The temporal stability of output pulse from the parallel grating pair compressor is discussed under two different conditions. The influence of the beam pointing variance, the retro-mirror misalignment, and the grating pair translational error on the output pulse width from the compressor is quantitatively analyzed by means of numerical calculation. When the grating groove density changes, the sensitivity of the output pulse to these factors is also discussed. Moreover, the allowable tolerance ranges of the aforementioned factors under different conditions are presented.

To improve the production of L-valine, the He-Ne laser is used for Brevibacterium flavum to induce mutagenesis. The He-Ne laser with different power and duration is tested, and the best laser treatment condition that the intensity and time of irradiation is 15 mW and 20 min, respectively is decided. The positive mutants are selected directionally and stepwise on the agar plates containing extra L-valine of different concentrations, and L-valine overproducing mutant is derived with production increased by 25.50%. The results show that the mutant strain can keep steady properties for at least 10 generations. He-Ne laser is an effective technique for microbial mutagenesis, and the He-Ne laser with low power is suitable for Brevibacterium flavum mutagenesis.

In order to provide scientific basis for preventing and controlling atmospheric pollution to environmental departments, the profile of aerosol backscattering coefficient is detected with high resolution by using the side scatter and backscatter lidar system. From the cases study, the profiles of near-ground aerosol backscattering coefficient with altitude are different, which can be classified into four categories, multilayer structure, increasing versus altitude, decreasing versus altitude and invariant versus altitude. Two continuous nights profiles of aerosol backscattering coefficient with altitude and time are analyzed. The average profiles of seasonal near ground aerosol backscattering coefficient at the western suburb of Hefei are calculated by averaging 82 nights data measured from March 2014 to February 2015 by season.

A GHz narrow-linewidth high-peak power nanosecond fiber laser based on the master oscillator power amplifier (MOPA) is reported. The modulated distributed feedback single-frequency diode laser seed delivers pulses with wavelength of 1064.12 nm, pulse width of 3.92 ns, and tunable repetition rate from 10 kHz to 50 kHz. A double-pass structure is used in the preamplifier to boost small signal and suppress amplification of spontaneous emission under intense pump. In the power amplifier the stimulated Brillouin scattering is avoided by shortening fiber length. Pulsed laser with 1.5 GHz linewidth and 15 kW peak power is obtained at a repetition rate of 10 kHz. The output pulse characteristics at different repetition rates are studied.

For miniaturization, light weight and high efficiency of the diode pumped all-solid-state laser (DPSSL), a new method is studied with laser diodes (LD) operating at high temperature (808 nm, 55 ℃) as the pumping source. The refrigeration press of the temperature control system is reduced, and the bulk, weight and power of the conductive cooling structure are optimized. The relationship between the cooling power of thermoelectric cooler used in DPSSL and the operation temperature of LD is studied. A highly reliable, miniaturized light-weight DPSSL is designed by using the pump source of high temperature LD array and the corner cube prism resonator. An engineering laser prototype is developed with parameters as follows: repetition rate 50 Hz, continuous operation time 6 min, pulse energy 85.7 mJ, pulse width 10.8 ns, divergence angle 2.8 mrad, electro-optic efficiency 4.3%, volume 170 mm×62 mm×80 mm, and weight 1.1 kg. The results show that the high temperature LD pumped all-solid-state laser is an effective way to realize small, light-weight highly effective DPSSL.

Unwanted cladding light in output of double-cladding fiber lasers has major adverse effect on the beam quality, spectral characteristics, and reliability and stability of lasers. It is of vital importance to strip the cladding light for engineering of the double-cladding fiber lasers. The thermal effect of high power cladding light strippers is studied theoretically and experimentally. The temperature distribution is simulated by means of computational fluid dynamics based on ANSYS. The experiments validate that the temperature simulation is effective and applicable for stripping power ranging from 15 W to 600 W. Furthermore, a kW-level cladding light stripper is optimized based on the thermal simulation. After optimization, the temperature of the stripper declines by up to 46 ℃, and the stripper can run stably at about 80 ℃.

In order to understand the effect of gas velocity distribution uniformity on injected energy and output energy of lasers, the flow field in discharge chamber is simulated by computational fluid dynamics (CFD) so that the gas velocity distribution in longitudinal and vertical directions can be acquired. According to the elements that affect the gas velocity uniformity, the whole flow field is optimized. Based on the fluid dynamics, laser structure and light weight requirement, three clapboards are added to improve the flow field distribution characteristics. After optimization, the average gas velocity reaches 99.3 m/s. The velocity non-uniformity in longitudinal direction is 5.2%, and that in vertical direction is 7.1%. The repetition rate of laser increases from 300 Hz to 365 Hz, the injected energy of single pulse increases from 160 J to 171.5 J, the output energy increases from 20 J to 21.8 J, and the divergence angle of laser beams decreases by 0.5 mrad. Improving flow field distribution characteristics in the discharge chamber will enhance the integral performance of lasers.

Optical sources at 1.7 μm region of optical coherent tomography (OCT) can reduce scattering and absorption in tissue, and enhance the imaging depth. A 300 m long high nonlinear optical fiber and a 10 km long dispersion shifted fiber are pumped by amplified spontaneous emission (ASE) beam to generate continuous spectrum. The continuous light source at peak wavelength of 1675 nm and 10 dB linewidth of 75 nm can be achieved after erbium-doped fiber absorption, by adjusting the peak wavelength and power of the tunable filter after ASE source. In order to improve OCT signal sensitivity without increasing the optical illumination power, a multi-wavelength broadband light source with 14 nm period is obtained by adding a Sagnac filter. The optical sources at the 1.7 μm region is achieved by the proposed method. Experimental results can provide reference for OCT new light source and 1.7 μm region fiber laser.

In the all-fiber optical parametric oscillator (OPO) based on four wave mixing (FWM), the signal laser gets effective gain only in a short distance. The increasing trend of signal laser in photonic crystal fiber (PCF) is measured by optical time domain reflecting (OTDR) technology, and the backscatter signal light is measured. The parametric gain distribution in PCF is calculated, and the distance between the start of the PCF and the sharp drop in the parametric gain is defined as the effective interaction length. Moreover, the results are verified by PCF heating experiment. The results show that there is only 10 m of the effective interaction length in the 34.5 m PCF. The signal laser keeps growing exponentially only in the first 10 m PCF, and cannot increase any more in the rest 24.5 m PCF for the signal and the pump light is no longer meet the phase matching conditions. An effective method for measuring the parametric gain distribution is proposed, and the result is useful for optimizing the configuration and improving the efficiency of all-fiber OPO.

Influences of external optical feedback effects on characteristic of the fiber Bragg grating external cavity semiconductor laser are investigated. When the effect of temperature on the wavelength of external cavity semiconductor laser is taken into account, an external optical feedback setup is built, and the optical feedback intensity can be adjusted. The linewidth of the external cavity semiconductor laser is measured by delayed self-homodyne technique, and the changes of linewidth and noise of the laser with different external optical feedback are analyzed. Experimental results show that the linewidth of the laser decreases when the optical feedback intensity increases. The linewidth is narrowed down to 15% of the original one when the feedback ratio reaches -22 dB. Meanwhile, within the same variation of the optical feedback, the relative intensity noise of the laser has no obvious change until the feedback ratio increases to -27 dB. As the feedback further increases, the relative intensity noise of laser increases significantly and the coherent collapse happens inside of lasers.

As a type of special light source, microelectronic mechanical system (MEMS) vertical cavity surface emitting laser (VCSELs) has the advantages such as low power consumption, high modulation speed, broad wavelength tuning range and easy coupling, which has been widely applied in the field of laser communication. To achieve better performance of the lasers, such as enlarging wavelength tuning range and increasing polarization contrast, optimizing the structure parameters of inter cavity subwavelength grating is indispensable to improve the distribution of optical field and polarization output mode. In this paper, based on effective medium theory (EMT), the optimal values of period, duty cycle and ridge height of subwavelength grating are designed combining with the theory of film (the center wavelength of tunable range is 850 nm and the grating material is GaAs). The impact of grating duty cycle, ridge height on the transmission efficiency for TE and TM are analyzed. Furthermore, the tuning ranges of MEMS VCSELs with no-etched grating, unoptimized grating and optimized grating are compared. The results indicate that, for the specific wavelength tuning range and grating material, optimization of the grating parameters can promote the increase of the grating transmission efficiency for TE or TM mode, enhance the coupling of optical field between the semiconductor cavity and air gap, thus enlarge the wavelength tuning range of the laser.

Laser bending experiments are performed on stainless steel-carbon steel laminated plates with thickness of 1.0, 1.5, and 2.0 mm. The element diffusion in the transition layer of the bending zone is analyzed by means of metallographic microscope, electron microprobe and energy dispersive spectrometer, the nano-indentation tests are used to obtain the nano-hardness and elastic modulus of the transition layer, and the variation in elastic modulus, nano-hardness and yield strength is analyzed eventually. The results demonstrate that the element in the transition layer diffuses continuously and steadily along the plate thickness, the diffusion ranges for Fe, Ni, and Cr are similar, and the transition layer thickness for 1.0, 1.5, and 2.0 mm thick laminated plates increases by 2.5, 3.5, and 3.0 μm, respectively. Good material properties in transition layer is ensured by element diffusion in the bending zone, and favorable metallurgical bond in laminated plates is also improved.

Hydrogen, oxygen, nitrogen and other gas impurities can be easily absorbed by zirconium alloys above 300 ℃ during the laser welding process, thus the mechanical property is drastically reduced. The key to ensuring the quality of zirconium alloy welding is to establish reasonable welding process parameters and strengthen the isolation protection for the welding seam and heat affected zone. Laser butt welding test on the 0.7 mm Zr-Sn-Nb-Fe alloy sheet with fiber laser as thermal source is carried out. Based on the ANSYS software, a nonlinear three dimensional (3D) transient heat conduction model is established by means of the finite element analysis. Simulation is done to get the temperature field distribution of the Zr-Sn-Nb-Fe alloy thin sheet. The results show that the weld profile and dimension obtained by simulations are basically consistent with those of the real welds, which verifies the rationality of adopting Gaussian heat source model to simulate the temperature field of Zr-Sn-Nb-Fe alloy thin sheet by laser butt welding. The temperature field presents a stable meteor-shaped distribution, the isotherms are dense near the weld, and the heat affected zone is narrow. The width of the area above 300 ℃ increases with the increment of laser power, but decreases with the increment of the welding speed. When the laser power is 1300~1500 W, the welding speed is 50~70 mm/s, the defocusing amount is +1~+2 mm, and the argon gas protection nozzle diameter is 8~12 mm, the tensile strength of Zr-Sn-Nb-Fe alloy welded specimens roughly equals to that of the base metal. The weld quality is good, air holes and cracks do not exist, the grain is small, and the contents of the main elements have no obvious changes if compared with that of parent metal. The welded specimens with a high mechanical property are obtained.

The effect of substrate preheating process on the residual stress, microstructure, and tensile property of GH4169 alloy samples repaired by laser deposition is studied. The results show that when the substrate is preheated to 300 ℃, as for the GH4169 alloy sample, the average decrease in amplitude of the residual stress along x direction (σx) is 59.4 MPa, which is reduced by 11.7%. The average decrease in amplitude of σy along y direction is 189.7 MPa, which is reduced by 44.6%. The Laves phase of the interdendritic precipitation in deposition layers presents a granular fragmentation phenomenon. Both the tensile strength and yield strength are basically equivalent to those of repaired samples without preheating, and the elongation rate is 12.3%, which is a onefold increase. These indicate the standard of forging is reached. The experimental results indicate that substrate preheating can effectively improve the tensile property of GH4169 alloys repaired by laser deposition at room temperature.

The surface roughness calculation model based on powder properties is established by using the least square mean line method. Selective laser is used to melt molded Ti6Al4V single track and parts. The surface roughness of molded parts is detected by surface profiler and the geometric size of molded single track is observed and measured with optical microscope. The cause of errors between the theoretical and experimental value of the built model is analyzed. The experimental results show that the laser remelting zone has a great impact on the precision of the surface roughness calculation model. The average relative error between the theoretical and experimental value is 5.7%. The error is small, and the model has a practical engineering application value. The molded parts are mainly composed of α and β phases, and the micro-hardness is 487.3 HV0.3.

An experimental study on laser transmission welding of transparent glass and 6061 aluminium alloy by a millisecond pulsed 300 W Nd:YAG laser is reported. The test of breaking strength is conducted by a universal tensile test machine, and the glass fracture morphology and weld sectional microstructure are analyzed by scanning electron microscopy. The effect of multipass welding and weld spacing on weld seam morphology, breaking strength, and fracture morphology is studied. The results indicate that a good glass-to-aluminium joint can be obtained if a millisecond pulsed laser is used. The maximum breaking strength of weldments is 159.93 N. When the distance between the welding seams is 0.8 mm, the aluminium alloy and glass can obtain good sealing.

TC4 titanium alloy surface is treated firstly by laser shock peening (LSP), then the shock of foreign object on the specimen edge is simulated by means of a gas gun test system, and finally the tension-tension fatigue experiment is applied to the foreign object damaged (FOD) specimen. The experimental results indicate that LSP can effectively improve the fatigue strength of FOD TC4 titanium alloy. The fatigue crack initiation (FCI) of FOD specimen without LSP locates in the notch root near the upper surface, while the FCI of FOD specimen with LSP transfers to the inside of material with a certain distance from the notch root, which makes it more difficult for crack to initiate. Numerical stress analysis results indicate that the maximum tensile stress decreases by 37.85% from 1076.2 MPa to 668.9 MPa as a result of LSP treatment. In addition, the residual tensile stress in the notch center of FOD specimen with LSP decreases by an average of 350 MPa compared with that of FOD specimen without LSP. With the loading of tension stress, in the notch of the FOD specimen with LSP, the maximum stress increases to 1124.37 MPa, in contrast, the maximum stress in FOD specimen without LSP increases to 1542.36 MPa. The former is reduced by 30.22% compared with the latter, which indicates the stress can slow the crack initiation obviously. The introduction of residual stress is one of the main reasons why LSP can improve the fatigue strength of the FOD specimen.

By means of arc-assisted laser welding and with the addition of Al-12%Si welding wire, the butt welding-brazing experiment of dissimilar metals joining aluminum alloy to galvanized steel sheet with filler powder is conducted. The influence of wire feeding speed on weld appearance is discussed. The microstructure, fracture morphology, phase composition, and mechanical property of welded joints are investigated by means of optical microscope, scanning electron microscope, X-ray diffractometer, and material testing machine. The results show that under suitable welding parameters and process conditions, a welding-brazing joint with high quality appearances at both front and back sides can be obtained. When the welding speed is fixed, with the increment of wire feeding speed, the spreading width of welds at the front side decreases, while that of the back side increases. Si element enriches mainly in the upper part of joints, and the welded seams consist of α(Al) substrate and Al-Si eutectic phases along grain boundaries. A kind of non-uniform intermetallic compounds is formed in the Al/steel interface layer which consists of Al8Fe2Si, Fe4Al13, and Fe2Al5. The maximum tensile strength of joints is up to 130 MPa, and the surface morphology of fractures possesses a ductile and brittle hybrid characteristic.

In order to study the formation mechanism of residual stress of 7050 aluminum alloy sheet surface by laser shock processing, the samples are processed by laser beam with five different power densities. The dynamic strain induced by laser shock processing sheet samples is measured by the polyvinylidene fluoride (PVDF) piezoelectric sensor. Residual stress distribution after laser shock processing is measured by the X-ray stress analyser. Three-dimensional microscopy system is used to observe the surface microstructure by laser shock processing. The results show that when the power density is 1.02 GW/cm2, the transverse deformation induced by laser shock is small. When the power density is 1.53 GW/cm2, both of the surface rarefaction wave and transverse deformation result in the equal-biaxial distribution of the maximum principal stress on the sample surface. When the power densities are 1.98 GW/cm2 and 2.77 GW/cm2, the center of the impact region is 5.680 μm and 10.800 μm higher than the adjacent part, respectively. The shock wave which is reflected back and forth and the surface rarefaction wave cause the occurrence of residual stress hole. When the power density is 4.07 GW/cm2, the sample impact area has a large plastic deformation and is relatively smooth, the maximum residual principal stress distribution is uniform.

The influence of overlapping rate of focused femtosecond laser spot on colorization of silicon surfaces is investigated. Experimental results indicate that the generation of periodic nanoscale ripple structures induced by femtosecond laser pulses is responsible for the silicon surfaces colorization. In the case of a certain femtosecond laser power, the colorization cannot be realized with too high or too low overlapping rate of focused laser spots.For a special range of laser power, the denser overlapping rate of focused laser spots, the more colors are observed, and the higher colorization efficiency is achieved. In addition, the measurement results of reflection indicate that the refection rate of colorized samples reduces about 50% in the visible light range. The window parameters for silicon surfaces colorization by femtosecond laser are obtained, which paves the way for femtosecond laser semiconductor colorizing technology.

Constraint condition and crystal temperature are two mainly factors that affect the performance of potassium dihydrogen phosphate (KDP) crystals. Under constraint conditions, the original phase matching condition is destroyed and third harmonic conversion efficiency is decreased as a result of thermal stress and thermal deformation caused by temperature variation. An analysis method of crystals thermal sensitivity is established to find the relationship between the temperature and phase matching angle. The finite analysis is used to obtain the distributions of thermal stress and thermal deformation, and then thermal-optic effect, stress-optic effect and thermal deformation are considered to calculate the change law of phase matching angle. The validity and feasibility of method are verified by using crystal constraint condition of SG-III prototype laser facility. The case indicates that the efficiency calculation results of method are better conforming to the actual change law of phase matching angle.

In order to solve shortcomings such as technology complexity, high cost and long cycle in the traditional microlens array, a new method to fabricate microlens array with low cost, high efficiency has been studied. The microlens array main mold structure is made with the main optical axis parallel to the silicon base , which uses SU-8 negative photoresist mold structure material as the material of main mold structure and two tilte-exposure technology. Polydimethylsiloxane (PDMS) soft lithography and NOA73 UV-exposure technology are taken to copy the main mold structure, and NOA73 microlens array and PDMS microlens array are made successively. The surface topography of the microlens array is observed by confocal microscope and an optical detection platform is built to test the imaging effect of the microlens array. The result shows that NOA73 microlens array has better optical characteristics. The microlens array has better imaging effect and surface topography with above technology process. It has advantages such as good repeatability, short processing cycle, and can be integrated in the micro-flow cytometry instrument to process fluorescence detection of the sample flow, which can enhance the detection precision.

Infrared thermography is introduced to measure the in-plane thermal conduction of woven carbon fiber reinforced polymers (CFRP). According to the principle of modulated laser heating, the relationship between the in-plane thermal diffusivity and the phase gradient is deduced, and the experiment platform is established to test the CFRP samples. Glassy carbon is used as the reference material, and the measured infrared radiation signal is normalized. The Gaussian filtering is applied to removing the noise of infrared thermal images. The experiment results show that the in-plane thermal conduction in various directions is related to the conduction direction owing to the anisotropy of CFRP. When the modulated frequency is lower than 2 Hz, the in-plane thermal diffusivity of samples in each direction can be measured.

An unmanned aerial vehicle（UAV） landing guidance system based on solar-blind ultraviolet imaging is designed to meet the demand of precise pose parameters of fuselage to the landing sites. The system utilizes a solar-blind UV imaging system with dual channel and a UV beacon consisted of several sources, which is imaged by the ultraviolet-channel imaging system, and the spot position of UV beacon is extracted based on light intensity. Camera perspective projection model and direct linear transformation algorithm are adopted to solve the initial pose parameters. The accurate pose parameters of UAV to the UV beacon are obtained by the iterative optimization, which guides UV safely landing. Because of the uniqueness of solar-blind ultraviolet near the ground, the advantages of system in poor visibility conditions are particularly obvious. Experimental results show that the relative accuracy of range location can reach 0.5% and the absolute accuracy of angle location is better than 1°. The proposed system can meet the requirements of UAV landing guidance.

The gradient method and standard deviation method are only the inversion of the atmospheric boundary layer height of the single moment. For this issue, an image edge detection method is presented using retrieve the height of atmospheric boundary layer over a period of time. Through the introduction and analysis of the conventional method, the necessity of the research on image edge detection is illustrated. The atmospheric boundary layer height of a day and night and four kinds of different weather backgrounds are analyzed by using the independent research and development lidar to measure real-time data. The minimum root mean square errors of the image edge detection method with the gradient method and the standard deviation method for the inversion of the atmospheric boundary layer height are 9.4 m and 11.4 m, respectively. Experimental results show that the proposed algorithm is simple, reliable and high accuracy, and there is no need to choose the initial value, which has lower sensitivity and stronger adaptability than the traditional methods.

In situ-fabricated three-dimensional micro/nanostructure TiO2 films over titanium substrates possess advantages such as large specific surface area, high light efficiency, and good electron conductivity. Titanium substrates are ablated by femtosecond laser for fabricating micron structures. Then it is processed by chemical oxidation with H2O2 to form nanoporous TiO2 on the surface of micron structures. The average reflectance of the as-fabricated three-dimensional TiO2 structure is below 5 % over the ultraviolet wavelength range. The photocatalytic degradation of methyl orange shows that catalytic performance is largely determined by the size and morphology of the micro structures prepared by femtosecond laser. Additionally, the photo degradation rate remains constant over several consecutive cycles, which indicates that the micro/nanostructure TiO2 film is mechanically stable. The fabrication method of femtosecond laser combined with chemical oxidation can fully play advantages of ultraprecise on titanium substrate surface and controllable fabrication. The insights from the study on the fabrication of micro/nanostructure TiO2 film can be extended to guide the fabrication of other functional metal oxides on metallic substrates.

To achieve high precision timing and punctuality in quasi national territory, time and frequency signal of high precision atomic clocks, such as cesium clock or hydrogen maser, is transferred by using fiber to examine the transfer performance of long distance on actual fiber link. By adopting the methods of wavelength division multiplexing and dual wavelength bi-directional transmitting, the related work of achieving high precision time and frequency signal is introduced on 275 km Beijing—Shanghai backbone. Based on the characteristics of long distance fiber link, the impact of loss and scattering of link, dispersion and frequency noise, feedback loop bandwidth and dynamic range of compensating system on time and frequency transfer performance is discussed. Results show that the frequency stabilities are up to 5×10-14 in one second averaging time and 7×10-14 in one day averaging time, and time synchronization stabilities are up to 2.4 ps in one thousand second averaging time.

The geostationary earth orbits（GEO） satellite-borne laser communication terminal is of an open-type structure and the laser communication antenna is uncovered out of the satellite. The effective operation time of the antenna is only 4 to 6 hours per day due to the strictly alternate orbital heat flux changing. The solar window is designed to weaken the effect of the orbital heat flux and to improve the thermal stability of the laser communication antenna based on existing thermal control technology, and the optimal design of the optical and thermal properties is obtained by the finite element analysis method as the thermal properties of the laser communication antenna is most stable. The solar window has a larger solar transmission, smaller absorption and larger infrared emissivity for inner and external surfaces. The laser communication antenna secondary mirror can meet the temperature index on whole day and the main mirror can be used for 12 hours per day by utilizing the optimal designed solar window. The effective operation time of the secondary mirror and the main mirror are twice and triple respectively as much as the time of the basic case without the solar window.

The electrode pulsed arc discharge excitation experiments on fiber Bragg grating (FBG) inscribed by ultraviolet exposure are reported. The effect of discharging at different grating positions on the spectral characteristics of fiber grating is studied especially. Experimental results show that the reflectivity of fiber grating is decreased,the grating spectrum is broadened and shows a red shift during electrode discharge excitation. The spectral changes induced by discharge are reversible. The spectral characteristics of fiber grating are analyzed in a non-uniform temperature field formed by electrode discharge with the transfer matrix method, and the theoretical results are in good agreement with the experimental results. Specifically, when the electrode discharges in the center of fiber grating, the decline amplitude of grating reflectivity reaches to maximum and the optical reflection spectrum is 12 dB less than initial spectrum, which shows switching characteristic.

Multiple input multiple output (MIMO) is used in order to decrease the effects of atmospheric attenuation and turbulence on free space optical communication (FSO). It is assumed that the intensity modulation direct detection (ID/DD) with on-off keying (OOK) is used in the FSO communication systems. The channel is memoryless, stationary ergodic, identically independent and distributed with additive white Gaussian noise (AWGN). The perfect channel state information (CSI) is available to both the transmitter and the receiver. Based on the hypothesis, the multiple input multiple output channel model and systems model are established under the integrated atmospheric effects. The closed form expressions of ergodic capacity and outage probability of FSO system with the equal gain diversity combined technique are derived. The influences of various weather conditions and turbulence strength on space optical communication link are analyzed. The results indicate that the performance of the communication system is increased and the atmospheric effect is decreased with the increase of the number of transmit aperture and receiving aperture. Considering the complexity and the increasing performance of the system, 2 transmission apertures and 2 receiving apertures are more suitable for FSO.

As a new type of optical fiber sensing technology, the optical fiber laser sensing system combines the advantages of fiber sensors of high sensitivity, distributed measurement capability and less susceptible to electromagnetic interference, with the advantages of fiber lasers of narrow linewidth and high optical signal to noise ratio, so it is used in measurement and security monitoring in the field of oil, mining, bridges, electricity, aircraft and so on. The research progress of optical fiber laser sensing system is introduced from two aspects. One is the optical fiber laser sensing system for the single-parameter measurement, including temperature, strain, refractive index, current, acoustic wave, wind speed and so on; the other is the optical fiber laser sensing system for the dual-parameter measurement, which is mainly used to solve the problem of cross sensitivity of temperature and transverse stress, strain and refractive index.

The coherent optical time domain reflectometry (COTDR) based on Rayleigh scattering is an important technique for the monitoring of ultra-long fiber link, for its high dynamic range and sensitivity. The principle and development of COTDR is introduced. The design method and key techniques such as the coherent fading noise suppression, measurement speed enhancement and multi-functions fusion system structure are discussed. And several important applications of COTDR are given, including ultra-long communication optical fiber link monitoring and distributed temperature/strain sensing.

A criterion is proposed to judge if the axes of the emergent beam and the telescope of coaxial Mie lidar are in the same line. Then the auto collimation system of coaxial Mie lidar is designed, in which the control target aims at the maximization of the criterion, the two-dimension electric driving mirror is used as actuator, and the variable step size regulation algorithm is used. The experiment results show that the system can realize auto collimation regulation with high precision and high speed, and the regulation accuracy is up to 0.05 mrad. The effectiveness of collimation criterion and regulation algorithm is verified. The system is benificial for realizing automation and unattended operation of lidar system.

The current phase generated carrier (PGC) demodulation system based on light source internal modulation method has phase noises of environmental and light source introduced by the interferometer arms difference and the high speed light source tuning. Aiming at this, a new PGC demodulation system based on microfiber all-optical phase modulator is studied. To satisfy the PGC demodulation requirements, the microfiber all-optical phase modulator based on light absorbing thermal effect is designed with bandwidth exceeding 10 kHz. Based on this, the PGC demodulation system of fiber hydrophone is constructed, and used to study the detection and demodulation performance of underwater acoustic signal. Experimental results show that the proposed system can realize good demodulation performance for the underwater signal with frequency below 3 kHz.

A method by low speed pre-acquiring and high speed accurate addressing of reference speckle acquisition is proposed to solve the limit of the low acquisition speed of reference arm array CCD. According to the method, an experimental system of prebuilt three-dimensional ghost imaging lidar is established. In order to ensure the twice sampling independence of preset trajectory scheme using the speckle size decided by translating a system once. The correlation of the speckle field is used as the evaluation function. Experimental results show that the programming is precise and repeatable. Meantime, an experiment with prebuilt three-dimensional ghost imaging lidar in 2 kHz acquisition speed is performed, and the results are almost same as the real-time acquired three-dimensional ghost imaging lidar with the real-time sampling rate of 100 Hz, which demonstrates the effectiveness of this method.

The research on performance improvement and efficiency promotion for the Rayleigh Doppler wind lidar is carried out. The optical efficiency of the improved receiver is 1.874 times higher than that of the original receiver by improving coupling efficiency, transmission efficiency and collection efficiency of optical signal in the receiver system, and the problem of low optical efficiency in the ultraviolet band in the current system is solved. The locking accuracy of laser frequency can meet the need of system detection accuracy by means of optimizing calibration parameters and increasing the surface reflectivity of etalon to 80%. Measurement deviation of line of sight wind velocity, which is owing to the Doppler zero offset, is eliminated by using a 60 m quasi static zero wind velocity real-time calibration pipeline, which improves the measurement accuracy. The comparison between the results obtained by lidar and radiosonde shows good agreement. The maximum deviation of wind velocity and direction is 6.73 m/s and 24.6°, respectively, and the average deviation of wind velocity and direction is 1.28 m/s and 2.65°, respectively. The wind field data with high spatial-temporal resolution is obtained in many areas, providing necessary data support for studying the change discipline of the middle and upper atmosphere and understanding the regional change characteristic of space environment.