， study the algorithm used to compensate for the synchronization signal circuit delay， and design experiments to verify it. The experimental results show that the proposed method can effectively compensate for an original circuit delay of approximately 190 ns in the synchronization signal.

In this paper， a optical fiber acoustic sensing system based on the Fabry-Perot （F-P） microcavity structure is studied. The sensor head composed of a zirconia （ZrO2） tube with a high melting point and low thermal expansion coefficient bonded to graphene oxide （GO） diaphragm. Test results show that the cavity length of the F-P microcavity is 92.943 μm. Further， the interference contrast of the reflection spectrum is 25 dB. Additionally， the sound sensing performance of the sensor head is tested. Combined with the multiframe comparison method， the automatic recognition of speech leading to silent frames is realized. The Wiener filtering method based on wavelet packet transform is used to denoise the collected signals. Moreover， the signal-to-noise ratio of this method improves by 1.5 dB compared with that of the Wiener filtering method only， thus improving the quality of the speech. The attributes of the entire system include a simple structure， low cost， high practicability， and wide application scenarios.

Aiming at the problems of harmonic crosstalk and small dynamic measurement range in traditional frequency modulation continuous wave laser interferometer displacement sensors， a three-channel displacement measurement system with three independent Fabry-Perot interferometer detection optical paths is developed. The system includes three chips， and the operation speed can be improved by using multi-chip processing. The simultaneous measurement of each chip is controlled by the equal length of the signal line and software， which can solve the problem of non-synchronization of the independent channel measurement. The experimental results show that the synchronization measurement error of each channel for a moving target with a velocity of 1 mm/s is only 0.46 nm， the standard deviation of the displacement measurement error of a 600 mm stroke is less than 3 nm， and the linear fitting coefficient is above 0.99997， indicating that this system can achieve the displacement measurement of three-dimensional moving targets.

Aiming at the recognition problem of robot adaptive grinding curved surface welding area， an algorithm based on linear structured light for automatic recognition of starting and ending points of grinding robot and a depth image enhancement operator are proposed in this work. The enhancement operator takes the sum of the absolute value of the intensity difference between the center pixel and the pixels in the 8 neighboring regions as the value of the center pixel to enhance the visualization features of the depth image and reveal the texture features of the polished area. First， the point cloud data is filtered and the hole filling is processed； second， the standard deviation of each scan line point cloud in the height direction is calculated； finally， the obtained features are identified to find the position with large feature change in a certain range， so as to extract the area that needs grinding. Experimental results show that the enhancement operator has excellent effect on depth image enhancement， and the average recognition accuracy of the algorithm for the start and end positions is less than 1 mm， which can achieve pixel level accuracy， and has strong robustness and is insensitive to noise. The effectiveness and feasibility of the algorithm are also verified in the field test.

In order to study the effect of laser shock peening （LSP） on the surface integrity of GH3039 superalloys， the GH3039 superalloy samples have been treated by LSP for different times. The surface roughness， microhardness， residual stress， microstructure and phase composition of GH3039 superalloys before and after LSP were characterized by the surface profiler， microhardness tester and X-ray diffractometer. The results indicate that compared with those of the base metal， after one， two， and three impacts， the surface roughness of LSP samples is increased by 28.5%， 54.1%， and 109.1%， the surface microhardness is increased by 21.4%， 26.5% ， and 28.6%， and the average grain size in the near surface layer is decreased by 54.5%， 57.1%， and 59.3%， respectively. The residual stress of the sample surface is transformed from tensile stress （75 MPa） into compressive stress （-275 MPa for one impact， -302.4 MPa for two impacts， and -335 MPa for three impacts）. The deformation layer depth of the LSP sample increases from around 1.2 mm （for one impact） to around 1.5 mm （for three impacts）. With the increase in the number of impacts， the full widths at half maximum of diffraction peaks increase obviously， which is attributed to grain refinement and micro-strain enhancement.

As a kind of new graphic laser processing method， the femtosecond laser spacing technology， based on the spatiotemporal interference， is more easy， flexible and efficient than traditional ones. However， the imaging distortion introduced by the shrink-beam system has huge influence on the accuracy of processing. This work simulates and analyzes the distortion of the system， and uses the phase hologram of the spatial light modulator to compensate the position change of the spatial light field and uneven light intensity distribution caused by the distortion. The method can make the maximum deviation of the interference pattern near the exposure point approach 0 from 10.66 μm， the relative maximum deviation reduces from 60.42% to under 8% and the two-dimension light intensity distribution get close to flat-top. This algorithm reduces the design requirements of the femtosecond laser space shaping technology of space-time interference for the reduced beam imaging system， and saves cost and time. Based on the above methods， a 1.5 mm×1.5 mm quick response code pattern with multi-level anti-counterfeiting capability was spliced and processed on the stainless steel surface.

The 35% TC4+65% Ni60 single-pass Ni-based laser cladding layer was prepared on the surface of TC4 substrate by the TruDisk4002 coaxial powder feeding fiber laser， and there were no cracks within the coating. The finite element model of the coating was established by the ABAQUS software. The simulation results of temperature field show that the molten pool has an obvious "coma tail" phenomenon， which is consistent with the motion characteristics of the actual laser heat source. The maximum temperature of the molten pool is about 3200 ℃. The temperature gradient is large at the front of the laser heat source， however that at the back end of the heat source is small. The shape characteristics of the molten pool and the coating are basically the same， and the size error is less than 5%. According to the residual stress field distribution of the coating， the stress concentration in the edge area and the bonding zone of the cladding layer is relatively large， where it is easy to form cracks. The cracks on both sides of the cladding layer are mainly related to the distribution characteristics of Gaussian heat source， and the cracks in the bonding zone are mainly caused by the difference of the thermo-physical properties of the cladding materials. The experimental results show that there occur obvious crack defects in both the edge area and the bonding zone of the coating， which is consistent with the finite element calculation results.

The centroid positioning accuracy is not only one of the key factors that affect the attitude accuracy of the spaceborne laser altimeter， but also a new type of technical means for satellite platform jitter detection. Aiming at the problems that the laser spot data of geo-science laser altimeter system（GLAS） satellite platform has noise， low spatial resolution， multiple maximum energy values， etc.， which affect the centroid positioning accuracy， the feasibility of improving the spatial resolution of spaceborne laser spots in satellite platform jitter detection is mainly studied in this paper. First， a non-spot dark channel template denoising method is proposed to denoise long-sequence laser spots. Second， the collaborative deconvolution and super-resolution reconstruction methods are used to improve the “tailing” phenomenon of laser spots while increasing its spatial resolution. Finally， a gray-weighted sub-pixel centroid method for centroid tracking is proposed to track and locate the long-time laser spot to achieve the preliminary detection of the satellite platform flutter. Experimental results show that the proposed residual fitting accuracy of centroid positioning method is about 0.08 pixel higher than gray-weighted method， 0.08 pixel higher than Gaussian surface fitting method， and 0.1 pixel higher than ellipse fitting method. It is preliminarily detected that there is a 94 min attitude change rule of glas satellite platform.

The change of the outgoing laser pulse power has a great influence on the accuracy of atmospheric lidar data collection， so it is necessary to detect the power of pulsed laser accurately. Based on the characteristics of the narrow pulse width and high repetition frequency of the outgoing laser pulse， we proposes a power detection system for narrow pulsed laser. First， the photoelectric conversion circuit composed of photodiodes is used to realize the photoelectric conversion of the laser signal. Second， the transconductance type peak hold circuit is used to hold the peak value of the laser single pulse. Finally， the synchronous trigger signal is used to collect the peak value of the single chip and measure the outgoing pulse power of the pulsed laser. After experimental testing， the system is suitable for power detection of laser pulses with a pulse width of 10 ns， a repetition frequency of 10 kHz， and a wavelength of 532 nm used by atmospheric lidar. The test results have good linear relationship and accuracy compared with standard laser power meters， which can meet application requirements.

To substantially strengthen and repair H13 die steel， an H13/Ni/ WC hybrid powder double-layer gradient cladding layer is prepared herein through laser cladding on an H13 steel matrix. The contents of the Ni/WC mixed powder in the initial powder of two cladding layers are 15% and 30%， respectively. Furthermore， the microhardness， high temperature hardness， thermal fatigue properties， and wear resistance of the matrix and cladding layer are tested and compared. Similarly， the wear mechanism of the cladding layer is discussed. The microvickers hardness of the cladding layer cross-section shows a gradient distribution from the matrix to the outer surface of the cladding layer. The average microhardness near the outer surface of the cladding layer is 1100 HV， whereas the average microhardness of the matrix is 400 HV. Results of the high temperature hardness test reveal that an increase in the test temperature decreases the vickers hardness of the matrix and area near the outer surface of the cladding layer. Moreover， the decrease extent of the cladding layer is larger than that of the matrix， which are 1347.68 HV and 1510.35 HV， respectively， at room temperature. Furthermore， when the test temperature reaches 800 ℃， the hardness of the matrix and the cladding layer drop to 1006.8 HV and 921.4 HV， respectively. When the test temperature is about 500 ℃， the hardness values of the two regions are observed to be equal. The wear resistance test also show that the wear resistance of the cladding layer is better than that of the matrix below 500 ℃， whereas the wear resistance of the cladding layer is lower than that of the matrix at 500 ℃ or above. This is attributed to the fact that when the temperature is below 500 ℃， the martensite in the cladding layer exhibits extremely high strength and hardness， leading to higher hardness of the cladding layer than that of the matrix. Conversely， when the temperature is above 500 ℃， the surface hardness and wear resistance of the cladding layer sharply decrease owing to the annealing phenomenon.

In order to solve the problem of poor thermal stability and large influence of background light noise of the airborne long-distance single photon ranging laser， a design of the miniaturized， narrow line width， and low-energy laser suitable for an airborne environment is proposed. The laser adopts the main oscillation power amplification configuration. The seed source laser uses a reflecting volume Bragg grating as the coupling output mirror. Under the condition of 10 kHz repetition rate， the laser output with a line width of 0.035 nm is obtained， and the beam quality is Mx2=1.303 and My2=1.271. The optical-optical conversion efficiency is 44.4%. The amplifier adopts the innoslab configuration to amplify the laser energy， effectively alleviating the thermal problem and improving the beam quality. The Monte Carlo method is used to establish a single-photon ranging echo signal model， which verifies that improving laser performance can reduce the impact of laser jitter on accuracy. At the same time， the narrower linewidth output can use a matched narrowband filter to reduce background noise interference， thereby improving the accuracy of ranging.

In this paper， a vanadium dioxide （VO2） based tunable broadband terahertz （THz） absorber is designed on the silicon plane， which is composed of a VO2 resonator and a metal layer， separated by a thin silicon dioxide （SiO2） dielectric layer. Numerical simulation results show that VO2 with high conductivity （30000 S/m） is at its metal phase， and when its absorptivity is greater than 90%， the 2.0 THz absorption bandwidth can be obtained. In addition， the perfect absorption is realized with absorptivity of 99.3% and 99.6% at 4.5THz and 5.8THz， respectively. In contrast， VO2 with low conductivity （100 S/m） is at its insulation phase， and the peak absorptivity in the corresponding broad absorption band is only 8%. Therefore， by altering the conductivity of VO2 in the absorber， one can switch between absorption and reflection and realize the dynamic tuning of absorptivity in a broad frequency band. In addition， the proposed absorber is polarization-insensitive under vertical incidence due to its structural symmetry. Moreover， the absorber maintains an excellent absorption performance over a wide incident angle range.

A low-loss waveguide structure composed of a silicon-on-insulator （SOI） structure and graphene-coated nanowires （GCNW） is proposed. The dependences of the transmission characteristics of fundamental graphene plasmon mode in the proposed structure on frequency， geometric， material parameters， and chemical potential of graphene are investigated in detail by use of the finite element method. Simulation results show that the low refractive index SiO2 dielectric layer in the waveguide can achieve high-performance deep subwavelength light confinement. Thanks to the high refractive index contrast between the low refractive index SiO2 dielectric layer and the SOI substrate as well as the graphene layer， a low-loss plasmon mode with a very small mode field area is obtained. The proposed waveguide structure provides a certain reference for the design of high-performance and deep sub-wavelength tunable integrated photonic devices.

In the present paper， based on the coupling principle of photonic crystal resonator， a six-channel wavelength division multiplexer is designed by combining ring cavity with micro cavity structure. By using the plane wave expansion method， the relationship between the band gap of photonic crystal and the radius of dielectric cylinder is studied when the lattice constant is 0.55 μm， and the band structure is obtained. The influence of the resonator structure on the wavelength is analyzed by using the finite difference time domain method. The transmission characteristic curve and electric field distribution of the resonator structure are obtained. Six-channel wavelength division multiplexing with wavelengths of 1.3298，1.4316，1.4419，1.5564，1.5966，1.6191 μm is realized. Experimental results show that the device can select the frequency by adjusting the radius of the dielectric column inside and outside the microcavity， reduce the crosstalk by increasing the micro cavity structure of the reflection point， and improve the output efficiency by changing the radius of the dielectric column at the output edge. A channel with a transmittance of more than 90% and an average bandwidth of 9 nm is finally realized， and the crosstalk between channels is small and the narrow band characteristic is good. The size of the device is only 13.4 μm×17.6 μm， which is conducive to optical device integration and has potential application value.

In the satellite-to-ship quantum key distribution system， the quantum satellite that sends the quantum key is a low-orbiting satellite. The receiver equipment that tracks the quantum satellite is installed on the ship. Because the receiver equipment needs to track the movement of the satellite， the basis vectors of receiver equipment are inevitably rotated. In this work， the reason for the rotation of the basis vector is analyzed， and the quantitative relationship between the rotation angle of the basis vector， the quantum error rate， and the amount of information obtained， is established for the BB84 protocol. The results show that when the transmission distance is 200 km and the rotation angle of the basis vector is 2° and 10°， respectively， the quantum error rate and the amount of information acquired are 8.255×10-5 and 0.99， 2.044×10-3 and 0.91， respectively. When the rotation angle of the basis vector is greater than 2°， the performance of the satellite-to-ship quantum key distribution system is significantly reduced. This indicates that when the satellite-to-ship quantum key is distributed， it is necessary to perform an adaptive correction in advance according to the rotation angle of the basis vector.

To effectively eliminate invalid targets and reduce the probability of false alarms， a dynamic threshold calculation method based on radar ambient noise statistics and target power distribution characteristics is proposed. The histogram statistical method is used to estimate the noise level of each range gate and search the peak value to find the local peak point. Simultaneously， the adaptive peak detection threshold is obtained by dynamically allocating the threshold gain according to the attenuation of different range gate radar signals. The local peak point is then taken as the signal of the reference unit and input into the detection method to solve the detection threshold. The experimental results show that the proposed method can effectively identify radar targets in complex application scenarios.

This paper proposed a real-time detection method for tunnel centerline by 16-line LiDAR based on particle swarm optimization （PSO） algorithm to solve too narrow of the beam''s two sides from the tunnel wall crossing the tunnel. We designed a three-dimensional LiDAR bracket for a certain type of beam truck， installed and calibrated the LiDAR， and constructed a real-time tunnel-center-detection platform for the beam transport vehicle. Using the PSO algorithm and point cloud filtering （PCL） in the point cloud library， we processed the LiDAR data and then calculated the real-time cross-swing angle of the vehicle''s forward direction relative to the vertical tunnel. Thus， we obtained the point cloud of the tunnel wall''s cross section and fitted the center point coordinate and radius of the circle using the PSO algorithm to fit the tunnel centerline. The experimental results show that the method can effectively detect the tunnel centerline. The errors between the tunnel radius calculated by this method and the actual tunnel radius and between the extracted tunnel centerline and actual tunnel centerline were both within 3 cm. Hence， the proposed method can effectively detect real-time tunnel centerline.

Node location technology is the key of wireless sensor networks. Aiming at the low accuracy of the traditional centroid localization algorithms， a three-dimensional iterative centroid localization algorithm based on received signal strength indication is proposed. Based on the location technology， the algorithm simplifies the location reference nodes of unknown nodes， obtains the distance offset coefficient through the spatial geometric relationship between the nodes， uses the optimal solution of the coefficient to modify the centroid coordinate， and updates the coefficient repeatedly to update the location result until the iteration criterion is reached. The simulation results show that compared with the traditional centroid positioning algorithm， the proposed algorithm can improve the positioning accuracy by 20%-28%， and effectively reduce the positioning error of three-dimensional positioning in wireless sensor networks.

High-order harmonics generated by the interaction between intense femtosecond laser and matter have the characteristics of high single photon energy， ultrashort pulse duration， and excellent spatiotemporal coherence. It can be used as the tabletop ultrafast vacuum ultraviolet laser source and soft X-ray laser source in laboratory. At the same time， high-order harmonics are used to generate attosecond pulses. The generation of these advanced laser sources has greatly enriched the research tools of material science. In this review article， we introduce the principle， optimization， and application of high-order harmonics of gases and solids.

As all countries pay more and more attention to marine security and territorial issues， underwater laser communication plays a more and more important role. The realization of high-speed transmission and confidentiality of information in a complex marine environment is an urgent subject for underwater laser communication. This paper sorts out the development history of underwater laser communication systems at home and abroad， summarizes their technical advantages， gives their communications principles， analyzes the key technologies influencing the system''s performance. Finally， the future underwater laser communication systems will not only possess high communication rate， low bit-error rate， long transmission distance， large capacity， low power consumption， and small size， but also develop from point-to-point communication to network-type communication.

High power ultra-short lasers have a wide range of applications in fields such as high-field physics， precise processing， and national defense. The output power of ultra-short fiber lasers has been improved significantly with the development of diode pump sources and fiber manufacturing process. Besides， the high power ultra-short fiber laser and its amplification and compression technologies have attracted more and more interest in recent years due to the advantages of fibers including good heat dissipation， high environment stability， good beam quality， and so on. This paper focuses on the advanced research of high power ultra-short fiber lasers and its amplification and compression technology. Several new methods， such as Mamyshev mode-locking，“9-type cavity” mode-locking， and spatiotemporal mode-locking， for producing high power and high energy ultra-short fiber lasers are reviewed. The recent progress of coherent combination and non-linear compression technology to produce high power ultra-short pulse in fibers is introduced.

In order to further quantitatively analyze the physical dose of laser to biological tissue， and to provide theoretical basis for related research in medical， military， and many other fields， it is necessary to accurately measure the optical characteristic parameters of biological tissue. This paper systematically summarizes the mainstream techniques and methods for measuring the optical parameters of biological tissues at home and abroad， summarizes the applicable scope of each method and compares the advantages and disadvantages of related methods horizontally and vertically. It reviews the theoretical basis of most simulations and experiments， and also summarizes the latest research results and applications in this field， which is convenient for readers to track the frontier quickly and accurately. Finally， it summarizes some major problems currently facing in this field research and makes a certain prospect for the future development direction.

Fiber evanescent wave sensor （FEWS） exhibits the advantages of a simple design， high sensitivity， and easy combination with other sensing technologies and is widely used in the field of biological and chemical sensing. Herein， we first describe the definition of FEWS and two types of common fibers and then summarize the optimization methods and principles of FEWS. Second， we review the research advancements of silica and chalcogenide fibers in the fields of gas， liquid， and biological sensing. Finally， the future development trends are prospected.

Segmented telescopes are among the expected future directions of large-diameter telescopes owing to the low difficulty of their processing technology， convenient carrying， and low manufacturing cost. The active support technology provides an important guarantee for the confocal and cophase adjustment of segmented telescopes. This study briefly summarizes the main technologies of the active support system for the main mirror of a segmented telescope and then summarizes and analyzes specific application examples. Finally， it provides design suggestions for the main mirror support system of a segmented telescope. It will provide a valuable reference for the future development of the segmented telescope support technology.

To simulate the scattering of photons in sea fog particles， in this study， we use a Stokes vector to describe the polarization state of the photons， Mie scattering theory to calculate the Muller matrix and reflect the scattering characteristics of sea fog particles， and the Monte Carlo method to solve the vector radiation transmission problem in the sea fog environment. This paper discusses the changes in transmittance and reflectance with the transmission distance for sea fog particles of different radii， and the changes in the degree of polarization with sea fog visibility under different incident light polarization states and different incident light wavelengths. The results of the simulation experiment presented in this paper provide useful reference for research on the polarization transmission characteristics of light in sea fog particles.

In this study， the terahertz time-domain spectroscopy （THz-TDS） technology was used to conduct experimental tests on seed samples of astragalus japonica. We obtained the terahertz time-domain spectra of five kinds of Astragalus adsurgens Pall. seeds in the effective frequency range of 0.2-1.2 THz， and used the fast Fourier transform analysis to study the optical parameters such as the absorption coefficient and refractive index of each grass-seed sample. It was found that in the effective frequency range， the peak intensity and delay time of the time-domain spectrum of the samples were different， and the average absorption coefficient and standard deviation of each spectrum line were significantly different. In addition， the average refractive index of the samples was significantly different. At the same time， this study proposes a hybrid model of optimized experimental data that combines principal component analysis （PCA） with random forest machine learning （RF）. Based on the terahertz refractive index spectrum， 200 datasets of five forage species were statistically calculated， and the calculated results were compared with the calculated results of the RF model. The results show that the average classification accuracy of principal component analysis-random forest （PCA-RF） in the mixed model is 91.20%. Compared with the RF model， both total average classification accuracy and the classification accuracy of each sample of the PCA-RF model are better than those of the RF model. The study shows that the PCA-RF model combining THz-TDS with the hybrid machine learning algorithm can be used as an effective method for the nondestructive identification of the authenticity of forage grass seeds. In particular， it can be used for the classification of forage grass varieties of the same family with little difference.

Tunable diode laser absorption spectroscopy （TDLAS） exploits the tunable and narrow linewidth characteristics of a diode laser to improve the selectivity and sensitivity of CO2 measurements in harsh environments， such as those with complex gas components and high temperature. The scanned-wavelength direct absorption spectroscopy technique based on TDLAS is commonly used for gas concentration inversion. However， the absorbance integral method in gas concentration inversion is applied in the frequency domain， which requires a complex system to retrieve accurate frequency-domain information. To avoid this problem， the present paper proposed a new gas-concentration inversion method based on calibration of the direct absorption peak value. First， the theoretical changes of absorption peak value with concentration（10%-20%） and temperature（298-338 K） were analyzed in a numerical model of the target gas （carbon dioxide， CO2）， which was established using the HITRAN2016 spectrum database and referring to high temperature and high concentration CO2 environment in tail flue of the power plant. The theoretical change was used as reference basis for establishing the actual absorption peak value concentration calibration model and temperature correction curve. To verify the feasibility of the method， a custom-built CO2 detection system based on TDLAS was built. The mean squared relative error of the inversed CO2 concentration by peak value calibration method was 1.08%， confirming that the gas-concentration inversion method based on direct absorption peak value calibration can accurately measure high concentrations of CO2.

Criminals typically adulterate drugs with other substances to increase drug-quality to obtain high profits， which has adverse effects on the society. The purpose of this study is to achieve fast and accurate， qualitative and quantitative analysis of adulterated drugs， and explore the influence of various factors on the results of model classification， e.g.， the modeling method， the spectral band， and dimension reduction. Here， 135 infrared spectrograms of caffeine， glucose， acetaminophen， phenacetin， and starch mixed with methamphetamine hydrochloride of different mass fraction were collected using attenuated total reflection-Fourier transform infrared spectrometer. A classification model was then constructed after data preprocessing. The results demonstrate that the characteristic variable has higher classification accuracy than the original variable. Multilayer perceptron （MLP） and radical basis function （RBF） could be used to classify and identify five additives； however， they were not able to distinguish methamphetamine samples with different mass fractions. A classification model of spectral fingerprint data was constructed using factor analysis and dimension reduction combined with Bayes discriminant analysis （BDA）， and the complete differentiation of five additives and methamphetamine samples with different mass fraction was achieved on 16 and 33 dimensional variables， with accuracy of up to 100%. We achieved fast and accurate qualitative and quantitative analysis of methamphetamine and five added components， which provide scientific data support for the inference of drug source. The results also provide theoretical support and method reference for drug-related cases.

Spectral data fusion， an emerging spectral analysis technology， can effectively combine the detection characteristics of different spectrum technologies. The classification of 288 handkerchief paper samples were studied with data fusion of infrared spectrum fingerprint region and strong signal region of Raman spectrum. A classification model was established on 189 training samples by linear discriminant analysis. Compared with the independent use of infrared spectroscopy or Raman spectroscopy， the accuracy was significantly improved. However， the distinguishing effect was still not ideal. The accuracy of 35 principal components extracted by principal components analysis for linear discriminant analysis can reach 100%. The model accuracy of the 32 principal components extracted from test samples reached 100%， and the classification results were ideal. The spectral fusion technology provides an effective tool for the classification and identification of handkerchief paper in the field of forensic science.

The reed leaves in Ebinur Lake Wetland Nature Reserve were sampled and water content was measured. The spectrum of reed leaves was measured by FieldSpec3 portable spectrometer. The original spectrum of reed leaves was smoothed and derivative transformation was carried out. The response characteristics of the first derivative of reed leaves to water content change under different differential window scales were analyzed， and the characteristics were extracted. The water content of reed leaves was characterized by characteristic parameters. The results show that when w = 1－15， the noise of the first-order spectral curve is difficult to be effectively reduced， which makes the contour of the spectral curve difficult to distinguish， and the spectral response characteristics caused by the change of leaf water content are not easy to judge. When w = 16－30， the first derivative spectrum noise obtained can be effectively removed. Compared to the correlation coefficient between the first derivative spectrum of phragmites reed leaves and the water content in the 1374－1534 nm band， the correlation coefficient in 1460－1500 nm band demonstrates small fluctuations， small standard deviations， and stable performance， the response feature between the whole first derivative spectrum within 1460 nm and 1500 nm， and water content variation has more consistency and significantly response. Take the average value of the first derivative spectral value in the 1460－1500 nm band as the characteristic parameter. Under the differential range of 1－30， compare the characteristic parameter with other differential ranges， it is more suitable to indicate the change of reed leaf water content， which provides a new way for quantitative detection of reed leaf moisture by short-wave infrared reflectance spectroscopy in remote sensing monitoring of reed water in arid area.

To analyze the main components of paper ashes and distinguish paper types， the experiment mentioned in this study prepared 30 brands of paper into paper ashes， using an X-ray fluorescent spectrometer to measure its main components. Using measurement data trained by support vector machine （SVM） classifier， the paper type and brand source were determined. The experiments accurately determined the main component data of 90 sets of paper ashes， and randomly and proportionally generated training and test sets. Using the MATLAB experimental platform， the best parameters c and g of radial-base core functions were determined by interactive testing method， and a support vector machine classification model was established. The reasons for the model misjudgment were analyzed using Pearson correlation coefficients. This study shows that an SVM classification model can effectively achieve sample classification， can be used to test the type of paper ashes and brand source， is beneficial to solve the court-science-related problems， and can provide assistance for police to collect physical evidence at a crime scene.